A phase I trial of continuous infusion of the multidrug resistance inhibitor zosuquidar with daunorubicin and cytarabine in acute myeloid leukemia.

Zosuquidar is a potent and specific inhibitor of P-glycoprotein (P-gp). In preliminary experiments, blockade of P-gp for at least 12 h was required to reverse daunorubicin resistance. Because of the short half-life of zosuquidar, we performed a phase I trial of this drug as a 72-h infusion (CIV) in 16 patients during leukemic induction with daunorubicin and cytarabine. Study goals were to establish safety and determine the dose required for P-gp inhibition in NK cells and AML blasts. > 90% P-gp inhibition was achieved within 2h at a plasma threshold of 132 ng/ml zosuquidar. The recommended phase II dose of zosuquidar is 700 mg/day.