Plasma concentration-dependent suppression of endogenous hydrocortisone in the horse after intramuscular administration of dexamethasone-21-isonicotinate.

This research study explores the correlation between plasma concentrations of the drug dexamethasone and the suppression of endogenous hydrocortisone in horses. The findings will aid in setting the screening limits for doping in equestrian sports.

Study Design and Administration

• The study involved administering a single dose of a suspension of dexamethasone-21-isonicotinate (0.03 mg/kg) intramuscularly to six horses.
• The dose and method of administration were chosen to simulate conditions that could occur in competitive equestrian scenarios, thus ensuring the findings of the research have practical relevance.

Data Collection and Analysis

• Following administration, both plasma and urine were regularly sampled and analyzed for levels of dexamethasone and hydrocortisone.
• The analysis was performed using sophisticated Ultra-Performance Liquid Chromatography-Mass Spectrometry/Mass Spectrometry (UPLC-MS/MS), capable of detecting very small concentrations of the substances.
• In addition to quantifying the concentrations, the time-profiles – how the concentration in the plasma and urine changes over time – were also determined.

Key Findings

• It was discovered that dexamethasone could be quantified in plasma for 8.3 ± 2.9 days and in urine for 9.8 ± 3.1 days after administration, providing an approximate window for viable detection.
• The maximum observed concentration of dexamethasone was 0.61 ± 0.12 μg/L in plasma and 4.2 ± 0.9 μg/L in urine.
• It was also found that hydrocortisone, a hormone, was significantly suppressed for 140 hours following dexamethasone administration.
• The research determined the potency, efficacy and sigmoidicity factor of dexamethasone for hydrocortisone suppression, vital indicators in understanding the drug’s pharmacodynamics.
• These data were instrumental in concluding that the suppression of hydrocortisone in the horse directly relates to the plasma concentration of dexamethasone.

Implications and Future Research

• Understanding this relationship makes it possible to determine irrelevant plasma concentrations and set accurate screening limits for use in equestrian sports.
• The study present important findings that may be used in the prevention of drug misuse in competitive scenarios.
• The paper also suggests future research might explore the possibility of utilizing hydrocortisone suppression as a biomarker for the clinical effect of dexamethasone.