Activable Cell-Penetrating Peptide Conjugated Prodrug for Tumor Targeted Drug Delivery

Recent Advances in Augmenting the Therapeutic Efficacy of Peptide-Drug Conjugates

There is an urgent need for the development of safe and effective modalities for the treatment of diseases owing to drug resistance, undesired side effects, and poor clinical outcomes. Combining cell-targeting and efficient cell-killing properties, peptide-drug conjugates (PDCs) have demonstrated superior efficacy compared with peptides and payloads alone. However, innovative molecular designs of PDCs are essential for further improving targeting precision, protease resistance and stability, cell permeability, and overall treatment efficacy. Several strategies have been developed to address these challenges, such as multivalency approaches, bispecific targeting, and long-acting PDCs. Other novel strategies, including overcoming biological barriers, conjugating novel functional payloads, and targeting macropinocytosis, have also shown promise. This perspective compiles the most recent strategies for enhancing PDC treatment efficacy, highlights key advancements in PDC, and provides insights on future directions for the development of novel PDCs.

Current progress and remaining challenges of peptide-drug conjugates (PDCs): next generation of antibody-drug conjugates (ADCs)?

Drug conjugates have emerged as a promising alternative delivery system designed to deliver an ultra-toxic payload directly to the target cancer cells, maximizing therapeutic efficacy while minimizing toxicity. Among these, antibody-drug conjugates (ADCs) have garnered significant attention from both academia and industry due to their great potential for cancer therapy. However, peptide-drug conjugates (PDCs) offer several advantages over ADCs, including more accessible industrial synthesis, versatile functionalization, high tissue penetration, and rapid clearance with low immunotoxicity. These factors position PDCs as up-and-coming drug candidates for future cancer therapy. Despite their potential, PDCs face challenges such as poor pharmacokinetic properties and low bioactivity, which hinder their clinical development. How to design PDCs to meet clinical needs is a big challenge and urgent to resolve. In this review, we first carefully analyzed the general consideration of successful PDC design learning from ADCs. Then, we summarised the basic functions of each component of a PDC construct, comprising of peptides, linkers and payloads. The peptides in PDCs were categorized into three types: tumor targeting peptides, cell penetrating peptide and self-assembling peptide. We then analyzed the potential of these peptides for drug delivery, such as overcoming drug resistance, controlling drug release and improving therapeutic efficacy with reduced non-specific toxicity. To better understand the potential druggability of PDCs, we discussed the pharmacokinetics of PDCs and also briefly introduced the current PDCs in clinical trials. Lastly, we discussed the future perspectives for the successful development of an oncology PDC. This review aimed to provide useful information for better construction of PDCs in future clinical applications.

Cysteine cathepsins: From diagnosis to targeted therapy of cancer

Cysteine cathepsins are a fascinating group of proteolytic enzymes that play diverse and crucial roles in numerous biological processes, both in health and disease. Understanding these proteases is essential for uncovering novel insights into the underlying mechanisms of a wide range of disorders, such as cancer. Cysteine cathepsins influence cancer biology by participating in processes such as extracellular matrix degradation, angiogenesis, immune evasion, and apoptosis. In this comprehensive review, we explore foundational research that illuminates the diverse and intricate roles of cysteine cathepsins as diagnostic markers and therapeutic targets for cancer. This review aims to provide valuable insights into the clinical relevance of cysteine cathepsins and explore their capacity to advance personalised and targeted medical interventions in oncology.

Recent Advances in Targeted Cancer Therapy: Are PDCs the Next Generation of ADCs?

Antibody-drug conjugates (ADCs) comprise antibodies, cytotoxic payloads, and linkers, which can integrate the advantages of antibodies and small molecule drugs to achieve targeted cancer treatment. However, ADCs also have some shortcomings, such as non-negligible drug resistance, a low therapeutic index, and payload-related toxicity. Many studies have focused on changing the composition of ADCs, and some have even further extended the concept and types of targeted conjugated drugs by replacing the targeted antibodies in ADCs with peptides, revolutionarily introducing peptide-drug conjugates (PDCs). This Perspective summarizes the current research status of ADCs and PDCs and highlights the structural innovations of ADC components. In particular, PDCs are regarded as the next generation of potential targeted drugs after ADCs, and the current challenges of PDCs are analyzed. Our aim is to offer fresh insights for the efficient design and expedited development of innovative targeted conjugated drugs.

Cell-Penetrating Peptides: A Powerful Tool for Targeted Drug Delivery

The cellular membrane hinders the effective delivery of therapeutics to targeted sites. Cellpenetrating peptide (CPP) is one of the best options for rapidly internalizing across the cellular membrane. CPPs have recently attracted lots of attention because of their excellent transduction efficiency and low cytotoxicity. The CPP-cargo complex is an effective and efficient method of delivering several chemotherapeutic agents used to treat various diseases. Additionally, CPP has become another strategy to overcome some of the current therapeutic agents' limitations. However, no CPP complex is approved by the US FDA because of its limitations and issues. In this review, we mainly discuss the cellpenetrating peptide as the delivery vehicle, the cellular uptake mechanism of CPPs, their design, and some strategies to synthesize the CPP complex via some linkers such as disulfide bond, oxime, etc. Here, we also discuss the recent status of CPPs in the market.

Peptides as carriers of active ingredients: A review

Bioactive compounds are highly valuable in the fields of food and medicine, but their application is limited due to easy deterioration after oral or skin administration. In recent years, the use of peptides as delivery systems for bioactive compounds has been intensively researched because of their special physicochemical characteristics. Peptides can be assembled using various preparation methods and can form several composite materials such as hydrogels, micelles, emulsions and particles. The composite material properties are determined by peptides, bioactive compounds and the construction methods employed. Herein, this paper provides a comprehensive review of the peptides used for active ingredients delivery, fabrication methods for creating delivery systems, structures, targeting characteristics, functional activities and mechanism of delivery systems, as well as their absorption and metabolism, which provided theoretical basis and reference for further research and development of functional composites.

