Abstracts

We and others have shown that the drug efflux protein P-glycoprotein (Pgp or MDR1) protects the brain from xenobiotics, but other transporters that may be involved in multiple drug resistance to antiepileptic drugs may also play a role. Unlike other selective transporters, Pgp recognizes a wide range of substrates including AED. However, there is a significant overlap between molecules transported by MDR1 and the non-ABC transporter RalA Binding Protein 1 (RLIP76). We used a combination of immunohistochemistry, Western blot analysis and pharmacokinetic assays to measure levels of expression of RLIP76; co-localization with MDR1; and relative contribution to AED extrusion. Data were obtained from 15 epileptics, age ranging from 3 month to 61 years. While MDR1 immunoreactivity was observed in neurons, glia and endothelial cells, RLIP76 was only found in endothelial and not in parenchymal cells. Experiments of drug extrusion using antibodies capable of selective inhibition of MDR1 or RLIP76 revealed that the latter mechanism was responsible for 72[plusmn]8% of 14C-phenytoin extrusion by epileptic BBB endothelial cells; MDR1 contributed to only 27[plusmn]8% of ATP-dependent drug extrusion. These findings are in agreement with the fact that transport of Pgp substrates in these cells is only weakly inhibited by the specific MDR1 blocker XR9576. Our findings suggest that RLIP76 and not MDR1 is the main multiple drug resistance mechanism at the blood-brain barrier of drug resistant epileptic patients. (Supported by NIH-2RO1 HL51614, NIH-RO1 NS 43284 and NIH-RO1 NS38195)