2,4-Diamino-5-cyano-6-halopyridines have been described previously as oral insulinotropic agents and have been found recently to have bronchodilatory properties. In the present report the synthesis of the iodo compound is newly described, and it is established that HI- or HBr-mediated condensation and cyclization of malononitrile in 1,2-dichloroethane yield selectively the 5-cyanopyridine derivatives. The pyridine derivatives were found to constitute a new class of potent cyclic AMP phosphodiesterase inhibitors. Inhibition of purified dog kidney cyclic AMP phosphodiesterase was of the mixed type. Since cyclic AMP phosphodiesterase inhibitors are known to enhance glucose-induced insulin secretion and to activate glucose production by the liver, the finding that the pyridine derivatives described here inhibited cyclic AMP phosphodiesterase opens new avenues of interpretation for their insulinotropic actions as well as for the paradoxical lack of improvement of glucose disposal by elevation of insulin after oral drug administration. Cyclic AMP phosphodiesterase inhibition also has the potential of explaining the bronchodilatory effects of these drugs.
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