Antihistamine | |
---|---|
Drug class | |
Class identifiers | |
Pronunciation | /ˌæntiˈhɪstəmiːn/ |
ATC code | R06 |
Mechanism of action | • Receptor antagonist • Inverse agonist |
Biological target | Histamine receptors • HRH1 • HRH2 • HRH3 • HRH4 |
External links | |
MeSH | D006633 |
Legal status | |
In Wikidata |
Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. [1] Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provides relief from nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy with few side effects. [1] Antihistamines are usually for short-term treatment. [1] Chronic allergies increase the risk of health problems which antihistamines might not treat, including asthma, sinusitis, and lower respiratory tract infection. [1] Consultation of a medical professional is recommended for those who intend to take antihistamines for longer-term use. [1]
Although the general public typically uses the word "antihistamine" to describe drugs for treating allergies, physicians and scientists use the term to describe a class of drug that opposes the activity of histamine receptors in the body. [2] In this sense of the word, antihistamines are subclassified according to the histamine receptor that they act upon. The two largest classes of antihistamines are H1-antihistamines and H2-antihistamines.
H1-antihistamines work by binding to histamine H1 receptors in mast cells, smooth muscle, and endothelium in the body as well as in the tuberomammillary nucleus in the brain. Antihistamines that target the histamine H1-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and sneezing). In addition, they may be used to treat insomnia, motion sickness, or vertigo caused by problems with the inner ear. H2-antihistamines bind to histamine H2 receptors in the upper gastrointestinal tract, primarily in the stomach. Antihistamines that target the histamine H2-receptor are used to treat gastric acid conditions (e.g., peptic ulcers and acid reflux). Other antihistamines also target H3 receptors and H4 receptors.
Histamine receptors exhibit constitutive activity, so antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptors. [2] [3] [4] [5] Only a few currently marketed H1-antihistamines are known to function as antagonists. [2] [5]
Histamine makes blood vessels more permeable (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. [6]
Antihistamines suppress the histamine-induced wheal response (swelling) and flare response (vasodilation) by blocking the binding of histamine to its receptors or reducing histamine receptor activity on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells. Antihistamines can also help correct Eustachian Tube dysfunction, thereby helping correct problems such as muffled hearing, fullness in the ear and even tinnitus. [7]
Itching, sneezing, and inflammatory responses are suppressed by antihistamines that act on H1-receptors. [2] [8] In 2014, antihistamines such as desloratadine were found to be effective to complement standardized treatment of acne due to their anti-inflammatory properties and their ability to suppress sebum production. [9] [10]
H1-antihistamines refer to compounds that inhibit the activity of the H1 receptor. [4] [5] Since the H1 receptor exhibits constitutive activity, H1-antihistamines can be either neutral receptor antagonists or inverse agonists. [4] [5] Normally, histamine binds to the H1 receptor and heightens the receptor's activity; the receptor antagonists work by binding to the receptor and blocking the activation of the receptor by histamine; by comparison, the inverse agonists bind to the receptor and both block the binding of histamine, and reduce its constitutive activity, an effect which is opposite to histamine's. [4] Most antihistamines are inverse agonists at the H1 receptor, but it was previously thought that they were antagonists. [11]
Clinically, H1-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H1-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, may therefore be used to treat insomnia. H1-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H1 receptor. [2]
A combination of these effects, and in some cases metabolic ones as well, lead to most first-generation antihistamines having analgesic-sparing (potentiating) effects on opioid analgesics and to some extent with non-opioid ones as well. The most common antihistamines utilized for this purpose include hydroxyzine, promethazine (enzyme induction especially helps with codeine and similar prodrug opioids), phenyltoloxamine, orphenadrine, and tripelennamine; some may also have intrinsic analgesic properties of their own, orphenadrine being an example.
Second-generation antihistamines cross the blood–brain barrier to a much lesser extent than the first-generation antihistamines. They minimize sedatory effects due to their focused effect on peripheral histamine receptors. However, upon high doses second-generation antihistamines will begin to act on the central nervous system and thus can induce drowsiness when ingested in higher quantity.
H2-antihistamines, like H1-antihistamines, exist as inverse agonists and neutral antagonists. They act on H2 histamine receptors found mainly 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 inhibit H2 signaling thus reduce the secretion of gastric acid.
