Mirtazapine


Mirtazapine, sold under the brand name Remeron among others, is an antidepressant primarily used to treat depression. Its full effect may take more than four weeks to occur, with some benefit possibly as early as one to two weeks. Often it is used in depression complicated by anxiety or trouble sleeping. It is taken by mouth.
Common side effects include increased weight, sleepiness, and dizziness. Serious side effects may include mania, low white blood count, and increased suicide among children. Withdrawal symptoms may occur with stopping. It is not recommended together with an MAO inhibitor. It is unclear if use during pregnancy is safe. How it works is not clear, but it may involve blocking certain adrenergic and serotonin receptors. Chemically, it is a tetracyclic antidepressant. It also has strong antihistamine effects.
Mirtazapine came into medical use in the United States in 1996. The patent expired in 2004, and generic versions are available. In 2017, it was the 119th most commonly prescribed medication in the United States, with more than six million prescriptions.

Medical uses

Depression

Mirtazapine is primarily used for major depressive disorder and other mood disorders. Onset of action appears faster than some SSRIs and similar to tricyclic antidepressants.
In 2010 NICE recommended generic SSRIs as first line choices, as they are "equally effective as other antidepressants and have a favourable risk–benefit ratio." With respect to mirtazapine, it found: "There is no difference between mirtazapine and other antidepressants on any efficacy measure, although in terms of achieving remission mirtazapine appears to have a statistical though not clinical advantage. In addition, mirtazapine has a statistical advantage over SSRIs in terms of reducing symptoms of depression, but the difference is not clinically important. However, there is strong evidence that patients taking mirtazapine are less likely to leave treatment early because of side effects, although this is not the case for patients reporting side effects or leaving treatment early for any reason."
A 2011 Cochrane review that compared mirtazapine to other antidepressants found that, while it appears to have a faster onset in people for whom it works, its efficacy is about the same as other antidepressants after 6 weeks' use.
A 2012 review focused on antidepressants and sleep found that in many people with sleep disorders caused by depression, mirtazapine reduces the time it takes to fall asleep and increases the quality of sleep, but that in some people it can disturb sleep, especially at higher doses, causing restless leg syndrome in 8 to 28% of people and in rare cases causes REM sleep behavior disorder.
A 2018 analysis of 21 antidepressants found them to be fairly similar overall. It found tentative evidence for mirtazapine being in the more effective group and middle in tolerability.
After one week of usage, mirtazapine was found to have an earlier onset of action compared to SSRIs.

Other

There is also some tentative evidence supporting its use in treating the following conditions, for which it is sometimes prescribed off-label:
A 2011 Cochrane review found that compared with other antidepressants, it is more likely to cause weight gain and sleepiness, but it is less likely to cause tremor than tricyclic antidepressants, and less likely to cause nausea and sexual dysfunction than SSRIs.
Very common adverse effects include constipation, dry mouth, sleepiness, increased appetite and weight gain.
Common adverse effects include weakness, confusion, dizziness, fasciculations, peripheral edema, and negative lab results like elevated transaminases, elevated serum triglycerides, and elevated total cholesterol.
Mirtazapine is not considered to have a risk of many of the side effects often associated with other antidepressants like the SSRIs, and may actually improve certain ones when taken in conjunction with them.
In general, some antidepressants, especially SSRIs, can paradoxically exacerbate some peoples' depression or anxiety or cause suicidal ideation. Despite its sedating action, mirtazapine is also believed to be capable of this, so in the United States and certain other countries, it carries a black box label warning of these potential effects, especially for people under the age of 25.
A case report published in 2000 noted an instance in which mirtazapine counteracted the action of clonidine, causing a dangerous rise in blood pressure.

