Sildenafil

Sildenafil, sold as the brand name Viagra among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension.[3] It is unclear if it is effective for treating sexual dysfunction in women.[3] It is taken by mouth or injection into a vein.[3] Onset is typically within 20 minutes and lasts for about 2 hours.[3]

Common side effects include headachesheartburn, and flushed skin.[3] Caution is advised in those with cardiovascular disease.[3] Rare but serious side effects include a prolonged erection that can lead to damage to the penis, vision problems, and hearing loss.[3] Sildenafil should not be taken by people on nitrates such as nitroglycerin (glycerin trinitrate), as this may result in a serious drop in blood pressure.[3]

Sildenafil acts by blocking phosphodiesterase 5 (PDE5), an enzyme that promotes breakdown of cGMP, which regulates blood flow in the penis.[3] It requires sexual arousal, however, to work.[3] It also results in dilation of the blood vessels in the lungs.[3]

Pfizer originally discovered the medication in 1989 while looking for a treatment for heart-related chest pain.[4] It was approved for medical use in the United States and Europe in 1998.[3][4] In the United States, about 2 million prescriptions were written for sildenafil in 2016.[5] In 2017, it became available as a generic medication.[6] In the United Kingdom, it is available over the counter.[7] As of 2018 in the United States, the wholesale cost is less than US$1 per dose.[8]

Medical uses

Pfizer Viagra tablet in the trademark blue diamond shape

Sexual dysfunction

The primary indication of sildenafil is treatment of erectile dysfunction (inability to sustain a satisfactory erection to complete intercourse). Its use is now one of the standard treatments for erectile dysfunction, including for men with diabetes mellitus.[9]

Antidepressant-associated sexual dysfunction

Tentative evidence suggests that sildenafil may help men who experience antidepressant-induced erectile dysfunction.[10]

Pulmonary hypertension

While sildenafil improves some markers of disease in people with pulmonary arterial hypertension, it does not appear to affect the risk of death or serious side effects as of 2014.[11]

Raynaud’s phenomenon

Sildenafil and other PDE5 inhibitors are used off-label to alleviate vasospasm and treat severe ischemia and ulcers in fingers and toes for people with secondary Raynaud’s phenomenon;[12][13] these drugs have moderate efficacy for reducing the frequency and duration of vasospastic episodes.[12] As of 2016, their role more generally in Raynaud’s was not clear.[13]

High-altitude pulmonary edema

Sildenafil has been studied for high-altitude pulmonary edema, but its use is currently not recommended for that indication.[14]

Adverse effects

In clinical trials, the most common adverse effects of sildenafil use included headacheflushingindigestionnasal congestion, and impaired vision, including photophobia and blurred vision.[2] Some sildenafil users have complained of seeing everything tinted blue (cyanopsia).[15] Some complained of blurriness and loss of peripheral vision. In July 2005, the FDA found that sildenafil could lead to vision impairment in rare cases,[16] and a number of studies have linked sildenafil use with non-arteritic anterior ischemic optic neuropathy.[17]

Rare but serious adverse effects found through postmarketing surveillance include prolonged erections, severe low blood pressuremyocardial infarction (heart attack), ventricular arrhythmiasstroke, increased intraocular pressure, and sudden hearing loss.[2] In October 2007, the FDA announced that the labeling for all PDE5 inhibitors, including sildenafil, required a more prominent warning of the potential risk of sudden hearing loss.[18]

Interactions

Care should be exercised by people who are also taking protease inhibitors for the treatment of HIV infection. Protease inhibitors inhibit the metabolism of sildenafil, effectively multiplying the plasma levels of sildenafil, increasing the incidence and severity of side effects. Those using protease inhibitors are recommended to limit their use of sildenafil to no more than one 25 mg dose every 48 hours.[2] Other drugs that interfere with the metabolism of sildenafil include erythromycin and cimetidine, both of which can also lead to prolonged plasma half-life levels.

The use of sildenafil and an α1 blocker (typically prescribed for hypertension or for urologic conditions, such as benign prostatic hypertrophy) at the same time may lead to low blood pressure, but this effect does not occur if they are taken at least 4 hours apart.[19]

Contraindications

Contraindications include:[2]:11

Sildenafil should not be used if sexual activity is inadvisable due to underlying cardiovascular risk factors[21]

Nonmedical use

Recreational use

Sildenafil’s popularity with young adults has increased over the years.[22] Sildenafil’s trade name, Viagra, is widely recognized in popular culture, and the drug’s association with treating erectile dysfunction has led to its recreational use.[23] The reasons behind such use include the belief that the drug increases libido, improves sexual performance,[23] or permanently increases penis size.[24] Studies on the effects of sildenafil when used recreationally are limited, but suggest it has little effect when used by those not suffering from erectile dysfunction. In one study, a 25-mg dose was shown to cause no significant change in erectile quality, but did reduce the postejaculatory refractory time.[25] This study also noted a significant placebo effect in the control group.[25]

Unprescribed recreational use of sildenafil and other PDE5 inhibitors is noted as particularly high among users of illegal drugs.[26] Sildenafil is sometimes used to counteract the effects of other substances, often illicit.[23] Some users mix it with methylenedioxymethamphetamine (MDMA, ecstasy), other stimulants, or opiates in an attempt to compensate for the common side effect of erectile dysfunction, a combination known as “sextasy“, “rockin’ and rollin'” or “trail mix”.[23] Mixing it with amyl nitrite, another vasodilator, is particularly dangerous and potentially fatal.[23]

