Influence of metabolic factors on the mutagenic effectiveness of cyclophosphamide in Drosophila melanogaster.

This paper describes the influence of changes in metabolic activity on the in-vivo mutagenic effectiveness of cyclophosphamide in Drosophila melanogaster. A dose-dependent increase in mutagenicity was observed until a plateau value is reached which was increased only slightly after enzyme induction with Aroclor 1254, whereas induction with phenobarbital resulted in a decrease, especially when cyclophosphamide was applied by injection. Treatment of the adult males with inhibitors of the monoamine oxidase (MAO, EC 1.4.3.4), such as iproniazid (Ipr), benzimidazole or tryptamine, led to a marked increase of the mutagenic effectiveness of cyclophosphamide especially in spermatocytes. This indicates the importance of metabolic de-activation processes for the limited mutagenicity of cyclophosphamide in Drosophila. The principal active metabolite of cyclophosphamide, phosphoramide mustard, is extensively de-activated by enzymes that can be inhibited by 1-phenylimidazole (PhI), presumably cytochrome P-450 (EC 1.14.14.1), but not by those blocked by MAO inhibitors. Inhibition of the FAD-containing dimethylaniline monooxygenase (FDMAM, EC 1.14.13.8) by N,N-dimethylbenzylamine (N,N-DMB) resulted in some increase in cyclophosphamide mutagenicity only in spermatids. The marginal mutagenicity of cyclophosphamide in Drosophila larvae could not be increased either by cytochrome P-450 induction with phenobarbital or by MAO inhibition with Ipr. In contrast to the failure of cyclophosphamide to induce rod-chromosome loss, a considerable activity was found when a ring-shaped chromosome was used. Similar to the sex-linked recessive lethal (SLRL) test, ring-X loss frequency could be enhanced by simultaneous treatment with MAO inhibitors. The observed ring-X loss frequency declined when males treated with cyclophosphamide were mated to DNA-repair deficient mei-9L1 females. Cyclophosphamide produces chromosome breaks, detected as 2-3 translocations, in Drosophila spermatocytes, the stage in spermatogenesis that is also the most sensitive to the induction of SLRL mutations.