H3B-6545 is a selective estrogen receptor covalent antagonist (SERCA) that is orally available. As a novel class of ERα antagonists, H3B-6545 is used in the study of metastatic breast cancers, such as estrogen receptor-positive (ER), human epidermal growth factor receptor 2-negative (HER2-) breast cancer.

H3B-6545 is a highly selective small molecule that potently antagonizes wild-type and mutant ERα in biochemical and cell-based assays. In vitro comparisons with standard of care and other experimental agents demonstrated increased cellular potency of H3B-6545 under both continuous and washout treatment conditions.[1]

H3B-6545, orally administered once daily, demonstrated potent activity and superior efficacy to fulvestrant in the MCF-7 xenograft model, with maximal antitumor activity at doses >10-fold below the maximum tolerated dose in mice. In addition, H3B-6545 demonstrated superior antitumor activity to tamoxifen and fulvestrant in xenograft models of estrogen receptor-positive breast cancer, including models harboring ERα mutations. In nonclinical safety studies in rats and monkeys, H3B-6545 was well tolerated over a wide dose range and at exposures significantly exceeding those required for efficacy in mouse xenograft models.[1]