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Targeted Treatments for Chronic Obstructive Pulmonary Disease (COPD) Using Low-Molecular-Weight Drugs (LMWDs) Huihui Ti, Yang Zhou, Xue Liang, Runfeng Li, Ke Ding, and Xin Zhao J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.8b01520 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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Journal of Medicinal Chemistry

Targeted Treatments for Chronic Obstructive Pulmonary Disease (COPD) Using Low-MolecularWeight Drugs (LMWDs) Huihui Ti,†,〇 Yang Zhou,†,‖,〇 Xue Liang, † Runfeng, Li,⊥ Ke Ding,*

,‡,⊥

and Xin Zhao*

,†,

§

†

Key Laboratory of Molecular Target & Clinical Pharmacology, State Key Laboratory of

Respiratory Disease, School of Pharmaceutical Sciences & The Fifth Affiliated Hospital, Guangzhou Medical University, Guangzhou 511436, P. R. China ‡

International Cooperative Laboratory of Traditional Chinese Medicine Modernization and

Innovative Drug Development of Chinese Ministry of Education (MOE), Guangzhou City Key Laboratory of Precision Chemical Drug Development, School of Pharmacy, Jinan University, Guangzhou 510632, P. R. China ⊥

State Key Laboratory of Respiratory Disease, National Clinical Research Center for

Respiratory Disease, Guangzhou Institute of Respiratory Health, The First Affiliated Hospital, Guangzhou Medical University, Guangzhou 510120, P. R. China §School

of Life Sciences, The Chinese University of Hong Kong, Shatin, N.T, Hong Kong SAR

999077, P. R. China

ACS Paragon Plus Environment

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of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of

Technology (KTH), AlbaNova University Center, Stockholm SE-100 44, Sweden ABSTRACT Chronic obstructive pulmonary disease (COPD) is a very common and frequently fatal airway disease. Current therapies for COPD depend mainly on long-acting bronchodilators, which cannot target the pathogenic mechanisms of chronic inflammation in COPD. New pharmaceutical therapies for the inflammatory processes of COPD are urgently needed. Several anti-inflammatory targets have been identified based on increased understanding of the pathogenesis of COPD, which raises new hopes for targeted treatment of this fatal respiratory disease. In this review, we discuss the recent advances in bioactive low-molecular-weight drugs (LMWDs) for the treatment of COPD and, in addition to the first-line drug bronchodilators, focus particularly on low-molecular-weight anti-inflammatory agents, including modulators of inflammatory mediators, inflammasome inhibitors, protease inhibitors, antioxidants, PDE4 inhibitors, kinase inhibitors, and other agents. We also provide new insights into targeted COPD treatments using LMWDs, particularly small-molecule agents.

1. INTRODUCTION What is COPD? Chronic obstructive pulmonary disease (COPD) is a highly prevalent disease1-3 that is associated with enhanced chronic inflammation of the respiratory tract.4 It is becoming a major global health problem with high morbidity and mortality.5,6 The Global Burden of Disease study 2015 (GBD 2015) reported that from 1990 to 2015, the prevalence of COPD increased by 44.2%. In 2015, 3.2 million people died from COPD worldwide,7 and the disease became the third-ranked cause of death and the fifth-ranked disease in terms of disease burden.1,8 This disease is increasing in importance and has been designated a high-profile respiratory disease.9,10


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However, it is difficult to define COPD, and the term is used to describe patients who suffer from airflow limitation (AL) accompanying small airway obstruction and have a low forced expiratory volume (FEV).11 AL, which is defined by a forced expiratory volume in 1 s (FEV1) to forced vital capacity (FVC) ratio 80%

79-50%

49-30%


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30 μM), moderate metabolic stability (half-life 3.27 h) and acceptable oral bioavailability (68%). A preliminary mechanism of action study indicated that MCC950 can specifically inhibit the activation of NLRP3 but not TLR signaling or NLRP3 priming and that it does not inhibit K+ efflux, Ca2+ influx or NLRP3-ASC interactions. In the NLRP3 inflammasome activation process, NLRP3-dependent ASC oligomerization is a key step. In the LPS- and nigericin-induced BMDMs model, ASC lcomplex formation was blocked by MCC950. In ATP-stimulated ASCcerulean cells, ASC speck formation also decreased in response to MCC950. These data suggest that MCC950 inhibits NLRP3 inflammasome activation by blocking NLRP3-induced ASC oligomerization. The intensive mechanism of action of MCC950 has not yet been fully explored. The molecular targets of MCC950 may be involved in the posttranslational modification of NLRP3.81 In mouse models, ASC specks can recruit neutrophils and monocytes, and ASC specks are abundant in bronchoalveolar lavage from COPD patients and cigarette smoke-induced murine models of COPD.81 This abundance indicates that NLRP3 inflammasome activation and subsequent ASC speck formation are important to the inflammatory progress of COPD. Furthermore, IL-1β response correlates with increased neutrophilic inflammatory response, which is the pathogenic characteristic of COPD.82 Thus, NLRP3 inflammation activation and IL1β responses may lead to the development of COPD, and MCC950 may have potent effects in COPD. Other NLRP3 inflammasome inhibitors (Chart 4). In addition to IL-1β, the proinflammatory cytokine TNF-α shows increased levels in the sputum of COPD patients and amplifies airway inflammation. Auranofin,83 a drug for rheumatoid arthritis, targets inhibitors of kappa B kinase (IKK) and suppresses the expression of IL-6 and TNF-α.84 Fc11a-2 is a benzimidazole derivative with in vivo activity in the mouse dextran sulfate sodium (DSS)-induced colitis model, in which


