P-glycoprotein Inhibition as Adjunct Treatment for Medically Refractory Epilepsy.

The Center for Disease Control reports that epilepsy afflicts 2.7 million Americans with annual costs of $15.5 billion. They estimate that 3% of Americans will have a diagnosis of epilepsy by age 80, and decided in 1997 to focus on treatment, with a motto of "no seizures, no side effects". Antiepileptic drugs (AED) can fail, despite being structurally unrelated and acting on different parts of the nervous system. This refractory state constitutes up to 35% of the epilepsy population, and may be due to pharmacoresistance. Efflux transporters, such as P-glycoprotein (Pgp), are present at the bloodbrain barrier and serve to pump out structurally unrelated compounds, likely serving as a method for the removal of toxins (and drugs). Upregulation of efflux transporters such as Pgp by tumor cells are thought to contribute to chemotherapy resistant cancer tumors, but Pgp has also been found focally at seizure foci. Its overexpression was also noted in blood vessel endothelial cells following temporal lobe resection for intractable epilepsy. Case series have shown mRNA for MDR1, the gene encoding Pgp, to be 10x greater in the medial temporal lobes of patients with temporal lobe epilepsy, as compared to those without epilepsy. Pathological examination following surgical resections have found that epilepsy causing lesions such as cortical dysplasias, encephalitis, tuberculous leptomeningitis, tuberous sclerosis and astrocytomas express Pgp in neurons and/or glia, whereas normal brain parenchyma does not. In animal and cell research, upregulation has been seen following seizure induction and status epilepticus. Many AEDs are validated substrates to Pgp in animal studies. Delivery of these medications to the brain is likely associated with Pgp and in some cases, presence of the substrate may upregulate Pgp. When Pgp inhibitors were added to animal models of drug resistant epilepsy, there were significant improvements in seizure frequency, duration and severity, providing proof-of-concept at the animal level. Carvedilol and verapamil, among other medications, have been found to be potent Pgp inhibitors. Verapamil and dexverapamil, either oral or intravenous, has been used as Pgp-inhibitors in clinical trials, with success as an adjuvant in malignant lymphoma and a phase III study as an adjunct in chemorefractory, metastatic breast carcinoma. There have been no clinical trials published using Pgp-inhibition in epilepsy.