This article reports the synthesis, spontaneous selfassembly, highly efficient drug encapsulation, and glutathione (GSH)triggered intracellular sustained drug delivery of an ABA-type amphiphilic triblock copolymer, namely, polyglycerol-b-poly-(disulfide)-b-polyglycerol (PG-b-PDS-b-PG). The bioreducible PDS block with reactive pyridyldisulfide groups present at the chain terminals was attached to thiol-terminated heterotelechelic PG by a thiol− disulfide exchange reaction producing the amphiphilic PG-b-PDS-b-PG. It formed a stable polymersome in aqueous medium with a critical aggregation concentration of 0.02 mg/mL and average hydrodynamic diameter (D h ) of 230 nm and showed highly efficient and stable encapsulation of doxorubicin (Dox) with a remarkably high drug loading efficiency (DLE) and drug loading content (DLC) of 54% and 16%, respectively. Fluorescence spectroscopy studies revealed GSH-triggered drug release and strong dependence of the release kinetics on the GSH concentration due to degradation of the amphiphilic block copolymer and disassembly of the polymersome. MTT assay indicated excellent biocompatibility of the block copolymer as >90% cells (HeLa or MDA-MB-231) were found to be alive after 96 h of incubation with a polymer concentration of up to 1.0 mg/mL, which was further validated by the hemolysis assay. Cytotoxicity assay of the Dox-loaded polymersome exhibited time and dose-dependent sustained killing of HeLa as well as MDA-MB-231 cells wherein after 48 h of incubation >50% cell killing was noticed with a Dox concentration of ∼4.0 and ∼8.7 μg/mL, respectively, while the free Dox showed faster cell killing. Flow cytometry and live cell fluorescence microscopy studies revealed time-dependent cellular uptake of the drug-loaded polymersome followed by diffusion of the drug to the nucleus. Cells with artificially enhanced GSH were killed at a much faster rate indicating that intracellular GSH-triggered disassembly is the key drug release mechanism.

We highlight the nanostructure assemblies containing disulfide linkages, dynamics, GSH triggered disassembly and implications in biological applications.

A bioreducible poly­(disulfide)-derived amphiphilic block copolymer–drug conjugate (loading content 31%) was synthesized by post-polymerization modification. It shows redox-responsive polymersome assembly in water with aggregation induced emission property arising from the appended Camptothecin (CPT) drug. Glutathione (GSH), a tripeptide overexpressed in cancer cells, triggers a cascade reaction resulting in simultaneous degradation of the polymer backbone (consisting of disulfide linkage) and the release of the pendant drug. The cascade reaction involves GSH trigger cleavage of the backbone disulfide bond producing free thiol followed by its intrachain nucleophilic attack to the adjacent carbonate group that links the appended drug molecule. The polymeric pro-drug exhibits killing efficiency to a cancer cell with remarkably low IC50 value of 3.1 μg/mL (based on the CPT concentration) while it shows negligible toxicity to a normal cell up to polymer concentration 300 μg/mL.

In two ABA type amphiphilic block copolymers (P1, P2), the hydrophobic B block consists of a bioreducible segmented poly(disulfide) (PDS), while poly-N-isopropylacrylamide (PNIPAM) or poly(triethyleneglycol)methylether-methacrylate (PTEGMA) serve as the hydrophilic A blocks in P1 and P2, respectively, leading to the formation of polymersome and micelle, owing to the difference in the packing parameters. Both exhibit comparable doxorubicin (Dox) encapsulation efficiency, but glutathione (GSH) triggered release appears much faster from the polymersome than micelle owing to the complete degradation of the PDS segment in polymersome morphology unlike in micelle. Dox-loaded polymers (P1-Dox and P2-Dox) exhibit minimum toxicity to normal cells like C2C12. By contrast, P1-Dox shows excellent killing efficiency to the HeLa cells (cancer cell) (in which the GSH concentration is significantly higher). However, P2-Dox reveals a rather poor activity even to HeLa cells. Fluorescence microscopy studies show comparable cellular uptake of P1-Dox and P2-Dox. But the polymersome entrapped dye escapes fast from the cargo and reach the nucleus, while the drug-loaded micelle remains trapped in the perinuclear zone explaining the significant difference in the drug delivery performance of polymersome and micelle.

Mucus hydrogels at biointerfaces are crucial for protecting against foreign pathogens and for the biological functions of the underlying cells. Since mucus can bind to and host both viruses and bacteria, establishing a synthetic model system that can emulate the properties and functions of native mucus and can be synthesized at large scale would revolutionize the mucus-related research that is essential for understanding the pathways of many infectious diseases. The synthesis of such biofunctional hydrogels in the laboratory is highly challenging, owing to their complex chemical compositions and the specific chemical interactions that occur throughout the gel network. In this perspective, we discuss the basic chemical structures and diverse physicochemical interactions responsible for the unique properties and functions of mucus hydrogels. We scrutinize the different approaches for preparing mucus-inspired hydrogels, with specific examples. We also discuss recent research and what it reveals about the challenges that must be addressed and the opportunities to be considered to achieve desirable de novo synthetic mucus hydrogels.