Acid-sensitive prodrugs; a promising approach for site-specific and targeted drug release

Drugs administered through conventional formulations are devoid of targeting and often spread to various undesired sites, leading to sub-lethal concentrations at the site of action and the emergence of undesired effects. Hence, therapeutic agents should be delivered in a controlled manner at target sites. Currently, stimuli-based drug delivery systems have demonstrated a remarkable potential for the site-specific delivery of therapeutic moieties. pH is one of the widely exploited stimuli for drug delivery as several pathogenic conditions such as tumor cells, infectious and inflammatory sites are characterized by a low pH environment. This review article aims to demonstrate various strategies employed in the design of acid-sensitive prodrugs, providing an overview of commercially available acid-sensitive prodrugs. Furthermore, we have compiled the progress made for the development of new acid-sensitive prodrugs currently undergoing clinical trials. These prodrugs include albumin-binding prodrugs (Aldoxorubicin and DK049), polymeric micelle (NC-6300), polymer conjugates (ProLindac™), and an immunoconjugate (IMMU-110). The article encompasses a broad spectrum of studies focused on the development of acid-sensitive prodrugs for anticancer, antibacterial, and anti-inflammatory agents. Finally, the challenges associated with the acid-sensitive prodrug strategy are discussed, along with future directions.

GSH-activable heterotrimeric nano-prodrug for precise synergistic therapy of TNBC

Combination chemotherapy is an effective approach for triple-negative breast cancer (TNBC) therapy, especially when drugs are administered at specific optimal ratios. However, at present, strategies involving precise and controllable ratios based on effective loading and release of drugs are unavailable. Herein, we designed and synthesized a glutathione (GSH)--responsive heterotrimeric prodrug and formulated it with an amphiphilic polymer to obtain nanoparticles (DSSC2 NPs) for precise synergistic chemotherapy of TNBC. The heterotrimeric prodrug was prepared using docetaxel (DTX) and curcumin (CUR) at the optimal synergistic ratio of 1: 2. DTX and CUR were covalently conjugated by disulfide linkers. Compared with control NPs, DSSC2 NPs had quantitative/ratiometric drug loading, high drug co-loading capacity, better colloidal stability, and less premature drug leakage. After systemic administration, DSSC2 NPs selectively accumulated in tumor tissues and released the encapsulated drugs triggered by high levels of GSH in cancer cells. In vitro and in vivo experiments validated that DSSC2 NPs released DTX and CUR at the predefined ratio and had a highly synergistic therapeutic effect on tumor suppression in TNBC, which can be attributed to ratiometric drug delivery and synchronous drug activation. Altogether, the heterotrimeric prodrug delivery system developed in this study represents an effective and novel approach for combination chemotherapy.

Research Progress on Stimulus-Responsive Polymer Nanocarriers for Cancer Treatment

As drug carriers for cancer treatment, stimulus-responsive polymer nanomaterials are a major research focus. These nanocarriers respond to specific stimulus signals (e.g., pH, redox, hypoxia, enzymes, temperature, and light) to precisely control drug release, thereby improving drug uptake rates in cancer cells and reducing drug damage to normal cells. Therefore, we reviewed the research progress in the past 6 years and the mechanisms underpinning single and multiple stimulus-responsive polymer nanocarriers in tumour therapy. The advantages and disadvantages of various stimulus-responsive polymeric nanomaterials are summarised, and the future outlook is provided to provide a scientific and theoretical rationale for further research, development, and utilisation of stimulus-responsive nanocarriers.

Covalent Adaptable Polymethacrylate Networks by Hydrazide Crosslinking Via Isosorbide Levulinate Side Groups

Reversible crosslinking offers an attractive strategy to modify and improve the properties of polymer materials while concurrently enabling a pathway for chemical recycling. This can, for example, be achieved by incorporating a ketone functionality into the polymer structure to enable post-polymerization crosslinking with dihydrazides. The resulting covalent adaptable network contains acylhydrazone bonds cleavable under acidic conditions, thereby providing reversibility. In the present work, we regioselectively prepare a novel isosorbide monomethacrylate with a pendant levulinoyl group via a two-step biocatalytic synthesis. Subsequently, a series of copolymers with different contents of the levulinic isosorbide monomer and methyl methacrylate are prepared by radical polymerization. Using dihydrazides, these linear copolymers are then crosslinked via reaction with the ketone groups in the levulinic side chains. Compared to the linear prepolymers, the crosslinked networks exhibit enhanced glass transition temperatures and thermal stability, up to 170 and 286 °C, respectively. Moreover, the dynamic covalent acylhydrazone bonds are efficiently and selectively cleaved under acidic conditions to retrieve the linear polymethacrylates. We next show that recovered polymers can again be crosslinked with adipic dihydrazide, thus demonstrating the circularity of the materials. Consequently, we envision that these novel levulinic isosorbide-based dynamic polymethacrylate networks have great potential in the field of recyclable and reusable biobased thermoset polymers.