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the expression levels of TNF, IL-1β and IL-18 are low.85 TAK-242 (TLR4 antagonist) and bromoxone are also effective NLRP3 inflammasome inhibitors that can suppress the expression and processing of TNF-α86,87 and may have the potential to treat COPD. The above compounds were selected from many reported NLRP3 inflammasome inhibitors because of their ability to inhibit the proinflammatory cytokines TNF and IL-1β, but this ability is not an absolute standard. The anti-TNF-α monoclonal antibody infliximab has been studied in clinical trials in COPD patients, but unfortunately, no positive results have been obtained.88 Furthermore, there were no LMWDs available for cytokine blocking, and the effects of lowmolecular-weight NLRP3 inflammasome inhibitors are unknown because they have not been applied to COPD research.

3.2.3. Leukotriene B4 (LTB4) receptor antagonists and LTB4 synthetic modulators. LTB4 is a proinflammatory mediator that is generated by the activation of 5-lipoxygenase (5LO) and leukotriene A4 (LTA4) hydrolase and is an activator of neutrophils and macrophages.89 LTB4 plays an important role in the recruitment of several types of inflammatory cells via binding to its receptors BLT1 and BLT2; therefore, several inflammatory diseases, including COPD, are associated with elevated levels of LTB4.90 The modulation of the biosynthetic routes of LTB4 via inhibiting 5-LO, 5-LO activating protein (FLAP), and LTA4 hydrolase or via antagonism of the LTB4 receptor are potential pharmacological strategies for COPD.91 Alternatively, the product of 15-lipoxygenase (15-LO) pathways, 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), modulates LTB4 levels and stimulates inflammation and mucus secretion (Figure S4).92,93 This key role of 15-LO makes it a drug target for the treatment of inflammatory diseases.


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Theoretically, LTB4 receptor inhibitors (Chart S4), 5-LO inhibitors (Chart S5), FLAP inhibitors (Chart S6), LTA4 hydrolase inhibitors, and 15-LO inhibitors that can target the LTB4 pathway are available for the treatment of COPD.94 However, these inhibitors either lack studies in COPD or showed negative results in clinical development.

3.3. LMWDs targeting proteases. An imbalance between proteases and antiproteases occurs in COPD and contributes to emphysema, mucus hypersecretion,95 and neutrophilic inflammation (Figure 7).45 Connective tissue components in the lung parenchyma, such as elastin, are important for maintaining lung function. Elastin can be digested by proteases, and the loss of elasticity in the lung parenchyma causes emphysema.96 In contrast, endogenous antiproteases, including α1-antitrypsin, secretory leukoprotease inhibitor (SLPI), elafin, and tissue inhibitors of MMP (TIMP), can limit this degradation.45 Ideally, the inhibition of such proteases and/or increasing the antiprotease levels would prevent lung destruction and slow the progress of inflammation. Antiproteases are protein drugs and must be applied through gene therapy. In contrast, treatment using low-molecularweight elastase inhibitors is more convenient and feasible.


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Figure 7. Imbalance between proteases and antiproteases in COPD. Neutrophil elastase inhibitors. Human neutrophil elastase (HNE), together with proteinase 3 (PR3) and cathepsin G, belongs to the neutrophil serine proteases. HNE is a worthwhile therapeutic target for the treatment of inflammatory diseases.97 Sivelestat (ONO-5046, Chart 5) is the only marketed low-molecular-weight HNE inhibitor, and it has been used to treat acute lung injury and acute respiratory distress.98 Some HNE inhibitors, including ONO-6818 (Chart S7), AZD9668 (Chart S7), and AZD6553 have been investigated in clinical trials; unfortunately, these developments have been discontinued. BAY85-8501 (Chart 5) is a new, highly potent and selective HNE inhibitor. BAY-85-8501 inhibits the activity of HNE at picomolar concentrations (65 pM) in vitro. BAY-85-8501 binds to HNE


