Tumor-Acidity-Cleavable Maleic Acid Amide (TACMAA) - American

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Tumor-Acidity-Cleavable Maleic Acid Amide (TACMAA): A Powerful Tool for Designing Smart Nanoparticles To Overcome Delivery Barriers in Cancer Nanomedicine Jin-Zhi Du,*,†,‡,∥ Hong-Jun Li,‡,# and Jun Wang*,†,§,∥,⊥ †

Guangzhou First People’s Hospital, School of Medicine, South China University of Technology, Guangzhou, Guangdong 510006, China Institutes for Life Sciences, School of Medicine, South China University of Technology, Guangzhou, Guangdong 510006, China § School of Biomedical Science and Engineering, South China University of Technology, Guangzhou International Campus, Guangzhou, Guangdong 510006, China ∥ National Engineering Research Center for Tissue Restoration and Reconstruction, Guangzhou, Guangdong 510006, China ⊥ Key Laboratory of Biomedical Engineering of Guangdong Province, South China University of Technology, Guangzhou, Guangdong 510006, China # Key Laboratory of Biomedical Materials and Engineering of the Ministry of Education, South China University of Technology, Guangzhou, Guangdong 510006, China

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CONSPECTUS: Over the past few decades, cancer nanomedicine has been under intensive development for applications in drug delivery, cancer therapy, and molecular imaging. However, there exist a series of complex biological barriers in the path of a nanomedicine from the site of administration to the site of action. These barriers considerably prevent a nanomedicine from reaching its targets in a sufficient concentration and thus severely limit its therapeutic benefits. According to the delivery process, these biological delivery barriers can be briefly summarized in the following order: blood circulation, tumor accumulation, tumor penetration, cellular internalization, and intracellular drug release. The therapeutic effect of a nanomedicine is strongly determined by its ability to overcome these barriers. However, advances in cancer biology have revealed that each barrier has its own distinct microenvironment, which imposes different requirements on the optimal design of nanocarriers, thus further complicating the delivery process. For example, the pH of blood is neutral, while the tumor extracellular environment features an acidic pH (pHe ≈ 6.5−7.0) and the endosome and lysosome are more acidic (pH 5.5−4.5). The nanoparticles (NPs) should be able to change their properties to adapt to each individual environment for robust and effective delivery. This demand promotes the design and development of smart delivery carriers that can respond to endogenous and exogenous stimuli. It is well-documented that tumors develop acidic extracellular microenvironments with pH ≈ 6.5−7.0 due to their abnormal metabolism in comparison with normal tissues. This provides a unique tool for designing smart NP drug delivery systems. Our studies have revealed that the NPs’ physiochemical properties, such as particle size and surface charge, have profound effects on their systemic transport in the body. In different delivery stages, the NPs should possess different sizes or surface charges for optimal performance. We developed a class of stimuli-responsive NPs by incorporating tumor-acidity-cleavable maleic acid amide (TACMAA) as a design feature. TACMAA is produced by the facile reaction of an amino group with 2,3-dimethylmaleic anhydride (DMMA) and its derivatives and can be cleaved under tumor acidity. By virtue of such characteristics, NPs containing TACMAA enable size or surface charge switching at tumor sites so that they can overcome those delivery barriers for improved drug delivery and cancer therapy. In this Account, we systemically review the development and evolution of TACMAA-based delivery systems and elaborate how TACMAA helps the innovation and design of intelligent nanocarriers for overcoming the delivery barriers. In particular, our Account focuses on five parts: TACMAA chemistry, tumor-acidity-triggered charge reversal, tumor-acidity-triggered shell detachment, tumor-acidity-triggered size transition, and tumor-acidity-triggered ligand reactivation. We provide detailed information on how tumor-acidity-triggered property changes correlate with the ability of NPs to overcome delivery barriers.