Design and Synthesis of Amphiphilic Graft Polyphosphazene Micelles for Docetaxel Delivery

The structural versatility of polydichlorophosphazene derived from the inestimable possibilities to functionalize the two halogens, attached to each phosphazene main chain unit, attracted increasing attention in the last decade. This uncountable chemical derivatization is doubled by the amphiphilic roleplay demonstrated by polyphosphazenes containing twofold side-chained hydrophilic and hydrophobic moieties. Thus, it is able to encapsulate specific bioactive molecules for various targeted nanomedicine applications. A new amphiphilic graft, polyphosphazenes (PPP/PEG-NH/Hys/MAB), was synthesized via the thermal ring-opening polymerization of hexachlorocyclotriphosphazene, followed by a subsequent two-step substitution reaction of chlorine atoms with hydrophilic methoxypolyethylene glycol amine/histamine dihydrochloride adduct (PEG-NH₂)/(Hys) and hydrophobic methyl-p-aminobenzoate (MAB), respectively. Fourier transform infrared spectroscopy (FTIR) and ¹H and ³¹P-nuclear magnetic resonance spectroscopy (NMR) have been used to validate the expected architectural assembly of the copolymer. Docetaxel loaded micelles based on synthesized PPP/PEG-NH/Hys/MAB were designed by dialysis method. The micelles size was evaluated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The drug release profiles from the PPP/PEG-NH/Hys/MAB micelles were established. In vitro cytotoxicity tests of PPP/PEG-NH/Hys/MAB micelles loaded with Docetaxel revealed that designed polymeric micelles exhibited an increased cytotoxic effect on MCF-7 cells.

Complex Structures Made Simple - Continuous Flow Production of Core Cross-Linked Polymeric Micelles for Paclitaxel Pro-Drug-Delivery

Translating innovative nanomaterials to medical products requires efficient manufacturing techniques that enable large-scale high-throughput synthesis with high reproducibility. Drug carriers in medicine embrace a complex subset of tasks calling for multifunctionality. Here, the synthesisof pro-drug-loaded core cross-linked polymeric micelles (CCPMs) in a continuous flow processis reported, which combines the commonly separated steps of micelle formation, core cross-linking, functionalization, and purification into a single process. Redox-responsive CCPMs are formed from thiol-reactive polypept(o)ides of polysarcosine-block-poly(S-ethylsulfonyl-l-cysteine) and functional cross-linkers based on dihydrolipoic acid hydrazide for pH-dependent release of paclitaxel. The precisely controlled microfluidic process allows the production of spherical micelles (D_h  = 35 nm) with low polydispersity values (PDI < 0.1) while avoiding toxic organic solvents and additives with unfavorable safety profiles. Self-assembly and cross-linking via slit interdigital micromixers produces 350-700 mg of CCPMs/h per single system, while purification by online tangential flow filtration successfully removes impurities (unimer ≤ 0.5%). The formed paclitaxel-loaded CCPMs possess the desired pH-responsive release profile, display stable drug encapsulation, an improved toxicity profile compared to Abraxane (a trademark of Bristol-Myers Squibb), and therapeutic efficiency in the B16F1-xenotransplanted zebrafish model. The combination of reactive polymers, functional cross-linkers, and microfluidics enables the continuous-flow synthesis of therapeutically active CCPMs in a single process.

Recent Progress in Bio-Responsive Drug Delivery Systems for Tumor Therapy

Spatially- and/or temporally-controlled drug release has always been the pursuit of drug delivery systems (DDSs) to achieve the ideal therapeutic effect. The abnormal pathophysiological characteristics of the tumor microenvironment, including acidosis, overexpression of special enzymes, hypoxia, and high levels of ROS, GSH, and ATP, offer the possibility for the design of stimulus-responsive DDSs for controlled drug release to realize more efficient drug delivery and anti-tumor activity. With the help of these stimulus signals, responsive DDSs can realize controlled drug release more precisely within the local tumor site and decrease the injected dose and systemic toxicity. This review first describes the major pathophysiological characteristics of the tumor microenvironment, and highlights the recent cutting-edge advances in DDSs responding to the tumor pathophysiological environment for cancer therapy. Finally, the challenges and future directions of bio-responsive DDSs are discussed.

