David G. Streeter

THE RELATIONSHIP BETWEEN THE METABOLISM OF RIBAVIRIN AND ITS PROPOSED MECHANISM OF ACTION

The synthetic nucleoside ribavirins has been shown to have broad spectrum antiviral activity both in vitro and in vivo. and recent clinical studies indicate that it is also efficacious in man.I-l2 Streeter et al.I3 have shown that the 5’-phosphate of ribavirin is a potent inhibitor of IMP-dehydrogenase, and have suggested that its antiviral effect may be mediated in part by the inhibition of this crucial biosynthetic enzyme. Streeter et aI.,l3 have also shown that ribavirin-5’-phosphate is found in the liver of rats after oral dosing, and that the enzyme most probably responsible for the phosphorylation of ribavirin is deoxyadenosine kinase. Further investigations, which are reported here, were directed towards determining if ribavirin-5’-phosphate could be further phosphorylated to the corresponding diand tri-phosphates, and towards finding out if any additional metabolites of ribavirin are produced in the whole animal. The primary purpose for studying the metabolism of ribavirin is to gain further understanding of the mechanism of action of the drug. Metabolites of ribavirin other than the 5’-phosphate may be acting at sites in the cell other than IMP-dehydrogenase. Knowledge of what metabolites are formed may give some insight into what those other sites may be.

Comparative cytotoxicities of various morpholinyl anthracyclines

SummaryA series of quinone- and sugar-modified analogs of adriamycin have been tested for growth inhibition of adriamycin-sensitive (P388/S) and -resistant (P388/ADR) sublines of P388 murine leukemia cells in vitro. P388/ADR is less resistant to analogs of adriamycin containing either a 3′-deamino-3′-(4″-morpholinyl) group, MRA; or a-(3″-cyano-4″-morpholinyl) group, MRA-CN, than to adriamycin. However, MRA-CN was the most potent growth inhibitor of either subline. This potency is reduced by either modification of the quinone unit with a 5-imino substituent or restriction of the cyano-morpholinyl ring by an oxygen bridge to the daunosamine sugar. The calcium antagonist verapamil substantially increases the cytotoxicity of adriamycin to P388/ADR but has no appreciable effect on the cytotoxicity of either MRA or MRA-CN. The results suggest that increased uptake and retention by both MRA and MRA-CN may contribute to their increased cytotoxicity, but that the intense potency of the cyano-morpholinyl analogs must be due to other unique properties of these compounds.

Uptake and retention of morpholinyl anthracyclines by adriamycin-sensitive and -resistant P388 cells.

Summary3′-Deamino-3′-(4-morpholinyl)adriamycin (MRA) and 3′-deamino-3′(3-cyano-4-morpholinyl)adriamycin (MRA-CN) were compared with adriamycin (ADR) in ADR-sensitive (P388/S) and-resistant (P388/ADR) murine leukemia cell lines with respect to cytotoxicity and cellular accumulation. MRA is only two- to threefold more cytotoxic to P388/S in culture than ADR, whereas MRA-CN is 500-fold more cytotoxic than ADR to this cell line. Yet both MRA and MRA-CN retain their potency against P388/ADR in spite of a 150-fold decrease in potency for ADR. The observed noncross-resistance of both MRA and MRA-CN in P388/ADR correlates with their increased cellular uptake and retention relative to ADR and the inability of P388/ADR to exclude these analogs as readily as it does ADR. The decreased uptake of MRA and MRA-CN in P388/ADR relative to P388/S (1.5 to 2.0-fold), the increased efflux, and the ability of verapamil to enhance cellular uptake of these analogs in P388/ADR, as it does with ADR, all indicate that the mechanism of ADR-resistance effects ADR and the morpholino analogs in a similar manner but to far different extents. The potent cytotoxicity of MRA-CN appears to be related to strong cellular interactions of the drug with macromolecules that are characterized by its nonextraction from cells by chloroform: methanol or 10 M urea and may therefore represent covalent binding.

The invitro inhibition of purine nucleotide biosynthesis by 2-β-D-ribofuranosylthiazole-4-carboxamide

Abstract A series of C-glycosylthiazoles were tested as inhibitors of purine nucleotide biosynthesis in in vitro cultures of Ehrlich ascites tumor cells. The thiazole C-nucleoside, 2-β-D-ribofuranosylthiazole-4-carboxamide, demonstrated the only significant activity of the series as a specific inhibitor of guanine nucleotide biosynthesis. At concentrations of 10–1000 μM the compound inhibits the activities of the enzymes IMP dehydrogenase and GDP kinase by 50–60% and 30–60%, respectively. The antiviral agent ribavirin demonstrated a similar pattern of enzyme inhibition at the same range of concentrations. The possible relevance of this inhibition to the recently discovered antitumor properties of 2-β-D-ribofuranosylthiazole-4-carboxamide is discussed.

7-ribosyl-3-deazaguanine—Mechanism of antibacterial action

Abstract The mechanism by which 7-ribosyl-3-deazaguanine [7 R 3 DG , 6- amino -3-β- d - ribofuranosylimidazo [4,5-c] pyridin -4(5H)- one ] exerts its antibacterial effect was examined. Escherichia coli was found to contain an enzyme that exhibited the properties of a nucleoside phosphorylase and that converted 7R3DG to 3-deazaguanine (3DG, 6-aminoimidazo[4,5- c ]pyridin-4(5 H )-one], but no mammalian system that was examined (Erilch ascites, rat liver and human liver) was able to convert 7R3DG to 3DG. The 3DG arising from the phosphorolysis of 7R3DG was converted to 3-deaza-GMP [3DGMP, 6-amino-l-β- D -riboluranosylimidazo [4,5- c ]pyridin-4(5 H )-one-5′-phosphate] by the guanine phosphoribosyltransferase present in E. coli . A strain of E. coli , resistant to 7R3DG, was found to lack this enzyme and, therefore, was unable to convert 3DG to 3DGMP.