Phenethylamine

This article is about 2-phenylethylamine. For its isomer, see 1-phenylethylamine. For its derivatives, see substituted phenethylamine.

Phenethylamine

Clinical data
Pronunciation	/fɛnˈɛθələmiːn/
Other names	Phenylethylamine; PEA; β-Phenylethylamine; β-Phenylethylamine; β-PEA; 2-Phenylethylamine; 2-PEA; Phetamine
Dependence liability	Psychological: low–moderate^[citation needed] Physical: none
Addiction liability	None–Low (w/o an MAO-B inhibitor)[1] Moderate (with an MAO-B inhibitor)[1]
Routes of administration	Oral (taken by mouth)
Drug class	Norepinephrine–dopamine releasing agent; Trace amine-associated receptor 1 (TAAR1) agonist; Psychostimulant; Sympathomimetic
ATC code	None
Physiological data
Source tissues	Substantia nigra pars compacta; Ventral tegmental area; Locus coeruleus; many others
Target tissues	System-wide
Receptors	Varies greatly across species; Human receptors: hTAAR1[2]
Precursor	L-Phenylalanine[3][4]
Biosynthesis	Aromatic L-amino acid decarboxylase (AADC)[3][4]
Metabolism	Primarily: MAO-B[3][4][5] Other enzymes: MAO-A,[5][6] SSAOs (AOC2 & AOC3),[5][7] PNMT,[3][4][5] AANAT,[5] FMO3,[8][9] and others
Legal status
Legal status	AU: Unscheduled CA: Unscheduled NZ: Unscheduled UK: Unscheduled US: Unscheduled UN: Unscheduled
Pharmacokinetic data
Metabolism	Primarily: MAO-B[3][4][5] Other enzymes: MAO-A,[5][6] SSAOs (AOC2 & AOC3),[5][7] PNMT,[3][4][5] AANAT,[5] FMO3,[8][9] and others
Elimination half-life	Exogenous: 5–10 minutes[10] Endogenous: ~30 seconds[3]
Excretion	Renal (kidneys)
Identifiers
IUPAC name 2-Phenylethan-1-amine
CAS Number	64-04-0^Y
PubChem CID	1001
IUPHAR/BPS	2144
DrugBank	DB04325^Y
ChemSpider	13856352^Y
UNII	327C7L2BXQ
KEGG	C05332^Y
ChEBI	CHEBI:18397^Y
ChEMBL	ChEMBL610^Y
NIAID ChemDB	018561
CompTox Dashboard (EPA)	DTXSID5058773
ECHA InfoCard	100.000.523
Chemical and physical data
Formula	C8H11N
Molar mass	121.183 g·mol^−1
3D model (JSmol)	Interactive image
Density	0.9640 g/cm^3 [10]
Melting point	−60 °C (−76 °F) [10]
Boiling point	195 °C (383 °F) [10]
SMILES NCCc1ccccc1
InChI InChI=1S/C8H11N/c9-7-6-8-4-2-1-3-5-8/h1-5H,6-7,9H2^Y Key:BHHGXPLMPWCGHP-UHFFFAOYSA-N^Y
(verify)

Phenethylamine^{[note 1]} (PEA) is an organic compound, natural monoamine alkaloid, and trace amine, which acts as a central nervous system stimulant in humans. In the brain, phenethylamine regulates monoamine neurotransmission by binding to trace amine-associated receptor 1 (TAAR1) and inhibiting vesicular monoamine transporter 2 (VMAT2) in monoamine neurons.^[1][11][12] To a lesser extent, it also acts as a neurotransmitter in the human central nervous system.^[13] In mammals, phenethylamine is produced from the amino acid L-phenylalanine by the enzyme aromatic L-amino acid decarboxylase via enzymatic decarboxylation.^[14] In addition to its presence in mammals, phenethylamine is found in many other organisms and foods, such as chocolate, especially after microbial fermentation.

