A Phase I Trial of the ABCB1 Inhibitor, Oral Valspodar, in Combination With Paclitaxel in Patients With Advanced Solid Tumors

Objectives: Multidrug resistance mediated by P-glycoprotein is a potential obstacle to cancer treatment. This phase 1 trial determined the safety of paclitaxel with valspodar, a P-glycoprotein inhibitor, in patients with advanced solid tumors. Patients and Methods: Patients were treated with single-agent paclitaxel Q3W 175 mg/m2 (or 135 mg/m2 if heavily pretreated) as a 3-hour infusion. If their disease was stable (SD) or progressive (PD), paclitaxel at 30% (52.5 mg/m2), 40% (70 mg/m2), or 50% (87.5 mg/m2) of 175 mg/m2 (full dose) was administered with valspodar 5 mg/kg orally 4 times daily for 12 doses. Pharmacokinetic sampling (PK) for paclitaxel and valspodar was performed during single-agent and combination therapy. Results: Sixteen patients had SD/PD after one cycle of paclitaxel and then received paclitaxel at 30% (n=3), 40% (n=3), and 50% (n=10) with valspodar. Hematologic adverse events (AEs) including myelosuppression at paclitaxel 40% were comparable to those of full-dose paclitaxel. Non-hematologic AEs consisted of reversible hepatic (hyperbilirubinemia and transaminitis) and neurologic AEs (ataxia and paresthesias). Eleven patients experienced SD with a median of 12.7 weeks (range, 5.4 to 36.0), 4 patients progressed, and 1 was inevaluable. Reduced dose paclitaxel with valspodar resulted in lower plasma peak concentrations of paclitaxel; otherwise, concentrations were similar to single-agent paclitaxel. Conclusion: Paclitaxel at 70 mg/m2 was administered safely with valspodar. Limited efficacy in hematologic and solid tumors resulted in discontinuation of its clinical development and other transporter inhibitors. Recently, the development of ATP-binding cassette transporter inhibitors has been reconsidered to mitigate resistance to antibody-drug conjugates.