We sought to characterize the interactions of flavopiridol with members of the ATP-binding cassette (ABC) transporter family, Cells overexpressing multidrug resistance-1 (MDR-1) and multidrug resistance-associated protein (MRP) did not exhibit appreciable flavopiridol resistance, whereas cell lines overexpressing the ABC half-transporter, ABCG2, (MXR/BCRP/ABCP1), were found to be resistant to flavopiridol, Flavopiridol at a concentration of 10 muM was able to prevent MRP-mediated calcein efflux, whereas Pgp-mediated transport of rhodamine 123 was unaffected at flavopiridol concentrations of up to 100 muM. To determine putative mechanisms of resistance to flavopiridol, we exposed the human breast cancer cell line MCF-7 to incrementally increasing concentrations of flavopiridol, The resulting resistant subline, MCF-7 FLV1000, is maintained in 1000 nM flavopiridol and was found to be 24-fold resistant to flavopiridol, as well as highly cross-resistant to mitoxantrone (675-fold), topotecan (423-fold), and SN-38 (950-fold), the active metabolite of irinotecan, Because this cross-resistance pattern is consistent with that reported for ABCG2-overexpressing cells, cytotoxicity studies were repeated in the presence of 5 muM Of the ABCG2 inhibitor fumitremorgin C (FTC), and sensitivity of MCF-7 FLV1000 cells to flavopiridol, mitoxantrone, SN-38, and topotecan was restored. Mitoxantrone efflux studies were performed, and high levels of FTC-reversible mitoxantrone efflux were found. Northern blot and PCR analysis revealed overexpression of the ABCG2 gene. Western blot confirmed overexpression of ABCG2; neither P-glycoprotein nor MRP overexpression was detected. These results suggest that ABCG2 plays a role in resistance to flavopiridol.