Tetracyclic guanines have been shown to be potent and selective inhibitors of the cGMP-hydrolyzing enzymes PDE1 and PDE5. In general, these compounds are inactive or only weakly active as inhibitors of PDE3, which is a major isozyme involved in cAMP hydrolysis. Structure-activity relationships are developed at N-1, C-2, N-3, and N-5 on the core nucleus. Compound 31, with an IC50 of 70 pM, is the most potent inhibitor of PDE1, while 50, with an IC50 of 4 nM, is the most potent inhibitor of PDE5. Compounds 20, 22, 30, and 50 are potent dual inhibitors with IC50 values below 30 nM for both PDE1 and PDE5. Compounds 12, 20, and 28 reduced blood pressure by more than 45 mmHg when administered orally at 10 mg/kg to the spontaneously hypertensive rat (SHR).

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