This is a single-center, phase 1b study evaluating the safety and feasibility of a neoadjuvant treatment with tucatinib, trastuzumab and pertuzumab in stage II-IIIA HER2-positive breast cancer.

High pathological complete response (pCR)-rates are seen using different neoadjuvant chemotherapy schedules with trastuzumab and pertuzumab in HER2-positive stage II - III breast cancer patients. However, a subset of patients with stage II-III HER2-positive breast cancer can be treated with HER2-blockade alone. These patients can potentially be totally spared from chemotherapy-associated toxicity. The proportion of patients whom can successfully be treated without chemotherapy could potentially be increased by selecting great responders using DCE-MRI and by adding tucatinib to trastuzumab and pertuzumab alone. The aim of this study is to evaluate the safety and efficacy of neoadjuvant treatment with tucatinib, trastuzumab and pertuzumab.