Abstract

Fluconazole is increasingly used in children receiving chemotherapy. Many of these patients are being treated with cyclophosphamide, which must undergo hepatic metabolism to produce active alkylating species. As a consequence of the cytochrome P-450 inhibitory properties of fluconazole, a potential interaction exists between these two agents that could influence the therapeutic effect of cyclophosphamide. To investigate this interaction, a retrospective case series of patients was chosen from a population of children with a previously established profile of cyclophosphamide metabolism. Twenty-two children who were not receiving other therapy known to influence drug metabolism were selected and analyzed in terms of fluconazole treatment; of these, nine were receiving fluconazole and thirteen were identified as controls. Study design was not randomized. The plasma clearance of cyclophosphamide was lower in patients receiving fluconazole [mean(SD) 2.4(0.71) versus 4.2(1.2) l/h/m², p = .001]. In vitro studies were performed to characterize the interaction between fluconazole and cyclophosphamide in six human liver microsomes. The concentration of fluconazole required to reduce the production of 4-hydroxycyclophosphamide to 50% of control values (IC₅₀) varied between 9 and 80 μM (median 38 μM). Further studies of the effect of fluconazole on 4-hydroxycyclophosphamide production in vivo are warranted to determine whether this interaction reduces the therapeutic effect of cyclophosphamide in clinical practice.