Clonazepam
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🧬 Receptor activity
| Target | Action | Affinity | Source | |
|---|---|---|---|---|
| GABA-A receptor; anion channel | — | Ki 0.85 nM | CHEMBL | TargetGABA-A receptor; anion channel Action— AffinityKi 0.85 nM SourceCHEMBL |
| Unchecked | — | Ki 0.85 nM | CHEMBL | TargetUnchecked Action— AffinityKi 0.85 nM SourceCHEMBL |
| Translocator protein | — | Ki 0.85 nM | CHEMBL | TargetTranslocator protein Action— AffinityKi 0.85 nM SourceCHEMBL |
Mechanism of action
Gamma-Aminobutyric acid (GABA) is considered the principal inhibitory neurotransmitter in the human body. When GABA binds to GABA(a) receptors found in neuron synapses, chloride ions are conducted across neuron cell membranes via an ion channel in the receptors. With enough chloride ions conducted, the local, associated neuron membrane potentials are hyperpolarized - making it more difficult or less likely for action potentials to fire, ultimately resulting in less excitation of the neurons. Subsequently, benzodiazepines like clonazepam can bind to benzodiazepine receptors that are components of various varieties of GABA(a) receptors. This binding acts to enhance the effects of GABA by increasing GABA affinity for the GABA(a) receptor, which ultimately enhances GABA ligand binding at the receptors. This enhanced ligand binding of the inhibitory neurotransmitter GABA to the receptors increases the aforementioned chloride ion conduction (perhaps reportedly via an increase in the frequency of the chloride channel opening), resulting in a hyperpolarized cell membrane that prevents further excitation of the associated neuron cells. Combined with the notion that such benzodiazepine receptor associated GABA(a) receptors exist both peripherally and in the CNS, this activity consequently facilitates various effects like sedation, hypnosis, skeletal muscle relaxation, anticonvulsant activity, and anxiolytic action. In particular, when out of the ordinary rapid and repetitive electrical signals are released in the CNS, it is proposed that the brain can become over-stimulated and ordinary functions are disrupted - resulting in seizure activity. By enhancing the neuro-inhibitory activity of GABA, it is believed that clonazepam can facilitate in decreasing any excessive electrical nerve activity in the CNS that might be contributing to seizures. Concurrently, it is also believed that clonazepam's actions in enhancing GABA effects may inhibit neuronal activity proposed to occur in amygdala-centered fear circuits - therefore assisting in the management of anxiety or panic.
Pharmacodynamics
The pharmacodynamic properties of clonazepam are common among benzodiazepines and include anticonvulsive, sedative, muscle relaxing and anxiolytic effects. Animal data and electroencephalographic investigations in man have shown that clonazepam rapidly suppresses many types of paroxysmal activity including the spike and wave discharge in absence seizures (petit mal), slow spike wave, generalized spike wave, spikes with temporal or other locations, as well as irregular spikes and waves. Moreover, the agent can also decrease the frequency, amplitude, duration, and spread of discharge in minor motor seizures. Generalized EEG abnormalities are more readily suppressed by clonazepam than are focal EEG abnormalities such as focal spikes. Clonazepam has beneficial effects in generalized and focal epilepsies.
Pharmacokinetics
Half-life
The mean elimination half-life determined for clonazepam is independent of the dose given and has been documented as being about 30-40 hours.
Elimination half-life ... 18.7 to 39 hr.
Absorption
Clonazepam is rapidly and almost entirely absorbed after oral administration as tablets. Peak plasma concentrations of clonazepam administered by the oral route are reached within 1-4 hours and the associated absorption half-life is about 25 minutes. The absolute bioavailability is approximately 90% - but with substantially large differences between individuals.
Approximately 50-70% of a clonazepam dose is excreted in the urine and 10-30% is excreted in the feces as metabolites. The excretion of unchanged clonazepam in the urine is typically less than 2% of the administered dose. Metabolites of clonazepam are present in urine as both free and conjugated (glucuronide and sulfate) compounds.
Clonazepam distributes very rapidly to various organs and body tissues with preferential uptake by brain structures. The apparent volume of distribution has been documented as approximately 3 L/kg.
The documented clearance for clonazepam is approximately 55 ml/min regardless of gender. Nevertheless, clearance values normalized by weight decline with increasing body weight.
