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Cyclophosphamide (CP) administration to rats produced a dose-dependent loss of hepatic NADPH-cytochrome-P450 reductase and microsomal mixed function oxidase (MFO) activities. In vitro CP, its metabolites (acrolein, phosphoramide mustard, 4-keto CP and nor-nitrogen mustard) and Ifosfamide, which is an analog of CP, were tested for their effects on the reductase activity. Only acrolein produced a significant loss of the reductase (66%). This loss of activity could be prevented by the presence of cysteine in the incubation mixture. Acrolein also produced a dose dependent loss of the activity when incubated with the purified reductase. These data suggest that CP-induced loss of the reductase results from interaction between CP metabolite acrolein and critical sulfhydryl groups in the reductase.

Summary The reaction of 2′-deoxyguanosine 3′-monophosphate with phosphoramide mustard resulted in the formation of several adducts. One of these adducts was formed by linking phosphoramide mustard to the phosphate group of 2′-deoxyguanosine 3′-monophosphate rather than by the generally accepted mechanism involving alkylation at the N7 position of guanine. This adduct served as an acceptor for the transfer of 32 P from[γ 32 P]ATP by polynucleotide kinase and thus could be detected by the sensitive 32 P-postlabeling assay.

Cyclophosphamide (CP) at high or repeated doses results in the depression of mixed function oxygenase activities of the liver. Recent studies have attributed this to the interaction between acrolein, a metabolite of CP, and sulfhydryl groups in cytochrome P-450. The present report demonstrates the protection afforded by N-acetylcysteine against acrolein-induced denaturation of cytochrome P-450 in vitro and CP-related depression of mixed function oxygenase in vivo . Co-administration of CP and innocuous chemicals that provide free sulfhydryl groups should, in the future, be useful in enhancing the therapeutic index of CP by either reducing some of the toxicity and/or by allowing the use of repeated treatment with lower but effective doses of CP.