Omed/Omed 20 Mechanism of Action

Pharmacology: Omed: Pharmacological Actions: 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.
Mechanism of Action: Omeprazole is activated at an acidic pH to a sulphenamide derivative that binds irreversibly to H+ K+ ATPase enzyme, an enzyme system found at the secretory surface of the parietal cells, it thereby inhibits the final transport of hydrogen ions (via exchange with potassium ions) into the gastric lumen thus inhibiting acid secretion. Omeprazole inhibits both basal and stimulated acid secretion irrespective of the stimulus.
Pharmacokinetic Profile: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.
Omed 20: Pharmacokinetics: Absorption is rapid, with peak plasma levels of Omeprazole occurring within 0.5 to 3.5 hours. Peak plasma concentrations and AUC are approximately proportional to doses up to 40 mg, but because of a saturable first-pass effect, a greater than linear response in peak plasma concentration and AUC occurs with doses greater than 40 mg. Absolute bioavailability (compared to intravenous administration) is about 30-40% at doses of 20-40 mg, due in large part to presystemic metabolism. In healthy subjects, the plasma half-life is 0.5 to 1 hour, and the total body clearance is 500-600 mL/min. Protein binding is approximately 95%. The bioavailability of Omeprazole increases slightly upon repeated administration of Omeprazole. Following single dose oral administration of a buffered solution of Omeprazole, little if any unchanged drug was excreted in urine. The majority of the dose (about 77%) was eliminated in urine as at least six metabolites. Two were identified as hydroxyOmeprazole and the corresponding carboxylic acid. The remainder of the dose was recovered in feces. This implies a significant biliary excretion of the metabolites of Omeprazole. These metabolites have very little or no antisecretory activity. In patients with chronic hepatic disease, the bioavailability increased to approximately 100% compared to an I.V. dose, reflecting decreased first-pass effect, and the plasma half-life of the drug increased to nearly 3 hours compared to the half-life in normals of 0.5-1 hour. Plasma clearance average 70 mL/min, compared to a value of 500-600 mL/min in normal subjects. In parents with chronic renal impairment, whose creatinine clearance ranged between 10 and 62 mL/min/1.73 m2 the disposition of Omeprazole was very similar to that in healthy volunteers, although there was a slight increase in bioavailability. Because urinary excretion is a primary route of excretion of Omeprazole metabolites, their elimination slowed in proportion to the decreased creatinine clearance. The elimination rate of Omeprazole was somewhat decreased in the elderly, and bioavailability was increased. Omeprazole was 76% bioavailable when a single 40 mg oral dose of Omeprazole (buffered solution) was administered to healthy elderly volunteers, versus 58% in young volunteers given the same dose. Nearly 70% of the dose was recovered in urine as metabolites of Omeprazole and no unchanged drug was detected. The plasma clearance of Omeprazole was 250 mL/min (about half that of young volunteers) and its plasma half-life averaged one hour, about twice that of young healthy volunteers.