pH-Responsive Polymer Nanomaterials for Tumor Therapy

The complexity of the tumor microenvironment presents significant challenges to cancer therapy, while providing opportunities for targeted drug delivery. Using characteristic signals of the tumor microenvironment, various stimuli-responsive drug delivery systems can be constructed for targeted drug delivery to tumor sites. Among these, the pH is frequently utilized, owing to the pH of the tumor microenvironment being lower than that of blood and healthy tissues. pH-responsive polymer carriers can improve the efficiency of drug delivery in vivo, allow targeted drug delivery, and reduce adverse drug reactions, enabling multifunctional and personalized treatment. pH-responsive polymers have gained increasing interest due to their advantageous properties and potential for applicability in tumor therapy. In this review, recent advances in, and common applications of, pH-responsive polymer nanomaterials for drug delivery in cancer therapy are summarized, with a focus on the different types of pH-responsive polymers. Moreover, the challenges and future applications in this field are prospected.

Hollow structures as drug carriers: Recognition, response, and release

Hollow structures have demonstrated great potential in drug delivery owing to their privileged structure, such as high surface-to-volume ratio, low density, large cavities, and hierarchical pores. In this review, we provide a comprehensive overview of hollow structured materials applied in targeting recognition, smart response, and drug release, and we have addressed the possible chemical factors and reactions in these three processes. The advantages of hollow nanostructures are summarized as follows: hollow cavity contributes to large loading capacity; a tailored structure helps controllable drug release; variable compounds adapt to flexible application; surface modification facilitates smart responsive release. Especially, because the multiple physical barriers and chemical interactions can be induced by multishells, hollow multishelled structure is considered as a promising material with unique loading and releasing properties. Finally, we conclude this review with some perspectives on the future research and development of the hollow structures as drug carriers.

DPD simulations on mixed polymeric DOX-loaded micelles assembled from PCL-SS-PPEGMA/PDEA-PPEGMA and their dual pH/reduction-responsive release

The design of mixed polymeric micelles by a combination of two or more dissimilar polymers is a potential strategy to achieve multiple stimuli-response for anti-cancer drug delivery. However, their drug loading co-micellization behavior and multiple stimuli-responsive drug release mechanism have been poorly understood at the mesoscopic level, especially in the system that involves reduction-response due to the difficulty of simulation on the cleavage of chemical bonds. In this work, the co-micellization behavior, drug distribution regularities and dual pH/reduction-responsive drug release process of mixed micelles formed by disulfide-linked polycaprolactone-b-polyethylene glycol methyl ether methacrylate (PCL-SS-PPEGMA) and poly(ethylene glycol) methyl ether-b-poly(N,N-diethylamino ethyl methacrylate) (PDEA-PPEGMA) were studied by dissipative particle dynamics (DPD) mesoscopic simulations. A dedicated bond-breaking script was employed to accomplish the disulfide bond-breaking simulations. The results showed that PCL₅₅-SS-PPEGMA₁₀ and PDEA₃₄-PPEGMA₁₁ could be well mixed to form superior DOX-loaded micelles with good drug-loading capacity and drug-controlled release performance. To prepare the DOX-loaded micelles with optimized properties, the simulation results suggested the feed ratio of DOX:PCL₅₅-SS-PPEGMA₁₀:PDEA₃₄-PPEGMA₁₁ set to 3:4:4. Compared with the two single stimuli-response, the dual pH/reduction-response process perfectly combined both pH-response and reduction-response together, providing a higher release rate of DOX. Therefore, this study provides theoretical guidance aimed at the property optimization and micellar structure design of the dual pH/reduction-responsive mixed micelles.

Emerging nanotaxanes for cancer therapy

The clinical application of taxane (including paclitaxel, docetaxel, and cabazitaxel)-based formulations is significantly impeded by their off-target distribution, unsatisfactory release, and acquired resistance/metastasis. Recent decades have witnessed a dramatic progress in the development of high-efficiency, low-toxicity nanotaxanes via the use of novel biomaterials and nanoparticulate drug delivery systems (nano-DDSs). Thus, in this review, the achievements of nanotaxanes-targeted delivery and stimuli-responsive nano-DDSs-in preclinical or clinical trials have been outlined. Then, emerging nanotherapeutics against tumor resistance and metastasis have been overviewed, with a particular emphasis on synergistic therapy strategies (e.g., combination with surgery, chemotherapy, radiotherapy, biotherapy, immunotherapy, gas therapy, phototherapy, and multitherapy). Finally, the latest oral nanotaxanes have been briefly discussed.

