Abstract
PURPOSE: A challenge in developing therapies for GBM is the inability of drugs to cross the BBB. Ispinesib, an inhibitor of kinesin spindle protein, has potent anti-cancer activity by causing mitotic arrest and growth inhibition. The objective was to determine the distribution of ispinesib across the BBB, and determine mechanisms responsible for limiting brain delivery. METHODS: Brain distribution studies were conducted in FVB wild-type (WT), triple-knockout (TKO; Mdr1a/b(-/-)Bcrp1(-/-)), P-gp knockout (PKO; Mdr1a/b(-/-)) and Bcrp knockout (BKO; Bcrp1(-/-)) mice. In a separate study, WT mice administered with and without elacridar (n=5) were co-dosed ispinesib. Plasma and brain were collected one-hour post dose. Brain distribution studies were conducted in WT and TKO mice, and plasma and brain were harvested at 7 time points. Rapid equilibrium dialysis assessed the unbound fractions of ispinesib in plasma and brain. All the samples were analyzed using LC-MS/MS. RESULTS: The single time point studies show that the brain concentrations in WT and BKO mice are significantly lower than in plasma, and brain-to-plasma ratios (B:P) in WT and BKO mice were below 2%. Brain concentrations in TKO and PKO mice are significantly higher than WT and BKO. The B:P was increased in the TKO and PKO mice [(WT: 0.0071 ± 0.01; BKO: 0.016 ± 0.01 (not significant); TKO: 0.35 ± 0.25 (p=0.0140); PKO: 0.54 ± 0.14 (p=0.0005)]. Plasma concentrations in WT and TKO mice following oral administration of ispinesib were about 33% of plasma concentrations following i.p. dosing and the B:P ratios were similar. Brain concentrations are less than 5% of plasma following elacridar co-administration, and did not cause a significant increase in ispinesib brain delivery. CONCLUSIONS: Brain delivery of ispinesib is restricted by active efflux transporters (P-gp). The pharmacokinetic studies in FVB mice post intravenous administration will confirm the findings from single time point studies.