Phenethylamine is sold as a dietary supplement for purported mood and weight loss-related therapeutic benefits; however, in orally ingested phenethylamine, a significant amount is metabolized in the small intestine by monoamine oxidase B (MAO-B) and then aldehyde dehydrogenase (ALDH), which converts it to phenylacetic acid.^[5] This means that for significant concentrations to reach the brain, the dosage must be higher than for other methods of administration.^[5][6][15] Some authors have postulated that phenethylamine plays a role in affection without substantiating these claims with any direct evidence.^[16][17]

Phenethylamines, or more properly, substituted phenethylamines, are the group of phenethylamine derivatives that contain phenethylamine as a "backbone"; in other words, this chemical class includes derivative compounds that are formed by replacing one or more hydrogen atoms in the phenethylamine core structure with substituents. The class of substituted phenethylamines includes all substituted amphetamines, and substituted methylenedioxyphenethylamines (MDxx), and contains many drugs which act as empathogens, stimulants, psychedelics, anorectics, bronchodilators, decongestants, and/or antidepressants, among others.

Natural occurrence

Phenethylamine is produced by a wide range of species throughout the plant and animal kingdoms, including humans;^[14][18] it is also produced by certain fungi and bacteria (genera: Lactobacillus, Clostridium, Pseudomonas and the family Enterobacteriaceae) and acts as a potent antimicrobial against certain pathogenic strains of Escherichia coli (e.g., the O157:H7 strain) at sufficient concentrations.^[19]

Chemistry

PEA powder and crystals

Phenethylamine is a primary amine, the amino-group being attached to a benzene ring through a two-carbon, or ethyl group.^[10] It is a colourless liquid at room temperature that has a fishy odor, and is soluble in water, ethanol and ether.^[10] Its density is 0.964 g/ml and its boiling point is 195 °C.^[10] Upon exposure to air, it combines with carbon dioxide to form a solid carbonate salt.^[20] Phenethylamine is strongly basic, pK_b = 4.17 (or pK_a = 9.83), as measured using the HCl salt, and forms a stable crystalline hydrochloride salt with a melting point of 217 °C.^[10][21] Its experimental log P is 1.41.^[10]

Substituted derivatives

Main article: Substituted phenethylamine

Substituted phenethylamines are a chemical class of organic compounds based upon the phenethylamine structure;^{[note 2]} the class is composed of all the derivative compounds of phenethylamine which can be formed by replacing, or substituting, one or more hydrogen atoms in the phenethylamine core structure with substituents.

Many substituted phenethylamines are psychoactive drugs, which belong to a variety of different drug classes, including central nervous system stimulants (e.g., amphetamine), hallucinogens (e.g., 2,5-dimethoxy-4-methylamphetamine), entactogens (e.g., 3,4-methylenedioxyamphetamine), appetite suppressants (e.g. phentermine), nasal decongestants and bronchodilators (e.g., pseudoephedrine), antidepressants (e.g. bupropion), antiparkinson agents (e.g., selegiline), and vasopressors (e.g., ephedrine), among others. Many of these psychoactive compounds exert their pharmacological effects primarily by modulating monoamine neurotransmitter systems; however, there is no mechanism of action or biological target that is common to all members of this subclass.

Numerous endogenous compounds – including hormones, monoamine neurotransmitters, and many trace amines (e.g., dopamine, norepinephrine, adrenaline, tyramine, and others) – are substituted phenethylamines. Dopamine is simply phenethylamine with a hydroxyl group attached to the 3 and 4 position of the benzene ring. Several notable recreational drugs, such as MDMA (ecstasy), methamphetamine, and cathinones, are also members of the class. All of the substituted amphetamines are phenethylamines, as well.

Pharmaceutical drugs that are substituted phenethylamines include phenelzine, phenformin, and fanetizole, among many others.

The N-methylated derivative of phenethylamine is N-methylphenethylamine.

