Combination chemotherapy: treatment of the Ehrlich ascites carcinoma and several drug-resistant sublines with combinations of 6-mercaptopurine and 6-(methylmercapto)purine ribonucleoside

The antitumor activities of 6-mercaptopurine (6MP) and 6-(methylmercapto)purine ribonucleoside (Me6MPR) are synergistically enhanced when these two agents are used together against the Ehrlich ascites carcinoma (EAC). This report describes the response of four thiopurine-resistant EAC sublines to therapy with this drug combination. The four sublines were resistant to treatment with 6MP and Me6MPR, whether these agents were used separately or together. Characteristics of the resistant sublines suggest that (a) the synergism requires the formation of ribonucleotide derivatives of both agents, (b) the cytotoxicity of the drugs in combination is achieved by the same mechanisms that are responsible for the cytotoxic of the agents used separately, and (c) the mechanisms of action of the two agents are related.