Self-Assembled pH-Responsive Polymeric Micelles for Highly Efficient, Noncytotoxic Delivery of Doxorubicin Chemotherapy To Inhibit Macrophage Activation: In Vitro Investigation

Hydrogen-bonded cytosine-endowed supramolecular polymeric nanogels: Highly efficient cancer cell targeting and enhanced therapeutic efficacy

We demonstrate that cytosine moieties within physically cross-linked supramolecular polymers not only manipulate drug delivery and release, but also confer specific targeting of cancer cells to effectively enhance the safety and efficacy of chemotherapy-and thus hold significant potential as a new perspective for development of drug delivery systems. Herein, we successfully developed physically cross-linked supramolecular polymers (PECH-PEG-Cy) comprised of hydrogen-bonding cytosine pendant groups, hydrophilic poly(ethylene glycol) side chains, and a hydrophobic poly(epichlorohydrin) main chain. The polymers spontaneously self-assemble into a reversibly hydrogen-bonded network structure induced by cytosine and directly form spherical nanogels in aqueous solution. Nanogels with a high hydrogen-bond network density (i.e., a higher content of cytosine moieties) exhibit outstanding long-term structural stability in cell culture substrates containing serum, whereas nanogels with a relatively low hydrogen-bond network density cannot preserve their structural integrity. The nanogels also exhibit numerous unique physicochemical characteristics in aqueous solution, such as a desirable spherical size, high biocompatibility with normal and cancer cells, excellent drug encapsulation capacity, and controlled pH-responsive drug release properties. More importantly, in vitro experiments conclusively indicate the drug-loaded PECH-PEG-Cy nanogels can selectively induce cancer cell-specific apoptosis and cell death via cytosine receptor-mediated endocytosis, without significantly harming normal cells. In contrast, control drug-loaded PECH-PEG nanogels, which lack cytosine moieties in their structure, can only induce cell death in cancer cells through non-specific pathways, which significantly inhibits the induction of apoptosis. This work clearly demonstrates that the cytosine moieties in PECH-PEG-Cy nanogels confer selective affinity for the surface of cancer cells, which enhances their targeted cellular uptake, cytotoxicity, and subsequent induction of programmed cell death in cancer cells.

HER2-Specific Peptide (LTVSPWY) and Antibody (Herceptin) Targeted Core Cross-Linked Micelles for Breast Cancer: A Comparative Study

This study aims to prepare a novel breast cancer-targeted micelle-based nanocarrier, which is stable in circulation, allowing intracellular drug release, and to investigate its cytotoxicity, apoptosis, and cytostatic effects, in vitro. The shell part of the micelle is composed of zwitterionic sulfobetaine ((N-3-sulfopropyl-N,N-dimethylamonium)ethyl methacrylate), while the core part is formed by another block, consisting of AEMA (2-aminoethyl methacrylamide), DEGMA (di(ethylene glycol) methyl ether methacrylate), and a vinyl-functionalized, acid-sensitive cross-linker. Following this, a targeting agent (peptide (LTVSPWY) and antibody (Herceptin®)), in varying amounts, were coupled to the micelles, and they were characterized by 1H NMR, FTIR (Fourier-transform infrared spectroscopy), Zetasizer, BCA protein assay, and fluorescence spectrophotometer. The cytotoxic, cytostatic, apoptotic, and genotoxic effects of doxorubicin-loaded micelles were investigated on SKBR-3 (human epidermal growth factor receptor 2 (HER2)-positive) and MCF10-A (HER2-negative). According to the results, peptide-carrying micelles showed a higher targeting efficiency and better cytostatic, apoptotic, and genotoxic activities than antibody-carrying and non-targeted micelles. Also, micelles masked the toxicity of naked DOX on healthy cells. In conclusion, this nanocarrier system has great potential to be used in different drug-targeting strategies, by changing targeting agents and drugs.

