Bretazenil

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Bretazenil
Bretazenil.svg
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Pharmacokinetic data
Elimination half-life 2.5 hours
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Chemical and physical data
Formula C19H20BrN3O3
Molar mass 418.291 g·mol−1
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Bretazenil (Ro16-6028) is an imidazopyrrolobenzodiazepine [1] anxiolytic drug which is derived from the benzodiazepine family, and was invented in 1988. It is most closely related in structure to the GABA antagonist flumazenil, although its effects are somewhat different. It is classified as a high-potency benzodiazepine due to its high affinity binding to benzodiazepine binding sites [2] [3] where it acts as a partial agonist. [4] Its profile as a partial agonist and preclinical trial data suggests that it may have a reduced adverse effect profile. [5] In particular bretazenil has been proposed to cause a less strong development of tolerance and withdrawal syndrome. [6] Bretazenil differs from traditional 1,4-benzodiazepines by being a partial agonist and because it binds to α1, α2, α3, α4, α5 and α6 subunit containing GABAA receptor benzodiazepine receptor complexes. 1,4-benzodiazepines bind only to α1, α2, α3 and α5 GABAA benzodiazepine receptor complexes. [1]

Contents

History

Bretazenil was originally developed as an anti-anxiety drug and has been studied for its use as an anticonvulsant [7] [8] but has never commercialised. It is a partial agonist for GABAA receptors in the brain. David Nutt from the University of Bristol has suggested bretazenil as a possible base from which to make a better social drug, as it displays several of the positive effects of alcohol intoxication such as relaxation and sociability, but without the bad effects such as aggression, amnesia, nausea, loss of coordination, liver disease and brain damage. The effects of bretazenil can also be quickly reversed by the action of flumazenil, which is used as an antidote to benzodiazepine overdose, [9] in contrast to alcohol for which there is no effective and reliable antidote.

Traditional benzodiazepines are associated with side effects such as drowsiness, physical dependence and abuse potential. It was hoped that bretazenil and other partial agonists would be an improvement on traditional benzodiazepines which are full agonists due to preclinical evidence that their side effect profile was less than that of full agonist benzodiazepines. For a variety of reasons however, bretazenil and other partial agonists such as pazinaclone and abecarnil were not clinically successful. However, research continues into other compounds with partial agonist and compounds which are selective for certain GABAA benzodiazepine receptor subtypes. [10]

Tolerance and dependence

In a study in rats, cross-tolerance between the benzodiazepine drug chlordiazepoxide and bretazenil has been demonstrated. [11] In a primate study bretazenil was found to be able to replace the full agonist diazepam in diazepam dependent primates without precipitating withdrawal effects, demonstrating cross tolerance between bretazenil and benzodiazepine agonists, whereas other partial agonists precipitated a withdrawal syndrome. The differences are likely due to differences in instrinsic properties between different benzodiazepine partial agonists. [12] Cross-tolerance has also been shown between bretazenil and full agonist benzodiazepines in rats. [13] In rats tolerance is slower to develop to the anticonvulsant effects compared to the benzodiazepine site full agonist diazepam. However, tolerance developed to the anticonvulsant effects of bretazenil partial agonist more quickly than they developed to imidazenil. [14]

Pharmacology

Bretazenil has a more broad spectrum of action than traditional benzodiazepines as it has been shown to have low affinity binding to α4 and α6 GABAA receptors in addition to acting on α1, α2, α3 and α5 subunits which traditional benzodiazepine drugs work on. The partial agonist imidazenil does not, however, act at these subunits. [15] [16] [17] 0.5mg of bretazenil is approximately equivalent in its psychomotor-impairing effect to 10 mg of diazepam. Bretazenil produces marked sedative-hypnotic effects when taken alone and when combined with alcohol. This human study also indicates that bretazenil is possibly more sedative than diazepam. The reason is unknown, but the study suggests the possibility that a full-agonist metabolite may be generated in humans but not animals previously tested or else that there are significant differences in benzodiazepine receptor population in animals and humans. [18]

In a study of monkeys bretazenil has been found to antagonize the effects of full agonist benzodiazepines. However, bretazenil has been found to enhance the effects of neurosteroids acting on the neurosteroid binding site of the GABAA receptor. [19] Another study found that bretazenil acted as an antagonist provoking withdrawal symptoms in monkeys who were physically dependent on the full agonist benzodiazepine triazolam. [20]

Partial agonists of benzodiazepine receptors have been proposed as a possible alternative to full agonists of the benzodiazepine site to overcome the problems of tolerance, dependence and withdrawal which limits the role of benzodiazepines in the treatment of anxiety, insomnia and epilepsy. Such adverse effects appear to be less problematic with bretazenil than full agonists. [21] Bretazenil has also been found to have less abuse potential than benzodiazepine full agonists such as diazepam and alprazolam, [22] [23] however long-term use of bretazenil would still be expected to result in dependence and addiction.

