Metformin a Therapy in Progress for Equine Insulin Resistance

Oregon State Researchers suggest that a low bioavailability and a rapid rate of elimination of the drug metformin might explain the "varied reports of clinical success" for improving insulin sensitivity in horses.

Metformin is an oral antihyperglycemic drug used to treat type-2 diabetes mellitus in humans. It also appears to increase insulin sensitivity in human patients with type-1 diabetes.

In horses, insulin resistance is thought to play an important role in a number of conditions such as equine metabolic syndrome, pars intermedia pituitary dysfunction, hyperlipemia, laminitis, endotoxemia, and osteochondrosis dissecans.

To date, there are few well-studied pharmacologic therapies for insulin resistance in horses. Current management recommendations include limiting carbohydrate intake, increasing the horse's exercise level, and in some cases, administering thyroid supplements or chromium picolinate.

"Anecdotal reports in horses suggest that metformin could be used to treat horses with insulin resistance. The current body of evidence supporting this hypothesis is scant, however, and conflicting data regarding the efficacy of metformin in horses exists," explained Anna Firshman, BVSc, PhD, Dipl. ACVIM, now an assistant professor in large animal internal medicine at the University of Minnesota's College of Veterinary Medicine (formerly of Oregon State University).

To study the bioavailability and drug distribution of metformin in horses, Firshman and colleagues administered 6 g of metformin intravenously (IV) or orally in both fed and fasted horses.

"Our study found that the bioavailability of metformin in horses is very low. Only 7% of the drug in fasted horses and 3.9% of the drug in fed horses was available in the bloodstream to have an effect on the horses' insulin resistance," said Firshman. "Also, horses appear to remove the drug from their blood much faster than in other species"

This very low bioavailability and quick elimination rate of metformin is likely the reason for the varied reports of clinical efficacy of this drug in horses with insulin resistance.

"Since metformin appears to be safe to administer to horses and is an inexpensive drug, additional research is certainly warranted. Studies evaluating larger doses, more frequent administration, or even a sustained-release formulation would be an asset to the equine industry," summarized Firshman.

The study, "Pharmacokinetics and bioavailability of metformin in horses," was published in the May 15th edition of the American Journal of Veterinary Research. Co-authors were Jaime L. Hustace, DVM and John E. Mata, PhD.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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