Difference between revisions of "Phenobarbital"

Latest revision as of 15:42, 27 September 2010

Phenobarbital (INN) or phenobarbitone (former BAN) is a barbiturate, first marketed as Luminal by Friedr. Bayer et comp. It is the most widely used anticonvulsant worldwide ^[1] and the oldest still commonly used.^[2] It also has sedative and hypnotic properties but, as with other barbiturates, has been superseded by the benzodiazepines for these indications. The World Health Organization recommends its use as first-line for partial and generalized tonic-clonic seizures (those formerly known as Grand Mal) in developing countries. It is a core medicine in the WHO Model List of Essential Medicines, which is a list of minimum medical needs for a basic health care system.^[3] In more affluent countries, it is no longer recommended as a first or second-line choice anticonvulsant for most seizure types,^[2][4] though it is still commonly used to treat neonatal seizures.

Phenobarbital (and phenobarbital sodium) is manufactured and supplied in various forms: In Tablets of 15, 30, 60 and 100 mg (though not all are available in all countries: for example, in Australia only the 30 mg strength tablets are available); in an oral elixir (commonly 3 mg/mL in strength); and in a form for injection (as phenobarbital sodium - usually 200 mg/mL). The injectable form is used principally to control status epilepticus, while the oral forms are used for prophylactic and maintenance therapy. The dose range for epilepsy is 60–320 mg/day; its very long active half-life means that for some patients, doses do not have to be taken every day, particularly once the dose has been stabilised over a period of several weeks or months and seizures are effectively controlled. It is occasionally still used as a sedative/hypnotic in anxious or agitated patients who may be intolerant of or do not have access to benzodiazepines, neuroleptics and other, newer drugs. For this purpose phenobarbital has a lower dose range - around 30–120 mg/day; however this practice is uncommon in developed countries.^[5]

History

The first barbiturate drug, barbital, was synthesized in 1902 by German chemists Emil Fischer and Joseph von Mering at Bayer. By 1904 several related drugs, including phenobarbital, had been synthesized by Fischer. Phenobarbital was brought to market in 1912 by the drug company Bayer using the brand Luminal. It remained a commonly prescribed sedative and hypnotic until the introduction of benzodiazepines in the 1950s.^[6]

Phenobarbital's soporific, sedative and hypnotic properties were well known in 1912, but nobody knew it was also an effective anticonvulsant. The young doctor Alfred Hauptmann^[7] gave it to his epilepsy patients as a tranquiliser and discovered that their epileptic attacks were susceptible to the drug. Hauptmann performed a careful study of his patients over an extended period. Most of these patients were using the only effective drug then available, bromide, which had terrible side effects and limited efficacy. On phenobarbital, their epilepsy was much improved: The worst patients suffered fewer and lighter seizures and some patients became seizure free. In addition, they improved physically and mentally as bromides were removed from their regime. Patients who had been institutionalised due to the severity of their epilepsy were able to leave and, in some cases, resume employment. Hauptman dismissed concerns that its effectiveness in stalling epileptic attacks could lead to patients suffering a build-up that needed to be "discharged". As he expected, withdrawal of the drug led to an increase in seizure frequency – it was not a cure. The drug was quickly adopted as the first widely effective anticonvulsant, though World War I delayed its introduction in the U.S.^[8]

Phenobarbital was used to treat neonatal jaundice by increasing liver metabolism and thus lowering bilirubin levels. In the 1950s, phototherapy was discovered, and became the standard treatment.^[9]

In 1940, Winthrop Chemical produced sulfathiazole tablets that were contaminated with phenobarbital. This occurred because both tablets were produced side-by-side and equipment could be interchanged. Each antibacterial tablet contained more than twice the required dose of phenobarbital necessary to induce sleep. Hundreds of patients died or were injured as a result. A U.S. Food and Drug Administration investigation was highly critical of Winthrop and the scandal led to the introduction of Good Manufacturing Practice for drugs.^[9]

The drug predates the FDA approval processes and has failed to be formally cleared for use in subsequent years. Guidance was issued in June 2006 of plans to enforce US approval for unapproved drugs.^[10]

Phenobarbital was used for over 25 years as prophylaxis in the treatment of febrile seizures.^[11] Although an effective treatment in preventing recurrent febrile seizures, it had no positive effect on patient outcome or risk of developing epilepsy. The treatment of simple febrile seizures with anticonvulsant prophylaxis is no longer recommended.^[12][13]

Indications

Phenobarbital is indicated in the treatment of all types of seizures except absence seizures.^[4][14] Phenobarbital is no less effective at seizure control than more modern drugs such as phenytoin and carbamazepine. It is, however, significantly less well tolerated.^[15][16]

The first line drugs for treatment of status epilepticus are fast acting benzodiazepines such as diazepam or lorazepam. If these fail then phenytoin may be used, with phenobarbital being an alternative in the U.S. but used only third line in the UK.^[17] Failing that, the only treatment is anaesthesia in intensive care.^[14][18]

Phenobarbital is the first line choice for the treatment of neonatal seizures.^[5][19][20] Concerns that neonatal seizures in themselves could be harmful make most physicians treat them aggressively. There is, however, no reliable evidence to support this approach.^[21]

Other uses

Phenobarbital is often used for the treatment of acute withdrawal from benzodiazepines.^{[citation needed]} Phenobarbital properties can effectively reduce tremors and seizures associated with abrupt withdrawal from benzodiazepines.

