Many oncogenes encode tyrosine kinases and the identification of appropriate inhibitors is an appealing proposition for the development of selective anticancer agents. These studies outline some screening models which we are currently developing in which we have evaluated a variety of compounds, reported to have tyrosine kinase inhibitory activity, as antitumor agents. In an enzyme based assay, using isolated pp60c-src, a variety of indolocabazoles, flavonoids, tyrophostins, herbimycin A and natural product cytotoxic agents were tested. Genistein, quercetin, staurosporine and herbimycin A were selected for comparison in whole cell studies. Soft agar colony formation of an activated c-src transformed NIH3T3 line was inhibited (IC50) by genistein (4.5 mug/ml) and quercetin (6.5 mug/ml) at similar concentrations and by herbimycin A (7 ng/ml) and staurosporine (1.3 ng/ml) at much lower concentrations. However, these agents inhibited colony formation of v-raf, H-ras and polyoma transformed NIH3T3 lines at comparable concentrations suggesting no selective inhibition of the src mediated transformation was being produced. Furthermore, quercetin failed to inhibit tyrosine phosphorylation of cellular proteins at concentrations shown to inhibit colony formation. Nevertheless. daily quercetin treatment (ip) for seven days at 1000 and 500 mg/kg/day did produce a meaningful increase in median survival time of athymic mice inoculated ip with 106 c-src transformed NIH3T3 cells. The screening models described are now being used to identify and develop new tyrosine kinase inhibitors from natural products and assess their potential as antitumor agents.

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