Discontinuation syndrome

Mirtazapine and other antidepressants may cause a discontinuation syndrome upon cessation. A gradual and slow reduction in dose is recommended to minimize discontinuation symptoms. Effects of sudden cessation of treatment with mirtazapine may include depression, anxiety, tinnitus, panic attacks, vertigo, restlessness, irritability, decreased appetite, insomnia, diarrhea, nausea, vomiting, flu-like symptoms such as allergies and pruritus, headaches and sometimes hypomania or mania.

Overdose

Mirtazapine is considered to be relatively safe in the event of an overdose, although it is considered slightly more toxic in overdose than most of the SSRIs. Unlike the tricyclic antidepressants, mirtazapine showed no significant cardiovascular adverse effects at 7 to 22 times the maximum recommended dose. Case reports of overdose with as much as 30 to 50 times the standard dose described the drug as relatively nontoxic, compared to tricyclic antidepressants.
Twelve reported fatalities have been attributed to mirtazapine overdose. The fatal toxicity index for mirtazapine is 3.1. This is similar to that observed with SSRIs.

Interactions

Concurrent use with inhibitors or inducers of the cytochrome P450 isoenzymes CYP1A2, CYP2D6, and/or CYP3A4 can result in altered concentrations of mirtazapine, as these are the main enzymes responsible for its metabolism. As examples, fluoxetine and paroxetine, inhibitors of these enzymes, are known to modestly increase mirtazapine levels, while carbamazepine, an inducer, considerably decreases them. Liver impairment and moderate chronic kidney disease have been reported to decrease the oral clearance of mirtazapine by about 30%; severe kidney disease decreases it by 50%.
Mirtazapine in combination with an SSRI,, or TCA as an augmentation strategy is considered to be relatively safe and is often employed therapeutically, with a combination of venlafaxine and mirtazapine, sometimes referred to as "California rocket fuel". Several case reports document serotonin syndrome induced by the combination of mirtazapine with other agents.
According to information from the manufacturers, mirtazapine should not be started within two weeks of any monoamine oxidase inhibitor usage; likewise, MAOIs should not be administered within two weeks of discontinuing mirtazapine.
The addition of mirtazapine to an MAOI, while potentially having typical or idiosyncratic reactions not herein described, does not appear to cause serotonin syndrome. This is in accordance with the fact that the 5-HT2A receptor is the predominant serotonin receptor thought to be involved in the pathophysiology of serotonin syndrome. Mirtazapine is a potent 5-HT2A receptor antagonist, and cyproheptadine, a drug that shares this property, mediates recovery from serotonin syndrome and is an antidote against it.

Pharmacology

Pharmacodynamics

Mirtazapine is sometimes described as a noradrenergic and specific serotonergic antidepressant, although the actual evidence in support of this label has been regarded as poor. It is a tetracyclic piperazine-azepine.
Mirtazapine has antihistamine, α2-blocker, and antiserotonergic activity. It is specifically a potent antagonist or inverse agonist of the α2A-, α2B-, and α2C-adrenergic receptors, the serotonin 5-HT2A, 5-HT2C, and the histamine H1 receptor. Unlike many other antidepressants, it does not inhibit the reuptake of serotonin, norepinephrine, or dopamine, nor does it inhibit monoamine oxidase. Similarly, mirtazapine has weak or no activity as an anticholinergic or blocker of sodium or calcium channels, in contrast to most TCAs. In accordance, it has better tolerability and low toxicity in overdose. As an H1 receptor antagonist, mirtazapine is extremely potent, and is in fact the most potent of all the TCAs and TeCAs. Antagonism of the H1 receptor is by far the strongest activity of mirtazapine, with the drug acting as a selective H1 receptor antagonist at low concentrations.
The - enantiomer of mirtazapine is responsible for antagonism of the serotonin 5-HT2A and 5-HT2C receptors, while the - enantiomer is responsible for antagonism of the 5-HT3 receptor. Both enantiomers are involved in antagonism of the H1 and α2-adrenergic receptors, although the - enantiomer is the stronger antihistamine.
Although not clinically relevant, mirtazapine has been found to act as a partial agonist of the κ-opioid receptor at high concentrations.

