The discovery of benzoxazine sulfonamide inhibitors of Na V 1.7: Tools that bridge efficacy and target engagement

Discovery of novel cyclopentane carboxylic acids as potent and selective inhibitors of NaV1.7

Discovery efforts leading to the identification of cyclopentane carboxylic acid 31, a potent inhibitor of NaV1.7 that showed high selectivity over NaV1.5 and exhibited robust analgesic effects in an inherited erythromelalgia (IEM) transgenic mouse assay, are described herein. Key design elements that culminated in the discovery of 31 include exploration of proline substituents, replacement of the proline warhead with cyclopentane carboxylic acid, that led to significantly boosted NaV1.7 potency, and replacement of the metabolically labile adamantane motif with the 2,6-dichlorobenzyl substituted piperidine system, that addressed metabolic instability and led to a significant improvement in PK.

Multiple machine learning methods aided virtual screening of NaV 1.5 inhibitors

Na_v 1.5 sodium channels contribute to the generation of the rapid upstroke of the myocardial action potential and thereby play a central role in the excitability of myocardial cells. At present, the patch clamp method is the gold standard for ion channel inhibitor screening. However, this method has disadvantages such as high technical difficulty, high cost and low speed. In this study, novel machine learning models to screen chemical blockers were developed to overcome the above shortage. The data from the ChEMBL Database were employed to establish the machine learning models. Firstly, six molecular fingerprints together with five machine learning algorithms were used to develop 30 classification models to predict effective inhibitors. A validation and a test set were used to evaluate the performance of the models. Subsequently, the privileged substructures tightly associated with the inhibition of the Na_v 1.5 ion channel were extracted using the bioalerts Python package. In the validation set, the RF-Graph model performed best. Similarly, RF-Graph produced the best result in the test set in which the Prediction Accuracy (Q) was 0.9309 and Matthew's correlation coefficient was 0.8627, further indicating the model had high classification ability. The results of the privileged substructures indicated Sulfa structures and fragments with large Steric hindrance tend to block Na_v 1.5. In the unsupervised learning task of identifying sulfa drugs, MACCS and Graph fingerprints had good results. In summary, effective machine learning models have been constructed which help to screen potential inhibitors of the Na_v 1.5 ion channel and key privileged substructures with high affinity were also extracted.

Inhibition of NaV1.7: the possibility of ideal analgesics

The selective inhibition of NaV1.7 is a promising strategy for developing novel analgesic agents with fewer adverse effects. Although the potent selective inhibition of NaV1.7 has been recently achieved, multiple NaV1.7 inhibitors failed in clinical development. In this review, the relationship between preclinical in vivo efficacy and NaV1.7 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.

Histamine Sensitization of the Voltage-Gated Sodium Channel Nav1.7 Contributes to Histaminergic Itch in Mice

Itch, a common clinical symptom of many skin diseases, severely impairs the life quality of patients. Nav1.7, a subtype of voltage-gated sodium channels mainly expressed in primary sensory neurons, is responsible for the amplification of threshold currents that trigger action potential (AP) generation. Gain-of-function mutation of Nav1.7 leads to paroxysmal itch, while pharmacological inhibition of Nav1.7 alleviates histamine-dependent itch. However, the crosstalk between histamine and Nav1.7 that leads to itch is unclear. In the present study, we demonstrated that in the dorsal root ganglion (DRG) neurons from histamine-dependent itch model mice induced by compound 48/80, tetrodotoxin-sensitive (TTX-S) but not TTX-resistant Na⁺ currents were activated at more hyperpolarized membrane potentials compared to those on DRG neurons from vehicle-treated mice. Meanwhile, bath application of histamine shifted the activation voltages of TTX-S Na⁺ currents to the hyperpolarized direction, increased the AP frequency, and reduced the current threshold required to elicit APs. Further mechanistic studies demonstrated that selective activation of H1 but not H2 and H4 receptors mimicked histamine effect on TTX-S Na⁺ channels in DRG neurons. The protein kinase C (PKC) inhibitor GO 8963, but not the PKA inhibitor H89, normalized histamine-sensitized TTX-S Na⁺ channels. We also demonstrated that histamine shifted the activation voltages of Na⁺ currents to the hyperpolarized direction in Chinese hamster ovary (CHO) cells expressing Nav1.7. Importantly, selective inhibition of Nav1.7 by PF-05089771 significantly relieved the scratching frequency in a histamine-dependent itch model induced by compound 48/80. Taken together, these data suggest that activation of H1 receptors by histamine sensitizes Nav1.7 channels through the PKC pathway in DRG neurons that contributes to histamine-dependent itch.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Discovery of Acyl-sulfonamide Nav1.7 Inhibitors GDC-0276 and GDC-0310

