ZB716, also known as fulvestrant-3-boronic acid, is a synthetic, steroidal, orally active antiestrogen which is under development for the treatment of estrogen receptor (ER)-positive metastatic breast cancer. The drug is a silent antagonist of the ERα (IC50 = 4.1 nM) as well as a selective estrogen receptor degrader (SERD). It is an analogue and prodrug of fulvestrant in which the C3 hydroxyl group has been replaced with a boronic acid moiety. In accordance, the two drugs have similar pharmacodynamic properties. However, whereas fulvestrant is not orally active and must be administered via intramuscular injection, ZB716 is less susceptible to first-pass metabolism, and in relation to this, is orally active.