PR-A isoform as a predictive marker of antiprogestin therapy in breast cancer: A tissue culture method to evaluate drug responsiveness

MAY M. MD1, ROJAS P. PHD1, SEQUEIRA G.1, MARTINEZ P. MD2, GONZÁLEZ P. MD2, GASS H. MD2, LANARI C. PHD1

Congreso; San Antonio Breast Cancer Symposium 2013; 2013

Two thirds of breast cancers express steroid hormone receptors and are susceptible to an endocrine therapy usually aimed to target the estrogen receptors (ER). There is however compelling evidence suggesting that progesterone receptors (PR) participate together with ER, modulating breast cancer growth. Based on our results obtained in an experimental breast cancer model in which antiprogestins induce complete regression of carcinomas with higher levels of PR isoform A (PR-A) than isoform B (PR-B), we propose that antiprogestins such as mifepristone (MFP) might be a possible therapeutic option for patients expressing high levels of PR-A. The goal of our study is to correlate the ratio of PR isoform expression, with the in vitro responsiveness of human breast cancer samples to MFP. Preliminary studies indicated that primary cultures of breast cancer samples overexpressing PR-A were responsive to MFP (10 nM) in vitro. However, since the rate of successful primary cultures was not encouraging (7%), we decided to develop a tissue culture assay to evaluate MFP responsiveness of breast cancer samples in vitro. Forty five breast cancer samples obtained from the General Pacheco´s Hospital, Buenos Aires, were kept in culture medium at room temperature and in dry ice. The former were cut in slices of 100 μm using a tissue chopper usually used to slice mouse nervous tissue for short incubations, and were incubated on filters in the presence of growth medium (fetal calf serum 10%) with or without MFP (10 nM), for 48 hs at 37º C. The slices were then fixed, paraffin embedded, and processed for Ki-67 staining to evaluate proliferating cells. Frozen samples were processed for Western blotting and the ratio of both PR isoforms was determined in nuclear extracts. In 17/45 (38%) of the samples valuable information was obtained. This was related with the quality (size and amount of viable cells) of the sample. The number of Ki-67 positive cells over the total viable cells was evaluated by a pathologist (M. May) in at least six different control or MFP-treated slices. From nine cases in which the Ki-67 index in control slices doubled the value obtained in MFP-treated slices (p