Enhanced Liposomal Drug Delivery Via Membrane Fusion Triggered by Dimeric Coiled-Coil Peptides

An ideal nanomedicine system improves the therapeutic efficacy of drugs. However, most nanomedicines enter cells via endosomal/lysosomal pathways and only a small fraction of the cargo enters the cytosol inducing therapeutic effects. To circumvent this inefficiency, alternative approaches are desired. Inspired by fusion machinery found in nature, synthetic lipidated peptide pair E4/K4 is used to induce membrane fusion previously. Peptide K4 interacts specifically with E4, and it has a lipid membrane affinity and resulting in membrane remodeling. To design efficient fusogens with multiple interactions, dimeric K4 variants are synthesized to improve fusion with E4-modified liposomes and cells. The secondary structure and self-assembly of dimers are studied; the parallel PK4 dimer forms temperature-dependent higher-order assemblies, while linear K4 dimers form tetramer-like homodimers. The structures and membrane interactions of PK4 are supported by molecular dynamics simulations. Upon addition of E4, PK4 induced the strongest coiled-coil interaction resulting in a higher liposomal delivery compared to linear dimers and monomer. Using a wide spectrum of endocytosis inhibitors, membrane fusion is found to be the main cellular uptake pathway. Doxorubicin delivery results in efficient cellular uptake and concomitant antitumor efficacy. These findings aid the development of efficient delivery systems of drugs into cells using liposome-cell fusion strategies.

Research advances in peptide‒drug conjugates

Peptide‒drug conjugates (PDCs) are drug delivery systems consisting of a drug covalently coupled to a multifunctional peptide via a cleavable linker. As an emerging prodrug strategy, PDCs not only preserve the function and bioactivity of the peptides but also release the drugs responsively with the cleavable property of the linkers. Given the ability to significantly improve the circulation stability and targeting of drugs in vivo and reduce the toxic side effects of drugs, PDCs have already been extensively applied in drug delivery. Herein, we review the types and mechanisms of peptides, linkers and drugs used to construct PDCs, and summarize the clinical applications and challenges of PDC drugs.

Cell-Penetrating Peptide-Based Delivery of Macromolecular Drugs: Development, Strategies, and Progress

Cell-penetrating peptides (CPPs), developed for more than 30 years, are still being extensively studied due to their excellent delivery performance. Compared with other delivery vehicles, CPPs hold promise for delivering different types of drugs. Here, we review the development process of CPPs and summarize the composition and classification of the CPP-based delivery systems, cellular uptake mechanisms, influencing factors, and biological barriers. We also summarize the optimization routes of CPP-based macromolecular drug delivery from stability and targeting perspectives. Strategies for enhanced endosomal escape, which prolong its half-life in blood, improved targeting efficiency and stimuli-responsive design are comprehensively summarized for CPP-based macromolecule delivery. Finally, after concluding the clinical trials of CPP-based drug delivery systems, we extracted the necessary conditions for a successful CPP-based delivery system. This review provides the latest framework for the CPP-based delivery of macromolecular drugs and summarizes the optimized strategies to improve delivery efficiency.

The Recent Advance of Cell-Penetrating and Tumor-Targeting Peptides as Drug Delivery Systems Based on Tumor Microenvironment

Cancer has become the primary reason for industrial countries death. Although first-line treatments have achieved remarkable results in inhibiting tumors, they could have serious side effects because of insufficient selectivity. Therefore, specific localization of tumor cells is currently the main desire for cancer treatment. In recent years, cell-penetrating peptides (CPPs), as a kind of promising delivery vehicle, have attracted much attention because they mediate the high-efficiency import of large quantities of cargos in vivo and vitro. Unfortunately, the poor targeting of CPPs is still a barrier to their clinical application. In order to solve this problem, researchers use the various characteristics of tumor microenvironment and multiple receptors to improve the specificity toward tumors. This review focuses on the characteristics of the tumor microenvironment, and introduces the development of strategies and peptides based on these characteristics as drug delivery system in the tumor-targeted therapy.

Peptide Drug Conjugates and Their Role in Cancer Therapy

Drug conjugates have become a significant focus of research in the field of targeted medicine for cancer treatments. Peptide-drug conjugates (PDCs), a subset of drug conjugates, are composed of carrier peptides ranging from 5 to 30 amino acid residues, toxic payloads, and linkers that connect the payload to the peptide. PDCs are further broken down into cell-penetrating peptides (CPPs) and cell-targeting peptides (CTPs), each having their own differences in the delivery of cytotoxic payloads. Generally, PDCs as compared to other drug conjugates-like antibody-drug conjugates (ADCs)-have advantages in tumor penetration, ease of synthesis and cost, and reduced off-target effects. Further, as compared to traditional cancer treatments (e.g., chemotherapy and radiation), PDCs have higher specificity for the target cancer with generally less toxic side effects in smaller doses. However, PDCs can have disadvantages such as poor stability and rapid renal clearance due to their smaller size and limited oral bioavailability due to digestion of its peptide structure. Some of these challenges can be overcome with modifications, and despite drawbacks, the intrinsic small size of PDCs with high target specificity still makes them an attractive area of research for cancer treatments.

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

Peptide-Drug Conjugates: A New Hope for Cancer Management

Cancer remains the leading cause of death worldwide despite advances in treatment options for patients. As such, safe and effective therapeutics are required. Short peptides provide advantages to be used in cancer management due to their unique properties, amazing versatility, and progress in biotechnology to overcome peptide limitations. Several appealing peptide-based therapeutic strategies have been developed. Here, we provide an overview of peptide conjugates, the better equivalents of antibody-drug conjugates, as the next generation of drugs for required precise targeting, enhanced cellular permeability, improved drug selectivity, and reduced toxicity for the efficient treatment of cancers. We discuss the basic components of drug conjugates and their release action, including the release of cytotoxins from the linker. We also present peptide-drug conjugates under different stages of clinical development as well as regulatory and other challenges.

