Interactions of N-desmethyl imatinib, an active metabolite of imatinib, with P-glycoprotein in human leukemia cells.

We measured intracellular accumulation of N-desmethyl imatinib (CGP 74588), the main pharmacologically active metabolite of imatinib (Gleevec or STI-571), in Bcr--Abl-positive cells. Using a sensitive and robust non-radioactive in vitro assay, we observed that CGP74588 accumulates in significantly higher amount than imatinib in sensitive K562 cells. In contrast, the intracellular level of CGP74588 was significantly lower than that of imatinib in K562/Dox cells, which represent a multidrug-resistant variant of K562 cells due to the P-glycoprotein (P-gp, ABCB1, MDR1) overexpression. An in vitro enzyme-based assay provided evidence that CGP74588 might serve as an excellent substrate for P-gp. Accordingly, we found that CGP74588 up to 20 μM concentration neither induced apoptosis nor inhibited substantially cell proliferation in resistant K562/Dox cells. In contrast, CGP74588 was capable to inhibit cell proliferation and induced apoptosis in sensitive K562 cells, although its effect was approximately three to four times lower than that of imatinib in the same cell line. Our results indicate that CGP74588 could hardly positively contribute to the treatment of chronic myeloid leukemia (CML) where ABCB1 gene overexpression represents a possible mechanism of resistance to imatinib in vivo.