Abstract GS1-01: Her2climb-05: a randomized, double-blind, phase 3 study of tucatinib versus placebo in combination with trastuzumab and pertuzumab as maintenance therapy for her2+ metastatic breast cancer

Abstract Background: The current standard of care for first-line (1L) treatment of human epidermal growth factor receptor 2-positive (HER2+) metastatic breast cancer (MBC) consists of induction chemotherapy combined with trastuzumab (TRAS) and pertuzumab (PERT) followed by maintenance therapy with TRAS + PERT. Although this regimen has been associated with sustained disease control, most patients eventually experience disease progression. The HER2CLIMB-05(NCT05132582) study is investigating the efficacy and safety of adding tucatinib to TRAS + PERT as maintenance therapy in patients with HER2+ MBC after completion of 1L chemotherapy-based induction therapy. Methods: Patients with centrally confirmed HER2+MBC without evidence of progression after 1L induction therapy with 4 to 8cycles of a taxane combined with TRAS + PERT, Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1, and no or asymptomatic brain metastases (BM) were enrolled in HER2CLIMB-05. Contrast-enhanced brain magnetic resonance imaging was performed at screening and regular intervals during the study. Patients were randomly assigned 1:1 to receive either tucatinib 300 mg or placebo twice daily, in combination with either TRAS (6 mg/kg IV or 600 mg SC) plus PERT (420 mg/kg IV) or a SC fixed-dose combination of TRAS, PERT, and hyaluronidase administered every 21 days. Endocrine therapy was permitted for patients with hormone receptor-positive tumors. The primary endpoint is investigator-assessed progression-free survival (PFS) per Response Evaluation Criteria in Solid Tumors v1.1. Secondary endpoints include overall survival (OS), PFS by blinded independent central review, investigator-assessed central nervous system (CNS)-PFS, and safety. Results: Between March 2022 and July 2024, 654 patients were randomly assigned to receive tucatinib (n = 326) or placebo (n = 328) with1L maintenance therapy. All patients were female (median age: 54 years [range, 24-83]);45.0% were White, 35.2% were Asian, 2.9% were Black/African American, and 19.3%were Hispanic/Latino(a)/Spanish origin. Most patients had ECOG PS of 0 (64.1%)and de novo metastatic disease (69.3%). At baseline, 12.4% of patients had presence or history of BM and 52.6% had hormone receptor-positive MBC. As of September5, 2025, PFS was statistically significantly improved with the addition of tucatinib versus placebo to 1L maintenance therapy (hazard ratio [HR] = 0.641 [95%confidence interval (CI), 0.514-0.799]; 2-sided P < 0.0001). All prespecified patient subgroups demonstrated a PFS benefit with tucatinib combination 1L maintenance therapy consistent with the overall population and regardless of presence or absence of BM at baseline or hormone-receptor status. OS is still immature (observed number of deaths = 51;tucatinib arm versus control arm HR = 0.539 [95% CI, 0.303-0.957; 2-sided P= 0.032]). In the safety analysis set, the most common treatment-emergent adverse events (TEAEs) in the tucatinib arm (n = 326) were diarrhea (72.7%), nausea (33.1%), and elevated liver enzymes (alanine aminotransferase: 28.2%; aspartate aminotransferase:25.8%), of which 6.1%, 0.9%, 13.5%, and 7.1% were, respectively, grade ≥ 3. In the tucatinib arm, 13.5% of patients discontinued tucatinib due to TEAEs. Conclusions: In the HER2CLIMB-05 trial, the addition of tucatinib to TRAS + PERT as 1L maintenance therapy demonstrated a statistically significant and clinically meaningful improvement in PFS with a manageable safety profile in patients with HER2+ MBC. Citation Format: E. Hamilton, G. Curigliano, M. Martin, F. Lerebours, J. Tsurutani, M. Savard, K. Jerzak, X. Hu, L. Martins de Aquino Pimentel, C. O'Sullivan, E. Tokunaga, A. Okines, C. Huang, W. Jacot, J. Sohn, E. Cronemberger Silva, V. Mueller, S. Yang, G. Granata, Q. Shen, L. Santarpia, V. Dieras Her2climb-05: a randomized, double-blind, phase 3 study of tucatinib versus placebo in combination with trastuzumab and pertuzumab as maintenance therapy for her2+ metastatic breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr GS1-01.