Abstract P1-17-10: H3B-6545, a novel selective estrogen receptor covalent antagonist (SERCA), in estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer - A phase II study
Authors: Hamilton Erika P , Wang Judy S , Pluard Timothy , Morikawa Aki , Dees E Claire , Jones Robert H , Haley Barbara , Armstrong Anne , Cohen Adam L , Munster Pamela , Wright Gail S , Kayali Fadi , Cantagallo Lisa , Korpal Manav , Long Jenny , Xiao Jianjun , Destenaves Benoit , Gao Lei , Sahmoud Tarek , Gualberto Antonio , Juric Dejan 
Published: March 2022

Purpose: H3B-6545, a novel Selective ERα Covalent Antagonist (SERCA), inactivates both and wild-type and mutant ERα by targeting cysteine 530 and enforcing antagonist conformation. H3B-6545 demonstrated preclinical high activity in breast cancer models with constitutively active ESR1 mutations (Furman C, SABCS 2020) and clinical activity in pretreated women with ER+ breast cancer (Hamilton EP, ASCO 2021). Patients and Methods: The primary objective of the phase II is to estimate the objective response rate (ORR), clinical benefit rate (CBR), progression-free survival (PFS), and secondary objectives include safety. Results: 94 pts were treated with 450 mg daily, the recommended phase II dose: 11 in the phase I and 83 in the phase II parts of the trial. Patients and tumor characteristics were presented previously (Hamilton EP, ASCO, 2021). As of March 31, 2021, grade (gr) 2 or higher adverse events (AE) reported in ≥10% were anemia (19%), nausea (17%), fatigue (16%), and diarrhea (12%). Laboratory gr 2 or higher abnormalities reported in ≥10% pts were creatinine clearance decrease (39%), hemoglobin decrease (38%), Lymphocytes decrease (37%), ALT increase (14%), AST increase (13%), bilirubin increase (12%), and creatinine increase (12%). AE of gr 1 sinus bradycardia (asymptomatic) was reported in 35% and gr 2 (symptomatic, no intervention needed) was reported in 5%. Gr 2 and 3 QTcF prolongation were reported in 2 and 3 pts, respectively. There were no treatment-related deaths. Efficacy estimates are presented in Table 1.

CI: Confidence interval. N: total number of pts in full analysis set, used in PFS analysis. n: number of response-evaluable pts, used in ORR and CBR analysis. Efficacy estimates were consistent across the various subgroups. Responses were demonstrated in heavily pretreated pts, pts with visceral metastases, pts with constitutionally active ESR1 Y537S mutations, and in pts who received chemotherapy in the metastatic setting. Numerically higher response rate and longer PFS were observed in pts with ESR1 Y537s. Conclusions: H3B-6545 has a manageable safety profile and demonstrated single-agent anti-tumor activity in heavily pretreated ER+, HER2- mBC patients. Clinical activity was consistent across the various subgroups. Tumors harboring the constitutionally active ESR1 Y537S mutation may present higher ERα activity, and consequently enrich for luminal A traits and demonstrate greater lineage dependence on ERα.

Table 1.Consistency of H3B-6545 activity across the key subgroupsEfficacyClinical CharacteristicORR % (95% CI)CBR % (95% CI)Median PFS mo (95% CI)Overall population (N=94, n=72)16.7 (8.9, 27.3)40.3 (28.9, 52.5)5.1 (3.2, 6.2)Liver and/or lung metastases (N=76, n=62)17.7 (9.2, 29.5)41.9 (29.5, 55.2)5.4 (1.8, 7.2)≥3 prior regimens (N=49, n=39)20.5 (9.3, 36.5)48.7 (32.4, 65.2)5.4 (3.5, 7.3)Prior chemotherapy (N=47, n=35)17.1 (6.6, 33.6)51.4 (34.0, 68.6)5.5 (3.6, 7.3)PgR+ (N=38, n=32)15.6 (5.3, 32.8)50.0 (31.9, 68.1)5.4 (2.0, 8.8)ESR1 clonal Y537S (N=10, n=10)30.0 (6.7, 65.2)60.0 (26.2, 87.8)7.3 (0.8, 11.2)ESR1 clonal D538G (N=19, n=17)0.0 (0.0, 19.5)35.3 (14.2, 61.7)5.4 (1.7, 7.2)

Citation Format: Erika P Hamilton, Judy S Wang, Timothy Pluard, Aki Morikawa, E Claire Dees, Robert H Jones, Barbara Haley, Anne Armstrong, Adam L Cohen, Pamela Munster, Gail S Wright, Fadi Kayali, Lisa Cantagallo, Manav Korpal, Jenny Long, Jianjun Xiao, Benoit Destenaves, Lei Gao, Tarek Sahmoud, Antonio Gualberto, Dejan Juric. H3B-6545, a novel selective estrogen receptor covalent antagonist (SERCA), in estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer - A phase II study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-17-10.