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07-SEPTEMBER-2008 03:17:44 - H2 antagonist The correct title of this article is H2 antagonist. It features superscript or subscript characters that are substituted or omitted because of technical limitations. Ball-and-stick model of cimetidine, the prototypical H2-receptor antagonist. Ball-and-stick model of cimetidine, the prototypical H2-receptor antagonist. The H2-receptor antagonists H2RA, often shortened to H2 antagonist are a class of drugs used to block the action of histamine on parietal cells in the stomach, decreasing the production of acid by these cells. H2 antagonist are used in the treatment of dyspepsia, although they have largely been surpassed in popularity by the more effective proton pump inhibitors. In the United States, all four FDA-approved members of the group-cimetidine, ranitidine, famotidine, and nizatidine-are available over the counter in relatively low doses. The prototypical H2 antagonist was cimetidine, developed by Smith, Kline French now GlaxoSmithKline in the mid-to-late 1960s and first marketed in 1976; sold under the trade name Tagamet, cimetidine would later become the first ever blockbuster drug. The use of quantitative structure-activity relationships QSAR led to the development of other agents-starting with ranitidine, first sold as Zantac-which had fewer adverse effects and drug interactions and were more potent. Contents 1 History and development 2 Pharmacology 3 Clinical use 4 Adverse effects 5 Drug interactions 6 Notes 7 References History and development Cimetidine was the prototypical histamine H2-receptor antagonist from which the later members of the class were developed. Cimetidine was the culmination of a project at Smith, Kline French SKF; now GlaxoSmithKline to develop a histamine receptor antagonist that would suppress stomach acid secretion. In 1964 it was known that histamine was able to stimulate the secretion of stomach acid, but also that traditional antihistamines had no effect on acid production. From these facts the SKF scientists postulated the existence of two histamine receptors. They designated the one acted on by the traditional antihistamines H1, and the one acted on by histamine to stimulate the secretion of stomach acid H2. The SKF team used a rational drug design process starting from the structure of histamine. Hundreds of modified compounds were synthesised in an effort to develop a model of the then-unknown H2 receptor. The first breakthrough was Nα-guanylhistamine, a partial H2-receptor antagonist. From this lead the receptor model was further refined and eventually led to the development of burimamide, a specific competitive antagonist at the H2 receptor 100-times more potent than Nα-guanylhistamine, proving the existence of the H2 receptor. Burimamide was still insufficiently potent for oral administration and further modification of the structure, based on modifying the pKa of the compound, led to the development of metiamide. Metiamide was an effective agent; however, it was associated with unacceptable nephrotoxicity and agranulocytosis. It was proposed that the toxicity arose from the thiourea group, and similar guanidine analogues were investigated until the discovery of cimetidine, which would become the first clinically successful H2 antagonist. Ranitidine common brand name Zantac was developed by Glaxo also now GlaxoSmithKline in an effort to match the success of Smith, Kline French with cimetidine. Ranitidine was also the result of a rational drug design process utilising the by-then-fairly-refined model of the histamine H2 receptor and quantitative structure-activity relationships QSAR. Glaxo refined the model further by replacing the imidazole-ring of cimetidine with a furan-ring with a nitrogen-containing substituent, and in doing so developed ranitidine. Ranitidine was found to have a far-improved tolerability profile i.e. fewer adverse drug reactions, longer-lasting action, and ten times the activity of cimetidine. Ranitidine was introduced in 1981 and was the world's biggest-selling prescription drug by 1988. The H2-receptor antagonists have since largely been superseded by the even more effective proton pump inhibitors, with omeprazole becoming the biggest-selling drug for many years. Pharmacology The H2 antagonists are competitive inhibitors of histamine at the parietal cell H2 receptor. They suppress the normal secretion of acid by parietal cells and the meal-stimulated secretion of acid. They accomplish this by two mechanisms: histamine released by ECL cells in the stomach is blocked