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Development of resistance against protein-kinase inhibitors in long-term protein-free culture.

In two distinct cell lines, the protein kinase inhibitors K-252a and staurosporine did not inhibit the proliferation of two protein-free subclones as compared with their parental serum-dependent clones in a time- or concentration-dependent manner. Resistance to protein kinase inhibitors appeared in the selection of protein-free subclones from serum dependent clones. This concomitant appearance of drug resistance and autonomic proliferation suggests that cell proliferation in a protein-free culture does not depend on signal transduction through the protein kinase system. It is of interest that this growth factor independence is not caused by autocrine growth factor secretion in the protein-free medium, with this phenomenon possibly reflecting the tumor progression from sensitive to resistant in clinical trials of these inhibitors.