Correlation between NK function and response to trastuzumab in metastatic breast cancer

Abstract

3036 Background: Trastuzumab is a monoclonal antibody selectively directed against Her2 approved for the treatment of Her2 overexpressing breast cancer patients. Its proposed mechanisms of action include also a role in mediating antibody-dependent cellular cytotoxicity (ADCC), through the triggering of the FcγRIII on natural killer (NK) cells. This study addressed the correlation between overall NK function and clinical trastuzumab activity. Methods: Between March and September 2006 22 metastatic patients in treatment with trastuzumab alone (8 mg/kg load and then 6 mg/kg every 3 weeks until disease progression) as maintaining therapy after chemotherapy were analyzed for clinical and immunological responses. According to RECIST criteria, 14 patients obtained a response to trastuzumab, while 8 patients had a disease progression. Patient’s peripheral blood mononuclear cells were tested for cytotoxic activity against standard NK target (the MHC class I-negative K562 cell line) and trastuzumab-coated MCF7 (Her2-negative) and SKBR3 (Her2-positive) human cell lines in a 4-h 51Cr-release cytotoxicity assay in the presence of grading concentrations of effector cells. Results: NK activity was significantly (p<0.05) higher in responder compared to non responder patients at all the four effector:target (50:1 to 6:1) ratios tested. NK activity of non responder patients was significantly lower than that of 25 sex and age matched controls (p<0.02) and this was not merely due to chemotherapy- or tumor-associated immunosuppression, since the values of responder patients did not significantly differ from those of controls. ADCC activity against Her2-positive SKBR3 cells was also significantly higher in responder compared to non responder patients (p<0.05) and markedly so when compared to controls (p<0.001). Conclusions: The fact that, as shown here, normal levels of FcγR- independent and higher than normal levels of FcγR-dependent cytotoxicity are required for trastuzumab response, lends support to a paramount role of NK cells in the mechanism of action of this drug. No significant financial relationships to disclose.