Synthesis

One method for preparing β-phenethylamine, set forth in J. C. Robinson and H. R. Snyder's Organic Syntheses (published 1955), involves the reduction of benzyl cyanide with hydrogen in liquid ammonia, in the presence of a Raney-Nickel catalyst, at a temperature of 130 °C and a pressure of 13.8 MPa. Alternative syntheses are outlined in the footnotes to this preparation.^[22]

A much more convenient method for the synthesis of β-phenethylamine is the reduction of ω-nitrostyrene by lithium aluminium hydride in ether, whose successful execution was first reported by R. F. Nystrom and W. G. Brown in 1948.^[23]

Phenethylamine can also be produced via the cathodic reduction of benzyl cyanide in a divided cell.^[24]

Electrosynthesis of phenethylamine from benzyl cyanide^[24]

Assembling phenethylamine structures for synthesis of compounds such as epinephrine, amphetamines, tyrosine, and dopamine by adding the beta-aminoethyl side chain to the phenyl ring is possible. This can be done via Friedel-Crafts acylation with N-protected acyl chlorides when the arene is activated, or by Heck reaction of the phenyl with N-vinyloxazolone, followed by hydrogenation, or by cross-coupling with beta-amino organozinc reagents, or reacting a brominated arene with beta-aminoethyl organolithium reagents, or by Suzuki cross-coupling.^[25]

Detection in body fluids

Reviews that cover attention deficit hyperactivity disorder (ADHD) and phenethylamine indicate that several studies have found abnormally low urinary phenethylamine concentrations in ADHD individuals when compared with controls.^[26] In treatment-responsive individuals, amphetamine and methylphenidate greatly increase urinary phenethylamine concentration.^[26] An ADHD biomarker review also indicated that urinary phenethylamine levels could be a diagnostic biomarker for ADHD.^[26]

Thirty minutes of moderate- to high-intensity physical exercise has been shown to induce an increase in urinary phenylacetic acid, the primary metabolite of phenethylamine.^[3][27][28] Two reviews noted a study where the mean 24 hour urinary phenylacetic acid concentration following just 30 minutes of intense exercise rose 77% above its base level;^[3][27][28] the reviews suggest that phenethylamine synthesis sharply increases during physical exercise during which it is rapidly metabolized due to its short half-life of roughly 30 seconds.^{[3][27][28][4]} In a resting state, phenethylamine is synthesized in catecholamine neurons from L-phenylalanine by aromatic amino acid decarboxylase at approximately the same rate as dopamine is produced.^[4] Monoamine oxidase deaminates primary and secondary amines that are free in the neuronal cytoplasm but not those bound in storage vesicles of the sympathetic neurone. Similarly, β-PEA would not be completely deaminated in the gut as it is a selective substrate for MAO-B, which is not primarily found in the gut. Brain levels of endogenous trace amines are several hundred-fold below those for the classical neurotransmitters noradrenaline, dopamine, and serotonin, but their rates of synthesis are equivalent to those of noradrenaline and dopamine and they have a very rapid turnover rate.^[14] Endogenous extracellular tissue levels of trace amines measured in the brain are in the low nanomolar range. These low concentrations arise because of their very short half-life. Because of the pharmacological relationship between phenethylamine and amphetamine, the original paper and both reviews suggest that phenethylamine plays a prominent role in mediating the mood-enhancing euphoric effects of a runner's high, as both phenethylamine and amphetamine are potent euphoriants.^[3][27][28]

Skydiving has also been shown to induce a marked increase in urinary phenethylamine concentrations.^[10][29]

Biological activity

See also: Neurobiological effects of physical exercise § β-Phenylethylamine

Monoamine release of phenethylamine and related agents (EC₅₀Tooltip Half maximal effective concentration, nM)