Charge reversal hairpin peptide modified synergy therapeutic nanoplatforms for tumor specific drug shuttling

Given the distinct pathological features of neoplasm tissues, multifunctional responsive nanocarriers have been recently considered as promising candidates to optimize the chemotherapy regime. As a result, we propose a graphene oxide-based pH-responsive drug delivery system via covalent assembly of "hairpin-like" cell penetrating peptides with acid sensitive and charge reversal properties to realize superior tumor specificity and lower toxicity. Graphene oxide here can serve as high doxorubicin-loading nanosheets and facilitate swift drug release in response to laser irradiation, which provides an efficient platform for the synergy of photo-chemotherapy. Structurally, polyglycol conjugation on the graphene oxide surface fulfils the function of nanocomposite stabilization. After administration, the elaborately acid sensitive cell penetrating peptides maintain the hairpin structure under physiological conditions, while after entering the tumor acidic microenvironment, they undergo charge reversal and structural conversion to promote the cellular uptake of nanoparticles. The evaluation of nanocomposites in vitro revealed their negligible systematic toxicity and remarkable antitumor effects. In vivo experiments also confirmed the impressive stability and tumor-specific targeting for alleviating breast cancer. In conclusion, hairpin peptide modified graphene oxide nanoparticles show multiple merits including high drug carrying capacity, selective tumor penetration, responsive drug release and effective combination oncotherapy.

New Technologies Bloom Together for Bettering Cancer Drug Conjugates

Drug conjugates, including antibody-drug conjugates, are a step toward realizing Paul Ehrlich's idea from over 100 years ago of a "magic bullet" for cancer treatment. Through balancing selective targeting molecules with highly potent payloads, drug conjugates can target specific tumor microenvironments and kill tumor cells. A drug conjugate consists of three parts: a targeting agent, a linker, and a payload. In some conjugates, monoclonal antibodies act as the targeting agent, but new strategies for targeting include antibody derivatives, peptides, and even small molecules. Linkers are responsible for connecting the payload to the targeting agent. Payloads impact vital cellular processes to kill tumor cells. At present, there are 12 antibody-drug conjugates on the market for different types of cancers. Research on drug conjugates is increasing year by year to solve problems encountered in conjugate design, such as tumor heterogeneity, poor circulation, low drug loading, low tumor uptake, and heterogenous expression of target antigens. This review highlights some important preclinical research on drug conjugates in recent years. We focus on three significant areas: improvement of antibody-drug conjugates, identification of new conjugate targets, and development of new types of drug conjugates, including nanotechnology. We close by highlighting the critical barriers to clinical translation and the open questions going forward. SIGNIFICANCE STATEMENT: The development of anticancer drug conjugates is now focused in three broad areas: improvements to existing antibody drug conjugates, identification of new targets, and development of new conjugate forms. This article focuses on the exciting preclinical studies in these three areas and advances in the technology that improves preclinical development.

Autophagy responsive intra-intercellular delivery nanoparticles for effective deep solid tumor penetration

Deep tumor cells (cells in the center of solid tumors) play a crucial role in drug tolerance, metastasis, recurrence and microenvironment immune suppression. However, their deep location endows them with an untouched abdomen and makes them refractory to current treatments. Herein, we exploited the characteristic of higher autophagy in deep tumor cells than in superficial tumor cells and designed autophagy-responsive multifunctional nanoparticles (PGN) to enhance drug accumulation in deep tumor cells. PGNs were prepared by densely coating poly (lactic-co-glycolic acid) (PLGA) with cationic autophagy-responsive cell-penetrating peptide (GR9) and anionic 2,3-dimethylmaleic anhydride (DMA)-modified DSPE-PEG. The suitable nanoparticle size (122.4 nm) and charge-neutral surface (0.21 mV) of the NPs enabled long blood circulation. The hydrolysis of surface-anchored anionic DMA in the acidic microenvironment led to the exposure of the GR9 peptide and enhance tumor penetration. Once the PGN arrived in deep tumor cells with strong autophagy, GR9 was cut off by an autophagy shear enzyme, and the nanoparticles remained in the cells to undergo degradation. Furthermore, we prepared docetaxel (DTX) and chloroquine (CQ) loaded d-PGN. CQ inhibits autophagosome fusion with lysosomes, resulting in autophagosome accumulation, which further enhances the sensitivity of d-PGN to autophagy and their deep tumor retention. In vivo experiments showed that drug-loaded d-PGN achieved excellent antitumor efficacy with a peak inhibition rate of 82.1%. In conclusion, autophagy-responsive multifunctional nanoparticles provide a novel potential strategy for solid tumor treatment.

Design, Synthesis, and Bioevaluation of Novel Enzyme-Triggerable Cell Penetrating Peptide-Based Dendrimers for Targeted Delivery of Camptothecin and Cancer Therapy

Novel enzyme-triggerable cell penetrating peptide (ETCPP) dendrimers with a camptothecin (CPT) warhead were designed and synthesized based on an amphiphilic penetrating peptide (FKKFFRKLL, discovered by us before). Among the newly synthesized ETCPP dendrimer conjugates, BL_Oc-SS-CPT (a high-generation dendrimer) exhibited the highest activity with IC₅₀s in the nanomolar range (31-747 nM) against a panel of cancer cells, which is 3-10 times better than that of CPT. BL_Oc-SS-CPT remained intact during transit to target cells and in normal tissues with a plasma half-life of 4.2 h, 2.3-fold longer than that of the monomer (1.8 h). Once reaching the tumor site, BL_Oc-SS-CPT gradually released CPT in the presence of excessive matrix metalloproteinase-2/9 and GSH in cancer cells. Importantly, BL_Oc-SS-CPT exhibited excellent in vivo tumor targeting capability and antitumor efficacy with benign toxicity profiles. Thus, the novel ETCPP dendrimer-based drug delivery system (e.g., BL_Oc-SS-CPT) represents a safe and effective strategy for targeted cancer therapy.

Prostate-Specific Membrane Antigen and Esterase Dual Responsive Camptothecin-Oligopeptide Self-Assembled Nanoparticles for Efficient Anticancer Drug Delivery

Background

The clinical utility of camptothecin (CPT) is restricted by poor aqueous solubility, high lipophilicity, active lactone ring instability, and off-targeted toxicities. We report here a prostate-specific membrane antigen (PSMA) and esterase dual responsive self-assembled nanoparticles (CPT-WT-H NPs) for highly efficient CPT delivery and effective cancer therapy.

