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 series of various pyrazolo[1,5-a]-1,3,5-triazines have been prepared and studied as inhibitors of cAMP phosphodiesterase isolated from bovine brain, bovine heart, and rabbit lung. A number of compounds were found to be superior to theophylline. 2-Ethyl-7-phenylpyrazolo[1,5-a]-1,3,5-triazine (35) was found to be 97 times more potent than theophylline as an inhibitor of bovine brain PDE. 8-Bromo-2,4-dimethyl-7-phenylpyrazolo[1,5-a]-1,3,5-triazine (52) showed alpha lung = 40 compared to alpha heart = 3.0. Thus, various substituents could increase or decrease the inhibition relative to the type and source of tissue from which the PDE was isolated. The most active compound was 8-bromo-4-(diethylamino)-7-phenylpyrazolo[1,3-a]-1,3,5-triazine (25), which was 185 times more potent than theophylline as an inhibitor of PDE isolated from rabbit lung. The stepwise synthesis via ring-closure procedures of requisite pyrazole intermediates, followed by electrophilic substitution in the pyrazole ring and/or nucleophilic substitution in the 1,3,5-triazine moiety, resulted in the various pyrazolo[1,5-a]1,3,5-triazines listed in Tables I and II. Structure-activity relationships are reviewed.

A series of new 2-(alkylthio)-5,7-disubstituted-1,2,4-triazolo[1,5-a]pyrimidines have been prepared as inhibitors of cAMP phosphodiesterase from various tissues. These derivatives were prepared via ring closure of various requisite 3-amino-1,2,4-triazole intermediates. 2-(Benzylthio)-5-methyl-7-(dimethylamino)-1,2,4-triazolo[1,5-a]pyrimidine (15a) is 6.3 times as potent as theophylline in inhibiting cAMP PDE isolated from rabbit heart. Treatment of dogs intravenously with 5 (mg/kg)/h of 15a gave a cardiac output increase of 69%, which was largely sustained for a 2-h period after administration of drug had ceased. There was no significant increase in heart rate upon administration of 15a. Related studies with 5,7-di-n-propyl-2-(benzylthio)-1,2,4-triazolo[1,5-a]pyrimidine (22a) in five dogs showed a 31.5% increase in cardiac output with an increase in stroke volume of 34.4% with no increase in heart rate. The specificity of action of these PDE inhibitors could be due to selective binding at a certain cAMP PDE site in the cardiovascular system. Several of these compounds are candidates for further studies with a view to clinical evaluation.

A number of 3,7-disubstituted 6-carbethoxypyrazolo [1,5-a] pyrimidines and 3,7-disubstituted 6-ethoxypyrazolo-[1,5-a]pyrimidines have been prepared and evaluated as adenosine cyclic 3',5'-phosphate (cAMP) phosphodiesterase (PDE) inhibitors vs. the low Km enzyme isolated from beef heart, rabbit lung, and kidney preparations. The results were found to be between 0.5 to 13 times as potent as theophylline as inhibitors of PDE, depending on the tissue source. A number of these PDE inhibitors exhibited significant physiological effects in different animal systems, suggesting it should be possible to obtain selective PDE inhibition in various tissues. Several of these heterocycles were found superior to adenosine in inhibiting ADP-induced platelet aggregation in vitro.