Selective inhibition of SCLC growth by the A12 anti-IGF-1R monoclonal antibody correlates with inhibition of Akt

4396 IGF-1 and IGF-2, via activation of the IGF-1R, play a prominent role in the growth and survival of small cell lung cancer (SCLC) by potently activating the PI3K-Akt signal transduction pathway, which is also an important factor in the resistance of SCLC to chemotherapy. We have shown that small molecule ATP-competitive inhibitors of IGF-1R effectively inhibit SCLC growth, promote apoptosis, and sensitize SCLC cell lines to chemotherapy. However, the clinical development of these agents has been hindered by concerns of toxicity, potentially due to inhibition of the highly related insulin receptor and other off-target activities. A12 is a fully humanized monoclonal antibody directed again the human IGF-1R that does not cross-react with the insulin receptor. In model systems it has been shown to block IGF-1R signaling by both competing for ligand binding and promoting receptor internalization and degradation. We have undertaken studies to determine the activity of A12 against SCLC cell lines. Incubation with 10 µg/ml A12 resulted in inhibition of IGF-1-mediated IGF-1R tyrosine phosphorylation by approximately 75%, with a similar decline in IGF-1-stimulated Akt activity in the H526 cell line. Serum-stimulated Akt activity was also decreased by a similar amount and was suppressed for at least 24 hours. The decline in IGF-1R levels in A12 treated cells compared to IgG controls was minor relative to the inhibition of IGF-1R signaling, suggesting that the predominant effect of A12 was mediated by inhibition of ligand binding. Growth of the H526 and H146 cell lines in serum was inhibited by a maximum of 60-70% in a dose-dependent fashion, as determined by 72-hr MTT assay. However, growth of the H69 and WBA cell lines was unaffected by A12. Interestingly, despite almost complete inhibition of IGF-1R phosphorylation by A12, Akt activity remained constitutively high in these cell lines. H69 is known to harbor an activating mutation in the PIK3CA gene and we have previously documented constitutive activation of Akt in the WBA cell line in the absence of all growth factors. Treatment with A12 additively enhanced response to carboplatin in the H526 and H146 cell lines but had no effect on the H69 and WBA cell lines. These data suggest that growth inhibition and chemosensitization of SCLC by A12 is directly correlated with the ability to inhibit PI3K-Akt signaling.