|Trade names||Pill form is generic and available under many brand names; transdermal patch is called Emsam|
|Bioavailability||10% (oral), 73% (patch)|
|Metabolism||In the gut wall and liver|
|Metabolites||N-desmethylselegiline, L-amphetamine and L-methamphetamine|
|Elimination half-life||10 hours (oral), 18–25 hours (transdermal)|
|CompTox Dashboard (EPA)|
|Chemical and physical data|
|Molar mass||187.281 g/mol g·mol−1|
|3D model (JSmol)|
Selegiline, also known as L-deprenyl, is a substituted phenethylamine. At normal clinical doses, it is a selective irreversible MAO-B inhibitor. In larger doses it loses its specificity and also inhibits MAO-A. It is available in pill form under many brand names and is used to reduce symptoms in early-stage Parkinson's disease. A transdermal patch (brand name, Emsam) is used to treat depression.
In its pill form, selegiline is used to treat symptoms of Parkinson's disease. It is most often used an adjunct to drugs such as L-DOPA, although it has been used off-label as a solo treatment. The rationale for adding selegiline to levodopa is to decrease the required dose of levodopa and thus reduce the motor complications of levodopa therapy. Selegiline delays the point when L-DOPA (levodopa) treatment becomes necessary from about 11 months to about 18 months after diagnosis. There is some evidence that selegiline acts as a neuroprotective and reduces the rate of disease progression, though this is disputed.
A quantitative review published in 2015 found that for the pooled results of the pivotal trials, the number needed to treat (a sign of effect size, so a low number is better) for the patch for symptom reduction was 11, and for remission, was 9. The number needed to harm (inverse of the NNT, a high number here is better) ranged from 387 for sexual side effects to 7 for application site reaction. With regard to the likelihood to be helped or harmed (LHH), the analysis showed that the selegiline patch was 3.6 times as likely to lead to a remission vs. a discontinuation due to side effects; the LHH for remission vs. incidence of insomnia was 2.1; the LHH for remission vs. discontinuation due to insomnia was 32.7. The LHH for remission vs insomnia and sexual dysfunction were both very low.
Side effects of the pill form include, in decreasing order of frequency, nausea, hallucinations, confusion, depression, loss of balance, insomnia, increased involuntary movements, agitation, slow or irregular heart rate, delusions, hypertension, new or increased angina pectoris, and syncope. Most of the side effects are due to a high dopamine activity, and can be alleviated by reducing the dose of levadopa.
The main side effects of the patch form for depression included application-site reactions, insomnia, diarrhea, and sore throat. The selegiline patch carries a black box warning about a possible increased risk of suicide, especially for young people, as do all antidepressants since 2007.
Both the oral and patch forms come with strong warnings against combining selegiline with drugs that could produce serotonin syndrome, such as SSRIs and the cough medicine dextromethorphan. Selegiline in combination with the opioid analgesic pethidine is not recommended, as it can lead to severe adverse effects. Several other synthetic opioids such as tramadol and methadone, as well as various triptans, are contraindicated due to potential for serotonin syndrome.
Oral contraceptives increases selegiline's bioavailability 10- to 20-fold. High levels can lead to loss of MAO-B selectivity, and selegiline may begin inhibition MAO-A as well. This increases susceptibility to side effects of unselective MAOIs, such as tyramine-induced hypertensive crisis and serotonin toxicity when combined with serotonergics.
Both forms of the drug carry warnings about food restrictions to avoid hypertensive crisis that are associated with MAO inhibitors. The patch form was created in part to overcome food restrictions; clinical trials showed that it was successful. Additionally, in post-marketing surveillance from April 2006 to October 2010, only 13 self-reports of possible hypertensive events or hypertension were made out of 29,141 exposures to the drug, and none were accompanied by objective clinical data. The lowest dose of the patch method of delivery, 6 mg/24 hours, does not require any dietary restrictions. Higher doses of the patch and oral formulations, whether in combination with the older non-selective MAOIs or in combination with the reversible MAO-A inhibitor moclobemide, require a low-tyramine diet.
Mechanism of action
Selegiline is a selective inhibitor of MAO-B in the nigrostriatal pathway of the brain, irreversibly inhibiting it by binding to it covalently. It exerts effects by blocking the breakdown of dopamine, thus increasing its activity. Its possible neuroprotective properties may be due to protecting nearby neurons from the free oxygen radicals that are released by MAO-B activity. At higher doses, selegiline loses its selectivity for MAO-B and inhibits MAO-A as well.
Selegiline also inhibits CYP2A6 and can increase the effects of nicotine as a result. Selegiline also appears to activate σ1 receptors, having a relatively high affinity of approximately 400 nM.
Selegiline has an oral bioavailability of about 10%, which increases when ingested together with fatty meal, as the molecule is fat soluble. Selegiline and its metabolites bind extensively to plasma proteins (at a rate of 94%). They cross the blood-brain barrier and enter the brain, where they most concentrated at the thalamus, basal ganglia, midbrain, and cingulate gyrus.
