molecules Article New Selective Progesterone Receptor Modulators and Their Impact on the RANK/RANKL Complex Activity Katarzyna Błaszczak-Swi´ ˛atkiewicz Department of Applied Pharmacy, Medical University of Lodz, Muszynskiego 1, 90-151 Lodz, Poland; [email protected]; Tel.: +48-426779240 Academic Editors: Simona Collina and Mariarosaria Miloso Received: 8 February 2020; Accepted: 11 March 2020; Published: 13 March 2020 Abstract: Breast cancer depends on women’s age. Its chemotherapy and hormone therapy lead to the loss of bone density and disruption of the skeleton. The proteins RANK and RANKL play a pivotal role in the formation of osteoclasts. It is also well established that the same proteins (RANK and RANKL) are the main molecules that play an important role in mammary stem cell biology. Mammary stem cells guarantee differentiation of the epithelial mammary cells, the growth of which is regulated by the progesterone-induced RANKL signaling pathway. The crosstalk between progesterone receptor, stimulated by progesterone and its analogues results in RANKL to RANK binding and activation of cell proliferation and subsequently unlimited expansion of the breast cancer cells. Therefore downstream regulation of this signaling pathway is desirable. To meet this need, a new class of selective estrogen receptor modulators (SPRMs) with anti- and mesoprogestin function were tested as potential anti-RANK agents. To establish the new feature of SPRMs, the impact of tested SPRMs on RANK-RANKL proteins interaction was tested. Furthermore, the cells proliferation upon RANKL stimulation, as well as NFkB and cyclin D1 expression, induced by tested SPRMs were analyzed. Conducted experiments proved NFkB expression inhibition as well as cyclin D1 expression limitation under asoprisnil and ulipristal treatment. The established paracrine anti-proliferative activity of antiprogestins together with competitive interaction with RANK make this class of compounds attractive for further study in order to deliver more evidence of their anti-RANK activity and potential application in the breast cancer therapy together with its accompanied osteoporosis. Keywords: RANK-RANKL complex; antiprogestins; mesoprogestins; osteoporosis; breast cancer 1. Introduction Breast cancer is a disorder which depends on the women’s age and hormonal system activity [1]. Estradiol determines the growth of mammary stem cells. It is well established that receptor activator of nuclear factor κB (RANK, the receptor for RANKL) and receptor activator of nuclear factor kappa-B ligand (RANKL) are the main molecules that play an important role in mammary stem cell biology. Differentiation of epithelial mammary cells is regulated by the progesterone-induced RANKL signaling pathway. Crosstalk between progesterone receptor, stimulated by progesterone, is expressed by the interaction between the progesterone receptor and RANKL, results in RANKL to RANK binding and activates cell proliferation and subsequently unlimited expansion of the breast cancer cells. Moreover, the same RANK-RANKL complex plays a pivotal role in osteoclastogenesis [2]. Osteoporosis often accompanies breast cancer treatment with selective estrogen receptor modulators (SERMs) and is a common disease, especially among menopausal women [3]. Estradiol deficiency during menopause or surgical removal of the ovaries is responsible for the loss of bone density in the female body. Although a similar bone density reduction effect is observed in correlation to low testosterone levels in the male body, osteoporosis is more common for women than men. 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