Various derivatives of adenosine 3',5'-cyclic phosphate, most notably vV^'-O-dibutyryl-cAMP1 and several 8-alkylthio-cAMP analogs (Free et al., 1972), have been shown to be more effective than cAMP itself in many whole cell systems (see reviews by Robison et Severson, 1971). Reasons for the low activity of cAMP might include the low permeability of cAMP across cell membranes and its intracellular or extracellular hydrolysis by cAMP phosphodiesterase. It has been proposed that 7V6,2'-0-dibutyryl-cAMP is both more rapidly transported across cell membranes and less rapidly hydrolyzed by the phosphodiesterase (Posternak et al., 1962; Robison et al., 1971). It has further been suggested that the acylated derivatives of cAMP

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.

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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.