Novel pH-sensitive charge-reversal cell penetrating peptide conjugated PEG-PLA micelles for docetaxel delivery: In vitro study

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In order to create a pH-sensitive charge-reversal system for cell penetrating peptides (CPP) to prevent non-specific internalization of the drug; and concomitantly enhance the physical stability and tumor targetability of poly(ethylene glycol)-poly(d,l-lactide) (PEG-PLA) micelles, two sets of novel PEG-PLA micelles were developed. Cell penetrating decapeptide arginine-glycine (RG)5 and a pH-sensitive masking decapeptide histidine-glutamic acid (HE)5 were conjugated at the PEG free end to produce pH sensitive with peptides outside micelles (PHPO), while the pH sensitive with peptides inside micelles (PHPI) are the micelles obtained with the two peptides conjugated to the free end of the PLA block. The polymers were successfully synthesized and characterized by 1H NMR and GPC. The mixed micelles were prepared and characterized for their loading efficiency, particle size and zeta potential. The surface charge of PHPO was greatly affected by the pH of the solution and (RG)5:(HE)5 ratio at the surface. The pH value of the solution at which the surface charge of PHPO reversed could be manipulated by the feed ratio of (RG)5-PEG-PLA (RGO) and (HE)5-PEG-PLA (HEO), hence, HEO:RGO molar ratio of 45:55 was selected for tumor targeting. Docetaxel (DTX) was sufficiently solubilized by DTX-PHPO with a loading efficiency of 90.18 ± 1.65%. At pH 7.4, DTX loaded mPEG-PLA (DTX-PM) (41.2 ± 0.3 nm), DTX-PHPO (195.3 ± 1.9 nm) and DTX-PHPI (190.9 ± 4.5 nm) showed sustained DTX release of less than 55% within 48 h. However, at pH 6.8 DTX-PHPI released 87.29 ± 0.24%, while DTX-PHPO released 70.49 ± 0.39% of the initial DTX amount within 48 h. Moreover, the physical stability of DTX-PHPO was increased due to the electrostatic interaction of the two peptides. The cellular uptake of DTX-PHPO in SGC-7901 cells and the cell killing effect tested on MCF-7 cells were enhanced by 2 folds at pH 6.8 compared to pH 7.4. Hence, DTX-PHPO is highly pH-sensitive in mildly acidic pH and exhibited higher internalization, but DTX-PHPI exhibited accelerated release. Meanwhile, both formulations displayed low internalization and release at pH greater than 7. This pH sensitive charge reversal design can offer a promising safe carrier using both CPPs and PEG-PLA micelles.

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