OR17-3 Cartilage-Targeted IGF-1 Treatment to Promote Longitudinal Bone Growth

Recombinant human growth hormone (GH) is commonly used to treat short stature in children. However, GH treatment has limited efficacy particularly in severe, non-GH deficient conditions such as chondrodysplasias, and has potential off-target effects. Because short stature results from decreased growth plate chondrogenesis, we previously developed a cartilage-targeting single-chain human antibody fragment (CaAb) (1), aiming to deliver therapeutic molecules to the growth plate, thereby increasing treatment efficacy while minimizing adverse effects on other tissues. In the current study, we created fusion proteins of these CaAbs conjugated with insulin-like growth factor-I (IGF-1), an endocrine/paracrine factor which positively regulates chondrogenesis. We first used ELISA to assess the ability of the CaAb-IGF1 fusion proteins to bind tissue lysates from different mouse organs and found that the fusion proteins bound to cartilage with high tissue specificity. We also found that these fusion proteins retain potent IGF-1 biological activity, as demonstrated by their ability to stimulate AKT and ERK phosphorylation in cell culture and stimulate metatarsal bone growth in organ culture. To assess in vivo efficacy, we administered daily subcutaneous injections to growth hormone-deficient (lit) mice for 5 days. In this model, twice-daily IGF-1 increased growth plate height but daily IGF-1 did not. In contrast, daily equimolar injections of the targeted fusion proteins did increase growth plate height whereas a fusion protein lacking the cartilage-targeting domain did not (P<0.01 targeted vs non-targeted or saline). Even alternate-day injections of the targeted fusion proteins were sufficient to produce a therapeutic effect (P<0.01) similar to daily injection. To assess off-target effects, we measured proliferation in kidney cortical cells by BrdU incorporation. Daily injections of the targeted fusion proteins did not increase kidney cell proliferation whereas twice-daily IGF-1 itself (the minimum IGF-1 regimen that stimulated the growth plate) did stimulate kidney proliferation (P<0.01). In summary, the fusion proteins, unlike IGF-I itself, were able to stimulate growth plate activity without significant off-target effects on kidney. Our findings therefore provide proof-of-principle that targeting therapeutics to growth plate cartilage can potentially improve treatment for childhood growth disorders. Reference: (1) Cheung et al. Pharma Res 2015 Jul;32(7):2439-49. Sources of Funding: This work was supported by the Intramural Research Programs of NICHD and NCI, National Institutes of Health. Additional funding through Grant for Growth Innovation from EMD Serono, Inc. a biopharmaceutical business of Merck KGaA, Germany, and Endocrine Scholars Award in Growth Hormone Research from the Endocrine Society.