Internal Medicine Resident Colorado State University Fort Collins, CO, United States
Abstract: Background – Ondansetron is a 5‐HT3 serotonin receptor antagonist which is commonly prescribed as an anti-nausea and anti-emetic medication for veterinary patients. Ondansetron bioavailability is highly variable amongst species and there have been no cross-over studies evaluating the bioavailability of ondansetron in client-owned dogs. Objectives – To evaluate the bioavailability of ondansetron, comparing oral (PO) to intravenous (IV) administration.Animals – Eight healthy client-owned dogs. Methods – Dogs were randomized to one of two protocols in a crossover design, receiving PO or IV ondansetron at a dose of 1mg/kg on Day 0 and the opposite formulation at an equal dose on Day 7. Plasma was collected at baseline and 1, 2, 4, and 8 hours post administration. Ondansetron concentrations were measured utilizing liquid chromatography.Results – For IV administration, mean Cmax was 630 +/- 399 ng/ml and AUC0–8h was 1181 +/- 619 ng/ml*h, with all dogs having detectable plasma concentrations at all time points. For PO administration, mean Cmax was 22 11.3 ng/ml and AUC0–8h was 61.7 +/- 45.4 ng/ml*h, with all dogs having undetectable concentrations at various time points. Oral mean bioavailability was less than 10%.Conclusions and clinical importance – The oral bioavailability of ondansetron is very low in healthy dogs, raising concern for the efficacy of this drug when given orally at 1 mg/kg. Future studies evaluating the pharmacodynamics of ondansetron in nauseous client-owned dogs should be performed to investigate whether plasma drug concentrations are the optimal way to assess the efficacy of oral ondansetron.
