Therapeutic Discovery Compensatory Insulin Receptor (IR) Activation on Inhibition of Insulin-Like Growth Factor-1 Receptor (IGF-1R): Rationale for Cotargeting IGF-1R and IR in Cancer Elizabeth Buck 1 , Prafulla C. Gokhale 3 , Susan Koujak 1 , Eric Brown 3 , Alexandra Eyzaguirre 1 , Nianjun Tao 4 , Maryland Rosenfeld-Franklin 3 , Lorena Lerner 4 , M. Isabel Chiu 4 , Robert Wild 3 , David Epstein 2 , Jonathan A. Pachter 2 , and Mark R. Miglarese 1 Abstract Insulin-like growth factor-1 receptor (IGF-1R) is a receptor tyrosine kinase (RTK) and critical activator of the phosphatidylinositol 3-kinase–AKT pathway. IGF-1R is required for oncogenic transformation and tumor- igenesis. These observations have spurred anticancer drug discovery and development efforts for both bio- logical and small-molecule IGF-1R inhibitors. The ability for one RTK to compensate for another to maintain tumor cell viability is emerging as a common resistance mechanism to antitumor agents targeting individual RTKs. As IGF-1R is structurally and functionally related to the insulin receptor (IR), we asked whether IR is tumorigenic and whether IR-AKT signaling contributes to resistance to IGF-1R inhibition. Both IGF-1R and IR(A) are tumorigenic in a mouse mammary tumor model. In human tumor cells coexpressing IGF-1R and IR, bidirectional cross talk was observed following either knockdown of IR expression or treatment with a selective anti–IGF-1R antibody, MAB391. MAB391 treatment resulted in a compensatory increase in phospho- IR, which was associated with resistance to inhibition of IRS1 and AKT. In contrast, treatment with OSI-906, a small-molecule dual inhibitor of IGF-1R/IR, resulted in enhanced reduction in phospho-IRS1/phospho-AKT relative to MAB391. Insulin or IGF-2 activated the IR-AKT pathway and decreased sensitivity to MAB391 but not to OSI-906. In tumor cells with an autocrine IGF-2 loop, both OSI-906 and an anti–IGF-2 antibody reduced phospho-IR/phospho-AKT, whereas MAB391 was ineffective. Finally, OSI-906 showed superior efficacy com- pared with MAB391 in human tumor xenograft models in which both IGF-1R and IR were phosphorylated. Collectively, these data indicate that cotargeting IGF-1R and IR may provide superior antitumor efficacy compared with targeting IGF-1R alone. Mol Cancer Ther; 9(10); 2652–64. ©2010 AACR. Introduction The role of insulin-like growth factor-1 receptor (IGF- 1R) in tumor cell proliferation and survival is well estab- lished (1). IGF-1R is a receptor tyrosine kinase (RTK) with a di-dimeric α 2 β 2 structure and is activated on binding the growth factor ligands IGF-1 and IGF-2 (2). IGF-1R couples to the phosphatidylinositol 3-kinase (PI3K) – AKT signaling pathway via interactions with the adaptor protein insulin receptor substrate (IRS). IGF-1R is re- quired for oncogenic transformation and tumorigenesis (3, 4), and disruption of IGF-1R activity by either genetic (5, 6) or pharmacologic (7–9) approaches can reduce tu- mor cell proliferation and promote apoptosis. Increased expression of IGF-1R and its ligands is associated with etiology, progression, and prognosis for many human cancer types (10, 11). IGF-1R signaling is a key contribu- tor of resistance to cytotoxic chemotherapeutics, ionizing radiation, and certain targeted agents, including inhibi- tors of epidermal growth factor receptor (EGFR), HER2, and mammalian target of rapamycin (12–15). IGF-1R has been intensely pursued as a cancer target, and both bio- logical and small-molecule tyrosine kinase domain in- hibitors (TKI) of IGF-1R are under investigation in oncology clinical trials (16–19). Given the important role for IGF-1R signaling as an adaptive survival mechanism against a diverse array of antitumor agents, combination therapies centered on IGF-1R inhibitors are being widely explored. IGF-1R is closely related to the IR, sharing 70% amino acid identity overall and 84% identity within the catalytic domain (20, 21). IR can exist as either of two isoforms [IR (A) and IR(B)] due to alternative splicing of exon 11 (22). IR(A) (short form) is a fetally expressed isoform that lacks a region within exon 11. IGF-1R and IR αβ monomers Authors' Affiliations: 1 Translational Research and 2 Cancer Biology, OSI Pharmaceuticals, Farmingdale, New York; 3 In Vivo Pharmacology, OSI Pharmaceuticals, Boulder, Colorado; and 4 Biology, AVEO Pharmaceuticals, Cambridge, Massachusetts Note: Supplementary material for this article is available at Molecular Cancer Therapeutics Online (http://mct.aacrjournals.org/). Corresponding Author: Elizabeth Buck, OSI Pharmaceuticals, Inc., 1 Bioscience Park Drive, Farmingdale, NY 11735. Phone: 631-962-0782; Fax: 631-845-5671. E-mail: ebuck@osip.com doi: 10.1158/1535-7163.MCT-10-0318 ©2010 American Association for Cancer Research. Molecular Cancer Therapeutics Mol Cancer Ther; 9(10) October 2010 2652 on June 3, 2020. © 2010 American Association for Cancer Research. mct.aacrjournals.org Downloaded from Published OnlineFirst October 5, 2010; DOI: 10.1158/1535-7163.MCT-10-0318