Amplifying the efficacy of ALA-based prodrugs for photodynamic therapy using nanotechnology

5-aminolevulinic acid (ALA) is a clinically approved prodrug involved in intracellular Heme biosynthesis to produce the natural photosensitizer (PS) Protoporphyrin IX (PpIX). ALA based photodynamic therapy (PDT) has been used to treat various malignant and non-malignant diseases. However, natural ALA has disadvantages such as weak lipophilicity, low stability and poor bioavailability, greatly reducing its clinical performance. The emerging nanotechnology is expected to address these limitations and thus improve the therapeutic outcomes. Herein, we summarized important recent advances in the design of ALA-based prodrugs using nanotechnology to improve the efficacy of PDT. The potential limitations and future perspectives of ALA-based nanomedicines are also briefly presented and discussed.

Doxorubicin-induced cardiotoxicity: causative factors and possible interventions

OBJECTIVES

Doxorubicin (Dox) belongs to the anthracycline drug classification and is a widely administered chemotherapeutic. However, Dox use in therapy is limited by its cardiotoxicity, representing a significant drawback of Dox treatment applicability. A large amount of current research is on reducing Dox-induced cardiotoxicity by developing targeted delivery systems and investigating cardiotoxicity mechanisms. Recently, discrepancies have challenged the traditional understanding of Dox metabolism, mechanisms of action and cardiotoxicity drivers. This review summarises the current knowledge around Dox's metabolism, mechanisms of anticancer activity, and delivery systems and offers a unique perspective on the relationships between several proposed mechanisms of Dox-induced cardiotoxicity.

KEY FINDINGS

While there is a strong understanding of Dox's pharmacokinetic properties, it is unclear which enzymes contribute to Dox metabolism and how Dox induces its cytotoxic effect in neoplastic and non-neoplastic cells. Evidence suggests that there are several potentially synergistic mechanisms involved in Dox-induced cardiotoxicity.

SUMMARY

It has become clear that Dox operates in a multifactorial fashion dependent on cellular context. Accumulation of oxidative stress appears to be a common factor in cardiotoxicity mechanisms, highlighting the importance of novel delivery systems and antioxidant therapies.

Biomedical Polyurethanes for Anti-Cancer Drug Delivery Systems: A Brief, Comprehensive Review

With the intensive development of polymeric biomaterials in recent years, research using drug delivery systems (DDSs) has become an essential strategy for cancer therapy. Various DDSs are expected to have more advantages in anti-neoplastic effects, including easy preparation, high pharmacology efficiency, low toxicity, tumor-targeting ability, and high drug-controlled release. Polyurethanes (PUs) are a very important kind of polymers widely used in medicine, pharmacy, and biomaterial engineering. Biodegradable and non-biodegradable PUs are a significant group of these biomaterials. PUs can be synthesized by adequately selecting building blocks (a polyol, a di- or multi-isocyanate, and a chain extender) with suitable physicochemical and biological properties for applications in anti-cancer DDSs technology. Currently, there are few comprehensive reports on a summary of polyurethane DDSs (PU-DDSs) applied for tumor therapy. This study reviewed state-of-the-art PUs designed for anti-cancer PU-DDSs. We studied successful applications and prospects for further development of effective methods for obtaining PUs as biomaterials for oncology.

Solid-Phase Synthesized Copolymers for the Assembly of pH-Sensitive Micelles Suitable for Drug Delivery Applications

