Tucatinib Maintenance Extends PFS Across HER2+ Breast Cancer Subgroups

Adding tucatinib (Tukysa) to first-line maintenance therapy with trastuzumab (Herceptin) and pertuzumab (Perjeta), commonly referred to as HP, more than doubled progression-free survival (PFS) in patients with HER2-positive (HER2+) metastatic breast cancer (mBC) and brain metastases, according to an updated subgroup analysis of the phase 3 HER2CLIMB-05 trial (NCT05132582) presented at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting. The benefit extended across every subgroup evaluated, including patients stratified by hormone receptor status and by de novo versus recurrent disease.

The analysis builds on the previously reported primary end point of investigator-assessed PFS from HER2CLIMB-05, a randomized, double-blind trial comparing tucatinib vs placebo, each added to HP maintenance therapy, in patients who had not progressed on induction treatment.

In an interview, Erika Hamilton, MD, director of Breast Cancer and Gynecologic Cancer Research at Sarah Cannon Research Institute, discussed the unmet need this analysis addresses, the key efficacy findings across subgroups, the significance of the brain metastases data, and questions that remain as the first-line treatment landscape for HER2+ mBC continues to evolve.

Targeted Oncology^TM: What unmet need does this analysis of HER2CLIMB-05 address in the first-line maintenance setting for HER2+ mBC?

Erika Hamilton, MD: Historically, patients with HER2+ mBC have been treated in the first-line setting with a taxane in combination with HP. After approximately 6 to 8 cycles, the taxane is stopped and patients transition to a maintenance period with HP alone. HER2CLIMB-05 was designed to ask whether adding tucatinib to that HP maintenance regimen extends PFS beyond what HP provides on its own.

What were the most significant efficacy findings from this subgroup analysis?

Among patients with brain metastases, who made up about 12.4% of the study population, PFS more than doubled with the addition of tucatinib, from a median of 4.2 months with placebo plus HP to 8.5 months with tucatinib plus HP. In patients without brain metastases, PFS improved from 18 months to 27 months, a benefit of about 9 months. A similar pattern was seen when patients were stratified by de novo vs recurrent disease, with tucatinib extending PFS by approximately 12 months in the de novo subgroup and approximately 9 months in the recurrent subgroup.

Why is the brain metastases subgroup particularly important to analyze in HER2+ mBC, and what does this update show?

Brain metastases are a particularly important end point in HER2+ mBC because up to about 50% of patients will develop brain metastases at some point during their metastatic disease course, and many HER2-targeted therapies do not cross the blood-brain barrier effectively. Tyrosine kinase inhibitors such as tucatinib do cross the blood-brain barrier, which generated considerable interest in this analysis. The original HER2CLIMB trial [NCT02614794], conducted in a later-line setting, showed that adding tucatinib to trastuzumab and capecitabine lengthened CNS-PFS and other CNS-related end points. In the first-line setting evaluated by HER2CLIMB-05, brain metastases were far less common, but PFS was still more than doubled in this subgroup with the addition of tucatinib.

What role did endocrine therapy play among patients with hormone receptor-positive disease?

Patients with hormone receptor-positive disease were permitted, though not required, to receive endocrine therapy, based on shared decision-making between the patient and physician at the local institutional level. Only about 45% of these patients received endocrine therapy, while the majority, 55%, did not. Patients who did not receive endocrine therapy had PFS that was about 15 months shorter than those who did, underscoring the importance of endocrine therapy in this population. Regardless of endocrine therapy use, tucatinib lengthened PFS compared with placebo.

What questions remain unanswered, and how does this analysis fit into the evolving first-line treatment landscape for HER2+ mBC?

Since HER2CLIMB-05 was designed, results have been reported from the DESTINY-Breast09 trial [NCT04784715], which evaluated [fam-trastuzumab deruxtecan (T-DXd; Enhertu)] in combination with pertuzumab in the first-line setting using a continue-until-progression design rather than a maintenance strategy. As a result, clinicians must now integrate multiple first-line data sets that were not generated sequentially. Patients continue to express a strong preference for maintenance strategies, largely because of the quality-of-life benefits of stopping cytotoxic therapy, particularly now that overall survival in this population exceeds 5 years. Data presented at the 2026 ASCO Annual Meeting on deep partial responses achieved with trastuzumab, tucatinib, and pertuzumab-based regimens suggest that patients who achieve a deep partial response have outcomes similar to those who achieve a complete response. This raises the possibility that treatment duration before transitioning to a maintenance regimen should be guided by depth of response rather than a fixed number of cycles.

REFERENCE

Hamilton EP, Curigliano G, Martin M, et al. Efficacy and safety of tucatinib (TUC) vs placebo (PBO) combined with trastuzumab and pertuzumab (HP) as maintenance therapy for HER2+ metastatic breast cancer by stratified subgroups. J Clin Oncol. 2026;44(suppl 16):1005. doi:10.1200/JCO.2026.44.16_suppl.1005