Phenobarbital

Medical uses

Phenobarbital is used in the treatment of all types of seizures except absence seizures. It is no less effective at seizure control than phenytoin, however phenobarbital is not as well tolerated. Phenobarbital may provide a clinical advantage over carbamazepine for treating partial onset seizures. Carbamazepine may provide a clinical advantage over phenobarbital for generalized onset tonic-clonic seizures. Its very long active half-life means for some people doses do not have to be taken every day, particularly once the dose has been stabilized over a period of several weeks or months, and seizures are effectively controlled.

The first-line drugs for treatment of status epilepticus are benzodiazepines, such as lorazepam or diazepam. 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. The World Health Organization gives phenobarbital a first-line recommendation in the developing world and it is commonly used there.

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 used for alcohol detoxification 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 is occasionally prescribed in low doses to aid in the conjugation of bilirubin in people with Crigler-Najjar syndrome (Type II), or in patients with Gilbert syndrome.

Phenobarbital can also be used to relieve cyclic vomiting syndrome symptoms.

Phenobarbital is a commonly used agent in high purity and dosage for lethal injection of "death row" criminals.

In infants suspected of neonatal biliary atresia, phenobarbital is used in preparation for a ^99mTc-IDA hepatobiliary study that differentiates atresia from hepatitis or cholestasis.

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.

Phenobarbital is a cytochrome P450 hepatic enzyme inducer. Drugs that are metabolized by the CYP450 enzyme system will decrease effectiveness because of faster clearance from the system.

Caution is to be used with children. Of anticonvulsant drugs, behavioural disturbances occur most frequently with clonazepam and phenobarbital.

Contraindications

Acute intermittent porphyria, hypersensitivity to any barbiturate, prior dependence on barbiturates, severe respiratory insufficiency and hyperkinesia in children are contraindications for phenobarbital use.

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, 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 mainly consists of 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. Also dying from an overdose in 1996 was actress/model Margaux Hemingway. The Japanese officers aboard the German submarine U-234 killed themselves with phenobarbital while the German crew members were on their way to the US to surrender (but before Japan had surrendered).

39 members of the Heaven's Gate UFO religious group committed mass suicide in March 1997 by drinking a lethal dose of phenobarbital and vodka "and then lay down to die" hoping to enter an alien spacecraft.

Mechanism of action

The net result is increased flux of chloride ions into the neuron, decreasing excitability. Direct blockade of excitatory glutamate signaling is also believed to contribute to the hypnotic/anticonvulsant effect that is observed with the barbiturates.

Pharmacokinetics

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

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.

It is also used to treat feline hyperesthesia syndrome in cats when antiobsessional therapies prove ineffective.

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

History

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 it was not yet known to be 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. Hauptmann 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 1939 a German family asked Adolf Hitler to have their disabled son killed, the five-month-old boy was given a lethal dose of Luminal after Hitler sent his own doctor to examine him. A few days later 15 psychiatrists were summoned to Hitler's Chancellery and directed to commence a clandestine euthanasia program. In 1940, at a clinic in Ansbach, Germany, around 50 intellectually disabled children were injected with Luminal and killed that way. A plaque was erected in their memory in 1988 in the local hospital at Feuchtwanger Strasse 38, although a newer plaque does not mention that patients were killed using barbiturates on site. Luminal was used in the Nazi "children's euthanasia" program until at least 1943. see also:

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.

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.

Names

Phenobarbital is the INN and phenobarbitone is the BAN.

Synthesis

Barbiturate drugs are obtained via condensation reactions between a derivative of diethyl malonate and urea in the presence of a strong base. The synthesis of Phenobarbital uses this common approach as well but differs in the way in which this malonate derivative is obtained. The reason for this difference is due to the fact that aryl halides do not typically act as nucleophiles in Malonic ester synthesis in the same way as aliphatic organosulfates or halocarbons do. To overcome this lack of chemical reactivity two dominant synthetic approaches using benzyl cyanide as a starting material have been developed:

The first of these methods consists of a Pinner reaction of benzyl cyanide, giving phenylacetic acid ethyl ester. Subsequently, this ester undergoes cross Claisen condensation using diethyl oxalate, giving diethyl ester of phenyloxobutandioic acid. Upon heating this intermediate easily loses carbon monoxide, yielding diethyl phenylmalonate. Malonic ester synthesis using ethyl bromide leads to the formation of α-phenyl-α-ethylmalonic ester. Finally a condensation reaction with urea gives phenobarbital.

The second approach utilizes diethyl carbonate in the presence of a strong base to give α-phenylcyanoacetic ester. Alkylation of this ester using ethyl bromide proceeds via a nitrile anion intermediate to give the α-phenyl-α-ethylcyanoacetic ester. This product is then further converted into the 4-iminoderivative upon condensation with urea. Finally acidic hydrolysis of the resulting product gives phenobarbital.

Regulation

The level of regulation includes Schedule IV Non-Narcotic (depressant) (ACSCN 2285) in the United States under the Controlled Substances Act 1970—but along with a few other barbiturates and at least one benzodiazepine, and codeine, dionine, or dihydrocodeine at low concentrations, it also has exempt prescription and had at least one exempt OTC combination drug now more tightly regulated for its ephedrine content. The phenobarbitone/phenobarbital exists in subtherapeutic doses which add up to an effective dose to counter the overstimulation and possible seizures from a deliberate overdose in ephedrine tablets for asthma, which are now regulated at the federal and state level as: a restricted OTC medicine and/or watched precursor, uncontrolled but watched/restricted prescription drug & watched precursor, a Schedule II, III, IV, or V prescription-only controlled substance & watched precursor, or a Schedule V (which also has possible regulations at the county/parish, town, city, or district as well aside from the fact that the pharmacist can also choose not to sell it, and photo ID and signing a register is required) exempt Non-Narcotic restricted/watched OTC medicine.