Methods and Results

In this study, smart self-assembled nanoparticles CPT-WT-H NPs were elaborately designed and synthesized by combing hydrophobic CPT with hydrophilic PSMA-responsive penta-peptide via a cleavable ester bond. This dual responsive nanoparticle with negatively charged surface first respond to the extracellular PSMA and then to the intracellular esterase, achieving a programmable release of CPT at the tumor site and producing the byproducts of biocompatible glutamic acid and aspartic acid. Our data demonstrated that CPT-WT-H NPs exhibited greatly improved water solubility and stability. Results from MTT and flow cytometry showed CPT-WT-H NPs exhibited significantly higher cytotoxicity as well as apoptosis-inducing activity against PSMA-expressing LNCaP-FGC cells than the non-PSMA-expressing cancer cells, showing excellent cytotoxic selectivity. Moreover, the unique nanostructure provided the efficient transportation of CPT to tumor site, which resulted in the effective inhibition of tumor growth and low systemic toxicity in vivo.

Conclusion

CPT-WT-H NPs exhibited excellent in vitro PSMA-response ability and in vivo antitumor activity and safety, holding the promise to become a new and potent anticancer drug. The current research presents a promising strategy for efficient drug delivery.

Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse amino acid sequences with the ability to cross cellular membranes. CPPs can deliver several bioactive cargos, including proteins, peptides, nucleic acids and chemotherapeutics, into cells. Ever since their discovery, synthetic and natural CPPs have been utilized in therapeutics delivery, gene editing and cell imaging in fundamental research and clinical experiments. Over the years, CPPs have gained significant attention due to their low cytotoxicity and high transduction efficacy. In the last decade, multiple investigations demonstrated the potential of CPPs as carriers for the delivery of therapeutics to treat various types of cancer. Besides their remarkable efficacy owing to fast and efficient delivery, a crucial benefit of CPP-based cancer treatments is delivering anticancer agents selectively, rather than mediating toxicities toward normal tissues. To obtain a higher therapeutic index and to improve cell and tissue selectivity, CPP-cargo constructions can also be complexed with other agents such as nanocarriers and liposomes to obtain encouraging outcomes. This review summarizes various types of CPPs conjugated to anticancer cargos. Furthermore, we present a brief history of CPP utilization as delivery systems for anticancer agents in the last decade and evaluate several reports on the applications of CPPs in basic research and preclinical studies.

Stimuli-responsive polypeptide nanoassemblies: Recent progress and applications in cancer nanomedicine

Stimuli-responsive polypeptide nanoassemblies exhibit great potentials for cancer nanomedicines because of desirable biocompatibility and biodegradability, unique secondary conformations, varying functionalities, and especially the stimuli-enhanced therapeutic efficacy and reduced side effect. This review introduces the design and fabrication of stimuli-responsive polypeptide nanoassemblies that exhibit endogenous stimuli (e.g., pH, reduction, reactive oxygen species, adenosine triphosphate and enzyme, etc.) and exogenous light stimuli (e.g., UV and near-infrared light), which are biologically related or applied in the clinic. We also discuss the applications and prospects of those stimuli-responsive polypeptide nanoassemblies that might overcome the biological barriers of cancer nanomedicines for in vivo administration. Much more effort is needed to accelerate the second-generation stimuli-responsive polypeptide nanomedicines for clinical transition and applications. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Cell-penetrating peptides (CPPs): an overview of applications for improving the potential of nanotherapeutics

In the field of nanotherapeutics, gaining cellular entry into the cytoplasm of the target cell continues to be an ultimate challenge. There are many physicochemical factors such as charge, size and molecular weight of the molecules and delivery vehicles, which restrict their cellular entry. Hence, to dodge such situations, a class of short peptides called cell-penetrating peptides (CPPs) was brought into use. CPPs can effectively interact with the cell membrane and can assist in achieving the desired intracellular entry. Such strategy is majorly employed in the field of cancer therapy and diagnosis, but now it is also used for other purposes such as evaluation of atherosclerotic plaques, determination of thrombin levels and HIV therapy. Thus, the current review expounds on each of these mentioned aspects. Further, the review briefly summarizes the basic know-how of CPPs, their utility as therapeutic molecules, their use in cancer therapy, tumor imaging and their assistance to nanocarriers in improving their membrane penetrability. The review also discusses the challenges faced with CPPs pertaining to their stability and also mentions the strategies to overcome them. Thus, in a nutshell, this review will assist in understanding how CPPs can present novel possibilities for resolving the conventional issues faced with the present-day nanotherapeutics.

Human serum albumin-based doxorubicin prodrug nanoparticles with tumor pH-responsive aggregation-enhanced retention and reduced cardiotoxicity

Doxorubicin (DOX) is a widely-used anticancer drug, but its cardiotoxicity severely hampers its potency in chemotherapy. Herein, human serum albumin (HSA) is engaged as a biocompatible nanocarrier to load a pH-sensitive DOX prodrug, DMDOX, generating HSA-DMDOX nanoparticles via self-assembly driven by hydrophobic interactions. HSA-DMDOX disperses well in a physiological environment (∼40 nm) but aggregates in a tumor acidic microenvironment (pH 6.5, ∼140 nm) owing to the hydrophobicity increase of DMDOX by protonation of carboxylic groups. In vitro anticancer study showed that HSA-DMDOX exhibited enhanced cellular uptake by 4T1 cells and superior cytotoxicity in comparison to HSA-DOX nanoparticles. In vivo study suggested that HSA-DMDOX achieved long blood circulation, aggregation enhanced tumor retention, comparable antitumor efficacy and reduced cardiotoxicity relative to free DOX. Our work presents a facile and effective approach to delivering anthracyclines by HSA-based tumor pH-responsive nanoparticles with aggregation-enhanced tumor retention and reduced toxicity.

