Sildenafil Viagra
Sildenafil Viagra - General Information
Sildenafil Viagra (in citrate form), sold under the names Viagra, Revatio and under various other names, is a drug used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH), developed by the pharmaceutical company Pfizer. Its primary competitors on the market are tadalafil (Cialis), and vardenafil (Levitra). [Wikipedia]
Pharmacology of Sildenafil Viagra
Sildenafil Viagra is used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH). Part of the physiological process of erection involves the release of nitric oxide (NO) in the corpus cavernosum. This then activates the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation in the corpus cavernosum, resulting in increased inflow of blood and an erection. Sildenafil Viagra is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. This means that, with Sildenafil Viagra on board, normal sexual stimulation leads to increased levels of cGMP in the corpus cavernosum which leads to better erections. Without sexual stimulation and no activation of the NO/cGMP system, Sildenafil Viagra should not cause an erection.
Sildenafil Viagra for patients
Patient Information:
Sildenafil should be used with caution in patients with anatomical deformation of the penis, sickle cell anemia,
multiple myeloma, or leukemia.
There is potential cardiac risk of sexual activity in patients with preexisting cardiovascular risk factors. Patients
should discontinue further sexual activity and contact the doctor if they experience chest pain, dizziness, or nausea
upon initiation of the activity.
Sildenafil may cause prolonged erections (> 4 hours) and priapism (painful erections greater than 6 hours in
duration). If patients experience prolonged erections or priapism, they should contact their doctors IMMEDIATELY!
Sildenafil Viagra Interactions
Effects of Other Drugs on Sildenafil Citrate
In Vitro Studies: Sildenafil metabolism is principally mediated by the cytochrome P450 (CYP) isoforms 3A4 (major route) and 2C9 (minor route). Therefore, inhibitors of these isoenzymes may reduce sildenafil clearance.
In Vivo Studies: Cimetidine (800 mg), a nonspecific CYP inhibitor, caused a 56% increase in plasma sildenafil concentrations when coadministered with sildenafil citrate (50 mg) to healthy volunteers.
When a single 100 mg dose of sildenafil citrate was administered with erythromycin, a specific CYP3A4 inhibitor, at steady state (500 mg bid for 5 days), there was a 182% increase in sildenafil systemic exposure (AUC). In addition, coadministration of the HIV protease inhibitor saquinavir, also a CYP3A4 inhibitor, at steady state (1200 mg tid) with sildenafil citrate (100 mg single dose) resulted in a 140% increase in sildenafil Cmax and a 210% increase in sildenafil AUC. Sildenafil citrate had no effect on saquinavir pharmacokinetics. Stronger CYP3A4 inhibitors such as ketoconazole or itraconazole would be expected to have still greater effects, and population data from patients in clinical trials did indicate a reduction in sildenafil clearance when it was coadministered with CYP3A4 inhibitors (such as ketoconazole, erythromycin, or cimetidine).
Coadministration with the HIV protease inhibitor ritonavir, which is a highly potent P450 inhibitor, at steady state (400 mg bid) with sildenafil citrate (100 mg single dose) resulted in a 300% (4-fold) increase in sildenafil Cmax and a 1000% (11-fold) increase in sildenafil plasma AUC. At 24 hours the plasma levels of sildenafil were still approximately 200 ng/mL, compared to approximately 5 ng/mL when sildenafil was dosed alone. This is consistent with ritonavir's marked effects on a broad range of P450 substrates. Sildenafil citrate had no effect on ritonavir pharmacokinetics.
It can be expected that concomitant administration of CYP3A4 inducers, such as rifampin, will decrease plasma levels of sildenafil.
Single doses of antacid (magnesium hydroxide/aluminum hydroxide) did not affect the bioavailability of sildenafil citrate.
Pharmacokinetic data from patients in clinical trials showed no effect on sildenafil pharmacokinetics of CYP2C9 inhibitors (such as tolbutamide, warfarin), CYP2D6 inhibitors (such as selective serotonin reuptake inhibitors, tricyclic antidepressants), thiazide and related diuretics, ACE inhibitors, and calcium channel blockers. The AUC of the active metabolite, N-desmethyl sildenafil, was increased 62% by loop and potassium-sparing diuretics and 102% by nonspecific beta-blockers. These effects on the metabolite are not expected to be of clinical consequence.
