This study was conducted to define the maximum tolerated dose (MTD), dose limiting toxicity (DLT), and pharmacokinetics of idarubicin when administered with and without the P-glycoprotein inhibitor PSC-833 in combination with cytarabine, and etoposide. Fifteen patients with relapsed and refractory acute leukemia were enrolled and received cytarabine as a 7-day continuous infusion, with etoposide and idarubicin administered for any three consecutive days during the cytarabine infusion. Two hours prior to the second dose of idarubicin, PSC-833 administration was initiated. The pharmacokinetics of idarubicin alone and with PSC-833 was assessed at three idarubicin dose levels (6, 8 and 10 mg/m(2)). The MTD of idarubicin in this combination was 8 mg/(m(2) day) with a DLT of oral mucositis. The complete remission rate (on an intent-to-treat basis) for this regimen was 33%, with a median duration of 6 months. The clearance of idarubicin was 140 +/- 200 and 181 +/- 94.3 l/h for idarubicin alone and with PSC-833, respectively. The volume of distribution of the central compartment was 423 +/- 443 and 337 +/- 394 l for idarubicin alone and in combination with PSC-833, respectively. This combination including PSC-833 was well tolerated. Although a pharmacokinetic interaction might have been expected, PSC-833 did not significantly alter the disposition of idarubicin.