S446 ERBB3 interacts with EGFR and is expressed in EGFR TKI sensitive NSCLC cell lines and may represent a poor prognostic marker in NSCLC. In this study we correlated the activity of the HDAC inhibitor vorinostat in NSCLC cell lines with ERBB3 expression and evaluated the synergistic effect of vorinostat with gefitinib. Further, we evaluated the interaction of ERBB3 and E-cadherin and their effect on response to gefitinib. Methods: Apoptosis: cell lines were incubated in control medium or in

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... e presence of various vorinostat and/or gefitinib concentrations and cell death and apoptosis were assessed using the vibrant apoptosis assay and evaluated with flow cytometry. Samples were assessed in the presence or absence of gefitinib and/or vorinostat. Gene expression analysis was done using real-time RT-PCR and microarray gene expression profiling. Protein expression and coimmunoprecipitation was evaluated with western blot. Results: We performed detailed analysis of the apoptotic activity of vorinostat in a panel of 21 NSCLC cell lines. The normalized apoptotic to life ratio was > 2.16 in 12 cell lines (sensitive) and < 2.16 in 11 cell lines (resistant). Using microarray analysis we found that ERBB3 expression was significantly higher in vorinostat sensitive compared to its expression in resistant cell lines (p<0.005). Vorinostat also induced erbB3 expression in gefitinib resistant cell lines. When gefitinib resistant cell lines (IC50>10uM) were treated with vorinostat and gefitinib, synergistic effects were detected in 4 of the 5 cell lines tested. Cotransfection of ERBB3 and E-cadherin in a gefitinib resistant cell line, H157, showed enhanced apoptotic response to gefitinib similar to what is detected in an EGFR mutant cell line H3255. Conclusion: ERBB3 may predict response to EGFR and HDAC inhibitors in NSCLC. For tumors with low ERBB3 and E-cadherin expression the combination increases expression of ERBB3 and E-cadherin and produce more than additive antitumor effects through increased apoptosis.