To provide a full pharmacokinetic/ pharmacodynamic (PK/PD) evaluation, Toutain and Lassourd (2002) recommended a system of drug modelling which relies on the calculation of the effective plasma concentration (EPC) of a drug based upon detailed pharmacokinetic studies. From this concentration it is possible to calculate the plasma concentration which would be ineffective and hence irrelevant. The Irrelevant Urine Concentration (IUC) can be calculated from the irrelevant plasma concentration (IPC). The application of the model to literature based data for pharmacokinetic studies of various drugs in horses has been published by Toutain and Lassourd (2002).
The Irrelevant Urinary (or plasma) Concentration (IUC) of the drug is determined by processing the study data in an established way (the Pierre-Louis Toutain Model). To establish a provisional screening limit based on this IUC, the member nations of the EHSLC agree an ordinal scale, Risk management is then use to apply the IUC to a point on this ordinal scale. Risk management is the process of weighting policy alternatives to accept, minimize or reduce the assessed risk and, on the basis of this process, to select and implement the appropriate option regarding prevention, control or regulation measurers. Factors considered in risk management include the extent of use of the drug in equine veterinary practice and its potential to affect the welfare of the horse or to improve its performance
In order to be applicable to the model, the drug must act systemically, i.e. the pharmacological effect should be directly related to the plasma concentration. Thus, local anaesthetics, substances administered intra-articular or by inhalation are not considered suitable candidates. Nevertheless, in some instances, a PK/PD approach for non-systemically acting substances can be applied to indicate minimum analytical performance requirements. In addition it was considered that:
•Substances which exert their delayed effect by a cascade of indirect mechanisms are generally not considered suitable candidates unless the effect of interest can be clearly related to plasma or urine concentrations;
•The presence of 2 or more pharmacologically related substances (cocktails) invalidated the model; and
•The presence of drug plus a diuretic invalidates the model.
A collaborative research programme has been undertaken by the racing laboratories of the nations of the EHSLC to generate reliable pharmacokinetic data for a number of therapeutic substances. Using the above criteria, drugs were chosen for study on the basis of their regular use in the treatment of racehorses. The laboratories of the Hong Kong Jockey Club and the Jockey Club of Southern Africa also participated in this experimental programme.
Robust guidelines apply to the research programme on: Test substance; Animal administrations; Sample collection; Assay validation; Sample storage; Sample analysis; Data analysis.
Animal administration studies Development and validation of appropriate quantitative methods Sample analysis following the administration studies
Careful control of test substance Details for preparation of calibration curve Samples to be analysed as rapidly as possible after collection
A single pilot administration study Testing for linearity Appropriate sample storage conditions to be applied
Main administration study - minimum of 6 animals Assay validation Analyse stability to be monitored
Administration studies completed in a short time- frame Intra – and Inter – assay precision Appropriate QC samples to be included in each batch analysis
Regular sample collection (plasma) Accuracy
Appropriate urinary collection period Specificity
Adherence to national and European regulation on welfare Sensitivity
Selection of an appropriate LOQ
Following these guidelines, pharmacokinetic data for a number of substances have been generated.
A minimum of six horses is preferred for each Detection Time study. The administered drug must be a proprietary preparation available for equine veterinary use or a preparation of a drug commonly used in the treatment of competition horses. The drug/preparation must be administered at a recommended dosing regimen used in equine veterinary practice. If several dosing regimens exit, the 'consensus' dose generally should be selected.
Following the administration, the collection period for biological fluids of interest (urine and/or blood) should be over a time period that ensures samples containing the drug, at concentrations below the provisionally determined screening sensitivity limit, are available from all horses in the study. For the analysis of the biological fluids, ideally the analytical method used should have the sensitivity to effectively detect the drug at a concentration corresponding to the next point lower on the ordinal scale than the provisionally agreed screening limit for the drug. If this is unattainable, the analytical method should effectively detect the drug at a concentration corresponding to ½ the provisionally agreed screening limit in the biological fluid under study. The analytical methods developed must be validated according to accepted practices. Analysis of all post administration samples by this approach provides a full excretion profile for the drug in the biological fluid under study and allows detection times to be determined for a range of proposed screening limits.