It has been reported that twice-weekly i.p.
injections of 4 mg
phorbol for 10 weeks, after a single feeding of 6 mg dimethylbenz(a)
anthracene (DMBA) in female Wistar rats, led to a significant augmentation of mammary
adenocarcinoma incidence and of lymphatic
leukemia incidence as compared to 6 mg DMBA alone. In an experiment reported here, in female Sprague-Dawley rats, using the same doses of DMBA and
phorbol and the same injection schedule,
phorbol given after DMBA did not augment mammary
adenocarcinoma incidence or lymphatic
leukemia incidence as compared to DMBA given alone. It thus appears that there is a strain-related sensitivity between Wistar and Sprague-Dawley rats with regard to the promoting activity of
phorbol when
phorbol treatment follows DMBA treatment, and mammary
adenocarcinoma incidence and lymphatic
leukemia incidence are studied. Further, in Sprague-Dawley rats,
phorbol did not promote mammary
fibroadenoma incidence in DMBA-treated rats, mammary
adenocarcinoma incidence in
procarbazine-treated rats, and mammary
adenocarcinoma incidence or mammary
fibroadenoma incidence in X-ray-treated rats. DMBA and
procarbazine, with or without
phorbol, tended to induce more
mammary neoplasms in the anterior (thoracic) than in the posterior (abdominal) mammary glands. X-irradiation tended to induce
mammary neoplasms in approximately equal numbers in the anterior and posterior mammary glands. It was suggested that regional differences in chemically induced mammary
carcinogenesis were due to a difference in the transport and delivery of the chemical
carcinogens to the regions rather than a difference in the amount of mammary gland tissue in the regions. An analysis of the numbers of Sprague-Dawley rats that developed either no
mammary neoplasms, or only mammary
adenocarcinomas, or only mammary
fibroadenomas, or both mammary
adenocarcinomas and mammary
fibroadenomas in response to DMBA,
procarbazine, and X-ray, suggested that the development of a mammary
adenocarcinoma or the development of a mammary
fibroadenoma are independent processes.