Hexobarbital

The chemical class of barbiturates are one of the oldest sedative-hypnotic agents known, dating back from the introduction of barbital in the early 20th century.{{cite journal | vauthors = Timmermann G, Czeizel AE, Bánhidy F, Acs N | title = A study of the teratogenic and fetotoxic effects of large doses of barbital, hexobarbital and butobarbital used for suicide attempts by pregnant women | journal = Toxicology and Industrial Health | volume = 24 | issue = 1–2 | pages = 109–19 | date = 2008-02-01 | pmid = 18818187 | doi = 10.1177/0748233708089004 | bibcode = 2008ToxIH..24..109T | s2cid = 36948994 }} In Hungary, hexobarbital (and other barbiturates) were 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.{{cite web|title=FDA Pregnancy Categories - CHEMM|url=https://chemm.nlm.nih.gov/pregnancycategories.htm|access-date=2021-02-27|website=chemm.nlm.nih.gov|archive-date=2021-08-27|archive-url=https://web.archive.org/web/20210827203300/https://chemm.nlm.nih.gov/pregnancycategories.htm|url-status=dead}} Some research however, indicate that ingestion of Hexobarbital might cause congenital abnormalities.

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.

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 post-traumatic stress disorder (PTSD){{cite journal|vauthors=Komelkova M, Manukhina E, Downey HF, Sarapultsev A, Cherkasova O, Kotomtsev V, Platkovskiy P, Fedorov S, Sarapultsev P, Tseilikman O, Tseilikman D, Tseilikman V|date=August 2020|title=Hexobarbital Sleep Test for Predicting the Susceptibility or Resistance to Experimental Posttraumatic Stress Disorder|journal=International Journal of Molecular Sciences|volume=21|issue=16|page=5900|doi=10.3390/ijms21165900|pmc=7460591|pmid=32824478|doi-access=free}} or to determine the effect of toxic compounds on sleep time.{{cite journal|vauthors=Bornheim LM, Borys HK, Karler R|date=March 1981|title=Effect of cannabidiol on cytochrome P-450 and hexobarbital sleep time|journal=Biochemical Pharmacology|volume=30|issue=5|pages=503–7|doi=10.1016/0006-2952(81)90636-5|pmid=7225146}}{{cite journal|vauthors=Schnell RC, Prosser TD, Miya TS|date=May 1974|title=Cadmium-induced potentiation of hexobarbital sleep time in rats|journal=Experientia|volume=30|issue=5|pages=528–9|doi=10.1007/BF01926332|pmid=4833683|s2cid=6402325}}

Hexobarbital can be synthesized by reacting cyclohex-1-enyl 2-cyanopropanoate with guanidine and sodium methylate. A hexobarbital sodium intermediate is then formed which can be  methylated with dimethyl sulfate.{{cite book | author = VCH Publishers |url=http://worldcat.org/oclc/50618230|title=Ullman's encyclopedia of industrial chemistry.|date=2002|publisher=Wiley-VCH|oclc=50618230}}

Another pathway for hexobarbital synthesis is reacting ethyl 2-cyano-2-(cyclohex-1-enyl)propanoate with N-methylurea.{{cite patent | country = US | number = 2015376136 | title = Compounds useful as modulators of trpm8 | inventor = Chumakova L, Patron A, Priest C, Karanewsky D, Kimmich R, Clayton B, Jeffrey B, Hammaker R, Chumakov V, Zhao W, Noncovich A, Ung J | assign1 = Senomyx Inc | pubdate = 31 December 2015 | gdate = 12 December 2017 }} This reaction is done in two stages, in the first stage the reactants are added with tert-butylate in tert-butyl alcohol at 20-50 °C. In the second stage hydrogen chloride is added with ethanol and water as solvent.

