Nitro Group Containing Drugs - Journal of Medicinal Chemistry (ACS

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Cite This: J. Med. Chem. XXXX, XXX, XXX−XXX

Nitro-Group-Containing Drugs Kunal Nepali, Hsueh-Yun Lee, and Jing-Ping Liou* School of Pharmacy, College of Pharmacy, Taipei Medical University, 250 Wuxing Street, Taipei 11031, Taiwan.

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S Supporting Information *

ABSTRACT: The nitro group is considered to be a versatile and unique functional group in medicinal chemistry. Despite a long history of use in therapeutics, the nitro group has toxicity issues and is often categorized as a structural alert or a toxicophore, and evidence related to drugs containing nitro groups is rather contradictory. In general, drugs containing nitro groups have been extensively associated with mutagenicity and genotoxicity. In this context, efforts toward the structure− mutagenicity or structure−genotoxicity relationships have been undertaken. The current Perspective covers various aspects of agents that contain nitro groups, their bioreductive activation mechanisms, their toxicities, and approaches to combat their toxicity issues. In addition, recent advances in the field of anticancer, antitubercular and antiparasitic agents containing nitro groups, along with a patent survey on hypoxia-activated prodrugs containing nitro groups, are also covered. histone deacetylase inhibition,16,17 DNA alkylation,18 or tubulin polymerization inhibition,19,20 and have significantly demonstrated hypoxia-induced effects that are attributed to their bioreductive activation potential.18,21−24 The antitubercular potential of some successful clinical or preclinical drugs also relies on their bioactivation, and many drugs are undergoing detailed preclinical investigations or are being examined in clinical trials.25 Nitroimidazole is one such versatile heterocyclic moiety that has been widely employed to exert diverse biological effects and has often been explored as a bioreductive arm.26−28 This is clearly evidenced by the number of clinically approved nitroimidazole-containing compounds, mostly for infectious diseases, and the large repository of such compounds endowed with exciting and optimistic preclinical potential.29−39 Not only the research literature but also the patent literature emphasizes the effects of compounds that contain nitro groups and are mediated through bioreduction.40−52 Despite these wide applications, it has also been well evidenced that drugs containing nitro groups can induce severe toxicity, and this is indubitably the reason in many cases for their being avoided. Concomitantly, selective toxicity with nitroaromatic and heteroaromatic compounds also forms the basis of chemotherapy that results in the poisoning of bacteria, parasites, or tumor cells without harming the host organism or normal cells.9 For this reason, medicinal chemists have been constantly striving and making great efforts to explore the bioactive potential of nitroaromatic and heteroaromatic

1. INTRODUCTION The nitro group is considered to be a versatile and unique functional group in medicinal chemistry. It possesses a strong electron attracting ability that creates localized electrondeficient sites within molecules and interacts with biological nucleophiles present in living systems, such as proteins, amino acids, nucleic acids, and enzymes. The interaction can occur by nucleophilic addition and electron transfer involving oxidation and reduction, or also simply by molecular complexation, to induce desired or undesired biological changes.1,2 Owing to this, numerous medicinal chemistry campaigns have been initiated to investigate compounds containing nitro groups. As such, drugs containing nitro groups have a long history of use as antineoplastic,3 antibiotic,4−6 and antiparasitic agents,7−9 as well as tranquilizers, fungicides, insecticides, and herbicides.2,10 Moreover, nitro groups can undergo reduction and may serve as prodrugs due to their bioactivation by enzymatic reduction, generating reactive species and ultimately inducing biological effects. It is well established that some nitroaromatic or nitroheteroaromatic compounds can induce therapeutic effects via redox cycling of single electron reduction by different flavoenzymes or from the alkylation of DNA and/or other cellular nucleophiles by the products of their bioreductive activation process.11 Recent publications indicate that the majority of efforts of medicinal chemists in the past decade have been directed toward the exploration of compounds containing nitro groups as anticancer agents, antitubercular agents, and antiparasitic agents. These compounds induce their cell-killing effects by diverse mechanisms, such as topoisomerase inhibition,12−15 © XXXX American Chemical Society

Received: January 29, 2018 Published: October 8, 2018 A

DOI: 10.1021/acs.jmedchem.8b00147 J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry

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Figure 1. Mutagenic pathway of nitroarenes.

major adverse effects encountered with such agents.54 Owing to this, these agents are often considered by researchers as structural alerts, which blocks exploration of their therapeutic utility.55 Nitro compounds require enzymatic reduction to induce the therapeutic and cytotoxic effects, and this reduction is usually mediated by nitroreductases using flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups and nicotinamide adenine dinucleotide (NADH) or nicotinamide adenine dinucleotide phosphate (NADPH) as reducing agents.54 A generalized sequence for the bioreductive pathway involves the formation of a nitroso derivative, a nitro radical anion, a nitroxyl radical, hydroxylamine, and a primary amine. The complete pathway is shown in Figure 1. Toxicity issues associated with nitro compounds have been attributed to each of these intermediates, but hydroxylamine derivatives are particularly responsible for methemoglobinemia, whereas the cumulative effects of the nitro radical anion, nitroso derivatives, or esterified hydroxylamine (e.g., sulfate derivatives) are held responsible for the mutagenic and carcinogenic activities. Superoxide anions, hydrogen peroxide, and hydroxyl radicals formed during the redox cycling of the nitro radical anion may also lead to carcinogenicity.54 Currently, drugs containing nitro groups are being investigated in detail as a part of a drug repurposing program for previously failed drugs and currently used clinical candidates, along with simultaneous efforts to develop new chemical classes. It is highly probable that we will find new treatments based on existing therapeutic drugs or previously failed clinical candidates, as the toxicity data in humans are

compounds in diverse applications, from the treatment of parasitic infections to cancer and in many enzyme expressiondependent diseases. Overall, the issues related to such agents are indeed contradictory, as the nitro group is considered as both a pharmacophore, or an integral part of the pharmacophore, and a toxicophore or a structural alert.53 In this Perspective, we focus on various aspects of agents containing nitro groups and discuss their toxicity issues and their role as prodrugs. This compilation emphasizes the bioreductive mechanisms leading to activation of nitro compounds, FDA-approved drugs containing nitro groups, their toxicities, and a clinical update, along with a section regarding recent developments in the field of anticancer, antitubercular, and antiparasitic agents that contain nitro groups. A library of agents containing nitro groups that have demonstrated their bioactive potential as anticancer, antitubercular, or antiparasitic potential in preclinical and preliminary screening assays is presented to create awareness about those structures rather than merely to treat them as missed opportunities. This Perspective also covers a recent patent survey and includes a discussion of the approaches used to combat the toxicity and selective targeting by nitro compounds.

2. DRUGS CONTAINING NITRO GROUPS: CURRENT SCENARIO, TOXICITY ISSUES, AND BIOREDUCTIVE THERAPY Toxicity has always been an issue with drugs, especially those containing nitro groups, and carcinogenicity, hepatotoxicity, mutagenicity, and bone marrow suppression have been the B



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Figure 2. Drugs containing nitro groups.

already available.56 As a part of a drug repurposing program to address the urgent need for the development of cost-effective drugs for visceral leishmaniasis, the antitubercular drug delamanid (OPC- 67683, 13) was investigated and found to be a potent inhibitor of Leishmania donovani, both in vitro and in vivo. The study revealed that delamanid at 30 mg/kg induced a sterile cure in the mouse model of visceral leishmaniasis. Further exploration of the mechanistic insights revealed the rapid metabolism of delamanid by parasites via an enzyme that is distinct from the nitroreductase activating fexinidazole (9). Overall, the results of the study found that delamanid presents enough promise to be repurposed as an oral therapy for VL.57 Furthermore, structural engineering investigations on delamanid have led to the identification of DNDI-0690 and DNDI2908 as promising agents that are endowed with significant potential for the treatment of VL. A recent study indicated that DNDI-0690 shows promising in vivo efficacy, better solubility, and reduced cardiotoxicity. The toxicity evaluation study further found that DNDI-0690 is a promising preclinical candidate for the treatment of VL.58 DNDI-VL-2098 is another delamanid analog that was identified as a VL preclinical candidate. DNDI-VL-2098 was found to possess potent submicromolar in vitro activity in a macrophage amastigote model against various L. donovani strains and demonstrated substantial in vivo activity in a HU3/BALB/c mouse model.59,60 Identification of novel bromo- and extra-terminal domain (BET) inhibitors by the drug repurposing of nitroxoline is also a motivating example.61 Other investigations on

drugs that contain nitro groups involve the bioisosteric replacement of the nitro group in an attempt to solve the toxicity issues. However, this approach has yielded contradictory results, in terms of enhancing the bioactivity and attenuating the toxicity. The investigation of flutamide exemplifies a case where the strategy of bioisosteric replacement of the nitro group with a cyano group was attempted, anticipating a decreased cytotoxic potential of the drug while preserving its antiandrogen activity. The replacement was anticipated to attenuate the reduction of the nitroaromatic group, simultaneously enabling the retention of a strong electron withdrawing group. The results were quite optimistic, as the cyano analog displayed equivalent anti-androgen activity in M12 AR cells to that of flutamide.62 However, a recent study on niclosamide (an FDA-approved anthelmintic agent) did not yield favorable results, in which the nitro group was replaced with groups such as acetyl, benzoyl, methyl sulfone, or amide. The replacements led to significant declines in activity, as evidenced in the Wnt-3-A-stimulated TOP flash assay.63 Furthermore, this strategy has been extensively attempted at the preclinical level by researchers; often, the analogs lacking the nitro group, the des nitro analogs (anticancer, antitubercular, and antiparasitic agents), were found to have a diminished activity potential. This is discussed in sections 4 and 5. Thus, it can be concluded that the results of bioisosteric replacement studies of the nitro group have demonstrated fluctuating outcomes in the context of the modulation of activity and toxicity. In summary, reinvestigations of some C



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Table 1. Drugs Containing Nitro Groups and Their Associated Clinical Updates drug name Azomycin (1)

Nifurtimox(2)

disease/use/indication Antimicrobial antibiotic produced by a strain of Nocardia mesenterica Chagas disease

toxicities/drug effects

Neurological

clinical studies

updates and other relevant information

1950

year

Cervical cancer, renal cancer (NCT00588276)

Has been employed as a lead for the design of nitroimidazole-based drugs.

72

1965

Refractory or relapsed neuroblastoma or medulloblastoma (NCT00601003) Human African trypanosomiasis (NCT01685827) Dosage regimens and chronic Chagas disease (NCT0238635, NCT0012391, NCT01549236) Safety and tolerability studies (NCT02392026)

Is the only available drug in use for the treatment of Chagas disease other than benznidazole.

73−76

Benznidazole (3)

Chagas disease

Hepatotoxicity, peripheral neuropathy, and angioedema

1971

Metronidazole (4)

Antibiotic and antiprotozoal

Anticipated to be a human carcinogen

1960

Secnidazole (5)

Antiprotozoal agent

Vulvo-vaginal candidiasis

Clostridium dif f icile diarrhea (NCT02200328) Bacterial vaginosis (NCT03099408) Vaginal discharge (NCT02111629)

1960−1970

Bacterial vaginosis (NCT02452866, (NCT02418845)

Tinidazole (6)

Antiparasitic

1972

Ornidazole (7)

Antiprotozoal

1977

Misonidazole (8)

Radiosensitizer

1974

Bacterial vaginosis (NCT0033463, NCT00229216, NCT00324142) Giardiasis (NCT02942485)

Crossover bioequivalence study (NCT01591889) Elective colorectal surgery (NCT02618720) Colorectal cancer (NCT00574353)

Induces sensitization in normally resistant hypoxic tumor cells

Fexinidazole (9)

HAT, VL, and Chagas disease

Prostate adenocarcinoma (NCT01898065) Malignant glioma (NCT01868906) Adult glioblastoma (NCT00902577) Visceral leishmaniasis (NCT01980199)

1970−1980

African trypanosomiasis (NCT03025789, NCT02169557, NCT02184689, NCT01685827, NCT02571062) Chagas disease (NCT02498782)

D

Is a component of nifurtimox−eflornithine combination therapy for human African trypanosomiasis (HAT). Accelerated approval was granted for benznidazole for the treatment of Chagas disease in children 2−12 years of age by the FDA (August 29, 2017).

ref

73−78

PYLERA, a triple-combination single-capsule product comprising tetracycline, metronidazole, and bismuth, is being developed by Aptalis as a first-line therapy for the eradication of Helicobacter pylori infection.

4, 31, 73 −75, 79

Phase III evaluation of OSTERVA (an oral tastemasked formulation of secnidazole developed by Ethypharm) for bacterial vaginosis is ongoing in the U.S. A single-dose oral granule formulation of secnidazole (SOLOSEC) is being developed by Lupin for bacterial vaginosis. The agent obtained approval in the U.S. for this indication. Tinidazole (TINDAMAX) received U.S. FDA approval on May 24, 2007, as announced by Mission Pharmacal for the treatment of bacterial vaginosis. Already marketed in the U.S. for the treatment of trichomoniasis. This approval was attributed to the results of a randomized, placebo-controlled, double-blind, multicenter phase III trial.

5, 73−75

Levornidazole (a levo-isomer of ornidazole) for anaerobic bacterial infections has been developed by Nanjing Sanhome Pharmaceutical. Under investigation in clinical trial NCT00038038 (Assessment of Head and Neck Tumor Hypoxia Using 18F-Fluoromisonidazole).

As a part of drugs for neglected diseases initiative (DNDi, Switzerland), sulfone derivative of fexinidazole is being developed for the treatment of visceral leishmaniasis. Fexinidazole sulfone, an active metabolite of fexinidazole, has shown promise to decrease the time required to attain effective blood levels of the drug.

