Development of a stealth liposomal formulation of d-Ino as 5-FU modulator: in vitro and in vivo study

Proc Amer Assoc Cancer Res, Volume 47, 2006 3087 Prodrug 5-fluorouracil (5-FU) exerts its antiproliferative action after conversion into cytotoxic metabolites, and optimizing its efficacy by modulating its activation in tumors is still an ongoing story in experimental oncology. We, and others, previously demonstrated that antiproliferative action of 5-FU could be greatly enhanced by boosting tumoral TP activity, the first of a multi-step enzymatic cascade known as the DNA pathway, that leads to the production of anti-TS FdUMP metabolite (Ciccolini et al., Mol Cancer Ther, 2001, and Fanciullino et al., Proc. AACR 95th Meeting, 2004). Using precursor of TP cofactor such as 2’-deoxy-inosine (d-Ino) proved to greatly enhance 5-FU efficacy indeed, both in vitro and in animals. In the present study, we developed a new, stealth liposomal formulation of d-Ino, designed to bypass an extensive erythrocytic metabolism and permits its use at low dose in animals. Pharmacokinetics studies performed in wistar rats confirmed a significant, 7-time clearance reduction of liposomal d-Ino, as compared with its free counterpart. Furthermore, ex-vivo studies fully confirmed the reduction of erythroctytic catabolism in the encapsulated form. Additionally, monitoring of 5-FU metabolism in SW620 tumors showed that when associated with liposomal d-Ino, 5-FU generates markedly higher levels of anti-TS FdUMP, an observation fully consistent with the evaluation of TS inhibition throughout time. Finally, when transposed in nude mice xenografted with the 5-FU-resistant SW620 cell line, liposomal d-Ino proved to reduce by 75% the tumor size with a doubling median survival time, whereas 5-FU alone was ineffective. Our data suggest therefore that liposomal d-Ino, through an optimized pharmacokinetics profile, can be efficient at low doses in vivo when combined with 5-FU, and that reversion of a resistant-phenotype can thus be achieved.