tiazofurin and acivicin

Topics: Animals; Antimetabolites, Antineoplastic; Bone Marrow; Cycloheximide; Enzyme Inhibitors; Fluorouracil; Half-Life; Humans; Isoxazoles; Methotrexate; Prodrugs; Purines; Pyrimidines; Ribavirin; Signal Transduction

(1) The currently used clinical anti-metabolites are targeted against-key enzymes of de novo purine and pyrimidine biosynthesis. However, the activities of salvage enzymes in each of the biosynthetic segments are markedly higher than those of the rate-limiting enzymes of de novo biosynthesis. Enzyme-pattern-targeted chemotherapy has been suggested to overcome the circumvention activity of salvage. Combination of inhibition of de novo and salvage pathways does provide a synergistic impact. Examples that enzyme-pattern-targeted drug treatment yields synergism include the following: tiazofurin (against IMP DH) and allopurinol (by raising serum hypoxanthine levels it inhibits GPRT); methotrexate or 5-FU lead to inhibition of the dTMP synthase reaction and AZT (a competitive inhibitor of thymidine kinase) or dipyridamole (a nucleoside transport inhibitor); acivicin, an inhibitor and inactivator of glutamine-utilizing enzymes in the de novo pathways of purine and pyrimidine biosynthesis, and dipyridamole. (2) Administration of MTX, 5-FU, tiazofurin or acivicin causes inhibition and/or inactivation of target enzymes. That these drugs are effective in spite of the presence of highly active salvage enzymes is now accounted for, at least in part, by new observations showing that these drugs markedly reduce (but do not eliminate) the activities (amounts) of CdR and TdR kinases, dTMP synthase and GPRT. This action is attributed to the rapid decay rate of these enzymes. (3) Studies on the bone marrow enzymic programs indicate that there is a window of opportunity for strengthening therapy and for the protection of bone marrow by administering salvage metabolites when the salvage enzymes are still present in high enough activities, i.e., 2-6 hr after administration of the blockers of de novo enzyme activities. (4) These results are a strong argument for discovering and utilizing inhibitors of purine and pyrimidine salvage enzymes to achieve more successful enzyme-pattern-targeted chemotherapy and to avoid development of resistant clones of cancer cells. (5) These approaches provide greater explanatory coherence than the previous accounts because recognition of (a) the importance of salvage and (b) rapid decay of key and salvage enzymes reveals a paradigm shift. The problem-solving process in chemotherapy should now be not only data-driven but also explanation-driven.

Topics: Animals; Antimetabolites, Antineoplastic; Bone Marrow; Cycloheximide; Deoxycytidine Kinase; Fluorouracil; Hypoxanthine Phosphoribosyltransferase; Isoxazoles; Methotrexate; Neoplasms, Experimental; Pyrimidines; Rats; Rats, Inbred Strains; Ribavirin; Thymidine Kinase; Thymidylate Synthase

Topics: Animals; Carbon-Nitrogen Ligases; Cycloheximide; Drug Interactions; IMP Dehydrogenase; Isoxazoles; Ketone Oxidoreductases; Ligases; Liver Neoplasms, Experimental; Oxazoles; Ribavirin; Ribonucleosides; Time Factors; Tumor Cells, Cultured

The in vivo actions of two antimetabolites, acivicin (NSC-163501) and tiazofurin (NSC-286193), were examined on the enzymic programs of rat bone marrow. From the bone marrow of the femurs, 100,000 g supernatant fractions were prepared; enzymic activities were measured by isotopic assays, and cellularity was determined. In the normal bone marrow, the specific activities of pyrimidine de novo synthetic enzymes, CDP reductase, dTMP synthase, CTP synthase, carbamoyl-phosphate synthase II (synthase II), orotidine 5'-phosphate decarboxylase and aspartate carbamoyltransferase, were 1, 2.7, 5, 10, 63 and 601 nmol/hr/mg protein, respectively, whereas those of the salvage enzymes, deoxycytidine, thymidine, cytidine and uridine kinases were 3, 43, 149, and 367 nmol/hr/mg protein, respectively. In purine biosynthesis, the activities of the de novo synthetic enzymes, IMP dehydrogenase, formylglycinamidine ribonucleotide (FGAM) synthase, GMP synthase, amidophosphoribosyl-transferase (AT) and adenylosuccinate synthase were 16, 8, 107, 78 and 124 nmol/hr/mg protein, respectively, and those of the salvage enzymes, adenine, hypoxanthine and guanine phosphoribosyl-transferases, were 340, 407, and 1018 nmol/hr/mg protein, respectively. The sequence of events was elucidated after a single i.p. injection of acivicin (5 mg/kg) or tiazofurin (200 mg/kg). Within 2 hr after acivicin injection, CTP, GMP and FGAM synthases lost 85-90%, while AT and synthase II lost 50 and 80%, respectively, of their activities. The activities rose to near normal range by 72-96 hr. The bone marrow cellularity decreased, reaching a nadir at 24 and 48 hr, and returning to normal range by 72 and 92 hr; thymidine kinase activity followed a similar pattern. Tiazofurin injection depressed IMP dehydrogenase activity to 20% by 2 hr with a rebound to normal range by 48 and 72 hr. The cellularity decreased more slowly, reaching its lowest point at 24 hr and returning to normal range at 72 hr. For acivicin the marked depletion of the activities of the glutamine-utilizing enzymes and for tiazofurin that of IMP dehydrogenase might account, in part at least, for the bone marrow toxicity of these antimetabolites. Because of the presence in the bone marrow of high activities of purine and pyrimidine salvage enzymes, it should be possible to design methods utilizing nucleosides and nucleobases to protect the bone marrow from the action of antimetabolites.

