A novel cell-based assay for the evaluation of anti-ras compounds.

In order to identify drugs active against mutated ras oncogenes we have developed an in vitro assay employing two clones of the human fibrosarcoma cell-line, HT1080 which carries an N-ras gene mutated at codon 61. Clone, HT1080scc2, retains the transformed phenotype of the parental line, whilst the other, HT1081c, is a morphologically flat, non-tumourigenic, revertant with under-representation of the chromosome carrying the transforming N-ras allele. The clear implication of mutant ras in maintaining the transformed nature of HT1080scc2 was confirmed when these cells were microinjected with the pan ras neutralising antibody Y13-259, which resulted in the morphological detransformation of these cells to a phenotype resembling that of the HT10801c clone. A number of known anti-cancer drugs with modes of action unrelated to ras function were found to be equipotent against both clones. However, when compounds chosen on the grounds of their potential selective cytotoxic or differentiating activity were tested some interesting results were obtained. Thus 8-bromo cAMP affected some morphological detransformation of HT1080scc2 cells and reduced their colony forming potential. The IMP-dehydrogenase inhibitors, tiazafurin and mycophenolic acid also flattened the morphology of the transformed clone. Fumagillin, an antibiotic reported to exhibit selective activity against ras transformed cells showed very marked and selective cytostatic effects against HT1080scc2 cells with IC50 values as low as 1 x 10(-11) M.