Chlorphenamine

Chlorphenamine (INN) or chlorpheniramine (USAN, former BAN), commonly marketed in the form of chlorpheniramine maleate (Chlorphen-12 ), is a first-generation alkylamine antihistamine used in the prevention of the symptoms of allergic conditions such as rhinitis and urticaria. Its sedative effects are relatively weak compared to other first-generation antihistamines. Chlorphenamine is one of the most commonly used antihistamines in small-animal veterinary practice as well. Although not generally approved as an antidepressant or anti-anxiety medication, chlorphenamine appears to have these properties as well (see below).

Chlorphenamine is part of a series of antihistamines including pheniramine (Naphcon) and its halogenated derivatives and others including fluorpheniramine, dexchlorpheniramine (Polaramine), brompheniramine (Dimetapp), dexbrompheniramine (Drixoral), deschlorpheniramine, dipheniramine (also known as triprolidine with the trade name Actifed), and iodopheniramine.

The halogenated alkylamine antihistamines all exhibit optical isomerism, and chlorphenamine in the indicated products is racemic chlorphenamine maleate, whereas dexchlorpheniramine is the dextrorotary stereoisomer.

Serotonergic and norepinephrinergic effects
In addition to being an histamine H1 receptor antagonist, chlorphenamine has been shown to work as a serotonin-norepinephrine reuptake inhibitor or SNRI. A similar antihistamine, brompheniramine, led to the discovery of the SSRI zimelidine. Limited clinical evidence shows that it is comparable to several antidepressant medications in its ability to inhibit the reuptake of serotonin and also norepinephrine (noradrenaline). However, extensive clinical trials of its psychiatric properties in humans have not been conducted. It inhibits serotonin reuptake less than norepinephrine reuptake, however the literature is not consistent in this respect.

Combination medications
Chlorphenamine is often combined with phenylpropanolamine to form an allergy medication with both antihistamine and decongestant properties. Brand names include Demazin, Allerest 12 Hour, Codral Nighttime, Chlornade, Contac 12 Hour, A. R. M. Allergy Relief, Ordrine, Ornade Spansules, Teldrin, Triaminic, and Tylenol Cold/Allergy.

Chlorphenamine is combined with a narcotic (hydrocodone) in the product Tussionex, which is indicated for treatment of cough and upper respiratory symptoms associated with allergy or cold in adults and children 6 years of age and older. This combination is manufactured as a time-released formula, which allows for administration every 12 hours, versus the more common 4-to-6-hour regimen for other narcotic cough suppressants.

Chlorphenamine/dihydrocodeine immediate-release syrups are also marketed. The antihistamine is helpful in cases where allergy or common cold is the reason for the cough; it is also a potentiator of opioids, allowing enhanced suppression of cough, analgesia, and other effects from a given quantity of the drug by itself. In various places in the world, cough & cold preparations containing codeine and chlorphenamine are available.

In the drug Coricidin, chlorphenamine is combined with the cough suppressant dextromethorphan.

Chemistry
Chlorphenamine, 3-(p-chlorophenyl)-3-(2-pyridyl)propyldimethylamine, is synthesized in two ways. The first is from 4-chlorbenzylcyanide, which is reacted with 2-chloropyridine in the presence of sodium amide to form 4-chlorophenyl(2-pyridyl)acetonitrile. Alkylating this with 2-dimethylaminoethylchloride in the presence of sodium amide gives γ-(4-chlorphenyl)-γ-cyano-N,N-dimethyl-2-pyridinepropanamine, the hydrolysis and decarboxylation of which lead to chlorphenamine. The second way is from pyridine, which undergoes alkylation by 4-chlorobenzylchloride, giving 2-(4-chlorobenzyl)pyridine. Alkylating this with 2-dimethylaminoethylchloride in the presence of sodium amide gives chlorphenamine.
 * D. Papa, E. Schwenk, N. Sperber, (1951).
 * D. Papa, E. Schwenk, N. Sperber, (1954).