:File:Hexobarbital synthesis.jpg

:File:Hexobarbital synthesis through ester and urea.png

One of the cytochrome P450 isozymes 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. the concentration of hexobarbital also plays a role in oxygenase and oxidase activity of hepatic microsomal cytochrome P450.{{cite journal | vauthors = Heinemeyer G, Nigam S, Hildebrandt AG | title = Hexobarbital-binding, hydroxylation and hexobarbital-dependent hydrogen peroxide production in hepatic microsomes of guinea pig, rat and rabbit | journal = Naunyn-Schmiedeberg's Archives of Pharmacology | volume = 314 | issue = 2 | pages = 201–10 | date = November 1980 | pmid = 7453835 | doi = 10.1007/BF00504539 | s2cid = 37489777}}

Triacetyl oleandomycin, an inhibitor for isozyme CYP3A4, also inhibits hexobarbital metabolism and biological activity, indicating a close relationship between hexobarbital and cytochrome P450.{{cite book | vauthors = Timbrell JA |title=Principles of biochemical toxicology|date=2009|publisher=Informa Healthcare|isbn=978-0-8493-7302-2|pages=181|oclc=243818515}}

File:Enantiomers of Hexobarbital.jpg

The biological effects of hexobarbital depend primarily on its ability to penetrate the central nervous system.{{cite journal | vauthors = Andrews PR, Mark LC | title = Structural specificity of barbiturates and related drugs | journal = Anesthesiology | volume = 57 | issue = 4 | pages = 314–20 | date = October 1982 | pmid = 6751157 | doi = 10.1097/00000542-198210000-00014 | doi-access = free }} Hexobarbital can potentiate GABA_A 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.{{cite journal | vauthors = Yamakura T, Bertaccini E, Trudell JR, Harris RA | title = Anesthetics and ion channels: molecular models and sites of action | journal = Annual Review of Pharmacology and Toxicology | volume = 41 | issue = 1 | pages = 23–51 | date = 2001-04-01 | pmid = 11264449 | doi = 10.1146/annurev.pharmtox.41.1.23 }} When GABA binds to the GABA_A receptor, the chloride ion channels open such that chloride ions can flow into the neuron. This causes a hyperpolarization in the membrane potential of the neuron, which makes it less likely for the neuron to start an action potential. Therefore, this type of receptor is the major inhibitory neurotransmitter receptor in the mammalian central nervous system.{{cite journal | vauthors = Sigel E, Steinmann ME | title = Structure, function, and modulation of GABA(A) receptors | journal = The Journal of Biological Chemistry | volume = 287 | issue = 48 | pages = 40224–31 | date = November 2012 | pmid = 23038269 | pmc = 3504738 | doi = 10.1074/jbc.R112.386664 | doi-access = free }} 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.{{cite journal | vauthors = Olsen RW, Sapp DM, Bureau MH, Turner DM, Kokka N | title = Allosteric actions of central nervous system depressants including anesthetics on subtypes of the inhibitory gamma-aminobutyric acidA receptor-chloride channel complex | journal = Annals of the New York Academy of Sciences | volume = 625 | pages = 145–54 | date = 1991 | pmid = 1711804 | doi = 10.1111/j.1749-6632.1991.tb33838.x | s2cid = 12448489 }} 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 receptors, neural acetylcholine receptors. And above all, barbiturates inhibit glutamate release by causing an open channel block on P/Q‐type high‐voltage activated calcium channels.{{cite journal | vauthors = Löscher W, Rogawski MA | title = How theories evolved concerning the mechanism of action of barbiturates | journal = Epilepsia | volume = 53 Suppl 8 | issue = s8 | pages = 12–25 | date = December 2012 | pmid = 23205959 | doi = 10.1111/epi.12025 | s2cid = 4675696 | doi-access = free }} All in all, hexobarbital causes an CNS-depressant effect on the brain by inhibiting the glutamate release and potentiating the GABA-effect.

The hepatic metabolism of hexobarbital (HB) can be divided into different pathways all forming different metabolites.{{cite web|title=Hexobarbital|url=https://go.drugbank.com/drugs/DB01355|access-date=2021-03-08|website=go.drugbank.com}} 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, CYP2B1.{{cite journal | vauthors = Takenoshita R, Toki S | title = [New aspects of hexobarbital metabolism: stereoselective metabolism, new metabolic pathway via GSH conjugation, and 3-hydroxyhexobarbital dehydrogenases] | journal = Yakugaku Zasshi | volume = 124 | issue = 12 | pages = 857–71 | date = December 2004 | pmid = 15577260 | doi = 10.1248/yakushi.124.857 | url = https://www.jstage.jst.go.jp/article/yakushi/124/12/124_12_857/_pdf/-char/ja | doi-access = free }} All 3HHB isomers formed can undergo further metabolism via glucuronidation or dehydrogenation.