73−75, 80

73−75, 81 73−75

73−75, 82

Being evaluated by Sanofi for the oral therapy of trypanosomiasis (DNDi; Switzerland).



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Table 1. continued drug name Pimonidazole (10)

disease/use/indication

toxicities/drug effects

Radiation sensitizer

Transient central nervous system syndrome

year

Megazol (11)

Used to treat protozoan infections

Mutagenic and genotoxic

1960−1970

Venetoclax (12)

Used for the treatment of chronic lymphocytic leukemia (CLL)

Tumor lysis syndrome

U.S. FDA granted breakthrough therapy designation to venetoclax for CLL in 2015

Delamanid (OPC- 67683, 13)

A mycolic acid biosynthesis inhibitor endowed with highly potent activity against TB, including MDR-TB

clinical studies

1970−1980

Refractory pulmonary tuberculosis (NCT00374517) Tongue cancer (NCT03181035) Prostate cancer (NCT02095249) Melanoma (NCT01992042)

Approved for medical use in 2014 in Europe, Japan, and South Korea

Entacapone (15)

Used for the treatment of tapeworm infections

Used for the treatment of Parkinson’s disease

Orthostatic hypotension, severe rhabdomyolysis

1999

A COMT inhibitor

Follicular lymphoma, Non-Hodgkin’s lymphoma, lollicular non-Hodgkin’s lymphoma (NCT03113422)

Being evaluated in a phase III combination study with rituximab (phase III) in patients of relapsed/refractory chronic lymphocytic leukemia.

73−75, 86

Relapsed or refractory malignancies (NCT03236857, NCT02966756, NCT02756897, NCT02980731) Safety, tolerability, and efficacy studies (NCT02573350)

Roche is conducting CLL clinical trials, and recent results revealed that the tested combination prolonged the progression-free survival in comparison to the combination of bendamustine with rituximab.

Methamphetamine abuse (NCT02058966)

approved drugs using structure−activity relationship studies, redesigning the dosage regimens, and evaluating the drug in a combination therapy may prove to be beneficial. Figure 2 and Table 1 present several representative examples of such drugs containing nitro groups and their clinical updates. The structures covered represent the drugs containing nitro groups that either have been employed as leads for the design of new

Has been categorized as an oxygen-mimetic radiosensitizer.

73, 84, 85

Pulmonary tuberculosis (NCT00401271) Multidrug-resistant refractive tuberculosis (NCT01131351). Metastatic prostate cancer (NCT02532114) In rheumatoid arthritis (NCT03160001) Resectable colon cancer (NCT02687009) Parkinson’s disease (NCT00373087, NCT00391898, NCT00237263) Chizophrenia (NCT00192855)

1982

ref 73−75, 83

More effective than benznidazole therapy for Chagas disease. Mutagenicity and genotoxicity have hindered its clinical applicability.

Multidrug-resistant tuberculosis (NCT01424670)

Niclosamide (14)

updates and other relevant information Under investigation for the diagnostic of prostate cancer and head and neck cancer.

In combination studies with standard drugs ethambutol, isoniazid, pyrazinamide, rifampicin, aminoglycoside antibiotics, and quinolones (phase II clinical trials), it was observed that the healing rates (measured as sputum culture conversion) were substantially better in patients who additionally took delamanid. Commercialization rights were acquired by Mylan, which was granted an exclusive license by Otsuka on August 24, 2017, for prioritizing access to delamanid (Deltyb) in South Africa and India.

73−75, 87

Reported to suppress the Wnt/β-catenin pathways and the mTOR/STAT3 pathway.

63, 73 −75, 88

Preclinical trials attempting to replace the nitro group of niclosamide have yielded analogs with mixed results.

Approval from the European Commission for the marketing of entacapone was received by Orion Pharma and Novartis in Europe.

73−75, 89

Entacapone has been launched by Novartis in the U.S. (Novartis, Oct 1999), Canada (Novartis, Oct 2001), and Russia (IMS, Feb 2009). It was launched by Orion Pharma in Russia in Feb 2009.

therapeutics or have been evaluated with respect to different indications, combinations, dosage regimens, or formulations. Notably, even some of the withdrawn drugs have been the subject of renewed clinical interest (Table 1S, Supporting Information), which may revive their utility. The risk of mutagenicity is one of the prime reasons why nitroarenes are less preferred in the area of drug design. The E



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Figure 3. Agents containing nitro groups with bioreductive potential.

the appropriate positioning of substituents in the vicinity of the nitro group (stereoelectronic factors) were found to be critical in modulating the mutagenic profile of the compounds.66 Recently, structure−genotoxicity relationship studies were carried out by Shamovsky et al. for aromatic amines. The investigation was based on a clear understanding that activation of ArNH2 generally is initiated with N-hydroxylation by P450 enzymes (CYP1A2). The same pathway is followed by nitroarenes after undergoing nitroreduction by a bacterial nitroreductase to hydroxylamines. Further, the generation of nitrenium ions by the heterolytic dissociation of hydroxylamines and their esters is catalyzed by protonation under acidic conditions. It was concluded that structural alterations in ArNH2 moieties enabling the disruption of the geometric compatibility with CYP1A2 either hinder the proton abstraction or strongly destabilize the nitrenium ion, thus preventing genotoxicity. The authors continued their investigations on the mechanistic aspects of N-hydroxylation by P450 enzymes leading to mutagenicity and deduced logical strategies to avoid genotoxicity issues by hindering Nhydroxylation.67 It is well-known that hydroxylamino, nitro,

nitro group and its location on aromatic rings influence the DNA binding and mutagenic profile of an aromatic compound. It has widely been suggested that nitro reduction plays a critical role in mutagenesis, as major DNA adducts formed with nitroaromatics have been isolated and characterized. However, there is a difference in their rates of formation and in the sites at which they may be preferentially introduced. There is a characteristic signature of mutagenic specificity associated with each mutagen, and thus they exert differential mutagenic effects.64 At the same time, not every drug containing a nitro group is mutagenic, and the antitubercular drug discovery field exemplifies this. Delamanid (13), BTZ043 (18), and pretomanid (PA-824, 24) represent antitubercular drugs in clinical development that have not been found to be mutagenic.65 Numerous studies are being conducted to solve the mutagenicity issue of drugs containing nitro groups. An interesting study aimed at mitigating the mutagenicity of nitro compounds was conducted by Landge et al.66 The investigation led to the identification of benzothiazole-based compounds containing nitro groups that were nonmutagenic and demonstrated improved safety profiles. Electronic factors and F



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Figure 4. Bioreductive activation of PR104.

of drugs containing nitro groups is somewhat better and more clarified, as the reasons for the toxicities attributed to their uncontrolled bioactivation are well-known. Thus, the available information could be employed as a tool with which to solve the issues of associated mutagenicity and genotoxicity. The term “bioreductive therapy” depends on enzymemediated reduction, which converts a nontoxic prodrug into a relatively cytotoxic reduced drug. This therapy can be utilized for attaining tumor tissue selectivity via the assistance of endogenous reductases that either are inhibited by oxygen or have the ability to metabolize the prodrug to a metabolite, a transient intermediate that can be back-scavenged by oxygen. This phenomenon laid the foundation for the design of selective cancer chemotherapeutic agents, particularly those directed toward solid tumors, which have a diminished oxygen supply, with compounds capable of undergoing anaerobic reduction to a cytotoxin anticipated to exert higher cell killing effects toward hypoxic tumor cells in comparison to oxygenated normal tissues.90−93 In this view, the design of hypoxiaactivated drugs or prodrugs appears to be an immediate need, and drugs containing nitro groups are the class most sought after, having been well explored mechanistically. Nitroaryl/heteroaryl-based compounds have demonstrated an immense potential, both as bioreductive components incorporated in the chemical architecture of the bioactive molecules and as triggers capable of inducing fragmentation. Numerous examples, such as PR-104A (16),94 TH4000 (17),95 BTZ-403 (18),96,97 pretomanid (24),98 SN-38 (27),99 CB1954 (25), 100,101 and TH-302 (26), 102 stand out as comprehensively explored bioreductive agents, while the concept has been expanded and applied to other established

and nitroso groups are able to generate amine groups by metabolic conversion. Moreover, the formation of N-hydroxylamines by microsomal and cytosolic enzyme-catalyzed reduction is also responsible for the initial activation of nitroaromatic hydrocarbons. In this context, microsomal nitroreduction also appears to depend on the cytochrome P450 complex.68 Thus, the results of these structure− genotoxicity studies can be of utility for the class of compounds containing nitro groups, which are essentially masked amines. Seger et al. also explored the CYP-mediated N-hydroxylation of primary and secondary amines and deduced that the hydrogen abstraction mechanism is preferred over a direct oxygen transfer mechanism. Their results could be quite useful in predicting those compounds that are likely to be N-hydroxylated by CYPs.69 Another study was recently conducted by Boechat et al. to evaluate a series of nitroimidazoles for genotoxicity (the comet assay) and mutagenicity (the Salmonella/microsome assay). The results of this study indicated that the placement of a nitro group at C-4 and a −CH3 group at C-2 in nitroimidazoles diminished the genotoxic effects.70 In view of these optimistic results, several programs to establish the structure−genotoxicity and mutagenicity relationship for drugs containing nitro groups must be conducted. Overall, the toxicities associated with drugs containing nitro groups have been extensively reported, but several other chemotherapeutics also have similar issues. Several drugs that do not contain nitro groups have been withdrawn due to these issues.71 The evidence clearly indicates the association of toxicity issues with most chemotherapeutics, but it is also clearly evident that toxicity issues are far more intensified in the case of drugs containing nitro groups. In fact, the understanding G



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Figure 5. Bioreductive activation of TH-4000.

Figure 6. Bioreductive activation of BTZ-043.

chemotherapeutic agents, such as paclitaxel prodrug (19),103 nitrochlormethylbenzinidazoles (nitroCBI, 20),104 nitrobenzylphosphoramide mustards (21),105 CH-01 (23),106 KS-119 (28),107,108 and O6-benzylguanine prodrugs (22),18 with results indicating optimistic outcomes. These agents have demonstrated a promising preclinical potential and are undergoing clinical trials for diverse tumor types. Figure 3 shows the structures, and Figures 4−16 represent the bioreductive metabolic pathway of these agents. Table 2 presents a brief overview of some selected nitroaryl/heteroarylbased bioreductive agents that are being examined at the clinical stage or are undergoing preclinical evaluation. In view of the aforementioned evidence, the nitroaryl- and nitroheteroaryl-based compounds appear to be logical and probable candidates for use in the development of bioreductive prodrugs. Thus, O’Connor et al., while working on bioreductive prodrugs of (1-methyl-2-nitro-1H-imidazol-5-yl)methanol, proposed various strategies for attaching the bioreductive nitro group to a range of functionalities by methods such as alkylation of nitrobenzyl halides, esterification of nitroaroyl halides, reduc-

tive amination with nitrobenzldehydes, and carbonate/ carbamate formation of nitrobenzyl carbonobromidates using alcohols and amines. Their attempts clarified many aspects of the design of molecularly targeted hypoxia-activated prodrugs, as the study revealed that the point of attachment of the bioreductive group is crucial for attaining the required difference in activity under aerobic and anaerobic conditions. The authors also emphasized the desired inactivity of the unreduced form in comparison to the active compounds generated after fragmentation.109 The structures of some other nitro-group-containing bioreductive compounds other than the agents presented in Figures 4−16 and Table 2 are shown in Figure 17. Among them, NCLQ-1/2 (72, 73) represents the DNA-targeted bioreductive prodrug for cancer therapy,110 BCCA621C (75)111 is a DNA-dependent protein kinase (DNA-PK) inhibitor, and nimorazole (76) is a nitroimidazole being investigated for effects in locally advanced head and neck cancer (NCT02976051). FSL-61 (74) is a hypoxia-selective POR-activated fluorogenic probe for one-electron reductases. It H



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Figure 7. Bioreductive activation of paclitaxel prodrug.

Figure 8. Bioreductive activation of nitroCBI as hypoxia activated prodrug.

that are primarily attributed to their bioreductive metabolism. Some selected exciting studies are covered that highlight the current focus of extracting the value of the nitro group based on its reduction potential. 3.1. Anticancer Prodrugs Containing Nitro Groups (Hypoxia-Activated Prodrugs and Others). The duocarmycins represent a small group of extremely cytotoxic natural

is employed to activate bioreductive prodrugs for targeting tumor hypoxia.112

3. RECENT ADVANCES OF ANTICANCER DRUGS CONTAINING NITRO GROUPS This section presents the diverse chemical architectures possessing nitro groups, which induce the cell-killing effects I



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Figure 9. Bioreductive activation of nitrobenzylphosphoramide mustards.

