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An open-label, clinical pilot study was performed to study the effect of cyclosporine A (CsA) on single-dose pharmacokinetics of itraconazole in patients with a hematologic malignancy. Patients (n = 10), admitted for allogeneic stem cell transplantation, received a single dose of 200 mg itraconazole in a 1-hour intravenous infusion during their treatment period before initiation of CsA. This was repeated during the period that CsA was administered and a steady-state concentration of CsA was achieved (trough whole blood level 200-400 ng/mL). After both administrations of itraconazole, serum pharmacokinetics of itraconazole and hydroxy (OH) itraconazole were determined during 24 hours. The results were compared with each patient acting as his or her own control. Exposure to itraconazole, as measured by the AUC[0-24h], was not significantly altered when combined with CsA. Large interindividual variations were observed in area under the concentration curve values among patients. In contrast, exposure to OH-itraconazole was significantly increased when itraconazole was coadministered with CsA (median increase of AUC[0-24h] 49%) with significant prolongation of T(max) and T1/2 (median increase of T(max) 37% and T1/2 176%). These differences may be the result of variability in affinity of itraconazole, OH-itraconazole, and CsA for the cytochrome P450 3A4 metabolic system and the occurrence of P-glycoprotein polymorphisms. In conclusion, exposure to OH-itraconazole, but not to itraconazole, is increased when itraconazole is coadministered with CsA. Although the interaction profile of itraconazole and CsA remains complex, these findings may be of importance in patients in whom monitoring of itraconazole serum levels is warranted, for example, in those with life-threatening fungal infections or in those who receive concurrent cytochrome inducers or inhibitors.