Impact of casopitant, a novel NK-1 antagonist, on the pharmacokinetics of ondansetron and dexamethasone.

INTRODUCTION: Pharmacokinetic interactions between casopitant (a substrate and weak to moderate inhibitor of CYP3A), dexamethasone (a substrate and weak inducer of CYP3A), and ondansetron (a mixed CYP substrate) were evaluated in a two-part, three-period, single-sequence study in two groups of healthy subjects.

MATERIALS AND METHODS: Part 1: subjects received oral casopitant (regimen A); oral dexamethasone and IV ondansetron (regimen B); and oral casopitant, a reduced dose of oral dexamethasone, and IV ondansetron (regimen C). Part 2: subjects received oral casopitant (regimen D); IV dexamethasone and oral ondansetron (regimen E); and oral casopitant, IV dexamethasone, and oral ondansetron (regimen F). Each regimen was separated by 14 days.

RESULTS: Casopitant AUC in regimen C was increased 28% on day 1 but decreased 34% on day 3 compared to casopitant alone in regimen A. When given with casopitant and ondansetron in regimen C, dexamethasone AUC was 17% lower on day 1, but similar on day 3, compared to regimen B (representing dose-normalized increases in exposure of 39% and 108%, respectively). Ondansetron exposure was equivalent in regimens B and C. Casopitant AUC in regimen F was similar to regimen D on days 1 and 3. Dexamethasone AUC increased 21% when given with oral casopitant and oral ondansetron (regimen F compared to regimen E). Ondansetron exposure was equivalent in regimens E and F.

CONCLUSION: When repeat-dose oral dexamethasone is to be coadministered with oral casopitant, a reduction in dexamethasone dose may be considered; however, no change in casopitant dose is required. Ondansetron exposure was not affected by coadministration with casopitant.