Compound	NETooltip Norepinephrine	DATooltip Dopamine	5-HTTooltip Serotonin	Ref
Phenethylamine	10.9	39.5	>10,000	[30][31][32]
Tyramine	40.6	119	2,775	[33][32]
Tryptamine	716	164	32.6	[34][35]
Dextroamphetamine	6.6–7.2	5.8–24.8	698–1,765	[33][36]
Levoamphetamine	9.5	27.7	ND	[31][32]
Dextromethamphetamine	12.3–13.8	8.5–24.5	736–1,292	[33][37]
Levomethamphetamine	28.5	416	4,640	[33]
Notes: The smaller the value, the more strongly the drug releases the neurotransmitter. See also Monoamine releasing agent § Activity profiles for a larger table with more compounds. Refs: [38][39]

Phenethylamine pharmacodynamics in a TAAR1–dopamine neuron v t e via AADC Both amphetamine and phenethylamine induce neurotransmitter release from VMAT2[11][40][41] and bind to TAAR1.[12][42] When either binds to TAAR1, it reduces neuron firing rate and triggers protein kinase A (PKA) and protein kinase C (PKC) signaling, resulting in DAT phosphorylation.[12][42] Phosphorylated DAT then either operates in reverse or withdraws into the axon terminal and ceases transport.[12][42]

Monoamine releasing agent

Phenethylamine, being similar to amphetamine in its action at their common biomolecular targets, is a releasing agent of norepinephrine and dopamine.^[11][12][42] It is roughly equipotent to amphetamine in this regard in vitro.^[32] Phenethylamine is inactive as a psychostimulant under normal circumstances due to rapid metabolism by monoamine oxidase (MAO), but can become active in the presence of a monoamine oxidase inhibitor (MAOI).^[32]

TAAR1 agonist

Phenethylamine has been shown to bind to human trace amine-associated receptor 1 (hTAAR1) as an agonist.^[2] β-PEA is also an odorant binding TAAR4 in mice thought to mediate predator avoidance.^[43]

Monoaminergic activity enhancer

Phenethylamine is a monoaminergic activity enhancer (MAE) of serotonin, norepinephrine, and dopamine in addition to its catecholamine-releasing activity.^[44][45][46] That is, it enhances the action potential-mediated release of these monoamine neurotransmitters.^[44][45][46] The compound is active as a MAE at much lower concentrations than the concentrations at which it induces the release of catecholamines.^[44][45][46] The MAE actions of phenethylamine and other MAEs may be mediated by TAAR1 agonism.^[47][48] Synthetic and more potent MAEs like phenylpropylaminopentane (PPAP) and selegiline (L-deprenyl) have been derived from phenethylamine.^[44][45]

Other activities

Unlike its derivatives norepinephrine (adrenaline) and epinephrine (adrenaline), phenethylamine is inactive as an agonist of the α- and β-adrenergic receptors.^[49]

Effects in animals and humans

According to Alexander Shulgin in PiHKAL, phenethylamine is completely inactive in humans at doses of up to 1,600 mg orally and 50 mg intravenously.^[50] This can be attributed to its extremely rapid metabolic breakdown rather than pharmacodynamic inactivity.^[50]

Although exogenous phenethylamine on its own is inactive, its metabolism can be strongly inhibited and it can thereby become active, showing psychostimulant effects, when combined with a monoamine oxidase inhibitor (MAOI), specifically monoamine oxidase B (MAO-B) inhibitors like selegiline.^[51][52] Oral L-phenylalanine (a precursor of phenethylamine) and/or phenethylamine itself in combination with selegiline has been studied in the treatment of depression and has been reported to be effective.^{[44][53][54][55][56]} Misuse of phenethylamine in combination with selegiline has also been reported.^[57][58]

The LD₅₀Tooltip median lethal dose values of phenethylamine include 175 mg/kg i.p. in mice, 320 mg/kg s.c. in mice, 100 mg/kg i.v. in mice, 100 mg/kg parenterally in mice, 39 mg/kg intracervically in mice, and 200 mg/kg i.p. in guinea pigs.^[10] Its LD_Lo values include 800 mg/kg p.o. in rats, 100 mg/kg i.p. in rats, 450 μg/kg s.c. in rats, and 300 mg/kg via an unspecified route in mice.^[10]

