New Data on Pergolide Dosing in Horses

New Data on Pergolide Dosing in Horses

Optimal management requires an individual patient approach. This means some horses might benefit from routine monitoring of ACTH concentrations to ensure they’re not receiving too little or too much pergolide.

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Pituitary pars intermedia dysfunction (PPID, sometimes called equine Cushing’s disease) occurs commonly in older horses, negatively impacting quality of life and longevity. Pergolide mesylate, historically deemed the “drug of choice” for PPID, is now widely available in an equine, rather than a human, formulation, yet ideal dosages for this compound—especially following long-term administration—remain unclear.

In the past, physicians used pergolide to treat humans with Parkinson’s disease; horses with PPID, therefore, benefitted from the availability of the human drug. When the U.S. Food and Drug Administration (FDA) recommended pergolide no longer be administered to humans in 2007, many horses with PPID were left reliant on compounded pergolide products. Those products’ stability and safety came under fire when researchers identified issues with compounded products, ranging from a wide variation (sometimes by as much as 200%) in how much active ingredient was in the preparations to their potency decreasing rapidly regardless of storage conditions.

In 2011, the FDA approved a new pergolide product (marketed as Prascend) for use in horses with PPID. Recommended initial dosing was 1 mg/horse orally once daily. Based on the knowledge of how pergolide “works” in humans, researchers raised questions regarding the product’s exact pharmacological impact in horses. While pergolide appears to control clinical signs of PPID in horses, data regarding “optimal” dosing, especially in older horses with PPID treated long-term, are lacking.

To shed some light on how the FDA-approved pergolide “behaves” in horses that receive it daily for an extended period, Dianne McFarlane, DVM, PhD, Dipl. ACVIM, an associate professor in physiological sciences at Oklahoma State University, and colleagues administered the recommended initial dose for two months to six horses with PPID, followed by a double dose (2 mg/horse) for four months. The team then stopped administering the medication and measured horses’ ACTH (adrenocorticotropic hormone, which is elevated in horses with PPID) levels.

They learned that:

  • Pergolide effectively decreased blood ACTH levels in horses with PPID;
  • The half-life (the time required for the drug's blood concentration to decrease by 50%) suggested that once-daily dosing was appropriate for most affected horses; and,
  • ACTH levels increased by 50% in all horses within two to 10 days of discontinuing pergolide administration.

“These results suggest that the once-daily dosing of pergolide at the recommended dose is appropriate for most horses with PPID,” McFarlane said.

Optimal management, however, requires an individual patient approach. This means some horses might benefit from routine monitoring of ACTH concentrations to ensure they’re not receiving too little or too much pergolide. Not all horses respond to the same medication in a similar manner.

The study, “Pharmacokinetic and pharmacodynamics properties of pergolide mesylate following long-term administration to horses with pituitary pars intermedia dysfunction,” will be published in an upcoming edition of Journal of Veterinary Pharmacology and Therapeutics

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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