If 3HHB undergoes a glucuronidation reaction, via UDP-glucuronosyl transferases (UGTs), it is readily excreted. 3HHB can also undergo dehydrogenation, forming a reactive ketone, 3'-oxohexobarbital (3OHB). The biotransformation of 3HHB into 3OHB is via the enzyme 3HHB dehydrogenase (3HBD), a NAD(P)+ linked oxidation.{{cite journal | vauthors = Endo S, Matsunaga T, Matsumoto A, Arai Y, Ohno S, El-Kabbani O, Tajima K, Bunai Y, Yamano S, Hara A, Kitade Y | title = Rabbit 3-hydroxyhexobarbital dehydrogenase is a NADPH-preferring reductase with broad substrate specificity for ketosteroids, prostaglandin D₂, and other endogenous and xenobiotic carbonyl compounds | journal = Biochemical Pharmacology | volume = 86 | issue = 9 | pages = 1366–75 | date = November 2013 | pmid = 23994167 | doi = 10.1016/j.bcp.2013.08.024 }} This enzyme is part of the aldo-keto reductase (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.{{cite journal | vauthors = Furner RL, McCarthy JS, Stitzel RE, Anders MW | title = Stereoselective metabolism of the enantiomers of hexobarbital | journal = The Journal of Pharmacology and Experimental Therapeutics | volume = 169 | issue = 2 | pages = 153–8 | date = October 1969 | pmid = 5824599 | url = https://jpet.aspetjournals.org/content/169/2/153 }}

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.{{cite journal | vauthors = Vermeulen NP, Rietveld CT, Breimer DD | title = Disposition of hexobarbitone in healthy man: kinetics of parent drug and metabolites following oral administration | journal = British Journal of Clinical Pharmacology | volume = 15 | issue = 4 | pages = 459–64 | date = April 1983 | pmid = 6849782 | pmc = 1427803 | doi = 10.1111/j.1365-2125.1983.tb01530.x }}{{cite journal | vauthors = Takenoshita R, Nakamura T, Toki S | title = Hexobarbital metabolism: a new metabolic pathway to produce 1,5-dimethylbarbituric acid and cyclohexenone-glutathione adduct via 3'-oxohexobarbital | journal = Xenobiotica; the Fate of Foreign Compounds in Biological Systems | volume = 23 | issue = 8 | pages = 925–34 | date = August 1993 | pmid = 8284947 | doi = 10.3109/00498259309059419 }} The formation of a 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.

:File:Metabolic pathway of hexobarbital.png

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.{{cite journal | vauthors = Smith DA, Chandler MH, Shedlofsky SI, Wedlund PJ, Blouin RA | title = Age-dependent stereoselective increase in the oral clearance of hexobarbitone isomers caused by rifampicin | journal = British Journal of Clinical Pharmacology | volume = 32 | issue = 6 | pages = 735–9 | date = December 1991 | pmid = 1768567 | pmc = 1368555 | doi = 10.1111/j.1365-2125.1991.tb03982.x }} Excretion is mainly via urine, for the three major metabolites. The cyclohexenone glutathione adduct is excreted in the bile.

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.

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 Agatha Christie's 1937 mystery Cards on the Table, Hexobarbital is used in conjunction with Veronal to induce overdose. It is referred to by Hercule Poirot as both N-methyl-cyclo-hexenyl-methyl-malonyl urea and Evipan.{{cite book| vauthors = Christie A | author-link1 = Agatha Christie |title=Cards on the Table|publisher=William Morrow|year=1937|isbn=978-0-06-207373-0|location=New York|pages=242}}

{{reflist}}{{General anesthetics}}

{{Hypnotics and sedatives}}

{{GABAAR PAMs}}

Category:Barbiturates

Category:General anesthetics

Category:Cyclohexenes