Melphalan-monomethoxypolyethylene glycol-based pH/enzyme double-response polymer prodrug nanoparticles for enhanced drug stability and anticancer efficacy

To improve hydrolytic stability and biological half-life and anticancer efficacy of the anti-cancer drug melphalan (Mel), a novel esterification strategy was developed to fabricate a pH/enzyme dual-responsive polymer prodrug (mPEG-Mel) via the formation of an ion bond between the carboxyl group in Mel and diazotized p-amino-benzoate-terminated poly(ethylene glycol) monomethyl ether (mPEG-N-2(+)) followed by light-induced conversion of the ionic bond to an ester bond. The prodrug could self-assemble into nanoparticles. Under physiological conditions (pH 7.4), the nanoparticles exhibited a regular core-shell structure with about 100 nm, which could effectively inhibit the hydrolysis of pharmacophores in Mel structure and showed the long-circulation ability. While pH dropped to 5.0, an obvious increase in the nanoparticle size was observed and enhanced by different concentrations of cathepsin B (CTB), reaching a diameter of 675 nm or more in the presence of 2.0 mu g/mL CTB. The nanoparticles showed minimal drug release at pH 7.4 (less than 10% in 24 h); however, drug release could be significantly accelerated in a CTB-containing medium at pH 5.0. Cell experiments showed that the half-inhibitory concentration (IC50) of nanoparticles was 1.00 mu g/mL, which was about 5 times lower than that of free Mel (5.00 mu g/mL), and indicating that the nanoparticles were more cytotoxic to cancer cells (4T1 cells). However, significantly reduced cytostatic effects were observed for normal cells (3T3 cells). In addition, cell uptake observation further supported the above experimental results, a stronger red fluorescence in 4T1 cells was observed for NR-loaded nanoparticles, which meant that the nanoparticles could release drug in response to pH/enzyme stimulation. Thus, mPEG-Mel based pH/enzyme double-response polymer prodrug nanoparticles could effectively enhance drug stability and anticancer efficacy.