Pyrilamine is an antihistamine used in human and veterinary medicine. As antihistamines produce central nervous system effects in horses, pyrilamine has the potential to affect the performance of racehorses. In the present study, O-desmethylpyrilamine (O-DMP) was observed to be the predominant equine urinary metabolite of pyrilamine. After intravenous (i.v.) administration of pyrilamine (300 mg/horse), serum pyrilamine concentrations declined from about 280 ng/mL at 5 min postdose to about 2.5 ng/mL at 8 h postdose. After oral administration of pyrilamine (300 mg/horse), serum concentrations peaked at about 33 ng/mL at 30 min, falling to <2 ng/mL at 8 h postdose. Pyrilamine was not detected in serum samples at 24 h postdosing by either route. After i.v. injection of pyrilamine (300 mg/horse) O-DMP was recovered at a level of about 20 ̄g/mL at 2 h postdose thereafter declining to about 2 ng/mL at 168 h postdose. After oral administration, the O-DMP recovery peaked at about 12 ̄g/mL at 8 h postdose and declined to <2 ng/mL at 168 h postdose. These results show that pyrilamine is poorly bioavailable orally (18%), and can be detected by sensitive enzyme-linked immunosorbent assay tests in urine for up to 1 week after a single administration. Care should be taken as the data suggest that the withdrawal time for pyrilamine after repeated oral administrations is likely to be at least 1 week or longer.

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Equine Drug Council

Kentucky Racing Commission

National, Florida, and Nebraska Horsemen’s Benevolent and Protective Associations

Ms. John Hay Whitney

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Published as Kentucky Agricultural Experiment Station Article # 08-14-085 with the approval of the Dean and Director, College of Agriculture and Kentucky Agricultural Experiment Station.

Published as 272 from the Equine Pharmacology and Experimental Therapeutics Program at the Maxwell H. Gluck Equine Research Center and the Department of Veterinary Science, University of Kentucky.