Pharmacokinetics of ifosfamide.

A multicompartment pharmacokinetic model for ifosfamide has been employed using a system of first-order differential equations, which includes a term for metabolism according to Michaelis-Menten kinetics in order to describe the distribution and elimination parameters of ifosfamide in man. The model satisfactorily accounts for all the administered drug. The pseudometabolic rate constant for ifosfamide in man is found to be less than 20 percent of that reported for cyclophosphamide in man, in agreement with the more extensive metabolism of cyclophosphamide than ifosfamide. A number of the pharmacokinetic parameters for ifosfamide differ substantially from those reported for cyclophosphamide. The volume of distribution for ifosfamide metabolites was found to be approximately equal to the plasma space volume. The central compartment volume for intact ifosfamide is slightly larger than for cyclophosphamide and includes the easily diffusible extravascular space of the body and suggests lack of protein binding. The renal clearance of ifosfamide is low and about twice that of cyclophosphamide. The model indicates that only a small fraction of the total metabolites distribute into the peripheral compartment and suggests that multiple doses of the drug may be useful.