pH-Sensitive Biomaterials for Drug Delivery

The development of precise and personalized medicine requires novel formulation strategies to deliver the therapeutic payloads to the pathological tissues, producing enhanced therapeutic outcome and reduced side effects. As many diseased tissues are feathered with acidic characteristics microenvironment, pH-sensitive biomaterials for drug delivery present great promise for the purpose, which could protect the therapeutic payloads from metabolism and degradation during in vivo circulation and exhibit responsive release of the therapeutics triggered by the acidic pathological tissues, especially for cancer treatment. In the past decades, many methodologies, such as acidic cleavage linkage, have been applied for fabrication of pH-responsive materials for both in vitro and in vivo applications. In this review, we will summarize some pH-sensitive drug delivery system for medical application, mainly focusing on the pH-sensitive linkage bonds and pH-sensitive biomaterials.

"Dual-Key-and-Lock" dual drug carrier for dual mode imaging guided chemo-photothermal therapy

Drug resistance and side effects are the two main problems of chemotherapy. In order to address these big challenges, p-PB@d-SiO₂, which has the ability to co-deliver both the hydrophobic drug doxorubicin hydrochloride (DOX) and the hydrophilic drug ibuprofen (IBU), is constructed to achieve synergistic treatment. The drug-loaded nanoparticle consists of porous Prussian blue (p-PB) as the core and dendrimer-like SiO₂ (d-SiO₂) as the shell, which is further thiolated and coated with polyethylene glycol thiol (HS-PEG) to form the "Dual-Key-and-Lock" drug carrier p-PB@d-SiO₂-SS-PEG. The locked drugs can only be released in the presence of cooperative triggers, i.e., a high glutathione concentration (the first key) and an acidic environment (the second key). The "dual key"-triggered release is much more significant in cancer lesions than in normal tissues, reducing side effects. Furthermore, cell viability experiments highlight the superior therapeutic efficacy of the dual-drug-loaded nanoparticles compared with the single-drug systems (60%, 73% and 86% vs. 56%, 68%, and 76% at 100, 200 and 500 μg mL^-1, respectively). In vitro and in vivo experiments demonstrate the potential application of p-PB@d-SiO₂-SS-PEG for dual-mode fluorescence and magnetic-resonance-imaging-guided chemo-photothermal therapy. The "Dual-Key-and-Lock" drug carrier system exhibits the "1 + 1 > 2" effect, demonstrating its excellent performance in synergy therapy for improved therapeutic efficiency and thereby reducing conventional drug resistance and side effects.

Novel pH-Sensitive Urushiol-Loaded Polymeric Micelles for Enhanced Anticancer Activity

Purpose

The aim of this study was to develop a means of improving the bioavailability and anticancer activity of urushiol by developing an urushiol-loaded novel tumor-targeted micelle delivery system based on amphiphilic block copolymer poly(ethylene glycol)-b-poly-(β-amino ester) (mPEG-PBAE).

Materials and Methods

We synthesized four different mPEG-PBAE copolymers using mPEG-NH₂ with different molecular weights or hydrophobicity levels. Of these, we selected the mPEG₅₀₀₀-PBAE-C₁₂ polymer and used it to develop an optimized means of preparing urushiol-loaded micelles. Response surface methodology was used to optimize this formulation process. The micellar properties, including particle size, pH sensitivity, drug release dynamics, and critical micelle concentrations, were characterized. We further used the MCF-7 human breast cancer cell line to explore the cytotoxicity of these micelles in vitro and assessed their pharmacokinetics, tissue distribution, and antitumor activity in vivo.

Results

The resulting micelles had a mean particle size of 160.1 nm, a DL value of 23.45%, and an EE value of 80.68%. These micelles were found to release their contents in a pH-sensitive manner in vitro, with drug release being significantly accelerated at pH 5.0 (98.74% in 72 h) without any associated burst release. We found that urushiol-loaded micelles were significantly better at inducing MCF-7 cell cytotoxicity compared with free urushiol, with an IC₅₀ of 1.21 mg/L. When these micelles were administered to tumor model animals in vivo, pharmacokinetic analysis revealed that the total AUC and MRT of these micelles were 2.28- and 2.53-fold higher than that of free urushiol, respectively. Tissue distribution analyses further revealed these micelles to mediate significantly enhanced tumor urushiol accumulation.

Conclusion

The pH-responsive urushiol-loaded micelles described in this study may be ideally suited for clinical use for the treatment of breast cancer.