α2-Adrenergic receptor

Antagonism of the α2-adrenergic receptors, which function largely as inhibitory autoreceptors and heteroreceptors, enhances adrenergic and serotonergic neurotransmission, notably central 5-HT1A receptor mediated transmission in the dorsal raphe nucleus and hippocampus; hence, mirtazapine's classification as a NaSSA. Indirect α1 adrenoceptor-mediated enhancement of serotonin cell firing and direct blockade of inhibitory α2 heteroreceptors located on serotonin terminals are held responsible for the increase in extracellular serotonin. Because of this, mirtazapine has been said to be a functional "indirect agonist" of the 5-HT1A receptor. Increased activation of the central 5-HT1A receptor is thought to be a major mediator of efficacy of most antidepressant drugs.

5-HT2 receptor

Antagonism of the 5-HT2 subfamily of receptors and inverse agonism of the 5-HT2C receptor appears to be in part responsible for mirtazapine's efficacy in the treatment of depressive states.
Mirtazapine increases dopamine release in the prefrontal cortex. Accordingly, it was shown that by blocking the α2-adrenergic receptors and 5-HT2C receptors mirtazapine disinhibited dopamine and norepinephrine activity in these areas in rats. In addition, mirtazapine's antagonism of 5-HT2A receptors has beneficial effects on anxiety, sleep and appetite, as well as sexual function regarding the latter receptor. Mirtazapine has been shown to lower drug seeking behaviour in various human and animal studies. It is also being investigated in substance abuse disorders to reduce withdrawal effects and improve remission rates.
Mirtazapine significantly improves pre-existing symptoms of nausea, vomiting, diarrhea, and irritable bowel syndrome in afflicted individuals. Mirtazapine may be used as an inexpensive antiemetic alternative to ondansetron. In conjunction with substance abuse counseling, mirtazapine has been investigated for the purpose of reducing methamphetamine use in dependent individuals with success. In contrast to mirtazapine, the SSRIs, SNRIs, MAOIs, and some TCAs increase the general activity of the 5-HT2A, 5-HT2C, and 5-HT3 receptors leading to a host of negative changes and side effects, the most prominent of which including anorexia, insomnia, sexual dysfunction, nausea, and diarrhea, among others. As a result, it is often combined with these drugs to reduce their side-effect profile and to produce a stronger antidepressant effect.
Mirtazapine does not have serotonergic activity and does not cause serotonergic side effects or serotonin syndrome. This is in accordance with the fact that it is not a serotonin reuptake inhibitor or MAOI, nor a serotonin receptor agonist. There are no reports of serotonin syndrome in association with mirtazapine alone, and mirtazapine has not been found to cause serotonin syndrome in overdose. However, there are a handful of case reports of serotonin syndrome occurring with mirtazapine in combination with serotonergic drugs like SSRIs, although such reports are very rare, and do not necessarily implicate mirtazapine as causative.

5-HT3 receptor

It is a potent 5-HT3 blocker. It may relieve chemotherapy-related and advanced cancer-related nausea.

H1 receptor

Mirtazapine is a very strong H1 receptor inverse agonist and, as a result, it can cause powerful sedative and hypnotic effects. A single 15 mg dose of mirtazapine to healthy volunteers has been found to result in over 80% occupancy of the H1 receptor and to induce intense sleepiness. After a short period of chronic treatment, however, the H1 receptor tends to desensitize and the antihistamine effects become more tolerable. Many patients may also dose at night to avoid the effects, and this appears to be an effective strategy for combating them. Blockade of the H1 receptor may improve pre-existing allergies, pruritus, nausea, and insomnia in afflicted individuals. It may also contribute to weight gain, however. In contrast to the H1 receptor, mirtazapine has only low affinity for the muscarinic acetylcholine receptors, although anticholinergic side effects like dry mouth, constipation, and mydriasis are still sometimes seen in clinical practice.