from binding on parietal cell H2 receptors which stimulate acid secretion, and other substances that promote acid secretion such as gastrin and acetylcholine have a reduced effect on parietal cells when the H2 receptors are blocked. Like the H1-antihistamines, the H2 antagonists are inverse agonists rather than true receptor antagonists. Clinical use H2-antagonists are clinically used in the treatment of acid-related Gastrointestinal conditions. Specifically, these indications may include:1 Peptic ulcer disease PUD Gastroesophageal reflux disease GERD/GORD Dyspepsia Prevention of stress ulcer a specific indication of ranitidine People that suffer from heartburn infrequently may take either antacids or H2-receptor antagonists for treatment. The H2-antagonists offer several advantages over antacids, including longer duration of action 6-10 hours vs 1-2 hours for antacids, greater efficacy, and ability to be used prophylactically before meals to reduce the chance of heartburn occurring. Proton pump inhibitors, however, are the preferred treatment for erosive esophagitis since they have been shown to promote healing better than H2-antagonists. Some studies suggest that H2-antagonists might be effective in treating herpes viruses, such as shingles and herpes simplex 1. Adverse effects The H2 antagonists are generally well-tolerated, except for cimetidine where all of the following adverse drug reactions ADRs are common. Infrequent ADRs include hypotension. Rare ADRs include: headache, tiredness, dizziness, confusion, diarrhea, constipation, and rash.1 Additionally, cimetidine may also cause gynecomastia in males, loss of libido, and impotence, which are reversible upon discontinuation. In a longitudinal study of elderly African Americans published in 2007, long-term use of H2 blockers appeared to increase the risk of cognitive decline.2 Drug interactions Skeletal formula of famotidine. Unlike cimetidine, famotidine has no significant interactions with other drugs. Skeletal formula of famotidine. Unlike cimetidine, famotidine has no significant interactions with other drugs. With regard to pharmacokinetics, cimetidine in particular interferes with some of the body's mechanisms of drug metabolism and elimination through the liver cytochrome P450 pathway. Specifically, cimetidine is an inhibitor of the P450 enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. By reducing the metabolism of drugs through these enzymes, cimetidine may increase their serum concentrations to toxic levels. Many drugs are affected, including warfarin, theophylline, phenytoin, lidocaine, quinidine, propranolol, labetalol, metoprolol, tricyclic antidepressants, some benzodiazepines, dihydropyridine calcium channel blockers, sulfonylureas, metronidazole,3 and some recreational drugs such as ethanol and MDMA. The more recently developed H2-receptor antagonists are less likely to alter CYP metabolism. Ranitidine is not as potent a CYP inhibitor as cimetidine, although it still shares several of the latter's interactions such as with warfarin, theophylline, phenytoin, metoprolol, and midazolam.4 Famotidine has negligible effect on the CYP system, and appears to have no significant interactions.3 Notes ^ a b Rossi S Ed. 2005. Australian Medicines Handbook 2005. Adelaide: Australian Medicines Handbook. ISBN 0-9578521-9-3 ^ Boustani M, Hall KS, Lane KA, et al August 2007. The association between cognition and histamine-2 receptor antagonists in African Americans. J Am Geriatr Soc 55 8: 1248-53. doi:10.1111/j.1532-5415.2007.01270.x. PMID 17661965. Lay summary - WebMD 2007-08-03. ^ a b Humphries TJ, Merritt GJ August 1999. Review article: drug interactions with agents used to treat acid-related diseases. Aliment. Pharmacol. Ther. 13 Suppl 3: 18-26. doi:10.1046/j.1365-2036.1999.00021.x. PMID 10491725. ^ Kirch W, Hoensch H, Janisch HD 1984. Interactions and non-interactions with ranitidine. Clin Pharmacokinet 9 6: 493-510. PMID 6096071. References Katzung, Bertram G. 2004. Basic and Clinical Pharmacology, 9th ed. ISBN 0-07-141092-9 v d e Major drug groups Gastrointestinal tract/metabolism A stomach acid Antacids, H2 antagonists, Proton pump inhibitors Antiemetics Laxatives Antidiarrhoeals/Antipropulsives Anti-obesity drugs Anti-diabetics Vitamins Dietary minerals Blood and blood forming organs B Antithrombotics Anticoagulants, Antiplatelets, Thrombolytics Antihemorrhagics Cardiovascular system C cardiac therapy/antianginals Cardiac glycosides, Antiarrhythmics, Cardiac stimulant Antihypertensives Diuretics Vasodilators Beta blockers renin-angiotensin system ACE inhibitors, Angiotensin