- Concurrent use of pimozide, terfenadine, astemizole, or cisapride (fosaprepitatnt is a dose-dependent inhibitor of cytochrome P450 isoenzyme 3A4 (CYP3A4). If used concurrently with above agents, there can be elevated plasma concentrations of these drugs, potentially causing serious or life-threatening reactions. Patients may be enrolled on the study if at least 7 days have elapsed since last dose of such a medication.