Pharmacokinetics

Biosynthetic pathways for catecholamines and trace amines in the human brain[59][60][61] L-Phenylalanine L-Tyrosine L-DOPA Epinephrine Phenethylamine p-Tyramine Dopamine Norepinephrine N-Methylphenethylamine N-Methyltyramine p-Octopamine Synephrine 3-Methoxytyramine AADC AADC AADC primary pathway PNMT PNMT PNMT PNMT AAAH AAAH brain CYP2D6 minor pathway COMT DBH DBH In humans, catecholamines and phenethylaminergic trace amines are derived from the amino acid L-phenylalanine.

By oral route, phenethylamine's half-life is 5–10 minutes;^[10] endogenously produced PEA in catecholamine neurons has a half-life of roughly 30 seconds.^[3] In humans, PEA is metabolized by phenylethanolamine N-methyltransferase (PNMT),^{[3][4][5][62]} monoamine oxidase A (MAO-A),^[5][6] monoamine oxidase B (MAO-B),^{[3][4][5][15]} the semicarbazide-sensitive amine oxidases (SSAOs) AOC2 and AOC3,^[5][7] flavin-containing monooxygenase 3 (FMO3),^[8][9] and aralkylamine N-acetyltransferase (AANAT).^[5][63] N-Methylphenethylamine, an isomer of amphetamine, is produced in humans via the metabolism of phenethylamine by PNMT.^[3][4][62] β-Phenylacetic acid is the primary urinary metabolite of phenethylamine and is produced via monoamine oxidase metabolism and subsequent aldehyde dehydrogenase metabolism.^[5] Phenylacetaldehyde is the intermediate product which is produced by monoamine oxidase and then further metabolized into β-phenylacetic acid by aldehyde dehydrogenase.^[5][64]

When the initial phenylethylamine concentration in the brain is low, brain levels can be increased 1000-fold when taking a monoamine oxidase inhibitor (MAOI), particularly a MAO-B inhibitor, and by 3–4 times when the initial concentration is high.^[65]

Notes

1. Synonyms and alternate spellings include: phenylethylamine, β-phenylethylamine (β-PEA), 2-phenylethylamine, 1-amino-2-phenylethane, and 2-phenylethan-1-amine.
2. In other words, all of the compounds that belong to this class are structural analogs of phenethylamine.