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with higher affinity (Ki = 80 pM) than to rat or mouse NEs and cannot inhibit 21 other serine proteases. These data show the high potency and selectivity of BAY-85-8501. In addition, BAY85-8501 has good pharmacokinetic properties, with a half-life of 8.5 h and 63% oral bioavailability, and it showed in vivo efficacy in an HNE-induced acute lung injury mouse model at a dose as low as 0.01 mg/kg.99 BAY-85-8501 has been developed in a phase II trial for safety and efficacy evaluation in patients with non-cystic fibrosis bronchodilation (ClinicalTrials.gov identifier: NCT01818544). Compound 2 (Chart 5) represents a series of novel HNE inhibitors with an N-benzoylindazole scaffold and can specifically inhibit HNE with an IC50 value of 7 nM.100 Testing of a silanediol peptide analog (compound 3, Chart 5) indicated that the incorporation of a silanediol isostere in the structure improved the selectivity for HNE over other serine proteases, including trypsin, chymotrypsin, cathepsin G, plasmin, thrombin, and porcine pancreatic elastase.101 The O3-pivaloyl derivative 4 (Chart 5) was identified as a potent HNE inhibitor with excellent selectivity and cytotoxicity profiles. In addition, its stability against hydrolytic enzymes from plasma and microsomes was tolerable, and its stability against microsomes was higher than that of sivelestat.102 Chart 5. Chemical structures of HNE inhibitors.


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O

H N

O

OH

O H N S O

O

NC

N N

NC

O

O

Sivelestat

F

O

CN

F F

N N

S O

CH3

O 2

BAY-85-8501

O O N H

H N O

O

OH OH Si

NHnBu

O

O

S

N

O N

3

COOH 4

These compounds provide promising leads or show therapeutic potential for the development of new HNE inhibitors for the treatment of COPD. Additional evidence of their effects in treating COPD must be provided by further studies. Other reported HNE inhibitors that represent different structural types are presented in Chart S7. Matrix metalloproteinase (MMP) inhibitors. MMPs are a family of structurally related and zinc-dependent metalloenzymes that mediate the breakdown of connective tissue, and they are targets for anti-inflammatory therapies.103 MMP-12, also known as macrophage metalloelastase, is involved in the pathological condition of emphysema. It was proposed that MMP-12 also mediates the release of TNF-α from macrophages and leads to smoke-induced inflammation.104 Moreover, MMP-12 plays a pivotal role in COPD because blocking MMP-12 can markedly impair macrophage and neutrophil recruitment and protect against emphysema in a cigarette smoke-induced mouse model;105 therefore, MMP-12 can serve as a target for the treatment of COPD.


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Marimastat (Chart 6) is a nonselective MMP inhibitor that significantly inhibits MMP-12induced early inflammatory responses, including neutrophil influx and cytokine and MMP-9 release, and inhibits late responses by decreasing macrophage recruitment.106 Marimastat underwent a clinical trial for cancer therapy, but it exhibited major side effects, such as arthralgia and musculoskeletal pain,107,108 which limited its development.50 A dual MMP-9/12 inhibitor, AZ11557272, ameliorates morphological emphysema and prevents small airway remodeling in a guinea pig model.109 Another dual MMP-9/12 inhibitor, AZD1236 (Chart 6), has been used in phase IIa studies against COPD. Although oral doses of AZD1236 were well tolerated in shortterm treatment in moderate to severe COPD patients, no clinical efficacy of AZD1236 was shown.110 Chart 6. Chemical structures of the nonselective MMP inhibitors marimastat and AZD1236.

O HO

N H

H N OH

O

O

Cl N H

N

N

O S

H N O O

O

O

NH

AZD1236

Marimastat


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Figure 8. The rational design of MMP-12 inhibitors. (A) Structural features of specific MMP-12 inhibitors. (B) Active site of MMP-12 (PDB entry, 1JK3). Due to the highly homologous structures of the MMP family, nonselective MMP inhibitors may interact with other MMPs and thus cause side effects. Therefore, the design of MMP-12 inhibitors with high selectivity and specificity has attracted the attention of researchers, and increased efforts have been made to identify specific MMP-12 inhibitors. The common features


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of the active site of MMPs consist of three parts: the Zn(II) ion, the backbone of the enzyme, and the S1’ pocket. The specificity of the S1’ pocket is responsible for the enzyme’s selectivity and is determined by the depth of the pocket.103,111 The rational design of specific MMP-12 inhibitors based on the structure and specificity of MMP-12 includes adding a functional group termed the zinc-binding group (ZBG) to chelate the active site Zn(II) ion, a H-bond receptor or donor for interacting with the amino acid backbone through hydrogen bonding, and a hydrophobic scaffold to fit into the S1’ pocket via hydrophobic interactions (Figure 8). The structure of marimastat is a hydroximic acid ZBG linked to an amino acid backbone. AS111793 (Chart 7) was designed by attaching a 5-(2-thiophenyl)-1,2,4-oxadiazolyl moiety to the backbone of marimastat, which may occupy the large S1’ pocket of MMP-12 to increase selectivity. This selective MMP-12 inhibitor, AS111793, not only significantly reduced the number of neutrophils and macrophages but also decreased the activity of inflammatory cytokines such as TNF, IL-6, and pro-MMP-9 in bronchoalveolar lavage fluids of mice exposed to cigarette smoke. This study indicated that AS111793 suppresses the lung inflammation induced by cigarette smoke via the inhibition of MMP-12 and demonstrated that the use of MMP-12 inhibitors may be a therapeutic approach for the treatment of inflammation associated with COPD.112 Compound 5 (Chart 7) is a substituted γ-keto carboxylic acid compound with carboxyl serving as a ZBG and the 4-position on the phenyl substituted by alkynyl groups, which fit into the bottom of the S1’ pocket. Structureactivity relationship (SAR) studies revealed that the groups at the end of the alkynyl are important for the interaction between inhibitors and the S1’ pocket. The IC50 value for the inhibition of MMP-12 by compound 5 is 0.2 µM, and 5 provides protection against emphysema in vivo. This compound may have the potential to be further developed for COPD therapy.113 The phenoxybenzenesulfonyl compound 6 (Chart 7) was identified as a selective and potent


