Inhibition of IGF-1R by OSI-906 potentiates efficacy of various molecular targeted agents by blocking feedback loops converging at the level of IRS-1

PR-1 The receptor for insulin-like growth factor (IGF-1R) is a critical mediator of tumor cell survival through its strong ability to couple to the PI3K-Akt signaling cascade, and both genetic and pharmacologic inhibition of the IGF-1R have been shown to promote the induction of apoptosis. OSI-906 is a selective small molecule inhibitor of the IGF-1R that is currently in phase I clinical development. OSI-906 achieves inhibition of Akt and tumor cell growth and survival in both in vitro and in vivo preclinical models. As a major promoter of tumor cell survival we sought to determine if resistance to other classes of molecular targeted therapeutics might be mediated by IGF-1R signaling and if combination with OSI-906 could potentiate their efficacy. Herein, we evaluated the effect of combining OSI-906 with inhibitors of 3 major oncogenic mediators; EGFR (erlotinib), MEK (PD98059 and MEK1), and mTORC1 (rapamycin). These agents produced varying extents of tumor cell growth inhibition in vitro , but not cell survival, when dosed individually. However, in combination with OSI-906 the inhibitors of EGFR, MEK, or mTORC1 promoted synergistic growth inhibition and apoptosis for select tumor cell lines. In vivo , we observed greater anti-tumor activity for the combination of OSI-906 with erlotinib in epithelial tumor cell models than that maximally achievable by either single agent, as demonstrated by >100% TGI and early tumor regression during treatment.
 Erlotinib as a single agent achieved inhibition of Akt for epithelial tumor cells, however we find that inhibition is not complete, nor sustained, following prolonged exposure. The combination of erlotinib with OSI-906 achieved a synergistic and sustained inhibition of Akt activity, associated with a synergistic inhibition of cell survival. Such observations were extended to in vivo models. For the other agents evaluated (PD98059, MEK1, rapamycin), we find that all could enhance phosphorylation of Akt, potentially limiting their single agent activity, and this could be inhibited in combination with OSI-906 to yield synergy.
 We sought to determine a mechanistic understanding for the observed synergy of OSI-906 in combination with this diverse panel of anti-cancer agents. Previous reports have demonstrated that rapamycin can promote IGF-driven Akt activity by affecting a negative feedback loop imposed on IGF-1R signaling. Here, blockade of the mTOR-p70S6K signaling pathway prevents serine phosphorylation of the IGF-1R adaptor protein IRS1. Serine phosphorylation of IRS-1 attenuates interactions with IGF-1R, important to convey PI3K-Akt signaling. Therefore, rapamycin, by blocking phosphorylation of IRS-1, promotes enhanced coupling from IGF-1R to PI3K. We now report here that both erlotinib and the MEK1 inhibitors also relieve IRS1 serine phosphorylation, potentiating IGF-driven Akt. These effects could be mediated by affecting pErk-p70S6K signaling, however, we will present data to suggest that pErk1/2 may directly phosphorylate IRS-1.
 Collectively these data show that resistance to a variety of molecular targeted anti-cancer agents is associated with enhanced IGF-1R-directed Akt signaling, where all affect feedback loops converging at the level of IRS-1.