H1 antagonist

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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.[ citation needed ]

Contents

In common use, the term "antihistamine" refers only to H1-antihistamines. Virtually all H1-antihistamines function as inverse agonists at the histamine H1-receptor, as opposed to neutral antagonists, as was previously believed. [1] [2] [3]

Medical uses

H1-antihistamines are clinically used in the treatment of histamine-mediated allergic conditions. These indications may include: [4]

H1-antihistamines can be administered topically (through the skin, nose, or eyes) or systemically, based on the nature of the allergic condition.

The authors of the American College of Chest Physicians Updates on Cough Guidelines (2006) recommend that, for cough associated with the common cold, first-generation antihistamine-decongestants are more effective than newer, non-sedating antihistamines. First-generation antihistamines include diphenhydramine (Benadryl), carbinoxamine (Clistin), clemastine (Tavist), chlorpheniramine (Chlor-Trimeton), and brompheniramine (Dimetane). However, a 1955 study of "antihistaminic drugs for colds," carried out by the U.S. Army Medical Corps, reported that "there was no significant difference in the proportion of cures reported by patients receiving oral antihistaminic drugs and those receiving oral placebos. Furthermore, essentially the same proportion of patients reported no benefit from either type of treatment." [5]

Side effects

Adverse drug reactions are most commonly associated with the first-generation H1-antihistamines. This is due to their relative lack of selectivity for the H1-receptor and their ability to cross the blood–brain barrier.

The most common adverse effect is sedation; this "side-effect" is utilized in many OTC sleeping-aid preparations. Other common adverse effects in first-generation H1-antihistamines include dizziness, tinnitus, blurred vision, euphoria, incoordination, anxiety, increased appetite leading to weight gain, insomnia, tremor, nausea and vomiting, constipation, diarrhea, dry mouth, and dry cough. Infrequent adverse effects include urinary retention, palpitations, hypotension, headache, hallucination, psychosis and erectile dysfunction. [4] [6] [7]

The newer, second-generation H1-antihistamines are far more selective for peripheral histamine H1-receptors and have a better tolerability profile compared to the first-generation agents. The most common adverse effects noted for second-generation agents include drowsiness, fatigue, headache, nausea and dry mouth. [4]

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. [8] [9]

Pharmacology

In type I hypersensitivity allergic reactions, an allergen (a type of antigen) interacts with and cross-links surface IgE antibodies on mast cells and basophils. Once the allergen cross-links Immunoglobulin E, tyrosine kinases rapidly signal into the cell, leading to cell degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil. Once released, the histamine can react with local or widespread tissues through histamine receptors.[ citation needed ]

Histamine, acting on H1-receptors, produces pruritus, vasodilation, hypotension, flushing, headache, bradycardia, bronchoconstriction, increase in vascular permeability and potentiation of pain. [2]

While H1-antihistamines help against these effects, they work only if taken before contact with the allergen. In severe allergies, such as anaphylaxis or angioedema, these effects may be of life-threatening severity. Additional administration of epinephrine, often in the form of an autoinjector, is required by people with such hypersensitivities. [10]

Comparison of selected sedating antihistamines
AntihistamineDoseaTime to peakHalf-lifebMetabolismAnticholinergic
Diphenhydramine 50 mg2–3 hours2–9 hoursCYP2D6, othersYes
Doxylamine 25 mg2–3 hours10–12 hoursCYP2D6, othersYes
Hydroxyzine 25–100 mg2 hours20 hoursADH, CYP3A4, othersNo
Doxepin 3–6 mg2–3 hours17 hourscCYP2D6, othersNo (at low doses)
Mirtazapine 7.5–15 mg2 hours20–40 hoursCYP2D6, othersNo
Quetiapine e25–200 mg1.5 hours7 hoursdCYP3A4No (at low doses)
Footnotes:a = For sleep/sedation. b = In adults. c Active metabolite nordoxepin half-life is 31 hours. d Active metabolite norquetiapine half-life is 9–12 hours. e Not recommended per literature reviews. Sources: See individual articles for references. See also selected reviews. [11] [12] [13]

First-generation (unselective)

These are the oldest H1-antihistaminergic drugs and are relatively inexpensive and widely available. They are effective in the relief of allergic symptoms, but are typically moderately to highly potent muscarinic acetylcholine receptor (anticholinergic) antagonists as well. These agents also commonly have action at α-adrenergic receptors and/or 5-HT receptors. This lack of receptor selectivity is the basis of the poor tolerability profile of some of these agents, especially when compared with the second-generation H1-antihistamines. Patient response and occurrence of adverse drug reactions vary greatly between classes and between agents within classes.

