Biochemical and antitumor activity of tiazofurin and its selenium analog (2-beta-D-ribofuranosyl-4-selenazolecarboxamide).

2-beta-D-Ribofuranosyl-4-selenazolecarboxamide (selenazofurin, CI-935), the selenium analog of tiazofurin (CI-909), was 3- to 10-fold more cytotoxic to murine or human tumor cells in vitro than tiazofurin and was also more active against P388 mouse leukemia in vivo. In vitro cytotoxicity could be reversed by guanosine or guanine but not by other purine nucleosides or bases. Three human tumor cell lines selected for selenazofurin or tiazofurin resistance showed cross resistance between selenazofurin and tiazofurin. Treatment with tiazofurin, selenazofurin, or mycophenolic acid decreased guanylate pools and caused an accumulation of IMP in WIL2 human lymphoma cells. The decrease in guanylate pools was accompanied by inhibition of RNA and DNA synthesis. The NAD analogs of tiazofurin and selenazofurin were inhibitors of L1210 IMP dehydrogenase (IMP:NAD oxidoreductase, EC 1.2.1.14), and both showed uncompetitive inhibition with respect to NAD having Kii values of 5.7 X 10(-8)M and 3.3 X 10(-8)M respectively.

Biochemical pharmacology of the lipophilic antifolate, trimetrexate.

Trimetrexate is a novel lipophilic folate antagonist that causes growth inhibition, inhibition of nucleic acid biosynthesis, and cytotoxicity at nanomolar concentrations in tissue cultures. The potency of trimetrexate cytotoxicity against most cell lines is greater than that of methotrexate. Trimetrexate has antitumor activity in vivo in several murine leukemia and solid tumor systems, including tumors in which methotrexate is inactive. Antitumor activity was seen following oral, intravenous, or intraperitoneal administration. Trimetrexate causes a pronounced and early depression in incorporation of deoxyuridine into DNA. In tumor cell lines resistant to methotrexate because of a drug transport defect, trimetrexate retains activity. In many such cases the methotrexate-resistant tumors show collateral sensitivity to trimetrexate. In methotrexate-resistant cells with impaired drug transport, trimetrexate sensitivity was even more pronounced when cells were grown in folate-free medium supplemented with physiological levels of tetrahydrofolate cofactor. In the human tumor stem cell colony assay, trimetrexate, at concentrations achievable in vivo, gave activity against many human tumors, including samples that were unresponsive to methotrexate. Trimetrexate crosses the blood-brain barrier, and at very high doses may cause neurotoxicity. At conventional doses the primary toxic effects in mice are gastrointestinal. This toxicity is reversible at therapeutic doses. Unlike earlier lipophilic antifolates, trimetrexate has rapid plasma clearance (t1/2 in mice of 45 minutes). Trimetrexate is a tight-binding competitive inhibitor of dihydrofolate reductase. The Ki,slope for inhibition of the human enzyme was 4 X 10(-11) M. A dose-dependent decrease in cellular purine ribonucleotide pools is given by trimetrexate. Pyrimidine ribonucleotide pools tend to increase in treated cells. Trimetrexate caused a marked depression of cellular pools of dTTP and dGTP, and a lesser depression in dATP. Cytotoxicity of trimetrexate in vitro was prevented by leucovorin. Leucovorin also protected mice from trimetrexate toxicity. Thymidine protected cells from lethal effects of low concentrations of trimetrexate, but not from high concentrations. The combination of thymidine and hypoxanthine completely protected cells from low and high concentrations of trimetrexate. A new, stable and highly water-soluble formulation of trimetrexate has been developed. Because of the interesting biochemical and pharmacological properties of trimetrexate, and its experimental antitumor activity, clinical trials are planned.