1. INTRODUCTION Nanomedicine has attracted great attention for delivering anticancer therapeutics to tumors because of its preferential © XXXX American Chemical Society

Received: May 5, 2018

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DOI: 10.1021/acs.accounts.8b00195 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

2. MAA CHEMISTRY It is generally believed that common amide bonds are nondegradable or can be degraded only at extreme pH values. However, the amide bond of MAA derivatives can be cleaved at mildly acidic pH values. The enhanced pH sensitivity is attributed to the reason that the neighboring carboxylate group can easily attack the carbonyl group of the amide to form a tetrahedral intermediate with a five-membered ring (Scheme 1).12

and selective accumulation at tumor sites. In the past several decades, numerous nanoparticulate delivery systems have been designed and developed, with many gaining approval for clinical applications and hundreds more at various stages of clinical investigation.1 However, the overall therapeutic efficacy of cancer nanomedicine is not satisfactory.2 Earlier studies overvalued improving the accumulation of nanoparticles (NPs) at tumor sites but failed to recognize that the overall antitumor efficacy of cancer nanomedicine is determined by the capability of NPs to overcome a series of delivery barriers from the site of injection to the site of action.3 These delivery processes can be briefly summarized as but are not limited to circulation in blood, extravasation and accumulation at the tumor site, penetration into the tumor parenchyma, internalization by target cells, and intracellular drug release.4 Therefore, it is fundamentally essential to improve the delivery efficacy of NPs at each of these stages. However, each delivery stage has its own distinct microenvironment.5 The optimal requirements for the properties of NPs at different stages can be significantly different and often conflicting. Each delivery stage imposes a delivery barrier to NP design. Conventional delivery systems, despite their many advantages, are not capable of overcoming these barriers in a holistic manner. An ideal nanoparticulate delivery system that is able to meet the requirements for all of these stages should possess a series of traits, including prolonged circulation, enhanced extravasation and accumulation, improved tumor penetration, facilitated cellular internalization, and controlled intracellular release of the active drug. Stimuli-responsive NPs show great promise in overcoming these barriers because of their intelligent property changes in accordance with the local environment.6−8 Among these stimuli, pH responsiveness is one of the most frequently used, as different tissues and cellular compartments show distinct pH values.9 For example, the tumor extracellular environment is more acidic (pH ≈ 6.5) than the blood (pH ≈ 7.4), while endo/lysosomes are more acidic (pH ≈ 4.5−5.0). By the use of these pH variations, a number of pH-responsive delivery systems have been developed to overcome these delivery barriers.6,10 However, many of these smart delivery carriers respond to the intracellular pH, whereas less attention has been paid to the development of nanomedicines that can respond to tumor acidity. Our group has been working on the development of pHresponsive cancer nanomedicine for years. Our recent progress has led to the development of a new class of tumor-acidityresponsive nanoparticulate delivery carriers based on maleic acid amide derivatives, here denoted as tumor-acidity-cleavable maleic acid amide (TACMAA).11 A critical feature of TACMAA is that the amide bond can be cleaved at the acidity of a tumor (pH ≈ 6.5−7.0), accompanied by the transition from carboxylic groups to amino groups. TACMAA has been validated by our results as a powerful tool for the design of smart NPs to overcome delivery barriers in cancer nanomedicine.10 In this Account, we introduce the evolution of TACMAA chemistry and then systemically review the development and TACMAAbased delivery systems and elaborate how TACMAA chemistry helps the innovation of intelligent nanocarriers to overcome the delivery barriers. In particular, this Account focuses on five parts: MAA chemistry, tumor-acidity-triggered charge reversal, tumoracidity-triggered shell detachment, tumor-acidity-triggered size transition, and tumor-acidity-triggered ligand reactivation. We provide detailed information on how the tumor-aciditytriggered property changes correlate with the ability of NPs to overcome delivery barriers in cancer treatment.

