Distribution of divalent small interfering RNA into neurons of sensory ganglia produces selective, durable knockdown of Nav1.7 and strong analgesia

ABSTRACT: Humans missing the SCN9A gene encoding the Nav1.7 sodium ion channel are wholly insensitive to pain, but Nav1.7 inhibitors to date have not produced correspondingly strong analgesia in clinical trials. In this study, we address whether potential challenges of selectivity, tissue distribution, and target engagement might be addressed by divalent small interfering RNA (di-siRNA), a modality for transcript knockdown throughout the CNS following dosing into cerebrospinal fluid. Small interfering RNA sequences catalyzing SCN9A transcript cleavage were identified, and in live-cell assays, a di-siRNA showed a minimum 500-fold selectivity against each of the 8 human Nav paralogs. Following intrathecal dosing in male rats, di-siRNA was internalized into over 90% of neuronal cell bodies within dorsal root ganglia, knocked down cytoplasmic Nav1.7 transcript selectively, and reduced Nav1.7 protein up to 75% in ganglia and up to 85% in spinal cord. Two di-siRNAs targeting SCN9A each produced a strong reduction in withdrawal response to noxious thermal and mechanical stimuli lasting over 3 months, suggesting the tissue distribution and level of Nav1.7 knockdown were sufficient to reduce pain. Finally, di-siRNA dosed directly at the trigeminal ganglion distributed to most or all cell bodies, reduced Nav1.7 levels, and produced dose-dependent self-lacerating behavior on the head, likely reflecting suppression of pain originating at areas innervated by the trigeminal nerve. The results in male animals show that di-siRNA reduces Nav1.7 in sensory neurons to a level that produces powerful analgesia, showing potential for the di-siRNA modality as a nonopioid analgesic applicable to either systemic or local reduction of pain.