H2-antihistamines 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 include:
An H3-antihistamine is a classification of drugs used to inhibit the action of histamine at the H3 receptor. H3 receptors are primarily found in the brain and are inhibitory autoreceptors located on histaminergic nerve terminals, which modulate the release of histamine. Histamine release in the brain triggers secondary release of excitatory neurotransmitters such as glutamate and acetylcholine via stimulation of H1 receptors in the cerebral cortex. Consequently, unlike the H1-antihistamines which are sedating, H3-antihistamines have stimulant and cognition-modulating effects.
Examples of selective H3-antihistamines include:
H4-antihistamines inhibit the activity of the H4 receptor. Examples include:
Receptor | Location | Mechanism of action | Function | Antagonists | Uses of antagonists |
---|---|---|---|---|---|
H1 | Throughout the body, especially in: [17]
| Gq |
| ||
H2 |
| Gs ↑ cAMP2+ |
| ||
H3 |
| Gi |
| ||
H4 | Gi |
| As of July 2021 [update] , no clinical uses exist. Potential uses include: [20] | ||
Inhibit the action of histidine decarboxylase:
Mast cell stabilizers are drugs which prevent mast cell degranulation. Examples include:
The first H1 receptor antagonists were discovered in the 1930s and were marketed in the 1940s. [22] Piperoxan was discovered in 1933 and was the first compound with antihistamine effects to be identified. [22] Piperoxan and its analogues were too toxic to be used in humans. [22] Phenbenzamine (Antergan) was the first clinically useful antihistamine and was introduced for medical use in 1942. [22] Subsequently, many other antihistamines were developed and marketed. [22] Diphenhydramine (Benadryl) was synthesized in 1943, tripelennamine (Pyribenzamine) was patented in 1946, and promethazine (Phenergan) was synthesized in 1947 and launched in 1949. [22] [23] [24] By 1950, at least 20 antihistamines had been marketed. [25] Chlorphenamine (Piriton), a less sedating antihistamine, was synthesized in 1951, and hydroxyzine (Atarax, Vistaril), an antihistamine used specifically as a sedative and tranquilizer, was developed in 1956. [22] [26] The first non-sedating antihistamine was terfenadine (Seldane) and was developed in 1973. [22] [27] Subsequently, other non-sedating antihistamines like loratadine (Claritin), cetirizine (Zyrtec), and fexofenadine (Allegra) were developed and introduced. [22]
The introduction of the first-generation antihistamines marked the beginning of medical treatment of nasal allergies. [28] Research into these drugs led to the discovery that they were H1 receptor antagonists and also to the development of H2 receptor antagonists, where H1-antihistamines affected the nose and the H2-antihistamines affected the stomach. [29] This history has led to contemporary research into drugs which are H3 receptor antagonists and which affect the H4 receptor antagonists. [29] Most people who use an H1 receptor antagonist to treat allergies use a second-generation drug. [1]
The United States government removed two second generation antihistamines, terfenadine and astemizole, from the market based on evidence that they could cause heart problems. [1]
Not much published research exists which compares the efficacy and safety of the various antihistamines available. [1] The research which does exist is mostly short-term studies or studies which look at too few people to make general assumptions. [1] Another gap in the research is in information reporting the health effects for individuals with long-term allergies who take antihistamines for a long period of time. [1] Newer antihistamines have been demonstrated to be effective in treating hives. [1] However, there is no research comparing the relative efficacy of these drugs. [1]
In 2020, the UK National Health Service wrote that "[m]ost people can safely take antihistamines" but that "[s]ome antihistamines may not be suitable" for young children, the pregnant or breastfeeding, for those taking other medicines, or people with conditions "such as heart disease, liver disease, kidney disease or epilepsy". [30]
Most studies of antihistamines reported on people who are younger, so the effects on people over age 65 are not as well understood. [1] Older people are more likely to experience drowsiness from antihistamine use than younger people. [1] Continuous and/or cumulative use of anticholinergic medications, including first-generation antihistamines, is associated with higher risk for cognitive decline and dementia in older people. [31] [32]
Also, most of the research has been on caucasians and other ethnic groups are not as represented in the research. [1] The evidence does not report how antihistamines affect women differently than men. [1] Different studies have reported on antihistamine use in children, with various studies finding evidence that certain antihistamines could be used by children 2 years of age, and other drugs being safer for younger or older children. [1]
Research regarding the effects of commonly used medications upon certain cancer therapies has suggested that when consumed in conjunction with immune checkpoint inhibitors some may influence the response of subjects to that particular treatment whose T-cell functions were failing in anti-tumor activity. Upon study of records in mouse studies associated with 40 common medications ranging from antibiotics, antihistamines, aspirin, and hydrocortisone, that for subjects with melanoma and lung cancers, fexofenadine, one of three medications, along with loratadine, and cetirizine, that target histamine receptor H1 (HRH1), demonstrated significantly higher survival rates and had experienced restored T-cell anti-tumor activity, ultimately inhibiting tumor growth in the subject animals. [33] Such results encourage further study in order to see whether results in humans is similar in combating resistance to immunotherapy.
