pH-triggered release of vancomycin from protein-capped porous silicon films

An in vitro model system for pH-triggered release of the antibiotic vancomycin from porous Si films is studied.

Vancomycin is infused into a mesoporous Si film from a mixed aqueous/acetonitrile solution and trapped by a capping layer containing the protein bovine serum albumin (BSA). The protein effectively traps vancomycin in the porous nanostructure at pH 4.0; the protein dissolves and vancomycin is released into solution when the pH increases to 7.4. The surface chemistry of porous Si exerts a substantial effect on the efficacy of drug loading. The amount of drug loading is larger in freshly-etched (hydrophobic, hydrogen-terminated) porous Si and smaller in methyl-modified, undecylenic acid-modified and thermally oxidized samples. The quantity of drug loaded in a freshly etched porous Si chip is proportional to the thickness of the porous layer, which exhibits a constant volume loading efficiency of 31% (v/v). Flow-cell experiments designed to mimic the transition from pH 4 to 7 that occurs when material moves from the stomach to the upper intestinal tract were performed on the freshly etched films and vancomycin- and BSA-release rates were quantified from the effluent of the flow cell by high-pressure liquid chromatography analysis.

There is a small, constant rate of vancomycin release at pH 4 that is independent of the amount of drug loaded in the pores. This is attributed to diffusion of vancomycin from the BSA-capping layer. The release rate increases five- to tenfold when the pH of the solution in the flow cell increases to 7.4; 100% of the drug is released within 3 h of this increase.