Org 34167 rescues voltage dependence of mutant channels and normalizes hyperexcitability in HCN1 epilepsy.

Objective: Developmental and epileptic encephalopathies (DEEs) are severe neurological conditions typically caused by pathogenic genetic variants. Variants in HCN1 can cause DEE by disrupting channel voltage sensitivity, leading to cation "leak." Even with current best-available medications, most patients with DEE struggle to achieve seizure control, and current treatments do not address major clinical morbidities of DEE. We aimed to study the capability of the small molecule Org 34167 to address phenotypes of HCN1 DEE.

Methods: Org 34167 is a blood-brain barrier-permeable, broad-spectrum HCN channel modulator. We used a range of assays at biophysical, cellular, network, and behavioral levels to explore the potential of Org 34167 as a precision medicine for HCN1 DEE.

Results: Org 34167 rescued voltage dependence of HCN1 channels carrying a range of pathogenic DEE variants in the Xenopus oocyte expression system, significantly reducing cation "leak." In layer V pyramidal neurons from the Hcn1^M249L knock-in mouse model of HCN1 DEE, Org 34167 significantly reduced "leak" current, hyperpolarized the resting membrane potential, increased input resistance, and caused a right-shift in rheobase, indicating reduced excitability. In vivo, Org 34167 caused a significant reduction in interictal spiking on electrocorticography in Hcn1^M249L mice and normalized performance in several behavioral assays. However, tremors emerged at therapeutic doses.

Significance: Our data provide strong proof-of-concept that a small molecule that rectifies a channel deficit caused by HCN1 pathogenic variants can correct cellular and consequently behavioral phenotypes of HCN1 DEE. The emergence of tremors is likely due the broad-spectrum nature of Org 34167 block. Therefore, more selective HCN1 modulators are needed to improve efficacy and reduce side-effects. Org 34167 acts as an exemplar of a brain-penetrant HCN modulator that can be used as a template for further drug development focused on greater HCN1 selectivity.