**5. Conclusions**

Clinical trials with IGF-targeting biologicals exposed several obstacles to their successful use in cancer therapy. Due to the homology and crosstalk between IGF-IR and IR, several inhibitors of IGF-IR signaling (including tyrosine kinase inhibitors) were found to also disrupt IR signaling, resulting in undesirable side e ffects such as hyperinsulinemia and hyperglycemia. The responses to more specific drugs, such as anti-IGF-IR antibodies, were also disappointing, and this has been attributed to several potential factors, including increased GH release, IGF-II/IR-A signaling, rescue signaling by alternate RTKs and increased IGF-IR nuclear translocation. Recently it was proposed that IGF-IR targeting by antibodies or kinase inhibitors may result in alternative, kinase-independent ERK signaling mediated via recruitment of interacting proteins such as β-arrestins, limiting the e ffectiveness of these inhibitors (reviewed in [124,125]).

The IGF-Trap o ffers key advantages over receptor targeting antibodies and small-molecule inhibitors. With high specificity for IGF-I and IGF-II, and poor a ffinity for insulin, the deleterious effects on the physiological functions of insulin can be minimized. Since the IGF-Trap binds circulating ligands, penetration and di ffusion into solid tumors are not major obstacles to e fficacy, although uptake at the tumor site, if achieved, could have the added benefit of neutralizing locally produced ligands. Moreover, the high binding a ffinity of the IGF-Trap for IGF-II should reduce IGF-II bioavailability for IR-A activation, bypassing one of the major resistance mechanisms to IGF-IR targeting drugs. In addition, the potential of anti-IGF-IR antibodies to act as natural agonists and activate alternate IGF-IR signaling can be circumvented with the use of an IGF-Trap [125], and targeting of the ligands rather than a cell surface receptor should minimize non-desirable side e ffects due to antibody-dependent cellular cytotoxicity (ADCC) that can be mediated by the Fc portion of cell bound antibodies [126]. Finally, our evidence suggests that the IGF-Trap, by reducing ligand bioavailability can target several components of the tumor microenvironment, further enhancing its inhibitory activity on tumor cell growth. Having established the utility of the KIRA for monitoring IGF-Trap e fficacy in vivo, our data sugges<sup>t</sup> that it could provide a surrogate marker for response evaluation and a potential tool for patient stratification. Collectively, there is therefore a compelling rationale for transitioning this technology to the clinic for treatment of malignant disease, either alone or in combination with other treatment modalities.

**Author Contributions:** All the authors contributed literature research and texts to this review. Y.M.C. wrote the introductory section on IGF-targeting strategies, S.Q. wrote the review on Trap technology. S.P. contributed to the section on IGF-Trap development and M.H. contributed to the section on the role of the IGF-axis in the tumor microenvironment. P.B. collated, revised and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a Terry Fox Frontiers Initiative Grant of the National Cancer Institute of Canada, grants MOP-77677 and PP2-141724 from the Canadian Institute for Health Research, gran<sup>t</sup> IT04483 from MITACS, and research support from Amorchem Inc.

**Acknowledgments:** The bioengineering, production and optimization of the IGF-Trap were made possible by generous grants from the Quebec Ministry of Economic Development and MSBiv, The Terry Fox Research Institute, grants MOP-77677 and PP2-141724 from the Canadian Institute for Health Research, gran<sup>t</sup> IT04483 from MITACS and funding by Amorchem Inc. The development of the IGF-Trap was a collaborative endeavor. The many contributions of the protein engineering team of the Human Health Therapeutics Research Centre, the National Research Council of Canada, Montreal, Quebec, Canada under the leadership of Bernard Massie are gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflicts of interest.