Diblock copolymers of polyhistidine are known for their self-assembly into micelles and their pH-dependent disassembly due to the amphiphilic character of the copolymer and the unsaturated imidazole groups that undergo a hydrophobic-to-hydrophilic transition in an acidic pH. This property has been largely utilized for the design of drug delivery systems that target a tumor environment possessing a slightly lower extracellular pH (6.8–7.2). The main purpose of this study was to investigate the possibility of designed poly(ethylene glycol)-polyhistidine sequences synthesized using solid-phase peptide synthesis (SPPS), to self-assemble into micelles, to assess the ability of the corresponding micelles to be loaded with doxorubicin (DOX), and to investigate the drug release profile at pH values similar to a malignant extracellular environment. The designed and assembled free and DOX-loaded micelles were characterized from a physico-chemical point of view, their cytotoxicity was evaluated on a human breast cancer cell line (MDA-MB-231), while the cellular areas where micelles disassembled and released DOX were assessed using immunofluorescence. We concluded that the utilization of SPPS for the synthesis of the polyhistidine diblock copolymers yielded sequences that behaved similarly to the copolymeric sequences synthesized using ring-opening polymerization, while the advantages of SPPS may offer facile tuning of the histidine site or the attachment of a large variety of functional molecules.

PH Responsive Polyurethane for the Advancement of Biomedical and Drug Delivery

Due to the specific physiological pH throughout the human body, pH-responsive polymers have been considered for aiding drug delivery systems. Depending on the surrounding pH conditions, the polymers can undergo swelling or contraction behaviors, and a degradation mechanism can release incorporated substances. Additionally, polyurethane, a highly versatile polymer, has been reported for its biocompatibility properties, in which it demonstrates good biological response and sustainability in biomedical applications. In this review, we focus on summarizing the applications of pH-responsive polyurethane in the biomedical and drug delivery fields in recent years. In recent studies, there have been great developments in pH-responsive polyurethanes used as controlled drug delivery systems for oral administration, intravaginal administration, and targeted drug delivery systems for chemotherapy treatment. Other applications such as surface biomaterials, sensors, and optical imaging probes are also discussed in this review.

Sequential post-polymerization modification of a pentafluorophenyl ester-containing homopolymer: a convenient route to effective pH-responsive nanocarriers for anticancer drugs

Recently, pH-responsive polymeric micelles have gained significant attention as effective carriers for anti-cancer drug delivery. Herein, pH-responsive polymeric micelles were constructed by a simple post-polymerization modification of a single homopolymer, poly(pentafluorophenyl acrylate) (PPFPA). The PPFPA was first subjected to modification with 1-amino-2-propanol yielding the amphiphilic copolymer of poly(pentafluorophenyl acrylate)-ran-poly(N-(2-hydroxypropyl acrylamide)). A series of amphiphilic random copolymers of different compositions could self-assemble into spherical micelles with a unimodal size distribution in aqueous solution. Then, 1-(3-aminopropyl)imidazole (API), a reagent to introduce charge conversional entities, was reacted with the remaining PPFPA segment in the micellar core resulting in API-modified micelles which can encapsulate doxorubicin (DOX), a hydrophobic anti-cancer drug. As monitored by dynamic light scattering, the API-modified micelles underwent disintegration upon pH switching from 7.4 to 5.0, presumably due to imidazolyl group protonation. This pH-responsiveness of the API-modified micelles was responsible for the faster and greater in vitro DOX release in an acidic environment than neutral pH. Cellular uptake studies revealed that the developed carriers were internalized into MDA-MB-231 cells within 30 min via endocytosis and exhibited cytotoxicity in a dose-dependent manner.

Photo-Responsive Supramolecular Micelles for Controlled Drug Release and Improved Chemotherapy

