Disposition, elimination, and bioavailability of phenytoin and its major metabolite in horses.

The research article discusses the study of how the drug phenytoin, and its major metabolite, is processed and excreted in horses. The results of the study provide information about the variability in how the drug is absorbed, distributed and excreted, making it challenging to predict the drug concentration in horses’ blood after administration.

Methodology and Results

• The study was conducted on six adult horses using a crossover design. This means that the same horses were used for all phases of the experiment to provide more reliable results and reduce variability.
• The drug phenytoin was administered both intravenously (IV) and orally (PO) at a dose of 8.8mg per kg of the horse’s body weight.
• The bioavailability of the drug, which is the proportion of the drug that enters the system and starts having an active effect, varied between 14.5% to 84.7% across the six horses.
• The average elimination half-life for phenytoin was calculated to be around 12.6 hours and 13.9 hours following single IV and PO administration respectively. The elimination half-life is the time it takes for the body to reduce the concentration of the drug by half. This was slightly lower, at 11.2 hours, after the drug was given orally for five consecutive days.
• The study also measured the peak plasma concentration of phenytoin (Cmax) following single oral administration, which was found to be 1.8 micrograms per milliliter.
• In terms of excretion, about 12% of the drug was found to be discharged during a 36-hour collection period, the majority of which was in the form of its metabolite 5-(phydroxyphenyl)-5-phenylhydantoin (p-HPPH).
• The research found that there was significant variability in results across the different horses and methods of administration.

Conclusion

The key takeaway from this study is the considerable variability in the absorption, distribution, metabolism and excretion of phenytoin in horses. This variability was observed in factors including the drug’s bioavailability, the terminal elimination phase half-life and peak plasma concentration. This inconsistency makes it difficult to accurately predict the concentration of phenytoin in plasma after administration in horses. This calls for further investigation into the factors influencing this variability.