Selegiline is metabolized by cytochrome P450 to L-desmethylselegiline and L-methylamphetamine, the latter being one of the enantiomers of methamphetamine. Desmethylselegiline has some activity against MAO-B, but it is much smaller than selegiline's. It thought to be further metabolized by CYP2C19. L-methamphetamine is converted to L-amphetamine (people taking selegiline may also test positive for amphetamine or methamphetamine on drug screening tests). While the amphetamine metabolites may contribute to selegiline's ability to inhibit reuptake of the neurotransmitters dopamine and norepinephrine, they have also been associated with orthostatic hypotension and hallucinations. The amphetamine metabolites are hydroxylated and, in phase II, conjugated by glucuronyltransferase.
Following application of the patch to humans, an average of 25% to 30% of the selegiline content is delivered systemically over 24 hours. Transdermal dosing results in significantly higher exposure to selegiline and lower exposure to all metabolites when compared to oral dosing; this is due to the extensive first-pass metabolism of the pill form and low first-pass metabolism of the patch form. The site of application is not a significant factor in how the drug is distributed. In humans, selegiline does not accumulate in the skin, nor is it metabolized there.
Selegiline belongs to a class of drugs called phenethylamines. Selegiline is an L-methamphetamine derivative with a propargyl group attached to the nitrogen atom. This detail is borrowed from pargyline, an older phenethylamine MAO-B inhibitor.
Following the discovery that the tuberculosis drug iproniazid elevated the mood of people taking it, and the subsequent discovery that the effect was likely due to inhibition of MAO, many people and companies started trying to discover MAO inhibitors to use as antidepressants. Selegiline was discovered by Z. Ecseri at the Hungarian drug company, Chinoin (part of Sanofi since 1993), which they called E-250.:66–67 Chinoin received a patent on the drug in 1962 and the compound was first published in the scientific literature in English in 1965.:67 Work on the biology and effects of E-250 in animals and humans was conducted by a group led by József Knoll at Semmelweis University which was also in Budapest.:67
Deprenyl is a racemic compound a mixture of two isomers called enantiomers. Further work determined that the levorotatory enantiomer was a more potent MAO-inhibitor, which was published in 1967, and subsequent work was done with the single enantiomer L-deprenyl.:67
In 1971, Knoll showed that selegiline selectively inhibits the B-isoform of monoamine oxidase (MAO-B) and proposed that it is unlikely to cause the infamous "cheese effect" (hypertensive crisis resulting from consuming foods containing tyramine) that occurs with non-selective MAO inhibitors. A few years later, two Parkinson's disease researchers based in Vienna, Peter Riederer and Walther Birkmayer, realized that selegiline could be useful in Parkinson's disease. One of their colleagues, Prof. Moussa B.H. Youdim, visited Knoll in Budapest and took selegiline from him to Vienna. In 1975, Birkmayer's group published the first paper on the effect of selegiline in Parkinson's disease.
In 1987 Somerset Pharmaceuticals in New Jersey, which had acquired the US rights to develop selegiline, filed a new drug application (NDA) with the FDA to market the drug for Parkinson's disease in the US. While the NDA was under review, Somerset was acquired in a joint venture by two generic drug companies, Mylan and Bolan Pharmaceuticals. Selegiline was approved for Parkinson's disease by the FDA in 1989.
In the 1990s, J. Alexander Bodkin at McLean Hospital, an affiliate of Harvard Medical School, began a collaboration with Somerset to develop delivery of selegiline via a transdermal patch in order to avoid the well known dietary restrictions of MAO inhibitors. Somerset obtained FDA approval to market the patch in 2006.
Society and culture
In E for Ecstasy (a book examining the uses of the street drug ecstasy in the UK) the writer, activist and ecstasy advocate Nicholas Saunders highlighted test results showing that certain consignments of the drug also contained selegiline. Consignments of ecstasy known as "Strawberry" contained what Saunders described as a "potentially dangerous combination of ketamine, ephedrine and selegiline," as did a consignment of "Sitting Duck" Ecstasy tablets.
In veterinary medicine, selegiline is sold under the brand name Anipryl (manufactured by Zoetis). It is used in dogs to treat canine cognitive dysfunction and, at higher doses, pituitary-dependent hyperadrenocorticism (PDH). Canine cognitive dysfunction is a form of dementia that mimics Alzheimer's disease in humans. Geriatric dogs treated with selegiline show improvements in sleeping pattern, reduced incontinence, and increased activity level; most show improvements by one month. Though it is labeled for dog use only, selegiline has been used off-label for geriatric cats with cognitive dysfunction.
Selegiline's efficacy in treating pituitary-dependent hyperadrenocorticism has been disputed. Theoretically, it works by increasing dopamine levels, which downregulates the release of ACTH, eventually leading to reduced levels of cortisol. Some claim that selegiline is only effective at treating PDH caused by lesions in the anterior pituitary (which comprise most canine cases). The greatest sign of improvement is lessening of abdominal distention.
Side effects in dogs are uncommon, but they include vomiting, diarrhea, diminished hearing, salivation, decreased weight and behavioral changes such as hyperactivity, listlessness, disorientation, and repetitive motions.
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