Dimethylbenz(a)anthracen (DMBA)-induced tumors in rats were studied for changes in size, cytosolic estrogen (ER) and progesterone (PgR) receptors, and for peroxidase activity before treatment, after 15 days of treatment with the anti-estrogen tamoxifen, and 15 days following cessation of the treatment. Mean size of the tumors decreased significantly during the treatment period to 1/3 the original size and ER and PgR decreased significantly to 1/5 their original levels. Peroxidase activity increased 30-fold. All the changes reversed significantly 15 days after cessation of tamoxifen treatment. These results are consistent with a previous study in which regression of the tumors was induced by castration and recovery was brought about by treatment with estradiol. In DMBA-induced mammary tumors in rats, peroxidase activity in regressing and growing tumors is in inverse relation to changes in tumor size, ER and PgR, which are accepted indicators of estrogenic activity.
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