Synthesis and Self-Assembly of the pH-Responsive Anionic Copolymers for Enhanced Doxorubicin-Loading Capacity

Polyelectrolyte complex micelles self-assembled from an ionic polymer and oppositely charged small molecules are a promising drug delivery system. In this study, the anionic block copolymers composed of poly(ethylene glycol), poly(ε-caprolactone), and carboxyl modified poly(ε-caprolactone), COOH-PCEC, were designed to encapsulate doxorubicin (DOX) via electrostatic and hydrophobic interactions to form spherical micelles with a particle size of 90-140 nm. The higher payload capacity of these micelles than noncharged micelles of PCL-poly(ethylene glycol)-PCL (PCEC) was achieved, and it was strongly dependent on the composition of the micelles. In vitro drug release studies showed that the release of DOX from the micelles was faster at pH 5.5 than at pH 7.4, which was mainly due to the protonation of carboxyl groups and the solubility of DOX. Studies of intracellular uptake demonstrated that the DOX-loaded micelles could be internalized effectively by HeLa cells. In vitro cytotoxicity revealed that the blank COOH-PCEC micelles had a low cytotoxicity against both L929 and HeLa cells. However, the DOX-loaded micelles inhibited the growth of HeLa cells remarkably, demonstrating their potential for use as an efficient carrier for the delivery of DOX.

Recent advances of dicyano-based materials in biology and medicine

We highlight the development of dicyano-based fluorescent materials in biology and medicine.

A sequential enzyme-activated and light-triggered pro-prodrug nanosystem for cancer detection and therapy

A sequential enzyme-activated and light-triggered pro-prodrug has been developed for cancer biomarker detection and on-demand therapy.

A self-immolative and DT-diaphorase-activatable prodrug for drug-release tracking and therapy

A DT-diaphorase-activatable theranostic prodrug has been developed for visualizing the release of active drug and enhancing the therapeutic effect.

A two-photon-activated prodrug for therapy and drug release monitoring

A two-photon-activated prodrug has been developed for drug release monitoring and photo-controllable therapy.

Physico-Chemical Strategies to Enhance Stability and Drug Retention of Polymeric Micelles for Tumor-Targeted Drug Delivery

Polymeric micelles (PM) have been extensively used for tumor-targeted delivery of hydrophobic anti-cancer drugs. The lipophilic core of PM is naturally suitable for loading hydrophobic drugs and the hydrophilic shell endows them with colloidal stability and stealth properties. Decades of research on PM have resulted in tremendous numbers of PM-forming amphiphilic polymers, and approximately a dozen micellar nanomedicines have entered the clinic. The first generation of PM can be considered solubilizers of hydrophobic drugs, with short circulation times resulting from poor micelle stability and unstable drug entrapment. To more optimally exploit the potential of PM for targeted drug delivery, several physical (e.g., π-π stacking, stereocomplexation, hydrogen bonding, host-guest complexation, and coordination interaction) and chemical (e.g., free radical polymerization, click chemistry, disulfide and hydrazone bonding) strategies have been developed to improve micelle stability and drug retention. In this review, the most promising physico-chemical approaches to enhance micelle stability and drug retention are described, and how these strategies have resulted in systems with promising therapeutic efficacy in animal models, paving the way for clinical translation, is summarized.

Hydrazone linkages in pH responsive drug delivery systems

Stimuli-responsive polymeric drug delivery systems using various triggers to release the drug at the sites have become a major focus area. Among various stimuli-responsive materials, pH-responsiveness has been studied extensively. The materials used for fabricating pH-responsive drug delivery systems include a specific chemical functionality in their structure that can respond to changes in the pH of the surrounding environment. Various chemical functionalities, for example, acetal, amine, ortho ester, amine and hydrazone, have been used to design materials that are capable of releasing their payload at the acidic pH conditions of the tumor or infection sites. Hydrazone linkages are significant synthons for numerous transformations and have gained importance in pharmaceutical sciences due to their various biological and clinical applications. These linkages have been employed in various drug delivery vehicles, such as linear polymers, star shaped polymers, dendrimers, micelles, liposomes and inorganic nanoparticles, for pH-responsive drug delivery. This review paper focuses on the synthesis and characterization methods of hydrazone bond containing materials and their applications in pH-responsive drug delivery systems. It provides detailed suggestions as guidelines to materials and formulation scientists for designing biocompatible pH-responsive materials with hydrazone linkages and identifying future studies.

Two-stage pH-sensitive doxorubicin hydrochloride loaded core–shell nanoparticles with dual drug-loading strategies for the potential anti-tumor treatment

Two-stage pH-sensitive DOX·HCl loaded core–shell nanoparticles (CPOD) with dual drug-loading strategies showed pretty in vivo anti-tumor efficacy.