Scheme 1. Acid-Triggered Structural Change of MAA Derivatives

More interestingly, the pH-sensitive degradability of MAA derivatives can be tuned by changing the substituents on the cis double bonds. Studies have discovered that the greater the number of substituents on the cis double bonds is, the more sensitive the MAA derivative is.13 For example, Lee et al.13 compared the degradability of MAA and citraconic acid amide at pH 7.4, 5.5, and 3.0. They found that MAA with two hydrogen substituents was stable at pH 7.4 but showed slow degradation at pH 5.5 with a half-life of ∼16 h and accelerated degradation at pH 3.0 with a half-life of ∼7 h. Citraconic acid amide with one methyl and one hydrogen substituent showed similar stability at pH 7.4 but much faster degradation at pH 5.5 with a half-life of ∼1.5 h. One other feature of MAA chemistry is that the charge density of MAA derivatives with a β-carboxylate group can be tuned by varying the pH. Generally, MAA derivatives are negatively charged at neutral or basic pH because of the presence of the carboxyl group, but they switch to positively charged because of the degradation of the amide bond at acidic pH. The pH-sensitive degradation and charge-reversal property of MAA derivatives have demonstrated great promise for the design of drug delivery systems. For example, Kataoka’s group and many other groups have used MAA derivatives as pH-labile charge-conversion systems for intracellular cargo delivery.14−17 However, these MAA derivatives can respond only to the endosomal/lysosomal pH or even lower pH values. MAA derivatives that can respond to tumor acidity went unexplored for a long time. Our group initiated an original study in this particular area by utilizing 2,3-dimethylmaleic anhydride (DMMA) as an amide agent for primary amine groups.11 The amide bond of the resultant 2,3-dimethylmaleic acid amide, which has two methyl substituents on the cis double bond, can be cleaved and undergo a negative-to-positive charge reversal at pH 6.8. This discovery greatly expands the application of MAA derivatives in the design of smart drug delivery systems to effectively overcome the delivery barriers in cancer nanomedicine, as will be exemplified in the following sections.

3. TUMOR-ACIDITY-TRIGGERED CHARGE REVERSAL Surface charge plays an important role in governing the in vivo fate of NPs. Generally, negative or neutral NPs tend to exhibit B


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Figure 1. (a) Tumor-acidity-activated charge reversal of PAMA−DMMA nanogel. (b, c) Chemical structure and tumor-acidity-triggered hydrolysis of the 2,3-dimethylmaleic acid amide bond. (d) Time-dependent change in the zeta potential of the PAMA−DMMA nanogel at pH 6.8. (e) Comparison of cellular uptakes of FITC-labeled PAMA−DMMA at pH 7.4 and 6.8 by flow cytometry. (f) Distribution of FITC-labeled PAMA−DMMA nanogels in tumor tissue following intratumoral injection. Reproduced with permission from ref 11. Copyright 2010 Wiley-VCH.

thereby effectively facilitating the cellular uptake of the nanogel by cancer cells (Figure 1e). Confocal imaging visually demonstrated that more fluorescein isothiocyanate (FITC)-labeled PAMA−DMMA nanogels were internalized by cells at pH 6.8 than at pH 7.4 as a result of the increased NP−cell interaction at pH 6.8. Intratumor injection of the NPs also indicated that more PAMA−DMMA was distributed inside the tumor cells, whereas the control group PAMA-SA, whose surface charge is always negative, remained mainly in the extracellular space or adhered to cell membranes (Figure 1f). This finding clearly demonstrates that the 2,3-dimethylmaleic acid amide bond can be activated by the in situ tumor acidity to achieve effective charge reversal from negative to positive, thereby facilitating the cellular internalization of NPs. On the basis of the proof-of-concept study, we further constructed a tailor-made dual pH-sensitive polyphosphoester− doxorubicin (DOX) conjugate to simultaneously enhance cellular uptake and achieve better control of intracellular drug release.21 The dual-sensitive polymeric carrier featured TACMAA chemistry and hydrazone bonds so that it could respond to both extracellular and intracellular pH gradients. It formed PEGylated NPs at neutral pH, favoring blood circulation and tumor accumulation. Then the TACMAA chemistry enabled the NPs to reverse their surface charge from negative to positive to facilitate cellular internalization, while the significantly increased acidity in the endosome cleaved the hydrazone bond to promote DOX release. Of particular interest, such dual pH-sensitive NPs overcame the notorious drug resistance of cancer stem cells and killed them, demonstrating the great promise of this strategy in mediating effective cancer therapy for drug resistance tumors. The in vivo application of TACMAA-chemistry-based NPs through the intravenous route was exemplified by zwitterionic polymeric NPs.22 Zwitterionic polymers can serve as a hydrophilic coating to endow NPs with long circulation and high tumor accumulation, which are promising alternatives to PEG.23