Bretazenil alters the sleep EEG profile and causes a reduction in cortisol secretion and increases significantly the release of prolactin. [24] Bretazenil has effective hypnotic properties but impairs cognitive ability in humans. Bretazenil causes a reduction in the number of movements between sleep stages and delays movement into REM sleep. At a dosage of 0.5 mg of bretazenil REM sleep is decreased and stage 2 sleep is lengthened. [25]

See also

Related Research Articles

Benzodiazepine Class of psychoactive drugs with a core chemical structure of benzene and diazepine rings

Benzodiazepines, sometimes called "benzos", are a class of psychoactive drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. The first such drug, chlordiazepoxide (Librium), was discovered accidentally by Leo Sternbach in 1955, and made available in 1960 by Hoffmann–La Roche, which, since 1963, has also marketed the benzodiazepine diazepam (Valium). In 1977 benzodiazepines were globally the most prescribed medications. They are in the family of drugs commonly known as minor tranquilizers.

Diazepam Benzodiazepine sedative

Diazepam, first marketed as Valium, is a medicine of the benzodiazepine family that typically produces a calming effect. It is commonly used to treat a range of conditions, including anxiety, seizures, alcohol withdrawal syndrome, benzodiazepine withdrawal syndrome, muscle spasms, trouble sleeping, and restless legs syndrome. It may also be used to cause memory loss during certain medical procedures. It can be taken by mouth, inserted into the rectum, injected into muscle, injected into a vein or used as a nasal spray. When given into a vein, effects begin in one to five minutes and last up to an hour. By mouth, effects begin after 15 to 60 minutes.

Flumazenil

Flumazenil is a selective GABAA receptor antagonist administered via injection, otic insertion, or intranasally. Therapeutically, it acts as both an antagonist and antidote to benzodiazepines, through competitive inhibition.

Clobazam Benzodiazepine class medication

Clobazam, sold under the brand name Frisium among others, is a benzodiazepine class medication that was patented in 1968. Clobazam was first synthesized in 1966 and first published in 1969. Clobazam was originally marketed as an anxioselective anxiolytic since 1970, and an anticonvulsant since 1984. The primary drug-development goal was to provide greater anxiolytic, anti-obsessive efficacy with fewer benzodiazepine-related side effects.

Clorazepate Benzodiazepine family medication

Clorazepate, sold under the brand name Tranxene among others, is a benzodiazepine medication. It possesses anxiolytic, anticonvulsant, sedative, hypnotic, and skeletal muscle relaxant properties. Clorazepate is an unusually long-lasting benzodiazepine and serves as a majoritive prodrug for the equally long-lasting desmethyldiazepam, which is rapidly produced as an active metabolite. Desmethyldiazepam is responsible for most of the therapeutic effects of clorazepate.

Pagoclone

Pagoclone is an anxiolytic agent from the cyclopyrrolone family, related to better-known drugs such as the sleeping medication zopiclone. It was synthesized by a French team working for Rhone-Poulenc & Rorer S.A. Pagoclone belongs to the class of nonbenzodiazepines, which have similar effects to the older benzodiazepine group, but with quite different chemical structures. It was never commercialised.

Chlordiazepoxide Benzodiazepine class sedative and hypnotic medication

Chlordiazepoxide, trade name Librium among others, is a sedative and hypnotic medication of the benzodiazepine class; it is used to treat anxiety, insomnia and symptoms of withdrawal from alcohol and other drugs.