Development of stimuli-responsive supramolecular micelles that enable high levels of well-controlled drug release in cancer cells remains a grand challenge. Here, we encapsulated the antitumor drug doxorubicin (DOX) and pro-photosensitizer 5-aminolevulinic acid (5-ALA) within adenine-functionalized supramolecular micelles (A-PPG), in order to achieve effective drug delivery combined with photo-chemotherapy. The resulting DOX/5-ALA-loaded micelles exhibited excellent light and pH-responsive behavior in aqueous solution and high drug-entrapment stability in serum-rich media. A short duration (1-2 min) of laser irradiation with visible light induced the dissociation of the DOX/5-ALA complexes within the micelles, which disrupted micellular stability and resulted in rapid, immediate release of the physically entrapped drug from the micelles. In addition, in vitro assays of cellular reactive oxygen species generation and cellular internalization confirmed the drug-loaded micelles exhibited significantly enhanced cellular uptake after visible light irradiation, and that the light-triggered disassembly of micellar structures rapidly increased the production of reactive oxygen species within the cells. Importantly, flow cytometric analysis demonstrated that laser irradiation of cancer cells incubated with DOX/5-ALA-loaded A-PPG micelles effectively induced apoptotic cell death via endocytosis. Thus, this newly developed supramolecular system may offer a potential route towards improving the efficacy of synergistic chemotherapeutic approaches for cancer.

CO2-Responsive Water-Soluble Conjugated Polymers for In Vitro and In Vivo Biological Imaging

Water-soluble conjugated polymers (WCPs) composed of a hydrophobic polythiophene main chain with hydrophilic tertiary amine side-chains can directly self-assemble into sphere-like nano-objects in an aqueous solution due to phase separation between the hydrophilic and hydrophobic segments of the polymeric structure. Due to the presence of gas-responsive tertiary amine moieties in the spherical structure, the resulting polymers rapidly and reversibly tune their structural features, surface charge, and fluorescence performance in response to alternating carbon dioxide (CO₂) and nitrogen (N₂) bubbling, which leads to significantly enhanced fluorescence and surface charge switching properties and a stable cycle of on and off switching response. In vitro studies confirmed that the CO₂-treated polymers exhibited extremely low cytotoxicity and enhanced cellular uptake ability in normal and tumor cells, and thus possess significantly improved fluorescence stability, distribution, and endocytic uptake efficiency within cellular organisms compared to the pristine polymer. More importantly, in vivo assays demonstrated that the CO₂-treated polymers displayed excellent biocompatibility and high fluorescence enhancement in living zebrafish, whereas the fluorescence intensity and stability of zebrafish incubated with the pristine polymer decreased linearly over time. Thus, these CO₂ and N₂-responsive WCPs could potentially be applied as multifunctional fluorescent probes for in vivo biological imaging.

Spontaneous Self-Assembly of Single-Chain Amphiphilic Polymeric Nanoparticles in Water

Single-chain polymeric nanoparticles (SCPNs) have great potential as functional nanocarriers for drug delivery and bioimaging, but synthetic challenges in terms of final yield and purification procedures limit their use. A new concept to modify and improve the synthetic procedures used to generate water-soluble SCPNs through amphiphilic interactions has been successfully exploited. We developed a new ultrahigh molecular weight amphiphilic polymer containing a hydrophobic poly(epichlorohydrin) backbone and hydrophilic poly(ethylene glycol) side chains. The polymer spontaneously self-assembles into SCPNs in aqueous solution and does not require subsequent purification. The resulting SCPNs possess a number of distinct physical properties, including a uniform hydrodynamic nanoparticle diameter of 10-15 nm, extremely low viscosity and a desirable spherical-like morphology. Concentration-dependent studies demonstrated that stable SCPNs were formed at high concentrations up to 10 mg/mL in aqueous solution, with no significant increase in solution viscosity. Importantly, the SCPNs exhibited high structural stability in media containing serum or phosphate-buffered saline and showed almost no change in hydrodynamic diameter. The combination of these characteristics within a water-soluble SCPN is highly desirable and could potentially be applied in a wide range of biomedical fields. Thus, these findings provide a path towards a new, innovative route for the development of water-soluble SCPNs.

