Fumitremorgin C is a potent and selective ABCG2/BRCP inhibitor. In Vitro: Multidrug resistance (MDR) is a major problem in cancer chemotherapy. Fumitremorgin C is extremely effective in reversing resistance to mitoxantrone, doxorubicin, and topotecan in multidrug-selected cell lines. In MCF-7/mtxR (a mitoxantroneselected cell line), fumitremorgin C reverses mitoxantrone resistance (114-fold) and doxorubicin resistance (3-fold). Fumitremorgin C (5/AM)significantly potentiates the toxicity of mitoxantrone (93-fold), doxorubicin (26-fold), and topotecan (24-fold) in S1M1-3.2 cells. Reversal of resistance is associated with an increase in drug accumulation. Fumitremorgin C does not reverse drug resistance in cells with elevated expression of Pgp or MRP^[1]. Fumitremorgin C almost completely reverses resistance mediated by BCRP in vitro and is a pharmacological probe for the expression and molecular action of this transporter. Fumitremorgin C also enhances the toxicity of mitoxantrone and topotecan in vector-transfected MCF-7 cells (2.5–5.6 fold). It reduces the IC₅₀ of topotecan in BCRP-overexpressing cells below that observed in the untreated vector-transfected cells. ^[2].