|AHFS/Drugs.com||Multum Consumer Information|
|Elimination half-life||2.1–3.4 hours|
|Chemical and physical data|
|Molar mass||151.206 g/mol|
|3D model (JSmol)|
Phenylpropanolamine (PPA) is a sympathomimetic agent which is used as a decongestant and appetite suppressant. It is commonly used in prescription and over-the-counter cough and cold preparations. In veterinary medicine, it is used to control urinary incontinence in dogs.
PPA is also known as β-hydroxyamphetamine, and is a member of the phenethylamine and amphetamine chemical classes. It is closely related to the cathinones (β-ketoamphetamines). The compound exists as four stereoisomers, which include d- and l-norephedrine and d- and l-norpseudoephedrine. d-Norpseudoephedrine is also known as cathine, and is found naturally in Catha edulis (khat). Pharmaceutical drug preparations of PPA have varied in their stereoisomer composition in different countries, which may explain differences in misuse and side effect profiles. Analogues of PPA include ephedrine, pseudoephedrine, amphetamine, methamphetamine, and cathinone.
Although originally thought to act as a direct agonist of adrenergic receptors, PPA was subsequently found to show only weak or negligible affinity for these receptors, and has been instead characterized as an indirect sympathomimetic which acts by inducing norepinephrine release and thereby activating adrenergic receptors.
In the United States, PPA is no longer sold due to a purported increased risk of stroke in younger women. In a few countries in Europe, however, it is still available either by prescription or sometimes over-the-counter. In Canada, it was withdrawn from the market on 31 May 2001. In India human use of PPA and its formulations was banned on 10 February 2011, but the ban was overturned by the judiciary in September 2011.
PPA acts primarily as a selective norepinephrine releasing agent. It also acts as a dopamine releasing agent with around 10-fold lower potency. The stereoisomers of the drug have only weak or negligible affinity for α- and β-adrenergic receptors.
Many sympathetic hormones and neurotransmitters are based on the phenethylamine skeleton, and function generally in "fight or flight" type responses, such as increasing heart rate, blood pressure, dilating the pupils, increased energy, drying of mucous membranes, increased sweating, and a significant number of additional effects.
Activity profiles of isomers
There are four optical isomers of PPA: dextro- and levo-norephedrine, and dextro- and levo-norpseudoephedrine. d-Norpseudoephedrine is also known as cathine, and occurs naturally in Catha edulis ("Khat").
PPA, structurally, is in the substituted phenethylamine class, consisting of a cyclic benzene or phenyl group, a two carbon ethyl moiety, and a terminal nitrogen, hence the name phen-ethyl-amine. The methyl group on the alpha carbon (the first carbon before the nitrogen group) also makes this compound a member of the substituted amphetamine class. Ephedrine is the N-methyl analogue of PPA.
Exogenous compounds in this family are degraded too rapidly by monoamine oxidase to be active at all but the highest doses. However, the addition of the α-methyl group allows the compound to avoid metabolism and confer an effect. In general, N-methylation of primary amines increases their potency; whereas β-hydroxylation decreases CNS activity, but conveys more selectivity for adrenergic receptors.
Society and culture
In Sweden, PPA is still available in prescription decongestants such as Rinexin, as well as over-the-counter medications such as Wick DayMed.
In the United Kingdom, PPA was available in many 'all in one' cough and cold medications which usually also feature paracetamol or another analgesic and caffeine and could also be purchased on its own; however, it is no longer approved for human use. A European Category 1 Licence is required to purchase PPA for academic use.
In the United States, the Food and Drug Administration (FDA) issued a public health advisory against the use of the drug in November 2000. In this advisory, the FDA requested that all drug companies discontinue marketing products containing PPA. The agency estimates that PPA caused between 200 and 500 strokes per year among 18-to-49-year-old users. In 2005, the FDA removed PPA from over-the-counter sale. Because of its potential use in amphetamine manufacture, it is controlled by the Combat Methamphetamine Epidemic Act of 2005. It is still available for veterinary use in dogs, however, as a treatment for urinary incontinence.
Internationally, an item on the agenda of the 2000 Commission on Narcotic Drugs session called for including the stereoisomer norephedrine in Table I of United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances.
- 4-Hydroxyamphetamine has been shown to be metabolized into 4-hydroxynorephedrine by dopamine beta-hydroxylase (DBH) in vitro and it is presumed to be metabolized similarly in vivo. Evidence from studies that measured the effect of serum DBH concentrations on 4-hydroxyamphetamine metabolism in humans suggests that a different enzyme may mediate the conversion of 4-hydroxyamphetamine to 4-hydroxynorephedrine; however, other evidence from animal studies suggests that this reaction is catalyzed by DBH in synaptic vesicles within noradrenergic neurons in the brain.
