Potassium penicillin and gentamicin pharmacokinetics in healthy conscious and anesthetized horses

To determine the effects of general anesthesia on the safety and efficacy of co-administered potassium penicillin G (PEN) and gentamicin (GENT) in horses.

Nonrandomized crossover.

Six adult, Thoroughbred horses.

Horses were administered PEN (22 000 IU/kg IV) and GENT (6.6 mg/kg IV). Plasma samples were collected over a 6 h period and synovial fluid was collected at 30 min and 6 h respectively. Drug administration and sample collection protocols were repeated after at least a 48 hour washout period and induction of anesthesia using xylazine/ketamine and maintenance with isoflurane gas. Drug concentrations were determined using ultrapressure liquid chromatography with mass spectrometry. A 2-compartment model was used to determine pharmacokinetics and differences were determined between conscious and anesthetized horses using paired t-tests (significance P < .05).

Potassium penicillin g and GENT had higher minimum plasma concentrations (PEN 0.44 vs. 0.11 μg/mL, P = .002; GENT 3.0 vs. 1.9 μg/mL, P = .009), longer half lives (PEN 71 vs. 59 min, P = .018; GENT 149 vs. 109 min, P = .038), and slower clearances (PEN 3.41 vs. 5.1 mL/kg/min, P = .005; GENT 1.18 vs. 1.48 mL/kg/min, P = .028) in anesthetized horses vs. conscious horses. The PEN concentrations remained above the breakpoint minimum inhibitory concentration (MIC, 0.5 μg/mL) for 332 min in anesthetized vs. 199 min in conscious horses. The GENT concentrations reached 10 times higher than the breakpoint MIC (2 μg/mL) in all horses and were maintained for 58 vs. 59 min in anesthetized and conscious states, respectively. Synovial fluid concentrations were higher in conscious horses vs. anesthetized horses at 30 min for PEN (7.0 vs. 0.93 μg/mL, P < .001) and 30 (5.3 μg/mL vs. 0.79 μg/mL, P < .001) and 360 min (3.4 vs. 1.82 μg/mL, P < .003) for GENT.

General anesthesia resulted in lower intrasynovial concentrations and delayed clearance of PEN/GENT in horses.

Redosing healthy anesthetized horses with PEN prior to 4-5 h is not necessary. When administered to anesthetized horses, intravenous PEN/GENT may not reach adequate intrasynovial concentrations to treat or prevent common pathogens. The doses or dosing intervals of antimicrobials administered to horses undergoing anesthesia may need to be adjusted to ensure maintenance of safe and effective plasma concentrations.