The Journey of Sildenafil

Sildenafil is rapidly absorbed orally with a bioavailability of
approximately 40% due to significant hepatic first-pass metabolism. Peak plasma concentrations are
reached within 30-120 minutes (median ~60 minutes) when taken on an empty stomach. A high-fat meal
delays the time to peak concentration by approximately 60 minutes and reduces Cmax by 29% — though
total absorption (AUC) is not significantly reduced. This delay explains the clinical recommendation
to take sildenafil 1 hour before anticipated sexual activity and to avoid high-fat meals. Higher
doses (≥100 mg) show slightly reduced bioavailability due to saturable first-pass metabolism.
Film-coated tablets and oral suspension have equivalent bioavailability. The drug can also be
administered intravenously (for pulmonary arterial hypertension when oral administration is
temporarily impossible).

Sildenafil distributes widely with a volume of distribution of
approximately 105 L, indicating significant tissue penetration beyond the vascular compartment.
Plasma protein binding is high at approximately 96%, primarily to albumin and alpha-1-acid glycoprotein.
Despite extensive protein binding, the free fraction is sufficient to achieve pharmacologically relevant
concentrations in target tissues including corpus cavernosum, pulmonary vasculature, and (via crossing
the blood-retinal barrier) the retina — where PDE6 inhibition by sildenafil at higher concentrations
causes the transient visual disturbances (blue-tinge, photophobia) reported by patients. Pulmonary
tissue concentrations significantly exceed plasma levels, supporting sildenafil's use in pulmonary
arterial hypertension (PAH) at higher, more frequent doses than for erectile dysfunction.

Nitric oxide (NO), produced by endothelial nitric oxide synthase
(eNOS) and neuronal NOS (nNOS) in response to sexual stimulation (parasympathetic, peptidergic
stimulation in the corpus cavernosum), activates soluble guanylyl cyclase (sGC) in vascular smooth
muscle cells, generating cyclic GMP (cGMP). cGMP activates protein kinase G (PKG), which
phosphorylates myosin light chain phosphatase (MLCP) activators and calcium channels, ultimately
reducing intracellular calcium and causing smooth muscle relaxation and vasodilation. PDE5
(phosphodiesterase type 5, PDE5A) is the primary cGMP-hydrolyzing enzyme in vascular smooth muscle
and corpus cavernosum. Sildenafil competitively and selectively inhibits PDE5 (IC50 ~3.9 nM),
preventing cGMP breakdown and amplifying and prolonging the NO-cGMP signal. Critically, sildenafil
requires NO to be present — it amplifies rather than replaces the endogenous signal, explaining
why the erection requires sexual stimulation. This NO dependence also explains the dangerous
hypotension when sildenafil is combined with nitrate drugs (both pathways converge on cGMP).

Sildenafil is extensively metabolized in the liver by CYP3A4 (primary)
and CYP2C9 (secondary) via N-demethylation to form UK-103,320 (N-desmethylsildenafil). This active
metabolite has a PDE5 IC50 of approximately 26 nM (6-7-fold less potent than sildenafil) and
contributes approximately 20% of the total pharmacological effect at steady state. UK-103,320 is
further metabolized to inactive products. CYP3A4 inhibitors (ketoconazole, erythromycin, ritonavir,
grapefruit juice) substantially increase sildenafil AUC (2-11-fold increases), requiring dose reduction.
CYP3A4 inducers (rifampicin) reduce sildenafil AUC by approximately 90%. Sildenafil also inhibits
PDE6 (retinal cGMP phosphodiesterase) at higher concentrations, and weakly inhibits PDE1 (cardiac).
Alpha-blocker co-administration risks additive hypotension.

Sildenafil is eliminated entirely by hepatic metabolism; no unchanged
drug appears in urine. Metabolites are excreted approximately 80% in feces (via biliary excretion)
and 13% in urine. Elimination half-life is 3-5 hours for both sildenafil and UK-103,320. Duration
of pharmacological effect for erectile dysfunction (4-6 hours) somewhat exceeds plasma elimination
due to sustained PDE5 inhibition at tissue concentrations. For PAH, three-times-daily dosing (20 mg)
maintains more consistent plasma concentrations. In elderly patients (>65 years), AUC is approximately
40% higher due to reduced hepatic clearance, supporting lower starting doses. Severe hepatic
impairment (Child-Pugh C) doubles sildenafil AUC. Renal impairment (CrCl <30 mL/min) increases
AUC by approximately 100% due to reduced hepatic blood flow and protein changes.

Sildenafil (Viagra for ED; Revatio for PAH) revolutionized treatment
of erectile dysfunction and provided a new therapeutic approach to pulmonary arterial hypertension.
The drug is effective in approximately 70-80% of men with organic and psychogenic ED. Absolute
contraindication exists with nitrate medications (nitroglycerin, isosorbide mono/dinitrate) due to
potentially life-threatening synergistic hypotension — the combination can drop systolic BP by
>25 mmHg. Caution is required with alpha-blockers (tamsulosin) and antihypertensives. The SUPER-1
trial established sildenafil efficacy in PAH (6-minute walk improvement, hemodynamic parameters).
Common adverse effects: headache (flushing — vasodilation), dyspepsia (esophageal smooth muscle
relaxation), nasal congestion, and transient visual disturbances (blue tinge — PDE6).