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Pharmacology: Pharmacodynamics: Omeprazole belongs to a class of antisecretory compounds, known as 'proton pump inhibitors' which are substituted benzimidazoles, that suppress gastric acid secretion by specific inhibition of the H+ K+ -ATPase enzyme system, the 'acid (proton) pump' of the gastric parietal cell.
After intravenous administration of omeprazole, the onset of the antisecretory effect occurs within one hour, with the maximum effect occurring within two hours. A single dose of 40 mg of omeprazole given intravenously has similar effect on intragastric acidity over a 24-hour period as repeated oral dosing with 20 mg once daily. Although the plasma half-life of omeprazole is very short (~ 50 minutes), the antisecretory effect lasts longer due to prolonged binding to the parietal H+ K+ ATPase enzyme. When the drug is discontinued, secretory activity returns gradually, over 3 to 5 days. The inhibitory effect of omeprazole on acid secretion is dose-related and increases with repeated once daily dosing, reaching a plateau after four days.
Pharmacokinetics: The apparent volume of distribution of omeprazole in healthy subjects and in patients with renal insufficiency is almost similar. The volume of distribution is slightly decreased in the elderly and in patients with hepatic insufficiency. The plasma protein binding of omeprazole is about 95%. The average half-life of the terminal phase of the plasma concentration time curve following intravenous administration of omeprazole is approximately 40 minutes. Omeprazole is completely metabolized by the cytochrome P450 system, mainly in the liver. No metabolite has been found to have any effect on gastric acid secretion. Almost 80% of an intravenous dose of omeprazole is excreted as metabolites in the urine, and the remainder is found in the faeces, primarily originating from biliary secretion.