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MMP-12 inhibitor; its phenoxybenzenesulfonyl scaffold may occupy the S1’ pocket well, its hydroxamic acid group can serve as a ZBG, and the sulfonyl interacts with the backbone of the enzyme via H-bonds. Compound 6 had an IC50 value of 0.2 nM, with good selectivity for MMP12 over MMP-1 and MMP-14. This compound also showed subnanomolar activity for MMP-9, which is another MMP related to COPD.114 The selectivity and activity of MMP408, MMP118, and compound 7 (Chart 7) revealed that substituted dibenzofuran groups were another core suitable for occupying the S1’ pocket of MMP-12. This series of compounds showed good oral efficacy in a lung inflammation mouse model. MMP408 has a half-life of 3 h and a bioavailability of 27% in C57BL/6 mice.115 MMP118 was generated by optimizing MMP408 through replacement of the amide group by a five-membered heterocycle in the dibenzofuran core. MMP118 maintained the potency, selectivity and half-life of MMP408 but had improved bioavailability (63%).116 Compound 7 represents the most potent MMP-12 inhibitor reported in the literature and has high metabolic stability. Moreover, 7 and two of its analogs were selected for the synthesis of their fluorine-18-labeled analogs, and the radiolabeled compounds were evaluated in vivo by imaging MMP-12 with positron-emission tomography (PET). This in vivo biodistribution approach provides a tool for the diagnosis of MMP-associated diseases.117 Compound 8 (Chart 7) was designed to represent novel MMP-12 inhibitors with a N-1hydroxypiperidine-2,6-dione group as a new ZBG. The in vitro inhibition of MMP-12 by 8 showed that 8 had an IC50 = 33 nM and exhibited 1200-, 176-, and 20-fold selectivity for MMP12 over MMP-14, MMP-9, and MMP-2, respectively.118 Compound 8 is a promising hit among MMP-12 inhibitors with a new ZBG for further optimization and evaluation against COPD. Compound 9 (Chart 7) was described as a thiophene-based MMP-12 inhibitor with nanomolar affinity. The X-ray structure of 9 complexed with the MMP-12 catalytic domain showed that its


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biaryl moiety fitted into the S1’-pocket well but that no groups interacted with the Zn(II) site; that is, 9 represents a series of new leads of MMP-12 inhibitors without a ZBG.119 The thiophene and its bioisosteric derivatives, a class of compounds with a β-hydroxy carboxylic acid template, and the newly reported radioiodinated probes of MMP-12 inhibitors are listed in Chart S8. The above cases indicated that the active site of MMP-12 may have a large and extended S1’ pocket, and the design of novel, highly selective MMP-12 inhibitors may focus on this specificity pocket to reduce side effects. Proteinase 3 (PR3) inhibitors. PR3 is another neutrophil serine protease that is also considered a target for the treatment of COPD. PR3 and HNE are homologous proteases that share up to 50% sequence and structural similarity.120 The ketomethylene-based short peptide PR3 inhibitor 10 (Chart 8) showed a low micromolar IC50 of 1.9 µM, with selectivity for PR3 over HNE, and exhibited competitive and reversible inhibition of PR3.121 Another peptide-based phosphonate inhibitor, 11 (Chart 8), inhibits human PR3 as well as macaque PR3 and has no effect on HNE. In addition, molecular probes were designed using biotinylated valine residues to provide a tool for exploring the function of PR3 both in vitro and in vivo.122 Compound 12 (Chart 8) is a reported hit among selective PR3 inhibitors based on the 1,2,3,5-thiatriazolidin-3one 1,1-dioxide scaffold.123 PR3 has specific functions in regulating the inflammatory response that are different from those of HNE; however, research of PR3 inhibitors is still in the initial stage. Chart 7. Chemical structures of selected MMP-12 inhibitors.


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O

S HO O HO

N H

N

H N

N

O

O

OH O Cl AS-111793

5

O

HO O HO

N H O

S

O

NH S O O

O

R

MMP408 R=

O



O N H

 MMP118 R=

O

7 R=

O

N N N

F N

O HO

N S



6

O

N

HOOC

S

NH S

O 8

O 9

Chart 8. Chemical structures of reported PR3 inhibitors.


