The excretion of theobromine in Thoroughbred racehorses after feeding compounded cubes containing cocoa husk--establishment of a threshold value in horse urine

Systematic analysis to assess the scientific validity of the international residue limits for caffeine and theophylline in horse-racing

Based on their performance-enhancing potential, caffeine and theophylline are prohibited substances in equine sports. Residues in horses can be caused by wilful application or by unintended uptake of contaminated feed. The International Federation of Horseracing Authorities recently introduced international residue limits (IRLs) to facilitate the discrimination between pharmacological relevant and irrelevant concentrations in doping samples. The objective of this study was to investigate the scientific validity of these IRLs. A systematic analysis was performed to assess the IRLs by different statistical approaches using published pharmacokinetic data. 31 out of 218 potentially relevant publications met the inclusion criteria. Thereby, both IRLs were found to be appropriate for the exclusion of the presence of a relevant pharmacological effect after a wilful application. The IRL of theophylline was also determined to be suitable for the prevention of positive doping tests caused by the ingestion of contaminated feed. In contrast, the IRL of caffeine is not suitable to prevent positive doping test caused by the ingestion of more than 10 mg caffeine per day per horse with contaminated feed. The lack of corresponding regulation for paraxanthine, a major active metabolite of caffeine and theophylline, was recognised as a substantial shortcoming of the current system, rendering both IRLs incomplete.

Theobromine and the pharmacology of cocoa

The effects of theobromine in man are underresearched, possibly owing to the assumption that it is behaviourally inert. Toxicology research in animals may appear to provide alarming results, but these cannot be extrapolated to humans for a number of reasons. Domestic animals and animals used for racing competitions need to be guarded from chocolate and cocoa-containing foods, including foods containing cocoa husks. Research ought to include caffeine as a comparative agent, and underlying mechanisms need to be further explored. Of all constituents proposed to play a role in our liking for chocolate, caffeine is the most convincing, though a role for theobromine cannot be ruled out. Most other substances are unlikely to exude a psychopharmacological effect owing to extremely low concentrations or the inability to reach the blood-brain barrier, whilst chocolate craving and addiction need to be explained by means of a culturally determined ambivalence towards chocolate.

Urinary excretion of dietary contaminants in horses

REASONS FOR PERFORMING STUDY

Presence of drugs is completely prohibited in post racing urine samples by most of racing and competition authorities, even if environmental contamination might occur.

OBJECTIVES

To assess the daily dose of several contaminants absorbed through the diet that would result in detectable concentrations in urine.

METHODS

Caffeine, theobromine, theophylline, atropine, scopolamine, bufotenine, DMT or morphine were administered orally to 6 horses, in different dosages, for 3 days before their urine was sampled for regular anti-doping tests.

RESULTS

Theobromine, theophylline, bufotenine and morphine daily intake >10 mg, 2 mg, 10 mg and 200 microg, respectively, by a performance horse, were found to result in detectable urinary concentrations. At the 2 tested doses, atropine (5 and 15 mg) and dimethyltryptamine (3 and 10 mg) were not detected in urine. For caffeine and scopolamine, even the lowest dosage tested (5 mg/horse/day and 2 mg/horse/day respectively) induced detectable concentrations of the molecule in urine.

CONCLUSIONS

Horses fed dietary contaminants, even at level much below the effective dosage, may be positive to antidoping urine analysis. Further research is needed to gain more confident results on a daily safe intake for caffeine and scopolamine.

POTENTIAL RELEVANCE

Selection of feed materials appears to be of great importance to prevent non voluntary positive result to anti-doping tests.

Some aspects of doping and medication control in equine sports

This chapter reviews drug and medication control in equestrian sports and addresses the rules of racing, the technological advances that have been made in drug detection and the importance of metabolism studies in the development of effective drug surveillance programmes. Typical approaches to screening and confirmatory analysis are discussed, as are the quality processes that underpin these procedures. The chapter also addresses four specific topics relevant to equestrian sports: substances controlled by threshold values, the approach adopted recently by European racing authorities to control some therapeutic substances, anabolic steroids in the horse and LC-MS analysis in drug testing in animal sports and metabolism studies. The purpose of discussing these specific topics is to emphasise the importance of research and development and collaboration to further global harmonisation and the development and support of international rules.

Antagonism of adenosine receptors by caffeine and caffeine metabolites in equine forebrain tissues

OBJECTIVE

To determine the presence of adenosine receptor subtypes A1 and A2a in equine forebrain tissues and to characterize the interactions of caffeine and its metabolites with adenosine receptors in the CNS of horses.

SAMPLE POPULATION

Brain tissue specimens obtained during necropsy from 5 adult male research horses.

PROCEDURE

Membrane-enriched homogenates from cerebral cortex and striatum were evaluated by radioligand binding assays with the A1-selective ligand [3H]DPCPX and the A2a-selective ligand [3H]ZM241385. Functional responses to adenosine receptor agonists and antagonists were determined by a nucleotide exchange assay using [35S]-guanosine 5'-(gamma-thio) triphosphate ([35S]GTPgammaS).

RESULTS

Saturable high affinity [3H]DPCPX binding (A1) sites were detected in cerebral cortex and striatum, whereas high-affinity [3H]ZM241385 binding (A2a) sites were detected only in striatum. Caffeine and related methylxanthines had similar binding affinities at A1 and A2a sites with rank orders of drug binding affinities (theophylline > paraxanthine > or = caffeine >> theobromine) similar to other species. [35S]GTPgammaS exchange revealed that caffeine and its metabolites act as pure adenosine receptor antagonists at concentrations that correspond to A1 and A2a receptor binding affinities.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of our study affirm the presence of guanine nucleotide binding protein linked adenosine receptors (ie, high-affinity A1 and A2a adenosine receptors) in equine forebrain tissues and reveal the antagonistic actions by caffeine and several biologically active caffeine metabolites. Antagonism of adenosine actions in the equine CNS by these stimulants may be responsible for some central actions of methylxanthine drugs, including motor stimulation and enhanced racing performance.