Monalizumab and Trastuzumab In Metastatic HER2-pOSitive breAst Cancer: MIMOSA-trial

In this phase II clinical trial with an explorative nature, the efficacy of the combination of monalizumab and trastuzumab is assessed in patients with metastatic or locally incurable HER2-positive breast cancer. Clinical efficacy will be assessed in patients with high stromal tumor-infiltrating lymphocytes (sTILs) or low sTILs in two separate cohorts (higher or equal to 5% versus lower than 5%). Since the combination of monalizumab and trastuzumab has not been administered before, dose limiting toxicities (DLTs) will be monitored throughout the trial using the Pocock-type boundary rules for continuous monitoring of toxicity in phase II trials. In the first stage, 11 patients will be accrued per cohort. If there are 1 or fewer responses in these 11 patients, the study will be stopped. Otherwise, 8 additional patients will be accrued for a total of 19 patients. The study will start with two cohorts (sTILs high and sTILs low), a total of 22 (2x11) patients will be included in the first stage. Dependent on the interim analysis (continuation of no cohorts, 1 or 2 cohorts), a maximum of 38 patients will be included.