II receptor antagonists, Renin inhibitors Antihyperlipidemics Skin D Emollients Cicatrizant Antipruritics Reproductive system G Hormonal contraception Fertility agents SERMs Sex hormones Endocrine system H Corticosteroids Sex hormones Thyroid hormones Antithyroid agent Infections and infestations J, P Antibiotics Antivirals Vaccines Antifungals Antiparasitic Antiprotozoals, Anthelmintics Malignant and immune disease L Anticancer agents Immunostimulators Immunosuppressants Muscles, bones, and joints M Anabolic steroids Anti-inflammatories NSAID Antirheumatics Corticosteroids Muscle relaxants Brain and nervous system N Anesthetics General, Local Analgesics Anticonvulsants Mood stabilizers Psycholeptic Anxiolytics, Antipsychotics, Hypnotics/Sedatives Psychoanaleptic Antidepressants, Stimulants/Psychostimulants Respiratory system R Bronchodilators Decongestants H1 antagonists Other ATC V Antidotes Contrast media Radiopharmaceuticals Dressing v d e Histamine antagonists H1 or non-selective Alkylamines Bepotastine Bamipine Brompheniramine Chlorpheniramine Dexbrompheniramine Dexchlorpheniramine Dimethindene Metron S Pheniramine Pyrrobutamine Talastine Thenaldine Tolpropamine Triprolidine Ethanolamines Aminoalkyl ethers Bietanautine Bromazine/Bromodiphenhydramine Carbinoxamine Chlorphenoxamine Clemastine Diphenylpyraline Diphenhydramine Doxylamine Embramine p-Methyldiphenhydramine Moxastine Orphenadrine Phenyltoloxamine Setastine Ethylenediamines Chloropyramine Chlorothen Histapyrrodine Methafurylene Mepyramine Methapyrilene Pyrilamine Talastine Thenyldiamine Thonzylamine Tripelennamine Pyribenzamine Phenothiazine Tricyclics Ahistan Etymemazine Hydroxyethylpromethazine Isopromethazine Isothipendyl Mequitazine Methdilazine Oxomemazine Promethazine Thiazinamium Other Tricyclics Azatadine Clobenzepam Cyproheptadine Deptropine Isothipendyl Loratidine Piperazine Buclizine Chlorcyclizine Cinnarizine Clocinizine Hydroxyzine Niaprazine Oxatomide Benzhydryl compounds Cyclizine, Meclizine Others including selective, 2nd gen, 3rd gen Antazoline Astemizole Azatadine Azelastine Bepottastine Bamipine Cetoxine Clemizole Clobenztropine Deptropine Desloratadine Dimebon Ebastine Emedastine Epinastine Ketotifen Levocabastine Loratadine Mebhydrolin Mizolastine Phenindamine Pimethixene Pyrrobutamine Rupatadine Thenalidine Tritoqualine Alkylamine Acrivastine Tricyclic Olopatadine Piperazine Levocetirizine, Cetirizine Benzhydryl compounds Fexofenadine, Terfenadine H2 Cimetidine Famotidine Ranitidine Roxatidine Lafutidine Niperotidine Nizatidine H3 A-349,821 ABT-239 Burimamide Ciproxifan Clobenpropit Conessine Impentamine Iodophenpropit Thioperamide VUF-5681 H4 JNJ 7777120 Thioperamide VUF-6002 v d e Drugs for acid related disorders: Drugs for peptic ulcer and GERD/GORD A02B H2 antagonists Cimetidine, Famotidine, Lafutidine, Niperotidine, Nizatidine, Ranitidine, Roxatidine Prostaglandins/analogues Misoprostol, Enprostil Proton pump inhibitors Esomeprazole, Lansoprazole, Omeprazole, Pantoprazole, Rabeprazole, Tenatoprazole Other Carbenoxolone, Cetraxate, Sucralfate, Pirenzepine, Troxipide v d e Receptor agonists, antagonists, and reuptake inhibitors BA/M 5-HT serotonin receptor Serotonin receptor agonist 5-HT4 Serotonin antagonist 5-HT3 Serotonin uptake inhibitor SSRI Dopamine receptor Dopamine agonist Dopamine antagonist Dopamine reuptake inhibitor Adrenergic receptor Adrenergic agonist Alpha, Beta 1/2 Adrenergic antagonist Alpha 1/2, Beta Adrenergic uptake inhibitor Histamine receptor Histamine agonist Histamine antagonist H1, H2, H3 AA GABA receptor GABA agonist GABA antagonist Glutamate receptor NMDA receptor NMDA receptor antagonist AMPA receptor Ampakine Excitatory amino acid agonist Excitatory amino acid antagonist ANS Acetylcholine receptor Cholinergic Muscarinic, Nicotinic Anticholinergic Muscarinic, Nicotinic/Ganglionic blocker/Neuromuscular-blocking drugs Acetylcholinesterase inhibitor Adrenergic receptor SANS only -- see above for details PANS Parasympathomimetic Cholinergic, Acetylcholinesterase inhibitor - Parasympatholytic Anticholinergic, Ganglionic blocker SANS Sympathomimetic Adrenergic agonist, Monoamine oxidase inhibitor - Sympatholytic Adrenergic antagonist, Alpha blocker, Ganglionic blocker Retrieved from http://en..org/wiki/H2_antagonist Categories: Antihistamines | H2 receptor antagonists Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Türkçe Español Français Hrvatski Nederlands 日本語 Polski Português РуÑ?Ñ?кий ไทย Türkçe This page was last modified on 4 August 2008, at 18:55
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