Peptides-based therapy and diagnosis. Strategies for non-invasive therapies in cancer

In recent years, remarkable progress was registered in the field of cancer research. Though, cancer still represents a major cause of death and cancer metastasis a problem seeking for urgent solutions as it is the main reason for therapeutic failure. Unfortunately, the most common chemotherapeutic agents are non-selective and can damage healthy tissues and cause side effects that affect dramatically the quality of life of the patients. Targeted therapy with molecules that act specifically at the tumour sites interacting with overexpressed cancer receptors is a very promising strategy for achieving the specific delivery of anticancer drugs, radioisotopes or imaging agents. This review aims to give an overview on different strategies for targeting cancer cell receptors localised either at the extracellular matrix or at the cell membrane. Molecules like antibodies, aptamers and peptides targeting the cell surface are presented with advantages and disadvantages, with emphasis on peptides. The most representative peptides are described, including cell penetrating peptides, homing and anticancer peptides with particular consideration on recent discoveries.

Recent Advances and Trends in Chemical CPP-Drug Conjugation Techniques

This review summarizes recent developments in conjugation techniques for the synthesis of cell-penetrating peptide (CPP)–drug conjugates targeting cancer cells. We will focus on small organic molecules as well as metal complexes that were used as cytostatic payloads. Moreover, two principle ways of coupling chemistry will be discussed direct conjugation as well as the use of bifunctional linkers. While direct conjugation of the drug to the CPP is still popular, the use of bifunctional linkers seems to gain increasing attention as it offers more advantages related to the linker chemistry. Thus, three main categories of linkers will be highlighted, forming either disulfide acid-sensitive or stimuli-sensitive bonds. All techniques will be thoroughly discussed by their pros and cons with the aim to help the reader in the choice of the optimal conjugation technique that might be used for the synthesis of a given CPP–drug conjugate

ConjuPepDB: a database of peptide-drug conjugates

Peptide–drug conjugates are organic molecules composed of (i) a small drug molecule, (ii) a peptide and (iii) a linker. The drug molecule is mandatory for the biological action, however, its efficacy can be enhanced by targeted delivery, which often also reduces unwanted side effects. For site-specificity the peptide part is mainly responsible. The linker attaches chemically the drug to the peptide, but it could also be biodegradable which ensures controlled liberation of the small drug. Despite the importance of the field, there is no public comprehensive database on these species. Herein we describe ConjuPepBD, a freely available, fully annotated and manually curated database of peptide drug conjugates. ConjuPepDB contains basic information about the entries, e.g. CAS number. Furthermore, it also implies their biomedical application and the type of chemical conjugation employed. It covers more than 1600 conjugates from ∼230 publications. The web-interface is user-friendly, intuitive, and useable on several devices, e.g. phones, tablets, PCs. The webpage allows the user to search for content using numerous criteria, chemical structure and a help page is also provided. Besides giving quick insight for newcomers, ConjuPepDB is hoped to be also helpful for researchers from various related fields. The database is accessible at: https://conjupepdb.ttk.hu/.

Challenge to overcome current limitations of cell-penetrating peptides

The penetration of biological membranes is a prime obstacle for the delivery of pharmaceutical drugs. Cell-penetrating peptide (CPP) is an efficient vehicle that can deliver various cargos across the biological membranes. Since the discovery, CPPs have been rigorously studied to unveil the underlying penetrating mechanism as well as to exploit CPPs for various biomedical applications. This review will focus on the various strategies to overcome current limitations regarding stability, selectivity, and efficacy of CPPs.

A polymeric nanocarrier with a tumor acidity-activatable arginine-rich (R9) peptide for enhanced drug delivery

Cell-penetrating peptides (CPPs) have been considered as a powerful tool to improve the intracellular and nuclear delivery efficiency of nanocarriers. However, their clinical application is limited because of their nonspecific targeting function, short half-life, and severe system toxicity. Herein, we have developed a polymeric nanocarrier with a tumor acidity-activatable arginine-rich (R9) peptide for targeted drug delivery. The nanocarrier is fabricated with a R9-conjugated amphiphilic diblock polymer of poly(ethylene glycol) (PEG) and poly(hexyl ethylene phosphate) (PHEP), and then further coated with tumor acidity-activatable polyanionic polyphosphoester through electrostatic interaction in order to block the nonspecific targeting function of the R9 peptide. In the slightly acidic tumor extracellular environment (∼pH 6.5), tumor acidity-activatable polyanionic polyphosphoester would be deshielded from the nanoparticles, resulting in the re-exposure of the R9 peptide to enhance tumor cellular uptake. As a result, intracellular concentration of payload in 4T1 tumor cells significantly increased at pH 6.5. And, we further demonstrate that such a delivery system remarkably promoted the anti-tumor efficiency of chemotherapeutic drugs in tumor-bearing mice, offering great potential for drug delivery and cancer therapy.

Synthetic strategies in construction of organic low molecular-weight carrier-drug conjugates

Inefficient transportation of polar metabolic inhibitors through cell membranes of eukaryotic and prokaryotic cells precludes their direct use as drug candidates in chemotherapy. One of the possible solutions to this problem is application of the 'Trojan horse' strategy, i.e. conjugation of an active substance with a molecular carrier of organic or inorganic nature, facilitating membrane penetration. In this work, the synthetic strategies used in rational design and preparation of conjugates of bioactive agents with three types of organic low molecular-weight carriers have been reviewed. These include iron-chelating agents, siderophores and cell-penetrating peptides. Moreover, a less known but very promising "molecular umbrella" conjugation strategy has been presented. Special attention has been paid on appropriate linking strategies, especially these allowing intracellular drug release after internalisation of a conjugate.

Activatable cell-penetrating peptides: 15 years of research

An important hurdle for the intracellular delivery of large cargo is the cellular membrane, which protects the cell from exogenous substances. Cell-penetrating peptides (CPPs) can cross this barrier but their use as drug delivery vehicles is hampered by their lack of cell type specificity. Over the past years, several approaches have been explored to control the activity of CPPs that can be primed for cellular uptake. Since the first report on such activatable CPPs (ACPPs) in 2004, various methods of activation have been developed. Here, we provide an overview of the different ACPPs strategies known to date and summarize the benefits, drawbacks, and future directions.

