Clearance times and the forensic significance of the dietary anthelmintic pyrantel tartrate in performance horses

Six performance bred (Thoroughbred or Standardbred) mares were fed the anthelmintic pyrantel tartrate as a daily supplement for a period of 21 days to assure steady state concentrations would be achieved. The forensic "clearance times" and potential for analytical interference of pyrantel tartrate were then investigated. This investigation was intend- ed to enable guidelines to be established for veterinarians and trainers to avoid a "positive" test result for pyrantel which might violate existing rules or regulations. The analysis of blood and urine samples from these horses was conducted by the University of Kentucky Equine Drug Testing Laboratory, and were performed on samples obtained on days -3, -2, -1, 7, 14, 21, +1, +2, +3 and +4. Pyrantel tartrate was readily detected by standard thin layer chromatography analysis in urine samples on days 7, 14 and 21 during the administration portion of the study, and on day + 1 of the post-adminislxation time period. Further analysis of the positive urine samples by direct probe mass spectrometry confirmed the presence of pyrantel. In our analysis of equine serum samples pyrantel tartrate was not detected. Pyrantel positive urine samples were additionally ana- lyzed using enzyme-linked immunosorbent assays designed to detect fluphenazine,butorphanol, morphine, oxymorphone, fentanyl, sufentanil and etorphine, with no cross-reactivity found. We concluded that, from a drug testing perspective, the use of pyrantel is unlikely to interfere with normal post- race drug testing.