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Chlorphenamine
Clinical data
Trade namesChlor-Trimeton; Piriton; Chlor-Tripolon; Allergex
AHFS/Drugs.comMonograph
MedlinePlusa682543
Pregnancy
category
  • AU: A
Routes of
administration		Oral, intravenous, intramuscular, subcutaneous
ATC code
Legal status
Legal status
Pharmacokinetic data
Bioavailability25 to 50%
Protein binding72%
MetabolismLiver (CYP2D6)
Elimination half-life13.9–43.4 hours[1]
ExcretionKidney
Identifiers
  • 3-(4-Chlorophenyl)-N,N-dimethyl-3-(pyridin-2-yl)-propan-1-amine
CAS Number
PubChem CID
IUPHAR/BPS
DrugBank
ChemSpider
UNII
KEGG
ChEBI
ChEMBL
CompTox Dashboard (EPA)
ECHA InfoCard100.004.596 Edit this at Wikidata
Chemical and physical data
FormulaC16H19ClN2
Molar mass274.79 g·mol−1
3D model (JSmol)
Solubility in water0.55 g/100 mL, liquid mg/mL (20 °C)
  • Clc1ccc(cc1)C(c2ncccc2)CCN(C)C
  • InChI=1S/C16H19ClN2/c1-19(2)12-10-15(16-5-3-4-11-18-16)13-6-8-14(17)9-7-13/h3-9,11,15H,10,12H2,1-2H3 checkY
  • Key:SOYKEARSMXGVTM-UHFFFAOYSA-N checkY
 ☒NcheckY (what is this?)  (verify)

Chlorphenamine (CP, CPM), also known as chlorpheniramine, is an antihistamine used to treat the symptoms of allergic conditions such as allergic rhinitis (hay fever).[2] It is taken orally (by mouth).[2] The medication takes effect within two hours and lasts for about 4–6 hours.[2] It is a first-generation antihistamine and works by blocking the histamine H1 receptor.[2]

Common side effects include sleepiness, restlessness, and weakness. Other side effects may include dry mouth and wheeziness.[2]

Chlorpheniramine was patented in 1948 and came into medical use in 1949.[3] It is available as a generic medication and over the counter.[2][4]

In 2023, it was the 318th most commonly prescribed medication in the United States, with more than 200,000 prescriptions.[5]

Medical uses

[edit]

Combination products

[edit]

Chlorphenamine is often combined with phenylpropanolamine to form an allergy medication with both antihistamine and decongestant properties, although phenylpropanolamine was removed from the U.S. market per studies concluding that it increased the risk of stroke in young women.[6] Vernate was a trade name of one such product available in the U.S. prior to the FDA ban; it was manufactured by Tutag and was among the medications prescribed to Elvis Presley.[7]

In the drug Coricidin, chlorphenamine is combined with the cough suppressant dextromethorphan. In the drug Cêgripe, chlorphenamine is combined with the analgesic paracetamol (also known as acetaminophen, sold as Tylenol).[8]

Side effects

[edit]

The adverse effects include drowsiness, dizziness, confusion, constipation, anxiety, nausea, blurred vision, restlessness, decreased coordination, dry mouth, shallow breathing, hallucinations, irritability, problems with memory or concentration, tinnitus and trouble urinating.[2]

Chlorphenamine produces less sedation than other first-generation antihistamines.[9]

A large study on people 65 years old or older linked the development of Alzheimer's disease and other forms of dementia to the "higher cumulative" use of chlorphenamine and other first-generation antihistamines, due to their anticholinergic properties.[10] Chlorphenamine is rated as a "high burden" anticholinergic by experts on a semi-subjective scale.[11] This is inconsistent with the in vitro experiments showing low affinity to muscarinic acetylcholine receptors (see below).

Pharmacology

[edit]

Pharmacodynamics

[edit]
Chlorphenamine[12]
Site Ki (nM) Species Ref
SERTTooltip Serotonin transporter 15.2 Human [13]
NETTooltip Norepinephrine transporter 1,440 Human [13]
DATTooltip Dopamine transporter 1,060 Human [13]
5-HT2A 3,130 Rat [14]
5-HT2C 3,120 Rat [15]
H1 2.5–3.0 Human [16][17]
H2 ND ND ND
H3 >10,000 Rat [18]
H4 2,910 Human [19]
M1 25,700 Human [20]
M2 17,000 Human [20]
M3 52,500 Human [20]
M4 77,600 Human [20]
M5 28,200 Human [20]
hERGTooltip Human Ether-à-go-go-Related Gene 20,900 Human [21]
Values are Ki, unless otherwise noted. The smaller the value, the more strongly the drug binds to the site. Values at the mAChRsTooltip muscarinic acetylcholine receptors and hERGTooltip Human Ether-à-go-go-Related Gene are IC50 (nM).

