Abstract RF02-03: Trastuzumab deruxtecan (T-DXd) in combination with anastrozole or fulvestrant in patients with HER2-low HR+ advanced/metastatic breast cancer: a Phase 1b, open-label, multicenter, dose-expansion study (DESTINY-Breast08)

Abstract Background: Trastuzumab deruxtecan (T-DXd) is approved in over 40 countries for the treatment of adult patients with unresectable or metastatic human epidermal growth factor receptor 2 (HER2)-low (a score of 1+ on immunohistochemistry [IHC] analysis or an IHC score of 2+ and negative results on in-situ hybridization) breast cancer who have received prior chemotherapy in the metastatic setting or who developed disease recurrence during or within 6 months of completing adjuvant chemotherapy. DESTINY-Breast08 (DB-08) was designed to establish the safety, tolerability, and preliminary activity of T-DXd in combination with widely used standard of care therapies in HER2-low hormone receptor (HR)+ metastatic breast cancer (mBC). Results reported here are from the dose-expansion stage of the Phase 1b, multicenter, open-label, parallel-assignment DB-08 study arms investigating T-DXd–endocrine therapy (ET) combinations [NCT04556773]. Methods: Eligible patients had centrally confirmed HER2-low advanced/mBC with measurable disease per RECIST 1.1 and were documented as HR+. Among patients with mBC, ≤1 prior line of ET ± a targeted therapy was allowed, and prior chemotherapy in the mBC setting was exclusionary. Patients received T-DXd 5.4 mg/kg intravenously (IV) every three weeks (Q3W) + anastrozole 1 mg orally daily (T-DXd + ANA) or T-DXd 5.4 mg/kg IV Q3W + fulvestrant 500 mg intramuscularly Q4W, with a 500 mg loading dose on Cycle 1 Day 15 (T-DXd + FUL). Primary endpoints were safety and tolerability; secondary endpoints included objective response rate (ORR), progression-free survival, duration of response (all evaluated by investigator per RECIST 1.1), and overall survival (OS). Data cutoff (DCO) was ~24 weeks after the last patient in each arm had received study treatment. Results: As of February 20, 2023, 21 patients in the T-DXd + ANA arm and 20 patients in the T-DXd + FUL arm had received study treatment. Median age was 55 and 66 years, 67% and 75% of patients received a prior line of ET ± a targeted therapy for mBC, 33% and 25% had no prior line of treatment for mBC, and median follow up in censored patients was 12.1 months (range 4.0, 17.3) and 8.5 months (range 1.3, 15.1) in the T-DXd + ANA and T-DXd + FUL arms, respectively. Adverse events (AEs) occurred in 95.2% (20/21) of patients in the T-DXd + ANA arm and 100% (20/20) of patients in the T-DXd + FUL arm, with AEs ≥Grade 3 occurring in 47.6% (10/21) and 55.0% (11/20) of patients in each arm, respectively. The most common any-grade AEs, occurring in ≥30% of patients in either arm, are reported in the Table. Three (15%) adjudicated drug-related interstitial lung disease/pneumonitis events were reported in the T-DXd + FUL arm (all Grade 2; at DCO, one case had resolved, one was resolving, and one was not resolved); none were reported in the T-DXd + ANA arm. AEs were manageable by drug interruption and dose reduction. Confirmed ORR was 71.4% (15/21; 95% confidence interval [CI] 47.8, 88.7) in the T-DXd + ANA arm, and 40.0% (8/20; 95% CI 19.1, 64.0) in the T-DXd + FUL arm (Table). OS data were not mature at this DCO. Conclusions: Safety profiles for T-DXd-ET combinations were generally comparable to T-DXd monotherapy and manageable with dose modification and routine clinical practice. T-DXd in combination with anastrozole or fulvestrant was active in first- or second-line treatment of patients with HER2-low HR+ mBC. Citation Format: Komal Jhaveri, Fabrice André, Erika Hamilton, Peter Schmid, Carey Anders, Laura Testa, Inna Ganshina, Yen-Shen Lu, Seock-Ah Im, Robyn Young, Magdalena Wrona, Caron Lloyd, Yiwen Zhang, Sherene Loi. Trastuzumab deruxtecan (T-DXd) in combination with anastrozole or fulvestrant in patients with HER2-low HR+ advanced/metastatic breast cancer: a Phase 1b, open-label, multicenter, dose-expansion study (DESTINY-Breast08) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr RF02-03.