Glycosylated Nanoparticles Derived from RAFT Polymerization for Effective Drug Delivery to Macrophages

The functional group tolerance and simplicity of reversible addition fragmentation chain transfer (RAFT) polymerization enable its use in the preparation of a wide range of functional polymer architectures for a variety of applications, including drug delivery. Given the role of tumor-associated macrophages (TAMs) in cancer and their dependence on the tyrosine kinase receptor FMS (CSF-1R), the key aim of this work was to achieve effective delivery of an FMS inhibitor to cells using a polymer delivery system. Such a system has the potential to exploit biological features specific to macrophages and therefore provide enhanced selectivity. Building on our prior work, we have prepared RAFT polymers based on a poly(butyl methacrylate-co-methacrylic acid) diblock, which were extended with a hydrophilic block, a cross-linker, and a mannose-based monomer scaffold, exploiting the abundance of macrophage mannose receptors (MMRs, CD206) on the surface of macrophages. We demonstrate that the prepared polymers can be assembled into nanoparticles and are successfully internalized into macrophages, in part, via the MMR (CD206). Finally, we showcase the developed nanoparticles in the delivery of an FMS inhibitor to cells, resulting in inhibition of the FMS receptor. As such, this study lays the groundwork for further drug-delivery studies aimed at specifically targeting TAMs with molecularly targeted therapeutics.

Adenine-Functionalized Supramolecular Micelles for Selective Cancer Chemotherapy

Functional supramolecular micelles containing self-complementary multiple hydrogen bonding adenine groups (A-PPG) can spontaneously self-assemble into stable nanosized micelles in aqueous solution. These micelles can be used to selectively deliver anticancer drugs to cancer cells and effectively promote tumor cell death via apoptosis, without harming normal cells. The drug-loaded micelles exhibit tunable drug-loading capacity and rapid pH-triggered drug release under acidic conditions, as well as a high drug-entrapment stability in serum-rich media due to the reversible hydrogen-bonded adenine-adenine interactions within the micellar interior; these properties are critical to achieving effective chemotherapeutic drug delivery and controlled drug release. In vitro assays show that the drug-loaded micelles exert significant cytotoxic effects on cancer cells, with minimal effects on normal cells under physiological conditions. Cytotoxicity assays using A-PPG micelles loaded with different anticancer drugs confirm these effects. Importantly, cellular internalization and flow cytometric analyses demonstrate that the adenine moieties within A-PPG micelles significantly increase selective endocytic uptake of the supramolecular micelles by cancer cells, which in turn induce apoptotic cell death and substantially enhance the response to chemotherapy. Thus, A-PPG micelles can improve the safety and efficacy of cancer chemotherapy.

Recent Advances in Nanocarrier-Assisted Therapeutics Delivery Systems

Nanotechnologies have attracted increasing attention in their application in medicine, especially in the development of new drug delivery systems. With the help of nano-sized carriers, drugs can reach specific diseased areas, prolonging therapeutic efficacy while decreasing undesired side-effects. In addition, recent nanotechnological advances, such as surface stabilization and stimuli-responsive functionalization have also significantly improved the targeting capacity and therapeutic efficacy of the nanocarrier assisted drug delivery system. In this review, we evaluate recent advances in the development of different nanocarriers and their applications in therapeutics delivery.

Photosensitive Supramolecular Micelle-Mediated Cellular Uptake of Anticancer Drugs Enhances the Efficiency of Chemotherapy

The development of stimuli-responsive supramolecular micelles with high drug-loading contents that specifically induce significant levels of apoptosis in cancer cells remains challenging. Herein, we report photosensitive uracil-functionalized supramolecular micelles that spontaneously form via self-assembly in aqueous solution, exhibit sensitive photo-responsive behavior, and effectively encapsulate anticancer drugs at high drug-loading contents. Cellular uptake analysis and double-staining flow cytometric assays confirmed the presence of photo-dimerized uracil groups within the irradiated micelles remarkably enhanced endocytic uptake of the micelles by cancer cells and subsequently led to higher levels of apoptotic cell death, and thus improved the therapeutic effect in vitro. Thus, photo-dimerized uracil-functionalized supramolecular micelles may potentially represent an intelligent nanovehicle to improve the safety, efficacy, and applicability of cancer chemotherapy, and could also enable the development of nucleobase-based supramolecular micelles for multifunctional biomaterials and novel biomedical applications.