Benzodiazepine withdrawal syndrome Signs and symptoms due to benzodiazepines discontinuation in physically dependent persons

Benzodiazepine withdrawal syndrome—often abbreviated to benzo withdrawal or BZD withdrawal —is the cluster of signs and symptoms that emerge when a person who has been taking benzodiazepines, either medically or recreationally, and has developed a physical dependence, undergoes dosage reduction or discontinuation. Development of physical dependence and the resulting withdrawal symptoms, some of which may last for years, may result from taking the medication as prescribed. Benzodiazepine withdrawal is characterized by sleep disturbance, irritability, increased tension and anxiety, panic attacks, hand tremor, shaking, sweating, difficulty with concentration, confusion and cognitive difficulty, memory problems, dry retching and nausea, weight loss, palpitations, headache, muscular pain and stiffness, a host of perceptual changes, hallucinations, seizures, psychosis, and increased risk of suicide. Further, these symptoms are notable for the manner in which they wax and wane and vary in severity from day to day or week by week instead of steadily decreasing in a straightforward monotonic manner. This phenomenon is often referred to as "waves" and "windows".

Imidazenil

Imidazenil is an experimental anxiolytic drug which is derived from the benzodiazepine family, and is most closely related to other imidazobenzodiazepines such as midazolam, flumazenil, and bretazenil.

QH-II-66

QH-II-66 (QH-ii-066) is a sedative drug which is a benzodiazepine derivative. It produces some of the same effects as other benzodiazepines, but is much more selective than most other drugs of this class and so produces somewhat less sedation and ataxia than other related drugs such as diazepam and triazolam, although it still retains anticonvulsant effects.

Abecarnil

Abecarnil (ZK-112,119) is an anxiolytic drug from the β-Carboline family. It is one of a relatively recently developed class of medicines known as the nonbenzodiazepines, which have similar effects to the older benzodiazepine group, but with quite different chemical structures. It is a partial agonist acting selectively at the benzodiazepine site of the GABAA receptor.

Pipequaline

Pipequaline (INN) is an anxiolytic drug that was never marketed. It possesses a novel chemical structure that is not closely related to other drugs of this type. The drug has a similar pharmacological profile to the benzodiazepine family of drugs, but with mainly anxiolytic properties and very little sedative, amnestic or anticonvulsant effects, and so is classified as a nonbenzodiazepine anxiolytic.

L-838,417

L-838,417 is an anxiolytic drug used in scientific research. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic. The compound was developed by Merck, Sharp and Dohme.

SL651498

SL651498 is an anxiolytic and anticonvulsant drug used in scientific research, with a chemical structure most closely related to β-carboline derivatives such as abecarnil and gedocarnil. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic.

CL-218,872

CL-218,872 is a sedative and hypnotic drug used in scientific research. It has similar effects to sedative-hypnotic benzodiazepine drugs such as triazolam, but is structurally distinct and so is classed as a nonbenzodiazepine hypnotic.

ELB-139

ELB-139 (LS-191,811) is an anxiolytic drug with a novel chemical structure, which is used in scientific research. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic.

NS-2710

NS-2710 (LS-193,970) is an anxiolytic drug with a novel chemical structure, developed by the small pharmaceutical company NeuroSearch. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic. NS-2710 is a potent but non-selective partial agonist at GABAA receptors, although with little efficacy at the α1 subtype and more at α2 and α3. It has anxiolytic effects comparable to chlordiazepoxide, and while it is a less potent anticonvulsant than the related drug NS-2664, it has a much longer duration of action, and similarly to other α2/α3-preferring partial agonists produces little sedative effects or physical dependence.

TP-13

TP-13 is an anxiolytic drug with a novel chemical structure, which is used in scientific research. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic. It is a subtype-selective partial agonist at GABAA receptors, binding selectively to GABAA receptor complexes bearing α2 and α3 subunits. It has modest anticonvulsant activity although less than that of diazepam, and its main effect is likely to be selective anxiolytic action, as seen with other related α2/3-preferring agonists such as L-838,417.

TP-003

TP-003 is an anxiolytic drug with a novel chemical structure, which is used in scientific research. It has similar effects to benzodiazepine drugs, but is structurally distinct and so is classed as a nonbenzodiazepine anxiolytic. It is a positive allosteric modulator at the benzodiazepine binding site of GABAA receptors. It has modest anticonvulsant activity although less than that of diazepam.

Divaplon

Divaplon (RU-32698) is a nonbenzodiazepine, anxiolytic and anticonvulsant drug from the pyrazolopyrimidine family of drugs. It acts as a partial agonist at the "benzodiazepine site" of the GABAA receptor in the brain.

References

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