The three available PDE5 inhibitors share a similar mechanism of action, but they have structural, pharmacologic, and clinical differences. The molecular structure of tadalafil is available in Figure 1. Tadalafil’s molecular structure is different than the similar structures of generic sildenafil and generic vardenafil. All three have a heterocyclic nitrogen-containing doublering system, with a central ring that is analogous to that of cGMP and allows for competitive binding of the drug with PDE5 at the catalytic site. Tadalafil is different in that it is a β-carboline-type PDE5 inhibitor with a piperazinedione ring formed from a modification of the hydantoin ring of sildenafil (Viagra).

Figure 1
Molecular structure of tadalafil.