State-dependent inhibition of Nav1.8 channels by VX-150 and VX-548.

Nav1.8 channels are an attractive therapeutic target for pain because they are prominent in primary pain-sensing neurons with little expression in most other kinds of neurons. Recently, two Nav1.8-targeted compounds, VX-150 and VX-548, have shown efficacy in clinical trials for reducing pain. We examined the characteristics of Nav1.8 inhibition by these compounds. The active metabolite form of VX-150 (VX-150m) inhibited human Nav1.8 channels with an IC50 of 15 nM. VX-548 (suzetrigine) was even more potent (IC50 0.27 nM). Both VX-150m and VX-548 had the unusual property of "reverse use-dependence", whereby inhibition could be relieved by repetitive depolarizations, a property seen before with another Nav1.8 inhibitor, A-887826. The relief of VX-548 inhibition by large depolarizations occurred with a time constant of ~40 ms that was not concentration-dependent. Re-inhibition at negative voltages occurred with a rate that was nearly proportional to drug concentration, consistent with the idea that relief of inhibition reflects dissociation of drug from the channel and re-inhibition reflects re-binding. The relief of inhibition by depolarization suggests a remarkably strong and unusual state-dependence for both VX-150m and VX-548, with very weak binding to channels with fully activated voltage sensors despite very tight binding to channels with voltage sensors in the resting state. Significance Statement The Nav1.8 sodium channel is a current target for new drugs for pain. This work describes the potency, selectivity, and state-dependent characteristics of inhibition of Nav1.8 channels by VX-150 and VX-548, compounds that have recently shown efficacy for relief of pain in clinical trials but whose mechanism of interaction with channels has not been described. The results show that the compounds share an unusual property whereby inhibition is relieved by depolarization, demonstrating a state-dependence different than most sodium channel inhibitors.

Scalable generation of sensory neurons from human pluripotent stem cells.

Development of new non-addictive analgesics requires advanced strategies to differentiate human pluripotent stem cells (hPSCs) into relevant cell types. Following principles of developmental biology and translational applicability, here we developed an efficient stepwise differentiation method for peptidergic and non-peptidergic nociceptors. By modulating specific cell signaling pathways, hPSCs were first converted into SOX10+ neural crest, followed by differentiation into sensory neurons. Detailed characterization, including ultrastructural analysis, confirmed that the hPSC-derived nociceptors displayed cellular and molecular features comparable to native dorsal root ganglion (DRG) neurons, and expressed high-threshold primary sensory neuron markers, transcription factors, neuropeptides, and over 150 ion channels and receptors relevant for pain research and axonal growth/regeneration studies (e.g., TRPV1, NAV1.7, NAV1.8, TAC1, CALCA, GAP43, DPYSL2, NMNAT2). Moreover, after confirming robust functional activities and differential response to noxious stimuli and specific drugs, a robotic cell culture system was employed to produce large quantities of human sensory neurons, which can be used to develop nociceptor-selective analgesics.

Use-Dependent Relief of Inhibition of Nav1.8 Channels by A-887826.

Sodium channel inhibitors used as local anesthetics, antiarrhythmics, or antiepileptics typically have the property of use-dependent inhibition, whereby inhibition is enhanced by repetitive channel activation. For targeting pain, Nav1.8 channels are an attractive target because they are prominent in primary pain-sensing neurons, with little or no expression in most other kinds of neurons, and a number of Nav1.8-targeted compounds have been developed. We examined the characteristics of Nav1.8 inhibition by one of the most potent Nav1.8 inhibitors so far described, A-887826, and found that when studied with physiologic resting potentials and physiologic temperatures, inhibition had strong "reverse use dependence", whereby inhibition was relieved by repetitive short depolarizations. This effect was much stronger with A-887826 than with A-803467, another Nav1.8 inhibitor. The use-dependent relief from inhibition was seen in both human Nav1.8 channels studied in a cell line and in native Nav1.8 channels in mouse dorsal root ganglion (DRG) neurons. In native Nav1.8 channels, substantial relief of inhibition occurred during repetitive stimulation by action potential waveforms at 5 Hz, suggesting that the phenomenon is likely important under physiologic conditions. SIGNIFICANCE STATEMENT: Nav1.8 sodium channels are expressed in primary pain-sensing neurons and are a prime current target for new drugs for pain. This work shows that one of the most potent Nav1.8 inhibitors, A-887826, has the unusual property that inhibition is relieved by repeated short depolarizations. This "reverse use dependence" may reduce inhibition during physiological firing and should be selected against in drug development.

Uncovering a new route to pain therapy.

High-voltage-activated calcium channels (HVACCs) are promising targets for developing analgesics given their roles in controlling synaptic transmission, neuronal excitability and neuropeptide release in primary nociceptive neurons. Despite previous efforts in developing HVACCs inhibitors of various drug modalities, it remains undetermined whether targeting HVACCs directly by a gene therapy approach could lead to pain alleviation in vivo. To test this, Sun and colleagues adopted a post-translational ubiquitination-based knockdown method targeting HVACCs in primary sensory neurons. They showed ablation of HVACC currents in a subset of primary sensory neurons, dampened hyperexcitability of sensory neurons after nerve injury and reduced pain behavior with no apparent adverse effects [1]. The results open the possibility of targeting ion channels with ubiquitination-based knockdown as a promising gene therapy candidate for pain treatment in future clinical studies.

