|Jmol-3D images||Image 1|
|Molar mass||137.18 g/mol|
|Appearance||pale yellow solid|
157-160°C at 17 mm Hg pressure
|Solubility in water||soluble|
| (what is: / ?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Phenylethanolamine (sometimes abbreviated PEOH), or β-hydroxyphenethylamine, is a biogenic amine related structurally to the major neurotransmitter norepinephrine, and the biogenic amine octopamine. As an organic compound, phenylethanolamine is a β-hydroxylated phenethylamine that is also structurally related to a number of synthetic drugs such as phenylpropanolamine, and the ephedrine family of alkaloids/drugs. In common with these compounds, phenylethanolamine has strong cardiovascular activity and, under the name Apophedrin, has been used as a drug to produce topical vasoconstriction.
In appearance, phenylethanolamine is a colorless solid.
Phenylethanolamine is perhaps best known in the field of bioscience as part of the enzyme name "phenylethanolamine N-methyl transferase", referring to an enzyme which is responsible for the conversion of norepinephrine into epinephrine, as well as other related transformations.
The pKa of phenylethanolamine hydrochloride, at 25°C and at a concentration of 10mM, has been recorded as 8.90.
The presence of the hydroxy-group on the benzylic C of the phenylethanolamine molecule creates a chiral center, so the compound exists in the form of two enantiomers, d- and l-phenylethanolamine, or as the racemic mixture, d,l-phenylethanolamine. The dextrorotatory isomer corresponds to the S-configuration, and the levorotatory isomer to the R-configuration The data given in the Chembox, at right, is for the racemate.
The synthesis of S-(+)-phenylethanolamine, from (+)-mandelic acid, via (+)-mandelamide, has been described. The physical constants reported in this paper are as follows: m.p. 55-57°C; [α] = + 47.9° (c 2.4, in ethanol).
Early, classical pharmacological studies of phenylethanolamine were carried out by Tainter, who observed its effects after administering it to rabbits, cats and dogs. The drug produced a rapid rise in blood pressure when administered i.v., but had little or no effect when given by any other route: doses as high as 200 mg given s.c. to rabbits did not alter blood pressure, nor were there any effects when the drug was intubated into the stomach.
In man, a total oral dose of 1 g also produced no effects.
Doses of 1–5 mg/kg, i.v., caused no definite changes in respiration in cats or rabbits, and additional experiments showed that phenylethanolamine had no broncho-dilatory properties in animals. There was a similar lack of effect when the drug was given s.c. to man.
In vivo and in vitro experiments involving cat and rabbit intestinal smooth muscle showed that the drug produced relaxation and inhibition.
Shannon and co-workers confirmed and extended some of Tainter's studies. After administering phenylethanolamine to dogs intravenously, these investigators observed that 10–30 mg/kg of the drug increased pupil diameter, and decreased body temperature; a dose of 10 or 17.5 mg/kg decreased heart rate, but a 30 mg/kg dose caused it to increase. Other effects that were noted included profuse salivation and piloerection. Phenylethanolamine also produced behavioral effects such as stereotyped head movement, rapid eye movement, and repetitive tongue extrusion. These and other observations were suggested to be consistent with an action on α- and β-adrenergic receptors.
Research by Carpéné and co-workers showed that phenylethanolamine did not significantly stimulate lipolysis in cultured adipocytes ("fat cells") from guinea pig or human. Moderate stimulation (intrinsic activities about half that of the reference standard, isoprenaline) was observed in adipocytes from rat or hamster. This lipolysis was inhibited completely by bupranolol (considered to be a non-selective β-blocker), CGP 20712A (considered to be a selective β1-antagonist), and ICI 118,551 (considered to be a selective β2-antagonist), but not by SR 59230A (considered to be a selective β3-antagonist).
Using a β2 adrenergic receptor preparation derived from transfected HEK 293 cells, Liappakis and co-workers found that in wild-type receptors, racemic phenylethanolamine had ~ 1/400 x the affinity of epinephrine, and ~ 1/7 x the affinity of norepinephrine in competition experiments with 3[H]-CGP-12177.
The two enantiomers of phenylethanolamine were studied for their interaction with the human trace amine receptor (TAAR1) by a research group at Eli Lilly. From experiments with human TAAR1 expressed in rGαsAV12-664 cells, Wainscott and co-workers observed that R-(-)-phenylethanolamine (referred to as "R-(-)-β-hydroxy-β-phenylethylamine") had an ED50 of ~1800 nM, with an Emax of ~ 110%, whereas S-(+)-phenylethanolamine (referred to as "S-(+)-β-hydroxy-β-phenylethylamine") had an ED50 of ~1720 nM, with an Emax of ~ 105%. In comparison, β-phenethylamine itself had an ED50 of ~106 nM, with an Emax of ~ 100%.
The pharmacokinetics of phenylethanolamine, after i.v. administration to dogs, were studied by Shannon and co-workers, who found that the drug followed the "two-compartment model", with T1/2(α) ≃ 6.8 mins and T1/2(β) ≃ 34.2 mins; the "plasma half-life" of phenylethanolamine was therefore about 30 minutes.
Phenylethanolamine was found to be an excellent substrate for the enzyme phenylethanolamine N-methyl transferase (PNMT), first isolated from monkey adrenal glands by Julius Axelrod, which transformed it into N-methylphenylethanolamine.
Subsequent studies by Rafferty and co-workers showed that substrate specificity of PNMT from bovine adrenal glands for the different enantiomers of phenylethanolamine was in the order R-(-)-PEOH > R,S-(racemic)-PEOH > S-(+)-PEOH.
The minimum lethal dose (m.l.d.) upon s.c. administration to guinea pigs was ~ 1000 mg/kg; the m.l.d. upon i.v. administration to rabbits was 25–30 mg/kg.; in rats, the m.l.d. after i.v. administration was 140 mg/kg.