Abstract 3994: Differential regulation of hormone receptors and Ki67 in luminal breast carcinomas and tumor-adjacent mammary glands by mifepristone treatment

Abstract Since antiprogestins inhibit the growth of preclinical mammary tumor models expressing higher levels of progesterone receptor (PR) isoform A (PRA) than isoform B (PRB), we designed a window-of-opportunity trial (MIPRA; NCT02651844) to study the benefits of mifepristone (MFP) in luminal breast carcinomas from postmenopausal patients selected by their PR+ expression (>50%) and their PR isoform ratio (PRA/PRB>1.5; PRA-H). A decrease in the cell proliferation marker Ki67 was registered after treatment in 14 out of the 20 tumors evaluated. This inhibition was associated with a decrease in PR, estrogen receptor (ER) and pSer118ER evaluated by immunohistochemistry while no changes in pSer167ER expression were observed (PMID: 36269797). In selected tissues from these samples we observed that the staining intensity in trapped glands within the MFP-treated tumors did not follow a similar trend as that of tumor cells. Thus, the aim of this study was to evaluate the expression of hormone receptors and Ki67 in non-neoplastic human mammary glands (nnMG) adjacent to the PRA-H tumor tissue of postmenopausal patients, treated (n=8) or not (n=9) with MFP. No differences in PR, ER and pSer118ER expression were observed between MFP-treated nnMG and those from untreated patients. Notably, contrarily to what occurred in tumors, there was a significant decrease of pSer167ER expression (p=0.008) in the MFP-treated nnMG compared to the untreated glands, suggesting a selective modulation in the AKT-mediated activation of ER. Regarding the proliferative state of the nnMGs, there was a slight but significant increase (p=0.02) in Ki67 expression in MFP-treated vs. non-treated nnMG. Data regarding the effect of antiprogestins in normal mammary glands is limited and points, in premenopausal women, to a decrease in the Ki67 index after treatment. The basal quiescent status of nnMG in postmenopausal women may explain this slight stimulatory effect. In conclusion, our results show that MFP exerts a specific regulatory effect in PRA-H tumors that is not observed in the nnMG, probably due to the fact that nnMG have presumably equimolar levels of PRA and PRB. In addition, to counteract the possible stimulatory effect that MFP may induce on nnMG, the combination of MFP and tamoxifen treatment is suggested. Citation Format: Leo Saldain, Silvia Vanzulli, Paula Martinez Vazquez, Javier Burruchaga, Eunice Spengler, Claudia Lanari, Paola Rojas. Differential regulation of hormone receptors and Ki67 in luminal breast carcinomas and tumor-adjacent mammary glands by mifepristone treatment. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3994.