277MO SAR439859, an oral selective estrogen receptor (ER) degrader (SERD), in ER+/ HER2- metastatic breast cancer (mBC): Biomarker analyses from a phase I/II study

SAR439859 has antitumor activity in patients (pts) with wild type (WT) and mutated ESR1 mBC. Here we describe tumor molecular features and evidence of on target activity in SAR439859-treated pts. Plasma circulating cell-free DNA (cfDNA) and paired biopsies were collected at baseline (BL), on treatment (OT) and end of treatment (EOT: cfDNA only) from heavily pretreated postmenopausal pts with ER+/HER2- mBC who received SAR439859 monotherapy (Part...