Pharmacokinetics and estimated bioavailability of amoxicillin in mares after intravenous, intramuscular, and oral administration.

The research examines how the body absorbs and eliminates the antibiotic amoxicillin when administered to horses in different ways. It was found that amoxicillin gets distributed and eliminated rapidly into the body, potentially requiring frequent dosing. This, along with the unpredictable absorption in horses with gastrointestinal issues, suggests the need for careful dosage and monitoring.

Administration Methods and Their Impact

• The study began by administering sodium amoxicillin intravenously (directly into the blood) at a dose of 10 mg/kg. This drug administration method, under a two-compartment open model, revealed a quick distribution phase followed by a rapid elimination phase. The average half-life of the dose was approximately 39.4 minutes. This time represents how long it took for half the dose to be eliminated from the body. Volume of distribution and body clearance rates were also noted.
• These findings suggest that to achieve consistently therapeutic concentrations of amoxicillin in a horse’s bloodstream, frequent intravenous dosing may be necessary. However, it’s suggested that IV use be reserved for initiating treatments or managing intensive care situations due to its rapid distribution and elimination.

Pharmacokinetics of Oral Administration

• Next, the antibiotic was administered orally in the form of a 5% cherry-flavored suspension of amoxicillin trihydrate (20mg/kg). This route showed that the drug was absorbed quickly but not entirely, and it was also eliminated quickly with an average half-life of about 51 minutes.
• The average estimated bioavailability, which refers to the fraction of the drug that successfully enters the bloodstream, was found to be 10.4%. It means that only about one-tenth of the administered dose was actually absorbed into the system.
• Peak plasma concentrations of amoxicillin were reached approximately 1.5 hours post-dosing, with mean concentrations of around 2.73 micrograms/ml. However, one horse exhibiting signs of abdominal pain and diarrhea demonstrated delayed and increased absorption, indicating that absorption might be inconsistent in horses suffering from gastrointestinal diseases.
• Given amoxicillin’s relatively low bioavailability and fast elimination rate, the researchers suggest its oral form should only be used for treating infections caused by bacteria that are highly susceptible to the antibiotic, and that frequent dosing would likely be required.