Figure 10. Bioreductive activation of 4-nitrobenzyloxycarbonyl derivatives of O6-benzylguanine.

products that alkylate adenine in the minor groove of DNA. Over the years, duocarmycin analogs have displayed positive antitumor activity in preclinical and clinical studies. With this background, Tercel et al. designed several nitro analogs of the duocarmycins and evaluated them for hypoxia-selective anticancer potential. Compound 77 at 42 μmol/kg in combination with a single dose of γ irradiation eliminated detectable clonogens in some SiHa cervical carcinoma xenografts. Compound 77 also induced the regression of all treated A2780 ovarian tumor xenografts in combination with a well-tolerated multidose schedule of gemcitabine (Figure 18).113 Duan et al. designed a series of achiral hypoxia-activated prodrugs bearing a 2-nitroimidazole moiety as the hypoxic trigger and an isophosphoramide mustard as the released toxin (a DNA cross-linking toxin of the prodrug, ifosfamide). Among the prodrugs synthesized, compound 78 was identified as a promising antitumor agent possessing substantial hypoxiadependent cytotoxicity and microsomal stability in addition to a striking efficacy in vivo in an MIA PaCa-2 pancreatic cancer orthotopic xenograft model (Figure 18).114

Naimi et al. designed a series of nitric oxide donor 3′-O-nitro derivatives of 2′-deoxyuridine, 2′-deoxycytidine, and 5′-Onitro-2′-deoxyuridine. Biological evaluation indicated that upon incubation in the presence of L-cysteine or serum, the designed compounds (79−81) released a greater percentage of NO in comparison to isosorbide dinitrate, and their cytotoxicity against a variety of cancer cell lines was comparable to that of 5-iodo-2′-deoxyuridine but weaker than that of 5-fluoro-2′deoxyuridine (Figure 19).115 In view of evidence that nitrochloromethylbenzindolines (nitroCBIs) as prodrugs are capable of undergoing hypoxiaselective metabolism forming potent DNA minor groove alkylating agents exerting antitumor effects, Tercel et al. reported a series of analogs with an extra electron-withdrawing substituent that serves to elevate the one-electron reduction potential of the nitroCBI. The most potent compound (82) was endowed with promising biological attributes, including (1) hypoxic cytotoxicity ratios of 275 and 330 in Skov3 and HT29 human tumor cell lines, respectively, (2) capability of being efficiently and selectively metabolized to the desired aminoCBI, (3) selective cytotoxic effects under hypoxia toward the panel of J



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Figure 11. Bioreductive activation of protected chk1 inhibitor.

Figure 12. Bioreductive activation of PA-824.

of adverse effects associated with these anticancer drugs in the normoxic region and improvement of the survival rate of mice as indicated by the results of in vivo studies, and (3) localization of the prodrugs at the hypoxic region compared to conventional anticancer drugs, which localize only in the normoxic region, as revealed by immunofluorescence analysis (Figure 20).22 Checkpoint kinase 1 (Chk1) is a promising cancer therapeutic target that is associated with high levels of hypoxia-induced replication stress. Coupled with the fact that hypoxic cells also exhibit similar levels of sensitivity to inhibition of the Aurora A kinase, this evidence motivated the

cell lines tested, and (4) showing in vivo potential against hypoxic tumor cells in a human tumor xenograft (Figure 20).104 Ikeda et al. proposed the development of novel hypoxiaactivated prodrugs of well-explored anticancer agents. This process involved the addition of a 2-nitroimidazole moiety to two standard anticancer drugs, doxorubicin and gemcitabine (83 and 84). An interesting finding was the formation of a sixmembered cyclic structure from the prodrug that enabled the release of the active drug under hypoxia. Various favorable trends were observed in the study, including (1) selective activation under hypoxic conditions, (2) a significant reduction K



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Figure 13. Bioreductive activation of CB-1954.

Figure 14. Bioreductive activation of TH-302.

Figure 15. Bioreductive activation of SN-38 prodrug activated via enzymatic metabolism under hypoxic conditions.

authors to design a bioreductive Chk1/Aurora A inhibitor, CH01 (27). The biological evaluation results of CH-01 showed the dual inhibitor-induced selective inhibition of Chk1/Aurora A in hypoxic conditions and a significant loss of viability in the cancer cell lines. In view of these optimistic results, compound 27 appears to be a promising bioreductive chemotherapeutic

agent, as it targets the therapy-resistant tumor fraction while shielding the normal tissue from therapy-induced genomic instability (Figure 21).106 Zhu et al. synthesized 4-nitrobenzyloxycarbonyl prodrugs of O6-benzylguanine (O6-BG), which is known to be an effective known inhibitor of O6-alkylguanine-DNA alkyltransferase L



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Figure 16. Bioreductive activation of KS119 prodrug activated via enzymatic metabolism under hypoxic conditions.

Table 2. Nitro-Group-Containing Agents with Bioreductive Potential and Their Clinical/Preclinical Update name PR-104 (16) Tarloxotinib (TH-4000, 17) BTZ043 (18) Paclitaxel prodrug (19)

Nitrochloromethylbenzindolines (NitroCBI, 20)

Nitrobenzylphosphoramide mustards (21) 4-Nitrobenzyloxycarbonyl derivatives of O6-benzylguanine (22) CH-01 (23)

Pretomanid (24)

(5-[Aziridin-1-yl]-2,4dinitrobenzamide (Prolarix, CB1954, 25) Evofosfamide (TH-302, 26)

SN-38 prodrug (27) KS119 (28)

clinical/preclinical update Solid tumors (phase 1, NCT00616213, NCT00349167, NCT00459836, NCT01358227). Improves selectivity and reduces some of the toxicities observed with existing EGFR TKIs.95 Has been employed as a lead molecule for several preclinical investigations.96,97 1. The prodrug releases paclitaxel after reduction and subsequent 1,6-elimination or 1,8-elimination. 2. The prodrug exhibits stability in buffer. 3. Prodrug exhibited diminished cytotoxic profile in aerobic cytotoxicity assays.103 1. Efficiently and selectively metabolized to the aminoCBI. 2. Displayed selective cytotoxicity under hypoxic conditions in several cell lines. 3. Hypoxic cytotoxicity ratios of 275 and 330 in Skov3 and HT29 human tumor cell lines. 4. Demonstrates activity against hypoxic tumor cells in a human tumor xenograft in vivo.104 1. The prodrugs exhibit low IC50 and the high selectivity toward E. coli nitroreductase-expressing cells. 2. Prodrugs exhibit low cytotoxicity before reduction and release the phosphoramide mustard upon bioreduction. 3. Also exhibits good bystander effects in NTR-expressing cells.105 1. Induced preferential release of the AGT inhibitor O6-BG under hypoxia. 2. The prodrug demonstrated significant enhancement on laromustine cytotoxicity under hypoxic conditions with little or negligible effects under aerobic conditions.18 1. Selectively inhibits Chk1/Aurora A in hypoxic conditions. 2. Induces substantial loss of viability in the cancer cell lines tested. 3. Demonstrates the potential for the bioreductive release of cancer chemotherapeutics.106 1. Safety and tolerability studies (phase1, NCT03202693, completed). 2. Bioavailability and pharmacokinetics study (phase1, NCT01828827, completed). 3. Pulmonary tuberculosis (phase 2, NCT00567840, completed). 4. Drug resistant pulmonary tuberculosis (phase 3, NCT02333799 NCT00567840, recruiting). 1. In rat Walker 256 carcinoma cells, CB-1954 undergoes reduction by NAD(P)H: quinone oxidoreductase (NQO1) to the cytotoxic derivative 5-(aziridin-1-yl)-4-hydroxylamino-2 nitrobenzamide, a bifunctional alkylating agent.100 2. Maximum tolerable dose is 24 mg/m2 by iv administration and 37.5 mg/m2 by ip route. 101 1. Neuroendocrine pancreatic tumors (phase 2, NCT02402062, recruiting). 2. Advanced solid tumors (phase 1, NCT02020226, active, not recruiting). Metastatic soft tissue sarcoma (phase 3, NCT01440088, completed). Advanced leukemias (phase 1, NCT01149915, completed). 3. Non-small-cell lung cancer (phase 1, phase 2, NCT00743379, completed). 1. Demonstrated significant hypoxia-selective antitumor effects, endowed with a 10-fold higher toxicity than evofosfamide. 2. Less toxic than SN-38 under normoxic conditions.99 1. KS119 is endowed with substantial potential as a hypoxia-selective tumor-cell cytotoxin and is unlikely to induce toxicity to well oxygenated normal tissues. 2. KS119 in combination with radiations decreased viability of cells in EMT6 tumors in comparison to KS119 or irradiation alone.107,108

(AGT). Among the synthesized compounds, compound 85 and its monomethyl (86) and gem-dimethyl analogs (87) were

evaluated for bioreductive activation by reductase enzymes under oxygen deficiency. The compound 87 released O6-BG in M



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Figure 17. Bioreductive agents containing nitro groups.

Figure 18. Prodrugs containing nitro groups.

Rami et al. designed and synthesized nitroimidazole-based inhibitors as radio- and chemosensitizing agents targeting the tumor-associated carbonic anhydrase (CA) isoforms. The designed compounds exhibited in vitro efficacy toward the inhibition of extracellular tumor acidification in colorectal HT29 and cervical HeLa carcinoma cell lines overexpressing CA IX. In both tumor types, a substantial decrease in hypoxiainduced tumor acidosis was also observed. Together with doxorubicin, compound 89 was endowed with sensitization potential toward radiotherapy and chemotherapy in vivo. (Figure 22).23 Karwa et al. employed an extremely interesting and practical approach to the design of phototherapeutic agents based on the incorporation of an S-N moiety in previously discovered phosensitizer structures. The authors synthesized novel photolabile sulfenamides and conducted cell viability studies of the compounds. The compounds were evaluated for free radical generation and cell killing effects toward a panel of cancer cell lines (U937, HTC11, KB, and HT29). Photoexcitation of the copious synthetic free radicals was confirmed by electron spin resonance spectroscopy. Sulfenamides 90 and 91 demonstrated striking effects against all of the cell lines. The results of this study further confirmed the role of nitrogen-centered radicals inducing cell death (Figure 22).117 In view of the clinical acceptance of 2-nitroimidazole-based molecules as hypoxic tumor radiodiagnostics, novel 6-Oglucoazomycin bioreductive agents were designed with 2nitroimidazole (bioreductive arm, 92−94) as putative substrates of glucose transport proteins (GLUTs) capable of inducing hypoxia-selective radiosensitization. The results of


the highest yield by the reduction of the nitro group trigger. In addition, it also induced significant enhancement of laromustine cytotoxicity under oxygen-deficient conditions in comparison to normoxic conditions, indicating the selective release of the AGT inhibitor O6-BG under hypoxia (Figure 21).18 Colon et al. conducted a comparative analysis of 3-nitro-10methylbenzothiazolo[3,2-a]quinolinium chloride (NBQ-91) and 10-methylbenzothiazolo[3,2-a]quinolinium chloride (BQ106) to ascertain the importance of the nitro functionality to the DNA binding ability under hypoxic conditions. The results of the dialysis experiments revealed that NBQ-91 (88) binds DNA under N2-saturated conditions with increasing concentrations of reducing agent due to the reduction of nitro functionality. The same was not observed with the compound lacking the nitro group. NBQ-91 exhibited covalent binding to DNA in the presence of hypoxanthine and xanthine oxidase under N2-saturated conditions, clearly showing that the presence of a nitro group is required for the activity (Figure 21).116 N



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these in vitro experiments clearly indicate the potential of these glucoazomycins as radiosensitizers with the ability to competitively inhibit glucose uptake. Among them, 94 induced moderate hypoxiaselective radiosensitization of MOO6X, EMT-6, and HeLa cancer cells (Figure 22).118 The potential of nitroimidazoles as radiosensitizers with which to increase the radiation response has been well explored. In this view, nitroimidazole alkylsulfonamides as hypoxic cell radiosensitizers were synthesized by Bonnet et al. The 2nitroimidazole sulfonamide-based synthetic compounds (95− 97) exhibited significant 6- to 64-fold increases in hypoxiaselective cytotoxicity. The results of the study were quite optimistic, as all of the analogs at nontoxic concentrations were able to sensitize anoxic HCT-116 human colorectal cells to radiation. The study also strengthened the optimistic future prospects of a “mixed mechanism” of radiosensitization and hypoxia-selective cytotoxicity (Figure 22).119

Cheng et al. incorporated the hypoxia-activated nitroimidazole moiety into a quinazoline scaffold for the design of EGFR inhibitors. Among the synthetic novel 6-(nitroimidazole1H-alkyloxyl)-4-anilinoquinazoline derivatives, compound 98, with an IC50 of 0.47 nM, was found to be the most potent, with substantial inhibitory effects against EGFR kinase and potent cell-killing effects against HT-29 cells under hypoxia with IC50 values of 1.62 μM. Compound 98 underwent reductive activation under hypoxia, as indicated by the mimic reductive activation study and could emerge as a potential cancer therapeutic agent (Figure 23).120 Zhu et al. designed 2-(4-nitrophenyl)propan-2-yl 6-((3((dimethylamino)methyl)benzyl)oxy)-9H-purin-2-yl) carbamate (99), a hypoxia-activated prodrug inhibitor of O6alkylguanine-DNA alkyl-transferase, employing an α,α-dimethyl-4-nitrobenzyloxycarbonyl moiety. The designed prodrug demonstrated hypoxia-selective conversion by EMT6 cells to the active drug. In addition, the prodrug was able to cause O