Effect of Induction Chemotherapy With Paclitaxel, Cisplatin, and Capecitabine vs Cisplatin and Fluorouracil on Failure-Free Survival for Patients With Stage IVA to IVB Nasopharyngeal Carcinoma: A Multicenter Phase 3 Randomized Clinical Trial.

Importance: Induction chemotherapy added to concurrent chemoradiotherapy significantly improves survival for patients with locoregionally advanced nasopharyngeal carcinoma, but the optimal induction regimen remains unclear. Objective: To determine whether induction chemotherapy with paclitaxel, cisplatin, and capecitabine (TPC) improves survival vs cisplatin and fluorouracil (PF) prior to chemoradiotherapy for patients with stage IVA to IVB nasopharyngeal carcinoma. Design, Setting, and Participants: This randomized, open-label, phase 3 clinical trial recruited 238 patients at 4 hospitals in China from October 20, 2016, to August 29, 2019. Patients were 18 to 65 years of age with treatment-naive, nonkeratinizing stage IVA to IVB nasopharyngeal carcinoma and an Eastern Cooperative Oncology Group performance status of 0 to 1. Interventions: Patients were randomly assigned (1:1) to receive induction chemotherapy with two 21-day cycles of TPC (intravenous paclitaxel [150 mg/m2, day 1], intravenous cisplatin [60 mg/m2, day 1], and oral capecitabine [1000 mg/m2 orally twice daily, days 1-14]) or PF (intravenous cisplatin [100 mg/m2, day 1] and fluorouracil [800 mg/m2 daily, days 1-5]), followed by chemoradiotherapy. Main Outcomes and Measures: The primary end point was failure-free survival in the intention-to-treat population. Secondary end points included distant metastasis-free survival, locoregional relapse-free survival, overall survival, tumor response, and safety. Results: Overall, 238 eligible patients (187 men [78.6%]; median age, 45 years [range, 18-65 years]) were randomly assigned to receive TPC (n = 118) or PF (n = 120). The median follow-up duration was 48.4 months (IQR, 39.6-53.3 months). Failure-free survival at 3 years was 83.5% (95% CI, 77.0%-90.6%) in the TPC group and 68.9% (95% CI, 61.1%-77.8%) in the PF group (stratified hazard ratio [HR] for recurrence or death, 0.47; 95% CI, 0.28-0.79; P = .004). Induction with the TPC regimen resulted in a significant reduction in the risk of distant metastases (stratified HR, 0.49 [95% CI, 0.24-0.98]; P = .04) and locoregional recurrence (stratified HR, 0.40 [95% CI, 0.18-0.93]; P = .03) compared with the PF regimen. However, there was no effect on early overall survival (stratified HR, 0.45 [95% CI, 0.17-1.18]; P = .10). The incidences of grade 3 to 4 acute adverse events and late-onset toxicities were 57.6% (n = 68) and 13.6% (16 of 118), respectively, in the TPC group and 65.8% (n = 79) and 17.9% (21 of 117), respectively, in the PF group. One treatment-related death occurred in the PF group. Conclusions and Relevance: This randomized clinical trial found that induction chemotherapy with 2 cycles of TPC for patients with stage IVA to IVB nasopharyngeal carcinoma improved failure-free survival compared with 2 cycles of PF, with no increase in the toxicity profile. Trial Registration: ClinicalTrials.gov Identifier: NCT02940925.

Rapid odor processing by layer 2 subcircuits in lateral entorhinal cortex.

Olfactory information is encoded in lateral entorhinal cortex (LEC) by two classes of layer 2 (L2) principal neurons: fan and pyramidal cells. However, the functional properties of L2 cells and how they contribute to odor coding are unclear. Here, we show in awake mice that L2 cells respond to odors early during single sniffs and that LEC is essential for rapid discrimination of both odor identity and intensity. Population analyses of L2 ensembles reveal that rate coding distinguishes odor identity, but firing rates are only weakly concentration dependent and changes in spike timing can represent odor intensity. L2 principal cells differ in afferent olfactory input and connectivity with inhibitory circuits and the relative timing of pyramidal and fan cell spikes provides a temporal code for odor intensity. Downstream, intensity is encoded purely by spike timing in hippocampal CA1. Together, these results reveal the unique processing of odor information by LEC subcircuits and highlight the importance of temporal coding in higher olfactory areas.

Cannabidiol activates neuronal Kv7 channels.

