OBJECTIVE To characterize polymorphisms of the gene for cytochrome P450 isozyme 2D50 (CYP2D50) and the disposition of 2 CYP2D50 probe drugs, dextromethorphan and debrisoquine, in horses.
ANIMALS 23 healthy horses (22 Thoroughbreds and 1 Standardbred).
PROCEDURES Single-nucleotide polymorphisms (SNPs) in CYP2D50 were identified. Disposition of dextromethorphan (2 mg/kg) and debrisoquine (0.2 mg/kg) were determined after oral (dextromethorphan) or nasogastric (debrisoquine) administration to the horses. Metabolic ratios of plasma dextromethorphan and total dextrorphan (dextrorphan plus dextrorphan-O-β-glucuronide) and 4-hydroxydebrisoquine concentrations were calculated on the basis of the area under the plasma concentration-versus-time curve extrapolated to infinity for the parent drug divided by that for the corresponding metabolite. Pharmacokinetic data were used to categorize horses into the phenotypic drug-metabolism categories poor, extensive, and ultrarapid. Disposition patterns were compared among categories, and relationships between SNPs and metabolism categories were explored.
RESULTS Gene sequencing identified 51 SNPs, including 27 nonsynonymous SNPs. Debrisoquine was minimally detected after oral administration. Disposition of dextromethorphan varied markedly among horses. Metabolic ratios for dextromethorphan ranged from 0.03 to 0.46 (mean, 0.12). On the basis of these data, 1 horse was characterized as a poor metabolizer, 18 were characterized as extensive metabolizers, and 3 were characterized as ultrarapid metabolizers.
CONCLUSIONS AND CLINICAL RELEVANCE Findings suggested that CYP2D50 is polymorphic and that the disposition of the probe drug varies markedly in horses. The polymorphisms may be related to rates of drug metabolism. Additional research involving more horses of various breeds is needed to fully explore the functional implication of polymorphisms in CYP2D50.
To characterize the pharmacokinetics of mycophenolate mofetil (MMF) following single-dose IV or PO administration, characterize the pharmacokinetics of MMF following long-term PO administration, and describe the clinicopathologic effects of long-term MMF administration in horses.
12 healthy adult horses.
In phase 1, 6 horses received a single IV (2.5 mg/kg) or PO (5 mg/kg) dose of MMF in a randomized balanced crossover assessment (≥ 2-week interval between administrations). In phase 2, 6 other horses received MMF for 60 days (5 mg/kg, PO, q 24 h for 30 days and then 5 mg/kg, PO, q 48 h for an additional 30 days).
Following IV (single-dose) or PO (single- or multiple-dose) administration, MMF was rapidly converted to mycophenolic acid. For single-dose PO administration, mean ± SD maximum plasma mycophenolic acid concentration was 1,778.3 ± 441.5 ng/mL at 0.71 ± 0.29 hours. For single-dose IV administration, mean systemic clearance and volume of distribution at steady state were 0.689 ± 0.194 L/h/kg and 1.57 ± 0.626 L/kg, respectively. Following single doses, mean terminal half-life was 3.99 ± 0.865 hours for IV administration and 4.02 ± 1.01 hours for PO administration. The accumulation index following long-term PO administration was 1.0 ± 0.002, and the terminal half-life was 4.59 ± 1.25 hours following the final dose on day 60. None of the horses developed abnormal clinical signs or had any consistently abnormal clinicopathologic findings.
CONCLUSIONS AND CLINICAL RELEVANCE
Further investigation of the clinical efficacy of long-term MMF treatment of horses with autoimmune diseases is warranted.
Objective—To develop a simple extractionless method for detection of rosiglitazone in canine plasma and test the method in a pharmacokinetic study after oral administration of rosiglitazone in dogs.
Animals—3 client-owned dogs with cancer.
Procedures—High-performance liquid chromatography-tandem mass spectrometry was performed on canine plasma. The 3 dogs with cancer in the pharmacokinetic study were assessed via physical examination and clinicopathologic evaluation and considered otherwise healthy. Food was withheld for 12 hours, and dogs were administered a single dose (4 mg/m2) of rosiglitazone. Plasma was collected at various times, processed, and analyzed for rosiglitazone.
Results—The developed method was robust and detected a minimum of 0.3 ng of rosiglitazone/mL. Mean ± SD maximum plasma concentration was 205.2 ± 79.1 ng/mL, which occurred at 3 ± 1 hours, and mean ± SD elimination half-life was 1.4 ± 0.4 hours. The area under the plasma rosiglitazone concentration-versus-time curve varied widely among the 3 dogs (mean ± SD, 652.2 ± 351.3 ng/h/mL).
Conclusions and Clinical Relevance—A simple extractionless method for detection of rosiglitazone in canine plasma was developed and was validated with excellent sensitivity, accuracy, precision, and recovery. The method enabled unambiguous evaluation and quantitation of rosiglitazone in canine plasma. This method will be useful for pharmacokinetic, bioavailability, or drug-drug interaction studies. Oral rosiglitazone administration was well tolerated in the dogs.