We evaluated the mechanism of antitumor effects of
buserelin, which is one of
LH-RH agonists, on a
hormone dependent
breast cancer model, using
7,12-dimethylbenz(a)anthracene (DMBA)-induced rat
mammary cancer. Rats developing solid mammary
tumors within 5-7 weeks following the DMBA administration were divided into groups weekly, and treated without delay. The
tumor bearing rats were randomized into five groups with regard to
tumor size or average weight (15 rats per group). Each group received one of the following treatments during 4 weeks: a) no treatment (NT); b)
ovariectomy (Ovx); c)
buserelin; d) Ovx and 17beta-
estradiol (E2) (Ovx+E2); e) Ovx+E2+
buserelin.
Tumor regression immediately began at one week after both
buserelin treatment and
ovariectomy. A significant reduction of
tumor size was observed in both
buserelin-treated rats and Ovx rats compared with NT rats (p<0.01). No significant difference of
tumor size was observed between
buserelin-treated rats and ovariectomized rats. No reduction of
tumor size was observed in Ovx+E2 rats and Ovx+E2+
buserelin rats. Although the mean uterine wet weight of the
buserelin group was significantly higher than that of the Ovx group, it was significantly lower than that of the NT group. The mean uterine wet weight of the NT group, the Ovx+E2 group and the Ovx+E2+
buserelin group was similar and was significantly higher than that of the Ovx group.
Buserelin did not inhibit exogenous
estrogen-dependent
tumor growth in DMBA-induced rat
mammary cancers. These results suggest that
buserelin has no direct effects on DMBA-induced rat
mammary cancers, and the main mechanism of action of
buserelin for
tumor-reduction is due to ovarian
estrogen deficiency.