Determination of in vivo steady-state unbound drug concentration in the brain interstitial fluid by microdialysis

Abstract

The in vivo unbound concentrations of [ 3H]water, caffeine and aminopyrine in the hippocampal interstitial fluid were directly determined by means of a brain microdialysis technique. The steady-state unbound concentration in the interstitial fluid (ISF) was determined from the dialysate concentration and the in vivo permeability rate constants of these compounds which were extrapolated from the in vitro permeability rate constant and from the effective dialysis coefficient of a reference compound such as antipyrine. The values of the unbound concentrations of [ 3H]water, caffeine and aminopyrine obtained by extrapolation were very close to those predicted from the plasma concentrations based on the assumption that the unbound concentration in the tissue interstitial fluid is the same as that in the plasma. Moreover, the damage due to implantation of transcranial-type microdialysis cannula was evaluated using [ 14C]sucrose as an extracellular marker. The average ISF/plasma ratios of [ 14C]sucrose estimated for the initial 30 min dialysis were 7% at l h and 3.7% at 48 h after implantation of the cannula, respectively, which correspond well with that (2.0–6.1%) estimated from the blood-brain barrier permeability rate constant reported previously. Therefore, we conclude that the microdialysis technique using the reference method for determination of the unbound concentration of ISF is a useful procedure for the reliable evaluation of the in vivo unbound concentration in brain ISF and that the blood-brain barrier is not significantly damaged by cannula implantation.