DENVER — , according to new data presented at the 2024 annual meeting of the American Academy of Neurology.
Of the two drugs evaluated in phase 2 trials, one is a highly targeted TARP-8–dependent AMPA receptor antagonist known as ES-481. The other is XEN1101, a novel potassium channel opener that was well tolerated as well as effective.
TARP inhibitors, which act on transmembrane AMPA (alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid) receptor regulatory proteins, are already available for the control of epilepsy, but ES-481 might be different, according to Terrence J. O’Brien, MD, department of neuroscience, Monash University, Melbourne, Australia.
First-in-Class TARP Inhibitor Is Tested
As a “first-in-class, potent and selective antagonist of the TARP-γ8 AMPA receptor,” ES-481 is “predicted to effectively suppress focal seizures arising from the hippocampus and limbic system,” he said. Dr. O’Brien claims that this specificity of action appears to circumvent central nervous system side effects in studies so far.
In the phase 2a multicenter, randomized trial, 22 patients with drug-resistant epilepsy of any type (focal, generalized, or mixed) were randomized to ES-481 or placebo. In the ES-481 arm, the dose was escalated each week, climbing from 25 mg once-daily, to 25 mg twice-daily, 50 mg twice-daily, and then to 75 mg twice daily. At the end of 4 weeks and after a 7-day washout, the randomized groups were crossed over to the opposite therapy for another 4 weeks.
When data were confined to the first treatment period to avoid a carry-over effect, there was a consistent advantage for active treatment over placebo. At the highest 75-mg twice-daily dose of ES-481, the reduction in seizure frequency was 80% vs 49% on placebo ( P < .05).
The rate of complete remission at the end of the study was not greater for ES-481, but higher proportions of patients on active therapy achieved reductions from baseline in seizure activity when defined as greater than 30% (72.77% vs 36.4%) or greater than 50% (36.4% vs 18.2%). P values for these differences were not provided.
Differences in EEG were not observed, but Dr. O’Brien reported that 18 of the subjects had no EEG activity at baseline, diminishing the opportunity to show a difference.
Open-Label Study Supports Controlled Data
Sixteen patients have entered an open-label extension with sustained suppression of seizure activity relative to baseline observed so far, Dr. O’Brien reported.
ES-481 was well tolerated. There were no significant changes in lab values, and all four of the adverse events leading to discontinuation occurred on placebo. There were higher rates of dizziness, insomnia, gait disturbance, and dysarthria associated with ES-481 than placebo, but the rate of serious adverse events was lower (4.8% vs 14.3%).
These response rates are noteworthy because patients had severe disease with diminishing therapeutic options, according to Dr. O’Brien. For entry, patients were required to be taking one to four antiseizure medications while still experiencing seizure activity. The patients averaged one interictal epileptiform discharge and/or seizure per hour on EEG.
Large-scale, double-blind, controlled studies are planned and warranted on the basis of these data, according to Dr. O’Brien, who emphasized that benefit was achieved with a low relative risk of significant adverse events.
New Potassium Channel Opener Shows Promise
Data with the selective potassium channel opener XEN1101 from the previously published phase 2b X-TOLE trial were reported in two parts. The first set of data involved an analysis of response by baseline activity. The other set of data were generated by an ongoing open-label extension (OLE) of X-TOLE.
In X-TOLE, which randomized 325 patients with treatment-resistant focal-onset seizures (FOS) to one of three doses of XEN-1011 or placebo, the median reduction in FOS at the highest dose of 25 mg once-daily XEN-1011 was 52.8% vs placebo (P < .001).
In the new analysis, the goal was to look at efficacy of the 25-mg dose across differences in baseline severity, reported Roger J. Porter, MD, adjunct professor of neurology, University of Pennsylvania, Philadelphia.
Generally, a greater response was observed for those with less severe disease. For example, the response rate defined as greater than 50% reduction in seizure frequency on XEN1101 was higher for those with a baseline seizure activity of 8.5 seizures/month or fewer relative to more (65.5% vs 50.6%) and six or fewer antiseizure medications relative to more (64.2% vs 40.0%).
Pointing out that the study enrolled a challenging group of patients, Dr. Porter said that the data do not rule out efficacy “across the spectrum of epilepsy severity,” but he did suggest that these data will provide context for the coming phase 3 trials.
In the OLE data presented by Jacqueline French, MD, professor of neurology at the Langone School of Medicine of New York University, the efficacy and safety of XEN1101 taken with food has been consistent with what was observed in the double-blind trial. With up to 2 years of follow-up in the planned 5-year OLE, which is evaluating 20 mg once-daily taken with food, 60% are still on therapy,
For those followed for 24 months, 23.6% are completely seizure free, according to Dr. French. For those followed at least 12 months, 31.5% have achieved a median percent reduction in monthly seizure activity of 90% or more; 41.8% a reduction of 75% or more; and 69.7% a reduction of 50% or more.
The side-effect profile has also been consistent with that seen in the phase 2b trial. Dizziness and other mild to moderate side effects that often accompany antiseizure medications have been observed, along with modest weight gain, but there have been no new safety signals over long-term use.
If a planned phase 3 study enrolling patients with localized and general epilepsy confirms these phase 2 data, Dr. French indicated that it has the potential to advance a potassium channel opener that is both efficacious and well tolerated.
