Hexobarbital or hexobarbitone, sold both in acid and sodium salt forms as Citopan, Evipan, and Tobinal, is a
barbiturate derivative having
hypnotic and
sedative effects. It was used in the 1940s and 1950s as an agent for inducing
anesthesia for surgery, as well as a rapid-acting, short-lasting hypnotic for general use, and has a relatively fast onset of effects and short duration of action. It was also used to murder women prisoners at
Ravensbrück concentration camp. Modern barbiturates (such as
Thiopental) have largely supplanted the use of hexobarbital as an anesthetic, as they allow for better control of the depth of anesthesia. Hexobarbital is still used in some scientific research.
History
The chemical class of
barbiturates are one of the oldest sedative-hypnotic agents known, dating back from the introduction of
barbital
Barbital (or barbitone), marketed under the brand names Veronal for the pure acid and Medinal for the sodium salt, was the first commercially available barbiturate. It was used as a sleeping aid (hypnotic) from 1903 until the mid-1950s. The chemic ...
in the early 20th century.
In Eastern Europe, hexobarbital (and other barbiturates) have been regularly used as drugs by pregnant women attempting suicide.
Hexobarbital was long thought to have potentially teratogenic and fetotoxic effects. The
FDA has classified them as Pregnancy Category D or C. Some research however, indicate that ingestion of Hexobarbital might cause congenital abnormalitie.
During World War II,
Herta Oberheuser was a
Nazi physician and convicted
war criminal, investigating the effects of hexobarbital. The experiments were mostly performed on woman prisoners in the
Ravensbrück concentration camp.
Application in research
Hexobarbital is used as the
narcotic in the Hexobarbital Sleep Test (HST). HST identifies rodents with high or low intensity of microsomal oxidation, so fast (FM) or slow metabolizers (SM). The sleep test is for example used to predict the susceptibility and resistance to
PTSD
Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threats on ...
or to determine the effect of toxic compounds on sleep time.
Synthesis
Hexobarbital can be synthesised by reacting methyl (cyclohexen-1-yl) cyanoacetate with
guanidine
Guanidine is the compound with the formula HNC(NH2)2. It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine predominantly in patients experie ...
and
sodium methylate. A hexobarbital sodium precursor is then formed which can be methylated with
dimethyl sulphate.
Another pathway for hexobarbital synthesis is reacting methylcyclohexenylcyanoacetic acid ethyl ester with N-methylurea. This reaction is done in two stages, in the first stage the reactants are added with tert-butylate in
tert-butyl alcohol
''tert''-Butyl alcohol is the simplest tertiary alcohol, with a formula of (CH3)3COH (sometimes represented as ''t''-BuOH). Its isomers are 1-butanol, isobutanol, and butan-2-ol. ''tert''-Butyl alcohol is a colorless solid, which melts near ...
at 20-50 °C. In the second stage
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
is added with
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
as
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
.
Reactivity
One of the
cytochrome
Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bi ...
P450
isozymes In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. dif ...
is coded by the gene CYP2B1, where hexobarbital is the substrate. Hexobarbital and the isozyme can form an enzyme-substrate-complex through a
hydroxylation reaction, which is involved in the metabolism of
xenobiotics
A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...
. the concentration of hexobarbital also plays a role in oxygenase and oxidase activity of hepatic microsomal cytochrome P450.
Triacetyl oleandomycin, an inhibitor for isozyme CYP3A4, also inhibits hexobarbital metabolism and biological activity, indicating a close relationship between hexobarbital and cytochrome P450.
Toxicity
Mechanism of actions
The biological effects of hexobarbital depend primarily on its ability to penetrate the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. Hexobarbital can potentiate
GABAA receptors, like all
barbiturates. It has been found over the years that the S(+)
enantiomer of hexobarbital potentiates GABA
A receptors more effectively than its R(-) enantiomer.
When GABA binds to the GABA
A receptor, the chloride ion channels open such that chloride ions can flow into the
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
. This causes a
hyperpolarization in the
membrane potential of the neuron, which makes it less likely for the neuron to start an
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
. Therefore, this type of receptor is the major inhibitory neurotransmitter receptor in the mammalian central nervous system. As a GABA
A receptor potentiator, hexobarbital binds to the barbiturate binding site localized in the chloride ion channel, thereby increasing the binding of GABA and benzodiazepines to their respective binding site, allosterically. Moreover, hexobarbital causes the chloride ion channel opening to their longest open state of 9 milli seconds, thereby causing the postsynaptic inhibitory effect to be extended.
In contrast to GABA,
glutamate is the major excitatory neurotransmitter in the mammalian brain. In addition to the inhibitory effect, hexobarbital blocks, like all barbiturates,
AMPA receptors,
kainate receptor
Kainate receptors, or kainic acid receptors (KARs), are ionotropic receptors that respond to the neurotransmitter glutamate. They were first identified as a distinct receptor type through their selective activation by the agonist kainate, a dru ...
s, neural
acetylcholine receptor
An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.
