
Background/Objective: Acute and chronic pain affect millions of individuals, yet there are currently no molecular imaging tools to directly assess pain-related mechanisms in the central nervous system (CNS). The voltage-gated sodium channel NaV1.8 plays a pivotal role in neuropathic pain by increasing the excitability of nociceptive neurons following nerve injury or inflammation. In this work, we aimed to develop a novel positron emission tomography (PET) imaging probe for NaV1.8 to facilitate noninvasive quantification of this target in the CNS and thereby advance our understanding of pain neurobiology. Methods: We selected the compound suzetrigine, a U.S. FDA-approved, highly selective non-opioid NaV1.8 inhibitor, as the first candidate for a NaV1.8-targeted PET tracer. The compound was first assessed using in silico docking and CNS multiparameter optimization (MPO) analysis to evaluate target binding and predicted brain penetrability. Radiolabeling was accomplished by O-methylation with [11C]methyl iodide to yield [11C]suzetrigine without structural modification. The tracer was then evaluated using in vitro binding assays, including autoradiography and saturation binding on rat brain tissues, to determine binding parameters (KD, Bmax), and using in vivo PET imaging in rats to assess brain uptake, time–activity curves (TACs), and tracer behavior under baseline and pretreatment conditions. Pretreatment was performed with unlabeled suzetrigine, the P-glycoprotein (P-gp) inhibitor verapamil, and the heterologous NaV1.8 inhibitor A-803467. Results: In silico docking demonstrated favorable binding of suzetrigine to the NaV1.8 active site, and the calculated CNS MPO score (>3.5) suggested adequate brain penetration. Radiochemical synthesis of [11C]suzetrigine via O-methylation yielded a high decay-corrected radiochemical yield (19.2 ± 2.7%, n = 3), excellent purity (>98%, n = 3), and moderate molar activity (62.9 ± 51.8 MBq/nmol, n = 3). Autoradiography on rat brain tissue confirmed saturable and selective binding of [11C]suzetrigine to NaV1.8. Saturation binding assays revealed a Bmax = 93 fmol/mg and a KD = 0.1 nM, supporting the imageability of NaV1.8 in the brain using this tracer. In vivo PET imaging in rats demonstrated rapid and sufficient brain uptake but revealed unexpected tracer behavior: signal intensity markedly increased following pretreatment with either unlabeled suzetrigine or the P-gp inhibitor verapamil, and showed a slight increase after pretreatment with the heterologous NaV1.8 inhibitor A-803467. Detailed analysis of PET images, TACs, and normalized area-under-curve (AUC) values indicated that these atypical uptake patterns were primarily attributable to P-gp-mediated effects, although additional factors may also contribute. Conclusions: [11C]Suzetrigine exhibits high affinity, good brain uptake, and selective target engagement in vitro, supporting its potential as a first-in-class NaV1.8-PET tracer. However, in vivo performance is confounded by P-gp-mediated efflux and possibly other mechanisms that limit accurate quantification of NaV1.8 in the living brain. These findings underscore the critical role of efflux transporters in CNS radiotracer development and highlight the need for design strategies that mitigate P-gp interaction when targeting ion channels in the brain. Future studies will include imaging under constant P-gp inhibition, arterial blood sampling for radiometabolite analysis and full kinetic modeling, and evaluation in non-human primates to assess translational feasibility.
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