Abstract PD7-05: A first-in-human phase I study to evaluate the oral selective estrogen receptor degrader (SERD), GDC-9545, in postmenopausal women with estrogen receptor-positive (ER+) HER2-negative (HER2-) metastatic breast cancer
Authors: Jhaveri Komal , Winer Eric Paul , Lim Elgene , Fidalgo Jose AlejandroPerez , Bellet Meritxell , Mayer Ingrid Alina , Boni Valentina , Patel Jaymin M , Bardia Aditya , Garcia Jose Manuel , Kabos Peter , Gates Mary , Chen Ya-Chi , Fredrickson Jill , Wang Xiaojing , Friedman Lori Sickels , Spoerke Jill , Gendreau Steven , Metcalfe Ciara , Liu Lichuan , Chang Ching-Wei , Monemi Sharareh , Gonzalez Monica , McCurry Ursula , Milan Sandra , Humke Eric William , Loi Sherene 
Published: March 2020

Background: Modulation of estrogen activity and/or synthesis is a mainstay therapeutic strategy for ER+ breast cancer (BC). However, despite the effectiveness of available endocrine therapies, many patients ultimately relapse or develop resistance to these agents via estrogen-dependent and estrogen-independent mechanisms, including mutations in ESR1 affecting the ER ligand-binding domain, that drive ER-dependent transcription and proliferation in the absence of estrogen. ER antagonists that robustly suppress ER signaling in both ESR1 wild-type and mutant tumors may be of substantial therapeutic benefit. GDC-9545 is a novel, potent, non-steroidal, orally bioavailable, selective ER antagonist/ER degrader (SERD) that induces tumor regression in ER+ BC patient-derived xenograft models.

Methods: A phase I dose escalation study with 3+3 design was conducted in postmenopausal women with ER+ (HER2−) advanced or metastatic BC (mBC). Patients were eligible if they had advanced or mBC that had progressed while being treated with adjuvant endocrine therapy for ≥ 24 months and/or endocrine therapy in the advanced or mBC setting for ≥ 6 months. No more than 2 prior treatments for advanced or mBC. Primary objective was to determine the safety, pharmacokinetics (PK) and the recommended Phase 2 dose (RP2D) of GDC-9545. Pharmacodynamic (PD) activity was assessed with [18F]-fluoroestradiol (FES)-PET scans and, when feasible, by performing biomarker analysis of pre- and on-study tumor biopsies. Plasma samples were collected for PK analyses and imaging for response assessment (CT and bone scans) were repeated every 8 weeks.

Results: From November 27 2017 to February 1 2019 patients (n = 29) with a median age of 58 years (range 42-75) and a median number of prior therapies for mBC of 2 (range 1-4) were enrolled at 4 total daily dose levels (10, 30, 90 and 250 mg) given once daily (QD) while fasting. Increases in GDC-9545 exposure were approximately dose proportional and t1/2 was ~30 hours. Treatment related adverse events (AEs) were all Grade 1 or 2. The most common treatment-related AEs in ≥10% of patients were fatigue (21%, n=6), nausea (21%, n=6), hot flushes (17%, n=5), diarrhea (17%, n=5), arthralgia (17%, n=5), constipation (10%, n=3) and dyspepsia (10%, n=3). No dose-limiting toxicities, treatment-related serious adverse events (SAEs), Grade ≥ 3 events, or fatal events have been reported. Treatment interruption was required for 3 patients: one patient for a short episode of dyspepsia, vomiting, diarrhea, and dizziness; and two patients due to unrelated SAEs.

In 14 patients with FES-PET avid disease at baseline and post-baseline scans, 11 (78%) showed complete or near complete (&gt; 90%) suppression of FES uptake to background levels, including patients with ESR1 mutations. Evidence of reduced ER, PR, and Ki67 IHC values and an ER activity signature was observed in paired pre- and on-treatment biopsies (n = 7). Twelve of 27 patients (10 mg [n=2], 30 mg [4], 90 mg [4], 250 mg [2]) had either confirmed partial responses (n = 3) or were on study ≥ 24 weeks (CBR = 44%), including patients pretreated with CDK4/6i, fulvestrant or harboring baseline ESR1 mutations. The R2PD was 100 mg QD and was selected for dose-expansion in post- and pre/peri-menopausal women with and without palbociclib, which is ongoing. Updated results will be presented.

Conclusions: GDC-9545 is well-tolerated to date with PK exposure supporting QD dosing, evidence of robust PD target engagement, and encouraging anti-tumor activity in patients with advanced or ER+ mBC, including patients with ESR1 mutations. Dose expansion cohorts of GDC-9545 with or without palbociclib in post-menopausal and pre/peri-menopausal women with mBC are ongoing.

Citation Format: Komal Jhaveri, Eric Paul Winer, Elgene Lim, Jose AlejandroPerez Fidalgo, Meritxell Bellet, Ingrid Alina Mayer, Valentina Boni, Jaymin M Patel, Aditya Bardia, Jose Manuel Garcia, Peter Kabos, Mary Gates, Ya-Chi Chen, Jill Fredrickson, Xiaojing Wang, Lori Sickels Friedman, Jill Spoerke, Steven Gendreau, Ciara Metcalfe, Lichuan Liu, Ching-Wei Chang, Sharareh Monemi, Monica Gonzalez, Ursula McCurry, Sandra Milan, Eric William Humke, Sherene Loi. A first-in-human phase I study to evaluate the oral selective estrogen receptor degrader (SERD), GDC-9545, in postmenopausal women with estrogen receptor-positive (ER+) HER2-negative (HER2-) metastatic breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr PD7-05.