N-nitroso-N-methylurea-induced mammary carcinogenesis: effect of pregnancy on preneoplastic cells.

The effect of pregnancy and lactation on mammary cancers induced with N-nitroso-N-methylurea (NMU) was determined in female outbred Sprague-Dawley rats. The animals received 5 mg NMU/100 g body weight at 50 days of age and were divided into the following groups: virgin, pregnancy (beginning 10 days after NMU administration), pregnancy and lactation (beginning 10 days after NMU), and pregnancy and lactation (beginning 82 days after NMU). The time of appearance of the first palpable cancers was shorter in rats undergoing an early pregnancy. Few cancers, however, were detected from rats after pregnancy or pregnancy and lactation was completed, and a decrease in cancer incidence from virgin rats was observed in these animals at termination of the study. Since NMU is a direct-acting carcinogen with a short half-life, no effect of pregnancy on carcinogen metabolism or binding could have occurred. Preneoplastic cells present before pregnancy appeared to have been either altered (such that their latent period was increased) or destroyed by the hormones associated with pregnancy.