🧬 Receptor activity
Mechanism of action
Escitalopram, like other selective serotonin re-uptake inhibitors, enhances serotonergic activity by binding to the orthosteric (i.e. primary) binding site on the serotonin transporter (SERT), the same site to which endogenous 5-HT binds, and thus prevents the re-uptake of serotonin into the presynaptic neuron. Escitalopram, along with [paroxetine], is also considered an allosteric serotonin re-uptake inhibitor - it binds to a secondary allosteric site on the SERT molecule to more strongly inhibit 5-HT re-uptake. Its combination of orthosteric and allosteric activity on SERT allows for greater extracellular 5-HT levels, a faster onset of action, and greater efficacy as compared to other SSRIs. The sustained elevation of synaptic 5-HT eventually causes desensitization of 5-HT<sub>1A</sub> auto-receptors, which normally shut down endogenous 5-HT release in the presence of excess 5-HT - this desensitization may be necessary for the full clinical effect of SSRIs and may be responsible for their typically prolonged onset of action. Escitalopram has shown little-to-no binding affinity at a number of other receptors, such as histamine and muscarinic receptors, and minor activity at these off-targets may explain some of its adverse effects.
The mechanism of antidepressant action of escitalopram, the S-enantiomer of racemic citalopram, is presumed to be linked to potentiation of serotonergic activity in the central nervous system (CNS) resulting from its inhibition of CNS neuronal reuptake of serotonin (5-HT).
Pharmacodynamics
Escitalopram belongs to a class of medications called selective serotonin re-uptake inhibitors (SSRIs). These agents cause an increase in serotonin levels in neuronal synapses by preventing the re-uptake of serotonin (5-HT) into the presynaptic terminals of serotonergic neurons. As compared to other SSRIs, it appears to have a relatively quick onset of effect due to its potency. SSRIs as a class have been associated with abnormal bleeding, particularly in patients receiving concomitant therapy with other medications affecting hemostasis, and with the development of serotonin syndrome. Use escitalopram with caution in patients with a higher-than-baseline risk of bleeding and in patients receiving concomitant therapy with other serotonergic drugs. Escitalopram may also cause a discontinuation syndrome with abrupt removal of the drug, and should be slowly tapered if discontinuation of therapy is warranted.
Pharmacokinetics
Half-life
The elimination half-life of escitalopram is 27-32 hours, though this is increased by approximately 50% in the elderly and doubled in patients with reduced hepatic function. The elimination half-life of escitalopram's primary metabolite, S-desmethylcitalopram, is approximately 54 hours at steady state.
Biotransformation of escitalopram is mainly hepatic, with a mean terminal half-life of about 27-32 hours.
Absorption
Absorption of escitalopram following oral administration is expected to be almost complete, with an estimated absolute bioavailability of approximately 80%. Tmax occurs after about 4-5 hours. Cmax and AUC appear to follow dose proportionality - at steady state, patients receiving 10mg of escitalopram daily had a Cmax of 21 ng/mL and a 24h AUC of approximately 360 ng*h/mL, while patients receiving 30mg daily had a roughly 3-fold increase in both Cmax and 24h AUC, comparatively.
After oral administration of escitalopram, approximately 8% of the total dose is eliminated in the urine as unchanged escitalopram and 10% is eliminated in the urine as S-desmethylcitalopram. The apparent hepatic clearance of escitalopram amounts to approximately 90% of the total dose.
Escitalopram appears to distribute extensively into tissues, with an apparent volume of distribution of approximately 12-26 L/kg.
The oral plasma clearance of escitalopram is 600 mL/min, of which approximately 7% is due to renal clearance.
/MILK/ Escitalopram is excreted in human breast milk. Limited data from women taking 10-20 mg escitalopram showed that exclusively breast-fed infants receive approximately 3.9% of the maternal weight-adjusted dose of escitalopram and 1.7% of the maternal weight-adjusted dose of desmethylcitalopram.
The absolute bioavailability of citalopram is about 80% relative to an intravenous dose, and the volume of distribution of citalopram is about 12 L/kg. Data specific on escitalopram are unavailable. The binding of escitalopram to human plasma proteins is approximately 56%.