/MILK/ A 2750-g female infant was born at 36 weeks' gestation to a 40-year-old woman treated with clonazepam throughout her pregnancy. The infant developed apnea, cyanosis, and hypotonia within a few hours of birth. The mother's serum clonazepam level at delivery was 32 ng/mL; the cord blood level was 19 ng/mL. The infant had no congenital malformations, evidence of infection, or seizures. Clinical episodes ceased by ten days of age. The woman elected to breastfeed; breast milk clonazepam levels were between 11 and 13 ng/mL. She was discharged with a cardiorespiratory monitor. The authors suggest that infants of mothers receiving this agent during pregnancy or while nursing have serum levels measured. Additionally, these infants should be monitored for central nervous system depression or apnea.
Clonazepam is rapidly and well absorbed from the GI tract. The absolute bioavailability is approximately 90%. In one study, peak blood concentrations of 6.5-13.5 ng/mL were usually reached within 1-2 hours following a single 2 mg oral dose of micronized clonazepam in healthy adults. In some individuals, however, peak blood concentrations were reached at 4-8 hours. Although the plasma concentration of clonazepam required for anticonvulsant effects has not been definitely established, some studies indicate it may be 20-80 ng/mL. Plasma concentrations in this range have been reported to be maintained in adults receiving 6 mg of clonazepam daily in 3 divided doses and in children 6-13 years of age receiving 1.5-4 mg of the drug daily in 3 divided doses. The onset of anticonvulsant action usually occurs within 20-60 minutes, and the duration of action usually is 6-8 hours in infants and young children and up to 12 hours in adults.
Metabolism
Clonazepam is metabolized principally in the liver. The metabolic pathways include hydroxylation, reduction of the nitro groups to amine groups, and the addition of acetate to the amino grouping. In particular, clonazepam is extensively metabolized by reduction to 7-amino-clonazepam and by N-acetylation to 7-acetamido-clonazepam. Hydroxylation at the C-3 position also occurs. Hepatic cytochrome P450 3A4 is implicated in the nitroreduction of clonazepam to pharmacologically inactive metabolites.
The shortcomings of clonazepam therapy include tolerance, withdrawal symptoms, and adverse effects such as drowsiness, dizziness, and confusion leading to increased risk of falls. Inter-individual variability in the incidence of adverse events in patients partly originates from the differences in clonazepam metabolism due to genetic and nongenetic factors. Since the prominent role in clonazepam nitro-reduction and acetylation of 7-amino-clonazepam is assigned to CYP3A and N-acetyl transferase 2 enzymes, respectively, the association between the patients' CYP3A status (CYP3A5 genotype, CYP3A4 expression) or N-acetyl transferase 2 acetylator phenotype and clonazepam metabolism (plasma concentrations of clonazepam and 7-amino-clonazepam) was evaluated in 98 psychiatric patients suffering from schizophrenia or bipolar disorders. The patients' CYP3A4 expression was found to be the major determinant of clonazepam plasma concentrations normalized by the dose and bodyweight (1263.5 +/- 482.9 and 558.5 +/- 202.4 ng/mL per mg/kg bodyweight in low and normal expressers, respectively, P<0.0001). Consequently, the dose requirement for the therapeutic concentration of clonazepam was substantially lower in low-CYP3A4 expresser patients than in normal expressers (0.029 +/- 0.011 vs 0.058 +/- 0.024 mg/kg bodyweight, P<0.0001). Furthermore, significantly higher (about 2-fold) plasma concentration ratio of 7-amino-clonazepam and clonazepam was observed in the patients displaying normal CYP3A4 expression and slower N-acetylation than all the others. Prospective assaying of CYP3A4 expression and N-acetyl transferase 2 acetylator phenotype can better identify the patients with higher risk of adverse reactions and can facilitate the improvement of personalized clonazepam therapy and withdrawal regimen.
Clonazepam is extensively metabolized in the liver to several metabolites including 7-aminoclonazepam, 7-acetaminoclonazepam, and 3-hydroxy derivatives of these metabolites and clonazepam. Clonazepam metabolites are excreted in urine by first-order kinetics, principally as their glucuronide and/or sulfate conjugates.
Hepatic (cytochrome P450, including CYP3A). Biotransformation occurs mainly by reduction of the 7-nitro group to the 4-amino derivative. This derivative can be acetylated, hydroxylated, and glucuronidated.
Route of Elimination: Clonazepam is highly metabolized, with less than 2% unchanged clonazepam being excreted in the urine. Metabolites of Klonopin are excreted by the kidneys. Clonazepam also undergoes acetylation via NAT2.
Half Life: 30-40 hours
Protein binding
The recorded plasma protein binding of clonazepam ranges between 82–86%.
External links
Fact-sheets from PsychonautWiki. Harm-reduction reference only — not medical advice.