Chlorphenamine acts primarily as a potent H1 antihistamine. It is specifically a potent inverse agonist of the histamine H1 receptor.[22][23] The drug is also commonly described as possessing weak anticholinergic activity by acting as an antagonist of the muscarinic acetylcholine receptors. The dextrorotatory stereoisomer, dexchlorpheniramine, has been reported to possess Kd values of 15 nM for the H1 receptor and 1,300 nM for the muscarinic acetylcholine receptors in human brain tissue.[24][25] The smaller the Kd value, the greater the binding affinity of the ligand for its target.

In addition to acting as an inverse agonist at the H1 receptor, chlorphenamine has been found to act as a serotonin reuptake inhibitor (Kd = 15.2 nM for the serotonin transporter).[13][26] It has only weak affinity for the norepinephrine and dopamine transporters (Kd = 1,440 nM and 1,060 nM, respectively).[13]

A study found that dexchlorphenamine had Ki values for the human cloned H1 receptor of 2.67 to 4.81 nM while levchlorphenamine had Ki values of 211 to 361 nM for this receptor, indicating that dexchlorphenamine is the active enantiomer.[27] Another study found that dexchlorphenamine had a Ki value of 20 to 30 μM for the muscarinic acetylcholine receptor using rat brain tissue while levchlorphenamine had a Ki value of 40 to 50 μM for this receptor, indicating that both enantiomers have very low affinity for it.[28]

Pharmacokinetics

[edit]

The elimination half-life of chlorphenamine has variously ranged between 13.9 and 43.4 hours in adults following a single dose in clinical studies.[1]

Chemistry

[edit]

Chlorphenamine is an alkylamine and is a part of a series of antihistamines including pheniramine (Naphcon) and its halogenated derivatives including fluorpheniramine, dexchlorphenamine (Polaramine), brompheniramine (Dimetapp), dexbrompheniramine (Drixoral), deschlorpheniramine, and iodopheniramine. The halogenated alkylamine antihistamines all exhibit optical isomerism, and chlorphenamine in the indicated products is racemic chlorphenamine maleate, whereas dexchlorphenamine is the dextrorotary stereoisomer.

Synthesis

[edit]

There are several patented methods for the synthesis of chlorphenamine. In one example, 4-chlorophenylacetonitrile 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.

Chlorpheniramine synthesis[29]

A second method boom starts from pyridine, which undergoes alkylation by 4-chlorophenylacetonitrile,[30] giving 2-(4-chlorobenzyl)pyridine. Alkylating this with 2-dimethylaminoethylchloride in the presence of sodium amide gives chlorphenamine.

Chlorpheniramine synthesis[31]

Society and culture

[edit]

Names

[edit]

Chlorphenamine is the INNTooltip International Nonproprietary Name while chlorpheniramine is the USANTooltip United States Adopted Name and former BANTooltip British Approved Name.

Brand names include Chlor-Trimeton, Demazin, Allerest 12 Hour, Piriton, Chlorphen-12, Tylenol Cold/Allergy, and numerous others according to country.[2]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Chlorphenamine

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Chlorpheniramine, also known as chlorphenamine, is a first-generation antihistamine medication that acts as a histamine H1 receptor antagonist to relieve symptoms of allergic reactions, such as hay fever, rhinitis, urticaria, and the common cold.[1][2] It is chemically classified as a tertiary amine compound with the molecular formula C16H19ClN2 and a molecular weight of 274.79 g/mol, commonly available as chlorpheniramine maleate in oral tablets, liquids, or extended-release formulations.[1] Introduced in the United States in 1950, chlorpheniramine was developed as part of the early wave of H1-antihistamines derived from anticholinergic compounds, following pioneering research in the 1940s that identified histamine's role in allergies.[3] It functions by competitively binding to and blocking H1 receptors on effector cells, thereby preventing the actions of histamine that cause symptoms like itching, sneezing, runny nose, and watery eyes, without addressing the underlying allergic cause.[1][2] While effective for short-term relief, its sedative properties stem from crossing the blood-brain barrier, distinguishing it from later non-sedating second-generation antihistamines.[4] Commonly prescribed for upper respiratory allergies and occasionally for motion sickness or as an antitussive adjunct, chlorpheniramine is dosed every 4-6 hours for immediate-release forms (typically 4 mg per dose for adults) or twice daily for extended-release versions, with a maximum of 24 mg in 24 hours to avoid toxicity.[2][5] Side effects include drowsiness, dry mouth, dizziness, and nausea, with more serious risks such as urinary retention or confusion in the elderly, infants, or those with glaucoma, prostate issues, or respiratory conditions.[1][2] Precautions advise against use in children under 4 years, combining with alcohol or sedatives, and during pregnancy or breastfeeding unless benefits outweigh risks, as small doses (2-4 mg) appear compatible with lactation.[6][2]

