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).

A number of new hypoxanthine analogs have been prepared as substrate inhibitors of xanthine oxidase. Most noteworthy inhibitory new hypoxanthine analogs are 3‐(m‐tolyl)pyrazolo[1,5‐a]pyrimidin‐7‐one (47), ID50 0.06 μM and 3‐phenylpyrazolo[1,5‐a]pyrimidin‐7‐one (46), ID50 0.40 μM. 5‐(p‐Chlorophenyl)pyrazolo[1,5‐a]pyrimidin‐7‐one (63) and the corresponding 5‐nitrophenyl derivative 64 exhibited an ID50 of 0.21 and 0.23 μM, respectively. 7‐Phenylpyrazolo[1,5‐a]‐s‐triazin‐4‐one (40) is shown to exhibit an ID50 of 0.047 μM. The structure‐activity relationships of these new phenyl substituted hypoxanthine analogs are discussed and compared with the xanthine analogs 3‐m‐tolyl‐ and 3‐phenyl‐7‐hydroxypyrazolo[1,5‐a]pyrimidin‐5‐ones (90) and (91), previously reported from our laboratory to have ID50 of 0.025 and 0.038 μM, respectively. The presence of the phenyl and substitutedphenyl groups contribute directly to the substrate binding of these potent inhibitors. This work presents an updated study of structure‐activity relationships and binding to xanthine oxidase. In view of the recent elucidation of the pterin cofactor and the proposed binding of this factor to the molybdenum ion in xanthine oxidase, a detailed mechanism of xanthine oxidase oxidation of hypoxanthine and xanthine is proposed. Three types of substrate binding are viewed for xanthine oxidase. The binding of xanthine to xanthine oxidase is termed Type I binding. The binding of hypoxanthine is termed Type II binding and the specific binding of alloxanthine is assigned as Type III binding. These three types of substrate binding are analyzed relative to the most potent compounds known to inhibit xanthine oxidase and these inhibitors have been classified as to the type of inhibitor binding most likely to be associated with specific enzyme inhibition. The structural requirements for each type of binding can be clearly seen to correlate with the inhibitory activity observed. The chemical syntheses of the new 3‐phenyl‐ and 3‐substituted phenylpyrazolo[1,5‐a]pyrimidines with various substituents are reported. The syntheses of various 8‐phenyl‐2‐substituted pyrazolo‐[1,5‐a]‐s‐triazines, certain s‐triazolo[1,5‐a]‐s‐triazines and s‐triazolo[1,5‐a]pyrimidine derivatives prepared in connection with the present study are also described.

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A general reaction of glycosyl cyanides with liquid hydrogen sulfide in the presence of 4-dimethylaminopyridine to provide the corresponding glycosylthiocarboxamides is described. These glycosylthiocarboxamides were utilized as the precursors for the synthesis of 2-D-ribofuranosylthiazole-4-carboxamide and 2-beta-D-ribofuranosylthiazole-5-carboxamide (23). The structural modification of 2-beta-D-ribofuranosylthiazole-4-carboxamide (12) into 2-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)thiazole-4-carboxamide (15), 2-beta-D-ribofuranosylthiazole-4-thiocarboxamide (17), and 2-(5-deoxy-beta-D-ribofuranosyl)thiazole-4-carboxamide (19) is also described. These thiazole nucleosides were tested for in vitro activity against type 1 herpes virus, type 3 parainfluenza virus, and type 13 rhinovirus and an in vivo experiment was run against parainfluenza virus. They were also evaluated as potential inhibitors of purine nucleotide biosynthesis. It was shown that the compounds (12 and 15) which possessed the most significant antiviral activity were also active inhibitors (40-70%) of guanine nucleotide biosynthesis.