Histamine antagonist
A histamine antagonist (commonly called an antihistamine) is a pharmaceutical drug that inhibits the action of histamine by blocking it from attaching to histamine receptors; or it may inhibit the enzymatic activity of histidine decarboxylase, catalyzing the transformation of histidine into histamine (atypical antihistaminics).
Antihistamines are commonly used for the relief of allergies caused by intolerance of proteins.[1]
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Clinical effects [edit]
Histamines produce increased vascular permeability, causing fluid to escape from capillaries into tissues, which leads to the classic symptoms of an allergic reaction — a runny nose and watery eyes. Histamine also promotes angiogenesis.[citation needed]
Antihistamines suppress the histamine-induced wheal response (swelling) and flare response (vasodilation) by blocking the binding of histamine to its receptors on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells. They exert a competitive antagonism to histamines.
Itching and sneezing are suppressed by antihistamine blocking of H1-receptors on nasal sensory nerves.[2] Antihistamines have also been used with great success in the treatment of Brown Recluse (genus Loxosceles) spider bites as well as other insect bites that cause necrosis.[3]
Clinical: H1- and H2-receptor antagonists [edit]
H1-receptor antagonists [edit]
In common use, the term antihistamine refers only to compounds that have inhibit action at the H1 receptor (and not H2, etc).
Rather than "true" antagonists, H1-antihistamines are actually inverse agonists at the histamine H1-receptor.[4] Clinically, H1 antagonists are used to treat allergic reactions. Sedation is a common side-effect, and some H1 antagonists, such as diphenhydramine and doxylamine, are also used to treat insomnia. However, second-generation antihistamines do not cross the blood–brain barrier, and as such are not supposed to cause drowsiness.
Examples:
- Acrivastine
- Azelastine
- Brompheniramine
- Buclizine
- Bromodiphenhydramine
- Carbinoxamine
- Cetirizine
- Chlorpromazine (antipsychotic)
- Cyclizine
- Chlorpheniramine
- Chlorodiphenhydramine
- Clemastine
- Cyproheptadine
- Desloratadine
- Dexbrompheniramine
- Dexchlorpheniramine
- Dimenhydrinate (most commonly used as an antiemetic)
- Dimetindene
- Diphenhydramine (Benadryl)
- Doxylamine (most commonly used as an OTC sedative)
- Ebastine
- Embramine
- Fexofenadine
- Levocetirizine
- Loratadine
- Meclozine (most commonly used as an antiemetic)
- Mirtazapine (primarily used to treat depression, also has antiemetic and appetite-stimulating effects)
- Olopatadine (used locally)
- Orphenadrine (a close relative of diphenhydramine used mainly as a skeletal muscle relaxant and anti-Parkinsons agent)
- Phenindamine
- Pheniramine
- Phenyltoloxamine
- Promethazine
- Pyrilamine
- Quetiapine (antipsychotic)
- Rupatadine
- Tripelennamine
- Triprolidine
H2-receptor antagonists [edit]
H2 antagonists, like H1 antagonists, are also inverse agonists and not true antagonists. They act on H2 histamine receptors found principally in the parietal cells of the gastric mucosa, which are part of the endogenous signaling pathway for gastric acid secretion. Normally, histamine acts on H2 to stimulate acid secretion; drugs that block H2 signaling thus reduce the secretion of gastric acid. H2 antagonists are among first-line therapy to treat gastrointestinal conditions including peptic ulcers and gastroesophageal reflux disease. Some formulations are available over the counter. Most side effects are due to cross-reactivity with unintended receptors. Cimetidine, for example, is notorious for antagonizing androgenic testosterone and DHT receptors at high doses.
Examples:
Experimental: H3- and H4-receptor antagonists [edit]
These are experimental agents and do not yet have a defined clinical use, although a number of drugs are currently in human trials. H3-antagonists have a stimulant and nootropic effect, and are being investigated for the treatment of conditions such as ADHD, Alzheimer's disease, and schizophrenia, whereas H4-antagonists appear to have an immunomodulatory role and are being investigated as anti-inflammatory and analgesic drugs.
H3-receptor antagonists [edit]
Examples:
H4-receptor antagonists [edit]
Examples:
Others [edit]
Atypical antihistaminics [edit]
They inhibit the enzymatic activity of histidine decarboxylase :
Mast cell stabilizers [edit]
Mast cell stabilizers appear to stabilize the mast cells to prevent degranulation and mediator release. These drugs are not usually classified as histamine antagonists, but have similar indications.
Examples:
- Cromoglicate (cromolyn)
- Nedocromil
- Beta2-adrenergic agonist
Other agents with antihistaminergic activity [edit]
Many drugs used for other indications possess unwanted antihistaminergic activity.[citation needed]
References [edit]
- ^ Sicherer, Scott H. M.D., Understanding and Managing Your Child's Food Allergy. Baltimore: The Johns Hopkins University Press, 2006 ISBN 0-8018-8492-6.
- ^ Monroe, EW; Daly, AF; Shalhoub, RF (1997). "Appraisal of the validity of histmine-induced wheal and flare is used to predict the clinical efficacy of antihistamines". The Journal of allergy and clinical immunology 99 (2): S798–806. PMID 9042073.
- ^ Paul K. Carlton Jr, MD, FACS The Texas A&M University Health Science Center, College Station, Tex
- ^ Leurs R, Church MK, Taglialatela M (2002). "H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects". Clin Exp Allergy 32 (4): 489–98. doi:10.1046/j.0954-7894.2002.01314.x. PMID 11972592.
External links [edit]
- Histamine antagonist at the US National Library of Medicine Medical Subject Headings (MeSH)
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