Potentiation of antimetabolite action by dibromodulcitol in cell culture.

The postulation that the activity of key enzymes that reveal marked increases should be potential targets for anticancer chemotherapy (47) was supported by new evidence on the alterations of CDP reductase, CTP synthetase and OMP decarboxylase in hepatoma 3924A cell cultures. Inhibitors of these enzymes (VF-122, acivicin, pyrazofurin) and that of IMP dehydrogenase (tiazofurin) efficiently killed hepatoma 3924A cells in culture, as demonstrated by the clonogenic assay. Acivicin, pyrazofurin, tiazofurin and VF-122 were lethal against tumor cells in the exponential phase of growth with IC50 of 1.5, 5, 10 and 4.5 microM, respectively. All these antimetabolites exhibited cytotoxicity preponderantly against exponential-phase cultures, indicating that all the four drugs belong to Class II (phase-specific agents) in the Kinetic Classification of Anticancer Agents (38). Dibromodulcitol, a bifunctional alkylating agent, revealed cycle-specific cytotoxicity (Class III agent) against hepatoma 3924A, yielding IC50 values of 2.3 and 5.5 microM for exponentially and stationary growing cells, respectively. Using isobologram analysis on the survival data of 3924A cells, synergistic interaction was observed when DBD in combination with acivicin, pyrazofurin and tiazofurin was examined. DBD in combination with VF-122 exhibited additive lethality against hepatoma cells in culture. The synergistic and additive cytotoxicity in combinations of DBD with these antimetabolites was accompanied by the concurrent depletion of ribonucleotide and/or deoxyribonucleotide pools. The synergistic biological results of drug combinations of acivicin with DBD can be accounted for by the action of acivicin in inhibiting CTP synthetase, resulting in a synergistic decrease in CTP content, and by inhibition of DNA synthesis caused by DBD. The synergistic and additive depletion of UTP, CTP, dTTP and dCTP pools in the combinations of DBD with pyrazofurin may be responsible for the synergistic lethality of these combinations. Synergism, in terms of pool depletion, was observed for GTP and dCTP; summation was detected for dGTP when DBD and tiazofurin were given concurrently. The synergistic cytotoxicity of this drug combination may be a consequence of these alterations. The additive lethality of DBD-VF-122 drug combinations was reflected in the additive elevations of the ribonucleoside diphosphate concentrations. These observations indicate that treatments based on the Kinetic Classification and on the biochemical targeting of the drug should have an impact on the design of in vivo chemotherapy.