Application of Muscle Microdialysis to Evaluate the Concentrations of the Fluoroquinolones Pazufloxacin and Ofloxacin in the Tissue Interstitial Fluids of Rats

Muscle microdialysis has been used to determine the unbound concentrations of the fluoroquinolones, pazufloxacin and ofloxacin, in tissue interstitial fluids (C_{isf, u}) of rats under steady state conditions. C_{isf, u} was estimated from the concentration in dialysate and the in-vitro permeability rate constant by the extrapolation method based on the clearance concept. Paper disks were inserted under the abdominal skin of rats, and the drug concentrations in the fluids penetrating into the disks (C_disk) were measured and compared with C_{isf, u}. The C_{isf, u} of pazufloxacin and ofloxacin in muscle were close to their unbound concentrations in the venous plasma; these were 75.3% and 77.1%, respectively, of the total concentrations in plasma at the steady state. The C_disk of pazufloxacin and ofloxacin were also close to their C_{isf, u}. These results indicate that the unbound concentrations of the fluoroquinolones in the tissue interstitial fluids were the same as those in the venous plasma. The disk insertion technique seems to be useful for evaluating drug concentrations in tissue interstitial fluid.