XEN496 for Epilepsy

XEN496, a Potassium Channel Modulator for the Treatment of Epilepsy

We are developing XEN496 (active ingredient ezogabine), a Kv7 potassium channel modulator for the potential treatment of epilepsy. Based on feedback from the U.S. FDA, we anticipate initiating a single, pivotal Phase 3 clinical trial in approximately mid-2019 examining XEN496’s efficacy as a precision medicine treatment of KCNQ2 epileptic encephalopathy (KCNQ2-EE) or EIEE7, which is a rare, severe, pediatric epilepsy caused by loss-of-function missense mutations in the KCNQ2 gene that encodes for the Kv7.2 channel. Published case reports where physicians have used ezogabine in infants and young children with KCNQ2-EE suggest that XEN496 may be efficacious in this often hard-to-treat patient population.

Ezogabine, also known as retigabine, is the only anti-epileptic drug previously approved by the FDA with a mechanism of action that potentiates Kv7-mediated potassium current. Ezogabine was originally approved by the FDA in June 2011 as an adjunctive treatment for adults with focal seizures with or without secondary generalization. GlaxoSmithKline (GSK) marketed ezogabine in various jurisdictions – as Potiga® in the U.S. and Trobalt™ in Europe – but withdrew the drug from the market worldwide in June 2017 citing commercial reasons.

Clinical Development of XEN496

As requested by Xenon, GSK authorized the FDA to refer to the information contained in GSK’s regulatory submissions for the purpose of the FDA’s review of Xenon’s regulatory filings. After consulting with clinical experts and patient advocacy groups, Xenon submitted a pre-IND briefing package to the FDA that outlined the proposed clinical development plans for XEN496. In response, the FDA indicated that it was acceptable to study XEN496 in infants and children up to 4 years old, and that a single pivotal trial in approximately 20 patients may be considered adequate in order to demonstrate XEN496’s efficacy in KCNQ2-EE. Recently, we received Orphan Drug Designation (ODD) from the FDA for ezogabine as a treatment of KCNQ2-EE.

We have formally established a steering committee made up of key opinion leaders in the epilepsy field to help guide the clinical development of XEN496. With input from this steering committee on the proposed trial design, dosing, and endpoints, the protocol development for the XEN496 Phase 3 clinical trial is currently underway. We are working on a pediatric-specific formulation for XEN496 that may also address certain pigmentation issues associated with ezogabine, and it is anticipated that the Phase 3 clinical trial examining XEN496’s efficacy as a treatment of KCNQ2-EE will be initiated in approximately mid-2019. Additional details regarding the proposed clinical development of XEN496 are anticipated over the coming months.

About KCNQ2-EE

KCNQ2-EE, otherwise known as EIEE7, is a rare, severe neurodevelopmental disorder with a significant seizure burden and profound developmental impairment. KCNQ2-EE is uniquely characterized by multiple, daily, refractory seizures presenting within the first week of life with a prominent tonic component and autonomic signs. Seizures are often accompanied by clonic jerking or complex motor behavior. The electroencephalogram (EEG) at onset of the disease shows a burst suppression pattern later evolving into multifocal epileptiform activity. The infants usually develop a severe to profound intellectual disability with axial hypotonia which can be accompanied by limb spasticity. The seizure activity typically decreases with age with patients often becoming seizure free or experiencing more minor seizure burden by 3 to 5 years of age; however, thereafter seizures can reoccur in clusters. The intellectual disability and other co-morbidities are not reversed or improved with age and patients generally require life-long care. Patients are often non-verbal and some children may also have autistic features. Seizure-related bradycardia and oxygen desaturation with cyanosis have been observed, and are thought to contribute to the significant risk of Sudden Unexpected Death in Epilepsy (SUDEP) in these children. KCNQ2-EE is rare, representing around 10% of patients with epileptic encephalopathy with onset in the first three months of life; however, the incidence of KCNQ2-EE is approximately 2.8/100,000 live births, which is roughly half the number of births of Dravet Syndrome, the most common genetic cause of early infantile epileptic encephalopathy.

Published Case Studies

There is strong human genetic validation and pharmacologic evidence, including published case studies that support the use of XEN496 as a potential treatment for KCNQ2-EE. The KCNQ2 gene encodes for the Kv7.2 voltage-gated potassium channel. Loss-of-function missense mutations in KCNQ2 can cause KCNQ2-EE, which is characterized in general, by multiple, daily, treatment-resistant seizures often presenting within the first week of life. XEN496 may have a greater potential to improve long term outcomes in KCNQ2-EE, as ezogabine enhances transmembrane potassium currents mediated by the Kv7.2/7.3 channels, thus potentially reversing the underlying genetic abnormality of KCNQ2-EE. By activating Kv7.2/7.3 channels, it is expected that XEN496 should stabilize the resting membrane potential and reduce brain excitability and may have the potential to improve brain function and cognitive development, in addition to decreasing seizures. In one previously published case report of 11 patients (Millichap 2016), ezogabine was associated with improvement in seizures and/or development in three of the four infants treated before six months of age, and two of the seven treated later. No serious adverse effects were observed in that study. Another study that included a review of medical records and structured interviews with families of eight children with KCNQ2-EE who had previously been prescribed ezogabine (Olson 2017), also suggested that ezogabine was effective and tolerable. Sustained improvement in seizure frequency was observed in five of the six patients with at least weekly seizures, along with improvements in development or cognition in all eight patients. The only adverse event reported was urinary retention in 3 patients and overall, ezogabine was well tolerated.