101P Proteome and RNA-Seq changes in breast cancer patients with higher levels of progesterone receptor A than B after mifepristone treatment

Abstract

Antiprogestins inhibit experimental breast cancer growth when progesterone receptor isoform A (PRA) levels are higher than those of isoform B (PRB). Thus, we conducted a window of opportunity trial to determine the therapeutic effects of oral mifepristone (MFP, 200mg) in 20 breast cancer patients selected by their high PRA/PRB isoform ratio (MIPRA; NCT02651844). We identified 14/20 patients that achieved the pre-specified response parameter (30% relative reduction) showing a 49.62% decrease in Ki67 staining after MFP treatment (p = 0.0003).