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 related to their high-affinity binding to tubulin, were studied in the immortalized 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 fitted biexponential kinetics with an initial fast phase (half-life = 2.2 min 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 colchicine, only the slow phase (half-life = 3.6 hr) could be observed. The colchicine and vinblastine uptake rate was increased by cyclosporin A, an inhibitor of the drug efflux pump P-glycoprotein, which is expressed at the blood-brain barrier. Whereas cyclosporin A decreased vinblastine efflux, its effect on colchicine efflux was apparent after only 13 hr washout and was associated with the re-uptake by cells of colchicine molecules. Differences in uptake kinetics of colchicine and vinblastine could be related to differences in their lipid solubility, and mainly in their binding affinities to tubulin. Differences in efflux kinetics could in addition be explained by the involvement of P-glycoprotein 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, which is tightly bound to intracellular receptors, the role of P-glycoprotein within the blood-brain barrier would be more to protect the brain against entry of this drug than to detoxify the brain by its extraction.