Plasma and serum concentrations of phenylbutazone and oxyphenbutazone in racing Thoroughbreds 24 hours after treatment with various dosage regimens.

The research article discusses a study conducted to measure the levels of phenylbutazone (PBZ) and oxyphenbutazone in the plasma and serum of 158 Thoroughbred racehorses after they were administered different doses of PBZ.

Objective and Methodology of the Study

• The aim of the research was to observe the plasma and serum concentrations of phenylbutazone (PBZ) and oxyphenbutazone in Thoroughbred racing horses following different dosage regimens.
• The study comprised 158 Thoroughbred horses that were either competing or training at various racetracks across the country.
• All the horses selected for the study had not been given any PBZ 24 hours before they were enrolled in the study and assigned a specific dosage schedule.
• The PBZ was administered via stomach tube, paste, or tablet for three days and on the fourth day, a 2g dose was given intravenously (IV), disregarding the initial dose and the method of administration.
• 24 hours after the last dose of PBZ was given, samples were collected from the horses to measure the plasma and serum concentrations of PBZ and oxyphenbutazone.

Results of the Study

• The study found that 24 hours after the last dose, the plasma PBZ concentrations registered at 3.51, 6.13, and 6.40 micrograms/ml when administered via stomach tube, paste, or tablet respectively.
• When 2g of PBZ was administered intravenously daily for four days, the plasma concentration was 4.16g/ml.
• Following a single 2-g IV administration, the serum PBZ concentration was at 0.87g/ml.
• Concentrations of oxyphenbutazone varied depending on the method of administration, with concentrations at 3.35 (stomach tube), 4.29 (paste), 3.60 (tablet), 3.65 (four-day IV), and 1.11 g/ml (single IV).
• However, no significant correlation was detected between the serum and urine concentrations at this 24-hour measurement.

Validation of Analytical Method

• The study also confirmed the stability of high-performance liquid chromatography as a method of analysis, via split samples sent off to various laboratories for testing.