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O

O

NH2

OH H N

O

NH2 O

O

H N

N H

O N H

O

H N

N H

O

O

O 2N

O N H

H N

NO2

N H

O

O HO 10

O HN H

HO OH

O Cl

NH H

H N

S O

O N H

11

H N

O N H O

O

O P O O

HO

N N N S O O N

N N S

Cl 12

3.4. Antioxidants. Oxidative stress is an imbalance between oxidant and antioxidant levels and is involved in many chronic diseases, including inflammation, cancer, and diabetes. ROS are chemical species in the oxidative stress response that contain oxygen and contribute to the pathophysiology of oxidative stress.42 When the levels of ROS exceed the levels of antioxidants, oxidative stress occurs. In COPD, particularly during exacerbations, the increased production of ROS is primarily stimulated by exposure to cigarette smoke, but they are also released from the activation of inflammatory cells, including macrophages, neutrophils and epithelial cells, in the lungs.124 Oxidative stress contributes to the pathogenesis of COPD in several ways and plays a key role in the development of therapeutic approaches for COPD (Figure 9). In the airway, ROS lead to the activation of proinflammatory/inflammatory cytokine and chemokine genes by activating NF‑κB and p38 mitogen-activated protein kinase (p38 MAPK); ROS contribute to increased elastolysis and accelerate lung damage by destroying the balance of endogenous proteases/antiproteases. ROS also activate TGFβ, which causes lung fibrosis. Additionally, the


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expression of histone deacetylase 2 (HDAC2) is reduced by oxidative stress, leading to corticosteroid resistance in vitro and in vivo.125 Therefore, antioxidant therapy is a promising area for COPD intervention.

Figure 9. Oxidative stress in COPD and potential therapeutic targets for antioxidants. Nrf2, nuclear factor erythroid 2-derived factor 2; NOX, NADPH oxidase; MPO, myeloperoxidase; SOD, superoxide dismutase; iNOS, inducible nitric oxide synthase; NO, nitric oxide Mucolytic Drugs. N-acetylcysteine (NAC), erdosteine, and carbocisteine (Chart 9) are mucolytic agents with antioxidant and anti-inflammatory properties. They have been used to reduce the mucus thickness in patients with COPD in the clinic. Recently, in China, the efficacy and safety of high-dose NAC (1200 mg daily) for the long-term treatment of patients with moderate to severe COPD was evaluated in a designed PANTHEON study, which is a prospective, ICS-stratified, randomized, double-blind, placebo-controlled, parallel-group, multicenter trial. This study found that high-dose long-term (1 year) NAC treatment can prevent


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COPD exacerbations.126 In a prospective randomized, double-blind, placebo-controlled study of erdosteine, enrolling patients aged 40–80 years with the Global Initiative for COPD stage II/III study (RESTORE study), the exacerbation rate was assessed as the primary outcome after longterm (1 year) treatment with a 300 mg twice-daily dose. This study demonstrated that erdosteine can decrease both the rate and duration of exacerbation, and the occurrence of adverse events in the treatment group was similar to that in the placebo group.127

Chart 9. Chemical structures of NAC, erdosteine, and carbocisteine. O

O

HS OH

O O NAC

S O

N H

S

Erdosteine

O

O OH

S HO

OH

NH2 Carbocysteine

Nuclear factor erythroid 2-derived factor 2 (Nrf2) activator. Nrf2 is a basic leucine zipper transcription factor that regulates the genes that express multiple antioxidant proteins.128 Nrf2 becomes functionally defective because of reduced stability due to reduced HDAC2 activity in COPD patients,129 and Nrf2 activators can protect against this functional defect. The Nrf2 activators sulforaphane and bardoxolone methyl (CDDO-Me) (Chart 10) were developed in clinical trials. Two dose levels of sulforaphane were administered orally in patients with COPD in a phase II trial, but sulforaphane affected neither the Nrf2 pathways nor the levels of antioxidants or inflammation.130 An analog of CDDO-Me and another synthetic triterpenoid, CDDO-imidazolide (Chart 10), was administered to a cigarette smoke-induced mouse model and delayed or prevented the progression of COPD.131 A novel chalcone, 13 (Chart 10), was


