Tumor dependence on the EGFR signaling pathway expressed by the p-EGFR:p-AKT ratio predicts erlotinib sensitivity in human Non-small Cell Lung Cancer (NSCLC) cells expressing wild-type EGFR gene

INTRODUCTION: This study was undertaken to identify molecular determinants of tumor dependency on the epidermal growth factor receptor (EGFR) signaling pathway for predicting clinical benefit from erlotinib monotherapy in non-small cell lung cancer (NSCLC) patients with tumors expressing wild-type EGFR gene. METHODS: The effect of erlotinib on the total and phosphorylated protein expression of EGFR and key downstream signaling molecules was determined by immunoblots in a panel of NSCLC cells expressing wild-type EGFR gene. The parameters that correlate with cell sensitivity and resistance to erlotinib was analyzed. RESULTS: Individual assessment of total or phosphorylated protein expression of EGFR or a downstream signaling molecule does not correlate with sensitivity to erlotinib in these NSCLC tumors. Resistance of NSCLC cells to erlotinib is associated with failed inhibition of at least one phosphorylated downstream signaling molecule. The dependency of NSCLC cells on the activated EGFR axis was measured by the ratio of p-EGFR to a phosphorylated downstream protein. A high ratio should indicate that activation of a downstream signaling molecule primarily results from the activation of upstream EGFR; and a low ratio should indicate that activation of a downstream signaling molecule primarily results from the activation of a upstream receptors other than EGFR. The p-EGFR:p-AKT ratio was 10-fold higher in erlotinib-sensitive cells than erlotinib-resistant cells (p = 0.03). It was the best predictor of erlotinib sensitivity among all parameters analyzed in this panel of NSCLC cell lines. CONCLUSIONS: The p-EGFR:p-AKT ratio deserves further investigation as a predictive parameter for clinical response to erlotinib in NSCLC tumors expressing wild-type EGFR gene.