Medical uses

Indications

Chlorpheniramine is primarily indicated for the treatment of allergic rhinitis, also known as hay fever, where it alleviates symptoms associated with seasonal or perennial allergies.[7] It is also approved for managing urticaria (hives), providing relief from acute skin reactions triggered by allergens.[8] Additionally, chlorpheniramine serves as adjunct therapy in angioedema and anaphylaxis, helping to mitigate histamine-mediated swelling and systemic allergic responses when used alongside epinephrine and other supportive measures.[8] The drug effectively relieves common symptoms of allergic conditions, including sneezing, runny nose, itchy or watery eyes, and itching of the nose or throat.[7] It is also used for the prevention and treatment of motion sickness.[9] Off-label, chlorpheniramine is commonly used for symptomatic relief of common cold and influenza symptoms, such as nasal congestion and rhinorrhea, though it does not treat the underlying viral infection. Evidence for the off-label use of chlorpheniramine in treating common cold symptoms is mixed. Randomized, double-blind, placebo-controlled studies have shown it to be effective in relieving symptoms such as sneezing and rhinorrhea. For example, a 1979 multicentric trial (Howard et al.) found chlorpheniramine maleate superior to placebo in lessening symptom severity, with significant differences from day 1 to day 7. A 1981 study (Crutcher et al.) reported significant relief when used four times daily for one week, with no notable increase in side effects. Some experimental rhinovirus studies also noted reductions in sneezing and symptom duration. However, a Cochrane systematic review (De Sutter et al., 2015) concluded that first-generation antihistamines like chlorpheniramine provide short-term beneficial effects on overall symptom severity in adults on days 1-2, but lack clinically significant impact on overall subjective improvement, nasal congestion, or other symptoms. Benefits appear more pronounced for sneezing and runny nose, but are modest and primarily symptomatic, with limited or no efficacy demonstrated in young children (e.g., Hutton et al., 1991). Chlorpheniramine does not shorten cold duration or treat the viral cause.[10] In veterinary medicine, it is applied off-label to manage allergic reactions in animals, including dogs, cats, and horses, for conditions like pruritus and acute inflammation from insect bites or environmental allergens.[11] Typical adult dosage is 4 mg orally every 4 to 6 hours, with a maximum of 24 mg per day for immediate-release formulations.[8] Safety and efficacy have not been established for children under 6 years. For children aged 6 to 12 years, the dose is 2 mg every 4 to 6 hours, not to exceed 12 mg per day.[12] Chlorpheniramine demonstrates efficacy with an onset of action within 1 to 2 hours after oral administration and a duration of effect lasting 4 to 6 hours for immediate-release forms.[13]

Combination therapies

Chlorpheniramine is frequently combined with decongestants such as pseudoephedrine or phenylephrine to provide relief from nasal congestion alongside its antihistaminic effects in treating allergic rhinitis and common cold symptoms.[14][15] These combinations allow for targeted management of multiple upper respiratory symptoms without solely relying on higher doses of the antihistamine.[16] It is also paired with antitussives like dextromethorphan for cough suppression or hydrocodone for more severe cases, enhancing control over cough and related respiratory irritation in cold and allergy presentations.[17][18] Combinations with analgesics such as acetaminophen address concurrent pain and fever, while additions of expectorants like guaifenesin aid in loosening mucus.[19][20] Notable over-the-counter products include Coricidin HBP Cough & Cold, which contains chlorpheniramine and dextromethorphan for cough and allergy relief, and formulations like acetaminophen-chlorpheniramine for pain and cold symptoms.[21][19] Prescription options, such as Tussionex Pennkinetic (hydrocodone polistirex and chlorpheniramine polistirex), are used for extended-release suppression of cough and upper respiratory symptoms.[22] Historically, chlorpheniramine was combined with phenylpropanolamine as a decongestant in allergy medications, but such products were banned in the United States in 2000 due to the increased risk of hemorrhagic stroke associated with phenylpropanolamine.[23] These multi-ingredient formulations offer broader symptom coverage for colds and allergies by synergistically targeting histamine-mediated responses, congestion, cough, and pain, thereby improving overall efficacy while maintaining standard antihistamine dosing.[16] However, combinations may elevate the risk of adverse interactions compared to monotherapy, necessitating careful patient selection.[24] Most such products are available over-the-counter, except those incorporating controlled substances like hydrocodone, which require a prescription.[22]

