The PDE5 inhibitors sildenafil, vardenafil and tadalafil are competitive and reversible inhibitors of cGMP hydrolysis by the catalytic side of PDE5. The structures of vardenafil and sildenafil are similar, they both contain similar structured purine ring of cGMP that contributes their features to act as a competitive inhibitor of PDE5. The difference of the molecular structures is the reason for interaction with the catalytic site of PDE5 and improves the affinity of these compounds compared with cGMP selectivity.