Self-Assembled Supramolecular Micelles with pH-Responsive Properties for More Effective Cancer Chemotherapy

pH-Responsive hydrogen-bonded supramolecular micelles, composed of a water-soluble poly(ethylene glycol) polymer with two terminal sextuple hydrogen bonding groups, can spontaneously organize in aqueous media to give well-defined, uniformly sized spherical micelles. The supramolecular micelles exhibit a number of unique physical characteristics, such as interesting amphiphilic behavior, desirable micellar size and nanospherical morphology, excellent biocompatibility, tailorable drug-loading capacities, and high structural stability in media containing serum or red blood cells. In addition, the drug release kinetics of drug-loaded micelles can be easily manipulated to achieve the desired release profile by regulating the environmental pH, thus these micelles are highly attractive candidates as an intelligent drug carrier system for cancer therapy. Cytotoxicity assays showed that the drug-loaded micelles induced pH-dependent intracellular drug release and exerted strong antiproliferative and cytotoxic activities toward cancer cells. Importantly, cellular uptake and flow cytometric analyses confirmed that a mildly acidic intracellular environment significantly increased cellular internalization of the drug-loaded micelles and subsequent drug release in the cytoplasm and nucleus of cancer cells, resulting in more effective induction of apoptotic cell death. Thus, this system may provide an efficient route toward achieving the fundamental properties and practical realization of pH-sensitive drug-delivery systems for chemotherapy.

pH-Responsive Micelles From an Oleic Acid Tail and Propionic Acid Heads Dendritic Amphiphile for the Delivery of Antibiotics

The aim of this study was to synthesize a novel biocompatible pH-responsive oleic acid-based dendritic lipid amphiphile (OLA-SPDA) which self-assembled into stable micelles (OLA-SPDA -micelles) with a relatively low critical micelle concentration (CMC) of 5.6 × 10^-6 M. The formulated micelles had particle size, polydispersity index (PDI) and zeta potential (ZP) of 84.16 ± 0.184 nm, 0.199 ± 0.011 and -42.6 ± 1.98 mV, respectively, at pH 7.4. The vancomycin (VCM) encapsulation efficiency was 78.80 ± 3.26%. The micelles demonstrated pH-responsiveness with an increase in particle size to 141.1 ± 0.0707 nm and a much faster release profile at pH 6.0, as compared to pH 7.4. The minimum inhibitory concentration (MIC) of VCM-OLA-SPDA-micelle against methicillin-resistant staphylococcus aureus (MRSA) was 8-fold lower compared to bare VCM, and the formulation had a 4-fold lower MIC at pH 6.0 when compared to the formulation's MIC at pH 7.4. MRSA viability assay showed the micelles had a percentage killing of 93.39% when compared bare-VCM (58.21%) at the same MIC (0.98 μg/mL). In vivo mice (BALB/c) skin infection models showed an 8-fold reduction in MRSA burden after treatment with VCM-OLA-SPDA-micelles when compared with bare VCM. The above results suggest that pH-responsive VCM-OLA-SPDA-micelles has the potential to be an effective carrier to enhance therapeutic outcomes against infections characterised by low pH.

Preparation of polyethylene glycol-polyacrylic acid block copolymer micelles with pH/hypoxic dual-responsive for tumor chemoradiotherapy