Nav1.7 is an extensively investigated target for pain with a strong genetic link in humans, yet in spite of this effort, it remains challenging to identify efficacious, selective, and safe inhibitors. Here, we disclose the discovery and preclinical profile of GDC-0276 (1) and GDC-0310 (2), selective Nav1.7 inhibitors that have completed Phase 1 trials. Our initial search focused on close-in analogues to early compound 3. This resulted in the discovery of GDC-0276 (1), which possessed improved metabolic stability and an acceptable overall pharmacokinetics profile. To further derisk the predicted human pharmacokinetics and enable QD dosing, additional optimization of the scaffold was conducted, resulting in the discovery of a novel series of N-benzyl piperidine Nav1.7 inhibitors. Improvement of the metabolic stability by blocking the labile benzylic position led to the discovery of GDC-0310 (2), which possesses improved Nav selectivity and pharmacokinetic profile over 1.

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.

Complementary roles of murine NaV1.7, NaV1.8 and NaV1.9 in acute itch signalling

Acute pruritus occurs in various disorders. Despite severe repercussions on quality of life treatment options remain limited. Voltage-gated sodium channels (Na_V) are indispensable for transformation and propagation of sensory signals implicating them as drug targets. Here, Na_V1.7, 1.8 and 1.9 were compared for their contribution to itch by analysing Na_V-specific knockout mice. Acute pruritus was induced by a comprehensive panel of pruritogens (C48/80, endothelin, 5-HT, chloroquine, histamine, lysophosphatidic acid, trypsin, SLIGRL, β-alanine, BAM8-22), and scratching was assessed using a magnet-based recording technology. We report an unexpected stimulus-dependent diversity in Na_V channel-mediated itch signalling. Na_V1.7^−/− showed substantial scratch reduction mainly towards strong pruritogens. Na_V1.8^−/− impaired histamine and 5-HT-induced scratching while Na_V1.9 was involved in itch signalling towards 5-HT, C48/80 and SLIGRL. Furthermore, similar microfluorimetric calcium responses of sensory neurons and expression of itch-related TRP channels suggest no change in sensory transduction but in action potential transformation and conduction. The cumulative sum of scratching over all pruritogens confirmed a leading role of Na_V1.7 and indicated an overall contribution of Na_V1.9. Beside the proposed general role of Na_V1.7 and 1.9 in itch signalling, scrutiny of time courses suggested Na_V1.8 to sustain prolonged itching. Therefore, Na_V1.7 and 1.9 may represent targets in pruritus therapy.

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.

Voltage-Gated Sodium Channels Regulating Action Potential Generation in Itch-, Nociceptive-, and Low-Threshold Mechanosensitive Cutaneous C-Fibers

We evaluated the effect of voltage-gated sodium channel 1 (NaV1) blockers in three nonoverlapping C-fiber subtypes in the mouse skin: chloroquine (CQ)-sensitive C-fibers with high mechanical thresholds-itch C-fibers; second, CQ-insensitive, capsaicin-sensitive C-fibers with high mechanical thresholds-nociceptors; and CQ and capsaicin-insensitive C-fibers with a very low mechanical threshold-C-LTMs. NaV1-blocking drugs were applied to the nerve terminal receptive fields using an innervated isolated dorsal mouse skin-nerve preparation where the drugs are delivered into the skin intra-arterially. We combined these studies with an analysis of the mRNA expression of the α-subunits of NaV1 in individual dorsal root ganglia neurons labeled from the same region of the skin. Our results show that virtually all nociceptors and itch C-fibers expressed the tetrodotoxin (TTX)-resistant channels NaV1.8 and NaV1.9. However, TTX applied selectively into the skin abolished the action potential firing in response to mechanical stimulation in 75% of the itch C-fibers, 100% of the nociceptors, and 100% of C-LTMs. NaV1.7 was the most commonly expressed TTX-sensitive NaV1 in all three C-fiber subtypes innervating the dorsal skin. Selectively blocking NaV1.7 abolished responses in about 40% of itch C-fibers, 65% of nociceptors, but only 20% of C-LTMs. Blocking NaV1.8 alone had no affect on the firing sensitivity of the C-fibers. However, in itch and nociceptive C-fibers where the activation was not inhibited with a NaV1.7 blocker, adding the NaV1.8 blocker silenced action potential discharge.