Inappropriate expression of the multidrug resistance (MDR1) gene encoding the
P-glycoprotein (Pgp) has been frequently implicated in resistance to different chemotherapeutic drugs. We have previously generated
chronic myeloid leukemia (CML) cell lines resistant to the
tyrosine kinase inhibitor imatinib mesylate (
STI571), and one line (LAMA84-r) showed overexpression not only of the Bcr-Abl
protein but also of Pgp. In the present study, we investigated this phenomenon in other cell lines overexpressing exclusively Pgp. Thus, cells from the K562/DOX line, described as resistant to
doxorubicin due to MDR1 gene overexpression, grew continuously in the presence of 1 microM
imatinib, but died in 4 to 5 days if the Pgp pump modulators
verapamil or
PSC833 were added to the
imatinib-treated culture. Analysis of cell proliferation by the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay confirmed the differential sensitivity of K562/DOX to
imatinib, which was also reversed by
verapamil or
PSC833. Flow cytometric analysis of the total
phosphotyrosine content by intracytoplasmic staining after a 2-hour incubation with escalating doses of
imatinib showed that the inhibitory concentrations of 50% (IC(50)) for inhibition of cellular
protein tyrosine phosphorylation were 15, 10, and 5 microM for K562/DOX, K562/DOX plus
verapamil, and K562, respectively. Retroviral-mediated transfection of the BCR-ABL(+) AR230 cell line with the MDR1 gene decreased its sensitivity to
imatinib, an effect that was also reversed by
verapamil. The possible role of MDR overexpression in clinical resistance to
imatinib remains to be defined. We therefore confirm that
imatinib should be added to the extensive list of drugs that can be affected by the MDR phenomenon.