Growth of Small Cell Lung Cancer Cells Indolinone Tyrosine Kinase Inhibitors Block Kit Activation and Updated Version

Six indolinone tyrosine kinase inhibitors were characterized for their ability to inhibit Kit kinase and for their effects on the growth of small cell lung cancer (SCLC) cell lines. All of the six compounds were potent inhibitors of Kit kinase in a biochemical assay. A homology model of compound binding to the ATP binding site could account for the increased potency observed with the addition of a propionate moiety to the indolinone core but not the increase observed with addition of a chloride moiety. Although all of the compounds tested were potent in the biochemical assay, several exhibited significantly less potency in cellular kinase assays. Their effects on stem cell factor (SCF)-dependent Kit autophosphorylation and SCLC cell growth were also examined. Inhibition of SCFstimulated Kit activation and cell growth in the H526 cell line was dosedependent. At concentrations that inhibited SCF-stimulated H526 cell growth, there was little effect on insulin-like growth factor-1-stimulated growth, suggesting that these compounds exhibit reasonable selectivity for inhibition of Kit-mediated proliferation. Higher doses of the compounds were needed to inhibit serum-stimulated growth. Of the six compounds examined, SU5416 and SU6597 demonstrated the best cellular potency and, therefore, their effect on the growth of multiple SCLC cell lines in serum-containing media was examined. In addition to inhibiting proliferation, these compounds also induced significant cell death of several SCLC cell lines, but not of normal human diploid fibroblasts, in complete media. These observations suggest that Kit kinase inhibitors such as these may offer a new approach for inhibiting Kit-mediated proliferation of tumors such as SCLC, gastrointestinal stromal tumors, seminomas, and leukemias.