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The simplest unsubstituted phenylisopropylamine, 1-phenyl-2-aminopropane, or amphetamine, serves as a common structural template for hallucinogens and psychostimulants. Amphetamine produces central stimulant, anorectic, and sympathomimetic actions, and it is the prototype member of this class (39). ... The phase 1 metabolism of amphetamine analogs is catalyzed by two systems: cytochrome P450 and flavin monooxygenase. ... Amphetamine can also undergo aromatic hydroxylation to p-hydroxyamphetamine. ... Subsequent oxidation at the benzylic position by DA β-hydroxylase affords p-hydroxynorephedrine. Alternatively, direct oxidation of amphetamine by DA β-hydroxylase can afford norephedrine.
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Dopamine-β-hydroxylase catalyzed the removal of the pro-R hydrogen atom and the production of 1-norephedrine, (2S,1R)-2-amino-1-hydroxyl-1-phenylpropane, from d-amphetamine.
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Table 5: N-containing drugs and xenobiotics oxygenated by FMO
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Hydroxyamphetamine was administered orally to five human subjects ... Since conversion of hydroxyamphetamine to hydroxynorephedrine occurs in vitro by the action of dopamine-β-oxidase, a simple method is suggested for measuring the activity of this enzyme and the effect of its inhibitors in man. ... The lack of effect of administration of neomycin to one patient indicates that the hydroxylation occurs in body tissues. ... a major portion of the β-hydroxylation of hydroxyamphetamine occurs in non-adrenal tissue. Unfortunately, at the present time one cannot be completely certain that the hydroxylation of hydroxyamphetamine in vivo is accomplished by the same enzyme which converts dopamine to noradrenaline.
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Figure 1. Glycine conjugation of benzoic acid. The glycine conjugation pathway consists of two steps. First benzoate is ligated to CoASH to form the high-energy benzoyl-CoA thioester. This reaction is catalyzed by the HXM-A and HXM-B medium-chain acid:CoA ligases and requires energy in the form of ATP. ... The benzoyl-CoA is then conjugated to glycine by GLYAT to form hippuric acid, releasing CoASH. In addition to the factors listed in the boxes, the levels of ATP, CoASH, and glycine may influence the overall rate of the glycine conjugation pathway.
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The biologic significance of the different levels of serum DβH activity was studied in two ways. First, in vivo ability to β-hydroxylate the synthetic substrate hydroxyamphetamine was compared in two subjects with low serum DβH activity and two subjects with average activity. ... In one study, hydroxyamphetamine (Paredrine), a synthetic substrate for DβH, was administered to subjects with either low or average levels of serum DβH activity. The percent of the drug hydroxylated to hydroxynorephedrine was comparable in all subjects (6.5-9.62) (Table 3).
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In species where aromatic hydroxylation of amphetamine is the major metabolic pathway, p-hydroxyamphetamine (POH) and p-hydroxynorephedrine (PHN) may contribute to the pharmacological profile of the parent drug. ... The location of the p-hydroxylation and β-hydroxylation reactions is important in species where aromatic hydroxylation of amphetamine is the predominant pathway of metabolism. Following systemic administration of amphetamine to rats, POH has been found in urine and in plasma.
The observed lack of a significant accumulation of PHN in brain following the intraventricular administration of (+)-amphetamine and the formation of appreciable amounts of PHN from (+)-POH in brain tissue in vivo supports the view that the aromatic hydroxylation of amphetamine following its systemic administration occurs predominantly in the periphery, and that POH is then transported through the blood-brain barrier, taken up by noradrenergic neurones in brain where (+)-POH is converted in the storage vesicles by dopamine β-hydroxylase to PHN.
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The metabolism of p-OHA to p-OHNor is well documented and dopamine-β hydroxylase present in noradrenergic neurons could easily convert p-OHA to p-OHNor after intraventricular administration.
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CHEMISTRY AND STRUCTURE-ACTIVITY RELATIONSHIP OF SYMPATHOMIMETIC AMINES
β-Phenylethylamine (Table 12–1) can be viewed as the parent compound of the sympathomimetic amines, consisting of a benzene ring and an ethylamine side chain. The structure permits substitutions to be made on the aromatic ring, the α- and β-carbon atoms, and the terminal amino group to yield a variety of compounds with sympathomimetic activity. ...N-methylation increases the potency of primary amines ...
Substitution on the α-Carbon Atom
This substitution blocks oxidation by MAO, greatly prolonging the duration of action of non-catecholamines because their degradation depends largely on the action of this enzyme. The duration of action of drugs such as ephedrine or amphetamine is thus measured in hours rather than in minutes. Similarly, compounds with an α-methyl substituent persist in the nerve terminals and are more likely to release NE from storage sites. Agents such as metaraminol exhibit a greater degree of indirect sympathomimetic activity.
Substitution on the β-Carbon Atom
Substitution of a hydroxyl group on the β carbon generally decreases actions within the CNS, largely because it lowers lipid solubility. However, such substitution greatly enhances agonist activity at both α- and β- adrenergic receptors. Although ephedrine is less potent than methamphetamine as a central stimulant, it is more powerful in dilating bronchioles and increasing blood pressure and heart rate.
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