Epilepsia, 46(6):849–857, 2005 Blackwell Publishing, Inc. C  2005 International League Against Epilepsy Localization of Breast Cancer Resistance Protein (BCRP) in Microvessel Endothelium of Human Control and Epileptic Brain ∗ Eleonora Aronica, ‡§Jan A. Gorter, ∗ Sandra Redeker, ‡§Erwin A. van Vliet, ∗ Marja Ramkema, ‖George L. Scheffer, ¶Rik J. Scheper, ¶Paul van der Valk, †Sieger Leenstra, ¶Johannes C. Baayen, ∗∗ Wim G. M. Spliet, and ∗ Dirk Troost Departments of ∗ (Neuro)Pathology and †Neurosurgery, Academic Medical Center, University of Amsterdam, and ‡Swammerdam Institute for Life Sciences, University of Amsterdam, Amsterdam; §Stichting Epilepsie Instellingen Nederland, Heemstede; Departments of // Pathology and ¶ Neurosurgery, VU University Medical Center, Amsterdam; and ∗∗ Department of Pathology, University Medical Center Utrecht, Utrecht, The Netherlands Summary: Purpose: Breast cancer resistance protein (BCRP) is a half adenosine triphosphate (ATP)-binding cassette (ABC) transporter expressed on cellular membranes and included in the group of multidrug resistant (MDR)-related proteins. Recently, upregulation of different MDR proteins has been shown in hu- man epilepsy-associated conditions. This study investigated the expression and cellular distribution of BCRP in human control and epileptic brain, including a large number of both neoplastic and nonneoplastic specimens from patients with chronic phar- macoresistant epilepsy. Methods: Several epileptogenic pathologies, such as hip- pocampal sclerosis (HS), focal cortical dysplasia (FCD), dysembryoplastic neuroepithelial tumor, oligodendroglioma as- trocytoma, and glioblastoma multiforme were studied by using Western blot and immunocytochemistry. Results: With Western blot, we could demonstrate the pres- ence of BCRP in both normal and epileptic human brain tissue. In contrast to P-glycoprotein (P-gp) and multidrug resistance– associated protein (MRP) 2, BCRP expression levels did not change in tissue from patients with HS, compared with con- trol hippocampus. No BCRP immunoreactivity was observed in glial or neuronal cells, including reactive astrocytes and dysplas- tic neurons in FCD. BCRP expression was, however, increased in tumor brain tissue. Immunocytochemistry demonstrated that BCRP was exclusively located in blood vessels and was highly expressed at the luminal cell surface and in newly formed tu- mor capillaries. This localization closely resembles that of P-gp. The higher expression observed in astrocytomas by Western blot analysis was related to the higher vascular density within the tu- mor tissue. Conclusions: These results indicate a constitutive expression of BCRP in human endothelial cells, representing an impor- tant barrier against drug access to the brain. In particular, the strong BCRP expression in the microvasculature of epilepto- genic brain tumors could critically influence the bioavailability of drugs within the tumor and contribute to pharmacore- sistance. Key Words: Epilepsy—Hippocampus—Glioma— Glioneuronal tumors—Blood vessels—Endothelium. Resistance to pharmacologic treatment with a broad range of antiepileptic drugs (AEDs) is a crucial clinical problem in human epilepsy (1,2). One possible general mechanism to account for this medical intractability is the inadequate drug concentration in the epileptogenic areas. In recent years, attention has been focused on multidrug transporters such as P-glycoprotein (P-gp) and the family of multidrug resistance–associated proteins [MRPs; reviewed in (3–6)]. These proteins are members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter family, and their overexpression was recently Accepted February 6, 2005. Address correspondence and reprint requests to Dr. E. Aronica at De- partment of (Neuro) Pathology, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. E-mail: e.aronica@ amc.uva.nl shown in several causes of human refractory epilepsy (7– 13). Localization and overexpression of multidrug trans- porter proteins at the level of the capillary endothelium, which represents the selective cellular barrier controlling drug delivery into the brain, has been proposed as the major mechanism responsible for multidrug resistance in epilepsy [for reviews, see (4,10)]. Overexpression of genes encoding P-gp and MRPs (MRP2 and MRP5) has been demonstrated in endothelial cells isolated from temporal lobe blood vessels of patients with refractory epilepsy (9). Confirmation of protein level in blood vessels of human epileptic tissue has been pro- vided for P-gp and MRP2 (7,9,10,13,14). Another mem- ber of the ABC transporter family is the breast cancer resistance protein (BCRP), also termed ABCG2/MXR/ ABCP (15). Like all members of the ABCG subfamily, 849