The current performance of analytical techniques used for drug control in horses lead the Regulatory Authorities to decide whether trace levels of drugs legitimately used for therapeutic medication should or should not be reported. Here, we propose a well-ordered and nonexperimental pharmacokinetic/pharmacodynamic approach for the determination of irrelevant drug plasma (IPC) and urine concentrations (IUC). The published plasma clearance is used to transform an effective (marketed) dose into an effective concentration (EPC). EPC is transformed into an IPC by applying a safety factor (SF). This method is based on several assumptions (eg, drug effects reversibly driven by plasma concentration, linearity of drug disposition). The suitability of the computed IPC and IUC can be checked by calculating the residual amount of drug at IPC and computing a minimal drug withdrawal time. It is concluded that controlling the drug effect (using drug or any analyte concentration as a marker) rather than the drug exposure will be more demanding and also makes urine a less than ideal matrix.

To quantify the extent of muscle alteration during prolonged exercise, the release rate of creatine kinase (CK) from striated muscle was measured in six horses during a rest period (6 h) and during three exercise tests (15, 30, and 60 km) at a constant speed of 200 m/min. CK clearance was measured after intravenous bolus administration (150 U/kg) of a CK solution obtained from horse muscle. The CK steady-state volume of distribution was 0.059 +/- 0.0215 l/kg, the terminal half-life was 123 +/- 28 min, and the plasma clearance was 0.36 +/- 0.10 ml.kg-1 x min-1. After an intramuscular CK administration, the CK systemic availability was 74.1 +/- 21.2% and the half time of absorption was 9.4 +/- 5.7 h, indicating a slow process for CK transit through the lymphatic system. The CK release rate was only significantly increased during the 60-km exercise test. The increase of CK plasma activity was observed after a delay of approximately 5 h and peaked after the end of the race; the estimated CK release rate was 9.92 +/- 2.62 U.kg-1 x h-1 over a mean duration period of 65.8 +/- 15.8 h. With the CK activity of horse striated muscle taken into account, a 60-km race released a quantity of CK corresponding to an equivalent of 18.8 +/- 4.3 g striated muscle. It is concluded that the equivalent amount of damaged muscle may be considered as negligible for a 60-km test and that only very high plasma CK activity levels (at least higher than 10,000 U/l) may provide some evidence of a myolysis.

The influence of a 56-km endurance exercise on cortisol kinetics and production rate was evaluated in six horses administered [3H]cortisol. Exercise resulted in an immediate two- to threefold increase in plasma cortisol, with values returning very rapidly to preexercise levels. During exercise, clearance and steady-state volume of distribution of total cortisol were greatly increased (338 +/- 95 vs. 137 +/- 34 ml.kg-1.h-1 for clearance and 359 +/- 82 vs. 229 +/- 18 ml/kg for volume of distribution), whereas the terminal half-life decreased significantly (0.97 +/- 0.16 vs. 1.55 +/- 0.33 h). The estimated cortisol production rate was 4.41 +/- 1.06 micrograms.kg-1.h-1 at rest and 26.75 +/- 5.11 micrograms.kg-1.h-1 during exercise. We conclude that exercise triggers a large increase (x 6) in the adrenal secretion rate, which is not accurately reflected by the more limited increase (x 2-3) in plasma cortisol concentration, the actual measurement of plasma cortisol clearance being a prerequisite to assessment of adrenal gland function during exercise.

The present study was undertaken to measure the weight of muscle destroyed by an intramuscular injection of phenylbutazone (PBZ) in horses. In six horses, CK disposition parameters were evaluated after intravenous (i.v.) and intramuscular (i.m.) administration of a CK horse preparation. The same horses received PBZ, a potentially irritating agent, by i.v. and i.m. (neck and hindquarter) routes. Data were analysed using compartmental approaches and instantaneous CK flux was calculated using a discrete deconvolution method. For a 150 U/kg CK dose, the steady-state volume of distribution was 0.050 +/- 0.0115 L/kg and the plasma half-life was 112 +/- 18 min. After CK i.m. administration, the half-life of the terminal phase was 11.8 +/- 5.3 h indicating a flip-flop process and the mean bioavailability of CK was close to 100%. After PBZ i.m. administration, the CK activity was significantly increased with peak values of 508 +/- 109 U/L after the neck administration and 873 +/- 365 U/L after the gluteal administration. By measuring the total amount of CK released from injured muscle, it was calculated that an equivalent of 0.044 +/- 0.029 g/kg of muscle was destroyed after PBZ administration in the neck. The corresponding figure was 0.118 +/- 0.048 g/kg after intragluteal PBZ administration. By deconvoluting plasma CK activity, it was shown that the CK entry rate was maximum for the first 30-60 min following PBZ administration, which then decreased slowly to return to the control value after a delay of 24-48 h after PBZ administration. It was concluded that the CK release pattern following a controlled muscular damage was a non-invasive approach useful for quantifying the amount of damaged muscle, and that the calculation of CK input rate by deconvolution was of potential interest in describing events at the muscle cell level.

Summary The purpose of the present report was to estimate the population parameters of cortisol concentrations in urine, an endogenous hormone used as a ‘doping’ agent and for which an international threshold (1.0 μg/ml) has been proposed. Two data bases (French and UK) corresponding to 112 and 142 samples, respectively were considered. Urine was collected under specific post competition conditions. Cortisol concentrations were obtained by validated methods (HPLC for the French samples, and GC‐MS for UK samples). No difference was observed between the 2 data sets and statistical analyses were carried out on the two merged flies. The overall geometric mean cortisol concentration was 48 ng/ml. Distribution was not Gaussian. A log‐normal distribution was not rejected (for P>0.05). Using the log‐normal distribution, it was calculated that the probability of exceeding a cortisol concentration in urine of 1.0 μg/ml was 1.1 times 10−4. It was concluded that the actual international threshold is specific i.e. robust with regard to the risk of erroneously declaring an unmedicated horse as positive.