Pharmacological studies of cisplatin encapsulated in long-circulating liposomes in mouse tumor models

We investigated the pharmacokinetics and therapeutic efficacy of cisplatin encapsulated in polyethyleneglycol-coated long-circulating liposomes in a formulation referred to as SPI-077, in three mouse tumor models (M-109 lung carcinoma inoculated s.c., J-6456 lymphoma inoculated i.p. and A-375 melanoma inoculated s.c.). Tumor-bearing mice were injected i.v. with single doses of SPI-077 and cisplatin. For pharmacokinetic experiments, mice were sacrificed at different timepoints post-treatment. Platinum levels were determined in plasma, spleen, liver, kidneys and tumors using flameless atomic absorption spectrophotometry. Survival times and/or tumor size were recorded for therapeutic studies. The pharmacokinetic studies revealed a prolonged circulation time and enhanced tumor uptake for SPI-077. In contrast to these results, no superior antitumor activity of SPI-077 over cisplatin could be observed in all tumor models. In vitro release experiments showed a negligible release (below 10%) of platinum from the liposomes. An in vitro cytotoxicity assay indicated a reduced cytotoxic activity of SPI-077 in comparison to cisplatin. We concluded that SPI-077 is being delivered to the tumor sites in a low bioavailability form, with extremely slow release kinetics. This explains the discrepant results of high platinum concentrations in tumors and reduced therapeutic activity after administration of SPI-077.