IFOSFAMIDE CLINICAL PHARMACOKINETICS

This article reviews the metabolism and pharmacokinetics of ifosfamide and their implications for the efficacy and toxicity pattern of this alkylating agent. Ifosfamide is a prodrug that requires biotransformation to become cytotoxic. It is a structural isomer of cyclophosphamide from which it differs only in having the chlorethyl functions on different nitrogen atoms. This causes a considerable change in initial metabolism, although overall metabolism remains the same. Beside the formation of 4-hydroxy-ifosfamide ('activated-ifosfamide'), a second pathway with liberation of chloroacetaldehyde exists. Therefore; less activated drug is formed than during cyclo-phosphamide metabolism, This fact may well explain why higher doses of ifosfamide are required during treatment. Chloroacetaldehyde may account for the adverse effects and therapeutic effects of the parent drug. This metabolite has been associated with central nervous system toxicity during ifosfamide treatment and was shown to;deplete intracellular glutathione concentrations. Glutathione depletion may support the activity of alkylating metabolites in tumour cells, thus overcoming the relative resistance of the cells to alkylating agents. Possibly, this mechanism explains the lack of complete cross-resistance between ifosfamide and cyclophosphamide as well as the greater antitumour activity of ifosfamide in some tumours. Urotoxicity of ifosfamide, which was the dose-limiting adverse effect, can be successfully attenuated by the use of mesna. Distinct pharmacokinetic properties of mesna are responsible for the fact that in contrast to other sulphydryl compounds the uroprotective activity of mesna does not imply a loss of therapeutic efficacy.