Sildenafil

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{{Drugbox

|IUPAC_name = 1-[4-ethoxy-3-(6,7-dihydro-1-methyl-
7-oxo-3-propyl-1H-pyrazolo[4,3-d]pyrimidin-5-yl)
phenylsulfonyl]-4-methylpiperazine citrate
| CAS_number=171599-83-0
| ATC_prefix=G04
| ATC_suffix=BE03
| PubChem=5281023
| DrugBank=APRD00556
| chemical_formula = C₂₂H₃₀N₆O₄S · C₆H₈O₇
| molecular_weight = base: 474.6 g/mol
salt: 666.7 g/mol
| bioavailability= ?
| metabolism = ?
| elimination_half-life=?
| excretion =  ?
| pregnancy_category = ?
| legal_status = ?
| routes_of_administration=Oral
}}

Sildenafil citrate, sold under the names Viagra, Revatio and (in the Indian subcontinent) Caverta, is a drug used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH), developed by the pharmaceutical company Pfizer. It is also capable of arousing women for greater sexual drive. Viagra pills, intended to treat impotence, are blue with the words "Pfizer" on one side and "VGR xx" (with xx representing 25, 50 or 100, the dose of that pill in milligrams) on the other. Its primary competitor on the market is tadalafil (Cialis).

1 History
2 Mechanism of action
3 Dosage and price
4 Contraindications and side effects
5 Uses outside of erectile dysfunction
- 5.1 Pulmonary hypertension
- 5.2 Raynaud's phenomenon
6 See also
7 References
8 Notes
9 External links

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History

Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists working at Pfizer's Sandwich, Kent research facility. It was initially studied for use in hypertension (high blood pressure) and angina pectoris (a form of ischaemic cardiovascular disease). Phase I clinical trials under the direction of Ian Osterloh suggested that the drug had little effect on angina, but that it could induce marked penile erections (Boolell et al 1996). Pfizer therefore decided to market it for erectile dysfunction, rather than for angina. The drug was patented in 1996, approved for use in erectile dysfunction by the FDA on March 27, 1998, becoming the first pill approved to treat erectile dysfunction in the United States, and offered for sale in the United States later that year (Kling 1998). It soon became a great success: annual sales of Viagra in the period 1999–2001 exceeded $1 billion.

The name "Viagra", like many drug names, is a marketing invention. It was possibly inspired by the Sanskrit word "vyāghra", which means "tiger". The word rhymes with "Niagara" (Niagara Falls is a popular honeymoon destination and Niagara, being such an impressive waterfall, evokes a sense of incredible erectile and ejaculatory strength). The sound of the word also suggests the words "vigor" and "virile".

Even though Viagra is only available by prescription from a doctor, it was advertised directly to consumers on TV (famously being endorsed by Bob Dole). Numerous sites on the Internet offer Viagra for sale after an "online consultation," a mere web questionnaire. The "Viagra" name has become so well known that many fake aphrodisiacs now call themselves "herbal Viagra" or are presented as blue tablets imitating the shape and colour of Pfizer's product. A mixture of Viagra and ecstasy, called sextasy, has become popular among rave partygoers. Viagra is also informally known as "Vitamin V", "the Blue Pill", and goes by various other nicknames.

It has been suggested that Viagra would lead to a marked drop in the demand for certain traditional remedies, such as tiger penises and rhinoceros horns and that the drug may therefore help to preserve these endangered species. However, this is unlikely in that these parts of endangered species are not only used to treat impotence. Rhinoceros horns, for example, are used as a treatment for high fever. Furthermore, since Viagra has not been shown to possess aphrodisiac properties, it is unclear that the natural remedies would compete with this new clinical drug.

Pfizer's worldwide patents on sildenafil citrate will expire in 2011–2013. The UK patent held by Pfizer on the use of PDE5 inhibitors (see below) as treatment of impotence has been invalidated in 2000 because of obviousness; this decision was upheld on appeal in 2002.

