Highly potent and selective NaV1.7 inhibitors for use as intravenous agents and chemical probes

Lessons learned in translating pain knowledge into practice

Introduction

During the past 2 decades, basic research deciphering the underlying mechanisms of nociception and chronic pain was thought to finally step beyond opioids and nonsteroidals and provide patients with new analgesics. But apart from calcitonin gene-related peptide antagonists, nothing arrived in hands of clinicians.

Objectives

To present existing evidence of 3 representative target molecules in the development of novel pain treatment that, so far, did not result in approved drugs.

Methods

This Clinical Update aligns with the 2022 IASP Global Year Translating Pain Knowledge into Practice and selectively reviews best available evidence and practice.

Results

We highlight 3 targets: a ion channel, a neuronal growth factor, and a neuropeptide to explore why these drug targets have been dropped in clinical phase II-III trials. Antibodies to nerve growth factor had very good effects in musculoskeletal pain but resulted into more patients requiring joint replacements. Blockers of NaV1.7 were often not effective enough-at least if patients were not stratified. Blockers of neurokinin receptor were similarly not successful enough. In general, failure was most often to the result of a lack of effect and to a lesser extend because of unexpected severe side effects. However, all studies and trials lead to an enormous move in the scientific community to better preclinical models and testing as well as revised methods to molecularly phenotype and stratify patients.

Conclusion

All stakeholders in the process can help in the future: better preclinical studies, phenotyping and stratifying patients, and participation in clinical trials to move the discovery of analgesics forward.

Nav1.7 is essential for nociceptor action potentials in the mouse in a manner independent of endogenous opioids

Loss-of-function mutations in Nav1.7, a voltage-gated sodium channel, cause congenital insensitivity to pain (CIP) in humans, demonstrating that Nav1.7 is essential for the perception of pain. However, the mechanism by which loss of Nav1.7 results in insensitivity to pain is not entirely clear. It has been suggested that loss of Nav1.7 induces overexpression of enkephalin, an endogenous opioid receptor agonist, leading to opioid-dependent analgesia. Using behavioral pharmacology and single-cell RNA-seq analysis, we find that overexpression of enkephalin occurs only in cLTMR neurons, a subclass of sensory neurons involved in low-threshold touch detection, and that this overexpression does not play a role in the analgesia observed following genetic removal of Nav1.7. Furthermore, we demonstrate using laser speckle contrast imaging (LSCI) and in vivo electrophysiology that Nav1.7 function is required for the initiation of C-fiber action potentials (APs), which explains the observed insensitivity to pain following genetic removal or inhibition of Nav1.7.

Inhibition of NaV1.7: the possibility of ideal analgesics

The selective inhibition of Na_V1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of Na_V1.7 has been recently achieved, multiple Na_V1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and Na_V1.7 coverage among three types of voltage-gated sodium channel (VGSC) inhibitors, namely conventional VGSC inhibitors, sulphonamides and acyl sulphonamides, is discussed. By demonstrating the PK/PD discrepancy of preclinical studies versus in vivo models and clinical results, the potential reasons behind the disconnect between preclinical results and clinical outcomes are discussed together with strategies for developing ideal analgesic agents.

Chemical and Biological Tools for the Study of Voltage-Gated Sodium Channels in Electrogenesis and Nociception

The malfunction and misregulation of voltage-gated sodium channels (NaV s) underlie in large part the electrical hyperexcitability characteristic of chronic inflammatory and neuropathic pain. NaV s are responsible for the initiation and propagation of electrical impulses (action potentials) in cells. Tissue and nerve injury alter the expression and localization of multiple NaV isoforms, including NaV 1.1, 1.3, and 1.6-1.9, resulting in aberrant action potential firing patterns. To better understand the role of NaV regulation, localization, and trafficking in electrogenesis and pain pathogenesis, a number of chemical and biological reagents for interrogating NaV function have been advanced. The development and application of such tools for understanding NaV physiology are the focus of this review.

