Omeprazole provides superior acid suppression because it most effectively shuts down acid production in the stomach, giving the ulcer time to heal. The cells of the stomach mucosa contain several acid-stimulating receptors, and when these receptors are activated they switch on the H+,K+ ATPase acid pump (an enzyme system) to secrete acid into the stomach. Omeprazole inhibits the acid pump, so when receptors are activated the acid pump does not initiate acid secretion. Omeprazole is the only medication approved for use in horses that acts directly on the acid pump, preventing acid secretion no matter what receptor is activated.6
Omeprazole was the first acid pump inhibitor to be approved for use in humans, and is now the first and only acid pump inhibitor approved for the treatment of stomach ulcers in horses.6 In a pharmacokinetic study involving thirteen healthy, mixed-breed horses (8 female, 5 male) receiving multiple doses of omeprazole paste (1.8 mg/lb once daily for 15 days) in either a fed or fasted state, there was no evidence of drug accumulation in the plasma when comparing the extent of systemic exposure (AUC-0-infinity). When comparing the individual bioavailability data (AUC-0-infinity, Cmax, and Tmax measurements) across the study days (doses 1 and 15 in fed animals), there was great inter- and intrasubject variability in the rate and extent of product absorption.
The antisecretory effects of omeprazole persist far longer than the drug’s serum level, because omeprazole becomes bound to the H+,K+ ATPase of the parietal cell, allowing it to exert its effect for 24 hours after a single dose.7 This prolonged duration of action enables once-daily dosing. Omeprazole, the active ingredient in GASTROGARD, is a potent inhibitor of gastric acid secretion. Inhibition of the acid pump by omeprazole results in near-complete suppression of acid secretion, regardless of stimulus, for a long period of time.6 In a study of pharmacodynamic effects using horses with gastric cannulae, secretion of gastric acid was inhibited in horses given 4 mg omeprazole/kg/day (1.8 mg omeprazole/lb/day). After the expected maximum suppression of gastric acid secretion was reached (5 days), the actual secretion of gastric acid was reduced by 99%, 95%, and 90% at 8, 16, and 24 hours, respectively.6