A pharmacodynamic study of docetaxel in combination with the P-glycoprotein antagonist tariquidar (XR9576) in patients with lung, ovarian, and cervical cancer

Purpose: P-glycoprotein (Pgp) antagonists have been difficult to develop because of complex pharmacokinetic interactions and a failure to show meaningful results. Here we report the results of a pharmacokinetic and pharmacodynamic trial using a third-generation, potent, noncompetitive inhibitor of Pgp, tariquidar (XR9576), in combination with docetaxel. Experimental Design: In the first treatment cycle, the pharmacokinetics of docetaxel (40 mg/m²) were evaluated after day 1 and day 8 doses, which were administered with or without tariquidar (150 mg). ^99mTc-sestamibi scanning and CD56 ⁺ mononuclear cell rhodamine efflux assays were conducted to assess Pgp inhibition. In subsequent cycles, 75 mg/m² docetaxel was administered with 150 mg tariquidar every 3 weeks. Results: Forty-eight patients were enrolled onto the trial. Nonhematologic grade 3/4 toxicities in 235 cycles were minimal. Tariquidar inhibited Pgp-mediated rhodamine efflux from CD56 ⁺ cells and reduced ^99mTc-sestamibi clearance from the liver. There was striking variability in basal sestamibi uptake; a 12% to 24% increase in visible lesions was noted in 8 of 10 patients with lung cancer. No significant difference in docetaxel disposition was observed in pairwise comparison with and without tariquidar. Four partial responses (PR) were seen (4/48); 3 in the non-small cell lung cancer (NSCLC) cohort, measuring 40%, 57%, and 67% by RECIST, and 1 PR in a patient with ovarian cancer. Conclusions: Tariquidar is well tolerated, with less observed systemic pharmacokinetic interaction than previous Pgp antagonists. Variable effects of tariquidar on retention of sestamibi in imageable lung cancers suggest that follow-up studies assessing tumor drug uptake in this patient population would be worthwhile.