Pharmacology: Antihistamines
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Histamine: endogenous compound synthesized, stored, & released primarily by mast cells, a cellular mediator of immediate hypersensitivity rxn, acute inflammatory response, primary stimulant of gastric acid & a central neurotransmitter.
Synthesis of histamine: decarboxylation of L-histidine catalyzed by L-histidine decarboxylase (pyridoxal phosphate-dependent enzyme).
H1-Receptor: direct contractile actions on smooth muscles & neuronal actions from stimulation of breakdown of inositol phospholipids.
Relaxation of vascular SM from synthesis & release of nitric oxide. Causes allergic rxn & anaphylactic shock, mediate pain & itching from cutaneous nerve endings.
H2-Receptor: activation of adenylate cyclase, mediate acid secretion (release HCl from gastric parietal cells), relaxation of vascular smooth muscle, neuronal excitation, inhibition of basophil degranuation & increases in myocardial contractility.
H3-Receptor: modulation of Ca2+ entry into nerve endings.
Anaphylaxis: causes hypotension, vasodilatation, myocardial depression, dysrhythmias, urticaria, angioedema & bronchospasm. Histamine is only one of the many mediators of anaphylasix, but it has regulatory roles in both production & limitation of other mediators.
| H1-R Antagonists | Brand Name | Chemical Class | Indications | Comments |
| Chlorpheniramine | Chlortrimeton | Alkylamines | Temporary relief of sneezing, itchy, watery eyes; rhinorrhea due to hay fever & other upper respiratory allergies | Moderately sedating, moderate antimuscarinic activity, no antiemetic / anti-motion sickness actions. |
| Clemastine | Tavist | Ethanolamines (Significant sedative actions, marked antimuscarinic & anti-motion sickness actions.) | Rhinorrhea, sneezing due to hay fever, common cold; reduces itching of nose due to allergies | |
| Dimenhydrinate | Dramamine | Prevention of nausea, vomiting or dizziness associated w/ motion sickness. | ||
| Diphenhydramine | Benadryl | P.O.: rhinnorrhea, sneezing, watery eyes due to hay fever, or above symptoms associated w/ common cold. IV,IM: symptoms of anaphylaxis; acute immediate allergic rxns; acute tx of motion sickness; anti-Parkinson.
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| Cetriizine | Zyrtec | Piperizines (varying degrees of anti-muscarinic act, anti-motion sickness & sedative actions.
| Seasonal & perennial rhinitis; chronic urticaria | A carboxy metabolite of hydroxyzine, nonsedating |
| Hydroxyzine | Atarax, Vustaril | Histamine-mediated pruitus, chronic urticaria, anti-anxiety. | Marked sedative & antimuscarinic actions | |
| Meclizine | Bonine, Antivert | Management of nausea & vomiting associated w/ motion sickness; vertigo associated w/ vestibular fcn. |
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| Fexofenadine | Allegra | Peperidines (non-sedating, no antimuscarinic activity) | Seasonal allergic rhinitis; chronic idiopathic urticaria | Safe w/ erythromycin / ketoconazole (P450 inhibitor), no cardiac effects. |
| Loratadine | Claritin | Nasal, non-nasal symptoms of seasonal rhinitis; chronic idiopathic urticaria | Tablet put on tongue: rapidly gets into bloodstream, as fast acting as injection. Claritin D = loratadine + psedoephedrine (a decongestant) | |
| Promethazine | Phenergan | Phenothiazine (anti-muscarinic & antiemetic & anti-motion sickness) | P.O.: Allergic & vasomotor rhinitis, conjunctivitis, dermographism, post anaphylaxis, adjunct to analgesics in post-op pain, tx of motion sickness, anti emetic post-op. IV, IM: sedation, hypnotic, post-op pain (w/ analgesics), adjunct to anesthesia, post-op nausea, vomiting | |
Category: Pharmacology Notes
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