Microdialysis as a tool to determine free kidney levels of voriconazole in rodents: a model to study the technique feasibility for a moderately lipophilic drug.

Microdialysis has been employed for the in vivo measurement of endogenous compounds and a variety of drugs in different tissues. The applicability of this technique can be limited by drug lipophilicity which can impair the diffusion through dialysis membrane. The objective of this study was to evaluate the feasibility of using microdialysis to study kidney penetration of voriconazole, a moderately lipophilic antifungal triazolic agent (LogD7.4=1.8). Microdialysis probe recoveries were investigated in vitro by dialysis and retrodialysis using four different drug concentrations (0.1-2 microg/mL) at five flow rates (1-5 microL/min). Recoveries were dependent on the method used for the determination as well as on the flow rate, but independent of drug concentration. The average apparent recoveries determined by dialysis and retrodialysis, at flow rate of 2 microL/min, were 21.1+/-1.5% and 28.7+/-2.0%, respectively. Recovery by retrodialysis was bigger than the recovery by dialysis. The average apparent dialysis/retrodialysis recovery ratio in vitro was 0.73 for all concentrations investigated. The differences between retrodialysis and dialysis recoveries were attributed to the drug's binding to the plastic tubing before and after the dialysis membrane which was experimentally evaluated and mathematically modeled. The in vivo apparent recovery determined by retrodialysis in healthy Wistar rats' kidney was 38.5+/-3.5%, similar to that observed in vitro using the same method (28.7+/-2.0%). The in vivo apparent recovery after correcting for plastic tubing binding (25.1+/-2.8%) was successfully used for determining free kidney levels of voriconazole in rats following 40 and 60mg/kg oral dosing. The results confirmed that microdialysis can be used as sampling technique to determine free tissue levels of moderately lipophilic drugs once the contribution of tubing binding and membrane diffusion on the apparent recovery are disentangled.