Pretreatment of tumor cells with an aromatase inhibitor increases sensitivity to human monocyte-mediated, antibody-dependent cellular cytotoxicity

5312 Recent results from a Phase III clinical trial of the breast cancer vaccine, Theratope®, which contains the carbohydrate determinant, STn, revealed that vaccinated patients treated concomitantly with hormone therapy survived significantly longer than patients treated with hormone therapy + a control vaccine lacking the STn epitope. The objective of this study was to investigate mechanisms to explain the apparent collaboration between anti-hormone treatment and vaccination observed in vivo in patients. Human peripheral blood mononuclear cells (PBMC) were activated in vitro with gamma interferon + lipopolysaccharide (IFN/LPS) and tested for the capacity of the monocyte (PBM) and lymphocyte (PBL) fractions to mediate cytolysis of STn-positive tumor cell lines in the presence and absence of monoclonal anti-STn antibodies (STn mAb). OvCar3 cells (ER+/STn+) cultured in medium were sensitive to lysis by activated PBM from normal donors (mean % cytolysis +/− SD = 54.6+/−11%); this level of cytolysis was not enhanced by treatment of the target cells with STn mAb (55.0 +/− 12%, n.s). OvCar3 cells treated with an I:C 50 concentration of the aromatase inhibitor (AI), formestane, exhibited decreased sensitivity to PBM-mediated cytolysis in the absence of STn mAb (44.8 +/− 12%, p=0.07 compared to media-cultured cells) but significantly increased sensitivity to cytolysis in the presence of STn mAb (64.8 +/− 8%; p=0.003 compared to formestane-treated cells in the absence of STn mAb and p=0.02 compared to media-cultured cells in the presence of STn mAb). The additive effects of formestane pretreatment and STn mAb treatment were not attributable to increased binding of STn mAbs as measured in cell-based ELISA or to Ab-mediated tumor cell cytostasis as measured in MTT assays. Additional experiements with MCF7 cells (ER+/STn-) and DU4475 cells (ER-/STn+) revealed that collaboration between AI pretreament and PBM-mediated ADCC appeared to require both estrogen sensitivity and STn positivity on the part of the target tumor cells. Although tumor cells were also sensitive to in vitro cytolysis by PBL from the same donors (mean % cytolysis +/− SD = 31.9 +/−4%) neither STn mAb treatment alone (30.2 +/− 4%); formestane pretreatment alone (28.8 +/− 3%); nor formestane pretreatment + STn mAb treatment (26.3 +/− 8%) significantly affected the level of tumor cytolysis. These results demonstrate that tumor cells which survive treatment with an AI exhibit increased sensitivity to human monocyte-mediated, antibody dependent cellular cytotoxicity. The capacity of an AI to “sensitize” tumor cells to this form of anti-tumor immunity represents a heretofore, undescribed mechanism whereby a hormone-based cytostatic/cytotoxic treatment may collaborate with antigen-specific tumor immunity to produce improved tumor control in vivo in patients.