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structural features. This was accomplished by the design of small molecules intended to inhibit HIF-2α−ARNT heterodimerization by binding to an internal cavity of the HIF-2α PAS-B domain. The overall structures of the compounds synthesized for the SAR study involved two aromatic/ heterocyclic rings connected by short linkers. The results indicated that the NO2-substituted benzoxadiazole was mandatory for high affinity and potency, and compound 103, a potently disrupting disubstituted analog, exhibited significant ability to interrupt HIF-2α−ARNT heterodimerization using the AlphaScreen, with an IC50 of 0.1 μM (Figure 24).122 Ambrosio et al. designed and synthesized a series of 2substituted-5-nitrobenzenesulfonamides as bioreducible inhibitors targeting the hypoxia-overexpressed tumor-associated isozymes, CA IX/XII. The designed compounds (104) were found to be potent inhibitors of the tumor-associated CA IX and XII (Ki of 7.4−653 nM against CA IX and 5.8−175 nM against CA XII). Some compounds even displayed excellent selectivity ratios toward the inhibition of the tumor-associated isozymes over the cytosolic isozymes (Figure 24).123 Jiang et al. reported nitrobenzylphosphoramide mustards with a strategically incorporated nitro group para to the benzylic carbon to facilitate reductive activation. The compounds were found to be good substrates of E. coli nitroreductase, with half-lives between 2.9 and 11.9 min at pH 7.0 and 37 °C. Among these, compound 105 was found to be the most active, exhibiting selective activation by NTR with 170 000× selective cytotoxicity toward NTR-expressing V79 cells and an IC50 of 0.4 nM. These results collectively indicated the future applicability of compound 105 in combination with E. coli nitroreductase in enzyme prodrug therapies (Figure 24).105 Reigan et al. reported aminoimidazolylmethyluracil analogs as potent inhibitors of thymidine phosphorylase in light of the


hypoxic sensitization of DU145 cells containing AGT to the cytotoxic actions of laromustine and was also endowed with improved solubility (Figure 23).24 Atwell et al. designed and synthesized a series of 2,4dinitrobenzamide mustards (100−102) as potential prodrugs for GDEPT. The results showed cytotoxic activation toward the four genetic backgrounds: human WiDr colon carcinoma, Skov3 ovarian carcinoma, EMT6 mouse mammary carcinoma, and V79 Chinese hamster fibroblasts. It was observed that analogs exhibited higher potencies and NTR selectivities than dichloromustards (Figure 23).121 In view of the role of hypoxia-inducible factors (HIFs) as heterodimeric transcription factors of cancer in tumorigenesis, a detailed investigation was performed to explore the prerequisite P



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overexpression of thymidine phosphorylase in the hypoxic regions of many tumors. This research group also reported the ability of their bioreductive nitroimidazolyl prodrugs to be converted to the amino analogs. Compound 106 (IC50 ≈ 20 nM) significantly inhibited E. coli and human TP, with IC50 values of ∼20 nM. The corresponding prodrug (107) of compound 106 was >1000-fold less active and underwent xanthine oxidase-mediated reduction to 106. The results of the study clearly demonstrated the potential of the designed prodrugs to selectively deliver the potent 2′-aminoimidazol-1′-

yl TP inhibitors into hypoxic solid tumors, because XO is highly expressed in many tumors (Figure 24).124 Wei et al. synthesized nitroimidazole-substituted 4-anilinoquinazoline derivatives as epidermal growth factor receptor (EGFR)/vascular endothelial growth factor (VEGF)-2 inhibitors. One of the synthesized compounds (108) exhibited a remarkable potential against A549 and H446 cells under hypoxic conditions (Figure 25).125 Thomas et al. synthesized glucuronide prodrugs of the histone deacetylase inhibitor CI-994 by incorporation of a Q



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Figure 26. Agents containing nitro groups.


oxidoreductase under anaerobic conditions. The majority of the synthesized prodrugs (110−113) were not tubulin polymerization inhibitors. This was a desirable feature of the study, as the bioreductively activatable prodrug conjugates were anticipated to be inactive prior to enzyme-mediated cleavage to release phenstatin (Figure 25).20 3.2. Anticancer Agents Containing Nitro Groups. This section presents a recent update on agents containing nitro groups and exerting cytotoxic effects either via a mechanism different from the hypoxia-induced effects or without studies to ascertain their effects as prodrugs. The agents covered in this section do not act as prodrugs and possess significant cytotoxic effects. In view of the striking cytotoxic potential of muramyldipeptide or normuramyldipeptide analogs modified with acridine/ acridone derivatives in their peptide, Dzierzbicka et al. reported

nitrobenzylphenoxy carbamate linker. The prodrug (109) was hypothesized to release CI-994, enabling easy recognition by βglucuronidase. Prodrug incubation with β-glucuronidase in the culture media efficiently released the parent drug CI-994, keeping the cell proliferation and the HDAC inhibitory and Ecadherin expression induction effects intact. The prodrug exhibited improved solubility and good stability under physiological conditions and is anticipated to be promising for use in tumor-targeting strategies such as ADEPT or PMT (Figure 25).16 Winn et al. synthesized bioreductively activated prodrug conjugates of phenstatin (110−113) that were designed to target tumor hypoxia. The results of biological evaluation revealed that dimethylnitrothiophene (110) and the gemdimethylnitrofuran (113) underwent efficient enzymatic cleavage in the presence of NADPH cytochrome P450 R



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imidazole-containing compounds led to the discovery of compound 118, which has striking cytotoxic potency (IC50 = 1 μM) and binding to eEF1A2. In addition, it exhibited selective binding to eEF1A2 and appeared to be an excellent candidate with a novel mechanism of action against breast cancer (Figure 27).130 The human selenoprotein, thioredoxin reductase, is a logical target for the development of cytostatic agents, as it is involved in antioxidant defense and DNA synthesis. This motivated Millet et al. to design cis-diamminedichloroplatinum complexes of the 5-nitro-2-furancarbohydrazide with the aim of overcoming the cisplatin resistance. The key fragments, 5-nitro-2furancarbohydrazide and cisplatin, for the design of the target compounds were identified by kinetic studies, which indicated μM binding affinity of these fragments at two different subsites of the human enzyme. The tethering of the two fragments led to the design of nitrofuran-based cis-diamminedichloroplatinum complexes (119−122), which demonstrated irreversible inhibition of the human enzyme with nanomolar affinities (Figure 27).131 Novel mycophenolic acid (MPA) conjugates containing nitroacridine/acridone derivatives were designed and synthesized by Malachowska-Ugarte et al. The length of the linker between MPA and the heterocyclic units was varied, and it was observed that the heterocyclic section had a significant influence on the cytotoxic and antiproliferative properties. Conjugates 122a−e displayed more potent cell-killing effects against the tested cell lines than the parent MPA. Mouse leukemia L1210 was the most sensitive to the exposure of the synthesized compounds, with IC50 values in the range of 0.042−0.087 μM. Preliminary investigations revealed that the compounds acted as IMPDH inhibitors (Figure 28).132 In view of the reported potential of SERT ligands to induce selective programmed cell death in Burkitt’s lymphoma, 1,3-

two analogs (114, 115) modified at the C-terminus and displaying potent in vitro cytotoxic activity against a panel of human prostate cancer cell lines. Analog 115 also demonstrated in vivo potential against sc UACC-62 melanoma in mice (Figure 26).126 Cholody et al. fused an imidazoacridone moiety to polycyclic heteroaromatics via linkers of diverse length and rigidity to accomplish the synthesis of unsymmetrical bifunctional antitumor agents. The most potent compound of the series (116, WMC79) induced substantial selective cytotoxic effects against colon cancers (GI50 = 0.5 nM, LC50 = 32 nM) and leukemias (GI50 = 3.5 nM, LC50 = 33 nM). The compounds also exhibited significant in vivo activity against HCT-116 colon cancer xenografts in nude mice (Figure 26).127 A group of researchers led by Hariprakasha et al. working on fused molecules containing different DNA binders included optimization of the unsymmetrical bifunctional antitumor agent WMC79 (116, previously reported)128 with a naphthalimideimidazoacridone scaffold. The investigation led to the identification of compound 117, which has significant inhibitory potential toward many tumor cell lines and is approximately 30-fold more active than 116 (median LC50 = 0.67 μM compared to 19.9 μM for WMC79) and induced rapid apoptosis. Unlike 116, compound 117 was toxic to both p53positive and -negative cancer cells and had potent in vivo activity against xenografts of human colon and pancreatic tumors in athymic mice. Due to these results, compound 117 was advanced into clinical development (Figure 26).129 A series of flavone analogs was designed, synthesized, and evaluated against breast cancer cells by Yao et al. Some of the primary compounds bearing an imidazole ring attached to the flavone framework were moderately cytotoxic against breast cancer cells (MDA-MB-231 (ER−) and MCF-7 (ER+). Structural optimization studies involving the simplification of S



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study were quite promising, as the 8- and 9-nitro analogs (129 and 130) displayed more pronounced TOP1-targeting activity and cytotoxicity than did camptothecin. A structure−activity relationship confirmed the importance of a nitro substituent at either the 8- or the 9-position of 5H-8,9-dimethoxy-5-(2-N,Ndimethylaminoethyl)-2,3-methylenedioxydibenzo[c,h][1,6]naphthyridin-6-one (Figure 30) for activity.14 Morrell et al. synthesized indenoisoquinoline analogs employing a combination of nitro groups, methoxy groups, and hydrogen atoms. The results of the study demonstrated that the nitro group was of paramount importance for the TOP1 inhibitory potential. The electron-rich oxygen atoms of the nitro group of compound 131 complemented the electrondeficient outer edges of the scissile strand DNA base pair, and the nitro group induced electron deficiency in the indenoisoquinoline aromatic ring system, leading to good electrostatic complementarity with the DNA base pairs. Thus, the stabilization of the complex from π-stacking interactions was associated with the benefits of appropriate placement of nitro groups (Figure 30).138

bis(aryl)-2-nitro-1-propene derivatives bearing a structural resemblance to MTA were synthesized by McNamara et al. Remarkable antiproliferative potential was exhibited by a synthetic compound (123) against the malignant cell lines DG-75, MUTU-I, and SHSY-5Y, showing apparent selectivity for BL-derived cell lines. In addition, the compounds caused caspase activation, PARP cleavage, chromatin condensation, and membrane blebbing in a cell line derived from Burkitt’s lymphoma and induced apoptosis in vitro. The role of the nitro group in inducing apoptosis was confirmed by the remarkable activity of the most potent compound lacking the nitro functionality and failing to induce apoptosis. (Figure 28).133 In view of the dysregulation of human cathepsin B activity, a lysosomal cysteine protease that is associated with cancers, the research group led by Sosic et al. designed and synthesized nitroxoline derivatives. The most potent inhibitor (124) demonstrated improved catB endopeptidase inhibition and enhanced selectivity toward catB over catH and catL, in comparison with the parent nitroxoline (Figure 29).134 Csuk et al. recently reported the potential of β-nitrosubstituted carboxylic acids as a new class of cytotoxic agents. Among the series of synthesized compounds, 125, 2-(4chlorophenyl)-3-nitropropionic acid ethyl ester with IC50 = 1.6 μM, displayed remarkable results against the human ovarian cancer cell line (A2780), suggesting the need for more detailed investigations (Figure 29).135 Tan et al. reported that 8-ethoxy-3-nitro-2H-chromene-based HDAC compounds have potent cell killing effects in K562, A549, MCF-7, PC3, and HeLa cell lines. The effects of the compounds were more potent than MS-275, with the majority of them exhibiting effects higher than or comparable to those of suberoylanilide hydroxamic acid (SAHA) against K562 cells. Compound 126 (IC50 = 128 nM, HDAC1) and compound 127 (IC50 = 179 nM, HDAC1) were endowed with more potent and selective inhibitory effects compared to HDAC2 than SAHA (Figure 29).136 A series of 1-indolyl acetate-5-nitroimidazole derivatives were designed, synthesized, and evaluated by Duan et al. as potential tubulin polymerization inhibitors. The most potent tubulin inhibitor (128) (IC50 = 2.4 μM) induced significant inhibition against the growth of cancer cells, with IC50 = 2.00, 1.05, 0.87 μM against A549, HeLa, and U251, respectively (Figure 29).137 Singh et al. synthesized 8-, 9-, and 10-nitro-5H-2,3methylenedioxy-5-(2-N,N-dimethylaminoethyl)dibenzo[c,h][1,6]naphthyridin-6-ones and their amino derivatives as potent TOP1-targeting agents and anticancer agents. The results of the

4. RECENT ADVANCES IN ANTITUBERCULAR AGENTS CONTAINING NITRO GROUPS There have been several investigations conducted on antitubercular agents with nitro functionality, and most of them revolve around structure modification and attempts to enhance the potential of well-explored antitubercular prodrugs containing nitro groups, including delamanid (13)87 and pretomanid (24).98 Pretomanid (24), possessing a 5-nitroimidazooxazine scaffold, and delamanid (13), bearing a 5nitroimidazooxazole skeleton, are potent inhibitors of active replication and latent Mycobacterium tuberculosis. Pretomanid (24) is a prodrug that is metabolized by Mycobacterium tuberculosis. It undergoes the bioreduction of the aromatic nitro group to a reactive nitro radical anion intermediate within the cell and is thus activated. Inhibition of the synthesis of cell wall lipids and proteins is considered to be the primary mechanism of action of pretomanid (24), but its effect against nonreplicating bacteria indicates that inhibition of cell wall biosynthesis is not the only mode of action.98 Delamanid (13) is also a prodrug that needs to be activated by M. tuberculosis.87 The selective attempts of medicinal chemists toward the exploration of antitubercular compounds containing nitro groups are presented in three categories: (1) structural engineering of prodrugs (pretomanid, 24, and delamanid, T



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Figure 31. Antitubercular prodrugs containing nitro groups.