Classes

The first H1-antihistamine discovered was piperoxan, by Ernest Fourneau and Daniel Bovet (1933) in their efforts to develop a guinea pig animal model for anaphylaxis at the Pasteur Institute in Paris. [14] Bovet went on to win the 1957 Nobel Prize in Physiology or Medicine for his contribution. Following their discovery, the first-generation H1-antihistamines were developed in the following decades. They can be classified on the basis of chemical structure, and agents within these groups have similar properties.

ClassDescriptionExamples
Ethylenediamines Ethylenediamines were the first group of clinically effective H1-antihistamines developed.
Ethanolamines Diphenhydramine was the prototypical agent in this group. Significant anticholinergic adverse effects, as well as sedation, are observed in this group but the incidence of gastrointestinal adverse effects is relatively low. [4] [15]
Alkylamines The isomerism is a significant factor in the activity of the agents in this group. E-triprolidine, for example, is 1000-fold more potent than Z-triprolidine. This difference relates to the positioning and fit of the molecules in the histamine H1-receptor binding site. [15] Alkylamines are considered to have relatively fewer sedative and gastrointestinal adverse effects, but relatively greater incidence of paradoxical central nervous system (CNS) stimulation. [4]
Piperazines These compounds are structurally related to the ethylenediamines and the ethanolamines, and produce significant anticholinergic adverse effects with the exception of hydroxyzine, which has low to no affinity for muscarinic acetylcholine receptors and therefore produces negligible anticholinergic side-effects. [16] Compounds from this group are often used for motion sickness, vertigo, nausea, and vomiting. The second-generation H1-antihistamine cetirizine also belongs to this chemical group. [15]
Tricyclics and Tetracyclics These compounds differ from the phenothiazine antipsychotics in the ring-substitution and chain characteristics. [15] They are also structurally related to the tricyclic antidepressants (and tetracyclics), explaining the H1-antihistaminergic adverse effects of those three drug classes and also the poor tolerability profile of tricyclic H1-antihistamines. The second-generation H1-antihistamine loratadine was derived from compounds in this group.

Common structural features

Antihistamine.svg
X = N, R1 = R2 = small alkyl groups
X = C
X = CO

Second-generation

Second-generation H1-antihistamines are newer drugs that are much more selective for peripheral H1 receptors as opposed to the central nervous system H1 receptors and cholinergic receptors. This selectivity significantly reduces the occurrence of adverse drug reactions, such as sedation, while still providing effective relief of allergic conditions. The reason for their peripheral selectivity is that most of these compounds are zwitterionic at physiological pH (around pH 7.4). As such, they are very polar, meaning that they are less likely to cross the blood–brain barrier and act mainly outside the central nervous system.

Examples of systemic second-generation antihistamines include:

Examples of topical second-generation antihistamines include:

Regulation

Over-the-counter

H1 receptor antagonists that are approved for over-the-counter sale in the United States, include the following. [30]

First-generation

Common/marketed:

Uncommon/discontinued:

Second-generation

See also

Related Research Articles

<span class="mw-page-title-main">Diphenhydramine</span> Antihistamine medication

Diphenhydramine (DPH) is an antihistamine and sedative mainly used to treat allergies, insomnia, and symptoms of the common cold. It is also less commonly used for tremors in parkinsonism, and nausea. It is taken by mouth, injected into a vein, injected into a muscle, or applied to the skin. Maximal effect is typically around two hours after a dose, and effects can last for up to seven hours.

<span class="mw-page-title-main">Loratadine</span> Antihistamine medication

Loratadine, sold under the brand name Claritin among others, is a medication used to treat allergies. This includes allergic rhinitis and hives. It is also available in drug combinations such as loratadine/pseudoephedrine, in which it is combined with pseudoephedrine, a nasal decongestant. It is taken orally.

<span class="mw-page-title-main">Chlorphenamine</span> Antihistamine used to treat allergies

Chlorphenamine, also known as chlorpheniramine, is an antihistamine used to treat the symptoms of allergic conditions such as allergic rhinitis. It is taken orally. 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 H1 receptor.

<span class="mw-page-title-main">Hydroxyzine</span> Antihistamine drug

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.

<span class="mw-page-title-main">Desloratadine</span> Allergy medication

Desloratadine. sold under the brand name Clarinex among others, is a tricyclic H1 inverse agonist that is used to treat allergies. It is an active metabolite of loratadine.