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reported as an Nrf2 activator in mice and human lung epithelial cells, and this study provides a promising lead for the treatment of COPD.132 Considering that the side effects and toxicity arise from the lack of specificity demonstrated by these Nrf2 activators, an alternative strategy for targeting selective Nrf2 activation mechanisms is focused on Kelch-like ECH-associated protein 1 (Keap-1)-Nrf2 protein−protein interaction (PPI) inhibition.133 Keap1 is selectively responsible for Nrf2 recognition as a substrate receptor. The PPI between Keap1 and Nrf2 is the first step of Nrf2 ubiquitination, and inhibition of this PPI results in the activation of Nrf2.134 Compound 16 (Chart 10) is a promising lead compound that tightly and selectively binds to Keap1 and disrupts the PPI between Keap1 and Nrf2 with nanomolar activity. Compound 14 upregulates Nrf2-dependent gene expression in bronchial epithelial cells from COPD patients and restores the ozone-induced depletion of reduced glutathione (GSH) levels in the lungs in vivo.135 Compound 15 (Chart 10) represents another series of Keap1−Nrf2 PPI inhibitors. Cell-based experiments showed that 15 activates Nrf2 and induces its downstream protein expression. In the LPS-challenged mouse model, 15 significantly reduces the levels of several proinflammatory cytokines and relieves the inflammatory response.136 These studies provide a selective and effective way to explore Nrf2-targeted antioxidant therapies.

Chart 10. Chemical structures of some known Nrf2 activators.


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O

O

CF3

O

OCH3

R

NCS

NC

Sulforaphane

O

O

13

R= OCH3

CDDO-Me

CDDO-imidazolide R= N 

O

HO

HO

OH O

O

O

N O S O

N N N

N S O O

O

14

N

N S O O

HN

NH O

O 15

NADPH Oxidase (NOX) inhibitors. NOXs are a family of enzymes that catalyze the transfer of electrons from NADPH to molecular oxygen and are a major source of ROS. There are seven isoforms of NOXs, including NOX1-5, Duox1 and Duox2;137 NOX4 is the predominant isoform and plays critical roles in pulmonary fibrosis.138 The identification of specific NOX4 inhibitors may provide a new opportunity for antioxidant therapies to treat pulmonary fibrosis and COPD. Although a large number of natural or synthetic NOX inhibitors have been reported, only a few selective NOX4 inhibitors are known. The known NOX2 and NOX4 inhibitor VAS2870 (Chart 11)137 inhibits oxidized low-density lipoprotein-induced ROS formation in human endothelial cells,139 and a later study indicated that VAS2870 may interfere with the protein and signaling pathways related to NOX enzymes and is an indirect NOX inhibitor.140 GKT136901 (Chart 11) is a first-in-class NOX4 inhibitor that showed double-digit nanomolar potency toward NOX4 with dual activity toward NOX1 and exhibited an excellent in vitro pharmacokinetic profile and


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high selectivity for NOX4 over the NOX2 isoform. This highly potent, orally bioavailable NOX4 inhibitor can serve as a lead compound for the development of novel antioxidants.147 Sulfonylurea compound 16 (Chart 11), which was newly designed using a pharmacophore modeling strategy, can inhibit Nox4-dependent signaling in a cell-based assay with submicromolar activity. Nevertheless, more studies are needed to directly evaluate Nox4 activity and determine the selectivity of this novel compound.141 Chart 11. Chemical structures of VAS2870, GKT136901 and compound 16.

O N

S

N

N

N

N N

N

VAS2870

Cl

H N

O N

OCH3

F3C

O

N N O

GKT136901

O S N O

16

Myeloperoxidase (MPO). MPO is released by neutrophils and macrophages and catalyzes the formation of potent oxidants, thereby further amplifying oxidative damage and the inflammation of lung tissue. The 2-thioxanthine MPO inhibitor AZD5904 can attenuate the oxidative stress promoted by MPO in vivo.142 In a 6-month cigarette smoke-induced COPD guinea pig model, AZD5904 retarded the progression of emphysema and small airway remodeling, which occur at both the early and late stages of smoke exposure.143 Some novel MPO inhibitors have been reported in the literature, including PF-06282999, whose human pharmacokinetics, safety/tolerability, and MPO inhibition have been in clinical trials;144 aromatic hydroxamates (17);145 3-alkylindole derivatives (18);146 and compounds with virtual screening-directed new


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patterns (19 and 20).147 The structures of the abovementioned compounds are presented in Chart 12. These chemical entities exhibited highly potent MPO inhibition at nanomolar levels, and additional studies are needed to evaluate their effects on COPD. Chart 12. Chemical structures of some MPO inhibitors. O

O N

HN S

HN

N H

N

O

CF3

S O

O N

F3C

F

O N H 18

Cl

O HN

PF-06282999 NH2

O

HN NH2

AZD5904

H N

17 H N

N H 2N

NH2 N

N

OH

HN O

HN N

N 19

F

20

Other antioxidants. Superoxide dismutase (SOD) is an enzymatic antioxidant that can protect the lungs and blood by neutralizing ROS.148 Therefore, SOD mimetics with SOD activity can be used as antioxidants. AEOL 10113149 and Tempol150 are known SOD mimetics that efficiently neutralize ROS. Spin trap compounds such as NXY-059 are another type of antioxidant that can scavenge free radicals; however, there is no evidence of their efficacy in COPD.151 Inducible nitric oxide synthase (iNOS) is implicated in many chronic inflammatory diseases. A study of smoke-exposed mice treated with the iNOS inhibitor L-NIL indicated that L-NIL protects against smoke-induced emphysema and pulmonary hypertension. This study demonstrated that iNOS inhibition may be a therapeutic option for COPD.152 Moderate levels of nitric oxide (NO) reduce neutrophil activation and suppress iNOS overexpression, and compounds with NO-releasing


