Half-life of oxazaphosphorines in biological fluids

Plasma AUC and half-life values for cyclophosphamide were determined in rats manipulated to hydroxylate cyclophosphamide at different rates; plasma AUC and apparent half-life values for two pharmacologically important metabolites of cyclophosphamide, viz. 4-hydroxycyclophosphamide/aldophosphamide and phosphoramide mustard, were also determined in these animals. Apparent plasma half-life values for 4-hydroxycyclophosphamide/aldophosphamide and phosphoramide mustard increased with an increase in plasma half-life values for cyclophosphamide. AUC values for cyclophosphamide increased approximately linearly with an increase in its plasma half-life but AUC values for 4-hydroxycyclophosphamide/aldophosphamide and phosphoramide mustard remained approximately constant with an increase in their respective apparent plasma half-life values. Given that the cytotoxic effects of cyclophosphamide are directly proportional to AUC values for 4-hydroxycyclophosphamide/aldophosphamide and/or phosphoramide mustard, we conclude that changes in the rate of cyclophosphamide hydroxylation will not alter the systemic toxic and therapeutic responses to a given dose of cyclophosphamide. Actual half-life values for 4-hydroxycyclophosphamide/aldophosphamide and phosphoramide mustard after the iv infusion of these agents were also determined. A comparison of the actual plasma half-life values for cyclophosphamide (29 min), 4-hydroxycyclophosphamide/aldophosphamide (14 min), and phosphoramide mustard (14 min) with the apparent plasma half-life values obtained for 4-hydroxycyclophosphamide/aldophosphamide (34 min) and phosphoramide mustard (55 min) following cyclophosphamide administration suggests that the major determinant with regard to the apparent plasma half-life of 4-hydroxycyclophosphamide/aldophosphamide is its rate of formation whereas in the case of phosphoramide mustard, an additional determinant, perhaps efflux from the cell, is operative.(ABSTRACT TRUNCATED AT 250 WORDS)