prolonged blood circulation and enhanced tumor accumulation but cannot be effectively internalized by cancer cells.18 By contrast, positively charged NPs are favorable for cellular uptake, but they are readily eliminated from blood circulation by the mononuclear phagocyte system. The different requirements for prolonged blood circulation and efficient tumor cell uptake form the “charge dilemma” in NP design. An ideal NP delivery system should be inert or stealthy in blood circulation but activated to become recognizable by tumor cells after accumulation at the tumor site. Site-specific charge reversal provides a promising means to address the charge dilemma. Earlier studies on charge-reversal NPs mainly focused on the intracellular pH environment, as documented by several reports.14,15,19,20 It should be noted that the intracellular pH-activated charge reversal is beneficial for intracellular cargo release but is unable to facilitate cellular uptake. As a proof-of-concept study, our group developed a nanogel system that can be activated by tumor acidity to achieve tumorspecific negative-to-positive charge reversal to enhance cellular internalization (Figure 1a).11 In this study, an amino-functionalized nanogel (denoted as PAMA) was prepared by reverse microemulsion. The primary amino groups of PAMA were reacted with DMMA, which generated PAMA−DMMA with abundant 2,3-dimethylmaleic acid amide bonds and carboxylic acid groups on the surface. This led to PAMA−DMMA with a zeta potential of −17 mV at neutral pH. However, the zeta potential could be quickly switched to around +5 mV after incubation at pH 6.8, indicating its rapid charge reversal at tumor acidity. Its 1H NMR spectrum indicated that the charge reversal resulted from the tumor-acidity-induced cleavage of 2, 3-dimethylmaleic acid amide bonds, which led to the removal of carboxylic acid groups and the concurrent recovery of the primary amino groups of the nanogel (Figure 1b−d). Fluorescence-activated cell sorting (FACS) analyses confirmed that the tumor acidity activated the charge reversal, C


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Figure 2. (a) Schematic illustration of DOX-loaded zwitterionic NPs and the change in surface charge in response to tumor acidity. (b) Plasma DOX concentrations of various formulations after intravenous administration. (c) Quantitative analyses of DOX concentrations in tumor tissue. (d) Tumor growth inhibition in nude mice bearing MDA-MB-231 breast cancer xenografts after intravenous injection. pHe stands for the tumor extracellular pH. Reproduced with permission from ref 22. Copyright 2012 Wiley-VCH.