References

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8. Krueger SK, Williams DE (June 2005). "Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism". Pharmacology & Therapeutics. 106 (3): 357–387. doi:10.1016/j.pharmthera.2005.01.001. PMC 1828602. PMID 15922018. The biogenic amines, phenethylamine and tyramine, are N-oxygenated by FMO to produce the N-hydroxy metabolite, followed by a rapid second oxygenation to produce the trans-oximes (Lin & Cashman, 1997a, 1997b). This stereoselective N-oxygenation to the trans-oxime is also seen in the FMO-dependent N-oxygenation of amphetamine (Cashman et al., 1999) ... Interestingly, FMO2, which very efficiently N-oxygenates N-dodecylamine, is a poor catalyst of phenethylamine N-oxygenation. The most efficient human FMO in phenethylamine N-oxygenation is FMO3, the major FMO present in adult human liver; the Km is between 90 and 200 μM (Lin & Cashman, 1997b).
9. Robinson-Cohen C, Newitt R, Shen DD, Rettie AE, Kestenbaum BR, Himmelfarb J, et al. (August 2016). "Association of FMO3 Variants and Trimethylamine N-Oxide Concentration, Disease Progression, and Mortality in CKD Patients". PLOS ONE. 11 (8): e0161074. Bibcode:2016PLoSO..1161074R. doi:10.1371/journal.pone.0161074. PMC 4981377. PMID 27513517. TMAO is generated from trimethylamine (TMA) via metabolism by hepatic flavin-containing monooxygenase isoform 3 (FMO3). ... FMO3 catalyzes the oxidation of catecholamine or catecholamine-releasing vasopressors, including tyramine, phenylethylamine, adrenaline, and noradrenaline [32, 33].
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20. O'Neil MJ, ed. (2001). The Merck Index – An Encyclopedia of Chemicals, Drugs, and Biologicals (13th ed.). Whitehouse Station, NJ: Merck and Co., Inc. p. 1296.
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23. Nystrom RF, Brown WG (November 1948). "Reduction of organic compounds by lithium aluminum hydride; halides, quinones, miscellaneous nitrogen compounds". Journal of the American Chemical Society. 70 (11): 3738–3740. doi:10.1021/ja01191a057. PMID 18102934.
24. Krishnan V, Muthukumaran A, Udupa HV (1979). "The electroreduction of benzyl cyanide on iron and cobalt cathodes". Journal of Applied Electrochemistry. 9 (5): 657–659. doi:10.1007/BF00610957. S2CID 96102382.
25. Molander GA, Vargas F (January 2007). "Beta-aminoethyltrifluoroborates: efficient aminoethylations via Suzuki-Miyaura cross-coupling". Organic Letters. 9 (2): 203–206. doi:10.1021/ol062610v. PMC 2593899. PMID 17217265.
26. Scassellati C, Bonvicini C, Faraone SV, Gennarelli M (October 2012). "Biomarkers and attention-deficit/hyperactivity disorder: a systematic review and meta-analyses". Journal of the American Academy of Child and Adolescent Psychiatry. 51 (10): 1003–1019.e20. doi:10.1016/j.jaac.2012.08.015. PMID 23021477. Although we did not find a sufficient number of studies suitable for a meta-analysis of PEA and ADHD, three studies^20,57,58 confirmed that urinary levels of PEA were significantly lower in patients with ADHD compared with controls. ... Administration of D-amphetamine and methylphenidate resulted in a markedly increased urinary excretion of PEA,^20,60 suggesting that ADHD treatments normalize PEA levels. ... Similarly, urinary biogenic trace amine PEA levels could be a biomarker for the diagnosis of ADHD,^20,57,58 for treatment efficacy,^20,60 and associated with symptoms of inattentivenesss.⁵⁹ ... With regard to zinc supplementation, a placebo controlled trial reported that doses up to 30 mg/day of zinc were safe for at least 8 weeks, but the clinical effect was equivocal except for the finding of a 37% reduction in amphetamine optimal dose with 30 mg per day of zinc.¹¹⁰
27. Szabo A, Billett E, Turner J (October 2001). "Phenylethylamine, a possible link to the antidepressant effects of exercise?". British Journal of Sports Medicine. 35 (5): 342–343. doi:10.1136/bjsm.35.5.342. PMC 1724404. PMID 11579070. The 24 hour mean urinary concentration of phenylacetic acid was increased by 77% after exercise. ... These results show substantial increases in urinary phenylacetic acid levels 24 hours after moderate to high intensity aerobic exercise. As phenylacetic acid reflects phenylethylamine levels³, and the latter has antidepressant effects, the antidepressant effects of exercise appear to be linked to increased phenylethylamine concentrations. Furthermore, considering the structural and pharmacological analogy between amphetamines and phenylethylamine, it is conceivable that phenylethylamine plays a role in the commonly reported "runners high" thought to be linked to cerebral β-endorphin activity. The substantial increase in phenylacetic acid excretion in this study implies that phenylethylamine levels are affected by exercise. ... A 30 minute bout of moderate to high intensity aerobic exercise increases phenylacetic acid levels in healthy regularly exercising men. The findings may be linked to the antidepressant effects of exercise.
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External links

• Phenethylamine MS Spectrum
• PEA (phenethylamine) - isomer design (PiHKAL entry)