Drug disposition and dosage determination of once daily administration of gentamicin sulfate in horses after abdominal surgery

Objective-To evaluate pharmacokinetics of once daily IV administration of g entamicin sulfate to adult horses that had abdominal surgery. Design-Prospective study. Animals-28 adult horses that underwent abdominal surgery for colic. Procedure-14 horses were treated with each dosage of gentamicin (ie, 6.6 or 4 mg/kg, IV, q 24 h) and blood samples were collected for pharmacokinetic analysis. Plasma gentamicin concentrations were measured by use of a fluore scence polarization immunoassay. Pharmacokinetic analysis measured the elim ination half-life, volume of distribution, and gentamicin total systemic cl earance. Treatment outcome, CBC, and serum creatinine concentrations were r ecorded. Results-1 horse in the high-dosage group died. Ail other horses successfull y recovered, and did not develop bacterial infection or have evidence of dr ug toxicosis resulting in renal injury. Mean pharmacokinetic variables for gentamicin administration at a high or low dosage (ie. 6.6 or 4 mg/kg, IV, q 24 h) were half-life of 1.47 and 1.61 hours, volume of distribution of 0. 17 and 0.17 L/kg, and systemic clearance of 1.27 and 1.2 ml/kg/min, respect ively. Mean serum creatinine concentration was 1.74 and 1.71 for the high a nd low dosages, respectively and serum creatinine concentration was not cor related with gentamicin clearance, Conclusions and Clinical Relevance-Gentamicin administration at a dosage of 4 mg/kg, IV, every 24 hours, will result in plasma concentrations that are adequate against susceptible bacteria with a minimum inhibitory concentrat ion (MIC) of less than or equal to 2.0 mu g/ml. Gentamicin administration a t a calculated dosage of 6.8 mg/kg, IV, every 24 hours will result in optim um plasma concentrations against susceptible bacteria with a MIC of less th an or equal to 4.0 mu g/ml.