Cyclophosphamide metabolism in children.

The alkylating agent cyclophosphamide is a prodrug which is metabolized in vivo to produce both therapeutic and toxic effects. Cyclophosphamide metabolism was investigated in 36 children with various malignancies. Concentrations of cyclophosphamide and its principal metabolites were measured in plasma and urine using a quantitative high-performance TLC method. The results indicated a high degree of inter-patient variation in metabolism. In contrast to previous adult studies on urinary metabolites, plasma carboxyphosphamide concentrations did not support the existence of polymorphic metabolism. Plasma concentrations of dechlorethylcyclophosphamide and carboxyphosphamide were correlated in individual patients, suggesting that the activity of both aldehyde dehydrogenase and cytochrome P450 enzyme(s) determine carboxyphosphamide production in vivo. The presence of ketocyclophosphamide in plasma was strongly associated with dexamethasone pretreatment and was also accompanied by a high clearance of the parent drug. Interpatient differences in metabolism reflect individual levels of enzyme expression and may contribute to variation in clinical effect.