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A method for the simultaneous determination of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) in canine plasma and urine has been developed. This method used derivatization of extracted samples with 9-anthrylmethane (ADAM) for analysis by reversed-phase high-performance liquid chromatography with fluorescence detection. Precision and accuracy were within the accepted limits of 15% and 85-115%, respectively, for both analytes in plasma and urine. Calibration curves for 2,4-D and MCPA in plasma were linear (r2 > 0.99) between 0.50 and 5.0 mg/L and 5.0 and 100 mg/L. Calibration curves for 2,4-D and MCPA in urine were linear (r2 > 0.99) between 5.0 and 70.0 mg 2,4-D/L and 10.0 and 70.0 mg MCPA/L. The lower limit of detection was 62.5 ng/mL for both 2,4-D and MCPA.

The objectives of the study were to compare various methods to determine flunixin in test samples collected periodically from horses after intramuscular (IM) and intravenous (IV) dosing at the maximum recommended dosage and to document detection times for this drug in test samples. Flunixin, a nonsteroidal anti-inflammatory drug approved for use in horses, was administered to eight mares in five consecutive daily doses of 1.1 mg per kilogram of body weight by the IM or IV route. Flunixin was detected in urine samples collected at various times after drug administration by flunixin enzyme-linked immunosorbent assay (ELISA), thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and gas chromatographic-mass spectrometric (GC-MS) methods. Detection time was defined as the time period over which flunixin was detected and was dependent on the method used. The shortest detection times were 24 to 48 h and were observed when the TLC method was used. On the other hand, detection times were as long as 15 days when HPLC, GC-MS, and flunixin ELISA methods were used. The use of these more sensitive tests to monitor official samples collected from racehorses could result in positive tests for flunixin when it is exerting no detectable clinical effects because it produces clinical effects lasting only 24-36 h in horses.

To compare pharmacokinetics of eltenac after first and last IV administrations (0.5 mg/kg), using a multiple dosing schedule.6 adult mares.Eltenac (50 mg/ml) was administered IV at a dosage of 0.5 mg/kg of body weight every 24 hours for days 0 through 4. On days 0 and 4, blood samples were collected before, then periodically for 8 hours after eltanac administration. Concentration of eltenac in plasma samples was determined by use of high-performance liquid chromatography.On day 0, median area under the plasma eltenac concentration versus time curve (AUC) was 6.77 microg.h/ml (range, 5.61 to 8.08 microg.h/ml), median plasma clearance was 1.23 ml/min/kg (range, 1.03 to 1.40 ml/min/kg), and median steady-state volume of distribution was 191 ml/kg (range, 178 to 218 ml/kg). Median terminal half-life of eltenac was 2.36 hours (range, 2.30 to 2.98 hours). On day 4, median eltenac AUC was 6.70 microg.h/ml (range, 5.21 to 7.44 microg.h/ml), median plasma clearance was 1.23 ml/min/kg (range, 1.12 to 1.53 ml/min/kg), and median steady-state volume of distribution was 193 ml/kg (range, 172 to 205 ml/kg). Median terminal half-life of eltenac was 2.40 hours (range, 2.11 to 3.25 hours). Protein binding of eltenac, determined by ultrafiltration, was99% at a total plasma concentration of 36 microg/ml.Pharmacokinetic variables determined for each horse were not different between days 0 and 4.Under conditions of this study, there was no clinically relevant accumulation of eltenac in equine plasma or alteration of pharmacokinetic variables after multiple IV dosing of 0.5 mg/kg of eltenac.

Cimetidine was administered intravenously and by the intragastric route to six mares at a dose of 4.0 mg/kg of body weight (bw). Specific and sensitive high performance liquid chromatographic methods for the determination of cimetidine in horse plasma and urine and cimetidine sulfoxide in urine are described. Plasma cimetidine concentration vs. time data were analysed by non-linear least squares regression analysis to determine pharmacokinetic parameter estimates. The median (range) plasma clearance (Cl) was 8.20 (4.96-10.2) mL/min.kg of body weight, that of the steady-state volume of distribution (Vdss) was 0.771 (0.521-1.15) L/kg bw, and that of the terminal elimination half-life (t1/2 beta) was 92.4 (70.6-125) minutes. The median (range) renal clearance of cimetidine was 4.08 (2.19-6.23) mL/min.kg bw or 55.4 (36.3-81.8)% of the corresponding plasma clearance. Cimetidine sulfoxide was excreted in urine and its urinary excretion through 8 h accounted for 12.0 (9.8-16.6)% of the plasma clearance of cimetidine. The median (range) extent of intragastric bioavailability was 14.4 (6.82-21.8)% and the maximum plasma concentration after intragastric administration was 0.31 (0.24-0.50) microgram/mL. Intravenous cimetidine had no effect on the disposition of intravenous phenylbutazone or its metabolites except that the maximum plasma concentration of gamma-hydroxyphenylbutazone was less after cimetidine treatment.

