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

Summary 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 K m for cyclophosphamide is 0.5 mm; nicotine, atropine, ephedrine, apomorphine, and cocaine are potent inhibitors. Phenobarbital, cytochrome c , 2-diethylaminoethyl-2,2-diphenylvalerate, 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 N,N -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 L1210 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 L1210 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. Lung homogenates accomplish the initial oxidation of both cyclophosphamide and nicotine but do not metabolize the products further. Kidney homogenates contain the amine oxidase producing nicotine 1′-oxide. Several other tissues are not active in the metabolism of either cyclophosphamide or nicotine.