Encapsulation in a rapid-release liposomal formulation enhances the anti-tumor efficacy of pemetrexed in a murine solid mesothelioma-xenograft model.

We recently developed a PEG-coated liposome encapsulating the anti-folate drug pemetrexed (PMX). Such liposomal formulations have shown potent cytotoxic effects against malignant pleural mesothelioma (MPM) cells in vitro. In the present study, we investigated the pharmacokinetics, bio-distribution and in vivo anti-tumor efficacy of two liposomal PMX formulations with different drug release rates in a murine mesothelioma-xenograft model. Liposomes with different PMX release rates were prepared via manipulating liposomal membrane fluidity through incorporating either a solid-phase (HSPC) or a fluid-phase (POPC) phospholipid. Both liposomal PMX formulations showed prolonged plasma pharmacokinetics and were accumulated to a similar extent in tumors and other tissues, presumably, due to surface modification with polyethylene glycol (PEG). In a murine mesothelioma-xenograft model, interestingly, PMX encapsulated in a fast-release POPC liposome produced superior tumor growth suppression compared with either free PMX or PMX encapsulated in a slow-release HSPC liposome. Such in vivo anti-tumor efficacy was accomplished mainly by a potent induction of apoptosis within tumor tissue by the released PMX from POPC liposomes. Our results clearly emphasize the therapeutic efficacy of liposomal PMX over free PMX in conquering aggressive solid tumors such as malignant mesothelioma. A guarantee of the targeted delivery of PMX to tumor cells helps overcome some of the major shortcomings encountered with the use of free PMX.