ROLE OF P-GLYCOPROTEIN IN COLCHICINE AND VINBLASTINE CELLULAR KINETICS IN AN IMMORTALIZED RAT-BRAIN MICROVESSEL ENDOTHELIAL-CELL LINE

Uptake and efflux of colchicine and vinblastine, whose effects are rel ated to their high-affinity binding to tubulin, were studied in the im mortalized rat brain microvessel endothelial cell line RBE4. At 10 nM extracellular drug concentration, uptake equilibrium was approached at 45 hr for colchicine, but at only 3.5 hr for vinblastine. After 1 hr preincubation with 200 nM colchicine or vinblastine, drug efflux fitte d biexponential kinetics with an initial fast phase (half-life = 2.2 m in and 9.6 min, respectively) and a later slow phase (half-life = 3.6 hr and 1.8 hr, respectively). After 6 hr preincubation with 200 nM col chicine, only the slow phase (half-life = 3.6 hr) could be observed. T he colchicine and vinblastine uptake rate was increased by cyclosporin A, an inhibitor of the drug efflux pump P-glycoprotein, which is expr essed at the blood-brain barrier. Whereas cyclosporin A decreased vinb lastine efflux, its effect on colchicine efflux was apparent after onl y 13 hr washout and was associated with the re-uptake by cells of colc hicine molecules. Differences in uptake kinetics of colchicine and vin blastine could be related to differences in their lipid solubility, an d mainly in their binding affinities to tubulin. Differences in efflux kinetics could in addition be explained by the involvement of P-glyco protein in the efflux of vinblastine, whereas efflux of colchicine was not influenced by this pump. Indeed, binding of colchicine to tubulin would imply that most intracellular colchicine may be inaccessible to P-glycoprotein. In the case of a cytotoxic drug such as colchicine, w hich is tightly bound to intracellular receptors, the role of P-glycop rotein within the blood-brain barrier would be more to protect the bra in against entry of this drug than to detoxify the brain by its extrac tion. (C) 1997 Elsevier Science Inc.