Expression and functional relevance of UGT1A4 in a cohort of human drug-resistant epileptic brains

The pharmacologic effects of drugs depend on their availability and levels reached in a given organ. When considering the brain, a combination of physical and dynamic barriers determines the pattern of drug distribution close to neuronal targets. The blood–brain barrier (BBB) plays a crucial role in the brain influx and efflux of several xenobiotics (Abbott, 2005; Oby & Janigro, 2006). Several hypotheses have been offered to elucidate the mechanisms regulating drug brain distribution under pathologic conditions. In the drug-resistant epileptic brain, extensive studies have focused on the possibility that overexpression of multi-drug transporters at the BBB impedes penetration of antiepileptic drugs (AEDs) into the brain parenchyma (Dombrowski et al., 2001; Abbott et al., 2002; Loscher & Potschka, 2005; Loscher & Sills, 2007; Marchi et al., 2010). However, the results obtained have not translated into clinical improvement in the management of drug-resistant seizures (Loscher & Sills, 2007; Perucca et al., 2007). Recent evidence has suggested that various cytochrome P450 (CYP) (phase I) and phase II metabolic enzymes are expressed in the human and rodent brain (Ghersi-Egea et al., 1993, 1995; Dauchy et al., 2008, 2009; Ghosh et al., 2011b; Shawahna et al., 2011). Metabolic enzymes are accountable for the hepatic metabolism of AEDs (Brodie et al., 2013). Levels of CYP1A1 and CYP1B1 can be detected in immortalized human brain microvascular cells (Dauchy et al., 2008, 2009), and CYP3A4 is upregulated in primary brain endothelial cells (EPI-ECs) derived from drug-resistant epileptic brain specimens (Ghosh et al., 2010, 2011a). Overexpression of CYP3A4 in EPI-ECs is accompanied by an increased metabolism of the AED carbamazepine (Ghosh et al., 2012). The expression and function of phase II enzymes in the human epileptic brain remains to be fully elucidated. Phase II conjugation enzymes (glutathione-S-transferases [GSTs], sulfotransferase, and UDP-glucuronosyl-transferase [UGTs]) are involved in the biotransformation of important AEDs, such as lamotrigine, which is a substrate of UGT1A4 (Anderson, 2004, 2008; Ghosh et al., 2012). It is unknown whether UGT1A4 is expressed in the drug-resistant human epileptic brain and whether this expression plays a role in drug metabolism. We evaluated the pattern of UGT1A4 expression in a cohort of brain specimens obtained from temporal lobectomies performed to relieve drug-resistant seizures. We then compared UGT1A4 expression and lamotrigine metabolism in EPI-ECs to commercially available, control brain endothelial cells. We evaluated the possible association between UGT1A4 expression and cell viability.