4. TUMOR-ACIDITY-TRIGGERED SHELL DETACHMENT In addition to inducing direct charge reversal, TACMAA chemistry can also be used to trigger shell detachment of the NPs. PEGylation is a prevailing solution for protecting NPs from quick clearance during blood circulation. However, PEGylation strongly inhibits cellular uptake, causing significant loss of delivery activity, which is the so-called “PEG dilemma”.29 A viable approach to addressing this dilemma is to remove the PEG shielding layer from the NPs at the tumor site, which has been typically documented by the multifunctional envelope-type nanodevice (MEND) platform,29 in which the PEG shell is detached from the NPs by enzymatic hydrolysis. However, the expression of specific enzymes in a particular location varies dramatically, which to an extent limits the application of the MEND system. By contrast, TACMAA chemistry can be a valuable choice to trigger PEG detachment from NPs because of the universality of slightly acidic extracellular pH in solid tumors. We developed a couple of delivery systems that can remove the PEG shell from NPs in response to the tumor acidic pH for siRNA and anticancer drug delivery. The first strategy that we developed is charge-reversal-mediated PEG shell detachment.30,31 The principle is that the PEG layer can be adsorbed on the core NPs with opposite charges through the electrostatic attraction effect. When the NPs arrive at the tumor site, the tumor acidity triggers charge reversal of the PEG segment, leading to detachment of the PEG layer due to electrostatic repulsion. In this strategy, a ternary sheddable NP (S-NP) was constructed by introducing a block copolymer of PEG and a tumor-acidityresponsive polyphosphoester onto the surface of positively charged PEI/siRNA polyplexes (Figure 3a).31 In blood circulation, the ternary complex was protected by the PEGylation and showed minimal nonspecific interactions with serum components to improve tumor accumulation, as evidenced by the pharmacokinetics and tissue distribution studies (Figure 3b).

However, NPs with zwitterionic surface coating are intrinsically unfavorable for cellular internalization because the NPs do not bind well to cell membranes. To address this problem, a zwitterionic polymer-based surface-charge-switchable NP was developed by employing TACMAA chemistry. In this system, an amphiphilic zwitterionic polymer was synthesized, and this polymer formed micellar NPs in aqueous solution with DOX encapsulation in the core (Figure 2a). The NPs showed a nearly neutral surface charge to avoid quick recognition by the immune system and exhibited prolonged blood circulation with a half-life (t1/2γ) of approximately 20 h (Figure 2b).22 Once the NPs accumulated at the tumor site, the negative DMMA moieties were removed, reversing the surface charge to positive and facilitating tumor cell uptake. In a mouse model bearing a MDA-MB-231 xenograft, the DOX-loaded charge-reversal NPs increased the tumor accumulation of DOX and significantly inhibited the tumor growth compared with other treatments (Figure 2c,d). This result once again demonstrated that the TACMAA-chemistrybased charge-reversal strategy is robust in addressing the contradictory designs needed for prolonged circulation time and efficient cellular uptake. In addition to our own studies, the ability of TACMAA chemistry to enhance cell uptake and improve cancer therapy has also been validated in various delivery systems.24−27 For example, Liu and his colleagues modified upconversion nanoparticles (UCNPs) with DMMA for improved cancer photodynamic therapy.27 The PEGylated NPs were negatively charged at pH 7.4 and became positively charged at pH 6.8, which significantly enhanced the cellular internalization of NPs and increased the efficacy of both in vitro and in vivo near-infrared-light-induced photodynamic therapy. In another example, Haag and co-workers prepared multiresponsive poly(vinyl alcohol) (PVA) nanogels for DOX delivery.28 The nanogels were modified with DMMA moieties by click chemistry, and the terminal groups allowed the nanogels to reverse their surface charge from negative to positive in the tumor extracellular environment, which facilitated cellular internalization. D


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Figure 3. (a) PEG shell detachment from positively charged NPs in the acidic tumor microenvironment. (b) Accumulation of Cy5-siRNA in tumor tissue after intravenous injection. (c) Tumor growth inhibition in nude mice bearing MDA-MB-231 breast cancer xenografts after intravenous injection. pHe stands for the tumor extracellular pH. Reproduced from ref 31. Copyright 2011 American Chemical Society.

uptake (Figure 4a). Thus, this system achieved a prolonged circulatory half-life of siRNA in blood with a terminal half-life (t1/2z) of nearly 12 h (Figure 4b) and resulted in effective accumulation in tumor cells as well as enhanced inhibition of nonsmall cell lung cancer growth (Figure 4c).34 However, the PEG detachment at pH 7.4 was minimal. Similarly, a tumor-pHlabile linkage-bridged copolymer of PEG and poly(D,L-lactide) (PEG-Dlinkm-PDLLA) was developed for effective anticancer drug delivery.35 Upon arriving at the tumor site, PEG-DlinkmPDLLA NPs lost their PEG layer and increased their zeta potential in response to the tumor acidity, which significantly enhanced the cellular uptake and improved the in vivo tumor inhibition rate to 78.1%, compared with 47.8% for conventional PEG-b-PDLLA NPs.

