The MDR1 transporter mediated efflux of different xenobiotics out of the cells serves as the most important mechanisms of the multidrug resistance in cancer cells, thus inhibition of the MDR1 transporter may increase the efficiency of anticancer drugs in the therapy. Here we describe some new phenothiazine derivatives, which possess strong in vitro MDR1 inhibitory activity. The effectiveness of the compounds on the MDR1 mediated calcein-AM efflux, ATPase activity, and colchicine resistance was proven by microplate assays and flow cytometry using recombinant and control cell lines. Some of these derivatives were more active than verapamil and one of them was at least as active as cyclosporin A. According to our results the new structural elements built in these phenothiazine type compounds increased their MDR1 inhibitory activity, which may serve as a basis of the development of an effective MDR1 inhibitor drug. (c) 2006 Elsevier Inc. All rights reserved.