Tetracyclic antidepressants (TeCAs) are a class of antidepressants that were first introduced in the 1970s. They are named after their tetracyclic chemical structure, containing four rings of atoms, and are closely related to the tricyclic antidepressants (TCAs), which contain three rings of atoms.
H2 antagonists, sometimes referred to as H2RAs and also called H2 blockers, are a class of medications that block the action of histamine at the histamine H2 receptors of the parietal cells in the stomach. This decreases the production of stomach acid. H2 antagonists can be used in the treatment of dyspepsia, peptic ulcers and gastroesophageal reflux disease. They have been surpassed by proton pump inhibitors (PPIs). The PPI omeprazole was found to be more effective at both healing and alleviating symptoms of ulcers and reflux oesophagitis than the H2 blockers ranitidine and cimetidine.
H1 antagonists, also called H1 blockers, are a class of medications that block the action of histamine at the H1 receptor, helping to relieve allergic reactions. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines.
Diphenhydramine (DPH) is an antihistamine and sedative first developed by George Rieveschl and put into commercial use in 1946. It is available as a generic medication, and also sold under the brand name Benadryl among others. In 2021, it was the 242nd most commonly prescribed medication in the United States, with more than 1 million prescriptions.
Mirtazapine, sold under the brand name Remeron among others, is an atypical tetracyclic antidepressant, and as such is used primarily to treat depression. Its effects may take up to four weeks but can also manifest as early as one to two weeks. It is often used in cases of depression complicated by anxiety or insomnia. The effectiveness of mirtazapine is comparable to other commonly prescribed antidepressants. It is taken by mouth.
A receptor antagonist is a type of receptor ligand or drug that blocks or dampens a biological response by binding to and blocking a receptor rather than activating it like an agonist. Antagonist drugs interfere in the natural operation of receptor proteins. They are sometimes called blockers; examples include alpha blockers, beta blockers, and calcium channel blockers. In pharmacology, antagonists have affinity but no efficacy for their cognate receptors, and binding will disrupt the interaction and inhibit the function of an agonist or inverse agonist at receptors. Antagonists mediate their effects by binding to the active site or to the allosteric site on a receptor, or they may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity. Antagonist activity may be reversible or irreversible depending on the longevity of the antagonist–receptor complex, which, in turn, depends on the nature of antagonist–receptor binding. The majority of drug antagonists achieve their potency by competing with endogenous ligands or substrates at structurally defined binding sites on receptors.
Chlorphenamine (CP, CPM), also known as chlorpheniramine, is an antihistamine used to treat the symptoms of allergic conditions such as allergic rhinitis (hay fever). It is taken orally (by mouth). The medication takes effect within two hours and lasts for about 4–6 hours. It is a first-generation antihistamine and works by blocking the histamine H1 receptor.
Hydroxyzine, sold under the brand names Atarax and Vistaril among others, is an antihistamine medication. It is used in the treatment of itchiness, anxiety, insomnia, and nausea. It is used either by mouth or injection into a muscle.
The histamine receptors are a class of G protein–coupled receptors which bind histamine as their primary endogenous ligand.
Amoxapine, sold under the brand name Asendin among others, is a tricyclic antidepressant (TCA). It is the N-demethylated metabolite of loxapine. Amoxapine first received marketing approval in the United States in 1980, approximately 10 to 20 years after most of the other TCAs were introduced in the United States.
