Omeprazole inhibits secretion of gastric acid by irreversibly blocking the enzyme system of hydrogen/potassium adenosine triphosphatase (H+/K+ATPase), the ‘proton pump’ of the gastric parietal cell.
Omeprazole is rapidly but variably absorbed after oral administration. Absorption of Omeprazole is not affected by food. Omeprazole is acid-labile and various formulations have been developed in an attempt to improve bioavailability from the gastro-intestinal tract. The absorption of Omeprazole, as well as being formulation-dependent, also appears to be dose-dependent, as increasing the dosage above 40mg has been reported to increase the plasma concentrations in a non-linear fashion. Bioavailability of Omeprazole may be increased in elderly patients and in patients with impaired hepatic function, but is not markedly affected in patients with renal impairment. Following absorption, Omeprazole is almost completely metabolised in the liver, primarily by cytochrome P450 isoenzyme CY2C19, and rapidly eliminated, mostly in the urine. Although the elimination half-life from plasma is short, being reported to be about 0.5 to 3 hours, its duration of action with regard to inhibition of acid secretion is much longer allowing it to be used in single daily doses. Omeprazole is highly bound (about 95%) to plasma proteins.
Omecos® is used in conditions where inhibition of gastric acid secretion may be beneficial, including aspiration syndrome, dyspepsia, gastro-oesophageal reflux disease, peptic ulcer disease, and the Zollinger-Ellison Syndrome.