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Pharmacology: Pharmacodynamics: Sildenafil inhibits phosphodiesterase type 5 (PDE-5) in smooth muscle of pulmonary vasculature where PDE-5 is responsible for the degradation of cyclic guanosine monophosphate (cGMP). Increased cGMP concentration results in pulmonary vasculature relaxation; vasodilation in the pulmonary bed and the systemic circulation (to a lesser degree) may occur.
Pharmacokinetics: Duration: Decrease blood pressure: <8 hours.
Absorption: Rapid; slower with a high-fat meal.
Distribution: Vdss: 105 L.
Protein binding, plasma: ~96% (Sildenafil and N-desmethyl metabolite).
Metabolism: Hepatic via CYP 3A4 (major) and CYP 2C9 (minor route): forms N-desmethyl metabolite (active).
Bioavailability: 41% (25 to 63%).
Half-life elimination: 4 hours.
Time to peak: 30-120 minutes; delayed by 60 minutes with a high-fat meal.
Excretion: Feces (~80%, as metabolites), urine (~13%).
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