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Pharmacological particulars
Pharmacotherapeutic group: Drugs for acid related disorders, Proton pump inhibitors.
ATCVet code: QA 02 BC 01
Pharmacodynamic properties
In studies lasting up to 28 days, treatment with Omeprazole at the dose rate of 1 mg Omeprazole per kg body weight per day has been shown to help prevent the occurrence of gastric ulcers in horses exposed to ulcerogenic conditions.
Omeprazole is a proton pump inhibitor belonging to the substituted benzimidazole class of compounds. It is an antacid, for treatment of peptic ulcers.
Omeprazole suppresses gastric acid secretion by specific inhibition of the H+/K+-ATPase enzyme system at the secretory surface of the parietal cell. The H+/K+- ATPase enzyme system is the acid (proton) pump within the gastric mucosa. Because H+/K+- ATPase is the final step involved in control of acid secretion, Omeprazole blocks secretion irrespective of the stimulus. Omeprazole irreversibly binds to the gastric parietal cell H+/K+- ATPase enzyme that pumps hydrogen ions into the lumen of the stomach in exchange for potassium ions.
At 8, 16 and 24 hours after dosing horses with Omeprazole at 4 mg/kg/day orally, pentagastrin-stimulated gastric acid secretion was inhibited by 99%, 95% and 90% and basal secretion was inhibited by 99%, 90% and 83%.
The full effect on the inhibition of acid secretion is reached by five days after the first administration.
Pharmacokinetic particulars
The median bioavailability of Omeprazole after oral administration as a paste is 10.5% (range 4.1 to 12.7%). The absorption is rapid with time to maximum plasma concentrations (Tmax) of approximately 1.25 hours after dosing. Cmax values for individual animals ranged between 121 ng/ml and 1470 ng/ml after one administration of the product at 4 mg/kg. There is a significant first-pass effect following oral administration. Omeprazole is rapidly metabolised principally into glucuronides of demethylated and hydroxylated Omeprazole sulphide (urinary metabolites) and methyl sulphide Omeprazole (biliary metabolite) as well as into reduced Omeprazole (both).
After oral administration at 4 mg/kg, Omeprazole is detectable in plasma for 9 hours after treatment and in urine as hydroxy Omeprazole and O-desmethyl Omeprazole at 24 hours but not at 48 hours. Omeprazole is eliminated quickly, mainly by urinary route (43 to 61% of the dose), and to a smaller extent by faecal route, with a terminal half-life ranging from approximately 0.5 to 8 hours.
After repeated oral administration, there is no evidence of accumulation.