(570c) Polymeric Prodrug Micelles for Gene-Directed Enzyme Prodrug Therapy

Acyclovir (ACV) is a guanosine-based prodrug used in anti-herpes virus therapy. Since ACV can be converted into its toxic form ACV-triphosphate by herpes simplex type-1 thymidine kinase enzyme (HSV-Tk), ACV has potential for cancer cell treatment via gene-directed enzyme prodrug therapy (GDEPT). This study showed that ACV can be used as an initiator for the ring-opening polymerization of ε-caprolactone (ε-CL) with the aid of tin(II) 2-ethylhexanoate (Sn(Oct)₂). Unlike commonly used grafting methods (i.e., conjugating ACV to biocompatible polymers), this innovative synthetic method simplified the process of making ACV-poly(ε-caprolactone) (ACV-PCL). The kinetics of the reaction was investigated by measuring the molecular weights of ACV-PCL at various time intervals via gel-permeation chromatography (GPC).  The hydrophobic ACV-PCL polymer was then grafted with hydrophilic chitosan to form amphiphilic copolymers for the preparation of stable micellar nanoparticles. The characteristics of these polymers were determined by GPC, proton nuclear magnetic resonance (¹H NMR) and Fourier transform infrared spectroscopy (FTIR). ¹H NMR analysis demonstrated that the ring-opening polymerization of ε-CL was caused by the hydroxyl group of ACV. FTIR spectra reveal characteristic absorption peaks associated with PCL and ACV are simultaneously present in ACV-PCL polymers. The average sizes of the micelles were approximately 210 nm and their zeta potentials were positive due to the chitosan segment deployed on the outer layer of the polymeric micelles.  The prepared polymeric prodrug micelles were used to challenge colon HT-29 cells with HSV-TK gene expression and showed toxic effects on cells by the MTT assay.