Resistance to trastuzumab in HER2-positive mucinous invasive ductal breast carcinoma

The human epidermal growth factor receptor 2 (HER2) is overexpressed in 20%25% of invasive breast cancer (BC) and is associated with a poor prognosis and resistance to certain chemotherapeutic agents. Treatment with trastuzumab, a recombinant humanized monoclonal antibody directed against the extracellular domain of the HER2 protein, improves outcomes of HER2-positive BC. However, a significant proportion of patients treated with trastuzumab either do not respond initially or relapse after experiencing a period of clinical response. ? We present 2 cases of patients with metastatic HER2- positive BC, in whom the presence of a mucin-producing component impaired the effectiveness of trastuzumab. ? Early identification of tumors resistant to trastuzumab and an understanding of responsible mechanisms are imperative in the care of patients with HER2-positive BC so that their therapeutic management can be changed as soon as possible. Because the presence of a mucinous component could act as a barrier against trastuzumab, surgical resection of disease should be considered early in cases of BC that have this pathologic feature. In addition, metastatic sites could become differentiated further during treatment, leading to increased production of mucin and acquired resistance to trastuzumab therapy.