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.

Allyl group-containing polyvinylphosphonates as a flexible platform for the selective introduction of functional groups via polymer-analogous transformations

Polyvinylphosphonates are highly promising candidates for (bio)medical applications as they exhibit a tunable lower critical solution temperature, high biocompatibility of homo- and copolymers, and a broad foundation for post-synthetic modifications. In this work we explored polymer-analogous transformations with statistical polyvinylphosphonates comprising diethyl vinylphosphonate (DEVP) and diallyl vinylphosphonate (DAlVP). The C[double bond, length as m-dash]C double bonds were used as a starting point for a cascade of organic transformations. Initially, the reactive moieties were successfully introduced via bromination, epoxidations with OXONE and mCPBA, or thiol-ene click chemistry with methyl thioglycolate (6). The obtained substrates were then employed in a variety of consecutive reactions depending on the introduced functional motif: (1) the brominated substrates were converted with sodium azide to enable the copper-mediated alkyne-azide coupling with phenylacetylene (1). (2) The epoxides were reacted with sodium azide for an alkyne-azide click coupling with 1 as well as small nucleophilic compounds (phenol (2), benzylamine (3), and 4-amino-2,1,3-benzothiadiazol (4)). Afterwards the non-converted allyl groups were reacted with thiochloesterol (5) to form complex polymer conjugates. (3) An acid-labile hydrazone-linked conjugate was formed in a two-step approach. The polymeric substrates were characterized by NMR, FTIR, and UV/Vis spectroscopy as well as elemental analysis and gel permeation chromatography to monitor the structural changes of the polymeric substrates and to prove the success of these modification approaches.

Facile construction of stabilized, pH-sensitive micelles based on cyclic statistical copolymers of poly(oligo(ethylene glycol)methyl ether methacrylate-st-N,N-dimethylaminoethyl methacrylate) for in vitro anticancer drug delivery

This study developed a facile approach to improve the colloidal stability of a cyclic polycation as well as presented a pH-sensitive cyclic copolymer-based nanoplatform with great potential for anticancer drug delivery.

Safe and Efficacious Diphtheria Toxin-Based Treatment for Melanoma: Combination of a Light-On Gene-Expression System and Nanotechnology

The controversy surrounding the use of diphtheria toxin (DT) as a therapeutic agent against tumor cells arises mainly from its unexpected harmfulness to healthy tissues. We encoded the cytotoxic fragment A of DT (DTA) as an objective gene in the Light-On gene-expression system to construct plasmids pGAVPO (pG) and pU5-DTA (pDTA). Meanwhile, a cRGD-modified ternary complex comprising plasmids, chitosan, and liposome (pG&pDTA@cRGD-CL) was prepared as a nanocarrier to ensure transfection efficiency. Benefiting from spatiotemporal control of this light-switchable transgene system and the superior tumor targeting of the carrier, toxins were designed to be expressed selectively in illuminated lesions. In vitro studies suggested that pG&pDTA@cRGD-CL exerted arrest of the S phase in B16F10 cells upon blue light irradiation and, ultimately, induced the apoptosis and necrosis of tumor cells. Such DTA-based treatment exerted enhanced antitumor activity in mice bearing B16F10 xenografts and displayed prolonged survival time with minimal side effects. Hence, we described novel DTA-based therapy combined with nanotechnology and the Light-On gene-expression system: such treatment could be a promising strategy against melanoma.

Designing Smart Polymer Conjugates for Controlled Release of Payloads

Incorporating labile bonds inside polymer backbone and side chains yields interesting polymer materials that are responsive to change of environmental stimuli. Drugs can be conjugated to various polymers through different conjugation linkages and spacers. One of the key factors influencing the release profile of conjugated drugs is the hydrolytic stability of the conjugated linkage. Generally, the hydrolysis of acid-labile linkages, including acetal, imine, hydrazone, and to some extent β-thiopropionate, are relatively fast and the conjugated drug can be completely released in the range of several hours to a few days. The cleavage of ester linkages are usually slow, which is beneficial for continuous and prolonged release. Another key structural factor is the water solubility of polymer-drug conjugates. Generally, the release rate from highly water-soluble prodrugs is fast. In prodrugs with large hydrophobic segments, the hydrophobic drugs are usually located in the hydrophobic core of micelles and nanoparticles, which limits the access to the water, hence lowering significantly the hydrolysis rate. Finally, self-immolative polymers are also an intriguing new class of materials. New synthetic pathways are needed to overcome the fact that much of the small molecules produced upon degradation are not active molecules useful for biomedical applications.

