Activity and resistance of trastuzumab according to different clinical settings

Trastuzumab, a humanized monoclonal antibody directed against HER2, has shown efficacy in breast cancers; however many patients do not respond to this reagent. Here, we discuss the potential mechanisms of trastuzumab efficacy and resistance in different clinical settings as a step toward optimizing the appropriate application of this antibody. The three major antitumor mechanisms of trastuzumab, i.e., inhibition of proliferation, antibody-dependent cell cytotoxicity (ADCC) and inhibition of DNA repair, appear to be differentially operative in different clinical settings. ADCC appears to be the prevalent mechanism in trastuzumab neoadjuvant monotherapy, whereas in neoadjuvant, adjuvant or metastatic settings in which trastuzumab is combined with chemotherapy, the relative role of ADCC is probably small, considering the compromising effects of chemotherapy on the immune cells that mediate this mechanism. In neoadjuvant and adjuvant settings involving concomitant use of trastuzumab and chemotherapy, the primary mechanism at play is presumably inhibition of DNA repair by the antibody, while in sequential protocols, the antibody acts mostly by exerting cytostatic activity through inhibition of HER2-mediated tumor cell proliferation. According to the ability of the antibody to induce cytotoxic or cytostatic antitumor effects depending on the clinical setting, different criteria, i.e., RECIST for cytotoxic effect, OS, and DFS for cytostatic, must be considered in accurately estimating antibody efficacy. Moreover, since trastuzumab resistance likely depends directly on the mechanisms responsible for its antitumor activity, resistance mechanisms must also be considered with respect to the different clinical settings.