Pharmacokinetics of gentamicin at steady-state in ponies: serum, urine, and endometrial concentrations

Gentamicin (GT) was administered IM to 6 healthy mature mare ponies at a dosage of 5 mg/kg of body weight every 8 hours for 7 consecutive days (total, 21 doses). Two venous blood samples were collected before (trough) and at 1 hour (peak) after the 5th, 10th, 14th, and 19th doses. An endometrial biopsy was done of each mare on days 4 and 7. On the 7th day, just before the 21st administration of GT, base-line blood samples were collected, and 22 blood samples were collected over a period of 48 hours after GT was given. The mares were catheterized on the 7th day, and urine was collected for 24 hours. Serum, urine, and endometrial GT concentrations were determined by a radioimmunoassay technique (sensitivity of 0.3 micrograms/ml of serum). Serum GT concentration data obtained from the terminal phase were best fitted by a 1-compartment open model with a biological half-life of 2.13 +/- 0.43 hours. Total body clearance and renal clearance were 1.69 +/- 0.41 and 1.40 +/- 0.26 ml/min/kg, respectively. Mean endometrial concentrations on day 4 and day 7 were 5.02 +/- 3.3 and 12.75 +/- 1.6 micrograms/g. To achieve mean serum GT concentrations (micrograms/ml) at steady state of 6.47 +/- 1.51, a maximum steady-state concentration of 12.74 +/- 1.60, and a minimum steady-state concentration of 1.43 +/- 0.57, a dosage of 5 mg/kg every 8 hours is recommended. Serum urea nitrogen, serum creatinine, and the fractional clearance of sodium sulfanilate were determined before and after GT treatment. Renal function remained within the base-line range during 7 days of GT administration.