The first participant has been enrolled in a Phase 2 clinical trial evaluating the safety and efficacy of GDC-9545 in combination with palbociclib as an investigational treatment for postmenopausal women with estrogen receptor (ER)-positive and HER2-negative untreated early breast cancer. "The initiation of enrolment is an important milestone as we aim to better understand the activity and safety of this therapy in early stage breast cancer," Sara Hurvitz, MD, medical director of the Jonsson Comprehensive Cancer Center Clinical Research Unit, in Canada, and the study's principal investigator, said in a press release. The growth of some breast cancers is driven by the hormone estrogen, with such tumors said to be ER-positive. Blocking estrogen signaling can be an effective treatment strategy for such tumors. "Although we have effective therapies for hormone receptor positive breast cancer, disease resistance to these therapies occurs in some women, leading to the diagnosis of incurable breast cancer sometimes years later," Hurvitz said.