The EGFR antagonist, erlotinib, combined with a small molecule inhibitor of IGF-1R acts synergistically to inhibit proliferation and induce apoptosis in ovarian and HNSCC cells

A130 Increasing evidence indicates that most cancers rely on multiple signaling networks to drive tumor proliferation and survival, suggesting that a combination of molecular targeted inhibitors may offer improved efficacy over single agent therapy. Here we provide evidence for the cooperativity between IGF-1R and EGFR signaling in head and neck and ovarian cancers, and a rationale for dual inhibition of these critical receptor signaling pathways. IGF-1R mediates multiple cellular responses, including activation of survival and proliferative pathways. IGF-1R is ubiquitously expressed in a panel of head and neck squamous cell carcinoma (HNSCC) cell lines and ovarian cancer cell lines. These cells lines exhibit varying sensitivity to PQIP, a small molecule inhibitor of IGF-1R, however sensitivity does not correlate with receptor expression. Since IGF-1R expression is prevalent in these tumor types, we reasoned that IGF-2 ligand expression may correlate more directly with the presence of activated receptor. We observed elevated levels of IGF-1 or IGF-II ligand in 27% of ovarian tumors surveyed, suggesting that a population of ovarian tumors may be driven by autocrine or paracrine IGF-IGF-1R signaling, potentially mediating sensitivity to an IGF-1R inhibitor. HNSCC and ovarian tumors have also been reported to express high levels of the epidermal growth factor receptor (EGFR). EGFR is over-expressed in over 90% of HNSCC tumors and up to 77% of ovarian primary tumors, implicating these tumor types as attractive targets for EGFR blockade. The HNSCC and ovarian cell lines surveyed here exhibit varying levels of sensitivity to erlotinib, and sensitivity does not correlate with EGFR expression. In HNSCC and ovarian cell lines treated in vitro with the combination of erlotinib and PQIP, synergistic inhibition of proliferation and induction of apoptosis was frequently observed, including cell lines which are relatively insensitive to erlotinib. To better understand the mechanism of synergy at the molecular level we assessed the phosphorylation of downstream effectors of EGFR and IGF-1R. Treatment of cells with PQIP enhanced phosphorylation of EGFR and conversely, treatment with erlotinib enhanced phosphorylation of IGF-1R suggesting that in cells where both pathways are active targeted inhibition can shift the cellular equilibrium toward reliance on the uninhibited receptor. Inhibition of the MAPK pathway with erlotinib reduces phospho-ERK and increases phospho-Akt, reflecting increased cellular reliance on the IGF-1R-PI3K axis for survival. This may be mediated through inhibition of the S6K-IRS1 negative feedback loop, resulting in increased PI3K signaling through Akt. Treatment with PQIP significantly decreased Akt phosphorylation and the combination of erlotinib and PQIP further reduced phospho-Akt, providing one possible mechanism for the observed synergy. Ovarian cell lines which are synergistically inhibited by the combination of PQIP and erlotinib express low basal levels of phospho-ERK, providing a potential biomarker to identify patients who may benefit from the combination of IGF-1R and EGFR inhibitors. Together, these data provide a mechanistic rationale for further exploring the combination of erlotinib and an inhibitor of IGF-1R in ovarian and head and neck cancers.