Oral Phosphodiesterase
- Inhibitors PDE5
- Pharmacokinetics and pharmacodynamics
- Side effects
- Safety
- Comparison
Phosphodiesterase, as mentioned in the previous section, is paramount to the breakdown of cGMP in the SMC. This enzyme is found in several dif ferent tissues throughout the body and has been categorized into 11 families. The family that is found in corpora cavernosum tissue as well as smooth muscle and vascular smooth muscle is PDE5. As the role of PDE5 was elucidated in the 1980s, drugs were targeted to inhibit this enzyme and increase the NO in SMCs. A result of this was an unexpected increase in NO in penile tissue and improvement in erectile function. PDE5 inhibitors were first marketed in 1998 with the dawn of sildenafil (Viagra®). This was followed closely in 2003 with vardenafil (Levitra®) and tadalafil (Cialis®). As the market for these drugs has increased, it has been impor tant to identify the safety, efficacy, and limita tions of these drugs. When choosing a PDE5 inhibitor for a patient, it is important to know the onset of action, efficacy, and duration. Studies usually will define the maximal plasma concentration (Cmax ), time to reach this plasma concentration (Tmax ), and the plasma halflife (t ½). How well these laboratory findings corre late to clinical findings is sometimes difficult to determine and the measuring tool most com monly used to evaluate efficacy is the International Index of Erectile Function (IIEF) domain score.
Viagra®
The most studied of these drugs is sildenafil (Viagra®). In several placebo controlled, ran domized trials, this drug has been shown to improve erectile function. The pivotal study for sildenafil was published in 1998, which showed a clinically and statistically significant improve ment in IIEF domain scores for men suffering from ED for greater than 5 years. The onset of action of sildenafil is approximately 20 min with a reported t ½ of 3–5 h and duration of action as long as 12 h. The important thing about sildenafil is that it should not be taken with a highfat meal because this will decrease the absorption of the medication. Female Viagra Australia
Levitra®
The FDA approved vardenafil (Levitra®) in 2003 after a study of 805 men with ED showed a clinically and statistically significant improve ment in IIEF scores when compared to placebo. In this study, different doses were evaluated and men were categorized into mild, moderate, and severe ED. Approximately 40% of the moder ate and severe ED men had improvement with the highest dose and over 79% of the mild ED men had improvement. Vardenafil is quickly absorbed with a Tmax of 45 min and a reported t ½ of 4–5 h. Onset of action has been recorded as early as 10 min [17]. Just as in sildenafil, it is recommended that highfat meals are avoided.