Phenylephrine

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Phenylephrine
Systematic (IUPAC) name
(R)-3-[-1-hydroxy-2-(methylamino)ethyl]phenol
Identifiers
CAS number 59-42-7
61-76-7 (hydrochloride)
ATC code C01CA06 R01AA04, R01AB01, R01BA03, S01FB01, S01GA05
PubChem 6041
DrugBank APRD00365
ChemSpider 5818
Chemical data
Formula C9H13NO2 
Mol. mass 167.205 g/mol
SMILES eMolecules & PubChem
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.

B2(AU) C(US)
Legal status

GSL(UK) OTC(US)
Routes Oral, intranasal, ophthalmic, intravenous, intramuscular

Phenylephrine or Neo-Synephrine is an α1-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 a placebo.[1]

Contents

[edit] Uses

[edit] Decongestant

Phenylephrine is used as a decongestant sold as an oral medicine, as a nasal spray, or as eye drops. Phenylephrine is now the most common over-the-counter (OTC) decongestant in the United States; 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 [2], an enzyme that is present in the gastrointestinal tract and in the liver. Therefore, compared to orally-taken pseudoephedrine, it has a reduced and variable bioavailability of only up to 38 percent,[3][4]. Because phenylephrine is a direct selective α-adrenergic receptor agonist, it does not cause the release of endogenous noradrenaline, as pseudoephedrine does. Therefore, phenylephrine is less likely to cause side-effects such as central nervous system stimulation, insomnia, anxiety, irritability, and restlessness.

Some popular cold remedies containing phenylephrine include:

Australia

Canada

Mexico

  • Histiacil Flu

New Zealand

  • Panadol Cold & Flu Max + Decongestant
  • Codral Cold & Flu (New Formula)

South Africa

  • Corenza C
  • Demazin
  • Flutex

South Korea

  • Happynose Tab

United Kingdom

United States

India

As a nasal spray, phenylephrine is available in 1 percent and 0.5 percent concentrations. It causes some rebound congestion effects, similar to oxymetazoline, although possibly to a lesser degree.

[edit] Mydriatic

Phenylephrine is used as an eye drop to dilate the pupil in order 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 to 3 hours.

[edit] Side-effects

The primary side-effect of phenylephrine is hypertension. Patients with congestion and hypertension are typically advised to avoid products containing phenylephrine.Prostatic hyperplasia can also be symptomatically worsened by use, and chronic use can lead to rebound hyperemia.[6]Patients with a history of epilepsy and on anticonvulsant medication should not take this substance. The drug interaction might produce seizures.Some patients have been shown to have an upset stomach, severe abdominal cramping, and vomiting issues connected to taking this drug.

[edit] Substitute for pseudoephedrine

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 in order 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.[7] Phenylephrine has been off patent for some time, and 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.[8] 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.[9][10] A subsequent meta-analysis by the same researchers concluded that there is insufficient evidence for its effectiveness,[11] though another meta-analysis published shortly thereafter by researchers from GlaxoSmithKline found the standard 10mg dose to be significantly more effective than a placebo.[12]

The matter is still under debate; however the Food and Drug Administration has stood by its 1976 approval.[7]

[edit] References

  1. ^ a b University of Florida (press release) (2006-07-19). "UF researchers question effectiveness of decongestant". http://news.ufl.edu/2006/07/19/decongensant/. 
  2. ^ Drug Properties
  3. ^ Pharmacogenetics and Pharmacogenomics Knowledge Base entry on phenylephrine
  4. ^ NZ Medicines and Medical Devices Safety Authority recommendation on phenylephrine (November 2004)
  5. ^ "Morepen acquires Lemolate The fifth largest brand in cough and cold Segment." Morepen Laboratories. 25 June 2002. Morepen Laboratories. 06 July 2009 <http://www.morepen.com/lemolate.htm>.
  6. ^ Shen, Howard (2008). Illustrated Pharmacology Memory Cards: PharMnemonics. Minireview. p. 3. ISBN 1-59541-101-1. 
  7. ^ a b Hilenmeyer, K. (30 January 2007). "All stuffed up". Southwest Florida Herald-Tribune. http://www.heraldtribune.com/apps/pbcs.dll/article?AID=/FP/20070130/HEALTHMATTERS/70129001/1025/NEWS06. 
  8. ^ Heldeles, L. and Hatton, R. (2006). "Oral phenylephrine: An ineffective replacement for pseudoephedrine?". Journal of Allergy and Clinical Immunology 118 (1): 279–280. doi:10.1016/j.jaci.2006.03.002. PMID 16815167. 
  9. ^ >Eccles R (May 2006). "Phenylephrine an ineffective replacement for pseudoephedrine in response to the methamphetamine problem in the USA". BMJ. http://www.bmj.com/cgi/eletters/332/7538/382-b. 
    Rapid Response to
    Tanne JH (February 2006). "Methamphetamine epidemic hits middle America". BMJ 332 (7538): 382. doi:10.1136/bmj.332.7538.382-b. PMID 16484253. 
  10. ^ 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:10.1111/j.1365-2125.2006.02833.x. PMID 17116124. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2000711.  (January 2007)
  11. ^ 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:10.1345/aph.1H679. PMID 17264159. http://www.theannals.com/cgi/content/abstract/aph.1H679v1. (published online Jan 2007)
  12. ^ 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:10.1016/j.clinthera.2007.05.021. 

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