Block copolymers of poly(ethylene glycol)-poly(acrylic acid) linked with metronidazole (MN-PAA-PEG) were prepared via carbodiimide and esterification methods, and self-assembled into core-shell micelles as nano radiosensitizers and carriers of doxorubicin (DOX) delivery. These DOX/MN-PAA-PEG micelles exhibited good pH value and hypoxia dual-responsive properties via analyzing the change of micelle size and drug‒release behavior under hypoxia humor condition. The results of the cell test indicated that DOX was efficiently delivered by DOX/MN-PAA-PEG micelles into the cell nuclei. Compared to 22.4 % of their DOX release under pH 7.4, the rate of DOX release from DOX/MN-PAA-PEG micelles under reducing condition (pH 5.0) was up to 55.9 %. DOX-loaded micelles under 600 MU electron radiation and hypoxia induced the rapidest apoptosis of the tumor-cells, indicating the synergistic effect of their radiotherapy and chemotherapy from the prepared micelles. In vivo investigation and fluorescence imaging revealed that MN-PAA-PEG possessed no toxicity on main organs, and DOX/MN-PAA-PEG micelles were mainly accumulated in the tumor site at 10 h of post-injection, suggesting their good passive tumor-targeted effect. These results suggested that DOX/MN-PAA-PEG micelles were promising candidates for chemoradiotherapy on tumor.

A glance over doxorubicin based-nanotherapeutics: From proof-of-concept studies to solutions in the market

Cancer is one of the leading causes of death worldwide and, as such, efforts are being done to find new chemotherapeutic drugs or, alternatively, novel approaches for the delivery of old ones. In this scope, when used as vehicles for drugs, nanomaterials may potentially maximize the efficacy of the treatment and reduce its side effects, for example by a change in drug's pharmacokinetics, cell targeting and/or specific stimuli-responsiveness. This is the case of doxorubicin (DOX) that presents a broad spectrum of activity and is one of the most widely used chemotherapeutic drugs as first-line treatment. Indeed, DOX is a very interesting example of a drug for which several nanosized delivery systems have been developed over the years. While it is true that some of these systems are already in the market, it is also true that research on this subject remains very active and that there is a continuing search for new solutions. In this sense, this review takes the example of doxorubicin, not so much with the focus on the drug itself, but rather as a case study around which very diverse and imaginative nanotechnology approaches have emerged.

Multifunctional adenine-functionalized supramolecular micelles for highly selective and effective cancer chemotherapy

Adenine-functionalized supramolecular micelles are rapidly endocytosed by cancer cells and enable selective induction of tumor cell death, without harming normal cells.

Hydrogen-bonded supramolecular micelle-mediated drug delivery enhances the efficacy and safety of cancer chemotherapy

Multiple hydrogen-bonded supramolecular polymers tend to form stable spherical micelles with oppositely charged anticancer drugs in biological environments, which improves cellular drug uptake and more effectively induces apoptosis in cancer cells.

Monoclonal antibody therapy of solid tumors: clinical limitations and novel strategies to enhance treatment efficacy

Monoclonal antibodies (mAbs) have become a cornerstone in the therapeutic guidelines of a wide range of solid tumors. The targeted nature of these biotherapeutics has improved treatment outcomes by offering enhanced specificity to reduce severe side effects experienced with conventional chemotherapy. Notwithstanding, poor tumor tissue penetration and the heterogeneous distribution achieved therein are prominent drawbacks that hamper the clinical efficacy of therapeutic antibodies. Failure to deliver efficacious doses throughout the tumor can lead to treatment failure and the development of acquired resistance mechanisms. Comprehending the morphological and physiological characteristics of solid tumors and their microenvironment that affect tumor penetration and distribution is a key requirement to improve clinical outcomes and realize the full potential of monoclonal antibodies in oncology. This review summarizes the essential architectural characteristics of solid tumors that obstruct macromolecule penetration into the targeted tissue following systemic delivery. It further describes mechanisms of resistance elucidated for blockbuster antibodies for which extensive clinical data exists, as a way to illustrate various modes in which cancer cells can overcome the anticancer activity of therapeutic antibodies. Thereafter, it describes novel strategies designed to improve clinical outcomes of mAbs by increasing potency and/or improving tumor delivery; focusing on the recent clinical success and growing clinical pipeline of antibody-drug conjugates, immune checkpoint inhibitors and nanoparticle-based delivery systems.