acelERA Breast Cancer (BC): Phase II study evaluating efficacy and safety of giredestrant (GDC-9545) versus physician’s choice of endocrine monotherapy in patients (pts) with estrogen receptor-positive, HER2-negative (ER+/HER2-) locally advanced or metastatic breast cancer (LA/mBC).

TPS1100 Background: The standard-of-care therapy for ER+ BC typically involves modulation of estrogen synthesis and/or ER activity. Despite disease progression with standard treatments, growth and survival of the majority of tumors are thought to remain dependent on ER signaling; therefore, pts with ER+ BC can still respond to second- or third-line endocrine treatment (ET) after progression on prior therapy (Di Leo 2010; Baselga 2012). ESR1 mutations may drive estrogen-independent transcription and proliferation leading to resistance. The highly potent, nonsteroidal oral selective ER degrader, giredestrant, achieves robust ER occupancy and is active regardless of ESR1 mutation status. Phase I data have shown that giredestrant is well tolerated and active both as a single agent and in combination with the cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), palbociclib (Lim 2020). Single-agent giredestrant has also shown encouraging antitumor activity in pts previously treated with fulvestrant and/or a CDK4/6i. Methods: acelERA BC (NCT04576455) is a randomized, open-label, multicenter phase II study evaluating the efficacy and safety of giredestrant vs. physician’s choice of ET (fulvestrant or aromatase inhibitor) in males, or postmenopausal or pre/perimenopausal females with ER+/HER2– LA/mBC who have received 1–2 prior lines of systemic therapy in the LA or mBC settings, at least one of which must be ET. Randomization: 1:1 to receive giredestrant (30 mg PO QD on Days 1–28 of each 28-day cycle) or physician’s choice of ET per local guidelines. Men and pre-/perimenopausal women will receive a luteinizing hormone-releasing hormone agonist. Eligibility: ≥18 years, ECOG PS 0–1, histologically or cytologically confirmed diagnosis of LA (recurrent or progressed) or metastatic adenocarcinoma of the breast, measurable disease (per modified RECIST v1.1) or evaluable bone lesions, and ER+/HER2– tumors (locally assessed). Primary endpoint: progression-free survival (PFS; investigator-assessed per RECIST v1.1). Secondary endpoints: overall survival, objective response rate, duration of response, clinical benefit rate, PFS in pts with baseline ESR1 mutations, and quality of life. Safety, pharmacokinetics, biomarkers, and health status utility will also be assessed. Stratification: site of disease (visceral vs. nonvisceral), prior treatment with CDK4/6i (yes vs. no), and prior treatment with fulvestrant (yes vs. no). PFS will be compared using a stratified log-rank test; median PFS, using Kaplan–Meier analyses. Recruitment for the global enrollment phase is ongoing, the first patient was enrolled November 27, 2020. Clinical trial information: NCT04576455 .