Adverse effects

Common effects

Chlorpheniramine, a first-generation antihistamine, commonly causes central nervous system effects at standard therapeutic doses, primarily due to its ability to cross the blood-brain barrier. The most frequent effect is sedation or drowsiness, reported as very common (affecting 10% or more of users), which can impair alertness and coordination.[25] Other CNS effects include dizziness, headache, and fatigue, occurring in 1% to 10% of users or with unreported frequency but noted as predictable adverse reactions.[25][2] Anticholinergic effects are also prevalent, stemming from the drug's blockade of muscarinic acetylcholine receptors. These include dry mouth (common, 1% to 10%), blurred vision, constipation, and urinary retention, which can exacerbate discomfort during treatment.[25][10] Additional common effects encompass gastrointestinal upset such as nausea (frequency not reported) and, less frequently, skin rash as a rare allergic response to the drug itself.[25][2] Elderly patients face a higher incidence of these effects, particularly anticholinergic ones such as dry mouth, constipation, urinary retention, cognitive impairment, drowsiness, and increased fall risk, due to age-related physiological changes and reduced clearance. The Beers Criteria explicitly lists chlorpheniramine as a drug to avoid in older adults because of these risks.[26][27] This leads to recommendations for cautious use or avoidance.[10][28] Regular use of first-generation antihistamines like chlorpheniramine has been associated with an increased dementia risk, with studies showing odds ratios of 1.5 to 2.0 for cumulative high exposure over years.[29][30] Management strategies include dose reduction to minimize sedation and anticholinergic burden, or switching to non-sedating second-generation antihistamines such as loratadine for better tolerability.[10][28]

Overdose

Chlorphenamine overdose, particularly with first-generation antihistamines like chlorphenamine, can lead to severe acute toxicity due to its anticholinergic and central nervous system (CNS) effects.[31] Symptoms typically manifest within 1-2 hours of ingestion and include severe CNS depression such as coma and seizures, anticholinergic delirium characterized by hallucinations and hyperthermia, tachycardia, hypertension, and potentially respiratory failure.[31] In children, these effects may occur at lower absolute doses due to smaller body weight, increasing vulnerability.[31] A toxic dose in adults is generally considered greater than 300 mg or exceeding 7.5 mg/kg body weight, though doses over 100 mg can cause serious symptoms; the estimated lethal dose is 25-50 mg/kg.[31][32] Pathophysiologically, this toxicity arises from excessive blockade of H1 histamine receptors and muscarinic acetylcholine receptors, resulting in unopposed sympathetic and anticholinergic effects that disrupt CNS, cardiovascular, and respiratory functions.[31] Management focuses on supportive care, including gastrointestinal decontamination with activated charcoal if ingestion was recent, and close monitoring of vital signs in an intensive care setting.[31] Benzodiazepines such as diazepam are used to control seizures, while physostigmine may be administered for severe anticholinergic delirium if other measures fail; sodium bicarbonate can address cardiac arrhythmias like prolonged QRS or QT intervals.[31][32] Prognosis is favorable with prompt intervention, as most patients recover fully, though fatalities can occur in severe cases or mixed overdoses involving other substances; survival beyond the first 24 hours significantly improves outcomes.[31][33]