Evaluation of their reduction potential indicated that although the nitroheterocyclic compounds exhibited values close to those of 24 and the nitroimidazothiazines were similar to the nitroimidazooxazines, these compounds were endowed with substantial in vitro antitubercular activity (Figure 31).139 Palmer et al. developed a series of biphenyl analogs of pretomanid (24). Their evaluation indicated that the paralinked biaryls (135) were the most active, with several compounds endowed with improved efficacies (>10-fold) over that of 24 in a mouse model of acute Mtb infection. The most potent was >200-fold more effective.140 In continuation of their program of investigation of 24, the same research group reported heterobiaryl analogs replacing one of the phenyl rings with several five-membered heterocycles in anticipation of new candidates with improved aqueous solubility. Two compounds (136, 137) displayed 4- to 41fold higher efficacy than 24 in a mouse model of acute Mtb infection, and the pyrazole analog (136) also demonstrated 2fold higher solubility than 24.141 Palmer et al. further attempted to improve the solubility and oral potential against chronic infection by Mycobacterium tuberculosis by designing novel nitroimidazooxazine analogs with extended side chains and the varying size, flexibility, and polarity of the linker. Several compounds exhibited good microsomal stabilities, but only compounds 138, 139, and 140 showed in vivo efficacies strikingly superior to that of 24 in an acute infection model.142 The same research group designed new pretomanid (24) analogs by employing side chain ether linkers of varying size and flexibility to obtain compounds with enhanced metabolic stability and high efficacy. The biological evaluation showed that compound 141 demonstrated excellent antitubercular effects, and the propenyloxy, propynyloxy, and pentynyloxy groups, categorized as extended linkers, resulted in greater potencies against the monoaryl analogs of replicating Mtb, with propynyl ethers and mono/biaryl analogs (142) displaying the most potent effects under anaerobic, nonreplicating conditions. Maximal aerobic activity was observed with the original

(13) and related heterocycles, (2) antitubercular agents containing nitro group, (3) benzothiazinones and related heterocycles as antitubercular prodrugs. The insights revealed in these studies appear to be beneficial for the future design of antitubercular agents and have also identified the structural prerequisites for accomplishing target specificities, particularly for nitroimidazoles. 4.1. Structural Engineering of Prodrugs (Delamanid (13) and Pretomanid (24)) and Related Heterocycles. The variation in the activity profile of pretomanid (24) instigated the authors led by Kim et al. to explore its structural features by synthesizing 4- and 5-nitroimidazoles analogs (132) and evaluating their antitubercular activities. The results of the biological evaluation revealed that the nitro group was essential for both aerobic and anaerobic activity, and this was evidenced by the diminished activity of the des-nitro derivatives (Figure 31).26 Kim et al., in a continuation of their study on pretomanid (24), investigated two areas of the nitroimidazooxazine pharmacophore for aerobic activity: the atom at the 2-position and the nature of the substituent at the 6-position on the oxazine ring. The key points regarding the SAR of 133 established that (1) a nitrogen or sulfur atom in place of oxygen at the 2-position yielded equipotent analogs; (2) a reduction in the activity profile was observed upon acylation of the amino functionality, oxidization of the thioether, or replacement of the ether oxygen with carbon; (3) N-benzylic functionality in place of O-benzylic functionality slightly elevated the potency and led to the more soluble 6-amino series; and (4) extending the length of the linker region between the 6-(S) position and the terminal hydrophobic aromatic substituent led to a significant improvement in the antitubercular potential (Figure 31).27 Thompson et al. investigated the antitubercular activity of ring A/B analogs of pretomanid (24). The authors established synthetic protocols for the designed chromophore analogs (134) using imidazoles, pyrazoles, and triazoles with diverse heteroatoms at the 4/8-position of the adjacent fused ring. U



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of the -OCH2- linkage in 24 with an amide, carbamate, and urea functionality. The bioisosteric replacement of the biaryl moiety by arylpiperazine yielded a soluble, orally bioavailable carbamate analog (145) (Figure 33).146 Cherian et al. explored the influence of the linker and lipophilic tail of designed derivatives at three positions of the 4(trifluoromethoxy)benzylamino tail against both replicating and nonreplicating Mtb. Out of three positions, modifications at R1 and R3 yielded an improved lead compound (146) in terms of both potency and solubility, and the third site, R2, was anticipated to be a site for modulation of the pharmacological properties of derivatives. The derivatives underwent deazaflavin-dependent nitroreductase (Ddn)-mediated reductive activation. Several active compounds displayed better activity than 24, with 40 nM aerobic whole-cell activity and 1.6 μM anaerobic whole-cell activity. Quantum chemistry suggested the pseudoequatorial orientation of the linker and lipophilic tail as the conformational preference of these analogs (Figure 33).147 With an aim of improving the aqueous solubility and maintaining high metabolic stability and efficacy, aza- and

(-OCH2-) linker for the benzyloxybenzyl and biaryl derivatives.143 Thompson et al., in a continuation of their backup program for the antitubercular drug pretomanid (24), prepared 6-nitro-2,3-dihydroimidazo[2,1-b][1,3]thiazoles and related 6nitro-2,3-dihydroimidazo[2,1-b][1,3]oxazoles in high yields. A part of the structure−activity relationship was deduced and indicated that substitution of the methyl group at the second position and extended aryloxymethyl side chains favored the antitubercular activity, while S-oxidized thiazoles generally displayed negative trends. Among the synthetic compounds, 143 exhibited good microsomal stability and was found to be potent in an acute Mycobacterium tuberculosis mouse model.144 Recently, while attempting to identify the two trace byproducts encountered during large-scale synthesis of pretomanid, Thompson et al. identified 144 (3′-methyl pretomanid) as a potent antitubercular agent, with 8-fold higher potency than pretomanid in the aerobic assay (Figure 32).145 Blaser et al. synthesized several analogs of compound 24 to address the issues of oxidative metabolism, minimal compound lipophilicity, and enhanced aqueous solubility by replacement V



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Figure 34. Antitubercular agents containing nitro groups.


diazabiphenyl analogs of pretomanid (24) were designed by Kmentova et al. The design strategy that was employed involved the synthesis of several heterocyclic analogs of the active biphenyl class derived from 24 and was accomplished by the replacement of phenyl groups by pyridine, pyridazine, pyrazine, or pyrimidine to decrease the lipophilicity. Some of the compounds demonstrated remarkable in vivo efficacies, for example, in an achronic infection model, along with high metabolic stabilities and excellent pharmacokinetics. The highlight of the study was the identification of the orally bioavailable pyridine analog (147), which is 3- to 4-fold more soluble than 24 at low pH values (Figure 33).148 Kang et al. attempted various modifications at the C-7 position of 2-nitroimidazooxazine pretomanid (24) analogs, such as placement of benzyl ether, phenyl ether, benzyl carbonate, and phenyl carbamate. Among the synthesized compounds, analog 148 bearing a trifluoromethoxy benzyl group (MIC = 0.078 μM) and analog 149 (phenyl ether-based analog, MIC = 0.050 μM) displayed striking antimycobacterial activity against Mtb in comparison to 24 (MIC = 0.390 μM) in the in vitro evaluation against the wild-type Mtb. Overall, the

results were quite encouraging, and most of the compounds exhibited low toxicity (Figure 34).149 4.2. Antitubercular Agents Containing Nitro Groups. This section presents studies that do not deal with structural modifications of the prodrugs (13 and 24) and thus have been separated from structures categorized as prodrugs in this perspective. Prompted by the positive preclinical outcomes attained with 2-nitroimidazole-linked chloroquinoline compounds, the research group led by Papadopoulou et al. investigated the antitubercular potential of 3-nitrotriazole-based and 2-nitroimidazole-based amides and sulfonamides against aerobic Mycobacterium tuberculosis H37Rv3 by using the BacTiter-Glo (BTG) microbial cell viability assay. Some of the sulfonamides (150) exhibited a positive trend with increasing lipophilicity; however, dependence of the antitubercular potential on one electron reduction potential was not observed. In general, the nitrotriazoles were endowed with unaltered antitubercular potential in the BTG assay against resistant strains (Figure 34).150 Motivated by these fruitful results, Papadopoulou et al. also recently evaluated some nitro(triazole/imidazole)-based compounds (mostly amides) for antitubercular activity against W



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Figure 37. Benzothiazinones and related heterocycles as antitubercular prodrugs.

Mycobacterium tuberculosis H37Rv (Mtb H37Rv) under aerobic or hypoxic conditions. This extensive program led by these authors resulted in the identification of compound 151 and 152, which exhibited potential against aerobic and hypoxic Mtb and were also found to possess bactericidal and intracellular antitubercular activities. Compounds 153, 154, and 155 were found to be selectively active against aerobic Mtb. Compound 154 also exhibited good in vitro ADMET characteristics, demonstrating excellent caco-2 permeability, an efflux ratio of 0.39, good microsomal, and chemical stability and a lack of hepatotoxicity (Figure 34).28 In view of the categorization of 1,3,4-oxadiazole as a privileged bioactive heterocycle, particularly for antitubercular agents, Rane et al. synthesized a series of novel 4-nitropyrrolebased 1,3,4-oxadiazole derivatives as antimicrobial and antitubercular agents. Compounds 156, 157, 158, and 159 were endowed with significant antitubercular potential, with MIC values of 0.81 μg/mL, 0.46 μg/mL, 0.72 μg/mL, and 1.8 mg/mL, respectively. Among them, compound 156 was almost equipotent to the standard drug isoniazid (0.40 mg/mL). The results of the study were encouraging, as all the compounds were found to be nontoxic when tested for mammalian cell toxicity using the VERO cell line (Figure 35).151

Working along similar lines and encouraged by the biopotential of bromopyrrole alkaloids and semicarbazide derivatives for antimicrobial activity, Rane et al. synthesized a series of 4-nitropyrrole-semicarbazide conjugates. Hybrids lacking a pyrrole-NH were more potent against Mycobacterium tuberculosis in comparison to the N-methylated analogs. Among the synthesized compounds, hybrids 160, 161, and 162 had MIC values of 0.50, 0.56, and 0.67 μM, respectively, against Mycobacterium tuberculosis H37RV (Figure 35).152 Kamal et al. synthesized the reported nitrofuramide-linked triazole hybrids and evaluated them for their antitubercular activity against a Mycobacterium tuberculosis H37Rv strain. The results of the investigation were encouraging, as a remarkable antitubercular potential was demonstrated by compound 163 (MIC = 0.25 μg/mL). Most of the other compounds synthesized in the study exhibited lower cytotoxicities (Figure 36).153 Tangallapally et al. synthesized a series of nitrofuranylamides and screened them against M. tuberculosis UDP-Gal mutase inhibition. The biological evaluation indicated that the MIC values did not correlate with UDP-Gal mutase inhibition, indicating that the antitubercular potential of the compounds could be attributed to an alternative primary cellular target. On X



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Figure 38. Benzothiazinones and related heterocycles as antitubercular prodrugs.

by conversion of the nitro group to a nitroso and a subsequent covalent reaction with Cys387 of DprE1. The study also indicated that BTZ-043 (18) and the other related nitroaromatic compounds (166−169), despite their structural diversity, induce their effects via a common mechanism of action, thus representing an emerging family of antimycobacterial agents (Figure 37A).156 BTZ-043 (18) can be defined as a BTZ core, bearing three substituents (R1, R2, and R3). The well-established structure− activity relationship indicates that there is little scope for modifications around groups R1 and R2 but that a variation in the structure of the R3 group is feasible. Karoli et al. employed an in silico ligand-based model based on SAR studies from 170 BTZ compounds to design a new series of compounds. Groups at the R1 and R2 positions remained as NO2 and CF3, and various alterations at position R3 were made. Biological assays led to the identification of several compounds with improved activity against MDR-TB. In addition, the compounds were endowed with favorable microsomal, metabolic, and plasma stability and low toxicity (Figure 37 B).157 Tiwari et al. designed nitroaromatic sulfonamide, reverseamide, and ester classes of anti-TB agents by employing a scaffold simplification strategy. The substantial effects of BTZ043 (18) led the authors to explore the influence of functional groups, such as sulfonamides, reverse-amides, and esters linked to the nitroaromatic rings, on anti-TB activity. Biological evaluation of the synthetic compounds against the H37Rv strain indicated that simple sulfonamides and nitrobenzoic acid ester analogs with dinitro substituents were more active than the reverse amides, but their potency was not close to that of BTZ043 (18). The difference in the activity profile was attributed to the electron-deficient aromatic ring in the nitroaromatics, as evidenced by the activity of compounds 170-172. The study concluded that electron-deficient aromatic rings in, for example, the nitroaromatics were instrumental in potentiating the bioactivity (Figure 38).158 Tiwari et al. designed and synthesized novel anti-TB agents, successors to BTZs and other nitroaromatic compounds. The compounds synthesized displayed promising antitubercular potential and were nontoxic. An important revelation of the study was the susceptibility of the unsubstituted aromatic

the basis of the in vitro results, the promising compounds were further evaluated in in vivo studies in a mouse model of tuberculosis infection. Among the tested compounds, compound 164 demonstrated substantial antitubercular effects (Figure 36).154 Karabanovich et al. synthesized a series of 1,3,4-oxadiazole and 1,3,4-thiadiazole scaffolds in view of their role as bioisosteric surrogates for 2,5-disubstituted tetrazoles that had previously proven their antitubercular potential. Excellent activity against drug-susceptible and multidrug-resistant Mtb was demonstrated by a majority of the compounds, and the 3,5dinitro substitution was found to be indispensable for antimycobacterial activity. In addition, a highly selective antimycobacterial effect was demonstrated by the compounds, as they possessed no inhibitory effects against other bacteria or fungi. Low in vitro toxicities against four proliferating mammalian cell lines and in isolated primary human hepatocytes, along with the absence of mutagenicity observed with the tested compounds, were among the salient features of the study. The active compound (165) had a favorable toxicity profile and exhibited a potency comparable to that of rifampicin against the nonreplicating streptomycin-starved M. tuberculosis 18b-Lux strain (Figure 36).155 4.3. Benzothiazinones and Related Heterocycles as Antitubercular Prodrugs. 1,3-Benzothiazin-4-ones (benzothiazinones, BTZs) have exhibited high activity against Mycobacterium tuberculosis both in vitro and in vivo. BTZ-043 (18), a benzothiazine analog, has demonstrated a remarkable potential to kill Mtb in vitro, ex vivo, and in mouse models of TB and induces its antitubercular effects at an MIC of 1 ng/mL against Mtb H37Rv. This is an extraordinary activity profile when compared to existing therapeutics such as isoniazid. Benzothiazines target the enzymes decaprenylphosphoryl-Dribose 2′-epimerase and DprE2 (Rv3791), which are critical for the synthesis of cell wall arabinans, and catalyze the epimerization of decaprenylphosphoryl-β-D-ribose to decaprenylphosphoryl-β-D-arabinose.96,97 Despite all these previous efforts, scope remains to fully investigate the detailed mechanism of BTZ-043 (18), and Trefzer et al. in their attempts to explore the mechanism of benzothiazine reported that BTZs act as suicide inhibitors, leading to enzyme inhibition Y



