|Systematic (IUPAC) name|
|Pregnancy cat.||D (US)|
|Legal status||Schedule IV (US) Class B (UK)|
|Routes||Oral, rectal, parenteral (intramuscular and intravenous)|
|Protein binding||20 to 45%|
|Metabolism||Hepatic (mostly CYP2C19)|
|Half-life||53 to 118 hours|
|Excretion||Renal and fecal|
|ATC code||N05 N03|
|Mol. mass||232.235 g/mol|
| (what is this?)
Phenobarbital (INN) is a barbiturate and the most widely used anticonvulsant worldwide, and the oldest still commonly used. It also has sedative and hypnotic properties, but as with other barbiturates, it has been superseded by the benzodiazepines for these indications.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. In more affluent countries, it is no longer recommended as a first- or second-line choice anticonvulsant for most seizure types, 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 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 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, but this practice is uncommon in developed countries.
The first barbiturate drug, barbital, was synthesized in 1902 by German chemists Emil Fischer and Joseph von Mering and was first marketed as Veronal by Friedr. Bayer et comp. By 1904, several related drugs, including phenobarbital, had been synthesized by Fischer. Phenobarbital was brought to market in 1912 by the drug company Bayer as the brand Luminal. It remained a commonly prescribed sedative and hypnotic until the introduction of benzodiazepines in the 1960s.
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 gave it to his epilepsy patients as a tranquilizer and discovered their seizures 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 regimen. 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 seizures 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.
In 1940, at a clinic in Ansbach, Germany, around 50 intellectually disabled children were injected with the drug Luminal and killed that way. A plaque was erected in their memory in 1988 in the local hospital at Feuchtwanger Strasse 38.
Phenobarbital was used for over 25 years as prophylaxis in the treatment of febrile seizures. 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.
Phenobarbital is indicated in the treatment of all types of seizures except absence seizures. It is no less effective at seizure control than more modern drugs such as phenytoin and carbamazepine. It is, however, significantly less well tolerated.
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 US, but used only third-line in the UK. Failing that, the only treatment is anaesthesia in intensive care.
Phenobarbital is the first-line choice for the treatment of neonatal seizures. Concerns that neonatal seizures in themselves could be harmful make most physicians treat them aggressively. No reliable evidence, though, supports this approach.
Phenobarbital is sometimes indicated for alcohol and benzodiazepine detoxification for its sedative and anticonvulsant properties. The benzodiazepines chlordiazepoxide (Librium) and oxazepam (Serax) have largely replaced phenobarbital for detoxification.
Other uses 
Phenobarbital properties can effectively reduce tremors and seizures associated with abrupt withdrawal from benzodiazepines. Phenobarbital is a cytochrome P450 inducer, and is used to reduce the toxicity of some drugs.
Phenobarbital can also be used to relieve Cyclic Vomiting Syndrome symptoms.
In infants suspected of neonatal biliary atresia, phenobarbital is used in preparation for a 99mTc-IDA hepatobiliary study that differentiates atresia from hepatitis or chondrostasis.
Phenobarbital is used as a secondary agent to treat newborns with neonatal abstinence syndrome, a condition of withdrawal symptoms from exposure to opioid drugs in utero.
Side effects 
Sedation and hypnosis are the principal side effects (occasionally, they are also the intended effects) of phenobarbital. Central nervous system effects, such as 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.
Special precautions 
Acute intermittent porphyria, oversensitivity for barbiturates, prior dependence on barbiturates, severe respiratory insufficiency and hyperkinesia in children are contraindications for phenobarbital use.
Mechanism of action 
GABAA receptors are the primary target for barbiturates in the central nervous system. As is the case for other clinically important barbiturates, phenobarbital prolongs and potentiates the action of GABA on GABAA receptors and at higher concentrations directly activates the receptors. In contrast to anesthetic barbitruates such as pentobarbital, phenobarbital is minimally sedating at effective anticonvulsant doses. Possible explanations for the reduced sedative effect of phenobarbital include more regionally restricted action; partial agonist activity; reduced propensity to directly activate GABAA receptors (possibly including extrasynaptic receptors containing δ subunits); and reduced activity at other ion channel targets, including voltage-gated calcium channels. Although the precise sites where barbiturates interact with GABAA receptors has not been defined, the second and third transmembrane domains of the β subunit appear to be critical; binding may involve a pocket formed by β-subunit methionine 286 as well as α-subunit methionine 236. In addition to effects on GABAA receptors, barbiturates block AMPA receptors, and they inhibit glutamate release through an effect on P/Q-type high-voltage activated calcium channels. The combination of these various actions likely accounts for their diverse clinical activities.
|Poisoning by barbiturates|
|Classification and external resources|
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, and can result in death.
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.
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%. No specific antidote for barbiturate poisoning is available.
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.
Phenobarbital has an oral bioavailability of about 90%. Peak plasma concentrations are reached eight 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 two to seven 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 
It is also used to treat feline hyperesthesia syndrome in cats when antiobsessional therapies prove ineffective. 
Phenobarbital (5-ethyl-5-phenylbarbituric acid or 5-ethyl-5-phenylhexahydropyrimindin-2,4,6-trione) has been synthesized in several different ways.
- Bayer & Co., DE 247952 (1911).
- M.T. Inman, W.P. Bitler, U.S. Patent 2,358,072 (1944).
- Chamberlain, J. S.; Chap, J. J.; Doyle, J. E.; Spaulding, L. B. (1935). Journal of the American Chemical Society 57 (2): 352. doi:10.1021/ja01305a036.
- Pinhey, J (1980). "The α-arylation of derivatives of malonic acid with aryllead triacetates. New syntheses of ibuprofen and phenobarbital". Tetrahedron Letters 21: 965. doi:10.1016/S0040-4039(00)77752-5.
There is no major difference between them.
The first method consists of ethanolysis of benzyl cyanide in the presence of acid, giving phenylacetic acid ethyl ester, the methylene group of which undergoes acylation using diethyloxalate, giving diethyl ester of phenyloxobutandioic acid, which upon heating easily loses carbon oxide and turns into phenylmalonic ester. Alkylation of the obtained product using ethyl bromide in the presence of sodium ethoxide leads to the formation of α-phenyl-α- ethylmalonic ester, the condensation of which with urea gives phenobarbital.
Another method of phenobarbital synthesis starts with condensation of benzyl cyanide with diethylcarbonate in the presence of sodium ethoxide to give α-phenylcyanoacetic ester. Alkylation of the ester using ethyl bromide gives α-phenyl-α-ethylcyanoacetic ester, which is further converted into the 4-iminoderivative upon treatment with urea. Acidic hydrolysis of the resulting product gives phenobarbital.
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- Taylor S, Tudur Smith C, Williamson PR, Marson AG (2003). "Phenobarbitone versus phenytoin monotherapy for partial onset seizures and generalized onset tonic-clonic seizures". In Taylor, Stephen. Cochrane Database Systematic Reviews (2): CD002217. doi:10.1002/14651858.CD002217. PMID 11687150. Retrieved 2006-09-06.
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