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The Effect of Ketoconazole on the Pharmacokinetics and Pharmacodynamics of Ixabepilone: A First in Class Epothilone B Analogue in Late-Phase Clinical Development

Authors' Affiliations: ¹ Albert Einstein Cancer Center, ² Department of Molecular Pharmacology, and ³ Division of Gynecologic Oncology, Department of Obstetrics and Gynecology, Albert Einstein College of Medicine, Bronx, New York; ⁴ Bristol-Myers Squibb Research and Development, Princeton, New Jersey; and ⁵ CEA, iBiTec-URA 2096 du Centre National de la Recherche Scientifique, SB²SM, CE-Saclay, Gif sur Yvette, France

Requests for reprints: Sridhar Mani, Albert Einstein College of Medicine, Albert Einstein Cancer Center, 1300 Morris Park Avenue, Chanin 302D-1, Bronx, NY 10461. Phone: 718-430-2871; Fax: 718-904-2830; E-mail: smani{at}montefiore.org.

Purpose: To determine if ixabepilone is a substrate for cytochrome P450 3A4 (CYP3A4) and if its metabolism by this cytochrome is clinically important, we did a clinical drug interaction study in humans using ketoconazole as an inhibitor of CYP3A4.

Experimental Design: Human microsomes were used to determine the cytochrome P450 enzyme(s) involved in the metabolism of ixabepilone. Computational docking (CYP3A4) studies were done for epothilone B and ixabepilone. A follow-up clinical study was done in patients with cancer to determine if 400 mg/d ketoconazole (inhibitor of CYP3A4) altered the pharmacokinetics, drug-target interactions, and pharmacodynamics of ixabepilone.

Results: Molecular modeling and human microsomal studies predicted ixabepilone to be a good substrate for CYP3A4. In patients, ketoconazole coadministration resulted in a maximum ixabepilone dose administration to 25 mg/m² when compared with single-agent therapy of 40 mg/m². Coadministration of ketoconazole with ixabepilone resulted in a 79% increase in AUC_0-. The relationship of microtubule bundle formation in peripheral blood mononuclear cells to plasma ixabepilone concentration was well described by the Hill equation. Microtubule bundle formation in peripheral blood mononuclear cells correlated with neutropenia.

Conclusions: Ixabepilone is a good CYP3A4 substrate in vitro; however, in humans, it is likely to be cleared by multiple mechanisms. Furthermore, our results provide evidence that there is a direct relationship between ixabepilone pharmacokinetics, neutrophil counts, and microtubule bundle formation in PBMCs. Strong inhibitors of CYP3A4 should be used cautiously in the context of ixabepilone dosing.