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Figure 39. Antiparasitic agents containing nitro groups.

parasite type I nitroreductase (NTR), which is absent from mammalian cells. This particular difference forms the basis for the drug selectivity, and the enzyme catalyzes the two-electron reduction of nitroheterocycles within the parasite. The reduction leads to the generation of toxic metabolites that preferentially target the parasite.76 Recent attempts toward the design of antiparasitic agents are presented in two categories: (1) triazoles, imidazoles and related heterocycles, (2) indazoles. 5.1. Nitrotriazoles, Imidazoles, and Related Heterocycles (Heterocycles as Prodrugs Containing Nitro Groups). The quest to explore nitro group-containing heterocycles led Papadopoulou et al. to design and evaluate a series of 42 novel 2-nitro-1H-imidazole- and 3-nitro-1H-1,2,4triazole-based aromatic and aliphatic amines. The results were quite good, as several compounds (175−181) were found to be endowed with significant growth inhibitory properties against T. cruzi amastigotes. IC50 values ranged from 40 nM to 1.97 μM and selectivity indices (SI) from 44 to 1320, with the majority displaying 33.8-higher potency than the standard (2). The striking feature of the study was that the compounds failed to induce toxicity to the host cells (L6 cells) (Figure 39).161 In view of the shortcomings associated with benzinidazole (3) as a therapeutic for Chagas disease, which includes side effects and limited efficacy, Papadopoulou et al. synthesized a series of novel 3-nitro-1H-1,2,4-triazole-based and 2-nitro-1Himidazole-based amides and sulfonamides. Several compounds (182−184) displayed striking activity against Trypanosoma cruzi intracellular amastigotes (IC50 values ranged from 28 nM to 3.72 μM, SI ranged from 66 to 2782) without concomitant toxicity to the L6 host cells. A moderate activity profile was also demonstrated by some nitrotriazoles against the axenic form of Leishmania donovani (Figure 39).162 Continuing their investigations, Papadopoulou et al. attempted the synthesis of bioreducible 3-nitrotriazole-based compounds as CYP51 enzyme inhibitors. The study aimed to exploit the anticipated benefits of fusing the nitroheterocycles with the concomitant benefits of inhibiting the ergosterol biosynthesis inhibitors with a single molecule. Linear, rigid 3nitrotriazole-based amides and 3-nitrotriazole-based carbinols, analogs of fluconazole, were synthesized and evaluated as

carbon between the electron withdrawing groups of compounds 173 and 18 to substitution by nucleophiles such as thiolates, cyanide, and even hydrides. This mediated the nonenzymatic reduction of the nitro groups to the corresponding nitroso intermediates by addition at the unsubstituted electrondeficient aromatic carbon present in these compounds. Overall, the study presents a possible alternative mechanism with which nitroaromatics might exert antitubercular effects. The mechanism proceeds via cysteine thiol(ate) or a hydride source at the active site of DprE1, triggering a nitro group reduction similar to the reaction proceedings in the von Richter reaction to the nitroso intermediates, thereby initiating the inhibition of DprE1 (Figure 38).159 HisG (an ATP-phosphoribosyl transferase (ATPPRTase)) represents a potential drug target for tuberculosis owing to its role in the biosynthetic pathway for histidine. For this reason, Cho et al. performed virtual screening for the identification of novel inhibitors for M. tuberculosis HisG using both GOLD and FlexX. A nitrobenzothiazole moiety was found to be a key structural unit present in several hits, and one of the new compounds was found to be endowed with significant enzyme inhibitory activity (IC50 = 6 μM). A docking study revealed that 174 occupies the PRPP binding site, and the nitro group forms a tight hydrogen-bonding network with the backbone nitrogen atoms of the P-loop, which leads to the stabilization of the molecule (Figure 38).160

5. RECENT ADVANCES IN ANTIPARASITIC AGENTS CONTAINING NITRO GROUPS This section encompasses an overview of some selected findings in the field of antiparasitic agents that contain nitro groups. Preclinical investigations of these compounds have mostly focused on synthesizing antiparasitic agents based on the structural features of well-known nitroheterocyclic prodrugs, namely, nifurtimox (2), benzinidazole (3), metronidazole (4), and fexinidazole (9), as a part of drugs for neglected diseases programs. Nitroheterocycles are considered to be prodrugs that require bioactivation of the nitro group to exert an antimicrobial effect. It has been established that the trypanocidal activity of nitroheterocyclic drugs depends on a Z



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potential antitrypanosomal/antichagasic agents, NTR substrates, and TcCYP51 inhibitors. The results of the investigation revealed that the synthetics were excellent substrates for type I nitroreductase (NTR), located in the mitochondria of trypanosomatids. Moreover, the compounds also exhibited inhibitory potential toward the sterol 14αdemethylase (T. cruzi CYP51) enzyme. Compounds 185 and 186 were found to be the most promising among all the synthetic compounds (Figure 40).163 Papadopoulou et al. explored the potential of aminothiazole as a heterocyclic unit with an appropriately placed nitro group to induce antitrypanosomal activity by synthesizing a series of 5-nitro-2aminothiazole-based amides containing arylpiperazine-, biphenyl-, or aryloxyphenyl functionalities. Compound 187 had a substantial activity profile (IC50 = 0.571 μM) and demonstrated a 4-fold higher potential than did 2. A moderate activity profile was also exhibited by some compounds against L. donovani axenic amastigotes (Figure 40).164 In a continuation of their investigations on nitro-based heterocycles, Papadopoulou et al. explored 3-nitrotriazole- and 2-nitroimidazole-linked quinolines and quinazolines for in vitro antitrypanosomal and antituber-

cular activities. The biological evaluation indicated that 3nitrotriazole-based compounds exhibited significant potency and parasite-selectivity in comparison to 2-nitroimidazole. Both of the classes demonstrated activity against T. cruzi amastigotes, with either similar or better activity than benzinidazole (3). A selective activity potential against T. b. rhodesiense was also demonstrated by 3-nitrotriazole-based analogs. Among all the compounds, 188 was the most promising agent for the Chagas disease, with an IC50 value of 0.038 μM and a selectivity index of 1937 against T. b. rhodesiense. Compound 189 was the most potent antitubercular agent, with an MIC value of 2.89 μM (Figure 40).34 Papadopoulou et al. also reported the synthesis and evaluation of novel nitro(triazole/imidazole)-based heteroarylamides/sulfonamides as potential antitrypanosomal agents. A striking inhibitory potential was demonstrated against T. cruzi at the nanomolar level by chlorothiophene-sulfonamides, benzothiophene-amides, and the benzothiazole-amide. The salient feature of the study was a 14-fold higher potential of these compounds than benzinidazole (3), with selectivity indices ranging from 216 to >1409. The structure of the most AA



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compounds against T. cruzi amastigotes at the nanomolar level. In addition, the compounds exhibited SI values of ≥200 and were found to be aligned with the Lipinski’s rule of 5, displaying drug-like characteristics. Among the synthesized compounds, 191−193 were identified as promising candidates for in vivo evaluation as anti-HAT agents (Figure 41).36 Papadopoulou et al. reported the in vitro and in vivo potential of 3-nitrotriazolebased aryloxyphenylamides as potently active and selective antiT. cruzi agents. 3-Nitrotriazole-based aryloxyphenylamides (194, 195) at 13 mg/kg/day (ip) reduced the parasite load after 5 days of treatment in infected mice. Metabolic stability and a good caco-2 permeability were demonstrated by these

promising compound (190) is shown in Figure 41. The compounds induced selective effects against T. b. rhodesiense. It was also observed that type I NTR activated most of the 3nitrotriazoles, as the parasites overexpressing the enzyme were substantially more sensitive (up to 73-fold) to exposure to the synthesized compounds (Figure 41).35 Extending their research program on heterocycles containing nitro groups with trypanocidal activity, Papadopoulou et al. investigated the in vitro trypanocidal activity of nitrotriazolebased piperazines and nitrotriazole-based 2-amino-1,3-benzothiazoles to ascertain the structure−activity relationship. The biological evaluation demonstrated a remarkable activity of the AB



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activity profile. The results of the biological evaluation indicated that compound 204 was 2-fold more potent than 2 against T. vaginalis and G. intestinalis and induced effects that were equipotent with those of 2 against E. histolytica. Selective toxicity was observed among sulfonamides against E. histolytica. Among the sulfonamides, compound 205 was the most active against E. histolytica. The compounds were found to be nontoxic in the MDCK cell line and merit a more detailed investigation (Figure 42).167 In view of the success of hybrid scaffolds that combine different pharmacophoric units to attain synergistic bioactivity, ́ Colin-Lozano et al. synthesized and evaluated 5-nitrothiazoleNSAID chimeras as analogs of nitazoxanide and evaluated them against a panel of protozoa. The chemical architectures of the desired hybrid scaffolds were accomplished by fusion of the pharmacophore, 2-amino-5-nitrothiazole, with other NSAIDs. All the fused scaffolds (206−210) were endowed with more potent effects than either 4 or nitrozoxanide. Among all the hybrids, the indomethacin hybrid (209) was found to be the most potent, possessing a striking inhibitory potential toward the growth of G. intestinalis, with an IC50 value of 0.145 μM. The effects of compound 206 were 38-fold higher than those of 4 and were 8-fold higher than those of nitazoxanide. The substantial giardicidal effect of 206 was also evidenced in in vivo studies in a CD-1 mouse model, with a median effective dose of 1.709 μg/kg (3.53 nmol/kg). In comparison to 4 and nitazoxanide, the in vivo potential of compound 206 is 321 times and 1015 times higher, respectively, after intragastric administration (Figure 42).168 Trunz et al. reported diverse 1-aryl-4-nitro-1H-imidazoles 211 and 212 with antitrypanosomal activity as compounds endowed with curative potential in mouse models of both acute and chronic African trypanosomiasis. The compounds were administered orally at doses of 25−50 mg/kg for 4 days in acute infection and 50−100 mg/kg (b.i.d.) for 5 days in the chronic model. There was no observed genotoxicity associated with these two compounds in mammalian cells. The structure− activity relationship delineated several structural features that led to increased or decreased activity. Of these, replacement of the 4-nitroimidazole moiety with 4-nitropyrazole reduced the activity potential (Figure 43).169

compounds, indicating a favorable profile for oral administration. Moreover, the compounds were found to be potential inhibitors of T. cruzi CYP51 and were excellent substrates for type I nitroreductase (NTR) (Figure 41).37 Phenotypic investigation of 6-nitro-2,3-dihydroimidazo[2,1b][1,3]oxazole derivatives led to the identification of a potential first-in-class drug candidate for visceral leishmaniasis: DNDIVL-2098. Thompson et al. improved its solubility and safety profile and attempted several modifications of the 6-nitro-2,3dihydroimidazo[2,1-b][1,3]oxazole scaffold. A structural engineering study indicated that the aryloxy side chain was open to modification, with attempts such as using nitrogen in place of the linking oxygen or the piperazine, biaryl extension, and a pyridine ring in place of phenyl rings leading to compounds that were well tolerated. However, the nitroheterocycle was extremely sensitive to modifications, as removal of the nitro group or switching its position and replacement of the imidazole ring by pyrazole or triazole all led to a diminished activity against VL. Compound 196 was identified as the most effective candidate, outshining DNDI-VL-2098 in the mouse model of acute Leishmania donovani infection (Figure 42).38 In view of previous work reporting the potential of delamanid analog DNDI-VL-2098 as a VL preclinical candidate, the 7substituted 2-nitro-5,6-dihydroimidazo[2,1-b][1,3]oxazine class was investigated by Thompson et al. Attempts were made to develop compounds with enhanced efficacy and improved solubility and safety. A biological evaluation revealed that the two racemic phenylpyridines (197, 198) had significant potential in the Leishmania donovani mouse model. Compound 198 induced >99% inhibition at 12.5 mg/kg (b.i.d., orally) in the Leishmania infantum hamster model. The configuration of compound 197 also had an influence on the activity profile, with the 7R enantiomer exhibiting more favorable pharmacokinetics, efficacy and safety (Figure 42).39 Thompson et al. screened a 900 compound nitroimidazolebased library established as a pretomanid backup program against human African trypanosomiasis (HAT) as a part of a drug for neglected diseases initiative. A stereoisomer (199) of the most potent compound (thiazine oxide) demonstrated substantial efficacy in the stage 1 HAT mouse model with oncedaily oral dosing. The structure−activity relationship studies indicated that removal of the benzylic methylene reduced the activity, but the addition of a proximal pyridine ring enhanced the potency of compound 200 (Figure 42).165 Crozet et al., while attempting to improve the antiparasitic pharmacophore by lowering the mutagenicity, synthesized a series of 5-nitroimidazoles with an arylsulfonylmethyl group and evaluated them for antiparasitic activity against Trichomonas vaginalis. The study revealed that several derivatives had SI values that were better than metronidazole (4), and compounds bearing an additional methyl group placed at the second position demonstrated a lower mutagenicity than metronidazole (4). A lower mutagenicity and significant antitrichomonal activity was exhibited by compounds 201−203 (Figure 42).166 Hernandez-Nunez et al. in their delineation of the structural prerequisites for antiparasitic activities of 2 prepared novel Nacetamide (sulfonamide)-2-methyl-4-nitro-1H-imidazoles as benzinidazole (3) analogs. The design strategy that was employed involved moving the nitro group from position 2 to position 4 and the addition of an extra methyl group at position 2 in the metronidazole core. The resulting 2-methyl-4nitroimidazole scaffold was fused with a sulfonamide pharmacophore in anticipation of an enhanced antiparasitic


