IntroductionThe phosphodiesterase-5 enzyme (PDE-5) inhibitor sildenafil(SIL) was developed for the treatment of cardiovascular disease(Revatio®, using in therapy of pulmonary arterial hypertension)and was also found to be effective in erectile dysfunction disorders(ED) therapy. SIL (Viagra®) was the only PDE-5 inhibitor approvedfor the treatment of ED until vardenafil (Levitra®, VAR) and tadalafil(Cialis®, TAD) were also approved in 2003 by the FDA [1].Erectile dysfunction is a highly prevalent inability to achieveand maintain adequate erection and sexual performance.