Additive interaction between tamoxifen and rifampicin in human biliary tract carcinoma cells.

A novel two-drug combination of tamoxifen and rifampicin has been investigated for the treatment of biliary tract-associated malignancies. Their effects, alone and in combination, on human tumour cells were studied using two pancreatic carcinoma cell lines. MIA PaCa-2 and AsPC-1, and a gall bladder carcinoma cell line, G-415. Inhibition of growth was used as an endpoint. The cells differed in sensitivity to equimolar doses of the two drugs given as single agents. Combined doses of rifampicin and tamoxifen resulted in growth inhibition to an extent that was suggestive of additive or synergistic drug effects. More detailed analysis was carried out, involving the production of an 'envelope of additivity' for each cell line. An additive effect of drug combination was shown to occur in all three cell lines, thus providing a basis for use of this novel two-drug combination in the treatment of biliary tract-associated malignancies.