Oral administration of tepoxalin in the horse: a PK/PD study.

This research paper focuses on the evaluation of tepoxalin, a non-steroidal anti-inflammatory drug, regarding its potential for clinical use in horses. The study examines the drug’s pharmacokinetics (the way the body processes the drug) and pharmacodynamics (the effects of the drug on the body) and raises concerns about its long-term use in equine treatment due to its inhibitory effects on COX-1 without significant anti-inflammatory activity.

Study Design and Administration of Tepoxalin

• The study involved six female horses, with some being fed and the others fasting.
• The horses were given an oral dose of 10mg/kg of tepoxalin in a cross-over study, meaning the horses would swap conditions to compare the drug’s effects.
• The researchers observed the conversion of tepoxalin to its metabolite, RWJ-20142, upon administration.

Analysis of Tepoxalin’s Absorption and Conversion

• The study noted that tepoxalin underwent a fast and wide-ranging conversion to RWJ-20142 after being administered.
• The presence of tepoxalin in the plasma was found to be nearly undetectable in fed animals, while in fasting animals, the levels were close to the lowest limit that the method could quantify.
• No differences were observed in the concentration levels of RWJ-20142 in the plasma between the fasting and fed horses, implying that the state of feeding didn’t significantly affect the metabolization of the drug.

Effects of Tepoxalin and its Metabolite on Horses

• It was found that tepoxalin had a strong and enduring ex vivo (outside the body) inhibitory activity against cyclooxygenase (COX)-1, predominantly due to its main metabolite RWJ-20142.
• Tepoxalin and RWJ-20142 didn’t display any notable inhibitory activity against COX-2 or 5-lipoxygenase in the horses, signifying that these aren’t the targeted sites of the drug.
• This information led the researchers to suggest that tepoxalin primarily serves as a selective COX-1 inhibitor in the horse, which means it specifically targets and inhibits the COX-1 enzyme, but does not provide substantial anti-inflammatory benefits.

Implications and Concerns

• Despite its analgesic and antipyretic properties, this characteristic of tepoxalin raises concerns about its long-term use in horses.
• Considering that COX-1 plays a role in several physiological functions such as maintaining the stomach lining and kidney function, long-term inhibition could have unforeseen consequences.
• Further research is necessary to fully understand the potential adverse effects of long-term use of tepoxalin in equine practice.