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capacity therefore have the potential to treat COPD. Compound 21, which contains NO donors, exhibited potent inhibitory activity against COPD-like inflammation in cigarette smoke- and LPS-induced mouse models,153 and this study proposes using NO-donor derivatives as antioxidants to mitigate COPD. The structures of the abovementioned compounds are presented in Chart 13. Chart 13. Chemical structures of AEOL 10113, Tempol, NXY-059, L-NIL, and compound 21.

N+

HO N

N

+

N

N Mn +

O N+ O-

Tempol

N

N

N+ NH

N

+

OH O

O

PhO2S

O

O O

N O N

O S O Na+ O

L-NIL

AEOL 10113

PhO2S

NXY-059

NH2

N H

O

O- Na+ O S O

N O N

O HO O

O

O

O

O OH O

O

O O

O

O

O

O 21

3.5. Kinase inhibitors. More than 500 kinase genes have been identified in humans, and kinases play essential roles in regulating cell proliferation, apoptosis, and inflammation. Several kinases have been implicated in orchestrating chronic inflammation by modulating inflammatory gene expression.154 In COPD,


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multiple inflammatory mediators and proteins are regulated by kinase signaling pathways. Therefore, these kinases may be targets for the development of kinase inhibitors for COPD treatment. To date, over 30 kinase inhibitors have been approved by the FDA, but few of these inhibitors have been applied to inflammatory diseases; however, kinase inhibitors are still a powerful tool for treating inflammatory diseases and full of possibility for use in future studies. Numerous kinase inhibitors have been reported in the literature; herein, we discuss only the kinases and their inhibitors related to COPD. IKKβ inhibitors. NF‑κB is a proinflammatory transcription factor that is activated in epithelial cells and macrophages, induces multiple inflammatory genes in patients with COPD and is involved in the activation of inflammasome and oxidative stress. IKKβ is an essential component in the NF‑κB pathway, which is activated by the overexpression of inflammatory mediators in COPD. IMD-3054 (Chart 14) is a selective IKKβ inhibitor that inhibits TNF-α-induced NF-κB transcription activity with an IC50 of 1.2 μM155 and counteracts allergic airway inflammation and hyperresponsiveness in a mouse model.156 Its prodrug, IMD-1041 (structure not disclosed), advanced to a phase II trial in patients with COPD for testing for anti-inflammation potency, but the study was halted due to side effects.157 Other IKKβ inhibitors, including BAY65-1942, TPCA-1, PS-1145 (Chart 14), and ainsliadimer-A, have been studied in lung inflammation-related models for their antiinflammatory effects in vitro or in vivo. These studies were introduced elsewhere157 and provide evidence for the potential of IKKβ inhibitors as anti-inflammatory agents against COPD. Chart 14. Chemical structures of some IKKβ inhibitors.


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CF3

O

OH O

N N H

CF3

H N

OH

Cl

O

H 2N

O

H 2N

O TPCA-1

BAY65-1942

O N

H N

O N

Cl

OH

H

H O OH

H H

PS-1145

H

O

O NH

S

HN

HN IMD-0354

F

O

H

O

Ainsliadimer A

p38 MAPK inhibitors. The p38 MAPKs represent a family of serine threonine kinases and comprise four isoforms, α, β, γ, and δ. Among these isoforms, p38α and p38β are highly homologous and widely expressed. p38 MAPK regulates the expression of multiple inflammatory genes, such as the cytokines TNF-α, IL-6, and IL-1β, and the chemokine IL-8, which is involved in COPD.158 p38 MAPKs have been investigated for COPD. Indeed, several p38 inhibitors have advanced in clinical trials in patients with COPD, and p38 inhibitors are the kinase inhibitors most extensively studied for COPD treatment. Acumapimod (BCT-197), PH-797804, losmapimod, and dilmapimod are oral p38 inhibitors that have been studied in phase II trials for COPD (Table 6). Although oral p38 inhibitors are efficient, high doses of oral administration may produce side effects. Directly delivering the agents to the lungs of COPD patients may avoid common adverse events. Therefore, discovering an inhaled p38 inhibitor is an alternative strategy for improving the therapeutic window.159 Several inhaled p38 MAPK inhibitors have also been evaluated clinically for use in COPD treatment (Table 7). Detailed information about the p38 inhibitors in clinical trials for COPD


