Effects of propylthiouracil on urinary metabolites of cyclophosphamide in rats.

Our previous studies have shown a protective effect of propylthiouracil (PTU) pretreatment against the toxicity of cyclophosphamide (CP). The present study was undertaken to investigate the mechanism of the PTU protection. CP is metabolized by the cytochrome P-450 drug-metabolizing enzyme system in the liver to alkylating metabolites, to active antineoplastic agents, and to acrolein, the most toxic and least antineoplastic metabolite. Measurements of CP metabolites in blood and urine during a 4-hr i.v. infusion of CP (50 mg/kg body weight/hr) showed urinary acrolein excretion to be 2.5 times higher in control rats as compared to PTU-treated rats. Since it has been reported that urinary acrolein levels are directly related to the frequency and severity of hemorrhagic cystitis, it is concluded from our observations that prevention of hemorrhagic cystitis is probably mediated by the PTU effect on lowering urinary acrolein concentration and excretion. Serum alkylating activity was significantly higher in the PTU-pretreated rats, which may enhance the antineoplastic potential of CP.