New evidence suggests that excessive glutamate released from glioma cells causes epileptic activity in peritumoral neurons, which may be stopped by a drug that blocks the release of glutamate from tumor cells.
Previous studies have suggested that tumor-associated seizures arise from increased levels of glutamate near areas of epileptiform activity in the peritumoral border where invading cells surround neurons, but this is the first study to establish that the activity is associated with glutamate release from the system xc- cystine-glutamate transporter that is expressed on tumor cells, according to Susan C. Buckingham, Ph.D., and her associates at the University of Alabama at Birmingham.
Most patients with glioma (80%) experience at least one seizure during the course of their illness, but about one-third have recurrent seizures, known as tumor-associated epilepsy, that are often refractory to antiepileptic drugs, the investigators said.
Dr. Buckingham and her colleagues detected abnormal EEG activity in 37% of immunodeficient mice 1 week after they underwent intracranial implantation of human glioma cells, but not in any mice that underwent sham implantation. This abnormal activity manifested itself as subtle changes in behavior such as freezing, facial automatisms, and tremor. Tumor-bearing cortical slices from these mice revealed a time-dependent increase in glutamate concentration (Nat. Med. 2011 Sept. 11 [doi:10.1038/nm.2453]).
Sulfasalazine (SAS), a Food and Drug Administration-approved drug that is known to inhibit system xc-, blocked the release of glutamate from the tumor cells but not from sham slices, which suggested to the investigators "that system xc- does not contribute substantially to glutamate release in tumor-free brain." Electrode recordings revealed spontaneous paroxysmal discharges near tumor cells in 23% of the cortical slices, but not in sham slices. Patch-clamp recordings from neurons in these areas demonstrated increased excitability. When the researchers applied SAS to these neurons, the mean duration of epileptiform activity declined significantly. Mice with xenografted tumors that received intraperitoneal injections of SAS also showed decreased epileptic activity on EEG.
"Together with previous studies, our findings establish that system xc- is a viable new target for seizure treatments," the authors concluded.
SAS did not inhibit tumor growth in an earlier clinical trial involving end-stage glioblastoma patients with poor neurological status, but the study did not assess its antiepileptic effects. Because "seizures often present early in disease progression, particularly among patients with low-grade, slow-growing tumors that can become refractory to traditional antiepileptic drugs," the investigators noted that such patients "would be most likely to benefit from SAS treatment" in a clinical trial.
Based on the approved status and tolerable side effects of SAS, the investigators are planning a trial using it as an adjuvant treatment for peritumoral epilepsy in approximately 50 patients with gliomas. Although the trial is open to patients with all grades of glioma, senior author Dr. Harald Sontheimer said in an interview that his team is "primarily interested in newly diagnosed patients with low-grade gliomas who present with seizures." The trial will use chemical shift MRI to determine the acute effect of oral sulfasalazine on glutamate release. A phase I trial in a several patients has already generated data showing that the treatment is feasible, Dr. Sontheimer said.
The current study was funded by grants from the National Institutes of Health. None of the authors had relevant financial disclosures.