Pharmacokinetics

The oral bioavailability of mirtazapine is about 50%. It is found mostly bound to plasma proteins, about 85%. It is metabolized primarily in the liver by demethylation and hydroxylation via cytochrome P450 enzymes, CYP1A2, CYP2D6, CYP3A4. One of its major metabolites is desmethylmirtazapine. The overall elimination half-life is 20–40 hours. It is conjugated in the kidney for excretion in the urine, where 75% of the drug is excreted, and about 15% is eliminated in feces.

Chemistry

Mirtazapine is a tetracyclic piperazinoazepine; mianserin was developed by the same team of organic chemists and mirtazapine differs from it via addition of a nitrogen atom in one of the rings. It is a racemic mixture of enantiomers. The --enantiomer is known as esmirtazapine.
Analogues of mirtazapine include mianserin, setiptiline, and aptazapine.

Synthesis

A chemical synthesis of mirtazapine has been published. The first step of synthesis is a condensation reaction between the molecule 2-chloro 3-cyanopyridine and the molecule 1-methyl-3-phenylpiperazine.

History

Mirtazapine was first synthesized at Organon and published in 1989, was first approved for use in major depressive disorder in the Netherlands in 1994, and was introduced in the United States in 1996 under the brand name Remeron.

Society and culture

Generic names

Mirtazapine is the English and French generic name of the drug and its,,,,, and. Its generic name in Spanish is mirtazapina and in German is Mirtazapin.

Brand names

Mirtazapine is marketed under many brand names worldwide, including Adco-Mirteron, Afloyan, Amirel, Arintapin Smelt, Avanza, Axit, Azapin, Beron, Bilanz, Calixta, Ciblex, Combar, Comenter, Depreram, Divaril, Esprital, Maz, Menelat, Mepirzapine, Merdaten, Meronin, Mi Er Ning, Milivin, Minelza, Minivane, Mirastad, Mirazep, Miro, Miron, Mirrador, Mirt, Mirta, Mirtabene, Mirtadepi, Mirtagamma, Mirtagen, Mirtalan, Mirtamor, Mirtamylan, Mirtan, Mirtaneo, Mirtapax, Mirtapil, Mirtapine, Mirtaron, Mirtastad, Mirtax, Mirtaz, Mirtazap, Mirtazapin, Mirtazapina, Mirtazapine, Mirtazapinum, Mirtazelon, Mirtazon, Mirtazonal, Mirtel, Mirtimash, Mirtin, Mirtine, Mirzapine, Mirzaten, Mirzest, Mitaprex, Mitaxind, Mitocent, Mitrazin, Mizapin, Motofen, Mytra, Norset, Noxibel, Pharmataz, Promyrtil, Rapizapine, Ramure, Razapina, Redepra, Reflex, Remergil, Remergon, Remeron, Remirta, Rexer, Saxib, Sinmaron, Smilon, Tazepin, Tazimed, Tetrazic, Tifona, U-Mirtaron, U-zepine, Valdren, Vastat, Velorin, Yarocen, Zania, Zapex, Zestat, Zismirt, Zispin, Zuleptan, and Zulin.
Although its name ends in -azepine and it is a sedative, it is not a benzodiazepine.

Research

The use of mirtazapine has been explored in several additional conditions:
Mirtazapine also has some veterinary use in cats and dogs. Mirtazapine is sometimes prescribed as an appetite stimulant for cats or dogs experiencing anorexia due to medical conditions such as chronic kidney disease. It is especially useful for treating combined poor appetite and nausea in cats and dogs.
Mirtazapine is indicated for bodyweight gain in cats experiencing poor appetite and weight loss resulting from chronic medical conditions. It is an ointment applied topically to the inner pinna.
The most common side effects include signs of local irritation or inflammation at the site where the ointment is applied and behavioural changes.