Abstract PO4-17-02: A multicenter, prospective, observational study of patients receiving trastuzumab deruxtecan for the treatment of HER2-positive and HER2-low unresectable and/or metastatic breast cancer: DESTINY-Breast-RESPOND

Abstract Background: Approximately 20% of breast cancer (BC) cases are human epidermal growth factor receptor 2-positive (HER2+; immunohistochemistry IHC 3+, IHC 2+/in-situ hybridization ISH+), and up to ~50% of patients with primary or metastatic BC (mBC) have HER2-low tumors (IHC 1+, IHC 2+/ISH−), a new therapeutically targetable subset of BC. Trastuzumab deruxtecan (T-DXd) is a HER2-directed antibody-drug conjugate approved in the USA, EU, and other countries for the treatment of adult patients with unresectable or metastatic HER2+ BC who have received a prior anti-HER2-based regimen, or with unresectable or metastatic HER2-low BC who have received a prior chemotherapy (CTx) in the metastatic setting or developed disease recurrence during or within 6 months of completing adjuvant CTx. Approvals followed positive results from the DESTINY-Breast03 and DESTINY-Breast04 clinical trials. In DESTINY-Breast03, T-DXd improved median progression-free survival (mPFS) compared with trastuzumab emtansine (28.8 vs 6.8 months, respectively; P 0.0001) in previously treated (prior trastuzumab and taxane) patients with HER2+ mBC. In DESTINY-Breast04, T-DXd significantly improved mPFS vs physician’s choice of CTx (9.9 vs 5.1 months, respectively; P 0.001) in all randomized patients with HER2-low mBC who were previously treated with one or two lines of CTx. Alongside these efficacy benefits, T-DXd has demonstrated an acceptable and generally manageable safety profile. To optimize care and maximize treatment benefit, insights into real-world T-DXd use and safety events of interest, including patient management and experience, are needed. This study aims to describe the effectiveness and tolerability of T-DXd in patients with HER2+ or HER2-low unresectable and/or mBC in a real-world setting. Trial design: DESTINY-Breast-RESPOND (NCT05592483) is a multicenter (120 sites planned as of June 2023), multi-country (HER2+ cohort: several regions globally; HER2-low cohort: North America only), prospective, observational study characterizing real-world clinical use, effectiveness, tolerability, and patient experience among adults who initiated T-DXd monotherapy, per standard of care, for HER2+ (second line or earlier) and HER2-low unresectable and/or mBC (T-DXd not provided by study sponsor). The decision to initiate T-DXd will be made by the physician prior to, and independent from, participation in this study. Approximately 750 patients with HER2+ and 250 patients with HER2-low disease will be enrolled. Patients must have received prior treatment with a trastuzumab-containing regimen in the metastatic setting or have evidence of disease progression within 6 months of neoadjuvant or adjuvant treatment (HER2+ cohort), or prior CTx in the metastatic setting or have evidence of disease progression within 6 months of adjuvant CTx (HER2-low cohort). Data will be collected via chart abstraction, patient-reported outcome questionnaires on tolerability and safety (PGI-TT, NCI PRO-CTCAE, and EORTC IL19), and a daily patient nausea/vomiting symptom diary. Endpoints will be analyzed separately for HER2+ and HER2-low cohorts. The primary endpoint of interest is time to next treatment from T-DXd initiation, a measure of real-world effectiveness. Other important endpoints include treatment patterns, safety events of interest (nausea/vomiting, fatigue, alopecia, interstitial lung disease/pneumonitis, left ventricular ejection fraction decrease) and their management, time to T-DXd discontinuation, and patient-reported tolerability. Patients will be observed until end of study (~60% of patients receive subsequent treatment or have died), withdrawal from study, or loss to follow up, whichever occurs first. Citation Format: Joyce O'Shaughnessy, Reva Basho, Maryam Lustberg, Gary H Lyman, Manoj Prahladan, Gareth D. James, Della Varghese, Flavia Lujan, Hans Tesch. A multicenter, prospective, observational study of patients receiving trastuzumab deruxtecan for the treatment of HER2-positive and HER2-low unresectable and/or metastatic breast cancer: DESTINY-Breast-RESPOND 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 PO4-17-02.