• These are the isoenzymes which are produced in different tissues of the body organs. These belong to the cyclic nucleotide phosphodiesterase inhibitors; these isoenzymes catalyses the hydrolysis reactions of cAMP and cGMP.

• These phosphodiesterase molecules they target the decrease in the levels of the cGMP that enhances the smooth muscle tissue to relax. The site specific reactions that are seen with this PDE are they inhibit the cGMP molecules causing natural erections.

• These isoenzymes can be inhibited by different inhibitors; they are classified into two major categories like selective inhibitors and non selective inhibitors.

 Selective inhibitors: The selective inhibitors are divided into five different types based on the site of the smooth muscle present. Type-5 inhibitors are mainly responsible for the erections in impotent men. The drugs which include are many; based on their activity and drug absorption in the body. They are classified into three types; they are,

 Long acting type-5 PDE inhibitors: The activity of the drug remains for about more than 24-36 hours in the body.

Example: Tadalafil (Cialis).

 Intermediate acting type-5 PDE inhibitors: The activity of the drug remains for about more than 4-5 hours in the body.

Example: Sildenafil (Viagra); Vardenafil (Levitra); Udenafil (Zydena); and Avanafil.

 Short acting type-5 PDE inhibitors: The activity of the drug remains less than

Example: Zaprinast.

 Non-selective inhibitors: The non-selective inhibitors include the drugs like Theophylline, Aminophylline, Caffeine, Paraxanthine, Pentoxifylline, and others like adenosine receptor antagonists. They increase the intracellular cAMP, and cGMP levels and innate the immune system.