Deoxyepinephrine, also known by the common names N-methyldopamine and epinine, is an organic compound and natural product that is structurally related to the important neurotransmittersdopamine and epinephrine. All three of these compounds also belong to the catecholamine family. The pharmacology of epinine largely resembles that of its "parent", dopamine. Epinine has been found in plants, insects and animals. It is also of significance as the active metabolic breakdown product of the prodrugibopamine, which has been used to treat congestive heart failure.[2][3]
The first total synthesis of epinine was reported by Buck, who prepared it from 3,4-dimethoxyphenethylamine ("homoveratrylamine") by first converting the latter to its Schiff base with benzaldehyde, then N-methylating this with methyl iodide; hydrolysis of the resulting product was followed by cleavage of the methyl ethers using hydriodic acid to furnish epinine.[10] A very similar synthesis, differing only in the use of dimethyl sulfate for the N-methylation, and HBr for the O-demethylation, but providing more extensive experimental details, was published by Borgman in 1973.[11]
An earlier semi-synthesis (so-called because it began with the natural product laudanosine) due to Pyman[1] is incorrectly cited by Buck,[10] and the error carried over to the entry for epinine (under the name deoxyepinephrine) in the Merck Index.[12]
Common salts of epinine are: hydrochloride, C9H13NO2.HCl, m.p. 179-180 °C; sulfate, (C9H13NO2)2.H2SO4, m.p. 289-290 °C;[1] hydrobromide, C9H13NO2.HBr, m.p. 165-166 °C.[11]
Structure
The X-ray structure of epinine hydrobromide has been reported.[13]
Pharmacology
One of the most prominent pharmacological characteristics of epinine, its ability to raise blood pressure, was noted as early as 1910, by Barger and Dale, who reported that "methylamino-ethyl-catechol", as they called it, had about 1/7 x the pressor potency of epinephrine, but about 5 x the potency of dopamine ("amino-ethyl-catechol") in cat preparations.[14] The Buroughs Wellcome Co., for which Barger, Dale and Pyman (see "Chemistry" section) worked, subsequently marketed the hydrochloride salt of "methylamino-ethyl-catechol", under the name "epinine", as a substitute for epinephrine.[15] Tainter further quantified the pressor activity of epinine in atropine-treated and anesthetized intact cats, showing that doses of 0.02-0.2 mg, given i.v., were about 1/12 as active as l-epinephrine, but that the effect lasted about twice as long (~ 3 minutes), and was accompanied by an increase in pulse rate.[15]
Eventually, epinine was determined to be a non-selective stimulant of dopamine (DA) receptors, α-, and β-adrenoceptors, with the stimulation of D2 receptors leading to inhibition of noradrenergic and ganglionic neurotransmission. These studies, conducted using anesthetized animals, were amplified by van Woerkens and co-workers, who compared the effects of epinine and dopamine in unanesthetized pigs, so as to avoid any possible influences of an anesthetic. Drug doses were in the range of 1-10 μg/kg/min, administered by i.v. infusion over a period of 10 minutes. The results of these experiments showed that, in pigs, over the dose-range employed, epinine was more potent than dopamine as an agonist on D2, α-, and β2-receptors, but was weaker than dopamine as a D1-agonist. The β1-agonist effect of both compounds was weak or non-existent.[16]
Comparable studies, in which blood pressure, heart rate and serum prolactin levels were measured after the administration of 0.5-4 μg/kg/min of epinine by i.v. infusion over a 15-minute period to healthy humans, were reported subsequently by Daul and co-workers.[17] These investigators found that at lower doses (0.5 or 1.0 μg/kg/min), which produced plasma concentrations of 20-80 nM/L, epinine, in common with dopamine, caused a fall in prolactin level, but did not affect blood pressure or heart rate. At higher doses (2.0 or 4.0 μg/kg/min), epinine significantly increased both systolic and diastolic blood pressure, as well as heart rate. In contrast, dopamine caused an increase in systolic blood pressure and heart rate only.
Both drugs increased diuresis and natriuresis - effects that are thought to be due to the activation of renal D1 receptors.
