N-Nitroso-N-methylurea elicits mammary cancer in resistant and sensitive rat strains

A single intravenous (i.v.) injection of the water-soluble mammary carcinogen N-nitroso-N-methylurea (NMU; 35 mg/kg of body weight) elicited cancer of the breast in young female rats of two strains in the following incidence: Long-Evans strain, 4%; Sprague-Dawley strain, 70%. In sisters of these rats, a set of 5 i.v. injections of NMU (35 mg/kg at biweekly intervals) evoked mammary carcinoma as follows: Long-Evans strain, 76%; Sprague-Dawley strain, 100%. In its effectiveness in evoking mammary cancer in Sprague-Dawley rats, the lipid-soluble mammary carcinogen 7,8,12-trimethylbenz[a]anthracene exceeded NMU in rapidity of development of cancer and in tumor yield.