Aspirin overcomes cisplatin resistance in lung cancer by inhibiting cancer cell stemness

BACKGROUND

Lung cancer is the leading cause of cancer death and is commonly treated by cisplatin. Although cisplatin treatment may initially be successful, its effectiveness usually reduces significantly in disease-recurrent patients. Aspirin, a nonselective COX inhibitor, has been shown to help reverse the status of cisplatin sensitivity in recurrent human ovarian cancer cells. This study aimed to explore the effect of aspirin on cisplatin resistance through the perspective of cancer cell stemness.

METHODS

We used clustering analysis to predict the H460 cisplatin resistance from the GSE21656 dataset. The increased lung cancer cell stemness may contribute to enhanced tolerance. In this study, we used aspirin, a nonselective COX inhibitor, with cisplatin for several hours in cells and days in vivo, and studied the inhibition against human cisplatin-resistant H460 cells. H460 cisplatin-sensitive and H460 cisplatin-resistant cells were treated with 16 μM aspirin or/and 0.3 μg/mL cisplatin for 72 hours.

RESULTS

H460 cisplatin-resistant cells showed stronger resistance, stemness, and invasiveness than H460 cisplatin-sensitive, and cisplatin significantly reduced the survival of cisplatin-sensitive cells, while cisplatin with aspirin dramatically reduced the surviving fractions of cisplatin-resistant cells.

CONCLUSIONS

This study revealed that stemness is a latent inhibitor of the resistance of lung cancer cisplatin-resistant cells and might be effectively inhibited by aspirin.

Protease-activated prodrugs: strategies, challenges, and future directions

Proteases play critical roles in virtually all biological processes, including proliferation, cell death and survival, protein turnover, and migration. However, when dysregulated, these enzymes contribute to the progression of multiple diseases, with cancer, neurodegenerative disorders, inflammation, and blood disorders being the most prominent examples. For a long time, disease-associated proteases have been used for the activation of various prodrugs due to their well-characterized catalytic activity and ability to selectively cleave only those substrates that strictly correspond with their active site architecture. To date, versatile peptide sequences that are cleaved by proteases in a site-specific manner have been utilized as bioactive linkers for the targeted delivery of multiple types of cargo, including fluorescent dyes, photosensitizers, cytotoxic drugs, antibiotics, and pro-antibodies. This platform is highly adaptive, as multiple protease-labile conjugates have already been developed, some of which are currently in clinical use for cancer treatment. In this review, recent advancements in the development of novel protease-cleavable linkers for selective drug delivery are described. Moreover, the current limitations regarding the selectivity of linkers are discussed, and the future perspectives that rely on the application of unnatural amino acids for the development of highly selective peptide linkers are also presented.

Dual-targeting nanotherapeutics antagonize hyperinsulinemia-promoted tumor growth via activating cell autophagy

Hyperinsulinemia, a concomitant symptom in type 2 diabetes mellitus (T2DM) promotes the migration, invasion and proliferation of tumors by inhibiting autophagy. Therefore, it is necessary to search for antitumor drugs that can effectively antagonize hyperinsulinemia by promoting autophagy. In this study, dual-targeting modified selenium nanoparticles (u/A-SeNPs) were proposed as a biocompatible tumor chemotherapeutic drug to antagonize high insulin. The modification of chitosan (CS) and grafting targeted peptides (uPA/ACPP) allowed SeNPs to exert better selectivity and higher antitumor activity. The nanotherapeutics entered tumor cells through receptor-mediated endocytosis and produced excessive ROS. Meanwhile, u/A-SeNPs significantly increased the level of autophagy in tumor cells, as detected by monodansylcadaverine (MDC) and mRFP-GFP-LC3. U/A-SeNPs cause mitochondrial fragmentation to induce the cell apoptosis via the synergistic action of overproduced ROS and activated autophagy. In conclusion, this study proposes a feasible method for the synthesis of dual-targeting nanomedicines, and it also provides a new strategy for the application of Se-based nanotherapeutics in tumor therapy under hyperinsulinemia conditions.

Design, synthesis, and in vitro antitumor activity of a transferrin receptor-targeted peptide-doxorubicin conjugate

In this study, a peptide-drug conjugate was designed and synthesized by connecting a transferrin receptor (TfR)-targeted binding peptide analog BP9a (CAHLHNRS) with doxorubicin (DOX) through N-succinimidyl-3-maleimidopropionate (SMP) as the cross-linker. Confocal laser scanning microscopy results indicated that free DOX mainly accumulated in the nuclei of both TfR overexpressed HepG2 hepatoma cells and L-O2 normal liver cells expressing low level of TfR; most of the BP9a-DOX conjugate displayed cytoplasmic location, and its cellular uptake by HepG2 cells was obviously reduced by TfR blockage test. Nevertheless, the cellular uptake of this conjugate by L-O2 cells was much less than that of free DOX. Meanwhile, the BP9a-DOX conjugate exhibited lower in vitro antiproliferative activity against HepG2 cells than free DOX, but its cytotoxic effect on L-O2 cells was decreased compared with that of free DOX. These results suggest that BP9a could be applied as a potential TfR-targeted peptide vector for selective drug delivery.

