Virazole is a synthetic nucleoside active in tissue culture against at least 16 DNA and RNA viruses. Applied topically, it inhibits herpetic keratitis in rabbits and tail lesions induced by herpes, vaccinia, and vesicular stomatitis viruses in mice. Injected intraperitoneally into mice, it inhibits splenomegaly and hepatomegaly induced by Friend leukemia virus and respiratory infections caused by influenza A(O), A(2), and B viruses and parainfluenza 1 virus. infections is also effective.

The antiviral activity of the synthetic nucleoside, Virazole (1-io-D-ribofuranosyl-1,2,4-triazole-3-carboxamide), against measles virus in Vero cell cultures was substantially reversed by xanthosine, guanosine, and to a slightly lesser extent by inosine. Virazole 5'-phosphate was subsequently found to be a potent competitive inhibitor of inosine 5'-phosphate dehydrogenase (IMP: NAD + oxidoreductase, EC 1.2.1.14) isolated from Escherichia coli (Km = 1.8 X 10-' M) with a Ki of 2.7 X 10-7 M.Guanosine 5'-phosphate (GMP) was a competitive inhibitor of this enzyme with a Ki of 7.7 X 106 M. Virazole 5'-phosphate was similarly active against IMP dehydrogenase isolated from Ehrlich ascites tumor cells, with a Ki of 2.5 X 10-7 M. The Km for this enzyme was 1.8 X 10-6 M, and the Ki for GMP was 2.2 X 10-4 M. These results suggest that the antiviral activity of Virazole might be due to the inhibition of GMP biosynthesis in the infected cell at the step involving the conversion of IMP to xanthosine 5'-phosphate. This inhibition would consequently result in inhibition of the synthesis of vital viral nucleic acid.The synthesis and development of a broad spectrum antiviral agent has been a challenging task because of the intimate association of virus replication and biochemical processes of the host cell. In addition, such an agent must inhibit a step in the process of virus replication that is common to a wide variety of RNA and DNA viruses (1). The synthesis and broad spectrum antiviral activity of 1-i3-D-ribofuranosyl-1,2,4-triazole-3-carboxamide (I, Virazole) have recently been reported (1,2).

Virazole (1-β- d -ribofuranosyl-1,2,4-triazole-3-carboxamide) is a highly soluble new synthetic nucleoside having significant, reproducible activity against a broad spectrum of deoxyribonucleic acid and ribonucleic acid viruses in vitro. The drug inhibited viral cytopathogenic effects in monolayers of cells infected for 3 days with type 3 adeno, types 1 and 2 herpes, myxoma, cytomegalo, vaccinia, infectious bovine rhinotracheitis, types 1A, 2, 8, 13, and 56 rhino, types 1 and 3 parainfluenza, vesicular stomatitis, subacute sclerosing panencephalitis, Semliki Forest, Newcastle disease, and measles viruses. Hemagglutinin production by influenza A 2 , influenza B, and type 1 parainfluenza viruses in chicken embryo cells was reduced by Virazole treatment. Recoverable intra- and extracellular virus titers were reduced by the drug in experiments with type 1 herpes, vaccinia, type 3 parainfluenza, and vesicular stomatitis viruses. Plaque formation by type 1 herpesvirus was also inhibited by exposure of the infected cells to Virazole. Pretreatment of cells with the compound, followed by its removal before addition of type 1 herpesvirus, severely lessened the antiviral activity; the compound was still moderately effective in reducing the viral effects on the cells when added as long as 22 hr after the virus. Parallel experiments, in which the antiviral activity of a number of known active drugs was compared, indicated Virazole to have at least a comparable degree of activity, and it was also active against a wider variety of viruses than any of these known active materials. The CCED 50 of Virazole to chicken embryo cells was approximately 1,000 μg/ml, although concentrations as low as 10 μg/ml caused slight (15%) inhibition in total cellular protein after 72 hr of incubation.

The syntheses of ribonucleosides structurally related to 1-ß-D-ribofuranosyl-1,2,4-triazole-3-carboxamide (1) are described and their antiviral activity is compared with that of 1. Both the acid-catalyzed fusion procedure and glycosylation of the appropriate trimethylsilyl derivative provided 3-cyano-l-(2,3,5-tri-0-acetyl-|3-D-ribofuranosyl)-l,2,4-triazole (4) in 80% yield. ß-D-Ribofuranosyl-l,2,4-triazole-3-thiocarboxamide (2) and 1-ß-D-ribofuranosyl-1,2,4-triazole-3-carboxamidine hydrochloride (3) and related nucleosides were prepared from 4. In tissue culture, 3 exhibited activity against herpes simplex, rhino, and parainfluenza viruses, similar to that shown by 1, while 2 was effective only against herpes simplex virus. Against lethal influenza A2 virus infections in mice, 3 was effective at 75 mg/kg/day compared with 1 which showed significant activity at 37.5 mg/kg/day.The synthetic nucleoside 1 -ß-D -ribofuranosyl-1,2,4-triazole-3-carboxamide 1,2 (virazole) (1) has been shown by our laboratory to exhibit a broad spectrum of activity against both DNA and RNA viruses in vitro and in vivo.2 Our interest in nucleosides structurally related to 1 has prompted us to investigate3'4 the synthesis and antiviral activity of 1ß-D-ribofuranosyl-l,2,4-triazole-3-thiocarboxamide (2) and 1-ß-D-ribofuranosyl-1,2,4-triazole-3-carboxamidine (3). These nucleosides, while retaining steric requirements very similar to those of the corresponding carboxamide nucleoside 1, differ in hydrogen bonding properties (S and N vs. 0) of the 3 substituent on the triazole.As a route to both the thiocarboxamide (2) and carboxamidine (3) triazole nucleosides, the synthesis of 3-cyano-l-(br s, 2, NH,); Xmax ( , ) 230 nm (e 6000), 287 (6800). Anal.

1-β- d -Ribofuranosyl-1,2,4-triazole-3-carboxamide (Virazole) was found to possess significant antiviral activity against aerosol-induced or intranasally induced influenza A 0 , A 2 , and B virus infections in mice. Significant protection was achieved by both oral and intraperitoneal routes of administration. Depending upon the level of virus infection, antiviral activity was best observed at the daily dose of 75 mg/kg. The efficacy of the compound was evidenced by an increase in survivor number, prolongation of mean survival time, suppression of lung consolidation, or decrease in hemagglutinin titer in the infected lung samples. The therapeutic value of this synthetic triazole nucleoside was evident as noted by a significant increase in survivor number even if the treatment was started as late as 24 h after infection with an aerosol of influenza A 2 virus.