Research on Intramuscular Omeprazole in Horses

More equine ulcer research is needed to determine possible use of an injectable omeprazole for U.S. horses.
grumpy horse head
More research is needed to determine possible successful use of an injectable omeprazole product in the United States based on a similar product used in Australia. iStock

Oral omeprazole has been the gold standard for treating equine gastric ulcer syndrome (EGUS). For best efficacy, horse owners need to follow certain protocols to give the medication on an empty stomach, then feed an hour later. There is variability in absorption between individual horses, management practices vary, hence results vary. Research has been looking into the use of a once-weekly intramuscular injectable form of omeprazole to improve serum concentrations and effect [Lehman, M.L.; Bass L, Gustafson, D.L.; et al. Clinical efficacy, safety and pharmacokinetics of a novel long-acting intramuscular omeprazole in performance horses with gastric ulcers. Equine Veterinary Education 2021; doi: 10.1111/eve.13546].

The study included 23 actively-ridden horses with EGUS that were administered 5 mg/kg body weight compounded IM omeprazole every seven days for four doses. An IM formulation is available in Australia with 100% efficacy by Day 28, but it is not available in the United States A Colorado study used a compounded formulation prepared in the United States.

Stomach assessment (fasted) was done with gastroscopy two days prior to start of study and on Days 14 and 28. Injection sites were monitored along with bloodwork screening prior to and following the study. Seventeen horses with ESGD (equine squamous gastric disease) and six EGGD (equine glandular gastric disease) were included in the study, but no control horses were used for comparison.

Injections were done on alternating sides of the neck with an 18 gauge 1-1/2” needle. Across the whole study, 23% of injection sites reacted with edema, heat and pain. Reactions increased with subsequent injections, with 8% of horses reacting on the first dose and 48% after the fourth dose. Routine vaccinations were given on the other side of the neck at the time of the fourth dose and might have amplified that higher proportion of adverse reactions.

Summarized results for managing EGUS with a compounded IM formulation of omeprazole:

  • 78% of horses with ESGD improved by at least one grade by Day 14, but had no further improvement by Day 28.
  • The treatment resulted in ESGD healing rate of 35%.
  • Three horses did not improve, and two had worsening of ESGD by Day 14.
  • Of horses with EGGD, 5/6 “healed” over time, but EGGD scores did not significantly change.
  • By the end of the study, six horses showed no improvement.
  • Three horses received concurrent firocoxib treatment—two had static ulcer grades for ESGD and EGGD, and one showed improvement in both ESGD and EGGD.
  • Serum omeprazole concentration peaked at six hours following injection and concentrations were maintained above 4.3 mg/ml for seven days following dosing. At this time, it is unknown what concentration provides a consistent therapeutic effect.
  • There were no adverse systemic effects noted.

Ongoing research is necessary to improve the therapeutic outcomes of this American formulation, and also for further FDA investigation and potential approval.

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