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Background: Interactions between cytochrome P450 3A4 (CYP3A4) and its substrates are complex with multiple binding sites within the active site. BI 11634 is a factor Xa inhibitor in drug development and its interaction with CYP3A4 was evaluated., Methods: Reaction phenotyping studies were conducted to determine human isoform(s) of cytochrome P450 responsible for BI 11634 metabolism using recombinant CYPs and specific chemical inhibitors. Metabolite identification and quantitation were performed for incubations of [14C]BI 11634 with human liver microsomes (HLMs) or recombinant CYP3A4 (rCYP3A4) using a high-performance liquid chromatography-mass spectrometry-radiomatic detector. Inhibition of the CYP3A4-mediated metabolism of BI 11634 by quinidine was further evaluated., Results: From the reaction phenotyping studies, it was shown that the metabolism of BI 11634 in HLM was inhibited by ketoconazole and quinidine, well-accepted specific inhibitors of CYP3A4 and CYP2D6, respectively. In contrast, BI 11634 metabolism was exclusively mediated by rCYP3A4. Additional studies confirmed that BI 11634 was metabolized by CYP3A4 to form one major metabolite and this reaction was inhibited by quinidine with a Ki of 7 µM., Conclusions: These data indicated that BI 11634 may interact with CYP3A4 similar to nifedipine. CYP3A4 substrates have been categorized into three subgroups, including a stand-alone subgroup for dihydropyridine calcium channel blockers such as nifedipine and felodipine. In addition, this study emphasizes the importance of using rCYP in conjunction with approaches relying on inhibition when conducting CYP450 reaction phenotyping studies, as one single method may generate misleading results. The specificity of quinidine as a CYP2D6 inhibitor is questionable as it can also significantly inhibit CYP3A4-mediated metabolism of some compounds.