Topics: Animals; Aspartate Carbamoyltransferase; Bone Marrow; Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing); Carbon-Nitrogen Ligases; Deoxycytidine Kinase; Isoxazoles; Ligases; Orotidine-5'-Phosphate Decarboxylase; Oxazoles; Rats; Ribavirin; Ribonucleoside Diphosphate Reductase; Ribonucleosides; Thymidine Monophosphate; Thymidylate Synthase

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

Topics: Amides; Animals; Antimetabolites, Antineoplastic; Cell Division; Cell Line; Drug Synergism; Hydroxamic Acids; Isoxazoles; Liver Neoplasms, Experimental; Mitolactol; Pyrazoles; Rats; Ribavirin; Ribonucleosides; Ribose

The mechanism of action of acivicin and tiazofurin was compared in hepatoma 3924A. The results were evaluated by assessing the impact of these drugs on primary targets, the activities of key enzymes, and on secondary and tertiary targets, the concentrations of pools of ribonucleotides and deoxyribonucleotides. The action of acivicin entails inhibition and inactivation of the key enzymes of glutamine utilization in the biosynthesis of purines and pyrimidines. As a result, the GTP and CTP pools were markedly depleted, whereas those of ATP and UTP were unaffected. Acivicin also markedly decreased the concentrations of all 4 deoxynucleoside triphosphates. The nucleotide pools returned to normal or near normal range within 2 to 3 days after a single acivicin injection. The pharmacologic targets of acivicin in anticancer chemotherapy include prominently the activities of glutamine-utilizing enzymes and the pools of GTP and CTP and all 4 dNTP's. These biochemical targets also serve as indicators of acivicin action in cancer cells. The action of tiazofurin in hepatoma cells entails the primary target, IMP dehydrogenase. The subsequent effects include marked enlargement of IMP and PRPP pools and depletion of the pools of GDP and GTP. The increased IMP concentration selectively inhibited the activities of hypoxanthine-guanine phosphoribosyltransferase, but did not affect that of adenine phosphoribosyltransferase. The markedly decreased GTP pool de-inhibited the activity of AMP deaminase which permitted the channeling of AMP to IMP. An important indicator of tiazofurin action is the prolonged depletion of dGTP pools and similar but less pronounced declines in the pools of dCTP and dATP. In contrast, dTTP pools were increased. The crucial biochemical targets and indicators of tiazofurin action in sensitive cancer cells include inhibition of IMP dehydrogenase, a decrease in the concentrations of GDP, GTP, dGTP, dCTP, dATP and marked rise in the pools of IMP, PRPP and dTTP. Measurements of the molecular targets and indicators of drug action should be helpful in identifying cancer cells and tissues sensitive or resistant to the action of acivicin or tiazofurin. Identification of the targets and indicators should also be helpful in the design of frequency of administration of the drugs in combatting animal and human neoplasia.

Topics: Animals; Antineoplastic Agents; Cell Line; Dose-Response Relationship, Drug; IMP Dehydrogenase; Isoxazoles; Liver; Liver Neoplasms, Experimental; Nucleotides; Oxazoles; Pentosephosphates; Pentosyltransferases; Phosphoribosyl Pyrophosphate; Rats; Ribavirin; Ribonucleosides