Molecular pharmacology of PDEs

     PDEs represent excellent targets for pharmacological interventions for several human diseases. A number of family-selective PDE inhibitor compounds targeting specific PDEs have already been  approved for clinical use in humans, and many more are in the pipeline. The most widely known group of PDE inhibitors--Viagra (sildenafil), Cialis (tadalafil), and Levitra (vardenafil)--were developed to target PDE5, a cGMP-specific PDE abundant in smooth muscle (among other tissues), and initially approved for the treatment of  male erectile dysfunction. One of the more common adverse side effects of certain PDE5 inhibitors are visual disturbances, which is not unexpected considering the close similarity of PDE5 and PDE6 in many of their biochemical and pharmacological properties.