Towards Structure-Guided Development of Pain Therapeutics Targeting Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels are critical molecular determinants of action potential generation and propagation in excitable cells. Normal Na_V channel function disruption can affect physiological neuronal signaling and lead to increased sensitivity to pain, congenital indifference to pain, uncoordinated movement, seizures, or paralysis. Human genetic studies have identified human Na_V1.7 (hNa_V1.7), hNa_V1.8, and hNa_V1.9 channel subtypes as crucial players in pain signaling. The premise that subtype selective Na_V inhibitors can reduce pain has been reinforced through intensive target validation and therapeutic development efforts. However, an ideal therapeutic has yet to emerge. This review is focused on recent progress, current challenges, and future opportunities to develop Na_V channel targeting small molecules and peptides as non-addictive therapeutics to treat pain.

In silico development of potential therapeutic for the pain treatment by inhibiting voltage-gated sodium channel 1.7

The voltage-gated sodium channel Nav1.7 can be considered as a promising target for the treatment of pain. This research presents conformational-independent and 3D field-based QSAR modeling for a series of aryl sulfonamide acting as Nav1.7 inhibitors. As descriptors used for building conformation-independent QSAR models, SMILES notation and local invariants of the molecular graph were used with the Monte Carlo optimization method as a model developer. Different statistical methods, including the index of ideality of correlation, were used to test the quality of the developed models, robustness and predictability and obtained results were good. Obtained results indicate that there is a very good correlation between 3D QSAR and conformation-independent models. Molecular fragments that account for the increase/decrease of a studied activity were defined and used for the computer-aided design of new compounds as potential analgesics. The final evaluation of the developed QSAR models and designed inhibitors were carried out using molecular docking studies, bringing to light an excellent correlation with the QSAR modeling results.

MicroRNA-96 is required to prevent allodynia by repressing voltage-gated sodium channels in spinal cord

Voltage-gated sodium channels (Na_vs) 1.7, 1.8, and 1.9 are predominately expressed in peripheral sensory neurons and are critical for action potential propagation in nociceptors. Unexpectedly, we found that expression of SCN9A, SCN10A, SCN11A, and SCN2A, the alpha subunit of Na_v1.7, Na_v1.8, Na_v1.9 and Na_v1.2, respectively, are up-regulated in spinal dorsal horn (SDH) neurons of miR-96 knockout mice. These mice also have de-repression of CACNA2D1/2 in DRG and display thermal and mechanical allodynia that could be attenuated by intrathecal or intraperitoneal injection of Na_v1.7 or Na_v1.8 blockers or Gabapentin. Moreover, Gad2::CreERT2 conditional miR-96 knockout mice phenocopied global knockout mice, implicating inhibitory neurons; nerve injury induced significant loss of miR-96 in SDH GABAergic and Glutamatergic neurons in mice which negatively correlated to up-regulation of Na_v1.7, Na_v1.8, Na_v1.9 and Scn2a, this dis-regulation of miR-96 and Na_vs in SDH neurons contributed to neuropathic pain which can be alleviated by intrathecal injection of Na_v1.7 or Na_v1.8 blockers. In conclusion, miR-96 is required to avoid allodynia through limiting the expression of VGCCs and Na_vs in DRG and Na_vs in SDH in naïve and nerve injury-induced neuropathic pain mice. Our findings suggest that central nervous system penetrating Na_v1.7 and Na_v1.8 blockers may be efficacious for pain relief.

Structure-activity relationship study of tryptophan-based butyrylcholinesterase inhibitors

A series of tryptophan-based selective nanomolar butyrylcholinesterase (BChE) inhibitors was designed and synthesized. Compounds were optimized in terms of potency, selectivity, and synthetic accessibility. The crystal structure of the inhibitor 18 in complex with BChE revealed the molecular basis for its low nanomolar inhibition (IC₅₀ = 2.8 nM). The favourable in vitro results enabled a first-in-animal in vivo efficacy and safety trial, which demonstrated a positive impact on fear-motivated and spatial long-term memory retrieval without any concomitant adverse motor effects. Altogether, this research culminated in a handful of new lead compounds with promising potential for symptomatic treatment of patients with Alzheimer's disease.