The respiratory stimulant prethcamide is a mixture of equal parts of crotethamide and cropropamide. A specific and sensitive gas chromatographic method for the determination of crotethamide and cropropamide in horse plasma and urine is described. Both components of prethcamide were extracted from plasma and urine into dichloromethane. The extracts were analyzed by capillary gas chromatography with thermionic detection in the nitrogen-specific detection mode. The lower limits of quantitation were 4.0 ng ml −1 of plasma and 10.0 ng ml −1 of urine. Calibration curves were linear from 2.0–100 ng ml −1 of plasma for both components. Pharmacokinetic parameters for crotethamide and cropropamide after intravenous and oral dosing were estimated by analysis of plasma concentration versus time data. The total plasma clearance of cropropamide was greater than that of crotethamide and both values were greater than 5 ml min −1 kg −1 . Renal clearance values of the two drugs were comparable and were much less than estimates of filtration clearance values in horses, indicating extensive re-absorption of both components from the renal tubules. Both compounds were metabolized by N-demethylation of the [(dimethylamino)-carbonyl]-propyl moiety and these metabolites were excreted in urine. The method was demonstrated to be suitable for detecting illicit administration of prethcamide to competition horses.

Sixteen young dairy cows were randomly allocated to 4 groups of 4 animals each: Group 1 cows were each given a single Ivomec SR Bolus; Group 2 cows were treated with the Ivomec Pour-on formulation; Group 3 cows were injected with the Ivomec Subcutaneous Injection; Group 4 cows were untreated controls. Blood and faecal samples were collected from all cows on the day before treatment and on days 1, 2, 3, 7, 14, 21, 28, 35, 42 and 49 after treatment for HPLC determination of plasma and faecal ivermectin concentrations. Group 1 (SR Bolus) cows had mean (+/- S.D.) plasma ivermectin concentrations ranging from 5.6 +/- 1.8 micrograms l-1 (p.p.b.) at 14 days to 11.0 +/- 4.7 micrograms l-1 at 49 days. Faecal ivermectin concentrations were little changed from 4.0 +/- 2.0 micrograms g-1 (p.p.m.) dry wt (dry weight) [0.5 +/- 0.2 microgram g-1 wet wt (wet weight)] at 14 days to 3.0 +/- 2.0 micrograms g-1 dry wt (0.5 +/- 0.4 microgram g-1 wet wt) at 49 days. Group 2 (Pour-on) cows showed a rapid rise in plasma concentrations to 32.9 +/- 15.7 micrograms l-1 2 days after treatment, followed by a gradual decline to 1.3 +/- 0.07 micrograms l-1 at 28 days. Faecal ivermectin concentrations rose sharply to 18.5 +/- 7.4 micrograms g-1 dry wt (2.8 +/- 1.2 micrograms g-1 wet wt) 2 days after treatment, then fell to 0.04 +/- 0.004 microgram g-1 dry wt (0.006 +/- 0.0004 microgram g-1 wet wt) at 28 days. Group 3 (Injection) cows also showed a rapid rise to an early plasma peak of 46.1 +/- 22.7 micrograms l-1 3 days after treatment, followed by a gradual decline to 1.3 micrograms l-1 at 35 days. Faecal ivermectin concentrations rose to 1.2 +/- 0.34 micrograms g-1 dry wt (0.2 +/- 0.05 microgram g-1 wet wt) at 3 days, declining to 0.08 +/- 0.0001 microgram g-1 dry wt (0.01 +/- 0.0008 microgram g-1 wet wt) at 28 days. No ivermectin was detected in the plasma or faeces of Group 4 (Control) cows. Concentrations of ivermectin potentially toxic to dung-breeding or dung-feeding invertebrates were excreted for the duration of the study in dung of cows treated with the SR Bolus and for 28 days in the dung of cows treated with the Pour-on or injectable formulations.

The disposition of florfenicol after single intravenous and intramuscular doses of 20 mg of florfenicol/kg of body weight (b.w.) to feeder calves was investigated. Serum florfenicol concentrations were determined by a sensitive high performance liquid chromatographic method with a limit of quantitation of 0.025 microgram/ml. The extent of serum protein binding of florfenicol was only 13.2% at a serum florfenicol concentration of 3.0 micrograms/ml. Serum concentration-time data after intravenous administration were best described by a triexponential equation. Total body clearance and steady state volume of distribution were 3.75 ml/min/kg b.w. and 761 ml/kg b.w., respectively. The terminal half-life after intravenous administration was 159 min. The absolute systemic availability after intramuscular administration was 78.5% (range: 59.3-106%) and the harmonic mean of the terminal half-life was 1098 minutes, indicating slow release of the florfenicol from the formulation at the intramuscular injection site.

