Influence of single or multiple doses of gentamicin and netilmicin on their cortical, medullary, and papillary distribution.

In this study, the comparative intrarenal distribution and accumulation of both gentamicin and netilmicin were investigated in normal rats. The animals received 1, 3, or 14 injections of 10 mg of gentamicin or netilmicin per kg. A total of 324 animals and 648 kidneys were analyzed. These animals were sacrificed at from 1 h to 25 days after the intraperitoneal injections. At each timed interval, the serum, urine, cortex, medulla, and papilla were analyzed for antimicrobial content. The peak serum values of gentamicin (8 to 12 mug/ml) and netilmicin (9 to 14 mug/ml) were close to those found in humans. There was a progressive increase in the cortical concentrations of both antibiotics from a low of 53 mug/g to a high of 719 mug/g when the animals were given increasing doses of the agents. At most timed intervals, the concentrations of gentamicin within the cortex were slightly lower than those of netilmicin. The accumulation of the drugs was also demonstrated in the medulla, where therapeutic levels could be detected for up to 25 days after the cessation of therapy. Even though gentamicin and netilmicin were transiently present in the papilla after 1 or 3 injections, both drugs could still be detected in the papilla on day 25 after 14 doses. The urinary concentration closely paralleled the papillary concentration. If applicable to humans, the persistence of high levels of both drugs within the medulla and papilla may have therapeutic implications.