Severe hyperglycemia induced by kinase inhibitors of insulin-like growth factor-1 receptor in mice is associated with inhibition of insulin receptor signaling in skeletal muscle

C156 Insulin-like growth factors (IGF1 and IGF2) and their receptors (IGF1R and IR) are implicated in the development and progression of cancer by contributing to transformation, proliferation, anti-apoptosis, migration, and angiogenesis. Monoclonal antibodies against IGF1R are being tested in the clinic and early evidence of anticancer activity has been observed for these agents. Several small molecule IGF1R kinase inhibitors also have shown anti-tumor activity in preclinical models. As IGF1R and IR are highly homologous and possess a nearly identical active site in the kinase domain, IGF1R inhibitors show little selectivity between IGF1R and IR kinases in both biochemical and cellular assays in vitro. Because IR plays an important role in blood glucose homeostasis, inhibition of IR by IGF1R inhibitors may result in abnormal blood glucose concentrations. In this study, we assessed the effect on blood glucose and insulin homeostasis in mice by multiple chemical classes of IGF1R inhibitors and investigated the possible underlying molecular mechanisms. The compounds we examined included a proprietary agent A-928605, as well as several literature reference IGF1R inhibitors. We used a mouse IGF1R PD model to determine doses required for complete IGF1R kinase inhibition. Blood glucose and insulin concentrations at different time points after dosing were measured. All the compounds caused elevation of blood glucose and insulin, but to varying degrees. Thus we saw a range of blood glucose responses induced by these agents at doses that could completely block IGF1R. We found that inhibitor doses which produced complete IGF1R inhibition in the PD assay also caused complete IR inhibition in the liver; however, we observed differences in the extent of IR signaling inhibition in skeletal muscle among IGF1R inhibitors and doses. There was an apparent correlation between IR inhibition in skeletal muscle and extreme changes in blood glucose. Agents that increased blood glucose concentrations to levels not exceeding 250 mg/dL did not show complete inhibition of IR phosphorylation and, especially, phosphorylation of the downstream signaling molecule Akt in skeletal muscle. In contrast, inhibitor doses that caused extremely high blood glucose elevations (>400 mg/dL) showed extensive inhibition of IR signaling in skeletal muscle. Differential effects on phosphorylation of IR in the liver and skeletal muscle were not related to compound distribution. When we examined the effects of the compounds on Akt isoforms, we found that the inhibitors had distinct effects on Akt2 protein levels in the muscle tissue. An increase of Akt2 protein seemed to be parallel to the decrease in IR phosphorylation and elevation of blood glucose. Taken together, these data indicate that severe hyperglycemia induced by IGF1R kinase inhibitors in mice is associated with inhibition of IR signaling in skeletal muscles and that selective inhibition of IGF1R over IR signaling in vivo could be achieved by a small molecule kinase inhibitor.