Plasma and Urine Pharmacokinetics of Long-Acting Injectable Omeprazole Following Intramuscular Administrations to Healthy Thoroughbred Horses

Abstract

Omeprazole is a gastric acid secretion inhibitor used as an effective anti-ulcer drug. Based on oral administration studies, its International Screening Limit (ISL) was established in plasma and urine at 1 ng/mL with a Detection Time (DT) of 48 h. A novel formulation of injectable omeprazole has since been released, and therefore, a pharmacokinetic study was performed to assess the DT above the ISL against current advice. Six Thoroughbred horses were given four repeated weekly intramuscular administrations of omeprazole (4 mg/kg). Plasma and urine omeprazole concentrations were measured by liquid chromatography–tandem mass spectrometry. Based on the current plasma and urine ISL (1 ng/mL), the DT for this long-acting omeprazole formulation administered at 4 mg/kg once per week is greater than 384 h (16 days) in both plasma and urine. Thus realistically, despite the appeal of giving an injection once per week rather than oral medication daily over a long period of time, this would make treatment for horses in training with the long-acting product challenging within the rules of racing. It would therefore most likely be used for horses outside of training, and the oral formulation would still be legitimately used during training.