Inactivation of Gentamicin by Carbenicillin in the Urinary Tract

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Abstract
Inactivation of gentamicin by carbenicillin was studied in the canine urinary tract and in vitro. One ureter was cannulated to measure the excretion of the two antibiotics under conditions of normal renal function following the administration of 2 mg/kg of gentamicin and 200 mg/kg of carbenicillin. Ureteral concentrations of both antibiotics were not significantly different when they were administered in combination from values obtained when each drug was given alone. After 6 h both ureters were tied, the Foley catheter draining the bladder clamped, and the degree of gentamicin inactivation in bladder urine measured 18 h later. Mean bladder gentamicin concentrations declined 93% and bladder carbenicillin levels fell an average of 46% during this period, but residual antibiotic activity was still adequate to inhibit most clinically important gram-negative bacilli. These results were comparable to those in vitro. Inactivation of gentamicin by carbenicillin becomes significant when urine flow is impaired and inadequate drainage could be a factor exerting an adverse influence on gentamicin activity in extrarenal sites of infection as well.

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