ROS-Responsive Polymeric Nanocarriers with Photoinduced Exposure of Cell-Penetrating Moieties for Specific Intracellular Drug Delivery

In situ modulation of the surface properties on the micellar drug delivery nanocarriers offers an efficient method to improve the drug delivery efficiency into cells while maintaining stealth and stability during blood circulation. Light has been demonstrated to be a temporally and spatially controllable tool to improve cellular internalization of nanoparticles. Herein, we develop reactive oxygen species (ROS)-responsive mixed polymeric micelles with photoinduced exposure of cell-penetrating moieties via photodynamic ROS production, which can facilitate cellular internalization of paclitaxel (PTX) and chlorin e6 (Ce6)-coloaded micelles for the synergistic effect of photodynamic and chemotherapy. The thioketal-bond-linked block polymers poly(ε-caprolactone)-TL-poly(N,N-dimethylacrylamide) (PCL-TL-PDMA) with a long PDMA block are used to self-assemble into mixed micelles with PCL-b-poly(2-guanidinoethyl methacrylate) (PCL-PGEMA) consisting of a short PGEMA block, which are further used to coencapsulate PTX and Ce6. After intravenous injection, prolonged blood circulation of the micelles guarantees high tumor accumulation. Upon irradiation by 660 nm light, ROS production of the micelles by Ce6 induces cleavage of PDMA to expose PGEMA shells for significantly improved cellular internalization. The combination of photodynamic therapy and chemotherapy inside the tumor cells achieves improved antitumor efficacy. The design of ROS-responsive mixed polymeric nanocarriers represents a novel and efficient approach to realize both long blood circulation and high-efficiency cellular internalization for combined photodynamic and chemotherapy under light irradiation.

Novel therapeutic interventions in cancer treatment using protein and peptide-based targeted smart systems

Cancer, being the most prevalent and resistant disease afflicting any gender, age or social status, is the ultimate challenge for the scientific community. The new generation therapeutics for cancer management has shifted the approach to personalized/precision medicine, making use of patient- and tumor-specific markers for specifying the targeted therapies for each patient. Peptides targeting these cancer-specific signatures hold enormous potential for cancer therapy and diagnosis. The rapid advancements in the combinatorial peptide libraries served as an impetus to the development of multifunctional peptide-based materials for targeted cancer therapy. The present review outlines benefits and shortcomings of peptides as cancer therapeutics and the potential of peptide modified nanomedicines for targeted delivery of anticancer agents.

Transforming stealthy to sticky nanocarriers: a potential application for tumor therapy

Nanomedicine has shown remarkable progress in preclinical studies of tumor treatment. Over the past decade, scientists have developed various nanocarriers (NCs) for delivering drugs into the tumor area. However, the average amount of accumulated drugs in tumor sites is far from satisfactory. This limitation is strongly related to the corona formation during blood circulation. To overcome this issue, NCs should be designed to become highly stealthy by modifying their surface charge. However, at the same time, stealthy effects not only prevent protein formation but also alleviate the cellular uptake of NCs. Therefore, it is necessary to develop NCs with switchable properties, which are stealthy in the circulation system and sticky when arriving at tumor sites. In this review, we discuss the recent strategies to develop passive and active charge-switchable NCs, known as chameleon-like drug delivery systems, which can reversibly transform their surface from stealthy to sticky and have various designs.

Mechanistic Landscape of Membrane-Permeabilizing Peptides

Membrane permeabilizing peptides (MPPs) are as ubiquitous as the lipid bilayer membranes they act upon. Produced by all forms of life, most membrane permeabilizing peptides are used offensively or defensively against the membranes of other organisms. Just as nature has found many uses for them, translational scientists have worked for decades to design or optimize membrane permeabilizing peptides for applications in the laboratory and in the clinic ranging from antibacterial and antiviral therapy and prophylaxis to anticancer therapeutics and drug delivery. Here, we review the field of membrane permeabilizing peptides. We discuss the diversity of their sources and structures, the systems and methods used to measure their activities, and the behaviors that are observed. We discuss the fact that "mechanism" is not a discrete or a static entity for an MPP but rather the result of a heterogeneous and dynamic ensemble of structural states that vary in response to many different experimental conditions. This has led to an almost complete lack of discrete three-dimensional active structures among the thousands of known MPPs and a lack of useful or predictive sequence-structure-function relationship rules. Ultimately, we discuss how it may be more useful to think of membrane permeabilizing peptides mechanisms as broad regions of a mechanistic landscape rather than discrete molecular processes.

pH-Dependent aggregation and pH-independent cell membrane adhesion of monolayer-protected mixed charged gold nanoparticles

Design of pH-responsive monolayer-protected gold nanoparticles (AuNPs) that are mixed charged, with the ability to switch their net surface charge, based on the stimuli of environmental pH is a promising technique in nanomedicine. However, understanding of pH-responsive mixed charged AuNP behavior in terms of their stability and cellular interaction are still limited. In this work, we study the aggregation of pH-responsive AuNPs and their interaction with model lipid bilayers by adopting Martini coarse-grained (CG) molecular dynamics simulations. The surface of these AuNPs is decorated by both positively and negatively charged ligands. The AuNP is positively charged at low pH values due to protonation of negatively charged ligands. Its net charge is lowered at higher pH by increasing the ratio of deprotonated negatively charged ligands. We find that the AuNPs are severely aggregated at moderate pH value, where each AuNP has an overall neutral charge, whereas they are stable and dispersed at both low and high pH values. Further free energy analysis reveals that the energy barrier at a larger separation distance than the location of the hydrophobic driving force potential well, plays a key role that determines the stability of monolayer-protected AuNPs at different pH values. This energy barrier is dramatically decreased at moderate pH value, leading to severe aggregation of AuNPs. By investigating the interaction between AuNPs and model lipid bilayers, we find that all the AuNPs adhere onto the lipid bilayer, independent of the pH value. Moreover, the lipids present originally in the bilayer are extracted by the AuNPs through a process of protrusion and upward climbing. The extraction of lipids can cause dehydration and disruption of the bilayers when multiple AuNPs are adhered. Free energy analysis reveals that the penetration of AuNPs will induce a dramatic free energy increase because of deformation of the ligands with hydrophilic functional end groups. We have systematically studied the stability of pH-responsive AuNPs and their interactions with lipid bilayers by simulation, which might pave the way for the design of pH-responsive monolayer protected AuNPs for biomedical applications.

