Etizolam
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Mechanism of action
Etizolam is selectively a full agonist at GABA-A receptors to increase GABAergic transmission and enhance GABA-induced Cl- currents. It is reported to bind to the benzodiazepine binding site which is located across the interface between the alpha and gamma subunits. Benzodiazapines are reported to only bind to receptors that contain gamma 2 and alpha 1/2/3/5 subunits. Alpha-1-containing receptors mediate the sedative effects of etizolam whereas alpha-2 and alpha-3 subunit-containing receptors mediate the anxiolytic effect. Etizolam shows high potency and affinity towards GABA-A receptor with alpha 1 beta 2 gamma 2S subunit combination. By binding to the regulatory site of the receptor, etizolam potentiates GABA transmission by facilitating the opening of GABA-induced chloride channels. Etizolam is a specific antagonist at PAFR. It inhibits PAF-induced platelet aggregation by inhibiting PAF binding to the receptors located on the surface of platelets with an IC50 of 22nM.
Pharmacodynamics
Etizolam is a CNS depressant with anxiolytic, anticonvulsant, sedative-hypnotic and muscle relaxant effects. It acts on the benzodiazepine site of the GABA-A receptor as an agonist to increase inhibitory GABAergic transmission throughout the central nervous system. Studies indicate that etizolam mediates its pharmacological actions with 6 to 10 times more potency than that of diazepam. Clinical human studies performed in Italy showed clinical effectiveness of etizolam in relieving symptoms in patients with generalized anxiety disorders with depressive symptoms. Etizolam also mediates imipramine-like neuropharmacological and behavioral effects, as well as minor effects on cognitive functioning. It is shown to substitute the actions of a short-acting barbiturate, pentobarbitol, in a drug discrimination study. Etizolam is an antagonist at platelet-activating-factor (PAF) receptor and attenuates the recurrence of chronic subdural hematoma after neurosurgery in clinical studies. It is shown to inhibit PAF-induced bronchoconstriction and hypotension.
Pharmacokinetics
Half-life
The average elimination half life of etizolam following a single oral dose of 0.5mg is 3.4 hours but may be increased up to 17 hours depending on the rate of metabolism. The main metabolite α-hydroxyetizolam displays a longer elimination half life of 8.2 hours.
Absorption
Etizolam is well absorbed from the intestines with a biological bioavailability of 93% following oral administration. After a single oral dosing of 0.5mg etizolam, it takes approximately 0.9 hours to reach the peak plasma concentration of 8.3 ng/mL.
In a rat study, the amounts of etizolam excreted was 30% in urine was 70% in feces, while the values in a mouse study were 40% in urine and 60% in feces.
Apparent distribution volume was 0.9 ± 0.2 L/kg following a single oral doing of 0.5mg etizolam.
Metabolism
Biotransformation of etizolam is extensive and involves hydroxylation and conjugation. The main metabolite formed via 1'-hydroxylation is α-hydroxyetizolam which retains pharmacological activity comparable to that of the parent drug, indicating that the action of metabolites may contribute to the clinical effects of etizolam. CYP3A4 is predicted to be the main CYP enzyme responsible for mediating etizolam metabolism. CYP2C18 and CYP2C19 are also involved in the metabolic pathways.
Etizolam has known human metabolites that include 7-(2-chlorophenyl)-4-ethyl-13-methyl-3-thia-1,8,11,12-tetrazatricyclo[8.3.0.02,6]trideca-2(6),4,7,10,12-pentaen-9-ol and alpha-Hydroxyetizolam.
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