Pharmacokinetics of Intrarectal Altrenogest in Horses.
Authors: Ellis Katelyn E, Council-Troche R McAlister, Von Dollen Karen A, Beachler Theresa M, Bailey C Scott, Davis Jennifer L, Lyle Sara K
Journal: Journal of equine veterinary science
Summary
# Intrarectal Altrenogest Administration in Horses: An Alternative Route for NPO Mares When pregnant mares require nil per os management during hospitalisation for medical or surgical complications, maintaining pregnancy becomes challenging since altrenogest—the standard progestin for pregnancy maintenance—is typically administered orally. Ellis and colleagues investigated whether rectal administration could achieve therapeutic plasma concentrations in horses, conducting a randomised crossover trial in six mares receiving 0.088 mg/kg altrenogest via oral or rectal routes daily for five days. Whilst rectal administration achieved therapeutic concentrations rapidly, bioavailability was considerably lower (5.47% compared to oral), with therapeutic levels persisting for only 5.5 hours versus 36 hours with oral dosing, and a half-life of 2.82 hours compared to 7.01 hours orally. For clinical practitioners, this means rectal administration represents a viable short-term option for NPO mares, though maintaining therapeutic concentrations would require dosing every 4–8 hours rather than the standard 24-hourly interval, making it a more labour-intensive protocol suitable primarily for hospitalised patients requiring intensive monitoring. These findings provide evidence-based guidance for managing pregnancy in mares with temporary oral feeding restrictions, expanding the therapeutic toolkit available to equine veterinarians in acute care settings.
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Practical Takeaways
- •Rectal altrenogest is a viable alternative for maintaining pregnancy in hospitalized mares unable to eat, reaching therapeutic levels despite lower bioavailability
- •More frequent dosing (every 4-8 hours) is required for rectal administration compared to once-daily oral dosing to maintain therapeutic coverage
- •This route offers a practical solution for pregnancy maintenance during medical/surgical crises when oral medication is contraindicated
Key Findings
- •Rectal altrenogest administration reaches therapeutic concentrations in horses with only 5.47% relative bioavailability compared to oral route
- •Oral altrenogest maintains therapeutic concentrations for mean 36 hours (range 24-72 hours) versus 5.5 hours (range 3-8 hours) for rectal administration
- •Half-life of oral administration (7.01 ± 3.13 hours) is 2.5 times longer than rectal administration (2.82 ± 1.07 hours)
- •Rectal administration at 0.088 mg/kg q 4-8 hours is necessary to maintain therapeutic concentrations over 24 hours in NPO mares