BRAIN EFFLUX INDEX AS A NOVEL METHOD OF ANALYZING EFFLUX TRANSPORT ATTHE BLOOD-BRAIN-BARRIER

A brain efflux index method has been developed to characterize an effl ux transport system for substrates from the cerebrum to the circulatin g blood across the blood-brain barrier. The brain efflux index value i s defined as the relative amount of test drug effluxed from cerebrum c ompared with that of a reference compound, [C-14]carboxyiinuiin, which has limited blood-brain barrier permeability. Microinjection of 0.2, 0.5 or 1.0 mu l into the parietal cortex area 2 region was found to be an appropriate procedure for obtaining a high recovery and for restri cting the test drug and reference compound to the ipsilateral cerebrum . No significant increase in influx clearance of [C-14]carboxylinulin into the brain was observed for the ipsilateral cerebrum after the sha m microinjection compared with tile contralateral cerebrum, which demo nstrated that only limited physical damage is caused. Microinjection o f [H-3]H2O into the cerebrum devoid of cerebral blood flow yielded no elimination from the brain. Analysis based on a pharmacokinetic model demonstrated that the elimination of a highly permeable compound [H-3] H2O from the brain was governed by cerebral blood flaw. The apparent e limination rate constant (k(el)) of [H-3]3-O-methyl-D-glucose from the brain was determined as 0.129 +/- 0.014 (min(-1)) and was reduced sig nificantly by a preinjection of excess cold 3-O-methyl-D-glucose and p hloridzin, whereas no significant elimination was observed for L[H-3]g lucose. The efflux clearance of 3-O-methyl-D-glucose was calculated fr om the k(el) value and the compound's distribution volume, the value b eing close to that of the influx clearance. These results demonstrate that the brain efflux index method is a useful technique for analyzing an efflux process from the brain across the blood-brain barrier invol ving a carrier-mediated transport system.