Since the discovery of sildenafil in 1989 as a highly selective inhibitor of the
phosphodiesterase type-5 (PDE-5) receptor, 2 additional PDE-5 inhibitors, tadalafil and
vardenafil, have emerged as safe and effective treatments of erectile dysfunction (ED).
Enzymes in the PDE family catalyze the hydrolysis of the intracellular signaling molecules
cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP),
which is the second messenger of nitric oxide (NO) and a principal mediator of smooth …