DNA synthesis during hormone-dependent mammary tumor regression in rats.

A serial biopsy method was developed to study DNA synthesis in 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumors during the regression process induced by bromocryptine (CB-154) administration in highly inbred female SD rats. With this technique the changes in tumor size could be correlated with those of DNA synthesis in single regressing tumors. DNA synthesis was estimated by the in vitro incorporation of tritiated thymidine into DNA which correlated well (correlation coefficient r = 0.95) with the in vivo mitotic activity of these neoplasms. Neither the biopsies themselves nor the estral status of the hosts affected significantly the rate of tumor DNA synthesis. DNA synthesis decreased sharply 4-8 days after the beginning of CB-154 treatment, whereas tumor shrinkage occurred more gradually. In conclusion, the serial biopsy method is a reliable technique for the estimation of changes in DNA synthesis in regressing DMBA-induced rat mammary tumors, whereas the measurement of the rate of tumor shrinkage is not.