Penicillin and Gentamicin Concentrations in the Uterine Fluid of Non‐Pregnant Mares Following a Single Intrauterine Infusion

Despite their widespread clinical use, there is limited pharmacokinetic data for many equine intrauterine antimicrobials. This study aimed to measure the concentration of gentamicin and penicillin in the uterine fluid of mares following infusion of either a standard (PPGent) or long-acting (LA-PPGent) compounded formulation. We hypothesized that both formulations would result in therapeutic concentrations, with total concentrations sustained for longer using the long-acting formulation. Mares were administered 2400 mg of procaine penicillin and 200 mg of gentamicin via a single intrauterine infusion in either a standard (n = 6) or a lyophilized formulation suspended in a slow-release matrix (n = 6). Intrauterine fluid was collected over a 72-h period and analyzed for antibiotic concentrations using high-performance liquid chromatography and ultra-high-performance liquid chromatography and tandem mass spectrometry. Mean maximal concentrations were seen at 0.5 h in group PPGent (Penicillin: 10,123.0 ± 4298.0 μg/mL, Gentamicin: 3397.3 ± 1338.5 μg/mL) and exceeded MIC for relevant organisms for 72 h (Penicillin: 2.59 ± 6.34 μg/mL, Gentamicin: 2.14 ± 2.4 μg/mL). Interestingly, maximal concentrations were lower in group LA-PPG (Penicillin: 2213.8 ± 967.8 μg/mL-p < 0.05, Gentamicin: 1859 ± 2413 μg/mL) and exceeded MIC for a shorter period of time than the unmodified mixture of commonly used FDA-approved antibiotics.