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Mechanism of action

Part of the physiological process of erection involves the parasympathetic nervous system causing the release of nitric oxide (NO) in the corpus cavernosum of the penis. NO binds to the receptors of the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation (vasodilation) in the corpus cavernosum, resulting in increased inflow of blood and an erection.

Sildenafil is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. The molecular structure of sildenafil is similar to that of cGMP and acts as a competitive binding agent of cGMP in the corpus cavernosum, resulting in better erections. Without sexual stimulation and no activation of the NO/cGMP system, sildenafil should not cause an erection. Other drugs that operate by the same mechanism include tadalafil (Cialis®) and vardenafil (Levitra®).

Sildenafil is metabolised by hepatic enzymes and excreted by both the liver and kidneys. If taken with a high fat meal, there may be a delay in absorption of sildenafil and the peak effect might be reduced slightly as the plasma concentration will be lowered.

Some reports have claimed that sildenafil causes enhanced sexual pleasure for women by increasing blood flow to the sexual organs. This lacks firm evidence, and it has also been suggested that this is part of an effort to invent a new disease - female sexual dysfunction - thus doubling the potential size of the market for this class of drugs.

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Dosage and price

As with all prescription drugs, proper dosage is at the discretion of a licensed medical doctor. The dose of sildenafil is 25 mg to 100 mg taken once per day between 30 minutes to 4 hours before sexual intercourse.

It is usually recommended to start with a dosage of 50 mg and then lower or raise the dosage as appropriate. The drug is sold in three dosages (25, 50, and 100 mg), all three costing about US$10 per pill. Sildenafil is not scored and it is not advisable to cut it to change dosage since the active compound is not distributed homogenously in the tablet.

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Contraindications and side effects

Contraindications include:

When taking nitric oxide donors, organic nitrites and nitrates, such as glyceryl trinitrate, sodium nitroprusside, amyl nitrite ("poppers") (Cheitlin et al 1999)
In men for whom sexual intercourse is inadvisable due to cardiovascular risk factors
Severe hepatic impairment (decreased liver function)
Severe impairment in renal function
Hypotension (low blood pressure)
Recent stroke or heart attack
Hereditary degenerative retinal disorders (including genetic disorders of retinal phosphodiesterases)

Amongst sildenafil's serious adverse effects are: priapism, severe hypotension, myocardial infarction, ventricular arrhythmias, sudden death, stroke and increased intraocular pressure.

Common side effects include sneezing, headache, flushing, dyspepsia, prolonged erections, palpitations and photophobia. Visual changes including blurring of vision and a curious bluish tinge have also been reported.

Care should be exercised by patients who are also taking Protease inhibitors for the treatment of HIV. Protease inhibitors inhibit the metabolism of sildenafil, effectively multiplying the plasma levels of sildenafil, increasing the incidence and severity of side-effects. It is recommended that patients using protease inhibitors limit their use of sildenafil to no more than one 25-mg dose every 48 hours.

Some Viagra users have complained of blurriness and loss of peripheral vision. In May of 2005, the U.S. Food and Drug Administration found that sildenafil could lead to vision impairmentTemplate:Fact and a number of studies have linked Viagra use with nonarteritic anterior ischemic optic neuropathy<ref>Pomeranz HD, Bhavsar AR. "Nonarteritic ischemic optic neuropathy developing soon after use of sildenafil (viagra): a report of seven new cases." J Neuroophthalmol. 2005 Mar;25(1):9-13. PMID 15756125.</ref><ref>Egan R, Pomeranz H. "Sildenafil (Viagra) associated anterior ischemic optic neuropathy." Arch Ophthalmol. 2000 Feb;118(2):291-2. PMID 10676804.</ref><ref>Pomeranz HD, Smith KH, Hart WM Jr, Egan RA. "Sildenafil-associated nonarteritic anterior ischemic optic neuropathy." Ophthalmology. 2002 Mar;109(3):584-7. PMID 11874765.</ref><ref>Cunningham AV, Smith KH. "Anterior ischemic optic neuropathy associated with viagra." J Neuroophthalmol. 2001 Mar;21(1):22-5. PMID 11315976.</ref><ref>Boshier A, Pambakian N, Shakir SA. "A case of nonarteritic ischemic optic neuropathy (NAION) in a male patient taking sildenafil." Int J Clin Pharmacol Ther. 2002 Sep;40(9):422-3. PMID 12358159.</ref><ref>Akash R, Hrishikesh D, Amith P, Sabah S. "Case report: association of combined nonarteritic anterior ischemic optic neuropathy (NAION) and obstruction of cilioretinal artery with overdose of Viagra." J Ocul Pharmacol Ther. 2005 Aug;21(4):315-7. PMID 16117695.</ref>.

