Clemastine

Clemastine, also known as meclastin, is a first generation H1 histamine antagonist (antihistamine) with anticholinergic properties (drying) and sedative side effects. [1] Like all first generation antihistamines, it is sedating. [2]

Clemastine
Clinical data
AHFS/Drugs.comMonograph
MedlinePlusa682542
Pregnancy
category
  • US: B (No risk in non-human studies)
    Routes of
    administration    		Oral
    ATC code
    Legal status
    Legal status
    Pharmacokinetic data
    Bioavailability39.2%
    MetabolismHepatic
    Elimination half-life21.3 hours
    ExcretionRenal
    Identifiers
    CAS Number
    PubChem CID
    IUPHAR/BPS
    DrugBank
    ChemSpider
    UNII
    KEGG
    ChEBI
    ChEMBL
    CompTox Dashboard (EPA)
    Chemical and physical data
    FormulaC21H26ClNO
    Molar mass343.90 g·mol−1
    3D model (JSmol)
      (verify)

    Patented in 1960, it came into medical use in 1967.[3]

    Medical uses

    Clemastine is used to relieve hay fever and allergy symptoms, including sneezing; runny nose; and red, itchy, tearing eyes. Prescription strength clemastine is also used to relieve the itching and swelling of hives. [4]

    Side effects

    Overdosage symptoms are paradoxical, ranging from CNS depression to stimulation. Stimulation is most common in children, and is usually followed by excitement, hallucinations, ataxia, loss of coordination, muscle twitching, athetosis, hyperthermia, cyanosis, convulsions, tremors, and hyperreflexia. This may be followed by postictal depression and cardiovascular/respiratory arrest. Other common overdose symptoms include dry mouth, fixed dilated pupils, flushing of the face, and pyrexia. In adults, overdose usually leads to CNS depression, ranging from drowsiness to coma.

    Pharmacology

    Clemastine is an antihistamine with anticholinergic and sedative effects. Antihistamines competitively bind to histamine receptor sites, thus reducing the neurotransmitter's effects. Effects of histamine (which are countered by antihistamines) include:

    • Increased capillary permeability
    • Increased capillary dilatation
    • Edema (i.e., swelling)
    • Pruritus (Itch)
    • Gastrointestinal/respiratory smooth muscle constriction

    Clemastine inhibits both the vasoconstrictor and vasodilator effects of histamine. Depending on the dose, the drug can produce paradoxical effects, including CNS stimulation or depression.

    Most antihistamines exhibit some type of anticholinergic activity. Antihistamines act by competitively binding to H1-receptor sites, thus blocking the binding endogenous histamine. Antihistamines do not chemically inactivate or prevent the normal release of histamine.

    Clemastine does also act as FIASMA (functional inhibitor of acid sphingomyelinase).[5]

    Clemastine is rapidly absorbed from the gastrointestinal tract and peak plasma concentrations are attained in 2–4 hours. Antihistamines are thought to be metabolized in the liver, mostly by mono-/didemethylation and glucuronide conjugation. It is an inhibitor of cytochrome P450 CYP2D6 and may interfere with other drugs metabolized by this isozyme.

    Mechanism of action

    Clemastine is a selective histamine H1 antagonist. It binds to the histamine H1 receptor, thus blocking the action of endogenous histamine, which leads to temporary relief of the negative symptoms caused by histamine.

    Society and culture

    Clemastine is an OTC drug, and is available under many names and dosage forms worldwide. Most common brand name is Tavegyl.[6]

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    gollark: Anyway, if speech's information rate is constrained by how fast the brain can process it... how can people read at different (faster, I think) speeds?
    gollark: I mean, as far as I know you get maybe 1 bit per 8 characters, and a syllable is maybe 4 characters usually.
    gollark: The random search result says that languages operate at ~40bps, which seems... wrong...
    gollark: ... 40 bits per second? What?

    References
    1. https://www.drugbank.ca/drugs/DB00283
    2. https://www.pharmacytimes.com/p2p/perspectives-on-second-generation-otc-antihistamines: Krouse JH. Allergic rhinitis--current pharmacotherapy. Otolaryngol Clin North Am. 2008;41:347-358.
    3. Fischer, Jnos; Ganellin, C. Robin (2006). Analogue-based Drug Discovery. John Wiley & Sons. p. 547. ISBN 9783527607495.
    4. https://medlineplus.gov/druginfo/meds/a682542.html
    5. Kornhuber J, Muehlbacher M, Trapp S, Pechmann S, Friedl A, Reichel M, Mühle C, Terfloth L, Groemer T, Spitzer G, Liedl K, Gulbins E, Tripal P (2011). "Identification of novel functional inhibitors of acid sphingomyelinase". PLOS One. 6 (8): e23852. doi:10.1371/journal.pone.0023852. PMC 3166082. PMID 21909365.
    6. drugs.com Clemastine at drugs.com international listings Page accessed May 10, 2015
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