Phenobarbital
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Mechanism of action
Phenobarbital acts on GABAA receptors, increasing synaptic inhibition. This has the effect of elevating seizure threshold and reducing the spread of seizure activity from a seizure focus. Phenobarbital may also inhibit calcium channels, resulting in a decrease in excitatory transmitter release. The sedative-hypnotic effects of phenobarbital are likely the result of its effect on the polysynaptic midbrain reticular formation, which controls CNS arousal.
/Phenobarbital/ produces anticonvulsant effects in subhypnotic doses. The drug lowers serum bilirubin concentrations in neonates and patients with congenital nonhemolytic unconjugated hyperbilirubinemia and patients with chronic intrahepatic cholestasis, presumably by induction of glucuronyl transferase, the enzyme which conjugates bilirubin.
The exact mechanism(s) by which barbiturates exert their effect on the CNS, has not been fully elucidated. However, it is believed that such effects are related, at least partially, to the drugs' ability to enhance the activity of gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the CNS, by altering inhibitory synaptic transmissions that are mediated by GABAA receptors. /Barbiturates General Statement/
Although the drugs act throughout the CNS, a site of particular sensitivity is the polysynaptic midbrain reticular formation which is concerned with the arousal mechanism. Barbiturates induce an imbalance in central inhibitory and facilitatory mechanisms influencing the cerebral cortex and the reticular formation. The significance of the effect of barbiturates on neurotransmitters is unclear. It appears that the drugs decrease the excitability of both presynaptic and postsynaptic membranes. It has not been determined which of the various actions of barbiturates at cellular and synaptic levels are responsible for their sedative and hypnotic effects. /Barbiturates General Statement/
Relatively low doses of the barbiturates depress the sensory cortex, decrease motor activity, and produce sedation and drowsiness. In some patients, however, drowsiness may be preceded by a period of transient elation, confusion, euphoria, or excitement, especially after subhypnotic doses of aprobarbital, pentobarbital, or secobarbital. /Barbiturates General Statement/
For more Mechanism of Action (Complete) data for Phenobarbital (12 total), please visit the HSDB record page.
Pharmacodynamics
Phenobarbital, the longest-acting barbiturate, is used for its anticonvulsant and sedative-hypnotic properties in the management of all seizure disorders except absence (petit mal).
Pharmacokinetics
Half-life
53 to 118 hours (mean 79 hours)
The plasma half-life for phenobarbital in adults ranges between 53 and 118 hours with an mean of 79 hours.
The plasma half-life for phenobarbital in children and newborns (less than 48 hours old) ranges between 60 to 180 hours with a mean of 110 hours.
Absorption
Absorbed in varying degrees following oral, rectal or parenteral administration. The salts are more rapidly absorbed than are the acids. The rate of absorption is increased if the sodium salt is ingested as a dilute solution or taken on an empty stomach.
About 70-90% of an oral dose of phenobarbital is absorbed slowly from the GI tract. Following rectal administration of phenobarbital sodium, the drug is readily absorbed from the colon. Following oral administration of phenobarbital, peak blood concentrations are reached in 8-12 hours and peak brain concentrations in 10-15 hours. ... When phenobarbital sodium is administered IV, the onset of action usually occurs within 5 minutes and maximum effects are achieved within 30 minutes. IM or subcutaneous administration of phenobarbital sodium results in a slightly slower onset of action. The duration of action of parenterally administered phenobarbital sodium is usually 4-6 hours.
Barbiturates are absorbed in varying degrees following oral, rectal, or im administration. The sodium salts are more rapidly absorbed by all routes of administration than are the acids. The rate of oral absorption is increased when the sodium salt is ingested as a dilute solution or taken on an empty stomach. Alcohol also enhances the rate of absorption, possibly by increasing blood flow through the gastric mucosa. /Barbiturates General Statement/
Following oral or rectal administration, the onset of action varies from 10-30 minutes for amobarbital, aprobarbital, butabarbital, pentobarbital, and secobarbital and from 20-60 minutes for metharbital, mephobarbital, and phenobarbital. IM administration results in a slightly faster onset of action. Following iv administration of the sodium salts of amobarbital, pentobarbital, phenobarbital, or secobarbital, the onset of action ranges from almost immediately for methohexital, pentobarbital, and thiopental to 5 minutes for phenobarbital. Maximum effects of thiopental or pentobarbital are achieved within about 1 minute while as much as 30 minutes may be required with administration of phenobarbital. /Barbiturates General Statement/
The duration of sedative effects of all the barbiturates is usually 3-6 hours following iv administration and 6-8 hours when the drugs are administered by other routes. There appears to be very little difference in duration of hypnotic action among barbiturates used orally as hypnotics. Therefore, most authorities now believe that barbiturates should be classified according to their intended pharmacologic action (ie, sedative-hypnotic barbiturates and anesthetic barbiturates [methohexital, thiamylal (no longer commercially available in the US), thiopental]), rather than as long-acting (mephobarbital, metharbital, and phenobarbital), intermediate-acting (amobarbital and butabarbital), short-acting (aprobarbital, pentobarbital, and secobarbital), and ultrashort-acting (methohexital, thiopental). /Barbiturates General Statement/
For more Absorption, Distribution and Excretion (Complete) data for Phenobarbital (15 total), please visit the HSDB record page.
Metabolism
Hepatic (mostly via CYP2C19).
Phenobarbital is hydroxylated by the liver to form p-hydroxyphenobarbital, an inactive metabolite. Phenobarbital is a potent inducer of the enzymes involved in the metabolism of other drugs, but there is no conclusive evidence that phenobarbital accelerates its own metabolism.
The inactive metabolites of the barbiturates are excreted as conjugates of glucuronic acid.
Biotransformation occurs primarily by the hepatic microsomal enzyme system. /Barbiturates/
Metabolites of phenobarbital sodium produced in rats and guinea pigs are 5-(3,4-dihydroxy-1,5-cyclohexadien-1-yl)-5-ethylbarbituric acid; 5-(1-hydroxyethyl)-5-phenylbarbituric acid; 5-(3,4-dihydroxyphenyl)-5-ethylbarbituric acid; and 5-(4-hydroxyphenyl)-5-ethylbarbituric acid ...
For more Metabolism/Metabolites (Complete) data for Phenobarbital (6 total), please visit the HSDB record page.
Protein binding
20 to 45%
External links
Fact-sheets from PsychonautWiki. Harm-reduction reference only — not medical advice.