PHENYTOIN SUPPRESSES EPILEPTIFORM FIELD POTENTIALS IN HUMAN BRAIN SLICES FROM PATIENTS WITH PHARMACORESISTANT EPILEPSY DESPITE INCREASED EXPRESSION OF MULTI-DRUG-RESISTANCE 1 (MDR1) GENE PRODUCT P-GLYCOPROTEIN (PGP)
Abstract number :
2.201
Submission category :
Year :
2003
Submission ID :
3856
Source :
www.aesnet.org
Presentation date :
12/6/2003 12:00:00 AM
Published date :
Dec 1, 2003, 06:00 AM
Authors :
Gabriel Moddel, Luca Cucullo, Nicola Marchi, William Bingaman, Stephen M. Dombrowski, Imad Najm, Damir Janigro Department of Neurology, The Cleveland Clinic Foundation, Cleveland, OH; Department of Neurosurgery, The Cleveland Clinic Foundation, Cleveland,
30% of epilepsy patients have seizures despite adequate antiepileptic drug (AED) levels. Many are resistant to multiple AEDs. It is therefore unlikely that alterations of neuronal drug targets (i.e. pharmacodynamic changes) are responsible for AED failure. Increased expression of the multi-drug resistance 1 (MDR1) gene product P-glycoprotein (PGP) in tissue from patients with intractable epilepsy has been demonstrated [1]. As PGP acts as an efflux pump for toxic substances and is located at the luminal membrane of brain capillary endothelial cells, inadequate drug levels in the brain interstitium are probably responsible for AED failure. The goal of this study was to test the hypothesis that epileptiform field potentials (EFP) in brain slices obtained from patients with pharmacoresistant epilepsy, who had previously failed phenytoin treatment, are suppressed by phenytoin [italic]in-vitro[/italic].
Human cortical slices (n=4) or slices from normal rats (controls; n=5) were superfused with Mg2+-free media or bicuculline to elicit EFP. Phenytoin was added to the bath solution in increasing concentrations (1-100 [mu]M), followed by washout. Field potentials were recorded and EFP frequency (F), burst integral (BI), and activity index (AI=F*BI) calculated. Neighboring slices were fixed in paraformaldehyde for protein analysis. Data are given as mean [plusmn] SEM.
Western blotting showed increased expression of PGP in human epileptic tissue, compared with normal human or rat brain.
In epileptic human brain, omission of Mg2+ from the bath solution elicited EFP occuring at a frequency of 0.70 [plusmn] 0.09 min-1 with BI=2078 [plusmn] 802 [mu]Vs and AI=1663 [plusmn] 791 [mu]Vs* min-1. Phenytoin concentrations of 1 [mu]M and 10 [mu]M did not significantly change bursting. 50 [mu]M of phenytoin reduced BI to 415 [plusmn] 58 [mu]Vs (-80%) and AI to 383 [plusmn] 155 [mu]Vs*min-1 (-77%). 100 [mu]M phenytoin lead to complete suppression of EFP. The ED50 was 60 [mu]M, compared with 80 [mu]M for rat slices.
These results demonstrate that the dose-dependency of EFP suppression by phenytoin is equal in normal rat brain and in human brain slices from patients with pharmacoresistant epilepsy despite increased expression of the MDR1 gene product PGP in human epileptic tissue. This suggests that (a) antiepileptic drug failure is a pharmacokinetic, rather than a pharmacodynamic phenomenon and (b) the mechanism of action of PGP is bypassed when brain tissue is directly exposed to the drug [italic]in-vitro[/italic], which indicates a vital role of the blood-brain-barrier in medically intractable epilepsy.
References:
[1] Dombrowski SM, Desai SY, Marroni M, Cucullo L, Goodrich K, Bingaman W, Mayberg MR, Bengez L, Janigro D: Overexpression of multiple drug resistance genes in endothelial cells form patients with refractory epilepsy. [italic]Epilepsia[/italic] (2001): 1501-1506
[Supported by: HL51614, NS43284, NS38195]