Abstract PO2-20-01: The STOP-HER2 Trial: A Phase 2 Study of Stopping Trastuzumab - Outcomes in Patients with HER2+ Metastatic Breast Cancer (TBCRC 062)

Abstract BACKGROUND: Anti-HER2 therapies have transformed the trajectory for patients with HER2-positive (HER2+) MBC. A subset of patients with HER2+ MBC do exceptionally well and remain on maintenance anti-HER2 therapy for many years. However, current therapy is non-curative and patients are treated indefinitely. While these therapies often are well-tolerated, they may cause significant toxicities, are expensive, and typically require frequent infusion and provider visits. Case reports and retrospective series demonstrate that some patients may do well after stopping anti-HER2 treatment. We currently lack prospective data on the likelihood of clinical impact of successful treatment discontinuation, outcomes among patients who progress and restart HER2-directed therapy, and tools to predict who may safely stop therapy. In this study, we will enroll pts with an exceptional response to anti-HER2 therapy. Pts may elect to either continue or stop anti-HER2 therapy and will be followed clinically and radiographically on a protocol-specified schedule. Considering the central role of patients in choosing the study intervention arm, and the unique nature of stopping therapy as a treatment intervention, patient advocates have been deeply involved in the study design to ensure patient concerns are being heard and addressed. We will retrospectively assess minimal residual disease (MRD), measured in plasma, as a biomarker of ongoing response, in a step toward shifting the treatment paradigm for pts with HER2+ MBC. METHODS: STOP-HER2 is a prospective, non-randomized, phase II, multicenter study of either continuation (cohort 1) or cessation (cohort 2) of anti-HER2 therapy in exceptional responders with HER2+ MBC. Eligible patients must have biopsy-proven HER2+ MBC by ASCO/CAP guidelines and must have been receiving first-line anti-HER2 therapy for MBC for at least 3 years without evidence of progressive disease (PD) as determined by the treating investigator. Patients with prior brain-only PD or oligo-PD outside the brain are eligible at least 2 years after local treatment to the central nervous system (CNS) and in absence of subsequent PD. Up to fifty-two participants will be enrolled into cohort 2, and an additional thirty participants enrolled into the cohort 1, at 10 US sites within the Translational Breast Cancer Research Consortium. Co-primary endpoints are the 1-year progression-free survival (PFS) in cohorts 2 and 1, separately. Secondary endpoints include the clinical benefit rate upon re-initiation of anti-HER2 therapy for patients in cohort 2 experiencing PD after treatment discontinuation; and 3-year overall survival in both cohorts. Correlative endpoints include MRD status at baseline and the correlation between MRD dynamics and outcomes. Patient-reported outcomes will be collected and analyzed, including quality of life, illness intrusiveness, financial toxicity, anxiety, and decision regret. The study will use a two-stage design to assess the primary objective of estimating the rate of participants who are free of progression at one year after discontinuing anti-HER2 therapy. If 50% or less are free of progression at one year, stopping HER2 therapy would not be considered a viable treatment plan (null hypothesis). The study is designed to have 80% power to reject the null hypothesis at the 10% type I error rate when 68% of exceptional responders are free of progression at one year after stopping HER2 therapy. STOP-HER2 began enrollment in Q2 2023 (NCT05721248). Citation Format: Heather Parsons, Kathryn Ruddy, Stefania Morganti, Karen Smith, Victoria Attaya, Eliza Kallfelz, Michelle DeMeo, Jamie LaScala, Shirley Mertz, Teri Pollastro, Patricia Spears, Adam Brufsky, Chau Dang, Susan Dent, Ahmed Elkhanany, WIlliam Gwin, Ciara O'Sullivan, Kathy Miller, Sara Nunnery, Elaine Walsh, Anna Maria Storniolo, Sara Tolaney, Nabihah Tayob, Antonio Wolff, Eric Winer, Mothaffar Rimawi, Ian Krop, Nancy Lin. The STOP-HER2 Trial: A Phase 2 Study of Stopping Trastuzumab - Outcomes in Patients with HER2+ Metastatic Breast Cancer (TBCRC 062) 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 PO2-20-01.