Tucatinib Triplet Regimen Shows Promise for HER2-Positive Breast Cancer Leptomeningeal Metastasis

Leptomeningeal metastasis (LM)—the seeding of tumor cells into the leptomeninges and cerebrospinal fluid (CSF)—is one of the most feared late-stage complications of metastatic breast cancer. Patients present with multifocal neurological deficits, and the median overall survival (OS) is historically only 4 to 5 months.^1,2 Women with HER2-positive (HER2+) breast cancer face a heightened risk of central nervous system (CNS) involvement, with CNS metastases developing in up to 55% of patients during the course of illness.³

Standard management, which includes radiation therapy, intrathecal chemotherapy, or select systemic agents, has yielded no robust prospective survival benefit, partly because conventional HER2-directed antibodies penetrate the blood-brain barrier poorly.⁴ Small-molecule tyrosine kinase inhibitors (TKIs) may overcome this limitation through enhanced CNS penetration.

Tucatinib and the HER2CLIMB Evidence Base

Tucatinib (Tukysa; Seagen Inc) is a highly selective, oral HER2 TKI with minimal off-target HER1 inhibition, reducing the dermatologic and gastrointestinal toxicities seen with earlier-generation agents. The pivotal HER2CLIMB (NCT02614794) trial established tucatinib combined with trastuzumab (Herceptin; Genentech, Inc) and capecitabine (Xeloda; Genentech, Inc) as a standard of care for patients with previously treated HER2+ metastatic breast cancer, including those with active brain metastases, improving median OS to 21.9 months vs 17.4 months with placebo.⁵ In the brain metastasis subgroup, tucatinib doubled the intracranial response rate and reduced the risk of intracranial progression or death by two-thirds.⁶ Patients with active LM, however, were excluded from HER2CLIMB, leaving a critical evidence gap.

TBCRC049: Trial Design and Results

According to data published in Nature Cancer, the TBCRC049 study (NCT03501979) was a phase 2, nonrandomized, single-arm, multicenter trial evaluating tucatinib-trastuzumab-capecitabine in 17 women with newly diagnosed, MRI-confirmed LM and HER2+ breast cancer.¹ At baseline, 15 patients (88%) were symptomatic, and 8 (47%) had abnormal CSF cytology. The primary end point was OS, benchmarked against a historical control of 4.4 months.

The trial met its prespecified interim efficacy threshold. At a median follow-up of 18 months, 6 of 17 patients (41%) remained alive. Median OS reached 10.0 months (95% CI, 4.1 months-not reached), more than double the historical comparator.¹ The median time to CNS progression was 6.9 months (95% CI, 2.8-13.8 months). Of 13 patients eligible for response evaluation, 5 (38%) had a composite LM objective response, while 7 of 12 evaluable patients (58%) showed improvement in neurological deficits. Findings from pharmacokinetic studies confirmed that tucatinib reached therapeutic concentrations in the CSF, thereby supporting the observed CNS activity from a mechanistic perspective.¹

Safety and Pharmacist Considerations

The safety profile was similar to that of HER2CLIMB, with the most significant adverse effects being diarrhea, hand-foot syndrome, nausea, fatigue, and transient elevations in hepatic aminotransferases.⁵ Pharmacists have a vital role in handling this oral intravenous treatment. Capecitabine has a high risk of drug interactions, and its use must be closely monitored. Tucatinib is mainly metabolized by CYP2C8, with some involvement of CYP3A4, so caution regarding drug-drug interactions is essential in a polypharmacy situation. Patient counseling on adherence, hydration, and the timely reporting of toxicity is necessary.

REFERENCES

1. Murthy RK, O'Brien BJ, Berry DA, et al. Tucatinib-trastuzumab-capecitabine for treatment of leptomeningeal metastasis in women with HER2+ breast cancer: TBCRC049 phase 2 study results. Nat Cancer. Published online March 18, 2026. doi:10.1038/s43018-026-01120-7

2. Ratosa I, Dobnikar N, Bottosso M, et al. Leptomeningeal metastases in patients with human epidermal growth factor receptor 2 positive breast cancer: real-world data from a multicentric European cohort. Int J Cancer. 2022;151(8):1355-1366. doi:10.1002/ijc.34135

3. Zimmer AS, Van Swearingen AED, Anders CK. HER2-positive breast cancer brain metastasis: a new and exciting landscape. Cancer Rep (Hoboken). 2022;5(4):e1274. doi:10.1002/cnr2.1274

4. Mills MN, King W, Soyano A, et al. Evolving management of HER2+ breast cancer brain metastases and leptomeningeal disease. J Neurooncol. 2022;157(2):249-269. doi:10.1007/s11060-022-03977-x

5. Murthy RK, Loi S, Okines A, et al. Tucatinib, trastuzumab, and capecitabine for HER2-positive metastatic breast cancer. N Engl J Med. 2020;382(7):597-609. doi:10.1056/NEJMoa1914609

6. Lin NU, Borges V, Anders C, et al. Intracranial efficacy and survival with tucatinib plus trastuzumab and capecitabine for previously treated HER2-positive breast cancer with brain metastases in the HER2CLIMB trial. J Clin Oncol. 2020;38(23):2610-2619. doi:10.1200/JCO.20.00775