5.2. Nitroindazolines: Heterocycles Containing Nitro Groups as Prodrugs. In view of the reported antichagasic potential of the 5-nitroindazolin-3-one scaffold, Fonseca-Berzal et al. explored 1-substituted 2-benzyl-5-nitroindazolin-3-ones and 3-alkoxy-2-benzyl-5-nitro-2H-indazoles against the protozoan parasites Trypanosoma cruzi and Trichomonas vaginalis. Among the series of 1-substituted 2-benzyl-5-nitroindazolin-3ones, compounds 213−215 demonstrated striking potential against amastigotes of the T. cruzi CL Brener strain (IC50 < 1 μM, SI > 1000). Some compounds also exhibited activity against amastigotes from Tulahuen and Y strains of T. cruzi and AC



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was found to possess low toxicity toward cardiac cells (LC50 > 100 μM). Delineation of the structural features indicated that electron-donating groups at position 1 of the indazolinone ring favored the activity. Among the 3-alkoxy-1-alkyl-2H-indazoles, compound 216 was endowed with similar activity profile against a metronidazole-sensitive (JH31A#4) or a metronidazole-resistant (IR78) isolate of T. vaginalis (IC50 = 7.25 and 9.11 μM, respectively) (Figure 44).170 Boiani et al. explored the activity of a series of 5nitroindazoles against T. cruzi bloodstream trypomastigotes and Leishmania promastigotes and reported compound 217 (general structure) to induce substantial trypanocidal potential (>80% lysis), equivalent to gentian violet. A complete lysis of promastigotes of Leishmania was caused by some compounds. The potent compounds were found to exert their effects via an oxidative stress-mediated mechanism of action against T. cruzi epimastigotes (Figure 44).171 Rodrıguez et al. investigated the 5-nitroindazole scaffold structural features that were mandatory for anti-Trypanosoma cruzi activity. The investigation revealed several ideas regarding the structure−activity relationship including 1) a 5-nitro substituent in the indazole ring is necessary for activity, as evidenced by the diminished activity potential of des-nitroanalogs of the potent compound; 2) a butylaminopentyl substituent at position 1 of the indazole ring favors the activity; 3) N-oxidation of the tertiary amino moiety leads to the loss of

activity; 4) the substituent at position 3 was found to be critical, with 3-OH substitution leading to complete loss of activity; and 5) intramolecular cyclization of the side chain at position 1 is not favored. The study also revealed that trypanocidal nitroanion radicals were generated by 5-nitroindazole derivatives through a one-electron process at physiological pH. The general structure of the potent compound (218) is shown in Figure 45.172 ́ Rodriguez et al. synthesized various 5-nitroindazole derivatives with remarkable in vitro activity against Trypanosoma cruzi. The general structures of the most potent compounds are shown in Figure 45. The active compound (e.g., 218) did not exhibit significant unspecific cytotoxicity against macrophages. It was also observed that new derivatives with a quaternized amino moiety between the 5- and 6-CH2 in their side chain resulted in an enhanced activity potential. The mechanism of action was associated with the generation of reduced species of the nitro moiety.173 Vega et al. synthesized a series of 1,2-disubstituted 5nitroindazolinones and evaluated them against epimastigote forms of Trypanosoma cruzi. Among the synthesized compounds, compounds 219, 220, and 221 were found to have significant trypanocidal activity and low unspecific toxicity. The authors assumed that, apart from the lipophilicity of the compounds, trypanocidal activity was also influenced by the susceptibility to the parasite nitroreductases (Figure 45).174 AD



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Figure 46. Patented agents containing nitro groups.


glycosomal or mitochondrial enzymes, be involved in the catabolism of T. cruzi disease. The substantial activity profile of 223 and 224 against T. cruzi was supported by the results of in vitro infectivity assays and in an in vivo murine model of Chagas disease (Figure 45).177

Fonseca-Berzal et al. reported a series of 5-nitroindazole derivatives and evaluated their trypanocidal activity and unspecific cytotoxicity. Among this series, compounds with general structure 222 induced complete inhibition of the growth of amastigotes at concentrations of 30). The in vivo evaluation of potent compounds caused 52% and 77% reductions in parasitemia levels when administered orally to infected mice.175 The potent compounds were further evaluated in diverse Trypanosoma cruzi strains belonging to two discrete typing units (DTUs) frequently associated with human infection (i.e., DTUs TcII and TcVI), and it was found that the compounds were endowed with high selectivity toward drug-sensitive (CL and Tulahuen) and drug-moderately resistant T. cruzi strains (Figure 45).176 Muro et al. reported the synthesis of 3-alkoxy-1-alkyl-and 3alkoxy-1-(ω-aminoalkyl)-5-nitroindazoles and tested them against the different morphological forms of Trypanosoma cruzi. The results of the biological evaluation identified some potent compounds, displaying striking in vitro potential, similar to or better than that of the reference drug (2) with low unspecific cytotoxicity against Vero cells and good selectivity indices (SI). Compounds 223 and 224 were the most potent of the series, and the metabolic changes induced by them evidenced by the complementary analyses and ultrastructural alterations indicate that they might, along with some

6. PATENT SURVEY ON ANTICANCER AGENTS CONTAINING NITRO GROUPS (HYPOXIA-ACTIVATED PRODRUGS) This section presents a recent patent survey on hypoxiaactivated prodrugs (HAPs) containing nitro groups that have displayed promising bioactivity as anticancer agents in in vitro and in vivo studies. In 2003, Longqin Hu et al. filed a patent describing the synthesis of nitroaryl-substituted phosphoramide compounds as potent cytotoxic agents. The results revealed that some of the compounds were substrates of E. coli nitroreductase, with T1/2 values of between 7 and 24 min and 2.9−6.4 min. The compounds were further assayed for cytotoxicity against T116 cells (E. coli nitroreductase) or hDT7 (human quinone oxidoreductase) using a maximum concentration of 100 μM. Compounds 225 and 226 demonstrated similar IC50 values and indicated that E. coli nitroreductase reduction was important for enhanced cytotoxicity. All of the compounds were also tested against human ovarian cancer cell lines (SKOV3), and the results revealed that the compounds with a nitro group at the position para to the benzylic carbon (227−233) showed AE



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four compounds were found to be active against the Miapaca cancer cell line. Furthermore, the HAPs were equally cytotoxic against the parental, sensitive, and resistant cell lines, suggesting that the compounds were not susceptible to the various resistance mechanisms and thus could be useful in cancer treatment (Figure 48).42 Xiao-Hong Cai et al. filed a patent in 2009 on hypoxiaactivated prodrug (HAP) compounds of anthracyclins. Four compounds (247−250) were found to be significantly active. Compound 248 was approximately 400 times more cytotoxic under hypoxic conditions than under normoxia, with the hypoxic IC50 value in the nanomolar range. Compound 248 was also found to be less toxic to the normal cells under normoxic conditions and exhibited selective inhibition of cancer cells. Moreover, compound 248 was administered to mice; the pharmacokinetic profile is depicted in Figure 49.43 In 2011, Li-Xi Yang et al. filed a patent on the discovery of camptothecin-based analogs for the treatment of tumors. Compound 251 emerged as a compound that significantly increased the radiosensitivity of cancer cells. The chemo-

significant inhibition against the ovarian cancer cell line (Figure 46).40 Denny et al. filed a patent on the discovery of novel nitro-1,2dihydro-3H-benzo[e]indoles and related analogs for the use of hypoxia-selective drugs and as radiosensitizing agents for cancer therapy. Almost all of the compounds showed selectivity values that were 200-fold greater for hypoxia against at least one cancer cell line. Most of the compounds bearing 7-SO2NHR or 7-CONHR substituents were found to be active. The selectivity of the compounds suggested that the compounds have possibilities as hypoxia-selective cytotoxins. Compounds 234, 235, 236, and 237 were found to be promising, with significant IC50 values against both of the tested cancer cell lines (Figure 47).41 Jiao et al. in 2008 filed a patent on hypoxia-activated prodrugs (HAPs) for the treatment of cancer. Their results demonstrated that prodrugs they produced were more cytotoxic under hypoxic conditions than under normoxia. Notably, nine of the compounds (238−246) exhibited potent cytotoxicity, with IC50 values within the nanomolar range, and AF



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therapeutic effect of 251 caused > 90% inhibition at 10 nM against human prostate cancer cell lines (PC-3 and DU-145). The toxicity profile in mice showed that compound 251 did not exert toxicity, even up to the dose level of 200 mg/kg (Figure 49).44 Charles et al. filed a patent on HAPs in combination with checkpoint kinase 1 (Chk1) inhibitors for the treatment of

cancer. The anticancer potential of the compounds was assessed individually against p53 and HT29 cells using Chk1 inhibitors PF477736, AZD7762 and LY2603618, and cell viability was observed by using alamarBlue. The results demonstrated that the activity of all the compounds was significantly enhanced in the presence of Chk1 inhibitors. Induction of apoptosis by the most potent compound (252) AG



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was assessed in HT29 cells. The treatment with 252 and AZD7762 in combination induced a 3-fold increase in apoptosis, which suggested that the Chk1 inhibitor sensitizes HT29 cells (Figure 49).45 Smaill et al. published a patent in 2014 on the synthesis of prodrugs of nitrobenzamide mustards, nitrobenzamide mustard alcohol, and their corresponding phosphate esters. Wild-type HCT116, H460, H1299, and SiHa cells were used, in addition to HC116 cells. Compounds 253, 254, 255, and 256 were

found to exhibit increased cytotoxicity in cells under hypoxic conditions. Nearly all of the compounds tested provided a profound tumor growth delay following a single intraperitoneal dose (Figure 50).46 In 2014, Nagasaki et al. filed a patent on the synthesis of a prodrug of 2-nitro-1-imidazolepropionic acid containing an amino group, a cyclic amino group or a hydroxyl group in the basic pharmacophore. Compound 257 exhibited a significantly lower survival rate of human pancreatic cancer cells, even at a AH



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enzyme. The results of the cell proliferation data indicated that compounds 272 and 273 had potent inhibitory activity, with IC50 values of ∼0.03 μM, even in the absence of a selective inhibitor of AKR IC3 (Figure 53).51 In 2016, Smaill et al. published a patent on the synthesis of various prodrug forms of kinase inhibitors. The results of the biological evaluation revealed that most of the compounds acted as reversible inhibitors of the erbB family and exhibited potent inhibitory activity against erbB1. It was also indicated that the quaternary ammonium salt prodrugs (274) of significant active compounds possessed potent inhibitory activity against erbB1, with IC50 values ranging from 0.22 to 0.56 nM (Figure 53).52

low oxygen concentration, compared to that of compound 258 (previously reported in WO 2009/018163 A1). These results revealed that compound 257 possessed higher cytotoxic activity against human pancreatic cancer cells (Figure 50).47 Nordihydroguaiaretic acid (NDGA) is an antioxidant and acts as a lipoxygenase inhibitor. It is isolated from creosote bush (Larrea tridentate). Previously, it was reported that tetra-Omethylnordihydroguaiaretic acid suppresses the growth of a variety of mouse and human tumor cells. On the basis of these findings, Huang et al. published a patent in 2014 on the synthesis of nitroimidazole conjugates by incorporating tri-Omethyl NDGA (M3N) with a nitroimidazole moiety to increase the effectiveness of tetra-O-methyl nordihydroguaiaretic acid that targets the hypoxic cancer cells. Among the synthesized compounds, 259−262 showed better inhibitory activity than the parent compound against Hep3B human hepatocellular carcinoma cells. The results indicated that all the conjugates, as well as the parent compound, were active in hypoxic conditions (Figure 51).48 In 2014, Matteucci et al. filed a patent on the synthesis of phosphoramidate alkylator prodrugs. The results of the biological evaluation indicated that compounds 263, 264, and 265 were significantly active only against H460 cell lines in all conditions, whereas compound 266 (in N2, IC50 = 2 μM; in O2, IC50 = 25 μM; in air, IC50 > 100 μM) showed activity against HT29 cell lines (Figure 51).49 In 2014, Yang et al. published a patent on the synthesis of nitropyridinyl ethyleneimine compounds. The results revealed that most of the compounds (267−271) possessed more intensive hypoxia-selective antitumor activity, and their working mechanism was, in general, associated with human cytochrome P450 reductase (CYPOR) but not with wild-type (WT) cell lines (Figure 52).50 Duan et al. filed a patent in 2016 on the synthesis and biological evaluation of nitrobenzyl derivatives as anticancer agents. All the synthetic compounds were tested for their ability to inhibit cell proliferation in an H460 human cancer cell line in the presence and absence of a specific inhibitor of the AKR IC3