<span class="mw-page-title-main">Hives</span> Skin disease characterized by red, raised, and itchy bumps

Hives, also known as urticaria, is a kind of skin rash with red, raised, itchy bumps. Hives may burn or sting. The patches of rash may appear on different body parts, with variable duration from minutes to days, and does not leave any long-lasting skin change. Fewer than 5% of cases last for more than six weeks. The condition frequently recurs.

<span class="mw-page-title-main">Cetirizine</span> Antihistamine medication

Cetirizine is a second-generation antihistamine used to treat allergic rhinitis, dermatitis, and urticaria (hives). It is taken by mouth. Effects generally begin within thirty minutes and last for about a day. The degree of benefit is similar to other antihistamines such as diphenhydramine, which is a first-generation antihistamine.

<span class="mw-page-title-main">Fexofenadine</span> Antihistamine medication

Fexofenadine, sold under the brand name Allegra among others, is an antihistamine pharmaceutical drug used in the treatment of allergy symptoms, such as hay fever and urticaria.

<span class="mw-page-title-main">Levocetirizine</span> Antihistamine drug

Levocetirizine, sold under the brand name Xyzal, among others, is a second-generation antihistamine used for the treatment of allergic rhinitis and long-term hives of unclear cause. It is less sedating than older antihistamines. It is taken by mouth.

<span class="mw-page-title-main">Ketotifen</span> Antihistamine medication

Ketotifen is an antihistamine medication and a mast cell stabilizer used to treat allergic conditions such as conjunctivitis, asthma, and urticaria (hives). Ketotifen is available in ophthalmic and oral forms: the ophthalmic form relieves eye itchiness and irritation associated with seasonal allergies, while the oral form helps prevent systemic conditions such as asthma attacks and allergic reactions. In addition to treating allergies, ketotifen has shown efficacy in managing systemic mast cell diseases such as mastocytosis and mast cell activation syndrome (MCAS), which involve abnormal accumulation or activation of mast cells throughout the body. Ketotifen is also used for other allergic-type conditions like atopic dermatitis (eczema) and food allergies.

Histamine H<sub>1</sub> receptor Histamine receptor

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.

<span class="mw-page-title-main">Chloropyramine</span> Chemical compound

Chloropyramine is a classical first-generation antihistamine drug approved in Eastern European countries 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.

<span class="mw-page-title-main">Azelastine</span> Chemical compound

Azelastine, sold under the brand name Astelin among others, is a H1 receptor-blocking medication primarily used as a nasal spray to treat allergic rhinitis (hay fever) and as eye drops for allergic conjunctivitis. Other uses may include asthma and skin rashes for which it is taken by mouth. Onset of effects is within minutes when used in the eyes and within an hour when used in the nose. Effects last for up to 12 hours.

<span class="mw-page-title-main">Acrivastine</span> Chemical compound

Acrivastine is a medication used for the treatment of allergies and hay fever. It is a second-generation H1-receptor antagonist antihistamine and works by blocking histamine H1 receptors.

<span class="mw-page-title-main">Ebastine</span> Antihistamine drug

Ebastine is a H1 antihistamine with low potential for causing drowsiness.

<span class="mw-page-title-main">Rupatadine</span> Second generation H1-antihistamine

Rupatadine is a second generation antihistamine and platelet-activating factor antagonist used to treat allergies. It was discovered and developed by Uriach and is marketed as Rupafin and under several other trade names.

<span class="mw-page-title-main">Antihistamine</span> Drug that blocks histamine or histamine agonists

Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic 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. Antihistamines are usually for short-term treatment. Chronic allergies increase the risk of health problems which antihistamines might not treat, including asthma, sinusitis, and lower respiratory tract infection. Consultation of a medical professional is recommended for those who intend to take antihistamines for longer-term use.

<span class="mw-page-title-main">Bilastine</span> Antihistamine medication

Bilastine is an antihistamine medication used to treat hives (urticaria), allergic rhinitis and itchy inflamed eyes (allergic conjunctivitis) caused by an allergy. It is a second-generation antihistamine and takes effect by selectively inhibiting the histamine H1 receptor, preventing these allergic reactions. Bilastine has an effectiveness similar to cetirizine, fexofenadine, and desloratadine.

Peripherally selective drugs have their primary mechanism of action outside of the central nervous system (CNS), usually because they are excluded from the CNS by the blood–brain barrier. By being excluded from the CNS, drugs may act on the rest of the body without producing side-effects related to their effects on the brain or spinal cord. For example, most opioids cause sedation when given at a sufficiently high dose, but peripherally selective opioids can act on the rest of the body without entering the brain and are less likely to cause sedation. These peripherally selective opioids can be used as antidiarrheals, for instance loperamide (Imodium).