Contraindications and interactions

Contraindications

Chlorpheniramine is contraindicated in patients with known hypersensitivity to chlorpheniramine maleate or any of its components, as it may provoke severe allergic reactions including anaphylaxis.[7] It is also absolutely contraindicated during acute asthma attacks, where its anticholinergic properties can thicken bronchial secretions and exacerbate airway obstruction.[7] Additionally, use is prohibited in individuals with narrow-angle glaucoma due to the risk of increased intraocular pressure from mydriasis induced by its anticholinergic effects.[7] Relative contraindications include benign prostatic hyperplasia, where chlorpheniramine's anticholinergic action may precipitate urinary retention by inhibiting bladder contraction.[28] Caution is advised in elderly patients with cognitive impairment, as cumulative exposure to anticholinergic agents like chlorpheniramine has been associated with an increased risk of dementia exacerbation and cognitive decline.[34][35] In neonates and premature infants, the drug is relatively contraindicated owing to the potential for severe reactions such as seizures, stemming from immature metabolic pathways; a 2024 study found first-generation antihistamines associated with a 22% higher seizure risk in children aged 6-24 months.[36][37] Regarding pregnancy, there is no evidence of fetal risk in animal studies and limited human data showing no overall increased chance of birth defects; however, it should be avoided in the third trimester to prevent neonatal anticholinergic effects like irritability or sedation.[38] During lactation, minimal amounts are excreted into breast milk, posing low risk, but infants should be monitored for sedation or other effects.[1] Furthermore, chlorpheniramine should not be used in patients who have received monoamine oxidase inhibitors (MAOIs) within the past 14 days due to the risk of enhanced anticholinergic and sympathomimetic effects.[39]

Drug interactions

Chlorpheniramine exhibits a wide range of drug interactions, with databases reporting over 300 known interactions that can alter its efficacy or increase the risk of adverse effects.[24] These primarily involve pharmacodynamic potentiation or pharmacokinetic alterations via cytochrome P450 enzymes.[8] Co-administration with central nervous system (CNS) depressants, such as alcohol, opioids (e.g., codeine), and benzodiazepines (e.g., alprazolam), can result in additive sedation and impaired psychomotor performance, potentially leading to increased risk of respiratory depression or accidents.[8][40] When combined with other anticholinergic agents, including atropine or tricyclic antidepressants (e.g., amitriptyline), chlorpheniramine enhances anticholinergic effects, exacerbating symptoms like dry mouth, constipation, urinary retention, and blurred vision.[8][41] Monoamine oxidase inhibitors (MAOIs), such as isocarboxazid or tranylcypromine, potentiate the anticholinergic and sympathomimetic effects of chlorpheniramine, raising the risk of hypertensive crisis, severe sedation, or serotonin syndrome; concurrent use is generally contraindicated.[8][10] Inhibitors of CYP2D6, the primary enzyme metabolizing chlorpheniramine (e.g., fluoxetine or quinidine), can prolong its half-life and plasma concentrations, leading to extended sedation and heightened adverse effects.[42] For instance, quinidine administration has been shown to significantly increase the maximum plasma concentration of the active (S)-(+)-enantiomer.[42] Chlorpheniramine may also reduce the absorption and therapeutic efficacy of levodopa in patients with Parkinson's disease, potentially worsening motor symptoms; monitoring or dose adjustments are recommended.

Pharmacology

Pharmacodynamics

Chlorphenamine acts primarily as a competitive antagonist at the histamine H1 receptor, with a binding affinity (Ki) of approximately 2.8 nM, thereby inhibiting histamine-mediated responses such as vasodilation, increased vascular permeability, and pruritus that contribute to allergic symptoms. This blockade prevents the activation of H1 receptors on endothelial cells, smooth muscle, and sensory nerves, reducing the physiological manifestations of type I hypersensitivity reactions. In addition to its H1 antagonism, chlorphenamine exhibits secondary pharmacological effects, including weak inhibition of serotonin reuptake with an IC50 of approximately 100 nM, which may contribute to its occasional use in mood-related contexts, though this activity is modest compared to dedicated serotonin reuptake inhibitors.[43] It also displays mild anticholinergic activity through blockade of muscarinic receptors (M1-M3 subtypes), with a Ki around 1.4 μM, potentially leading to side effects like dry mouth and blurred vision, while showing negligible inhibition of dopamine or norepinephrine reuptake transporters.[44] As a first-generation antihistamine, chlorphenamine demonstrates high selectivity for H1 receptors over H2, H3, and H4 subtypes, with substantially lower potency at the latter (Ki values exceeding 1 μM for H2 and H3). Its lipophilic structure allows it to readily cross the blood-brain barrier, resulting in central H1 receptor blockade that underlies its sedative properties, distinguishing it from second-generation agents that are more peripherally restricted.[45] Chlorphenamine belongs to the alkylamine class of antihistamines, characterized by a diarylalkylamine scaffold where the para-chlorine substitution on the phenyl ring enhances H1 receptor affinity by 20- to 50-fold relative to the unsubstituted analog pheniramine, improving potency and duration of action.[45] The antihistaminic effects of chlorphenamine typically peak between 2 and 6 hours post-administration, correlating with its plasma concentration profile, while sedative effects arise from central H1 blockade and may persist longer due to slower clearance from brain tissue.[46]

