Reversing multi-drug tumor resistance to Paclitaxel by well-defined pH-sensitive amphiphilic polypeptide block copolymers via induction of lysosomal membrane permeabilization

A series of hybrid di-block copolymers of poly(L- glutamic acid-b-L-leucine) (PGA-PLeu), methoxy poly (ethylene glycol)-b-poly(L-leucine) (PEG-PLeu), methoxy poly(ethylene glycol)-b-poly(gamma-benzyl-r-glutamic acid) (PEGPBLG) and tri-block copolymers of poly(ethylene glycol)-b-poly(L-glutamic acid-co-muuJeucine) (PEG-PGAPLeu) were synthesized through sequential HMDS-mediated ring-opening polymerization (ROP). Chemical structure of copolymers was studied by FTIR and H-1-NMR and their molecular weight was determined by H-1-NMR and gel permeation chromatography (GPC). Copolymers self-assembled into nanomicelles with particle size (PS) of 65 to 139 nm. Higher fraction of polyleucine (% f(PLeu)) led to significantly larger PS, lower critical aggregation concentration (CAC) and higher drug loading content (DLC%). In addition, introducing PEG segment led to significant decrease in PS, increase of CAC and DLC%. Apart from copolymer composition, DLC% changed by the method with significantly higher loading for solid dispersion. Remarkably, the release of PTX from PEG-PGA-PLeu tri-block copolymers was highly dependent on pH, revealing a relatively two-fold faster release at pH 5 than pH 7.4. CD spectroscopy showed transition to alpha-helix secondary structure at acidic pH. Hemocompatibility assay confirmed that copolymers were absolutely hemocompatible at physiological pH. MTT assays demonstrated that unlike MCF7 and 4T1 cells that PTX-loaded nanoparticles (PTX-NPs) exhibited similar antitumor activity, ten-fold higher toxicity was recognized in multidrug-resistant uterine sarcoma cells (MES-SA/ DX5). Fluorescent imaging and flow cytometric analysis of cellular uptake showed that nanoparticles' uptake was time-dependent. It was also revealed that higher toxicity of the PTX-NPs could be due to ability of copolymer to inhibit P-gp pumps and induce lysosomal membrane permeabilization (LMP).