A phase I/II dose-escalation and expansion study of ZN-c5, an oral selective estrogen receptor degrader (SERD), as monotherapy and in combination with palbociclib in patients with advanced estrogen receptor (ER)+/HER2-breast cancer

Most patients with ER+/HER2- breast cancer are diagnosed at an early stage, receive adjuvant endocrine therapy (ET), and have a good prognosis. However, a subset relapses while on or after adjuvant ET and are placed on first-line ET. After initially responding, they however will progress. These patients account for the majority of breast cancer-related deaths. Fulvestrant, currently the only approved drug that binds and selectively degrades the ER, shows significant anti-tumor activity following ET failure.