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Uses outside of erectile dysfunction

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Pulmonary hypertension

As well as erectile dysfunction, sildenafil citrate is also effective in the rare disease pulmonary arterial hypertension (PAH). It relaxes the arterial wall, decreasing pulmonary arterial resistance and pressure, and thus the strain on the right side of the heart thus improving symptoms of right-sided heart failure. Because the distribution of PDE-5, which is primarily in the arterial wall smooth muscle in the lungs and penis, sildenafil acts selectively in both these areas without inducing vasodilation in other areas of the body. Pfizer submitted an additional registration for sildenafil to the FDA, and sildenafil was approved for this indication in June 2005. The preparation is named Revatio, avoiding confusion with Viagra, and the 20 milligram tablets are white and round. Sildenafil joins bosentan and prostacyclin, both of which are given by continuous infusion, as therapies for this condition.

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Raynaud's phenomenon

In 2005, Dr. Roland Fries and colleagues reported that sildenafil cut the frequency of Raynaud's phenomenon attacks, reduced their duration by roughly one half, and more than quadrupled the mean capillary blood velocity. This was a double-blind, placebo-controlled crossover trial and the patients had both the primary and secondary forms and had all discontinued the more conventional treatments for this (Fries et al 2005). It had earlier been reported to be effective for Raynaud's in a 65-year-old woman with scleroderma and pulmonary hypotension (Rosenkranz et al 2003); (Rosenkranz et al 2004), and in nine women and one man treated by a Dr. Jack R. Lichtenstein of Annapolis, Maryland over a three-year period (Lichtenstein 2003).

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References

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Kling J. From hypertension to angina to Viagra. Mod Drug Discov 1998;1:31-38. Fulltext.
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{{cite journal

| first = Stephan | last = Rosenkranz
| Coauhtors = Frank Diet, Thomas Karasch, Julia Weihrauch, Klaus Wasserman and Erland Erdmann
| date = November 18, 2003
| title = Sildenafil improved pulmonary hypertension and peripheral blood flow in a patient with scleroderma-associated lung fibrosis and the raynaud phenomenon
| journal = Annals of Internal Medicine
| volume = 139 | issue = 10 | pages = 871-3
| id = PMID 14623635
| url = http://www.annals.org/cgi/reprint/139/10/871.pdf
}} (PDF)

{{cite journal

| first = Stephan | last = Rosenkranz
| Coauhtors = E. Caglayan, Frank Diet, Thomas Karasch, Julia Weihrauch, Klaus Wasserman and Erland Erdmann
| date = August 13, 2004
| title = Langzeiteffekte von Sildenafil bei Sklerodermie-assoziierter pulmonaler Hypertonie und Raynaud-Syndrom (Long-term effects of sildenafil in a patient with scleroderma-associated pulmonary hypertension and Raynaud’s syndrome)
| journal = Deutsche Medizinische Wochenschrift
| volume = 129 | issue = 33 | pages = 1736-40
| id = PMID 15295684 Template:Doi
| url =   http://www.thieme-connect.com/ejournals/html/dmw/doi/10.1055/s-2004-829025
}}

Terrett, N. K. et al. Sildenafil (Viagra), a potent and selective inhibitor of Type 5 cGMP phosphodiesterase with utility for the treatment of male erectile dysfunction. Bioorg. Med. Chem. Lett. 1996, 6, 1819-1824.

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