New Ponazuril Loading Dose Examined for EPM Treatment (AAEP 2012)

Achieving therapeutic drug levels sooner using a loading dose could minimize S. neurona-caused damage.
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Veterinarians have been treating equine protozoal myeloencephalitis (EPM) with ponazuril (Marquis) since the Food and Drug Administration approved the antiprotozoal in 2001 but, as with any pharmaceutical approach to disease, fine-tuning an effective treatment protocol is always a work in progress. In laboratory studies scientists have shown ponazuril must reach a particular concentration in the cerebrospinal fluid (CSF) to kill Sarcocystis neurona (a causative agent of EPM), so clinicians wondered if they could be more efficient by starting with a stronger loading dose.

Stephen Reed, DVM, Dipl. ACVIM, of Rood & Riddle Equine Hospital, and research colleagues re-examined the pharmacokinetics (how the drug levels are processed and maintained in the body) of ponazuril in the horse. He presented the results at the 2012 Annual Convention of the American Association of Equine Practitioners, Dec. 1-5in Anaheim, Calif.

The team administered ponazuril in eight adult horses, giving each a single oral dose at the labeled 5 mg/kg rate. They obtained serum and CSF samples from the horses over the next eight days

The researchers then calculated that with this dose, it would take approximately one week to reach a steady-state concentration of the ponazuril level that’s lethal to S. neurona–100 micrograms/L. They extrapolated from this model to conclude that a 15 mg/kg loading dose would produce steady-state concentrations immediately

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Written by:

Christy Corp-Minamiji, DVM, practices large animal medicine in Northern California, with particular interests in equine wound management and geriatric equine care. She and her husband have three children, and she writes fiction and creative nonfiction in her spare time.

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