After accumulating at tumor site, the ternary NPs lost the PEG protective layer because of the charge repulsion between the PEG segment and the core NPs resulting from the TACMAAinduced charge reversal. The improved in vivo siRNA delivery was confirmed in mice with MDA-MB-231 xenografts (Figure 3c). In addition to siRNA delivery, this charge-reversalmediated shell detachment strategy has also been adopted to deliver microRNA and chemotherapeutics, and both showed improved therapeutic efficacy in cancer treatment.32,33 Another strategy to remove the PEG protective shell is to cleave the linker between the PEG and the NPs at the target site. In this regard, we synthesized the bifunctional MAA derivative 2-propionic-3-methylmaleic anhydride (CDM), which can be used as a linker molecule for polymer preparation. An appealing feature of the CDM linker is that its carboxyl group can react with a hydroxyl group of the PEG chain, while the anhydride part reacts with an amino group to generate the TACMAA bond. At the tumor site, the tumor acidity triggers the cleavage of the amide bond and causes the discharge of the PEG segment accordingly. More interestingly, this strategy also causes a concurrent negative-to-positive charge reversal accompanying PEG detachment, which work together to facilitate the cellular uptake of NPs. Using this design, we developed two amphiphilic polymers for siRNA and anticancer drug delivery.34−36 For siRNA delivery, the polymer PEG-Dlinkm-R9-PCL was synthesized, in which the tumor-acidity-degradable CDM linker (Dlinkm) and a cationic cell-penetrating peptide having nine arginine residues (R9) was inserted between the hydrophilic PEG block and the hydrophobic polycaprolactone (PCL) segment. The polymer formed cationic PEGylated NPs in aqueous solution and also formed a complex with siRNA. The PEG layer protected the complex from clearance during blood circulation. At tumor site, the tumor acidity cleaved the linker and triggered PEG detachment, facilitating the exposure of the R9 peptide and enhancing cellular

5. TUMOR-ACIDITY-TRIGGERED SIZE TRANSITION Among various delivery barriers in cancer nanomedicine, tumor penetration is a long-term bottleneck that severely limits therapeutic efficacy. Tumors develop aberrant vasculature, elevated interstitial fluid pressure, and dense extracellular matrix in their microenvironment, all of which hinder the effective penetration and distribution of NPs in the tumor interstitium.37 It has been documented that small NPs (e.g., sub-30 nm) show better penetration than larger ones,38 but very small particles usually suffer from a short half-life and insufficient tumor accumulation because of their rapid clearance.39 Hence, NPs should maintain a large enough size during blood circulation to avoid rapid clearance but should transition to small particles when docking at the tumor site to facilitate tumor penetration. TACMAA chemistry once again shows its versatility in this regard. Using the bifunctional linker CDM, we developed a sizeswitchable clustered NP system named iCluster.40 iCluster was selfassembled from the coassembly of platinum prodrug-conjugated poly(amidoamine)-graf t-PCL (PCL-CDM-PAMAM/Pt), E


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Figure 4. (a) Tumor-acidity-triggered PEG detachment from the NPs. (b) Pharmacokinetic study of siRNA-loaded NPs. (c) Mean fluorescence intensity (MFI) of Cy5-siRNA in green fluorescent protein (GFP)-expressing A549 cells 24 h after intravenous injection. Reproduced from ref 34. Copyright 2015 American Chemical Society.

