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 suppressionof 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
