Neoadjuvant trastuzumab deruxtecan (T-DXd) with response-directed definitive therapy in early stage HER2-positive breast cancer: a phase II study protocol (SHAMROCK study)

Background The current standard of care in the neoadjuvant setting for high-risk HER2-positive (HER2 +) breast cancer is to combine systemic chemotherapy with dual HER2 blockade, trastuzumab and pertuzumab. Targeted therapies have significantly improved outcomes for patients with HER2-positive breast cancer. To improve treatment-associated toxicity, chemotherapy-sparing approaches are currently being investigated. Trastuzumab deruxtecan (T-DXd) is an HER2-directed antibody–drug-conjugate (ADC) with promising results in the metastatic setting for HER2-positive breast cancer. The SHAMROCK study investigates neoadjuvant T-DXd in early stage HER2-positive breast cancer, using pathological complete response (pCR) rate as the primary endpoint. Methods This is a phase II open-label, single arm, adaptive multi-centre trial of T-DXd in the neoadjuvant setting in stage 2–3 HER2-positive breast cancer. Eligible patients will receive 5.4 mg/kg of T-DXd intravenously every 3 weeks for up to 6 cycles. A repeat biopsy will performed after 2 cycles for the RNA disruption index (RDI) score assessment. According to their likelihood of pCR, as determined by the RDI score, patients will either undergo 4 or 6 cycles of T-DXd prior to imaging. Patients with imaging complete response (iCR) after either 4 or 6 cycles will proceed to surgery. Patients who do not achieve iCR will either undergo further systemic therapy or proceed to surgery. Discussion The SHAMROCK study is a chemotherapy-sparing approach to curative intent treatment, investigating neoadjuvant T-DXd. We hypothesise that neoadjuvant T-DXd will have a high pCR rate and be associated low toxicity in early stage HER2-positive breast cancer. Trial registration EudraCT Number: 2022–002485-32; ClinicalTrials.gov identifier: NCT05710666; Cancer Trials Ireland study number: CTRIAL-IE 22–01.