Pharmacokinetics

Chlorpheniramine is well absorbed from the gastrointestinal tract following oral administration, with an absolute bioavailability ranging from 25% to 50%, influenced by first-pass metabolism in the gut and liver.[47] The time to maximum plasma concentration (Tmax) is typically 2 to 6 hours for immediate-release formulations and 4 to 12 hours for extended-release forms, reflecting rapid onset of action despite variable absorption rates across individuals.[47] The drug exhibits wide distribution throughout the body, with a volume of distribution of approximately 3 to 5 L/kg in adults, indicating extensive tissue penetration.[48] Chlorpheniramine is highly bound to plasma proteins, at 69% to 72%, primarily to albumin.[49] As a lipophilic first-generation antihistamine, it readily crosses the blood-brain barrier, contributing to central nervous system effects such as sedation, and it also crosses the placenta.[13][50] Metabolism occurs predominantly in the liver via cytochrome P450 enzymes, with CYP2D6 as the major isoform responsible for N-demethylation to the active metabolite desmethylchlorpheniramine, and CYP3A4 playing a minor role.[51] Additional metabolites include didesmethylchlorpheniramine and glucuronide conjugates, which are pharmacologically inactive.[52] Elimination is primarily through renal excretion of metabolites, with less than 10% of the parent drug recovered unchanged in urine; conjugates such as glucuronides predominate in excreta.[52] The elimination half-life varies from 12 to 43 hours, with a mean of about 21 hours in adults, leading to steady-state concentrations achieved after 3 to 5 days of chronic dosing.[52][49] Pharmacokinetic variability is significant due to genetic polymorphisms in CYP2D6, where poor metabolizers exhibit reduced clearance, prolonged half-life, and higher plasma exposure compared to extensive metabolizers, resulting in extended therapeutic and adverse effects.[51]

Chemistry

Physical properties

Chlorpheniramine has the molecular formula C₁₆H₁₉ClN₂ and a molecular weight of 274.79 g/mol for the free base, while the commonly used maleate salt has a molecular weight of 390.9 g/mol.[1][53] The chemical structure is described by the IUPAC name 3-(4-chlorophenyl)-N,N-dimethyl-3-(pyridin-2-yl)propan-1-amine, featuring a chiral center at the carbon atom in position 3 of the propan-1-amine chain; the clinically employed form is a racemic mixture of (R)- and (S)-enantiomers.[1] Chlorpheniramine free base appears as a white crystalline powder or, in some reports, an oily liquid, with a melting point of 110–115 °C; the maleate salt is an odorless white crystalline powder with a higher melting point of 130–135 °C.[54][53] The free base exhibits limited water solubility at approximately 0.55 g/100 mL at 37 °C, whereas the maleate salt shows improved solubility of 1–5 g/100 mL in water at 21 °C and is also soluble in ethanol; the pKa of the pyridine nitrogen is 9.2.[1][55][49] The compound is stable at room temperature but light-sensitive, particularly in the maleate form, and the maleate salt is preferred in formulations due to its enhanced solubility.[53] Chlorpheniramine is lipophilic, with a logP value of 3.4, reflecting its partition behavior between octanol and water.[49]

