Tucatinib Moves Into Breast Cancer Maintenance. What Changes for Pharmacists?

The FDA’s approval of tucatinib (Tukysa; Seagen, a subsidiary of Pfizer) with trastuzumab and pertuzumab introduces a new maintenance option for adults with unresectable locally advanced or metastatic HER2-positive breast cancer after induction treatment. For pharmacists, implementation requires particular attention to the updated boxed warning for hepatotoxicity.¹

The regimen adds an oral HER2-directed tyrosine kinase inhibitor to ongoing antibody therapy after initial chemotherapy. Its benefit is supported by longer progression-free survival (PFS), while safe delivery depends on a monitoring plan that begins before the first prescription.

Establish Where Maintenance Fits

HER2CLIMB-05 enrolled 654 patients without disease progression after four to eight cycles of a taxane with trastuzumab and pertuzumab. Participants were randomized to tucatinib or placebo with continued dual HER2-directed therapy; patients with hormone receptor–positive disease could also receive endocrine therapy.^1,2

Median investigator-assessed PFS was 24.9 months with tucatinib versus 16.3 months with placebo, an 8.6-month difference. The hazard ratio was 0.64 (95% CI, 0.51-0.80; P < .0001). Overall survival data were immature at the PFS analysis, so a survival advantage has not been established.^1,2

This is a maintenance strategy following disease control with induction treatment. It should be distinguished from tucatinib’s existing combination with trastuzumab and capecitabine for previously treated HER2-positive advanced breast cancer. Order sets should identify the intended combination explicitly to prevent confusion between regimens.³

Make Liver Monitoring Part of Treatment Initiation

The October 2026 prescribing information carries a boxed warning for severe and fatal hepatotoxicity, particularly after rechallenge.³ Pharmacists reviewing a maintenance prescription should confirm that the current monitoring schedule is built into the treatment plan.

ALT, AST, and bilirubin should be assessed before treatment every 2 weeks for the first 2 months, then every three weeks and as clinically indicated. In HER2CLIMB-05, 18% of tucatinib-treated patients had ALT elevations above five times the upper limit of normal. Five confirmed Hy’s law cases occurred after rechallenge, including one fatal case.^3,4

These findings make follow-through after an abnormal result especially important. The label specifies repeat testing within 48 to 72 hours for defined abnormalities and dose interruption or discontinuation according to severity. If treatment resumes after recovery to grade 1 or lower, weekly liver function monitoring is required.^3,4

Pharmacy teams can help ensure that a held prescription is not automatically refilled before the oncology team documents the next step.

Prepare Patients for Early Diarrhea

Diarrhea occurred in 73% of patients receiving tucatinib in HER2CLIMB-05, including grade 3 events in 6%. Median onset was 11 days. Prophylactic antidiarrheal treatment was not required in the trial, but patients still need clear instructions for reporting symptoms and using prescribed supportive treatment.⁴

An early follow-up call offers an opportunity to assess bowel changes and oral intake before dehydration develops. Persistent or severe diarrhea warrants clinical assessment and label-directed treatment modification. Pharmacists should make the contact pathway explicit rather than leave patients to manage worsening symptoms until their next infusion.

Review Interactions Before Dispensing

Avoid strong CYP3A or moderate CYP2C8 inducers, which can reduce tucatinib exposure. Strong CYP2C8 inhibitors should also be avoided; if unavoidable, tucatinib requires a dosage reduction. Tucatinib can increase concentrations of CYP3A and P-glycoprotein substrates, requiring additional review when concentration changes could cause serious toxicity.³

Reconciliation should include medications added by outside clinicians and nonprescription products. Repeating that review at refills can identify changes made after the initial oncology assessment.

Coordinate Oral Therapy With Ongoing Care

The recommended tucatinib dose is 300 mg twice daily, approximately 12 hours apart, with or without food. Tablets should be swallowed whole. After a missed dose or vomiting, patients should take the next scheduled dose rather than an extra dose.³

Continuing trastuzumab also means continuing cardiac surveillance. Its prescribing information recommends left ventricular ejection fraction assessment at baseline and every three months during treatment. Pharmacy coordination should connect oral refills with required laboratory testing while preserving antibody-treatment monitoring.⁵

Maintenance can reduce exposure to cytotoxic chemotherapy while extending disease control. Pharmacists help make that transition workable by ensuring patients understand the daily regimen and that abnormal results reach the team before treatment continues.

REFERENCES

1. FDA approves tucatinib with trastuzumab and pertuzumab for the maintenance treatment of HER2-positive breast cancer. News release. FDA. October 7, 2026. Accessed October 19, 2026. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-tucatinib-trastuzumab-and-pertuzumab-maintenance-treatment-her2-positive-breast-cancer

2. Dieras V, Curigliano G, Martin M, et al. HER2CLIMB-05: A Phase III study of tucatinib versus placebo in combination with trastuzumab and pertuzumab as first-line maintenance therapy for HER2+ metastatic breast cancer. J Clin Oncol. 2026;44(17):1597-1607. doi:10.1200/JCO-25-02600

3. Tukysa. Prescribing information. Seagen Inc; October 2026. Accessed October 19, 2026. https://www.pfizermedical.com/tukysa/highlights

4. Pfizer’s TUKYSA regimen receives FDA approval as front-line maintenance treatment for HER2+ metastatic breast cancer. News release. Pfizer. October 7, 2026. Accessed October 19, 2026. https://www.pfizer.com/news/press-release/press-release-detail/pfizers-tukysa-regimen-receives-fda-approval-front-line

5. Herceptin. Prescribing information. Genentech, Inc. Accessed October 19, 2026. https://www.gene.com/download/pdf/herceptin_prescribing.pdf