LB-332

Inflammatory breast cancer (IBC) is an aggressive subtype with majority overexpressing epidermal growth factor (ErbB2) and therapeutically has the worst outcome among breast cancers. However, the clinical benefit of ErbB2 targeting using antibodies (Trastuzumab/HerceptinR, an FDA approved humanized anti-ErbB2 monoclonal antibody) or small molecule inhibitors (Lapatinib, a dual ErbB1/2 tyrosine kinase inhibitor showing a 50% clinical response rate in ErbB2-overexpressing IBC patients) in breast cancer is compromised since resistance mechanisms invariably develop. A distinct mechanism involving expression of survivin, an inhibitor of apoptosis (IAP) family member, in resistance to Lapatinib and Trastuzumab, has been reported. The study objectives were twofold 1) To elucidate the role of inhibitor of apoptosis proteins in resistance to ErbB2 targeted strategies in IBC vs. non-IBC preclinical cellular models; 2) Identify and validate the prominent IAP as a potential targeted therapy in combination with anti-ErbB2 agents. We report herein that Trastuzumab can bind with similar affinity to ErbB2 receptor on both IBC (SUM190) and non-IBC (SKBR3) ErbB2 overexpressing cell lines and elicit an antibody dependent cell cytotoxicity (ADCC) response. However, SUM190 is highly resistant to Trastuzumab-mediated decrease in cell viability in comparison to SKBR3 cells. In addition, SUM190 cells have high constitutively activated AKT and MAPK. Trastuzumab resistance in SUM190 corresponded with no change in p-AKT or ErbB2 protein expression compared to non-IBC SKBR3 cells. The prominent feature post-Trastuzumab treatment was an increase in X-linked inhibitor of apoptosis protein (XIAP), one of the most potent cellular caspase inhibitors. In contrast, the SUM190 cells are highly sensitive to cisplatin (IC50=13 +/- 3uM, 24h) and doxorubicin (IC50=1.8 +/- 0.02uM, 24h), which correlated with XIAP downregulation even in the presence of sustained expression of survival factors p-AKT, p-MAPK, survivin and phosphorylated NFkappaB. We show for the first time a signaling-mediated, immune-independent correlation of Trastuzumab resistance and XIAP in ErbB2 positive breast cancer. We also provide evidence that specific inhibition of XIAP with siRNA alone or in combination with Trastuzumab with appropriate controls revealed a significant decrease (45-50%) in cell viability along with decreased pro-caspase 9 levels in the XIAP siRNA+Trastuzumab combination. Further, use of a small molecule inhibitor that disrupts XIAP binding to pro-caspase 9 as a single agent caused apoptosis in the breast cancer cells. These results emphasize the merit of evaluating blockade of ErbB2 and the IAP anti-apoptotic pathway with combination therapy in human clinical trials of ErbB2 positive breast cancer.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA