Antihistamines mechanism

Clinically Efficacy. It is evident from the mechanism of action of antihistamines and the etiology of allergic diseases that antihistamines in no sense achieve a cure of the patient s allergy. After the adrninistration of a therapeutic dose, a temporal blockade of the effects of histamine is obtained. Whereas classical antihistamines needed at least twice daily adrninistration, for most of the more recently introduced agents adrninistration once daily is sufficient. 

The discovery of the antiulcer activity of H2 antihistamine antagonists has revolutionized the treatment of that disease. A benzimidazole. Omeprazole (55), inhibits gastric secretion and subsequent ulcer formation by a quite different mechanism. Studies at the molecular level suggest that this compound inhibits K /H dependent ATPase and consequently shuts down the proton pumping action of this enzyme system. 

Patients with chronic idiopathic urticaria, who develop cutaneous reactions in response to aspirin, display certain similarities in eicosanoid profile with AIA. The mechanism of the reactions is often related to COX-1 inhibition [18]. Therefore, aspirin and all drugs that inhibit COX-1 should be avoided in patients who already have had cutaneous reactions to NSAID. Coxibs are usually well tolerated, although occasional adverse reactions have been reported [19, 20]. For treatment of the reactions, antihistamines are usually sufficient, but in more severe cases adrenaline and corticosteroids may be warranted. 

Treatment of ocular allergy is aimed at slowing or stopping these processes. Antihistamines block the histamine receptors and some prevent histamine production and/or inhibit mediator release from the mast cells.15 Mast cell stabilizers inhibit the degranulation of mast cells, preventing mediator release. Some topical agents have multiple mechanisms of action, combining antihistaminic, mast cell stabilization, and antiinflammatory properties (Tables 60-3 and 60-4).16... 

Reiner, P. B. Kamondi, A. (1994). Mechanisms of antihistamine-induced sedation in the human brain HI receptor activation reduces background leakage potassium current. Neuroscience 59, 579-88. 

Sleep and sedative effects of the atypical antipsychotics could be related to different mechanisms antagonism of 5-HT2 receptors, antihistaminic and antimus-carinic effects, and probably an a-1 noradrenergic effect. The difference in the effect on sleep between risperidone and haloperidol may be due to their differential actions on serotoninergic receptors (Trampus and Ongini 1990 Trampus et al. 1993). 

Use of combination oral products containing a decongestant and antihistamine is rational because of the different mechanisms of action. 

Several studies support the notion that the basic mechanism by which many drugs prolong the QT interval is related to blockade of potassium currents. For instance, several antihistamines, antibacterial macrolides, fluoroquinolones and antipsychotics were shown to inhibit the rapid component of the delayed rectifier K+ current (fKr) in electrophysiological studies and to block potassium channels encoded by hERG [37-42]. 

The mechanisms of the beneficial effect of HRl antihistamines in asthma have been investigated in a mice model. Fexofenadine was found to suppress allergic immune/inflammatory responses in sensitized mice [60]. Treatment with fexofenadine diminished Th2-like response that typically follows sensitization and challenge with... 

Significant side effects have been estimated to occur in about 5% of patients on TCAs, most of these effects being attributed to their antimuscarinic properties, for example, blurred vision, dry mouth, tachycardia and disturbed gastrointestinal and urinary tract function. Orthostatic hypotension due to the block of alphai adrenoceptors and sedation resulting from antihistaminic activity frequently occur at therapeutic doses, particularly in the elderly. Excessive sweating is also a fairly common phenomenon, but its precise mechanism is uncertain. In the elderly patient, the precipitation of prostatic hypertrophy and glaucoma by the TCAs is also a frequent cause of concern. 

Most people have heard of antihistamines, even if they have little concept of the nature of histamine. Histamine is the decarboxylation product from histidine, and is formed from the amino acid by the action of the enzyme histidine decarboxylase. The mechanism of this pyridoxal phosphate-dependent reaction will be studied in more detail later (see Section 15.7). 

