Pyrilamine in the horse: Detection and pharmacokinetics of pyrilamine and its major urinary metabolite O-desmethylpyrilamine

L. Dirikolu, A. F. Lehner, J. D. Harkins, W. E. Woods, W. Karpiesiuk, R. S. Gates, M. Fisher, T. Tobin

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Pages (from-to)66-78
Number of pages13
JournalJournal of Veterinary Pharmacology and Therapeutics
Volume32
Issue number1
DOIs
StatePublished - Feb 2009

ASJC Scopus subject areas

  • Pharmacology
  • veterinary(all)

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