Jet lag research

The 2007 Ig Nobel Prize in Aviation went to Patricia V. Agostino, Santiago A. Plano, and Diego A. Golombek of Universidad Nacional de Quilmes, Argentina, for their discovery that sildenafil helps treat jet lag recovery in hamsters.[27][28]

Sports

Professional athletes have been documented using sildenafil, believing the opening of their blood vessels will enrich their muscles. In turn, they believe it will enhance their performances.[29][30]

Analogs

Acetildenafil and other synthetic structural analogs of sildenafil which are PDE5 inhibitors have been found as adulterants in a number of “herbal” aphrodisiac products sold over-the-counter.[31] These analogs have not undergone any of the rigorous testing that drugs like sildenafil have passed, and thus have unknown side-effect profiles.[32] Some attempts have been made to ban these drugs, but progress has been slow so far, as, even in those jurisdictions that have laws targeting designer drugs, the laws are drafted to ban analogs of illegal drugs of abuse, rather than analogs of prescription medicines. However, at least one court case has resulted in a product being taken off the market.[33]

The US FDA has banned numerous products claiming to be Eurycoma longifolia that, in fact, contain only analogs of sildenafil.[34][35][36] Sellers of such fake herbals typically respond by just changing the names of their products.

Detection in biological fluids

Sildenafil and/or N-desmethylsildenafil, its major active metabolite, may be quantified in plasma, serum, or whole blood to assess pharmacokinetic status in those receiving the drug therapeutically, to confirm the diagnosis in potential poisoning victims, or to assist in the forensic investigation in a case of fatal overdose.[37]

Mechanism of action

Crystal structure of human PDE5 with bound sildenafil, PDB entry 1udt[38]

Sildenafil protects cyclic guanosine monophosphate (cGMP) from degradation by cGMP-specific phosphodiesterase type 5 (PDE5) in the corpus cavernosumNitric oxide (NO) in the corpus cavernosum of the penis binds to guanylate cyclase receptors, which results in increased levels of cGMP, leading to smooth muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries. This smooth muscle relaxation leads to vasodilation and increased inflow of blood into the spongy tissue of the penis, causing an erection.[39] Robert F. FurchgottFerid Murad, and Louis Ignarro won the Nobel Prize in Physiology or Medicine in 1998 for their independent study of the metabolic pathway of nitric oxide in smooth muscle vasodilation.

The molecular mechanism of smooth muscle relaxation involves the enzyme CGMP-dependent protein kinase, also known as PKG. This kinase is activated by cGMP and it phosphorylates multiple targets in the smooth muscle cells, namely myosin light chain phosphataseRhoAIP3 receptorphospholipase C, and others.[40] Overall, this results in a decrease in intracellular calcium and desensitizing proteins to the effects of calcium, engendering smooth muscle relaxation.[40]

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 PDE5 in the corpus cavernosum, resulting in more cGMP and better erections.[39] Without sexual stimulation, and therefore lack of 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 broken down in the liver by hepatic metabolism using cytochrome p450 enzymes, mainly CYP450 3A4(major route), but also by CYP2C9 (minor route) hepatic isoenzymes. The major product of metabolisation by these enzymes is N-desmethylated sildenafil, which is metabolised further. This metabolite also has an affinity for the PDE receptors, about 40% of that of sildenafil. Thus, the metabolite is responsible for about 20% of sildenafil’s action. Sildenafil is excreted as metabolites predominantly in the feces (about 80% of administered oral dose) and to a lesser extent in the urine (around 13% of the administered oral dose). If taken with a high-fat meal, absorption is reduced; the time taken to reach the maximum plasma concentration increases by around one hour, and the maximum concentration itself is decreased by nearly one-third.[41]

Route of administration

  • When taken by mouth sildenafil for erectile dysfunction results in an average time to onset of erections of 27 minutes (ranging from 12 to 70 minutes).[42]
  • Under the tongue use of sildenafil for erectile dysfunction results in an average onset of action of 15 minutes and lasting for an average of 40 minutes.[43]

There are also mouth spray preparations of sildenafil for faster onset of action.

Chemical synthesis

The preparation steps for synthesis of sildenafil are:[44]

  1. Methylation of 3-propylpyrazole-5-carboxylic acid ethyl ester with hot dimethyl sulfate
  2. Hydrolysis with aqueous sodium hydroxide (NaOH) to free acid
  3. Nitration with oleum/fuming nitric acid
  4. Carboxamide formation with refluxing thionyl chloride/NH4OH
  5. Reduction of nitro group to amino group
  6. Acylation with 2-ethoxybenzoyl chloride
  7. Cyclization
  8. Sulfonation to the chlorosulfonyl derivative
  9. Condensation with 1-methylpiperazine.

History

Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists working at Pfizer’s Sandwich, Kent, research facility in England. It was initially studied for use in hypertension (high blood pressure) and angina pectoris (a symptom of ischaemic heart disease). The first clinical trials were conducted in Morriston Hospital in Swansea.[45] Phase I clinical trials under the direction of Ian Osterloh suggested the drug had little effect on angina, but it could induce marked penile erections.[46][47] Pfizer therefore decided to market it for erectile dysfunction, rather than for angina; this decision became an often-cited example of drug repositioning.[48][49] The drug was patented in 1996, approved for use in erectile dysfunction by the FDA on 27 March 1998, becoming the first oral treatment approved to treat erectile dysfunction in the United States, and offered for sale in the United States later that year.[50] It soon became a great success: annual sales of Viagra peaked in 2008 at US$1.934 billion.[51]

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