Hypnotics fall into different categories, including the benzodiazepines (e.g., triazolam, temazepam, clotiaze-pam, nitrazepam), barbiturates (e.g., hexobarbital, pentobarbital), chloral hydrate, and Hi-antihistamines with sedative activity (p. 114). Benzodiazepines act at specific receptors (p. 226). The site and mechanism of action of barbiturates, antihistamines, and chloral hydrate are incompletely understood. 

Cyclizine exhibits antihistamine and anticholinergic action and is used for vomiting and diarrhea. The exact mechanism of action is not known. Synonyms of this drug are marezine and migril. 

Arrhythmogenic mechanisms of nonsedating antihistamines. Clinical and Experimental Allergy, 29 (Suppl 3), 174-181. 

The indications for Hi-antihistaminics are derived from their mechanism of action all conditions in which a histamine release, mainly as sequel of an allergic reaction (bronchial asthma, hey fever, urticaria, allergic reactions to food or drugs), dominates the clinical symptoms. They can be used pro-phylactically or in acute situations, even by intravenous application. 

Hydroxyzine hydrochloride Atarax, Vistarit) is the antihistamine with the greatest use in the treatment of anxiety. It is often used to reduce the anxiety that is associated with anesthesia and surgery. It also produces sedation, dries mucous membranes (via an anticholinergic mechanism), and has antiemetic activity. A more extensive discussion of the pharmacology of the Hj-receptor antagonists is found in Chapter 38. 

Mechanism of Action An antihistamine that competes with histamine for histamine receptor sites on cells in the blood vessels, G1 tract, and respiratory tract. Therapeutic Effect Relieves symptoms associated with seasonal allergic rhinitis, such as increased mucus production and sneezing and symptoms associated with allergic conjunctivitis, such as redness, itching, and excessive tearing. 

Mechanism of Action A phenothiazine that blocks dopamine neurotransmission at postsynaptic dopamine receptor sites. Possesses strong anticholinergic, sedative, and antiemetic effects moderate extrapyramidal effects and slight antihistamine action. Therapeutic Effect Relieves nausea and vomiting improves psychotic conditions controls intractable hiccups and porphyria. 

Mechanism of Action An antihistamine that competes with histamine at histaminic... 

Mechanism of Action A nonsedating antihistamine that exhibits selective peripheral histamine Hi-receptor blocking action. Competes with histamine at receptor sites. Therapeutic Effect Prevents allergic responses mediated by histamine, such as rhinitis and urticaria. 

Mechanism of Action An antihistamine and anticholinergicthat competes for H,-receptor sites on effector cells of the G1 tract, blood vessels, and respiratory tract. The anticholinergic action diminishesvestibular stimulation and depresses labyrinthine function. Therapeutic Effect Prevents symptoms of motion sickness. Pharmacokinetics Well absorbed following PO administration. Metabolized in liver. Excreted in urine. Half-life Unknown. 

Mechanism of Action A phenothiazine that acts as an antihistamine, antiemetic, and CNS-antipsychotiC typical hypnotic. As an antihistamine, inhibits histamine at histamine receptor sites. As an antiemetic, diminishes vestibular stimulation, depresses labyrinthine function, and acts on the chemoreceptor trigger zone. As a sedative-hypnotic, produces CNS depression by decreasing stimulation to the brainstem reticular formation. Therapeutic Effect Prevents allergic responses mediated by histamine, such as rhinitis, urticaria, and pruritus. Prevents and relieves nausea and vomiting. Pharmacokinetics ... 

In the treatment of children and adolescents with anxiety disorders clinicians have a wide variety of pharmacologic options beyond the antidepressants (Shader and Greenblatt, 1995 Lydiard et ah, 1996 Riddle et ah, 1999). The benzodiazepines (BZs), with their favorable safety profile and quick onset of action, are attractive alternatives for the treatment of acute anxiety. While the clinical effectiveness of buspirone has not been proven in children, buspirone is used alone or in combination with other drugs in the treatment of anxiety disorders. The antihistamines are often used to treat insomnia and may reduce acute mild agitation. Zolpidem (Ambien) is occasionally used for its sedative properties. This chapter reviews the structure, proposed mechanisms of action, pharmacodynamic principles, and pharmacokinetic principles of these drugs. 

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