Source: Health Products Regulatory Authority (ZA) Revision Year: 2023 Publisher: Acino Pharma (Pty) Ltd, 106 16th Road, Midrand, 1686
Category and class: A 11.4.3. Medicines acting on the Gastrointestinal tract Other Pharmacotherapeutic group: Drugs for acid-related disorders, proton pump inhibitors.
ATC code: A02BC01
Omeprazole reduces gastric acid secretion. It is a specific inhibitor of the gastric proton pump in the parietal cell. It produces reversible control of gastric acid secretion with once daily dosing.
Omeprazole is a weak base and is concentrated and converted to the active form in the acid environment of intracellular canaliculi within the parietal cell, where it inhibits the enzyme H+, K+-ATPase–proton pump. This effect on the final step of the gastric acid formation process is dose-dependent and provides for effective inhibition of both basal acid secretion and stimulated acid secretion irrespective of the secretagogue.
Omeprazole has no effect on acetylcholine, histamine or gastrin receptors.
Oral dosing with omeprazole once daily provides inhibition of gastric acid secretion with maximum effect being achieved within 4 days of treatment. In duodenal ulcer patients, a mean decrease of approximately 80% in 24-hour intragastric acidity is then maintained, with the mean decrease in peak acid output after pentagastrin stimulation being about 70%, 24 hours after dosing with omeprazole.
Omeprazole is acid-labile and is administered orally as enteric-coated granules in capsules.
Absorption takes place in the small intestine and is usually completed within 3 to 6 hours. The systemic bioavailability of omeprazole from a single oral dose of omeprazole is approximately 35%. After repeated once-daily administration, the bioavailability increases to about 60%. Concomitant intake of food has no influence on the bioavailability.
The apparent volume of distribution in healthy subjects is approximately 0,3 litres/kg and a similar value is also seen in patients with renal insufficiency. In elderly and in patients with hepatic insufficiency, the volume of distribution is slightly decreased. Concomitant intake of food has no influence on the bioavailability.
The plasma protein binding of omeprazole is about 95%.
The average half-life of the terminal phase of the concentration-time curve is approximately 40 minutes. There is no change in half-life during treatment. The inhibition of acid secretion is related to the area under the plasma concentration-time curve (AUC) but not to the actual plasma concentration at a given time.
Omeprazole is entirely metabolised by the cytochrome P450 (CYP), mainly in the liver. The major part of its metabolism is dependent on the polymorphically expressed, specific isoform CYP2C19 (S-mephenytoin hydroxylase).
Identified metabolites in plasma are sulphone, sulphide and hydroxyl-omeprazole. These metabolites having no significant effect on acid secretion.
About 80% of the metabolites are excreted in the urine and the rest in the faeces. The two main urinary metabolites are hydroxyl-omeprazole and the corresponding carboxylic acid.
The systemic bioavailability of omeprazole is not significantly altered in patients with reduced renal function.
The area under the plasma concentration-time curve is increased in patients with impaired liver function, but no tendency to accumulation of omeprazole has been found.
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