Synthesis

Chlorpheniramine is synthesized through multi-step processes that typically involve condensation reactions followed by alkylation and functional group transformations. The original synthesis, developed and patented by Schering Corporation, utilizes 4-chlorophenylacetonitrile as a key starting material. This compound is deprotonated with sodium amide and reacted with 2-chloropyridine via nucleophilic aromatic substitution to form the intermediate (4-chlorophenyl)(pyridin-2-yl)acetonitrile.[56][57][58] The intermediate is then deprotonated again with sodium amide and alkylated with 2-(dimethylamino)ethyl chloride to introduce the propylamine side chain, yielding 2-(4-chlorophenyl)-2-(pyridin-2-yl)-3-(dimethylamino)propanenitrile (also referred to as γ-(4-chlorophenyl)-γ-cyano-N,N-dimethyl-2-pyridylpropan-1-amine). This nitrile undergoes hydrolysis under acidic conditions to form the corresponding carboxylic acid, followed by decarboxylation upon heating, which effectively replaces the cyano group with hydrogen and affords racemic chlorpheniramine. The process is inherently non-stereoselective, producing a racemic mixture of the (R)- and (S)-enantiomers.[58] An alternative synthetic route begins with the alkylation of pyridine using 4-chlorobenzyl chloride to generate 2-(4-chlorobenzyl)pyridine. This benzylic position is then deprotonated with sodium amide and alkylated with 2-(dimethylamino)ethyl chloride, directly yielding chlorpheniramine after workup. This method bypasses the nitrile intermediate and is also detailed in early Schering patents.[56][58] In industrial production, purification is commonly achieved by forming the maleate salt through reaction with maleic acid in a suitable solvent, which facilitates crystallization and isolation of the product with high purity. Overall multi-step yields typically range from 70% to 80%, depending on optimization of reaction conditions and purification steps. Modern variants incorporate catalytic hydrogenation for selective reductions in analogous routes, improving efficiency and reducing the use of harsh reagents like sodium amide.[57]

History

Development

Chlorpheniramine was developed in the late 1940s by a team of researchers at Schering Corporation, including Nathan Sperber, Domenick Papa, and Erwin Schwenk, as part of a broader search for improved antihistamines in the wake of diphenhydramine's (Benadryl) introduction in 1946.[56] The compound, chemically known as 3-(4-chlorophenyl)-3-(2-pyridyl)-N,N-dimethylpropylamine, emerged from synthetic efforts focused on aryl-(2-pyridyl)-amino alkane derivatives designed to exhibit potent H1 receptor antagonism with reduced toxicity compared to earlier agents.[56] Preclinical evaluation in the late 1940s confirmed chlorpheniramine's efficacy as an H1 antagonist through standard animal models of allergy, such as protection against histamine-induced bronchospasm and mortality in guinea pigs, where it demonstrated strong inhibitory effects at low doses.[59] Early testing also revealed its sedative properties, attributed to central nervous system penetration, which became a characteristic feature of first-generation antihistamines.[59] Initial clinical studies in the early 1950s assessed chlorpheniramine's effectiveness against hay fever and related allergic symptoms, with trials reporting rapid relief from sneezing, nasal congestion, and pruritus in patients, often at doses of 4 mg every 4–6 hours.[60] These investigations highlighted its favorable potency relative to contemporaries like tripelennamine, paving the way for its commercial launch as Chlor-Trimeton in July 1950, marking it as the inaugural member of the alkylamine antihistamine class.[59] In the 2020s, reviews have examined chlorpheniramine's potential repurposing for COVID-19 symptom management, particularly via intranasal formulations to alleviate early respiratory inflammation and viral pharyngitis, though evidence from small-scale studies remains preliminary and unproven for broad therapeutic efficacy.[61]

