Characterization of gentamicin renal disposition using isolated perfused rat kidney model

Gentamicin (GTM) is a broad spectrum antibiotic used to treat a variety of gram positive as well as gram negative bacterial infections with low incidences of tolerance. Therapeutic use of GTM is mainly restricted to life threatening infections as the compound is nephrotoxic at therapeutic doses. Despite the wealth of information available in literature regarding GTM, the cause of this toxicity has not been discovered. Elucidation of this pathway is believed to provide insight into the mechanism of GTM induced nephrotoxicity.; In the present investigation, the isolated perfused rat kidney (IPK) model was used to gain insight into the renal disposition kinetics of GTM. Several studies were performed including a dose range study, a urinary alkalization study, and a drug interaction study. Based on the findings from these studies, potential strategies for limiting GTM induced nephrotoxicity were developed (by reducing GTM kidney accumulation); the strategies were tested using the IPK model. While the IPK is a useful technique, it is important to assess correlation of IPK studies with in vivo drug excretion. In this dissertation, the relevance of IPK methodology as a model for studying drug excretion was also assessed by comparing the results generated from IPK studies (ex vivo) to a parallel study in rats in vivo.; The dose range study analyzed the effect of increasing GTM concentration on the kidney function, drug elimination and kidney accumulation. The urinary alkalization study employed sodium bicarbonate (NaHCO3), and analyzed the effect of increasing the urine pH on GTM disposition. The drug interaction study employed cimetidine (CMT), a classic organic cation transport (OCT) inhibitor, to assess the role of this transport pathway on GTM renal disposition.; Dose-linearity studies found that AUC0-t (%dose) did not increase linearly with GTM dose, where as urinary excretion was non-linear (↓as dose↑). To understand this phenomenon and identify the origin of nonlinear behavior, urinary alkalization studies were performed. Literature findings suggest that GTM accumulates in the proximal tubule cells by endocytosis and lysosomal sequestration, being preceded by binding to the luminal membrane. This phenomenon seems to be electrostatic in nature, and alteration in the cationic charge of the GTM molecule may influence the binding and subsequent epithelial uptake of GTM by a charged affinity for the luminal membrane. In the present study, urinary alkalization by co-administration of NaHCO 3 (which caused a ↓ ionization of GTM) caused an increase in the renal clearance of GTM, consistent with this hypothesis. On the other hand, co-administration of CMT did not alter the renal clearance, but decreased GTM kidney accumulation by 51%. This observation suggests that GTM enters the kidney cells via the OCT mediated transport.; Based on the results from the above-mentioned studies, the synergistic effect of transport inhibition and urinary alkalization (by co-administration of CMT and NaHCO3) on GTM disposition was studied in the IPK. The study revealed that GTM kidney accumulation decreased by 80%, indicating that GTM enters the kidney cells via both luminal as well as the basolateral membrane uptake. Overall, urinary alkalization together with transport inhibition could be a potential strategy to limit GTM induced nephrotoxicity, assuming kidney accumulation is the precursor for its nephrotoxicity.; Finally, the renal excretion of GTM was characterized in rats in vivo, and the results were compared to IPK findings. In vivo GTM pharmacokinetic parameter estimates compared favorably with clinical pharmacokinetic data, suggesting that rat is a useful model for studying the kidney disposition of GTM. Furthermore, based on the observed correlation to IPK data, a good prediction of renal clearance can be gleaned using IPK studies, provided that the limits of the IPK are considered and that the data is corrected for the GFR. Ho...