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treatment has previously been presented in the review literature.160 Talmapimod (Chart 15) has been investigated in a phase II trial for the treatment of rheumatoid arthritis. It is also effective in a corticoid-resistant cigarette smoke-induced mouse model.161 Another compound, SB-239063 (Chart 15), can reduce LPS-induced TNF-α production in vivo.162 A newly reported compound, 22 (Chart 15), suppresses the production of cytokines TNF-α and CXCL8 induced by LPS and inhibits both p38 and hematopoietic kinases in a nanomolar range. Compound 22 is also highly effective against cigarette smoke-induced pulmonary inflammation in mice and is active for a long time in vivo.163 Based on their structural features and their interaction with p38 MAPK, p38 inhibitors can be categorized into 3 types, teardrop binders, linear binders, and extended binders. Both the first two types interact competitively with the ATP-binding site, while the third undergoes allosteric binding with the kinase. The active sites of all p38 isoforms in which the p38 inhibitor and ATP competitively interact with have distinct features that include (1) a ligand-rejecting pocket and a special residue, Thr106, named the “gatekeeper pocket” and the “gatekeeper residue,” respectively; (2) a hydrophobic floor; and (3) a linker residue, Met109. All three types of ligands have aromatic ring or heterocyclic groups that fit into the gatekeeper pocket and hydrophobic floor and a hydrogen bond receptor for interacting with Met109 (Figure 10).164 For instance, SB239063 is a teardrop binder, dilmapimod is a linear binder, and PF-03715455 is an extended binder. These characteristics can guide the structure-based design of novel p38 MAPK inhibitors for COPD treatment.


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Figure 10. 2D model of the interactions between selected p38 inhibitors and p38α kinase. (A) Representative compound of the teardrop binders (PDB entry 1A9U). (B) Representative compound of the linear binders (PDB entry). (C) Representative compound of the extended binders (PDB entry 1KV2). Table 6. Oral p38 MAPK inhibitors in clinical trials for COPDa

Compound

Structure

Clinical stage

ClinicalTrials.gov identifier

Phase II

NCT02700919

O N H

acumapimod

N

H 2N

NC

N

O O Br

N

PH-797804 O F

NCT00559910

H N O

F

Phase II

NCT01321463 NCT01543919


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O O

losmapimod

N

NCT02299375

N H

Phase II

N H

NCT01541852 NCT01218126

F

F O

N

F N

H N

OH

N

dilmapimod

Phase II

OH

NCT00144859

F a Information

was collected from www.clinicaltrials.gov.

Table 7. Inhaled p38 MAPK inhibitors in clinical trials for COPDa

Compound

Clinical stage

Structure N N

N

N

N H

OH S

S

O

PF03715455

N

ClinicalTrials.gov identifier

Phase II

N H

(terminated)

NCT02366637

Cl OH


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O

N H

NH

AZD-7624

O

N

O

N

Phase II

NCT02238483

Phase I/II

NCT02815488

N H F

CHF-6297

—b N N

RV-568

O N

N H

GSK-610677 a Information b Structure

O

O N H

OH

N H

Phase II

—

Phase I

NCT01475292 NCT01867762

NCT00694902

was collected from www.clinicaltrials.gov.

not disclosed

Chart 15. Chemical structures of talmapimod, SB-239063, and compound 22.


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OH O O

O F

N

N N

N

N N

O

N

N H

Cl

F

Talmapimod

SB-239063

N N

O

O N

N H

O N H

O

N H

22

Phosphoinositide-3-kinase (PI3K) inhibitors. PI3Ks generate lipid second messengers and play an integral role in the immune system. PI3Ks are divided into three classes and have several isoforms, including PI3Kα, PI3Kβ, PI3Kδ, and PI3Kγ. Specifically, PI3Kδ and PI3Kγ were proven to be the main players in chemoattractant-stimulated neutrophil and macrophage migration, which are implicated in COPD.165 Therefore, PI3Kδ and PI3Kγ can be targeted pharmacologically for the treatment of COPD. GSK2269557 (Chart 16)166 is an inhaled selective PI3Kδ inhibitor that was evaluated in a phase II trial (NCT02294734). The inhalation of GSK2269557 suppressed airway inflammation, as reflected by decreased IL-8 and IL-6 levels in the sputum of patients with COPD.167 The PI3Kδ/γ inhibitor TG100-115168 (Chart 16) reduces the lung inflammatory response and pulmonary neutrophilia after aerosolization in the treatment of BALB/c mice induced by LPS or cigarette smoke exposure. Considering the favorable biological activity, pharmacokinetic, and safety profiles of this compound, it is suitable for development as a candidate for COPD therapy. The study also indicated that PI3Kδ and PI3Kγ are reasonable molecular targets for the treatment


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of COPD. Compound 23 (Chart 16) is a newly reported orally bioavailable PI3Kδ inhibitor169 derived from the inhaled clinical candidate GSK229276 (Chart 16), which is indicated for respiratory treatment. Replacement of the indazole core of GSK2292767 with a dihydroisobenzofuran and optimization at the 6-position of the dihydroisobenzofuran scaffold led to 23, which was identified and found to show a marked increase in oral bioavailability compared to GSK2269557 (54% vs

Targeted Treatments for Chronic Obstructive ... - ACS Publications

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