Potent modulator of P-glycoprotein-mediated multidrug resistance (MDR) that inhibits the binding of cytotoxics to P-glycoprotein (EC50 = 134 nM). Reverses resistance to a variety of cytotoxic drugs (including doxorubicin, etoposide and vincristine) in several murine and human P-gp-overexpressing cell lines. Orally active.