Effect of Capillary Efflux Transport Inhibition on the Determination of Probe Recovery During in Vivo Microdialysis in the Brain

摘要

Intracerebral microdialysis probe recovery (extraction fraction) may be influenced by several mass transport processes in the brain, including efflux and uptake exchange between brain and blood. Therefore, changes in probe recovery under various experimental conditions can be useful to characterize funda- mental drug transport processes. Accordingly, the effect of inhibiting transport on probe recovery was investigated for two capillary efflux transporters with potentially different membrane localization and transport mechanisms, P-glycoprotein and an organic anion transporter. Fluorescein/probenecid and quini- dine/L Y-335979 were chosen as the substrate/inhibitor combi- nations for organic anion transport and P-glycoprotein-medi- cated transport, respectively. Probenecid decreased the probe recovery of fluorescein in frontal cortex, from 0.21+-0.017 to 0.17+-0.020 {p < 0.01 ). Quantitative microdialysis calcula- tions indicated that probenecid treatment reduced the total brain elimination rate constant by 3-fold from 0.37 to 0.12 (ml/min .ml of extracellular fluid). In contrast, the microdialysis recovery of quinidine, delivered locally to the brain via the probe perfusate, was not sensitive to P-glycoprotein inhibition by systemically administered L Y -335979, a potent and specific inhibitor of P-glycoprotein. Recovery of difluorofluorescein, an analog of fluorescein, was also decreased by probenecid in the frontal cortex but not in the ventricle cerebrospinal fluid. These experimental observations are in qualitative agreement with microdialysis theory incorporating mathematical models of transporter kinetics. These studies suggest that only in certain circumstances will efflux inhibition at the blood-brain barrier and blood-cerebrospinal fluid barrier influence the microdialy- sis probe recovery, and this may depend upon the substrate and inhibitor examined and their routes of administration, the localization and mechanism of the membrane transporter, as well as the microenvironment surrounding the probe.