Figure 1.

The chemical structure of the cGMP (brown arrow indicates phosphodiester bond). The lower panel shows the chemical structure of sildenafil and vardenfil. Notice the similarity in structure, except the nitrogen atom's position and the change of sildenafil's piperazinering from a methyl group to an ethyl group (green arrows). The bottom structure is for tadalafil which is more different the other PDE5 inhibitors, which explains the binding affinity to the PDE5 enzyme and thus its longer half-life.