Clenbuterol plasma concentrations after therapeutic administration in fit Standardbred horses: threshold recommendations

Clenbuterol, (RS)-1-(4-amino-3,5-dichlorophenyl)-2-(tert-butylamino)ethan-1-ol, as Ventipulmin is an FDA approved β2 agonist medication for the management of airway obstruction in horses. Administration above the FDA approved doses for clenbuterol produces repartitioning effects, which have led to restrictions on its use in human athletics and Quarter Horse and Thoroughbred racing. Clenbuterol, however has long been used therapeutically at FDA approved doses in Harness racing. The goal of this study was to identify a withdrawal time guideline for its use at FDA approvsed dose levels in Harness racing, where horses may start at seven-day intervals. Eight healthy, moderately fit Standardbred horses (4 mares, 4 geldings, weight 491±40 kg, age 13±2 years) were administered 0.8 μg/kg of clenbuterol as Ventipulmin syrup twice daily (BID) for three days. Blood samples were collected prior to dosing and at 1, 24, 48 and 96 h post administration. Clenbuterol was quantified in all samples using the New York Drug Testing and Research Laboratory ISO-17025 Racing and Medication Testing Consortium (RMTC) accredited quantitative procedure. The lower limit of quantitation of the method was 1.0 pg/ml, and three data points at 96 h post administration were censored. One horse developed diarrhoea and data from this horse was excluded from the overall analysis. Plasma regulatory thresholds were calculated using the 95/95 tolerance method and Gauss Camp Meidell at P=0.05 and P=0.001. Horses were also evaluated for effects of clenbuterol on body composition using body mass and ultrasound measurements of rump fat thickness. There were no effects (P>0.05) of clenbuterol on any of the measures including fat mass and fat free mass and thus no repartitioning effect was observed. The pharmacokinetic data and the 96 h data set support the therapeutic use of clenbuterol in Harness horses at the FDA approved 0.8 μg/kg BID dose for three days and suggest a 41 pg/ml regulatory threshold for a 96 h withdrawal time with a P=0.001 probability of randomly exceeding this regulatory threshold.

Descriptive statistics and the pattern of horse racing in New Zealand. 2. Harness racing

The aim of the present study was to describe the pattern of pacing and trotting races and starts, including temporal trends, in Harness racing in New Zealand. Data on all race starts between 1 August 2005 and 31 July 2010 were supplied by Harness Racing New Zealand. Descriptive statistics were used to describe the data at both race and starts level, stratified by pacing and trotting races. In total, 78% of races run were pacing races and most races occurred in the Southern region. The start type varied by race, with most pacing races run with mobile starts and trotting races with standing starts. Regardless of race type most races were run on all-weather tracks and were run in the summer or autumn. There was little change in the median number of starts per horse over time and the median number of starters per trainer per year was 16 (interquartile range 7–35, max. 612) and 12 (interquartile range 5–24, max. 235) for pacing and trotting races, respectively. Differences in track surfaces, gait, region and drivers between the Standardbred and the Thoroughbred industry suggests a need for data specific to the Harness racing industry in New Zealand. This study provided baseline data on the pattern of Standardbred racing and highlighted factors unique to the Harness racing industry in New Zealand.