Objectives: A few observational studies have suggested that progesterone and dydrogesterone may have a lower risk of breast cancer than other progestogens. In our earlier xenograft animal experiments, progesterone did not stimulate breast tumors. The aim of this study was to test dydrogesterone for the first time. The study also evaluated the effects of PGRMC1 on proliferation with progestogens. METHODS (1): In-vitro study. The proliferative effects of dydrogesterone and of progesterone were assessed in vitro using T47D cells transfected with PGRMC1 or empty vector in the presence or absence of estradiol. Additionally, to find the strongest proliferator for inclusion as a comparator in the xenograft animal study, norethisterone, levonorgestrel, desogestrel, dienogest, drospirenone, nomegestrol, and cyproterone acetate were tested. METHODS (2): Xenograft main study. PGRMC1-transfected or empty-vector T47D and MCF7 xenotransplants were each treated with four different hormonal preparations: E2+placebo; E2+dydrogesterone; E2+progesterone; E2+norethisterone. A total of 112 castrated mice were randomly allocated to the 16 groups. This was thus a prospective, randomized, blinded, placebo-controlled four-arm study (45-50 days) with the two T47D and two MCF7 xenografts. Tumor volumes were monitored twice weekly. RESULTS (1): In-vitro study. The strongest proliferation was with norethisterone, but only with PGRMC1-transfected cells. There was significant proliferation with dydrogesterone, but not with progesterone in the absence of estradiol. However, no increase in proliferation was achieved by adding dydrogesterone to estradiol compared with the proliferation induced with estradiol alone, in contrast to norethisterone. RESULTS (2): Xenograft main study. There was significantly faster tumor growth with norethisterone + E2 than with E2+placebo in T47D and MCF7 PGRMC1 xenografts, but not with dydrogesterone + E2 or progesterone + E2. There was less tumor growth in empty-vector xenografts, without between-group differences.
Conclusion: PGRMC1 increases the breast-cell proliferation effects of certain progestogens, including dydrogesterone, in contrast to progesterone, but not during estradiol-induced proliferation, either in vitro or in a xenograft animal model, in contrast to norethisterone. Thus the proliferative potency of dydrogesterone may be similar to that of progesterone. Clinical studies in women overexpressing PGRMC1 are recommended.
Keywords: Breast-cell proliferation; Breast-tumor growth; Dydrogesterone; Mouse xenograft model; Progesterone; Progestins.
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