Abstract P5-14-01: Telapristone acetate abrogates PR-dependent paracrine-mediated mammary cell proliferation
Authors: Lee O , Sun L , Karavites LC , Clare SE , Khan SA 
Published: February 2018

Abstract
Background: Blockade of the progesterone (P)-progesterone receptor (PR) axis is a novel but untested strategy for breast cancer prevention. We report preclinical data evaluating telapristone acetate (TPA) compared with mifepristone (MFP), the prototype PR-antagonist. We hypothesize that the progesterone-PR blockade by TPA will inhibit PR-dependent paracrine expression (RANKL, WNT4, ID4, and calcitonin) attenuating cell proliferation and abrogating side branching and alveoli formation of mammary glands induced by hormones similar in extent or superior to MFP.
Methods:Adult virgin FVB mice at 12 weeks of age were randomized to four treatment groups: no treatment control, EP (0.3 mg E + 30 mg P), EP + TPA (30mg), and EP + MFP (30mg). Hormone and drug pellets were subcutaneously implanted in flank area between the neck and shoulder. After 28-day treatments, the mice were euthanized to collect mammary glands, and processed as mammary whole mounts and as formalin-fixed paraffin embedding (FFPE) specimens. We evaluated cell proliferation (Ki67) by immunohistochemistry. Total RNA was extracted from FFPE specimens and the Nanostring nCounter assay was utilized to assess paracrine gene expression. The Mann Whitney test was used to calculate statistical significance (p&lt;0.05).
Results: We observed a considerable increase in side branches and alveoli in EP treatment group compared to the controls. The growth of the mammary gland stimulated by EP treatment was corroborated by a significant increase in Ki67 compared to control mice (median 41% vs 24%, respectively, p &lt; 0.0001). Both TPA and MFP treatment abrogated side branching and alveoli formation and significantly reduced median Ki67 to 1/3 of that of control mice having endogenous estrus cycle hormone levels (24%, 8%, and 9% for control, EP+TPA and EP+MFP groups, respectively).TPA induced greater Ki67 reduction than MFP (7.5 ± 1.9% vs. 9.4 ± 1.9%, p= 0.04). As expected, EP treatment upregulated the RANKL expression compared to the control group while the administration of TPA and MFP significantly inhibited the RANKL expression stimulated by EP treatment. The mRNA expression measurement revealed that Rankl expression was 18 fold increased by EP treatment but completely inhibited by TPA and MFP treatment (p&lt;0.001 for both drugs). Similarly the Rank expression was increased two-fold by EP treatment (p&lt; 0.05) but down-regulated by TPA and MFP treatment (p&lt;0.01 for both drugs). We observed significant upregulation of Wnt4, and Calca expression by EP treatment compared to controls (p&lt;0.01 for both). Id4 expression showed the same trend but the change was non-significant. The addition of TPA completely opposed the hormone-stimulated increase of three paracrine molecules (Wnt4, Calca, and Id4) to a level below control group (p&lt; 0.001 for all).
Conclusions: We have demonstrated that TPA abrogates cell proliferation induced by exogenous EP hormones in an ovary intact mouse model. The blockade of PR-P binding was evident by complete inhibition of paracrine expression (not only RANKL/RANK expression but also WNT4, Calcitonin, and ID4 expression). TPA was efficacious as MFP in opposing paracrine-induced mammary cell proliferation, and warrants further testing in a breast cancer prevention trial.
Citation Format: Lee O, Sun L, Karavites LC, Clare SE, Khan SA. Telapristone acetate abrogates PR-dependent paracrine-mediated mammary cell proliferation [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-14-01.