Histamine H2-receptor antagonists
H2- antagonists or histamine H2-receptor antagonists were discovered many years later after H1 antagonists. The first drug released on the market was cimetidine followed by ranitidine, famotidine and nizatidine.
H2 antagonists inhibit competitively histamine H2 receptors. The principal consequence is inhibition of gastric acid secretion wheter it is basal, diurnal, nocturnal, or stimulated by meals. The antisecretory efficacy of the different H2 antagonists seems identical when they are used in advised dosages.
The bioavailability of H2-antagonists goes from 50% for ranitidine and famotidine to approximately 90% for nizatidine and advised dosages take this into account. They are taken especially in the evening to reduce night gastric acidity.
Their elimination is primarily renal.
Cimetidine has a characteristic: whereas others H2 antagonists have little effect on P-450 cytochrome, cimetidine inhibits it and increases the concentrations and the effects of many other drugs.
As gastric acidity induces outbreak of ulcers and delays their cicatrisation, H2-antihistamines are indicated in treatment of esophageal ulcers (by acid reflux) and of gastric and duodenal ulcers as of high digestive bleedings of ulcerous origin where one resorts especially to injectable H2-antagonist formulation.
Cimetidine, r anitidine,famotidine,nizatidine H2-antagonists constitute a possible treatment of gastroduodenal ulcer but there is another choice, proton pump inhibitors, to which antibiotics intended to eliminate Helicobacter pylori are added.
- All H2 antagonists can cause bradycardia by suppression of the positive chronotropic effect of histamine and different adverse effects, generallynon-severe.
- Cimetidine has endocrine adverse effects: it increases prolactin plasma concentration and can induce gynecomastia and galactorrhea. It also has an antiandrogenic effect by displacing dihydrotestosterone fromits binding sites and can cause impotence. Because of inhibition of P-450 cytochrome, cimetidine can slow down the rate of hepatic biotransformations of different drugs, like oral anticoagulants, benzodiazepines, beta-adrenergic antagonists, cyclosporine, theophylline, whose concentrations can rise beyond usual limits.