Metabolism
The metabolism of escitalopram is mainly hepatic, mediated primarily by CYP2C19 and CYP3A4 and, to a lesser extent, CYP2D6. Oxidative N-demethylation by the CYP enzyme system results in S-desmethylcitalopram (S-DCT) and S-didesmethylcitalopram (S-DDCT) - these metabolites do not contribute to the pharmacologic activity of escitalopram, and exist in the plasma in small quantities relative to the parent compound (28-31% and <5%, respectively). There is also some evidence that escitalopram is metabolized to a propionic acid metabolite by monoamine oxidase A and B in the brain, and that these enzymes constitute the major route of escitalopram metabolism in the brain.
The antidepressant escitalopram is predominantly metabolized by the polymorphic CYP2C19 enzyme. The authors investigated the effect of CYP2C19 genotype on exposure and therapeutic failure of escitalopram in a large patient population. A total of 4,228 escitalopram serum concentration measurements from 2,087 CYP2C19-genotyped patients 10-30 hours after drug intake were collected retrospectively from the drug monitoring database at Diakonhjemmet Hospital in Oslo. The patients were divided into subgroups based on CYP2C19 genotype: those carrying inactive (CYP2C19Null) and gain-of-function (CYP2C19*17) variant alleles. The between-subgroup differences in escitalopram exposure (endpoint: dose-harmonized serum concentration) and therapeutic failure (endpoint: switching to another antidepressant within 1 year after the last escitalopram measurement) were evaluated by multivariate mixed model and chi-square analysis, respectively. Compared with the CYP2C19*1/*1 group, escitalopram serum concentrations were significantly increased 3.3-fold in the CYP2C19Null/Null group, 1.6-fold in the CYP2C19*Null/*1 group, and 1.4-fold in the CYP2C19Null/*17 group, whereas escitalopram serum concentrations were significantly decreased by 10% in the CYP2C19*1/*17 group and 20% in the CYP1C19*17/*17 group. In comparison to the CYP2C19*1/*1 group, switches from escitalopram to another antidepressant within 1 year were 3.3, 1.6, and 3.0 times more frequent among the CYP2C19Null/Null, CYP2C19*1/*17, and CYP1C19*17/*17 groups, respectively. The CYP2C19 genotype had a substantial impact on exposure and therapeutic failure of escitalopram, as measured by switching of antidepressant therapy. The results support the potential clinical utility of CYP2C19 genotyping for individualization of escitalopram therapy.
Escitalopram is metabolized to S-demethylcitalopram (S-DCT) and S-didemethylcitalopram (S-DDCT). In humans, unchanged escitalopram is the predominant compound in plasma. At steady state, the concentration of the escitalopram metabolite S-DCT in plasma is approximately one-third that of escitalopram. The level of S-DDCT was not detectable in most subjects. In vitro studies show that escitalopram is at least 7 and 27 times more potent than S-DCT and S-DDCT, respectively, in the inhibition of serotonin reuptake, suggesting that the metabolites of escitalopram do not contribute significantly to the antidepressant actions of escitalopram. S-DCT and S-DDCT also have no or very low affinity for serotonergic (5-HT1-7) or other receptors including alpha- and beta-adrenergic, dopamine (D1-5), histamine (H1-3), muscarinic (M1-5), and benzodiazepine receptors. S-DCT and S-DDCT also do not bind to various ion channels including Na+, K+, Cl-, and Ca++ channels. In vitro studies using human liver microsomes indicated that CYP3A4 and CYP2C19 are the primary isozymes involved in the Ndemethylation of escitalopram.
Mainly hepatic. Escitalopram undergoes <i>N</i>-demethylation to <i>S</i>-demethylcitalopram (S-DCT) and <i>S</i>-didemethylcitalopram (S-DDCT). CYP3A4 and CYP2C19 are the enzymes responsible for this <i>N</i>-demethylation reaction.
Route of Elimination: Following oral administrations of escitalopram, the fraction of drug recovered in the urine as escitalopram and <i>S</i>-demethylcitalopram (S-DCT) is about 8% and 10%, respectively. The oral clearance of escitalopram is 600 mL/min, with approximately 7% of that due to renal clearance. Escitalopram is metabolized to S-DCT and <i>S</i>-didemethylcitalopram (S-DDCT).
Half Life: 27-32 hours
Protein binding
Escitalopram exhibits relatively low protein binding at approximately 55-56%.
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