Classification
Like other transmembrane receptors, acetylcholine receptors are classified according ...
s. And above all, barbiturates inhibit glutamate release by causing an open channel block on P/Q‐type high‐voltage activated
calcium channels. All in all, hexobarbital causes an CNS-depressant effect on the brain by inhibiting the glutamate release and potentiating the GABA-effect.
Metabolism
The hepatic metabolism of hexobarbital (HB) can be divided into different pathways all forming different metabolites.
The S(+) enantiomer of HB preferentially metabolizes into β-3'-hydroxyhexobarbital and the R(-) enantiomer preferentially metabolizes into α-3'-hydroxyhexobarbital, the reaction thus is stereoselective. Both enantiomers, however, form both α- and β-isomers. In total four enantiomers for 3'-hydroxyhexobarbital (3HHB) can be metabolized. This reaction is catalyzed by a
cytochrome P450
Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...
, CYP2B1.
All 3HHB isomers formed can undergo further metabolism via
glucuronidation
Glucuronidation is often involved in drug metabolism of substances such as drugs, pollutants, bilirubin, androgens, estrogens, mineralocorticoids, glucocorticoids, fatty acid derivatives, retinoids, and bile acids. These linkages involve gl ...
or
dehydrogenation
In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At ...
.
If 3HHB undergoes a
glucuronidation
Glucuronidation is often involved in drug metabolism of substances such as drugs, pollutants, bilirubin, androgens, estrogens, mineralocorticoids, glucocorticoids, fatty acid derivatives, retinoids, and bile acids. These linkages involve gl ...
reaction, via UDP-glucuronosyl
transferase
A transferase is any one of a class of enzymes that catalyse the transfer of specific functional groups (e.g. a methyl or glycosyl group) from one molecule (called the donor) to another (called the acceptor). They are involved in hundreds of ...
s (UGTs), it is readily excreted. 3HHB can also undergo
dehydrogenation
In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At ...
, forming a reactive
ketone, 3'-oxohexobarbital (3OHB). The
biotransformation Biotransformation is the biochemical modification of one chemical compound or a mixture of chemical compounds. Biotransformations can be conducted with whole cells, their lysates, or purified enzymes. Increasingly, biotransformations are effected w ...
of 3HHB into 3OHB is via the enzyme 3HHB dehydrogenase (3HBD), a NAD(P)+ linked oxidation. This enzyme is part of the
aldo-keto reductase
The aldo-keto reductase family is a family of proteins that are subdivided into 16 categories; these include a number of related monomeric NADPH-dependent oxidoreductases, such as aldehyde reductase, aldose reductase, prostaglandin F synthase, xy ...
(AKR) superfamily. In humans, 3HBD has a high preference for NAD
+.
These reactions are also stereospecific, the R(-) conformation preferentially forms 3OHB as 3HBD has the highest activity for this enantiomer in both alpha and beta form.
New evidence proved the further metabolism of 3OHB into 1,5-dimethylbarbituric acid and a cyclohexenone glutathione adduct.
This biotransformation step takes place via an epoxide-diol mechanism.
The formation of an reactive epoxide, leads to the formation of the compounds mentioned.
Experiments in man indicated the major metabolites to be 3HHB, 3OHB and 1,5-dimethylbarbituric acid.
Health effects in man
Excretion
The plasma half-life of HB in man is estimated at 222±54 min.
The clearance of HB differs between the two enantiomers and the age of the human subject. The clearance of the R(-) enantiomer is almost 10-fold greater than the clearance of the S(+) enantiomer. Clearance on average in elderly people, compared to young subjects, is slower. Excretion is mainly via urine, for the three major metabolites.
The cyclohexenone glutathione adduct is excreted in the bile.
Symptoms
An intoxication in man with hexobarbital can result in sluggishness, incoordination, difficulty in thinking, slowness of speech, faulty judgment, drowsiness or coma, shallow breathing and staggering. In some severe cases coma and death can be the result of an overdose.
Effects on animals
The following table presents the studies about the effects of hexobarbital on animals, which are done in the 1900s. Most of these studies showed that hexobarbital has short-term toxicity effects and that it can induce hypnotic effects in mice, rabbits and frogs.
In popular culture
In
Agatha Christie's 1937 mystery ''
Cards on the Table
''Cards on the Table'' is a detective fiction novel by the English author Agatha Christie, first published in the UK by the Collins Crime Club on 2 November 1936 and in the US by Dodd, Mead and Company the following year. The UK edition retail ...
'', Hexobarbital is used in conjunction with
Veronal
Barbital (or barbitone), marketed under the brand names Veronal for the pure acid and Medinal for the sodium salt, was the first commercially available barbiturate. It was used as a sleeping aid (hypnotic) from 1903 until the mid-1950s. The chemic ...
to induce overdose. It is referred to by
Hercule Poirot
Hercule Poirot (, ) is a fictional Belgian detective created by British writer Agatha Christie. Poirot is one of Christie's most famous and long-running characters, appearing in 33 novels, two plays ('' Black Coffee'' and ''Alibi''), and more ...
as both N-methyl-cyclo-hexenyl-methyl-malonyl urea and Evipan.
References
{{GABAAR PAMs
Barbiturates
General anesthetics
GABAA receptor positive allosteric modulators
Cyclohexenes