P-Glycoprotein (Pgp) in the blood-brain barrier limits the uptake of substrate drugs into the brain. We have determined the efficacy of several (putative) inhibitors of Pgp (cyclosporin A, PSC833, GF120918, and Cremophor EL) on the penetration of paclitaxel into the mouse brain. Pgp inhibitors were administered p.o. before i.v. paclitaxel. Plasma and tissues were collected at 1, 4, 8, and 24 h and analyzed for paclitaxel by high-performance liquid chromatography. Pgp knockout mice were used as reference for complete blockade of Pgp and to determine the selectivity of Pgp inhibitors on the pharmacokinetics of paclitaxel. Cremophor EL had no effect at all. Increased brain uptake was observed with cyclosporin A (3-fold), PSC833 (6.5-fold), and GF120918 (5-fold), although the levels were lower than that observed in Pgp knockout mice (11-fold increase). Both cyclosporin A and PSC833 also markedly increased the plasma levels of paclitaxel in contrast to GF120918. Obviously, cyclosporin A and PSC833 markedly inhibited several elimination pathways of paclitaxel, whereas the reduced clearance of paclitaxel by GF120918 was most probably related to the inhibition of Pgp alone. After further optimization of the dose and schedule of GF120918, we could achieve paclitaxel brain levels of about 80-90% of those reached in Pgp knockout mice. It is warranted to test paclitaxel in combination with GF120918 in experimental brain tumor models and in clinical trials.