unsuitable for many in vivo applications. To address this challenge, strategies have been developed to tune the display of ligands on NPs. An ideal scenario is that the targeting ability of these ligands is shielded during delivery and then reactivated once they arrive at the tumor site. TACMAA chemistry can also be used in this regard. As an example, a block copolymer of PEG and TAT-modified polyphosphoester was developed in which TAT was masked with DMMA.44 At neutral pH, TAT temporarily lost its nonspecific interaction with biological entities. Upon accumulation at the tumor site, TACMAA-induced reactivation of TAT promoted the cellular internalization of NPs. In addition to TAT, the TACMAA chemistry can also be used to address the same problem with other cell-penetrating peptides. Zhang’s group constructed a novel prodrug by conjugating DOX to an activatable cell-penetrating peptide (CR8G3PK6, ACPP) with a DMMA shielding group.45 The DMMA shielding group linked to the primary amines of K6 through the maleic acid amide bond, which was used to block the cell-penetrating function of the polycationic CPP (R8) through intramolecular electrostatic attraction at physiological pH 7.4. At the tumor extracellular pH of 6.8, hydrolysis of DMMA led to charge reversal, activating the pristine function of CPP for improved cellular uptake by tumor cells. This strategy was also applicable to nuclear localization sequence (R8NLS) and ligands such as folic acid for improved cancer therapy.25,46

PCL homopolymer, and PEG-b-PCL. In iCluster, PAMAM/Pt was coupled to the PCL segment via the pH-labile CDM linker. The initial size of iCluster was ∼100 nm, which was favorable for long blood circulation and a high propensity of extravasation through tumor vascular fenestrations. Once accumulated at tumor sites, the tumor acidity cleaved the pH-labile linker and triggered the discharge of PAMAM/Pt (diameter ∼5 nm) in the tumor microenvironment (Figure 5a). PAMAM/Pt carried the chemotherapeutic drug and penetrated deeply into the tumor interstitium because of its small size, as validated by confocal imaging and tissue immunostaining (Figure 5b). The in vivo antitumor activities of iCluster were validated in poorly permeable pancreatic cancer, in which iCluster reached 88% tumor suppression, compared with 57%, 38%, and 45% tumor inhibition of Cluster/Pt, free cisplatin, and PAMAM/Pt, respectively (Figure 5c). Furthermore, iCluster also significantly prolonged the survival of mice with metastatic breast tumor in comparison with other treatments (Figure 5d). Similar strategy has also been validated in other systems to improve the tumor penetration of drug delivery systems.41−43

6. TUMOR-ACIDITY-TRIGGERED LIGAND REACTIVATION Surface functionalization of NPs with biologically active ligands or cell-penetrating peptides (e.g., transactivator of transcription peptide (TAT)) increases the interactions of NPs with cells. However, these moieties may lead to immune recognition or undesirable cellular uptake in the bloodstream and normal tissue. For example, TAT is known to mediate strong interactions with biological membranes, promoting cell penetration and nuclear targeting of various cargos. However, their interactions with biological systems are nonspecific, making them

7. CONCLUSION AND OUTLOOK In this Account, we have summarized our research progress over the past 8 years in developing tumor-acidity-responsive NPs, particularly the tumor-acidity-cleavable maleic acid amide (TACMAA)-based delivery nanocarriers for improved cancer treatment. Four strategies, namely, tumor-acidity-triggered F


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revealed that several responsive polymers containing tertiary amines exhibit super-pH-sensitive structural changes under the stimulation of tumor acidity, which can reach completion within seconds.47 These polymers have been used for tumor imaging and image-guided tumor surgery as well as improved drug delivery.48,49 An open question is how the pH-sensitive rate of a nanomedicine affects its in vivo performance. If faster is better, structural optimization of TACMAA chemistry should be performed to further increase the reactive sensitivity in the future. Finally, more attention is needed to promote the translational study of TACMAA chemistry and polymers based on TACMAA. To do so, the chemical structure of the polymers containing TACMAA should be simple, biodegradable, biocompatible, and easy to scale up.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jin-Zhi Du: 0000-0003-4037-1212 Hong-Jun Li: 0000-0002-1765-8445 Jun Wang: 0000-0001-9957-9208 Notes