Side effects

Sedation and hypnosis are the principal side effects of phenobarbital. Central nervous system effects like dizziness, nystagmus and ataxia are also common. In elderly patients, it may cause excitement and confusion while in children, it may result in paradoxical hyperactivity. Another very rare side effect is amelogenesis imperfecta.^{[citation needed]}

Special precautions

Caution is to be used with children. Of anticonvulsant drugs behavioural disturbances occur most frequently with clonazepam and phenobarbital.^[22]

Contraindications

Acute intermittent porphyria, oversensitivity for barbiturates, prior dependence on barbiturates, severe respiratory insufficiency and hyperkinesia in children.

Mechanism of action

See barbiturates.

Overdose

Phenobarbital causes a "depression" of the body's systems, mainly the central and peripheral nervous systems; thus, the main characteristic of phenobarbital overdose is a "slowing" of bodily functions, including decreased consciousness (even coma), bradycardia, bradypnea, hypothermia, and hypotension (in massive overdoses). Overdose may also lead to pulmonary edema and acute renal failure as a result of shock.

The electroencephalogram of a person with phenobarbital overdose may show a marked decrease in electrical activity, to the point of mimicking brain death. This is due to profound depression of the central nervous system, and is usually reversible.^[23]

Treatment of phenobarbital overdose is supportive, and consists mainly in the maintenance of airway patency (through endotracheal intubation and mechanical ventilation), correction of bradycardia and hypotension (with intravenous fluids and vasopressors, if necessary) and removal of as much drug as possible from the body. Depending on how much time has elapsed since ingestion of the drug, this may be accomplished through gastric lavage (stomach pumping) or use of activated charcoal. Hemodialysis is effective in removing phenobarbital from the body, and may reduce its half-life by up to 90%.^[23] There is no specific antidote for barbiturate poisoning.^[24]

British veterinarian Donald Sinclair, better known as "Siegfried Farnon" in the "All Creatures Great and Small" books of James Herriot committed suicide at the age of 84 by injecting himself with an overdose of phenobarbital. Activist Abbie Hoffman also committed suicide by consuming phenobarbital, combined with alcohol, on April 12, 1989; the residue of around 150 pills was found in his body at autopsy.^[25]

Pharmacokinetics

Phenobarbital has an oral bioavailability of approximately 90%. Peak plasma concentrations are reached 8 to 12 hours after oral administration. It is one of the longest-acting barbiturates available – it remains in the body for a very long time (half-life of 2 to 7 days) and has very low protein binding (20 to 45%). Phenobarbital is metabolized by the liver, mainly through hydroxylation and glucuronidation, and induces many isozymes of the cytochrome P450 system. Cytochrome P450 2B6 (CYP2B6) is specifically induced by Phenobarbital via the CAR/RXR nuclear receptor heterodimer. It is excreted primarily by the kidneys.

Veterinary uses

Phenobarbital is one of the initial drugs of choice to treat epilepsy in dogs, and is the initial drug of choice to treat epilepsy in cats.^[26]

It may also be used to treat seizures in horses when benzodiazepine treatment has failed or is contraindicated.^[27]

References

cs:Fenobarbital de:Phenobarbital es:Fenobarbital eu:Fenobarbital fr:Phénobarbital gl:Fenobarbital it:Fenobarbital hu:Fenobarbitál mk:Фенобарбитон nl:Fenobarbital ja:フェノバルビタール no:Fenobarbital pl:Fenobarbital pt:Fenobarbital ro:Fenobarbital ru:Фенобарбитал fi:Fenobarbitaali sv:Fenobarbital tr:Fenobarbital

1. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
2. ↑ ^{2.0 2.1} "Phenobarbital". Epilepsy Foundation. Retrieved 2006-09-07. 
3. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
4. ↑ ^{4.0 4.1} NICE (2005-10-27). "CG20 Epilepsy in adults and children: NICE guideline". NHS. Retrieved 2006-09-06. 
5. ↑ ^{5.0 5.1} British Medical Association, Royal Pharmaceutical Society of Great Britain, Royal College of Paediatrics and Child Health and Neonatal and Paediatric Pharmacists Group (2006). "4.8.1 Control of epilepsy". British National Formulary for Children. London: BMJ Publishing. pp. 255–6. ISBN 0-85369-676-4. CS1 maint: Multiple names: authors list (link)
6. ↑ Sneader, Walter (2005-06-23). Drug Discovery. John Wiley and Sons. p. 369. ISBN 0-471-89979-8.  |access-date= requires |url= (help)
7. ↑ Ole Daniel Enersen. "Alfred Hauptmann".  Text " accessdate 2006-09-06 " ignored (help)
8. ↑ Scott,, Donald F (1993-02-15). The History of Epileptic Therapy. Taylor & Francis. pp. 59–65. ISBN 1-85070-391-4.  |access-date= requires |url= (help)
9. ↑ ^{9.0 9.1} Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
10. ↑ Michelle Meadows (January–February 2007). "The FDA Takes Action Against Unapproved Drugs". FDA Consumer magazine. Retrieved 2007-09-27. CS1 maint: Date format (link)
11. ↑ John M. Pellock, W. Edwin Dodson, Blaise F. D. Bourgeois (2001-01-01). Pediatric Epilepsy. Demos Medical Publishing. p. 169. ISBN 1-888799-30-7.  |access-date= requires |url= (help)CS1 maint: Multiple names: authors list (link)
12. ↑ Robert Baumann (2005-02-14). "Febrile Seizures". eMedicine. WebMD. Retrieved 2006-09-06. 
13. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
14. ↑ ^{14.0 14.1} British National Formulary 51
15. ↑ Taylor S, Tudur Smith C, Williamson PR, Marson AG (2003). "Phenobarbitone versus phenytoin monotherapy for partial onset seizures and generalized onset tonic-clonic seizures". Cochrane Database Systematic Reviews (2): CD002217. doi:10.1002/14651858.CD002217. PMID 11687150. Retrieved 2006-09-06. CS1 maint: Multiple names: authors list (link)
16. ↑ Tudur Smith C, Marson AG, Williamson PR (2003). "Carbamazepine versus phenobarbitone monotherapy for epilepsy". Cochrane Database of Systematic Reviews (1): CD001904. doi:10.1002/14651858.CD001904. PMID 12535420. Retrieved 2006-09-06. CS1 maint: Multiple names: authors list (link)
17. ↑ British Medical Association, Royal Pharmaceutical Society of Great Britain, Royal College of Paediatrics and Child Health and Neonatal and Paediatric Pharmacists Group (2006). "4.8.2 Drugs used in status epilepticus". British National Formulary for Children. London: BMJ Publishing. p. 269. ISBN 0-85369-676-4. CS1 maint: Multiple names: authors list (link)
18. ↑ Kälviäinen R, Eriksson K, Parviainen I (2005). "Refractory generalised convulsive status epilepticus : a guide to treatment". CNS Drugs. 19 (9): 759–68. doi:10.2165/00023210-200519090-00003. PMID 16142991. CS1 maint: Multiple names: authors list (link)
19. ↑ John M. Pellock, W. Edwin Dodson, Blaise F. D. Bourgeois (2001-01-01). Pediatric Epilepsy. Demos Medical Publishing. p. 152. ISBN 1-888799-30-7.  |access-date= requires |url= (help)CS1 maint: Multiple names: authors list (link)
20. ↑ Raj D Sheth (2005-03-30). "Neonatal Seizures". eMedicine. WebMD. Retrieved 2006-09-06. 
21. ↑ Booth D, Evans DJ (2004). "Anticonvulsants for neonates with seizures". Cochrane Database of Systematic Reviews (3): CD004218. doi:10.1002/14651858.CD004218.pub2. PMID 15495087. Retrieved 2006-09-06. 
22. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
23. ↑ ^{23.0 23.1} Rania Habal (2006-01-27). "Barbiturate Toxicity". eMedicine. WebMD. Retrieved 2006-09-14. 
24. ↑ "Barbiturate intoxication and overdose". MedLine Plus. Retrieved 15 July 2008. 
25. ↑ King, Wayne (April 19, 1989). "Abbie Hoffman Committed Suicide Using Barbiturates, Autopsy Shows". The New York Times. Retrieved 2008-04-09. 
26. ↑ Thomas, WB (2003). "Seizures and narcolepsy". In Dewey, Curtis W. (ed.). A Practical Guide to Canine and Feline Neurology. Ames, Iowa: Iowa State Press. ISBN 0-8138-1249-6. CS1 maint: Extra text: editors list (link)
27. ↑ Editor, Cynthia M. Kahn; associate editor Scott Line (February 8, 2005). Kahn, Cynthia M., Line, Scott, Aiello, Susan E. (ed.), ed. The Merck Veterinary Manual (9th ed.). John Wiley & Sons. ISBN 0-911910-50-6. CS1 maint: Multiple names: authors list (link) CS1 maint: Extra text: authors list (link)