Effect of capillary efflux transport inhibition on the determination of probe recovery during in vivo microdialysis in the brain

Intracerebral microdialysis probe recovery (extraction fraction) may be inf luenced by several mass transport processes in the brain, including efflux and uptake exchange between brain and blood. Therefore, changes in probe re covery under various experimental conditions can be useful to characterize fundamental drug transport processes. Accordingly, the effect of inhibiting transport on probe recovery was investigated for two capillary efflux tran sporters with potentially different membrane localization and transport mec hanisms, P-glycoprotein and an organic anion transporter. Fluorescein/probe necid and quinidine/LY-335979 were chosen as the substrate/inhibitor combin ations for organic anion transport and P-glycoprotein-medicated transport, respectively. Probenecid decreased the probe recovery of fluorescein in fro ntal cortex, from 0.21 +/- 0.017 to 0.17 +/- 0.020 (p < 0.01). Quantitative microdialysis calculations 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 quin idine, delivered locally to the brain via the probe perfusate, was not sens itive to P-glycoprotein inhibition by systemically administered LY-335979, a potent and specific inhibitor of P-glycoprotein. Recovery of difluorofluo rescein, an analog of fluorescein, was also decreased by probenecid in the frontal cortex but not in the ventricle cerebrospinal fluid. These experime ntal observations are in qualitative agreement with microdialysis theory in corporating mathematical models of transporter kinetics. These studies sugg est that only in certain circumstances will efflux inhibition at the blood- brain barrier and blood-cerebrospinal fluid barrier influence the microdial ysis probe recovery, and this may depend upon the substrate and inhibitor e xamined and their routes of administration, the localization and mechanism of the membrane transporter, as well as the microenvironment surrounding th e probe.