Modulating angiogenesis with integrin-targeted nanomedicines

Targeting angiogenesis-related pathologies, which include tumorigenesis and metastatic processes, has become an attractive strategy for the development of efficient guided nanomedicines. In this respect, integrins are cell-adhesion molecules involved in angiogenesis signaling pathways and are overexpressed in many angiogenic processes. Therefore, they represent specific biomarkers not only to monitor disease progression but also to rationally design targeted nanomedicines. Arginine-glycine-aspartic (RGD) containing peptides that bind to specific integrins have been widely utilized to provide ligand-mediated targeting capabilities to small molecules, peptides, proteins, and antibodies, as well as to drug/imaging agent-containing nanomedicines, with the final aim of maximizing their therapeutic index. Within this review, we aim to cover recent and relevant examples of different integrin-assisted nanosystems including polymeric nanoconstructs, liposomes, and inorganic nanoparticles applied in drug/gene therapy as well as imaging and theranostics. We will also critically address the overall benefits of integrin-targeting.

RGD-decorated redox-responsived-α-tocopherol polyethylene glycol succinate–poly(lactide) nanoparticles for targeted drug delivery

A novel kind of copolymer, TPGS-SS-PLA, was successfully synthesized and applied in targeted drug delivery.

A tumor-targeting protein nanoparticle based on Tat peptide and enhanced green fluorescent protein

A protein-based nanoparticle containing cell penetrating peptides (CPPs) and enhanced green fluorescent protein (EGFP) was developed through a genetic engineering method.

Targeted polymer-drug conjugates: Current progress and future perspective

The combination of polymer technology and targeted drug delivery may pave the way for more effective yet safer therapeutic options for cancer therapy. Polymer-drug conjugates belonging to polymer therapeutics represent an emerging approach for drug delivery. The development of smart targeted polymer-drug conjugates that can specifically deliver drugs at a sustained rate to tumor cells may substantially improve the therapeutic index of anticancer agents. In this update, we provide an overview of the most important targeting molecules, and systemically summarize the recent advances in the development of tumor-targeted polymer-drug conjugates. Additionally, several promising approaches for the future will also be presented.

Glycyrrhetinic Acid-Mediated Polymeric Drug Delivery Targeting the Acidic Microenvironment of Hepatocellular Carcinoma

PURPOSE

The major hurdle of current drug carrier against hepatocellular carcinoma (HCC) is the lack of specific and selective drug delivery to HCC. In this study, a novel glycyrrhetinic acid (GA) and poly(L-Histidine) (PHIS) mediated polymeric drug delivery system was developed to target HCC that have GA binding receptors and release its encapsulated anticancer drug in the acidic microenvironment of HCC.

METHODS

Firstly, GA and PHIS were conjugated to form poly (ethylene glycol)-poly(lactic-co-glycolic acid) (GA-PEG-PHIS-PLGA, GA-PPP) micelles by grafting reaction between active terminal groups. Secondly, andrographolide (AGP) was encapsulated to GA-PPP to make AGP/GA-PPP using the solvent evaporation method. The pH-responsive property of AGP/GA-PPP micelles was validated by monitoring its stability and drug release behavior in different pH conditions. Furthermore, selective hepatocellular uptake of GA-PPP micelles in vitro, liver specific drug accumulation in vivo, as well as the enhanced antitumor effects of AGP/GA-PPP micelles confirmed the HCC targeting property of our novel drug delivery system.

RESULTS

Average size of AGP/GA-PPP micelles increased significantly and the encapsulated AGP released faster in vitro at pH 5.0, while micelles keeping stable in pH 7.4. AGP/GA-PPP micelles were uptaken more efficiently by human Hep3B liver cells than that by human MDA-MB-231 breast cancer cells. GA-PPP micelles accumulated specifically in the liver and possessed long retention time in vivo. AGP/GA-PPP micelles significantly inhibited tumor growth and provided better therapeutic outcomes compared to free AGP and AGP/PEG-PLGA(AGP/PP) micelles without GA and PHIS decoration.

CONCLUSIONS

This novel GA-PPP polymeric carrier is promising for targeted treatment of HCC.

cRGD targeted and charge conversion-controlled release micelles for doxorubicin delivery

A biodegradable polymeric micelle with cRGD targeting and charge-conversional moiety could enhance the cellular uptake of pharmaceuticals and result in high cytotoxicity to tumor cells.

Targeted photosensitizer nanoconjugates based on human serum albumin selectively kill tumor cells upon photo-irradiation

Small and non-toxic nanoconjugates RGD–HSA–Ce6 could provide targeted and effective photodynamic therapy of tumor cells.