Enzymatic metabolism of cyclophosphamide and nicotine and production of a toxic cyclophosphamide metabolite.

Cyclophosphamide is converted by enzymes of mouse liver into two metabolites. Production of the first (aldophosphamide), which is uncharged, requires TPNH, is inhibited by CO, and is accomplished predominantly by the microsomal fraction. With the microsomal enzyme, the Km for Cyclophosphamide is 0.5 mM; nicotine, atropine, ephedrine, apomorphine, and cocaine are potent inhibitors. Phénobarbital, cytochrome c, 2-diethylaminoethy 1-2,2diphenylvalerate, and some steroid hormones also inhibit the reaction. Aldophosphamide is very toxic, as judged by inhibition of clone formation of human epidermoid carcinoma No. 2 cells and by toxicity to L1210 leukemia cells. The initial metabolite is further converted to 2 -carboxyethyl 7V,A^-bis-(2-chloroethyl)phosphorodiamidate (carboxyphosphamide) by an enzyme in the soluble portion of the cell. This enzyme can be replaced by purified aldehyde oxidase (aldehyde: oxygen oxidoreductase, EC 1.2.3.1). Carboxyphosphamide, which has little or no antitumor effect, is much less toxic to clone formation of human epidermoid carcinoma No. 2 cells and to LI210 cells. Administration of pyridoxal, which could saturate the endogenous aldehyde oxidase and thus delay the production of carboxyphosphamide, in combination with Cyclophosphamide increases the life-span of mice implanted with LI210 cells. The metabolic conversion of nicotine to cotinine by liver proceeds in the same manner as Cyclophosphamide oxidation. Nicotine is also oxidized by an amine oxidase to nicotine 1'-oxide.