Glioblastoma multiforme (GBM) is the most aggressive of brain tumors and is extremely insensitive to anticancer drugs. Studies have attributed the ABC transporter Mrp1 (ABCC1, multiple-drug resistance protein 1) with conferring chemoresistance in this tumor by extrusion of a wide spectrum of anticancer drugs. Therefore it is crucial to search for and investigate inhibitors of Mrp1 activity in GBM cells, particularly those that could be safe as chemosensitizers to anticancer drugs in clinical studies. We find that in primary cultured or T98G GBM cells exposed to therapeutic plasma concentrations of FK506 (tacrolimus), the expression of Mrp1 was decreased in a dose-dependent manner. The activity of this transporter, measured by CFDA fluorescent substrate extrusion, decreased significantly in primary cultured GBM cells on exposure to FK506 at concentrations of 15 ng/ml. When GBM cells were exposed to anticancer drugs vincristine, etoposide or taxol, cell viability was not affected. However when the anticancer drugs were assayed in combination with FK506, cell viability was significantly decreased by as much as 50% in GBM primary culture. We conclude that FK506 could be a valuable tool for chemosensitization of GBM cells, offering a possible improvement to the current poor therapy available for high-grade human gliomas. (C) 2011 Elsevier Inc. All rights reserved.