Phenylephrine
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Phenylephrine
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| Systematic (IUPAC) name | |
| 3-(1-hydroxy-2-methylamino-ethyl)phenol | |
| Identifiers | |
| CAS number | 61-76-7 (hydrochloride) |
| ATC code | C01 R01AA04, R01AB01, R01BA03, S01FB01, S01GA05 |
| PubChem | |
| DrugBank | |
| Chemical data | |
| Formula | C9H13NO2 |
| Mol. mass | 167.205 g/mol |
| Pharmacokinetic data | |
| Bioavailability | 38% through GI tract |
| Protein binding | 95% |
| Metabolism | Hepatic (monoamine oxidase) |
| Half life | 2.1 to 3.4 hours |
| Excretion | ? |
| Therapeutic considerations | |
| Pregnancy cat. | |
| Legal status | |
| Routes | Oral, intranasal, ophthalmic, intravenous, intramuscular |
Phenylephrine or Neo-Synephrine is an α-adrenergic receptor agonist used primarily as a decongestant, as an agent to dilate the pupil and to increase blood pressure. Phenylephrine has recently been marketed as a substitute for pseudoephedrine, (e.g. Pfizer's Sudafed (Original Formulation)) but there are recent claims that oral phenylephrine may be no more effective as a decongestant than placebo.[1]
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[edit] Uses
[edit] Decongestant
Phenylephrine is used as a decongestant, available as an oral medicine or as a nasal spray. Phenylephrine is now the most common over-the-counter (OTC) decongestant; oxymetazoline is a more common nasal spray.[citation needed] Pseudoephedrine was historically more common, although its notoriety as a methamphetamine precursor has led some governments to restrict its sale.
Oral phenylephrine is extensively metabolised by monoamine oxidase [1], an enzyme which is present in the gastrointestinal tract and the liver. Therefore, compared to orally-taken pseudoephedrine, it has a reduced and variable bioavailability of only up to 38 percent,[2][3] and is therefore less effective as a nasal decongestant than pseudoephedrine. Because phenylephrine is a direct selective α-adrenergic receptor agonist, it does not cause the release of endogenous noradrenaline like pseudoephedrine does. Therefore, phenylephrine is less likely to cause side-effects like central nervous system stimulation, insomnia, anxiety, irritability and restlessness.
Some popular cold remedies containing phenylephrine:
Australia
- Codral Cold & Flu (New Formula)
Canada
- Hot lemon Neocitran,
United Kingdom
United States
- Alka-Seltzer Cold Effervescent formula
- Sudafed PE Non-Drowsy Nasal Decongestant
- Robitussin CF, Tylenol Sinus
- DayQuil Capsules.
As a nasal spray, phenylephrine is available in 1% and 0.5% concentrations. It suffers some rebound congestion effects, although to a lesser degree than oxymetazoline.
[edit] Mydriatic
Phenylephrine is used as an eye drop to dilate the pupil to facilitate visualization of the retina. It is often used in combination with tropicamide. Narrow angle glaucoma is a contraindication to phenylephrine use.
[edit] Vasopressor
Phenylephrine is sometimes used as a vasopressor to increase the blood pressure in unstable patients with hypotension. Such use is more common in anesthesia or critical care practices; phenylephrine is especially useful in counteracting the hypotensive effect of epidural and subarachnoid anesthetics. It also has the advantage of not being inotropic or chronotropic, and so it strictly elevates the blood pressure without increasing the heart rate or contractility (reflex bradycardia may result from the blood pressure increase, however). This is especially useful if the heart is already tachycardic and/or has a cardiomyopathy. The elimination half life of Phenylephrine is about 2.5-3hours.
[edit] Side effects
The primary side effect of phenylephrine is hypertension. Patients with congestion and hypertension are typically advised to avoid products containing phenylephrine.
Patients with a history of epilepsy and on anticonvulsant medication should not take this substance. The drug interaction might produce seizures.
Some patients have shown to have stomach upset, severe abdominal cramping, and vomiting issues connected to taking this drug
[edit] Legal issues
Pseudoephedrine and phenylephrine are both used as decongestants; and until recently, pseudoephedrine was much more commonly available in the United States. This has changed because provisions of the Combat Methamphetamine Epidemic Act of 2005 placed restrictions on the sale of pseudoephedrine products to prevent the clandestine manufacture of methamphetamine. Since 2004, phenylephrine has been increasingly marketed as a substitute for pseudoephedrine; some manufacturers have changed the active ingredients of products to avoid the restrictions on sales.[4] Phenylephrine has been off patent for a long time; there are many generic brands available.
[edit] Questions about effectiveness
Pharmacists Leslie Hendeles and Randy Hatton of the University of Florida suggested in 2006 that oral phenylephrine is ineffective as a decongestant at the 10 mg dose used, arguing that the studies used for the regulatory approval of the drug in the United States in 1976 were inadequate to prove effectiveness at the 10 mg dose and safety at higher doses.[5] Other pharmacists have expressed concerns over phenylephrine's effectiveness as a nasal decongestant,[1] and other clinicians have indicated concern for regulatory actions that reduced the availability of pseudoephedrine.[6][7] A subsequent meta-analysis by the same researchers concluded that there is insufficient evidence for its effectiveness,[8] though another meta-analysis published shortly thereafter by researchers from GlaxoSmithKline found the standard 10mg dose to be significantly more effective than a placebo.[9]
The matter is still under debate, though the Food and Drug Administration has stood by its 1976 approval.[4]
[edit] References
- ^ a b University of Florida (press release) (2006-07-19). UF researchers question effectiveness of decongestant.
- ^ Pharmacogenetics and Pharmacogenomics Knowledge Base entry on phenylephrine
- ^ NZ Medicines and Medical Devices Safety Authority recommendation on phenylephrine (November 2004)
- ^ a b Hilenmeyer, K.. "All stuffed up", Southwest Florida Herald-Tribune, 30 January 2007.
- ^ Heldeles, L. and Hatton, R. (2006). "Oral phenylephrine: An ineffective replacement for pseudoephedrine?". Journal of Allergy and Clinical Immunology 118 (1): 279-280. doi:. PMID 16815167.
- ^ >Eccles R (May 2006). Phenylephrine an ineffective replacement for pseudoephedrine in response to the methamphetamine problem in the USA. BMJ.
Rapid Response to
Tanne JH (February 2006). "Methamphetamine epidemic hits middle America". BMJ 332 (7538): 382. doi:. PMID 16484253. - ^ Eccles, R. (2007). "Substitution of phenylephrine for pseudoephedrine as a nasal decongestant. An illogical way to control methamphetamine abuse". British Journal of Clinical Pharmacology 63: 10-14. doi:. PMID 17116124. (January 2007)
- ^ Hatton, R.C. et al. (2007). "Efficacy and Safety of Oral Phenylephrine: A Systematic Review and Meta-Analysis" (abstract). Annals of Pharmacotherapy 41: 381-390. doi:. PMID 17264159.(published online Jan 2007)
- ^ Kollar C, Schneider H, Waksman J, Krusinska E. (2007). "Meta-analysis of the efficacy of a single dose of phenylephrine 10 mg compared with placebo in adults with acute nasal congestion due to the common cold". Clinical Therapeutics 29: 1057-1070. doi:.
[edit] See also
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