The authors declare no competing financial interest. Biographies Jin-Zhi Du is a Professor at the Institutes for Life Sciences in the School of Medicine at South China University of Technology. He obtained his B.Sc. and Ph.D. in Polymer Physics and Chemistry at the University of Science and Technology of China in 2006 and 2011, respectively, under the supervision of Prof Jun Wang. From 2012 to 2016 he did his postdoctoral research at Emory University School of Medicine with Prof. Shuming Nie. His current research interests include nanomedicine and drug delivery.

Figure 5. (a) Structure of PCL-CDM-PAMAM/Pt, its self-assembly into iCluster/Pt, and iCluster/Pt’s response to tumor acidity and the intracellular reductive environment. (b) Immunofluorescence images showing the microdistribution of iCluster in BxPC-3 xenograft tumor at 4 h postinjection. PAMAM, green; iCluster core, red; blood vessels, yellow. The scale bars represent 50 μm. (c) Growth inhibition of BxPC-3 xenograft tumor model by different treatments. (d) Kaplan− Meier plots of animal survival in the 4T1 xenograft tumor model. Reproduced with permission from ref 40. Copyright 2016 National Academy of Sciences.

Hong-Jun Li obtained his B.Sc. and Ph.D. in Polymer Physics and Chemistry at the University of Science and Technology of China in 2012 and 2017, respectively, under the supervision of Prof. Jun Wang. He is currently doing postdoctoral research with Prof. Jun Wang at South China University of Technology. His research is focused on nanomedicine.

charge reversal, shell detachment, size transition, and ligand reactivation, have been used to design a variety of delivery systems. In multiple systems, we have demonstrated that TACMAA chemistry has considerable potential for the design of effective nanocarriers to overcome multiple delivery barriers and improve the treatment efficacy of cancer. Looking forward, more attention should be paid to the following points. First, all of the above-mentioned delivery systems based on TACMAA chemistry focused on the treatment of primary tumors. How effective those systems would be in the treatment of metastatic tumors is not yet fully understood. Several reports have demonstrated that metastatic tumor lesions with very small size develop tumor acidity, which means that the TACMAA chemistry might be viable for treating tumor metastasis. Second, our previous studies mainly focused on developing tumor-acidity-responsive nanocarriers for improved drug delivery, whereas we did not exploit their potential to design nanoprobes for tumor imaging and diagnosis as well as for tumor theranostics. Since tumor imaging and diagnosis share many common features in particle design, TACMAA chemistry can be used in this particular area, including the imaging of tumor metastasis. Third, although the degradation of TACMAA at tumor pH is quite fast, the full degradation of the amide bond still needs tens of minutes or even hours. Recent studies have

Jun Wang is a Professor in the School of Biomedical Science and Engineering at South China University of Technology. He received a joint B.Sc. in Chemistry and Cell Biology at Wuhan University in 1993 and a Ph.D. in Polymer Chemistry and Physics in 1999 under the direction of Prof. Ren-Xi Zhuo. He has been a postdoctoral fellow at Johns Hopkins Singapore and the Johns Hopkins University School of Medicine under the direction of Prof. Kam Leong. His research interests include biomaterials, nanomedicine, and cancer immunotherapy.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2017YFA0205600), the National Natural Science Foundation of China (51633008, 31771091, and 51390482), the Program for Guangdong Introducing Innovative and Enterpreneurial Teams (2017ZT07S054), Guangdong Natural Science Funds for Distinguished Young Scholar (2017A030306018), the China Postdoctoral Science Foundation (2017M622673), and the Fundamental Research Funds for the Central Universities. G


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Tumor-Acidity-Cleavable Maleic Acid Amide (TACMAA) - American

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