Methylnitrosophenylurea--a highly carcinogenic compound.

In vitro, 1-methyl-1-nitroso-3-phenylurea (MNPU) was easily formed from 1-methyl-3-phenylurea (MPU) and sodium nitrite in an acid environment. In rats MNPU showed a strong carcinogenic effect, inducing tumors of the forestomach. MNPU was formed endogenously after combined administration of MPU and nitrite to rats, as indicated by the induction of forestomach tumors. MPU itself was not carcinogenic. In the metabolism of the herbicide 1,1-dimethyl-3-phenylurea (fenuron) MPU resulted as a demethylation product. The possible significance for preventive oncology and the role of MNPU as an environmental carcinogen are briefly discussed.