Age and species reportedly affect the pharmacokinetic variables of nonsteroidal anti-inflammatory drugs. We determined the effect of age on flunixin pharmacokinetic variables in foals during the first month of life. We also estimated the physiologic activity of the drug in neonatal foals by determining the effect of flunixin on thromboxane production during clotting of blood taken from the foals. Flunixin disposition and clearance were determined after IV administration of 1.1 mg of drug/kg of body weight to 5 healthy foals when they were 24 to 28 hours, 10 to 11 days, and 27 to 28 days old. The area under the curve (2,471 micrograms.min/ml), mean residence time (477 minutes), and zero-time intercept of the elimination phase (4,853 ng/ml) were significantly (P = 0.05) greater, the elimination half-life (339 minutes) and slope of the elimination phase (0.002 L/min) were significantly (P = 0.05) longer, and total body clearance (0.482 ml/min/kg) and zero-time intercept for the distribution phase (2,092 ng/ml) were significantly (P = 0.05) lower at 24 to 28 hours. At each age, a biexponential equation was best fitted to the plasma flunixin concentration from each foal. Thromboxane B2 production during clotting of blood was significantly (P = 0.05) suppressed for 12 hours after flunixin meglumine administration at all ages. Therefore, it appears that although age does alter the disposition and elimination of flunixin in neonatal foals, this effect may be of little consequence because the drug's physiologic activity in foals appears similar to that in mature horses.

Flunixin meglumine and phenylbutazone are nonsteroidal anti-inflammatory drugs commonly used for the management of colic, endotoxemia, and musculoskeletal disorders in equids. Although it is not usually recommended, there appears to be an increasing trend to use nonsteroid anti-inflammatory drugs in combination to enhance or prolong their effects. Therefore, we studied the effect of concurrent administration of flunixin (1.1 mg/kg of body weight, IV) as flunixin meglumine and phenylbutazone (2.2 mg/kg, IV) on the pharmacokinetics of each drug and on in vitro thromboxane B2 production. Pharmacokinetic variables calculated for each drug when given alone and in combination were similar to those reported. Serum thromboxane B2 production was significantly (P = 0.05) suppressed for 12, 8, and 24 hours after administration of flunixin, phenylbutazone, and the drugs in combination, respectively. These results indicate that although concurrent administration of these drugs at the aforementioned dosages does not alter either drug disposition or clearance, it prolongs their pharmacologic effect.

Summary The haemodynamic, respiratory and behavioural effects and pharmacokinetics of methocarbamol were studied in eight healthy, adult horses after intravenous (iv) and oral administration of large dosages. Heart rate, cardiac output, mean pulmonary arterial blood pressure, systolic, diastolic and mean aortic blood pressure, respiratory rate and arterial blood gases did not change after either iv (30 mg/kg bodyweight [bwt]) or oral (50 and 100 mg/kg bwt) dosages of methocarbamol. Mild to moderate depression was observed in five of eight horses administered iv methocarbamol, and in all horses administered oral methocarbamol. Plasma methocarbamol concentration declined very rapidly during the initial or rapid disposition phase after iv administration; the terminal elimination half-life ranged from 59 to 90 mins. Peak plasma methocarbamol concentrations following oral administration occurred within 15 to 45 mins and oral bioavailability ranged from 50.7 to 124 per cent.

The pharmacokinetics and bioavailability of probenecid given IV and orally at the dosage level of 10 mg/kg of body weight to mares were investigated. Probenecid given IV was characterized by a rapid disposition phase with a mean half-life of 14.0 minutes and a subsequent slower elimination phase with a mean half-life of 87.8 minutes in 5 of 6 mares. In the remaining mare, a rapid disposition phase was not observed, and the half-life of the elimination phase was slower (172 minutes). The mean residence time of probenecid averaged 116 minutes for all 6 mares and 89.2 minutes for the 5 mares with biphasic disposition. The total plasma clearance of probenecid averaged 1.18 +/- 0.49 ml/min/kg, whereas renal clearance accounted for 42.6 +/- 9.3% of the total clearance. The steady-state volume of distribution of probenecid averaged 116 +/- 28.2 ml/kg. Plasma protein binding of probenecid was extensive, with 99.9% of the drug bound at plasma probenecid concentrations of 10 micrograms/ml. The maximum plasma probenecid concentration after 10 mg/kg orally averaged nearly 30 micrograms/ml. The half-life of probenecid after oral administration was approximately 120 minutes. Oral bioavailability was good with greater than 90% of the dose absorbed. The effect of probenecid on tubular secretion of organic anions was evaluated by determining the pharmacokinetics of IV cefazolin (11 mg/kg) administered alone and 15 minutes after probenecid (10 mg/kg orally). Treatment with probenecid did not affect pharmacokinetic values of cefazolin. This failure of probenecid to alter the pharmacokinetics of cefazolin may be caused by insufficient plasma probenecid concentrations after the oral dose.