Trial of Neoadjuvant Trastuzumab Emtansine in Patients With HER2-Equivocal Breast Cancer

NATURE is an open label, neoadjuvant, phase II study designed to evaluate the objective response rate of trastuzumab emtansine in patients with newly diagnosed HER2-equivocal breast cancer. Patients will receive trastuzumab emtansine at a dose of 3.6 mg/kg via intravenous infusion every 3 weeks for a total of 6 weeks (2 21-day cycles). Patients who achieve partial or complete response (responders) after the 6-week treatment will continue on trastuzumab emtansine for an additional 12 weeks (4 cycles). The primary objective will be objective response rate after 6 weeks of neoadjuvant trastuzumab emtansine. Secondary objectives will include imaging response (ultrasound and magnetic resonance imaging) after six weeks of neoadjuvant trastuzumab emtansine and toxicity and efficacy of trastuzumab emtansine. After completion of continued trastuzumab emtansine treatment, pathological complete response rate of responders as a whole and according to estrogen receptor status will be explored. Markers related to the mechanism of action of trastuzumab emtansine (HER2 copy number in circulating tumor cells; tissue expression of PTEN, PI3K, and other potential candidate markers) will also be explored.