Anti-allergic agents are medications used to treat allergic reactions. Anti-allergic agents have existed since 3000 B.C in countries such as China and Egypt. It was not until 1933 when antihistamines, the first type of anti-allergic agents, were developed. Common allergic diseases include allergic rhinitis, allergic asthma and atopic dermatitis with varying symptoms, including runny nose, watery eyes, itchiness, coughing, and shortness of breath. More than one-third of the world's population is currently being affected by one or more allergic conditions.

References

  1. ↑ Leurs R, Church MK, Taglialatela M (April 2002). "H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects". Clinical and Experimental Allergy. 32 (4): 489–98. doi:10.1046/j.0954-7894.2002.01314.x. PMID   11972592. S2CID   11849647.
  2. 1 2 Simons FE (November 2004). "Advances in H1-antihistamines". The New England Journal of Medicine. 351 (21): 2203–17. doi:10.1056/NEJMra033121. PMID   15548781.
  3. ↑ Khilnani G, Khilnani AK (September 2011). "Inverse agonism and its therapeutic significance". Indian Journal of Pharmacology. 43 (5): 492–501. doi: 10.4103/0253-7613.84947 . PMC   3195115 . PMID   22021988.
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  14. ↑ Fourneau, Ernest; Daniel Bovet (1933). "Recherches sur l'action sympathicolytique d'un nouveau dérivé du dioxane". Archives Internationales de Pharmacodynamie et de Thérapie. 46: 178–91. ISSN   0003-9780.
  15. 1 2 3 4 Nelson, Wendel L. (2007). "Antihistamines and Related Antiallergic and Antiulcer Agents". In William O. Foye; Thomas L. Lemke; David A. Williams (eds.). Foye's Principles of Medicinal Chemistry. Hagerstown, Maryland: Lippincott Williams & Wilkins. pp. 1004–1027. ISBN   978-0-7817-6879-5. OCLC   149596645.
  16. ↑ Kubo, Nobuo; Shirakawa, Osamu; Kuno, Takayoshi; Tanaka, Chikako (1987). "Antimuscarinic Effects of Antihistamines: Quantitative Evaluation by Receptor-Binding Assay". The Japanese Journal of Pharmacology. 43 (3): 277–282. doi: 10.1254/jjp.43.277 . PMID   2884340.
  17. ↑ "Bepotastine Monograph for Professionals".
  18. ↑ "Cetirizine Monograph for Professionals".
  19. ↑ Howell G, West L, Jenkins C, Lineberry B, Yokum D, Rockhold R (August 2005). "In vivo antimuscarinic actions of the third generation antihistaminergic agent, desloratadine". BMC Pharmacology. 5: 13. doi: 10.1186/1471-2210-5-13 . PMC   1192807 . PMID   16109168.
  20. ↑ "Desloratadine Monograph for Professionals".
  21. ↑ Vena GA, Cassano N, Filieri M, Filotico R, D'Argento V, Coviello C (September 2002). "Fexofenadine in chronic idiopathic urticaria: a clinical and immunohistochemical evaluation". International Journal of Immunopathology and Pharmacology. 15 (3): 217–224. doi:10.1177/039463200201500308. PMID   12575922. S2CID   23060714.
  22. ↑ "Fexofenadine Monograph for Professionals".
  23. ↑ "Ketotifen Monograph for Professionals".
  24. ↑ Nettis E, Colanardi MC, Barra L, Ferrannini A, Vacca A, Tursi A (March 2006). "Levocetirizine in the treatment of chronic idiopathic urticaria: a randomized, double-blind, placebo-controlled study". The British Journal of Dermatology. 154 (3): 533–8. doi:10.1111/j.1365-2133.2005.07049.x. PMID   16445787. S2CID   35041518.
  25. ↑ "Levocetirizine Monograph for Professionals".
  26. ↑ "Loratadine Monograph for Professionals".
  27. ↑ "Azelastine Monograph for Professionals".
  28. ↑ Al-Ahmad, Mona; Hassab, Mohammed; Al Ansari, Ali (21 December 2020). "Allergic and Non-allergic Rhinitis". Textbook of Clinical Otolaryngology. Cham: Springer International Publishing. pp. 241–252. doi:10.1007/978-3-030-54088-3_22. ISBN   978-3-030-54087-6. S2CID   234142758. Intranasal H1 antihistamines such as azelastine are effective for controlling nasal symptoms. They need to be applied twice daily.
  29. ↑ "Olopatadine Monograph for Professionals".
  30. ↑ "OTC Active Ingredients" (PDF). United States Food and Drug Administration. 7 April 2010.
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