Drugs and their metabolites often produce undesirable effects. These may be due to a number of mechanisms, including biotransformation by P450 enzymes which are not exclusively expressed by hepatocytes but also by endothelial cells in brain from epileptics. The possibility thus exists that the potency of systemically administered central nervous system therapeutics can be modulated by a metabolic blood-brain barrier (BBB).

Surgical brain specimens and blood samples (ex vivo) were obtained from drug-resistant epileptic subjects receiving the antiepileptic drug carbamazepine prior to temporal lobectomies. An in vitro blood-brain barrier model was then established using primary cell culture derived from the same brain specimens. The pattern of carbamazepine (CBZ) metabolism was evaluated in vitro and ex vivo using high performance liquid chromatography-mass spectroscopy. Accelerated mass spectroscopy was used to identify 14C metabolites deriving from the parent 14C-carbamazepine.

Under our experimental conditions carbamazepine levels could not be detected in drug resistant epileptic brain ex situ; low levels of carbamazepine were detected in the brain side of the in vitro BBB established with endothelial cells derived from the same patients. Four carbamazepine-derived fractions were detected in brain samples in vitro and ex vivo. HPLC-accelerated mass spectroscopy confirmed that these signals derived from 14C-carbamazepine administered as parental drug. Carbamazepine 10, 11 epoxide (CBZ-EPO) and 10, 11-dihydro-10, 11-dihydrooxy-carbamazepine (DiOH-CBZ) were also detected in the fractions analyzed. 14C-enriched fractions were subsequently analyzed by mass spectrometry to reveal micromolar concentrations of quinolinic acid (QA). Remarkably, the disappearance of carbamazepine-epoxide (at a rate of 5% per hour) was comparable to the rate of quinolinic acid production (3% per hour). This suggested that quinolinic acid may be a result of carbamazepine metabolism. Quinolinic acid was not detected in the brain of patients who received antiepileptic drugs other than carbamazepine prior to surgery or in brain endothelial cultures obtained from a control patient.

Our data suggest that a drug resistant BBB not only impedes drug access to the brain but may also allow the formation of neurotoxic metabolites.