AZT - A BIOCHEMICAL RESPONSE MODIFIER OF METHOTREXATE AND 5-FLUOROURACIL CYTOTOXICITY IN HUMAN OVARIAN AND PANCREATIC-CARCINOMA CELLS

In ovarian and pancreatic carcinoma cell lines, the activity of the salvage enzyme, thymidine kinase (EC 2.7.1.21), was 2- to 13-fold higher than that of the key enzyme of thymidylate de novo biosynthesis, thymidylate synthase (dTMP synthase, EC 2.1.1.45). AZT (3'-azido-3'-deoxythymidine, zidovudine) competitively inhibited thymidine kinase activity in extracts of human ovarian and pancreatic carcinoma cells, with Dixon plots yielding K(i) = 1.1-mu-M in both cell lines. AZT (20-mu-M) yielded synergistic cytotoxicity with methotrexate (0.4-mu-M) in human pancreatic carcinoma cells in clonogenic assay and also with methotrexate (0.02-mu-M) in human ovarian carcinoma cells, as measured by cell counts. Thymidine (10-mu-M) and hypoxanthine (100-mu-M) reversed these inhibitions. AZT (20 or 40-mu-M) also provided synergistic cytotoxicity with 5-fluorouracil (0.5 and 1.0-mu-M) in human pancreatic carcinoma cells in clonogenic assay. These studies suggest a new role for AZT, which, as an inhibitor of thymidine salvage, should be useful as a biochemical response modifier to provide a synergistic clinical anticancer impact on de novo biosynthesis of thymidylates in conjunction with methotrexate or 5-fluorouracil.