DESTINY-Breast07: Dose-expansion interim analysis of T-DXd monotherapy and T-DXd + pertuzumab in patients with previously untreated HER2+ mBC.

1009 Background: Trastuzumab deruxtecan (T-DXd) is approved for adult patients (pts) with HER2+ advanced/metastatic breast cancer (mBC) who received a prior anti-HER2–based regimen. DESTINY-Breast07 is a Phase 1b/2 multicenter, open-label, modular study exploring the safety, tolerability, and antitumor activity of T-DXd alone or in combination with other anticancer agents (NCT04538742). These results are from an interim analysis of the dose-expansion phase assessing T-DXd ± pertuzumab (P) as first-line (1L) treatment in HER2+ mBC. Methods: Pts had locally assessed HER2+ mBC with measurable disease and no or stable brain metastases. A disease-free interval of ≥12 months from (neo)adjuvant therapy was required; no prior therapy for mBC was allowed. Pts were stratified by hormone receptor and disease status (recurrent vs de novo), and PD-L1 expression. Pts received T-DXd 5.4 mg/kg intravenously (IV) every 3 weeks (Q3W) as monotherapy or in combination with P 420 mg IV Q3W, with an 840 mg loading dose. Primary endpoints were safety and tolerability; key secondary endpoints included objective response rate (ORR) and progression-free survival (PFS), per RECIST 1.1 by investigator. Results: Seventy-five pts were treated in the T-DXd module and 50 pts in the T-DXd + P module; median follow up was 19.2 months (range 8.7–29.2) and 20.6 months (range 13.3–26.7), respectively. Median age was 57 years in both modules. As of August 1, 2023, the most common adverse event (AE) was nausea (T-DXd, 70.7% [4.0% Grade 3]; T-DXd + P, 68.0% [0% Grade 3]). Diarrhea was reported in 34.7% (2.7% Grade 3) and 60.0% (6.0% Grade 3) of pts in the T-DXd and T-DXd + P modules, respectively. There were no Grade ≥4 nausea or diarrhea events. Adjudicated drug-related interstitial lung disease (ILD) / pneumonitis was reported in six (8.0%) and five (10.0%) pts in the T-DXd and T-DXd + P modules, respectively (all Grade ≤2). One non-treatment-related AE with outcome of death was reported in the T-DXd module (post-acute COVID-19 syndrome); none with T-DXd + P. The confirmed ORR was 77.3% (80% confidence interval [CI] 70.0, 83.6) with T-DXd and 82.0% (80% CI 73.1, 88.8) with T-DXd + P. PFS rate at 12 months was 77.3% (80% CI 69.0, 83.7) with T-DXd and 89.4% (80% CI 81.9, 93.9) with T-DXd + P. Conclusions: Safety profiles were consistent with the known profiles for T-DXd and P, with no Grade ≥3 ILD events. Early data showed promising efficacy in both modules. The DESTINY-Breast07 study is ongoing; analyses from the Phase 3 DESTINY-Breast09 clinical trial will provide definitive data on TDXd ± P in 1L HER2+ mBC. Clinical trial information: NCT04538742 . [Table: see text]