It was concluded that at the lower doses, epinine and dopamine exerted their effects only at DA (D2) receptors, but did not activate α- or β-adrenoceptors. At the higher doses, epinine activated α-, β1- and β2-receptors to about the same extent, whereas dopamine showed only a mild stimulation of β1-receptors, without any effects on α- or β2-receptors. Additionally, it was observed that the effects of epinine were largely due to its direct action on receptors, while dopamine also produced some of its effects indirectly, by stimulating norepinephrine release.
Toxicity
LD50 for HCl salt: 212 mg/kg (mouse; i.p.). For comparison, it might be noted that dopamine has a LD50 of 1978 mg/kg under the same conditions.[18]
^ abcF. L. Pyman (1910). "XXVIII. Isoquinoline derivatives. Part III. o-Dihydroxy-bases. The conversion of 1-keto-6,7-dimethoxy-2-methyltetrahydroisoquinolines into 3:4-dihydroxyphenylethylalkylamines." J. Chem. Soc., Trans.97 264-280.
^P. A. Zwieten (1994). "Pharmacotherapy of congestive heart failure." Pharmacy World & Science16 334 - 342.
^R. Gifford, W. C. Randolph, F. C. Heineman and J. A. Ziemniak (1986). "Analysis of epinine and its metabolites in man after oral administration of its pro-drug ibopamine using high-performance liquid chromatography with electrochemical detection." Journal of Chromatography B381 83-93. doi 10.1016/S0378-4347(00)83567-7
^J. Lundstrom (1971). "Biosynthesis of mescaline and tetrahydroisoquinoline alkaloids in Lophophora williamsii (Lem.) Coult. Occurrence and biosynthesis of catecholamine and other intermediates." Acta Chem. Scand.25 3489-3499. http://actachemscand.dk/pdf/acta_vol_25_p3489-3499.pdf
^B. A. Clement, C. M. Goff and T. D. A. Forbes (1998). "Toxic amines and alkaloids from Acacia rigidula." Phytochemistry49 1377-1380.
^T. A. Smith (1977). "Phenethylamine and related compounds in plants." Phytochemistry16 9-18.
^P. Laduron, P. van Gompel, J. Leysen and M. Claeys (1974). " In vivo formation of epinine in adrenal medulla. A possible step for adrenaline biosynthesis." Naunyn-Schmiedebergs Arch. Pharmacol.286 227-238.
^F. Märki, J. Axelrod and B. Witkop (1962). "Catecholamines and N-methyltransferase in the South American toad (Bufo marinus)." Biochim. Biophys. Acta58 367-369.
^S. Tanaka and N. Takeda (1997). "Biogenic monoamines in the brain and the corpus cardiacum between albino and normal strains of the migratory locust, Locusta migratoria." Comp. Biochem. Physiol. Pt. C: Comp. Pharmacol. Toxicol.117 221-227.
^ abJ. S. Buck (1930). "Synthesis of lodal and epinine." J. Am. Chem. Soc.52 4119-4122.
^ abR. Borgman et al. (1973). "Synthesis and pharmacology of centrally acting dopamine derivatives and analogs in relation to Parkinson's Disease." J. Med. Chem.16 630-633.
^J. Giesecke (1976). "The structure of the catecholamines. V. The crystal and molecular structure of epinine hydrobromide." Acta Crystallographica Section B32 2337-2340.
^G. Barger and H. H. Dale (1910)."Chemical structure and sympathomimetic action of amines." J. Physiol.41 19-59.
^ abM. L. Tainter (1930). "Comparative actions of sympathomimetic compounds: catechol derivatives." J. Pharmacol. Exp. Ther.40 43-64.
^L. J. van Woerkens, F. Boomsma, A. J. Man in 't Veld, M. M. Bevers, P. D. Verdouw (1992). "Differential cardiovascular and neuroendocrine effects of epinine and dopamine in conscious pigs before and after adrenoceptor blockade." Br. J. Pharmacol.107 303–310.
^A. Daul et al. (1995). "Dose-dependent separation of dopaminergic and adrenergic effects of epinine in healthy volunteers." Naunyn-Schmiedebergs Arch. Pharmacol.352 429-437
^J. Z. Ginos et al. (1975). "Cholinergic effects of molecular segments of apomorphine and dopaminergic effects of N,N-dialkylated dopamines." 18 1194-1200.