Effects of Sildenafil Citrate on Other Drugs
In Vitro Studies: Sildenafil is a weak inhibitor of the cytochrome P450 isoforms 1A2, 2C9, 2C19, 2D6, 2E1 and 3A4 (IC50 >150 mM). Given sildenafil peak plasma concentrations of approximately 1 mcM after recommended doses, it is unlikely that sildenafil citrate will alter the clearance of substrates of these isoenzymes.
In Vivo Studies: When sildenafil citrate 100 mg oral was coadministered with amlodipine, 5 mg or 10 mg oral, to hypertensive patients, the mean additional reduction on supine blood pressure was 8 mmHg systolic and 7 mmHg diastolic.
No significant interactions were shown with tolbutamide (250 mg) or warfarin (40 mg), both of which are metabolized by CYP2C9.
Sildenafil citrate (50 mg) did not potentiate the increase in bleeding time caused by aspirin (150 mg).
Sildenafil citrate (50 mg) did not potentiate the hypotensive effect of alcohol in healthy volunteers with mean maximum blood alcohol levels of 0.08%.
Sildenafil (100 mg) did not affect the steady state pharmacokinetics of the HIV protease inhibitors, saquinavir and ritonavir, both of which are CYP3A4 substrates.
Sildenafil Viagra Contraindications
Consistent with its known effects on the nitric oxide/cGMP pathway, sildenafil citrate was shown to potentiate the hypotensive effects of nitrates, and its administration to patients who are using organic nitrates either regularly and/or intermittently in any form is therefore contraindicated.
After patients have taken sildenafil citrate, it is unknown when nitrates, if necessary, can be safely administered. Based on the pharmacokinetic profile of a single 100 mg oral dose given to healthy normal volunteers, the plasma levels of sildenafil at 24 hours post dose are approximately 2 ng/mL (compared to peak plasma levels of approximately 440 ng/mL). In the following patients: age >65, hepatic impairment (e.g., cirrhosis), severe renal impairment (e.g., creatine clearance <30 mL/min), and concomitant use of potent cytochrome P450 3A4 inhibitors (erythromycin), plasma levels of sildenafil at 24 hours post dose have been found to be 3 to 8 times higher than those seen in healthy volunteers. Although plasma levels of sildenafil at 24 hours post dose are much lower than at peak concentration, it is unknown whether nitrates can be safely coadministered at this time point.
Sildenafil citrate is contraindicated in patients with a known hypersensitivity to any component of the tablet.
Additional information about Sildenafil Viagra
Sildenafil Viagra Indication: For the treatment of erectile dysfunction
Mechanism Of Action: Sildenafil Viagra inhibits the cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum located around the penis. Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and corpus cavernosal smooth muscle. This response is mediated by the release of nitric oxide (NO) from nerve terminals and endothelial cells, which stimulates the synthesis of cGMP in smooth muscle cells. Cyclic GMP causes smooth muscle relaxation and increased blood flow into the corpus cavernosum. The inhibition of phosphodiesterase type 5 (PDE5) by Sildenafil Viagra enhances erectile function by increasing the amount of cGMP.
Drug Interactions: Not Available
Food Interactions: Take on empty stomach: 1 hour before or 2 hours after meals.
Generic Name: Sildenafil
Synonyms: Sildenafil Citrate
Drug Category: Vasodilator Agents; Phosphodiesterase Inhibitors
Drug Type: Small Molecule; Approved; Investigational
Other Brand Names containing Sildenafil: Sildenafil Viagra; Viagra; Revatio;
Absorption: ~40%
Toxicity (Overdose): Not Available
Protein Binding: 96%
Biotransformation: Hepatic
Half Life: 4 hours
Dosage Forms of Sildenafil Viagra: Tablet Oral
Chemical IUPAC Name: 5-[2-ethoxy-5-(4-methylpiperazin-1-yl)sulfonylphenyl]-1-methyl-3-propyl-4H-pyrazolo[5,4-e]pyrimidin-7-one
Chemical Formula: C22H30N6O4S
Sildenafil on Wikipedia: https://en.wikipedia.org/wiki/Sildenafil
Organisms Affected: Humans and other mammals