7. FUTURE PERSPECTIVES The investigation of drugs that contain nitro groups is an area of study with diverse opinions. A majority of the scientific community considers it to be a bioreductive trigger as well as a toxicophore. It often has been labeled as a structural alert, but this categorization has not hindered the focus of chemists on the exploration of nitro-group-based prodrugs. This Perspective presents the various aspects of agents that contain nitro groups, along with an overview of the recent advances in antitubercular, anticancer, and antiparasitic agents containing these groups. The anticancer effects of nitro-based compounds have been demonstrated to be induced via diverse mechanisms, but primarily the impact lies in its hypoxia-selective effects. The hypoxia-selective cell killing effects have been observed with chemically varied architectures including duocarmycins, isophosphoramide mustard, muramyldipeptide, imidazoacridone moiety, 2′-deoxyuridine, 2′-deoxycytidine, 5′-O-nitro-2′-deoxyuridines, nitrochloromethylbenzindolines, doxorubicin, gemcitabine, O6-benzylguanine, 6-O-glucoazomycin, campthothecin, quinazolines, phenstatins, and several hybrid scaffolds. This clearly indicates the feasible incorporation of aryl and heteroaryl functionalities containing a nitro group as the bioreductive arm or trigger in diverse scaffolds, ultimately leading to pronounced AI



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nitro-containing drugs. Nifurtimox (2) excellently exemplifies this strategy, as the drug is an alcohol dehydrogenase 2 (ALDH2) substrate, and therefore the concomitant administration of ALDH2 inhibitors can reduce the toxicity issues. This approach presents enough scope in the fabrication of such strategies to reduce toxicity. Another way to combat such issues could be the establishment of tumor tissue selectivity, which evidence indicates can be achieved by bioreductive therapy. In this context, application of stimuli-responsive nanocarriers would appear to be a promising approach, particularly in view of the large number of nitro-based drugs that fail clinical trials due to toxicity issues. A formulation design program for the numerous drugs containing nitro groups covered in this review that can release the anticancer drugs in the tumor microenvironment (e.g., hypoxic regions) appears to be a practical approach that could lead to therapeutic benefits. The cleavage or reduction of hypoxia-responsive nitro functional groups at low oxygen levels is likely to induce conformational variations in hypoxia-responsive nanocarriers, which are required to be deeply explored for the efficient design of nanocarriers for the “drugs containing nitro groups” pipeline. Formulation chemists should also direct their attention to the antitubercular agents containing nitro groups, such as PA-824 (24) and OPC-67683 (13), which are not mutagenic and genotoxic but rather are handicapped by solubility issues, which are a barrier to their therapeutic utility. In summary, agents containing nitro groups have been designed and employed as prodrugs. This is clear from recent advances in the fields of anticancer, antitubercular, and antiparasitic agents. It appears that controlling the bioactivation of nitro compounds could prove to be a genuine boon in the maximization of the bioactive potential of such compounds, with a concomitant minimization of toxicity issues. Toxicity related hindrance in the therapeutic growth of drugs containing nitro groups should be handled by finding ways to combat it rather than avoiding it.

hypoxia-selective inhibitory effects. The research literature for antitubercular and antiparasitic agents also exemplifies the therapeutic benefits of bioreduction or induced fragmentation mediated by nitro groups in augmenting the biological effects. There have been a number of investigations conducted on antitubercular agents with nitro group functionality, and most of them revolve around the attempts involving structural engineering on PA-824 and OPC-67683. Several alterations have been attempted and explored in the 5-nitroimidazooxazole skeleton, along with the placement of several heteroaryl bioisosteric surrogates, and in most cases, the results have exhibited favorable trends. Nitrotriazoles, imidazoles, and several antitubercular leads have also been an area of investigation for the development of antiparasitic agents. The development of drugs for neglected disease initiative programs has been very efficient in developing libraries of nitro-based heterocyclic compounds, and the results have been quite optimistic, indicating a high probability of effectiveness replicated in clinical studies. The patent literature is in accordance with the research literature, showing the contribution of the bioreductive activation of nitro-based compounds to exert potent effects. The role of the nitro group in inducing these effects was often confirmed by researchers by comparing the activity profile with the des-nitro analogs or derivatives in a majority of the cases. The influence of nitro groups toward the stabilization of the compounds within the catalytic site by hydrogen bonding and other interactions, as indicated by molecular modeling studies, was also observed. Among the nitro heterocycles, the nitroimadozolyl ring has emerged as the most privileged and versatile heterocycle, exerting potent cellkilling effects in addition to its well-proven potential against infectious diseases. In view of the aforementioned evidence, along with the fact that hypoxic regions in tumors are often associated with invasiveness, metastasis, and resistance to radiotherapy and chemotherapy, the development of nitrobased hypoxia-activated prodrugs appears to be a most logical and urgently desired area of research. Despite the promising evidence of nitro-based bioreductive prodrugs targeting tumor hypoxia, it has not been possible to replicate and amplify their preclinical promise into clinical efficacy. Thus, several alternatives must be conceptualized to fully exploit the benefits of drugs containing nitro groups. One of the logical strategies could be the combination of radiation therapy and chemotherapy that employs bioreductive drugs containing nitro groups to induce synergistic effects. Previously reported evidence of such combinations has provided positive results for the rationalization of this approach, as prodrugs activated by hypoxia target radiation-resistant hypoxic cells. Furthermore, the potential of the role of nitroimidazoles as radiosensitizers should also be investigated in detail. It appears that solid tumors exhibiting extreme vulnerability to this combination can be substantially inhibited. Several agents covered in this Perspective have shown remarkable efficacy against solid tumors, and this further lays the foundation for the evaluation of these agents in combination with radiation. Another challenge associated with the use of drugs containing nitro groups concerns toxicity issues such as carcinogenicity, hepatotoxicity, mutagenicity, and bone-marrow suppression. In this context, future constructive attempts should be directed toward capitalizing the biopotential and reducing the toxicities of these drugs through structure genotoxicity/mutagenicity studies. A relevant approach could be the identification of the human enzymes that activate the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jmedchem.8b00147.

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Nitro containing drugs/agents and their clinical/ preclinical update (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-2-2736-1661, extension 6130. ORCID

Jing-Ping Liou: 0000-0002-3775-6405 Notes

The authors declare no competing financial interest. Biographies Kunal Nepali obtained his Ph.D. degree in Pharmaceutical Chemistry in the year 2012 from ISF College of Pharmacy, Moga, Punjab, India, and is currently a postdoctoral researcher in the Department of Medicinal Chemistry, Taipei Medical University, Taiwan. His scientific interests are centered in the design and synthesis of novel molecular entities as future therapeutics to address pharmacological problems. AJ



Perspective

(6) (a) Lopez Nigro, M. M.; Carballo, M. A. Genotoxicity and cell death induced by tinidazole (TNZ). Toxicol. Lett. 2008, 180, 46−52. (b) Fung, H. B.; Doan, T. L. Tinidazole: a nitroimidazole antiprotozoal agent. Clin. Ther. 2005, 27, 1859−1884. Tinidazole News. https:// www.drugs.com/answers/support-group/tinidazole/news/ (accessed December 21, 2017). (7) Raether, W.; Hänel, H. Nitroheterocyclic drugs with broad spectrum activity. Parasitol. Res. 2003, 90 (Suppl. 1), S19−S39. (8) Wilkinson, S. R.; Bot, C.; Kelly, J. M.; Hall, B. S. Trypanocidal activity of nitroaromatic prodrugs: current treatments and future perspectives. Curr. Top. Med. Chem. 2011, 11, 2072−2084. (9) Whitmore, G. F.; Varghese, A. J. The biological properties of reduced nitroheterocycles and possible underlying biochemical mechanisms. Biochem. Pharmacol. 1986, 35, 97−103. (10) Boelsterli, U. A.; Ho, H. K.; Zhou, S.; Leow, K. Y. Bioactivation and hepatotoxicity of nitroaromatic drugs. Curr. Drug Metab. 2006, 7, 715−727. (11) Wardman, P. Electron transfer and oxidative stress as key factors in the design of drugs selectively active in hypoxia. Curr. Med. Chem. 2001, 8, 739−761. (12) Ruchelman, A. L.; Singh, S. K.; Wu, X.; Ray, A.; Yang, J.-M.; Li, T.-K.; Liu, A.; Liu, L. F.; LaVoie, E. J. Diaza- and triazachrysenes: Potent topoisomerase-targeting agents with exceptional antitumor activity against the human tumor xenograft, MDAMB-435. Bioorg. Med. Chem. Lett. 2002, 12, 3333−3336. (13) Ruchelman, A. L.; Singh, S. K.; Ray, A.; Wu, X.; Yang, J.-M.; Li, T.-K.; Liu, A.; Liu, L. F.; LaVoie, E. J. 5H-Dibenzo[c,h][1,6]naphthyridin-6-ones: Novel topoisomerase I-targeting anticancer agents with potent cytotoxic activity. Bioorg. Med. Chem. 2003, 11, 2061−2073. (14) Singh, S. K.; Ruchelman, A. L.; Li, T. K.; Liu, A.; Liu, L. F.; LaVoie, E. J. Nitro and amino substitution in the D-Ring of 5-(2dimethylaminoethyl)-2,3-methylenedioxy-5H-dibenzo[c,h][1,6]naphthyridin-6-ones: Effect on topoisomerase-I targeting activity and cytotoxicity. J. Med. Chem. 2003, 46, 2254−2257. (15) Morrell, A.; Placzek, M.; Parmley, S.; Antony, S.; Dexheimer, T. S.; Pommier, Y.; Cushman, M. Nitrated indenoisoquinolines as topoisomerase I inhibitors: a systematic study and optimization. J. Med. Chem. 2007, 50, 4419−4430. (16) Thomas, M.; Clarhaut, J.; Tranoy-Opalinski, I.; Gesson, J. P.; Roche, J.; Papot, S. Synthesis and biological evaluation of glucuronide prodrugs of the histone deacetylase inhibitor CI-994 for application in selective cancer chemotherapy. Bioorg. Med. Chem. 2008, 16, 8109− 8116. (17) Tan, S.; He, F.; Kong, T.; Wu, J.; Liu, Z. Design, synthesis and tumor cell growth inhibitory activity of 3-nitro-2H-chromene derivatives as histone deacetylase inhibitors. Bioorg. Med. Chem. 2017, 25, 4123−4132. (18) Zhu, R.; Liu, M. C.; Luo, M. Z.; Penketh, P. G.; Baumann, R. P.; Shyam, K.; Sartorelli, A. C. 4-Nitrobenzyloxycarbonyl derivatives of O6-benzylguanine as hypoxia-activated prodrug inhibitors of O6alkylguanine-DNA alkyltransferase (AGT), which produces resistance to agents targeting the O-6 position of DNA guanine. J. Med. Chem. 2011, 54, 7720−7728. (19) Zheng, Y. B.; Gong, J. H.; Liu, X. J.; Wu, S. Y.; Li, Y.; Xu, X. D.; Shang, B. Y.; Zhou, J. M.; Zhu, Z. L.; Si, S. Y.; Zhen, Y. S. A novel nitrobenzoate microtubule inhibitor that overcomes multidrug resistance exhibits antitumor activity. Sci. Rep. 2016, 6, 31472. (20) Winn, B. A.; Shi, Z.; Carlson, G. J.; Wang, Y.; Nguyen, B. L.; Kelly, E. M.; Ross, D.; Hamel, E.; Chaplin, D. J.; Trawick, M. L.; Pinney, K. G. Bioreductively activatable prodrug conjugates of phenstatin designed to target tumor hypoxia. Bioorg. Med. Chem. Lett. 2017, 27, 636−641. (21) Tercel, M.; Atwell, G. J.; Yang, S.; Stevenson, R. J.; Botting, K. J.; Boyd, M.; Smith, E.; Anderson, R. F.; Denny, W. A.; Wilson, W. R.; Pruijn, F. B. Hypoxia-activated prodrugs: substituent effects on the properties of nitro seco-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one (nitrocbi) prodrugs of DNA minor groove alkylating agents. J. Med. Chem. 2009, 52, 7258−7272.

Hsueh-Yun Lee received his Ph.D. from National Taiwan University in 2010. After 2 years of postdoctoral training, he joined in faculty at Taipei Medical University in 2012. His major scientific interests focus on the discovery of small molecules targeting epigenetic disorders, oncogenesis, and neurological diseases. Jing-Ping Liou has expertise spanning medicinal chemistry, natural product chemistry, and organic synthesis, with a >20-year experience in the development of cancer therapeutics. Presently, he is a Professor of Medicinal Chemistry in Taipei Medical University, Taiwan, and has a Ph.D. degree from the College of Medicine, National Taiwan University. His research work employs a rational structure based drug design approach that lies at the interface of chemistry and biology to treat cellular processes dealing with chronic molecular stress. His career as a medicinal chemist extends to the industrial sector also where he has advised several pharmaceutical companies and start-ups for developing novel clinical drug candidates. His publication profile includes 20 contributions to the Journal of Medicinal Chemistry along with numerous other publications and patents.

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ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technology of the Republic of China (Grant MOST 106-2113M-038-002).

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ABBREVIATIONS USED

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REFERENCES

Mtb, Mycobacterium tuberculosis; MIC, minimum inhibitory concentration; MBC, minimum bactericidal concentration; ADMET, absorption, distribution, metabolism, excretion, toxicity; BTZ, benzothiazine; TB, tuberculosis; MPA, mycophenolic acid; TKI, tyrosine kinase inhibitor; POR, cytochrome P450 oxidoreductase; ADEPT, antibody-directed directed enzyme prodrug therapy; GDEPT, gene-directed enzyme prodrug therapy; AGT, O6-alkylguanine-DNA alkyltransferase; CTA, clinical trial application; IND, investigational new drug application; NO, nitric oxide; CLL, chronic lymphocytic leukemia; FDA, Food and Drug Administration; DNDi, drugs for neglected diseases initiative; IMPDH, inosine 5′-monophosphate dehydrogenase; HAPS, hypoxia activated prodrug; HAT, human African trypanosomiasis; VL, visceral leishmaniasis

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