Cannabidiol (CBD), a chemical found in the Cannabis sativa plant, is a clinically effective antiepileptic drug whose mechanism of action is unknown. Using a fluorescence-based thallium flux assay, we performed a large-scale screen and found enhancement of flux through heterologously expressed human Kv7.2/7.3 channels by CBD. Patch-clamp recordings showed that CBD acts at submicromolar concentrations to shift the voltage dependence of Kv7.2/7.3 channels in the hyperpolarizing direction, producing a dramatic enhancement of current at voltages near -50 mV. CBD enhanced native M-current in mouse superior cervical ganglion starting at concentrations of 30 nM and also enhanced M-current in rat hippocampal neurons. The potent enhancement of Kv2/7.3 channels by CBD may contribute to its effectiveness as an antiepileptic drug by reducing neuronal hyperexcitability.

Anthrax toxins regulate pain signaling and can deliver molecular cargoes into ANTXR2+ DRG sensory neurons.

Bacterial products can act on neurons to alter signaling and function. In the present study, we found that dorsal root ganglion (DRG) sensory neurons are enriched for ANTXR2, the high-affinity receptor for anthrax toxins. Anthrax toxins are composed of protective antigen (PA), which binds to ANTXR2, and the protein cargoes edema factor (EF) and lethal factor (LF). Intrathecal administration of edema toxin (ET (PA + EF)) targeted DRG neurons and induced analgesia in mice. ET inhibited mechanical and thermal sensation, and pain caused by formalin, carrageenan or nerve injury. Analgesia depended on ANTXR2 expressed by Nav1.8+ or Advillin+ neurons. ET modulated protein kinase A signaling in mouse sensory and human induced pluripotent stem cell-derived sensory neurons, and attenuated spinal cord neurotransmission. We further engineered anthrax toxins to introduce exogenous protein cargoes, including botulinum toxin, into DRG neurons to silence pain. Our study highlights interactions between a bacterial toxin and nociceptors, which may lead to the development of new pain therapeutics.

Cannabidiol Inhibition of Murine Primary Nociceptors: Tight Binding to Slow Inactivated States of Nav1.8 Channels.

The nonpsychoactive phytocannabinoid cannabidiol (CBD) has been shown to have analgesic effects in animal studies but little is known about its mechanism of action. We examined the effects of CBD on intrinsic excitability of primary pain-sensing neurons. Studying acutely dissociated capsaicin-sensitive mouse DRG neurons at 37°C, we found that CBD effectively inhibited repetitive action potential firing, from 15-20 action potentials evoked by 1 s current injections in control to 1-3 action potentials with 2 µm CBD. Reduction of repetitive firing was accompanied by a reduction of action potential height, widening of action potentials, reduction of the afterhyperpolarization, and increased propensity to enter depolarization block. Voltage-clamp experiments showed that CBD inhibited both TTX-sensitive and TTX-resistant (TTX-R) sodium currents in a use-dependent manner. CBD showed strong state-dependent inhibition of TTX-R channels, with fast binding to inactivated channels during depolarizations and slow unbinding on repolarization. CBD alteration of channel availability at various voltages suggested that CBD binds especially tightly [Kd (dissociation constant), ∼150 nm] to the slow inactivated state of TTX-R channels, which can be substantially occupied at voltages as negative as -40 mV. Remarkably, CBD was more potent in inhibiting TTX-R channels and inhibiting action potential firing than the local anesthetic bupivacaine. We conclude that CBD might produce some of its analgesic effects by direct effects on neuronal excitability, with tight binding to the slow inactivated state of Nav1.8 channels contributing to effective inhibition of repetitive firing by modest depolarizations.SIGNIFICANCE STATEMENT Cannabidiol (CBD) has been shown to inhibit pain in various rodent models, but the mechanism of this effect is unknown. We describe the ability of CBD to inhibit repetitive action potential firing in primary nociceptive neurons from mouse dorsal root ganglia and analyze the effects on voltage-dependent sodium channels. We find that CBD interacts with TTX-resistant sodium channels in a state-dependent manner suggesting particularly tight binding to slow inactivated states of Nav1.8 channels, which dominate the overall inactivation of Nav1.8 channels for small maintained depolarizations from the resting potential. The results suggest that CBD can exert analgesic effects in part by directly inhibiting repetitive firing of primary nociceptors and suggest a strategy of identifying compounds that bind selectively to slow inactivated states of Nav1.8 channels for developing effective analgesics.

Novel charged sodium and calcium channel inhibitor active against neurogenic inflammation.

Voltage-dependent sodium and calcium channels in pain-initiating nociceptor neurons are attractive targets for new analgesics. We made a permanently charged cationic derivative of an N-type calcium channel-inhibitor. Unlike cationic derivatives of local anesthetic sodium channel blockers like QX-314, this cationic compound inhibited N-type calcium channels more effectively with extracellular than intracellular application. Surprisingly, the compound is also a highly effective sodium channel inhibitor when applied extracellularly, producing more potent inhibition than lidocaine or bupivacaine. The charged inhibitor produced potent and long-lasting analgesia in mouse models of incisional wound and inflammatory pain, inhibited release of the neuropeptide calcitonin gene-related peptide (CGRP) from dorsal root ganglion neurons, and reduced inflammation in a mouse model of allergic asthma, which has a strong neurogenic component. The results show that some cationic molecules applied extracellularly can powerfully inhibit both sodium channels and calcium channels, thereby blocking both nociceptor excitability and pro-inflammatory peptide release.