Regulatory milestones

Chlorpheniramine maleate received approval from the U.S. Food and Drug Administration (FDA) on August 15, 1950, as a safe and effective antihistamine for treating allergic conditions, marking one of the early approvals for first-generation antihistamines in oral dosage forms.[62] In 1976, the FDA approved its switch to over-the-counter (OTC) status for allergy relief uses, allowing non-prescription access for symptoms such as sneezing, itching, and runny nose, as part of the broader OTC drug review process for antihistamines.[63] A significant regulatory change occurred in 2000 when the FDA requested the removal of phenylpropanolamine (PPA) from all drug products, including combinations with chlorpheniramine, following evidence from the Hemorrhagic Stroke Project linking PPA to an increased risk of hemorrhagic stroke; this led to the market withdrawal of numerous cough and cold remedies containing these ingredients.[64] Internationally, chlorpheniramine was included on the World Health Organization (WHO) Model List of Essential Medicines starting from the 14th edition in 2005 for the symptomatic treatment of allergic conditions but was removed in 2013 during the 18th edition update, replaced by cetirizine due to the latter's improved safety profile and lower sedating effects.[65] In the European Union, approvals for chlorpheniramine have historically been handled through national competent authorities rather than centralized procedures, with ongoing pharmacovigilance assessments confirming its authorization for allergy treatment in various member states as of 2024.[66] As of 2023, chlorpheniramine continued to see substantial use in the U.S., with approximately 202,412 prescriptions dispensed annually, primarily in tablet form for short-term therapy.[67] By November 2025, no major regulatory changes had been implemented regarding its approval status, though there has been heightened scrutiny on first-generation antihistamines like chlorpheniramine due to their anticholinergic properties, particularly in guidelines advising caution for use in older adults to mitigate risks of cognitive decline.[68] In veterinary medicine, chlorpheniramine has been used off-label for managing allergic reactions in companion animals such as dogs and cats since the 1960s, with no specific FDA approval for animal indications but permitted under veterinary discretion for non-food animals.[49] Restrictions apply to its use in food-producing animals to prevent potential drug residues in meat or milk, aligning with FDA guidelines on extra-label drug use that prohibit applications risking human food contamination.[11] Regulatory controversies in the 2010s centered on emerging evidence linking long-term use of anticholinergic drugs, including chlorpheniramine, to increased dementia risk in older adults, prompting its inclusion on the American Geriatrics Society's Beers Criteria as a medication to avoid due to strong anticholinergic effects that may exacerbate cognitive impairment.[69] This led to updated clinical guidelines recommending alternatives like second-generation antihistamines for elderly patients, influencing prescribing practices without altering core approval status.

Society and culture

Brand names

Chlorphenamine is marketed under various brand names in the United States, including Chlor-Trimeton by Merck (successor to Schering-Plough), Aller-Chlor as a generic option, and PediaCare for pediatric formulations.[70][71][38] Internationally, prominent brands include Piriton by GlaxoSmithKline in the United Kingdom, Teldrin in Canada, and generic chlorphenamine products in the European Union and Australia.[49][72][38] Combination products featuring chlorphenamine include Coricidin, which pairs it with acetaminophen for cold and allergy relief, and Tussionex, an extended-release formulation combined with hydrocodone available by prescription only.[49][73] Major manufacturers encompass the original developer Schering-Plough (now part of Merck) for branded versions, alongside generic producers such as Teva and Mylan (now Viatris).[71][74] Formulations vary by release mechanism, with immediate-release options like standard Chlor-Trimeton tablets and extended-release versions such as Chlor-Trimeton 12 Hour providing prolonged symptom relief.[71][70]

Availability and legal status

In the United States, chlorphenamine is available over-the-counter (OTC) for standalone use in treating allergy symptoms, as approved under the FDA's OTC monograph system.[75] However, formulations combined with opioids (such as hydrocodone) or certain decongestants require a prescription due to controlled substance regulations.[76] Combinations with pseudoephedrine are subject to additional restrictions under the Combat Methamphetamine Epidemic Act, requiring sales behind the pharmacy counter and purchaser identification to prevent diversion for illicit drug production.[77] It ranked as the 318th most commonly prescribed medication in the US in 2023, with approximately 202,000 prescriptions.[67] In the United Kingdom, chlorphenamine in tablet or syrup form is classified as a pharmacy medicine (P), meaning it can be purchased without a prescription from a pharmacist, who must provide advice on appropriate use.[78] In Australia, oral formulations of chlorphenamine are scheduled as Schedule 3 (pharmacist only) medicines, requiring direct supervision and counseling by a pharmacist for sale. Certain topical preparations may fall under Schedule 2 (pharmacy medicine) in some contexts, allowing limited self-selection with pharmacist oversight. In Canada, low-dose chlorphenamine is generally available OTC as part of non-prescription cough, cold, and allergy products under Health Canada's labelling standards.[79] Similarly, in the European Union, it is authorized for OTC sale in low doses through national procedures, with periodic safety updates confirming ongoing availability.[66] However, combinations with pseudoephedrine face restrictions in various countries due to regulatory scrutiny over precursor chemicals for methamphetamine production, akin to US DEA oversight.[77] Globally, chlorphenamine is widely accessible as a low-cost generic medication in both human and veterinary formulations.[80] In many regions, including the US, it is sold OTC for veterinary applications to manage allergic conditions in pets such as dogs and cats, though veterinary consultation is recommended.[81] As of 2025, no major international bans on chlorphenamine itself have been implemented, though regulatory agencies have issued warnings about the risks of OTC antihistamine misuse, including overdose leading to sedation, cardiac effects, and dependency, particularly among adolescents.[82]

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