Inactivation of Gentamicin by Carbenicillin in the Urinary Tract

Abstract

Inactivation of gentamicin by carbenicillin was studied in the canine urinary tract and <i>in vitro</i>. 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 <i>in vitro</i>. 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.