Abstract PS11-05: Updated data from SERENA-1: A Phase 1 dose escalation and expansion study of the next generation oral SERD AZD9833 as a monotherapy and in combination with palbociclib, in women with ER-positive, HER2-negative advanced breast cancer

Background: AZD9833 is an oral selective estrogen receptor (ER) antagonist and degrader (SERD) in Phase 2 clinical development for the treatment of ER+ HER2− breast cancer. Here we report data from Parts C and D of the ongoing Phase 1 study (SERENA-1) examining AZD9833 in combination with palbociclib, together with updated data from Parts A and B examining AZD9833 monotherapy. Methods: SERENA-1 (NCT03616587) is an ongoing open-label Phase 1 study of AZD9833 in pre- and post-menopausal women with ER+, HER2− advanced breast cancer who have previously received ≥1 endocrine therapy and ≤2 prior chemotherapies. Prior treatment with fulvestrant and/or CDK4/6 inhibitors was permitted. The primary objective is to determine the safety and tolerability of once daily (QD) AZD9833, with dose-limiting toxicities (DLTs) in the first 28 days defining the maximum tolerated dose. Secondary objectives include anti-tumor response (including circulating tumor [ct] DNA response) and pharmacokinetics. Parts A (escalation) and B (expansion) assess AZD9833 as a monotherapy, and Parts C (escalation) and D (expansion) assess AZD9833 in combination with palbociclib. Results: At a data cut-off of March 24 2020, 17 patients had received either 150 mg or 300 mg AZD9833 in combination with palbociclib, given according to its product labeling. Eighty patients had received AZD9833 monotherapy at doses of 25, 75, 150, 300, and 450 mg QD. In patients treated with AZD9833 and palbociclib, treatment-related adverse events (AEs; experienced by ≥10% of patients) included visual disturbances*, bradycardia*, asthenia, anemia, QTcF prolongation, nausea, neutropenia, decreased white blood cell count, and vomiting (*combined terms). All instances of AZD9833-related bradycardia were Grade 1. One DLT was observed in the 150 mg cohort: CTCAE Grade 2 visual disturbances, which began on Cycle 1 Day 8 and resolved by Cycle 1 Day 9 following dose interruption. The patient restarted treatment on Cycle 1 Day 15 at 75 mg and continued this dose until data cut-off. No causally related AEs led to discontinuation of AZD9833. The tolerability of AZD9833 with palbociclib was consistent with the observed tolerability profile of AZD9833 monotherapy, and the known tolerability profile of palbociclib. Pharmacokinetic analysis showed similar AZD9833 exposure for monotherapy and palbociclib combination therapy. Similarly, palbociclib exposure was comparable with simulations using a published population pharmacokinetic model. In Part A, ESR1 hotspot mutations were detected in ctDNA at baseline in 26/56 (46%) patients; 13/26 (50%) of these patients achieved a partial response or stable disease at 24 weeks, including 5/10 (50%) with a Y537S ESR1 mutation. Further, in patients with ESR1 mutations and samples available for longitudinal ctDNA analysis, 17/20 (85%) exhibited a reduction or loss of mutant ESR1 on treatment with AZD9833. Efficacy data to be presented include objective response rate and clinical benefit rate at 24 weeks. Of note, unconfirmed partial responses have been observed in Part C after the data cut-off for this abstract. Conclusions: AZD9833 continues to show an encouraging efficacy and dose-dependent safety profile as a monotherapy or in combination with palbociclib. A Phase 2 study comparing the efficacy and safety of three doses of AZD9833 versus fulvestrant (NCT04214288), and a Phase 2 pre-surgical ‘window of opportunity’ study (EUDRA-CT; 2019-003706-2) are ongoing. Citation Format: Richard Baird, Mafalda Oliveira, Eva Maria Ciruelos Gil, Manish R Patel, Begona Bermejo de las Heras, Manuel Ruiz-Borrego, Javier Garcia-Corbacho, Anne Armstrong, Udai Banerji, Chris Twelves, Valentina Boni, Jason Incorvati, Peter Kabos, Adam L Cohen, Bruno de Paula, Marta Capelan Rodriguez, Judy S Wang, Christina Hernando, Alejandro Falcon Gonzalez, Ivan Victoria Ruiz, Julia Lai-Kwon, Anosheh Afghani, Christos Vaklavas, Tim Brier, Steven Fox, Bistra Kirova, Teresa Klinowska, Chris Leach, Justin PO Lindemann, Richard Mather, Rhiannon Maudsley, Christopher J Morrow, Nitharsan Sathiyayogan, Andy Sykes, Li Zhang, Erika Hamilton. Updated data from SERENA-1: A Phase 1 dose escalation and expansion study of the next generation oral SERD AZD9833 as a monotherapy and in combination with palbociclib, in women with ER-positive, HER2-negative advanced breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS11-05.