NADPH-dependent enzyme-catalyzed reduction of aldophosphamide, the pivotal metabolite of cyclophosphamide.

One of the metabolites found in the urine of mammals given the prodrug cyclophosphamide is alcophosphamide, an alcohol. It is most probably generated from cyclophosphamide via aldophosphamide, an aldehyde which otherwise can directly give rise to phosphoramide mustard; the latter effects the cytotoxic action of cyclophosphamide and other oxazaphosphorines. It has already been demonstrated that horse liver alcohol dehydrogenase catalyzes the reduction of aldophosphamide to alcophosphamide. Herein, we report that aldose reductase and aldehyde reductase purified from human placenta also catalyze this reaction. The Km values for aldose reductase- and aldehyde reductase-catalyzed reduction of aldophosphamide to alcophosphamide were 0.15 and 1.6 mM, respectively. Aldose reductase and aldehyde reductase accounted for 94 and 6%, respectively, of total placental pyridine nucleotide-dependent enzyme-catalyzed aldophosphamide (160 microM) reduction. Aldose reductase-catalyzed reduction of aldophosphamide appeared to be noncompetitively inhibited by sorbinil; the Ki value was 0.4 microM. The in vivo significance of these observations is uncertain but could be of some magnitude since alcophosphamide is known to be only weakly cytotoxic.