Purpose: Leptomeningeal carcinomatosis is a devastating complication of malignant disease. In this study, we evaluated the safety and pharmacokinetics of intrathecally administered pemetrexed in rats.
Methods: Three levels of pemetrexed (0.3, 1, and 3 mg/kg) were administered to 15 rats per level (45 rats in total) twice a week for 2 weeks through specifically designed indwelling subarachnoid catheters. Presence of clinical and pathological neurotoxicity was evaluated. To evaluate the pharmacokinetics of pemetrexed, independent cohorts of 30 rats were treated with 1 mg/kg of pemetrexed and its concentration in cerebrospinal fluid (CSF) and blood was measured using UPLC/MS/MS.
Results: There were no cases of clinical or pathologic neurotoxicity after intrathecal administrations of pemetrexed at levels of 0.3 and 1 mg/kg; however, 5 of 15 (33%) rats died after administration of 3 mg/kg pemetrexed. The distribution/elimination of pemetrexed in CSF was best described by a two-compartment model, with initial and terminal half-lives of 0.43 and 1.43 h, respectively. The predicted maximal concentration in CSF was 588 μM, and high levels of pemetrexed appeared to be maintained for a long time. Area under the curve and volume of distribution at steady state were 560 μM h and 1.14 ml, respectively.
Conclusions: The no observed adverse effect level of intrathecal administration of pemetrexed was 1 mg/kg in rats. At this level, therapeutically high and durable pemetrexed concentrations could be achieved. Based on these results, further research on intrathecal pemetrexed in humans or non-human primates should be considered.