Doxylamine is an antihistamine medication used to treat insomnia and allergies, and—in combination with pyridoxine (vitamin B6)—to treat morning sickness in pregnant women. It is available over-the-counter and is typically sold under such brand names as Equate or Unisom, among others; and it is used in nighttime cold medicines (e.g., NyQuil) and pain medications containing acetaminophen and/or codeine to help with sleep. The medication is delivered chemically by the salt doxylamine succinate and is taken by mouth. Doxylamine and other first-generation antihistamines are the most widely used sleep medications in the world. Typical side effects of doxylamine (at recommended doses) include dizziness, drowsiness, grogginess, and dry mouth, among others.
Histamine H3 receptors are expressed in the central nervous system and to a lesser extent the peripheral nervous system, where they act as autoreceptors in presynaptic histaminergic neurons and control histamine turnover by feedback inhibition of histamine synthesis and release. The H3 receptor has also been shown to presynaptically inhibit the release of a number of other neurotransmitters (i.e. it acts as an inhibitory heteroreceptor) including, but probably not limited to dopamine, GABA, acetylcholine, noradrenaline, histamine and serotonin.
The histamine H4 receptor, like the other three histamine receptors, is a member of the G protein-coupled receptor superfamily that in humans is encoded by the HRH4 gene.
Chlorprothixene, sold under the brand name Truxal among others, is a typical antipsychotic of the thioxanthene group.
The H1 receptor is a histamine receptor belonging to the family of rhodopsin-like G-protein-coupled receptors. This receptor is activated by the biogenic amine histamine. It is expressed in smooth muscles, on vascular endothelial cells, in the heart, and in the central nervous system. The H1 receptor is linked to an intracellular G-protein (Gq) that activates phospholipase C and the inositol triphosphate (IP3) signalling pathway. Antihistamines, which act on this receptor, are used as anti-allergy drugs. The crystal structure of the receptor has been determined (shown on the right/below) and used to discover new histamine H1 receptor ligands in structure-based virtual screening studies.
H2 receptors are a type of histamine receptor found in many parts of the anatomy of humans and other animals. They are positively coupled to adenylate cyclase via Gs alpha subunit. It is a potent stimulant of cAMP production, which leads to activation of protein kinase A. PKA functions to phosphorylate certain proteins, affecting their activity. The drug betazole is an example of a histamine H2 receptor agonist.
Chloropyramine is a first-generation antihistamine drug approved in several Eastern European countries such as Russia for the treatment of allergic conjunctivitis, allergic rhinitis, bronchial asthma, and other atopic (allergic) conditions. Related indications for clinical use include angioedema, allergic reactions to insect bites, food and drug allergies, and anaphylactic shock.
Mianserin, sold under the brand name Tolvon among others, is an atypical antidepressant that is used primarily in the treatment of depression in Europe and elsewhere in the world. It is a tetracyclic antidepressant (TeCA). Mianserin is closely related to mirtazapine, both chemically and in terms of its actions and effects, although there are significant differences between the two drugs.
An H3 receptor antagonist is a type of antihistaminic drug used to block the action of histamine at H3 receptors.
Clorotepine, also known as octoclothepin or octoclothepine, is an antipsychotic of the tricyclic group which was derived from perathiepin in 1965 and marketed in the Czech Republic by Spofa in or around 1971 for the treatment of schizophrenic psychosis.
The H1-receptor is a transmembrane protein belonging to the G-protein coupled receptor family. Signal transduction from the extracellular to the intracellular environment occurs as the GCPR becomes activated after binding of a specific ligand or agonist. A subunit of the G-protein subsequently dissociates and affects intracellular messaging including downstream signaling accomplished through various intermediaries such as cyclic AMP, cyclic GMP, calcium, and nuclear factor kappa B (NF-κB), a ubiquitous transcription factor thought to play an important role in immune-cell chemotaxis, proinflammatory cytokine production, expression of cell adhesion molecules, and other allergic and inflammatory conditions.1,8,12,30–32 ... For example, the H1-receptor promotes NF-κB in both a constitutive and agonist-dependent manner and all clinically available H1-antihistamines inhibit constitutive H1-receptor-mediated NF-κB production ...
Importantly, because antihistamines can theoretically behave as inverse agonists or neutral antagonists, they are more properly described as H1-antihistamines rather than H1-receptor antagonists.15