Objective—To compare pharmacokinetics of triamcinolone acetonide (TA) following IV, intra-articular (IA), and IM administration and determine its effect on plasma concentrations of hydrocortisone and cortisone.
Procedures—TA (0.04 mg/kg) was administered IV, IM, or IA, and plasma TA, hydrocortisone, and cortisone concentrations were determined.
Results—IV administration of TA was fitted to a 2-compartment model. Median distribution half-life was 0.50 hours (range, 0.24 to 0.67 hours); elimination half-life was 6.1 hours (range, 5.0 to 6.4 hours). Transfer half-life of TA from joint to plasma was 5.2 hours (range, 0.49 to 73 hours); elimination half-life was 23.8 hours (range, 18.9 to 32.2 hours). Maximum plasma concentration following IA administration was 2.0 ng/mL (range, 0.94 to 2.5 ng/mL), and was attained at 10 hours (range, 8 to 12 hours). Maximum plasma concentration following IM administration was 0.34 ng/mL (range, 0.20 to 0.48 ng/mL) and was attained at 13.0 hours (range, 12 to 16 hours); concentration was still quantifiable at 360 hours. Hydrocortisone plasma concentrations were significantly different from baseline within 0.75, 2, and 1 hours after IV, IA, and IM administration, respectively, and remained significantly different from baseline at 96 and 264 hours for IV and IA administration. Following IM administration of TA, plasma concentrations of hydrocortisone did not recover to baseline concentrations by 360 hours.
Conclusions and Clinical Relevance—Pharmacokinetics of TA and related changes in hydrocortisone were described following IV, IA, and IM administration. A single administration of TA has profound effects on secretion of endogenous hydrocortisone.
Objective—To determine the pharmacokinetics of methylprednisolone (MP) and develop a pharmacokinetic-pharmacodynamic model of the related changes in plasma concentrations of endogenous hydrocortisone (HYD) and cortisone (COR) following intra-articular administration of methylprednisolone acetate (MPA) in horses.
Procedures—In each horse, 200 mg of MPA was injected intrasynovially into a carpal joint, and plasma MP, HYD, and COR concentrations were determined via liquid chromatography-mass spectrometry.
Results—A 5-compartment pharmacokinetic-pharmacodynamic model was used to describe the concatenated changes in the plasma concentrations of MP, HYD, and COR and to estimate the instantaneous rate of endogenous HYD production. The median transfer half-life (t1/2t) of methylprednisolone from the joint to plasma and elimination half-life (t1/2e) from plasma were 1.7 and 19.2 hours, respectively. Maximum plasma concentration of methylprednisolone was 7.26 ± 3.3 ng/mL at 8 hours, which decreased to 0.11 ± 0.08 ng/mL at 144 hours after injection. At 3 hours after MPA administration, plasma COR and HYD concentrations were significantly decreased from baseline values (from 2.9 ± 0.28 ng/mL to 2.10 ± 1.0 ng/mL and from 61.1 ± 18.9 ng/mL to 25.7 ± 12.1 ng/mL, respectively).
Conclusions and Clinical Relevance—The sensitivity of the analytic method used allowed complete description of the related kinetics of MP, HYD, and COR following intra-articular administration of MPA. A single intra-articular administration of MPA profoundly affected the secretion of HYD and COR in horses; secretion of endogenous corticosteroids remained suppressed for as long as 240 hours after injection.
Objective—To evaluate whether urine supernatant contains amplifiable DNA and to determine factors that influence genotyping of samples from racehorses after storage and transportation.
Sample Population—580 urine, 279 whole blood, and 40 plasma samples obtained from 261 Thoroughbreds and Standardbreds.
Procedures—Genomic DNA was isolated from stored blood and urine samples collected from racehorses after competition. Quantified DNA was evaluated to determine whether 5 equine microsatellite loci (VHL20, HTG4, AHT4, HMS6, and HMS7) could be amplified by use of PCR techniques. Fragment size of each amplified locus was determined by use of capillary electrophoresis.
Results—High–molecular-weight and amplifiable DNA were recovered from refrigerated blood samples, but recovery from urine varied. Deoxyribonucleic acid was recovered from both urine supernatant and sediment. Freeze-thaw cycles of urine caused accumulation of amplifiable DNA in the supernatant and clearance of naked DNA. Repeated freeze-thaw cycles significantly decreased DNA yield and induced DNA degradation, which resulted in failure to detect microsatellite loci. Select drugs detected in test samples did not affect PCR amplification. Contaminants in DNA isolates inhibited PCR amplification and resulted in partial microsatellite profiles.
Conclusions and Clinical Relevance—Properly stored urine and blood samples were successfully genotyped, but subjecting urine to freeze-thaw cycles was most detrimental to the integrity of DNA. Increasing the volume of urine used improved recovery of DNA.
Objective—To determine pharmacokinetics and
excretion of phenytoin in horses.
Animals—6 adult horses.
Procedure—Using a crossover design, phenytoin
was administered (8.8 mg/kg of body weight, IV and
PO) to 6 horses to determine bioavailability (F).
Phenytoin also was administered orally twice daily for
5 days to those same 6 horses to determine steadystate
concentrations and excretion patterns. Blood
and urine samples were collected for analysis.
Results—Mean (± SD) elimination half-life following a
single IV or PO administration was 12.6 ± 2.8 and 13.9
± 6.3 hours, respectively, and was 11.2 ± 4.0 hours following
twice-daily administration for 5 days. Values for
F ranged from 14.5 to 84.7%. Mean peak plasma concentration
(Cmax) following single oral administration
was 1.8 ± 0.68 µg/ml. Steady-state plasma concentrations
following twice-daily administration for 5 days
was 4.0 ± 1.8 µg/ml. Of the 12.0 ± 5.4% of the drug
excreted during the 36-hour collection period, 0.78 ±
0.39% was the parent drug phenytoin, and 11.2 ±
5.3% was 5-(p-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). Following twice-daily administration for 5
days, phenytoin was quantified in plasma and urine
for up to 72 and 96 hours, respectively, and p-HPPH
was quantified in urine for up to 144 hours after
administration. This excretion pattern was not consistent
in all horses.
Conclusion and Clinical Relevance—Variability in F,
terminal elimination-phase half-life, and Cmax following
single or multiple oral administration of phenytoin
was considerable. This variability makes it difficult to
predict plasma concentrations in horses after phenytoin
administration. (Am J Vet Res 2001;62:483–489)