Discovery of Potent, Selective, and State-Dependent NaV1.7 Inhibitors with Robust Oral Efficacy in Pain Models: Structure-Activity Relationship and Optimization of Chroman and Indane Aryl Sulfonamides

Voltage-gated sodium channel Na_V1.7 is a genetically validated target for pain. Identification of Na_V1.7 inhibitors with all of the desired properties to develop as an oral therapeutic for pain has been a major challenge. Herein, we report systematic structure-activity relationship (SAR) studies carried out to identify novel sulfonamide derivatives as potent, selective, and state-dependent Na_V1.7 inhibitors for pain. Scaffold hopping from benzoxazine to chroman and indane bicyclic system followed by thiazole replacement on sulfonamide led to identification of lead molecules with significant improvement in solubility, selectivity over Na_V1.5, and CYP2C9 inhibition. The lead molecules 13, 29, 32, 43, and 51 showed a favorable pharmacokinetics (PK) profile across different species and robust efficacy in veratridine and formalin-induced inflammatory pain models in mice. Compound 51 also showed significant effects on the CCI-induced neuropathic pain model. The profile of 51 indicated that it has the potential for further evaluation as a therapeutic for pain.

Challenges and Opportunities for Therapeutics Targeting the Voltage-Gated Sodium Channel Isoform NaV1.7

Voltage-gated sodium ion channel subtype 1.7 (NaV1.7) is a high interest target for the discovery of non-opioid analgesics. Compelling evidence from human genetic data, particularly the finding that persons lacking functional NaV1.7 are insensitive to pain, has spurred considerable effort to develop selective inhibitors of this Na+ ion channel target as analgesic medicines. Recent clinical setbacks and disappointing performance of preclinical compounds in animal pain models, however, have led to skepticism around the potential of selective NaV1.7 inhibitors as human therapeutics. In this Perspective, we discuss the attributes and limitations of recently disclosed investigational drugs targeting NaV1.7 and review evidence that, by better understanding the requirements for selectivity and target engagement, the opportunity to deliver effective analgesic medicines targeting NaV1.7 endures.

Identification of CNS-Penetrant Aryl Sulfonamides as Isoform-Selective NaV1.6 Inhibitors with Efficacy in Mouse Models of Epilepsy

Nonselective antagonists of voltage-gated sodium (Na_V) channels have been long used for the treatment of epilepsies. The efficacy of these drugs is thought to be due to the block of sodium channels on excitatory neurons, primarily Na_V1.6 and Na_V1.2. However, these currently marketed drugs require high drug exposure and suffer from narrow therapeutic indices. Selective inhibition of Na_V1.6, while sparing Na_V1.1, is anticipated to provide a more effective and better tolerated treatment for epilepsies. In addition, block of Na_V1.2 may complement the anticonvulsant activity of Na_V1.6 inhibition. We discovered a novel series of aryl sulfonamides as CNS-penetrant, isoform-selective Na_V1.6 inhibitors, which also displayed potent block of Na_V1.2. Optimization focused on increasing selectivity over Na_V1.1, improving metabolic stability, reducing active efflux, and addressing a pregnane X-receptor liability. We obtained compounds 30-32, which produced potent anticonvulsant activity in mouse seizure models, including a direct current maximal electroshock seizure assay.

In silico exploration of aryl sulfonamide analogs as voltage-gated sodium channel 1.7 inhibitors by using 3D-QSAR, molecular docking study, and molecular dynamics simulations

It has been demonstrated by human genetics that the voltage-gated sodium channel Nav1.7 is currently a promising target for the treatment of pain. In this research, we performed molecular simulation works on a series of classic aryl sulfonamide Nav1.7 inhibitors using three-dimensional quantitative structure-activity relationships (3D-QSAR), molecular docking and molecular dynamics (MD) simulations for the first time to explore the correlation between their structures and activities. The results of the relevant statistical parameters of comparative molecular field analyses (CoMFA) and comparative molecular similarity indices analyses (CoMSIA) had been verified to be reasonable, and the deep relationship between the structures and activities of these inhibitors was obtained by analyzing the contour maps. The generated 3D-QSAR model showed a good predictive ability and provided valuable clues for the rational modification of molecules. The interactions between compounds and proteins were modeled by molecular docking studies. Finally, accuracy of the docking results and stability of the complexes were verified by 100 ns MD simulations. Detailed information on the key residues at the binding site and the types of interactions they participate in involved was obtained. The van der Waals energy contributed the most in the molecular binding process according to the calculation of binding free energy. All research results provided a good basis for further research on novel and effective Nav1.7 inhibitors.