Renal handling of fleroxacin in rabbits, dogs, and humans

The renal handling of fleroxacin was studied by renal clearance and stop-flow techniques in rabbits and dogs and by analyzing the pharmacokinetics with and without probenecid in humans. In rabbits the excretion ratios (fleroxacin intrinsic renal clearance/glomerular filtration rate) were greater than unity (2.01) without probenecid and were decreased to a value below unity (0.680) with probenecid. In dogs, on the other hand, the excretion ratios were less than unity (0.608 and 0.456) both without and with probenecid, and so were not affected by probenecid. This fact suggested that fleroxacin was excreted into urine by both glomerular filtration and renal tubular secretion in rabbits, but only by glomerular filtration in dogs, accompanied by partial renal tubular reabsorption in both species; these mechanisms were also supported by stop-flow experiments. In humans probenecid treatment induced increases in the elimination half-life and area under the serum concentration-time curve and decreases in apparent serum clearance, renal clearance, and urinary recovery of fleroxacin. The excretion ratio without probenecid was 1.13, which was significantly decreased to 0.750 with probenecid. These results indicated that both renal tubular secretion and reabsorption contributed to renal excretion of fleroxacin in humans. The contribution of tubular secretion was species dependent and was extensive in rabbits, minimal in dogs, and moderate in humans. Renal tubular reabsorption was commonly found in every species. The long elimination half-life of fleroxacin in humans might be explained by its small total serum clearance and small renal clearance, which are attributed to less tubular secretion and more tubular reabsorption.