Recent Advances in Cell Penetrating Peptide-Based Anticancer Therapies

Cell-penetrating-peptides (CPPs) are small amino-acid sequences characterized by their ability to cross cellular membranes. They can transport various bioactive cargos inside cells including nucleic acids, large proteins, and other chemical compounds. Since 1988, natural and synthetic CPPs have been developed for applications ranging from fundamental to applied biology (cell imaging, gene editing, therapeutics delivery). In recent years, a great number of studies reported the potential of CPPs as carriers for the treatment of various diseases. Apart from a good efficacy due to a rapid and potent delivery, a crucial advantage of CPP-based therapies is the peptides low toxicity compared to most drug carriers. On the other hand, they are quite unstable and lack specificity. Higher specificity can be obtained using a cell-specific CPP to transport the therapeutic agent or using a non-specific CPP to transport a cargo with a targeted activity. CPP-cargo complexes can also be conjugated to another moiety that brings cell- or tissue-specificity. Studies based on all these approaches are showing promising results. Here, we focus on recent advances in the potential usage of CPPs in the context of cancer therapy, with a particular interest in CPP-mediated delivery of anti-tumoral proteins.

Rational Design of Cancer Nanomedicine for Simultaneous Stealth Surface and Enhanced Cellular Uptake

Owing to the complex and still not fully understood physiological environment, the development of traditional nanosized drug delivery systems is very challenging for precision cancer therapy. It is very difficult to control the in vivo distribution of nanoparticles after intravenous injection. The ideal drug nanocarriers should not only have stealth surface for prolonged circulation time but also possess enhanced cellular internalization in tumor sites. Unfortunately, the stealth surface and enhanced cellular uptake seem contradictory to each other. How to integrate the two opposite aspects into one system is a very herculean but meaningful task. As an alternative drug delivery strategy, chameleon-like drug delivery systems were developed to achieve long circulation time while maintaining enhanced cancer cell uptake. Such drug nanocarriers can "turn off" their internalization ability during circulation. However, the enhanced cellular uptake can be readily activated upon arriving at tumor tissues. In this way, stealth surface and enhanced uptake are of dialectical unity in drug delivery. In this review, we focus on the surface engineering of drug nanocarriers to obtain simultaneous stealth surfaces in circulation and enhanced uptake in tumors. The current strategies and ongoing developments, including programmed tumor-targeting strategies and some specific zwitterionic surfaces, will be discussed in detail.

New Cell-Penetrating Peptide (KRP) with Multiple Physicochemical Properties Endows Doxorubicin with Tumor Targeting and Improves Its Therapeutic Index

Cell-penetrating peptides (CPPs) are considered as promising drug carriers by virtue of their potent cell-penetrating capacity. However, lack of targetability still represents a bottleneck for their systemic administration. Here, we synthesized a lysine-rich CPP named KRP and developed a tumor-targeted drug delivery system (DDS) by linking KRP and doxorubicin (DOX) with stable covalent bonds (thioether bond and amide bond). Through in vitro and in vivo tests, we confirmed that the multiple physicochemical properties of KRP endow KRP-DOX with multiple synergistic functions, including good biocompatibility and biodistribution, selective accumulation in tumor tissues, inclination to remain in tumor tissues and be internalized by tumor cells; stable covalent bonds prevent free DOX release from KRP-DOX in blood stream, shield normal tissues from the toxic effect of DOX, and lead to the majority of DOX delivery into tumor cells by KRP; lysosome escape of KRP-DOX ensures its tumor-killing effect. In addition, the simple chemical composition and modification reduce the risk of immunogenicity and metabolite toxicity. Our study provides a simple, safe, and efficient platform for tumor-targeted DDS.

A Self-Delivery Chimeric Peptide for Photodynamic Therapy Amplified Immunotherapy

In this paper, a self-delivery chimeric peptide PpIX-PEG₈ -KVPRNQDWL is designed for photodynamic therapy (PDT) amplified immunotherapy against malignant melanoma. After self-assembly into nanoparticles (designated as PPMA), this self-delivery system shows high drug loading rate, good dispersion, and stability as well as an excellent capability in producing reactive oxygen species (ROS). After cellular uptake, the ROS generated under light irradiation could induce the apoptosis and/or necrosis of tumor cells, which would subsequently stimulate the anti-tumor immune response. On the other hand, the melanoma specific antigen (KVPRNQDWL) peptide could also activate the specific cytotoxic T cells for anti-tumor immunity. Compared to immunotherapy alone, the combined photodynamic immunotherapy exhibits significantly enhanced inhibition of melanoma growth. Both in vitro and in vivo investigations confirm that PDT of PPMA has a positive effect on anti-tumor immune response. This self-delivery system demonstrates a great potential of this PDT amplified immunotherapy strategy for advanced or metastatic tumor treatment.

Tumor heterogeneity and nanoparticle-mediated tumor targeting: the importance of delivery system personalization

After the discovery of the enhanced permeability and retention effect in 1986, it was envisioned that nanoparticle-mediated tumor-targeted delivery of chemotherapeutics would make a radical change in cancer therapy. However, after three decades of extensive research, only a few nanotherapeutics have been approved for clinical use. Although significant advantages of nanomedicines have been demonstrated in pre-clinical studies, clinical outcome was found to be variable. Advanced research has revealed that significant biochemical and structural variations exist between (and among) different tumors. These variations can considerably affect the tumor delivery and efficacy of nanomedicines. Tumor penetration is an important determining factor for positive therapeutic outcome and same nanomedicine can show diverse efficacy against different tumors depending on the extent of tumor accumulation and penetration. Recent research has started shading light on how the tumor variations can influence nanoparticle tumor delivery. These findings indicate that there is no "ideal" design of nanoparticles for exhibiting equally high efficacy against a broad spectrum of tumors. For achieving maximum benefit of the nanotherapeutics, it is necessary to analyze the tumor microenvironment for understanding the biological and structural characteristics of the tumor. Designing of the nanomedicine should be done according to the tumor characteristics. In this comprehensive review, we have first given a brief overview of the design characteristics of nanomedicine which impact their tumor delivery. Then we discussed about the variability in the tumor architecture and how it influences nanomedicine delivery. Finally, we have discussed the possibility of delivery system personalization based on the tumor characteristics.

Tumor-Acidity-Cleavable Maleic Acid Amide (TACMAA): A Powerful Tool for Designing Smart Nanoparticles To Overcome Delivery Barriers in Cancer Nanomedicine

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 (pH_e ≈ 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.