Disposition and excretion of 6-methoxy-2-naphthylacetic acid, the active metabolite of nabumetone in horses.

This research article investigates how horses metabolize and expel 6-methoxy-2-naphthylacetic acid (6MNA), a compound found in the anti-inflammatory drug nabumetone. The study discovers that horses can metabolize nabumetone to 6MNA, similar to other species, although some metabolites in the urine were not fully identified.

Methodology

• The research used a group of 5 horses administered with 3.7 mg of nabumetone per kg of their body weight orally. This setup was designed to understand the pharmacokinetics of 6MNA following the oral administration of nabumetone.
• After a three-week washout period (a period necessary to ensure no traces of the initial drug remained), 4 horses were given an intravenous (IV) injection of 2.5 mg of 6MNA per kg of body weight. The purpose of this phase was to study the dispersion of 6MNA when it is directly introduced into the bloodstream, bypassing the metabolic process.

Results

• The absorption of nabumetone into the horses’ system and subsequent metabolism into 6MNA had a median appearance half-life of 0.88 hour. This means that it took this much time for half of the nabumetone to be converted to 6MNA. The elimination half-life, i.e., the time taken for the concentration of 6MNA to reduce by half in the body, was 11 hours.
• The study found that orally administered nabumetone resulted in an area under the plasma concentration time curve (AUC) for 6MNA, calculated to be 120.6 mg/h/L. The AUC is a key measurement in pharmacokinetics, indicating the total drug exposure over time.
• By counterpoint, an equivalent plasma concentration of 6MNA was achieved by the IV-administered dose of 2.5 mg/kg. This directly injected dose produced an AUC of 117.0 mg/h/L, which is slightly lower than the oral dosage. This finding suggests that 6MNA is readily absorbed and metabolized in horses, whether administered orally or intravenously.
• The method found that 7.4% of the total dose was recoverable in the urine after oral administration and 5.3% after IV administration over a 36-hour collection period, hinting at the excretion patterns of the 6MNA in horses.

Conclusions

• The research concludes that horses do metabolize nabumetone into 6MNA, which aligns with findings in other species. Some additional metabolites were found in the urine post-administration, but these were not completely identified or characterized, suggesting further areas for investigation.