Selective block of Na1.7 promises to produce non-narcotic analgesic activity without motor or cognitive impairment. Several Na1.7-selective blockers have been reported, but efficacy in animal pain models required high multiples of the IC for channel block. Here, we report a target engagement assay using transgenic mice that has enabled the development of a second generation of selective Nav1.7 inhibitors that show robust analgesic activity in inflammatory and neuropathic pain models at low multiples of the IC. Like earlier arylsulfonamides, these newer acylsulfonamides target a binding site on the surface of voltage sensor domain 4 to achieve high selectivity among sodium channel isoforms and steeply state-dependent block. The improved efficacy correlates with very slow dissociation from the target channel. Chronic dosing increases compound potency about 10-fold, possibly due to reversal of sensitization arising during chronic injury, and provides efficacy that persists long after the compound has cleared from plasma.

This Account is dedicated to Professor George Just (McGill University) and to the memory of the late Professors Guido Perold (University of the Witwatersrand) and Satoru Masamune (University of Alberta and M.I.T.) for giving me the initial opportunities to undertake research in chemistry under their guidance; and also to Janette, Melissa, Andreas, Noulla and Menelaos. SYNLETT 2005, No. 6, pp 0879-08910 6 . 0 4 . 2 0 0 5 Advanced online publication: 23.03.2005Abstract: The first naphthalene ring-based analogues of the betterknown and extensively studied calixarenes were synthesized in 1993. Since that time, our group has focused its research on the design and synthesis of other 'calixnaphthalenes'. These endeavours are motivated partly due to the synthetic challenges which they provide, and to the challenges in deciphering their structural and complexation properties. Calixnaphthalenes offer several advantages over their calixarene analogues. Among these are the facts that they can form deeper, more electron-rich and in some cases, chiral, cavities. They also provide a wide range of potential new scaffolds upon which to design and build new receptors for neutral, or charged guest species. Their supramolecular complexation properties have barely been explored as yet since in most cases, only relatively small quantities only of these compounds have been obtained. In this review, all of the known, and previously unreported calixnaphthalenes are described. As well, some of the compounds, which are described, are still subjects of ongoing research.

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ABSTRACT:We report on a novel series of aryl sulfonamides that act as nanomolar potent, isoform-selective inhibitors of the human sodium channel hNa V 1.7. The optimization of these inhibitors is described. We aimed to improve potency against hNa V 1.7 while minimizing off-target safety concerns and generated compound 3. This agent displayed significant analgesic effects in rodent models of acute and inflammatory pain and demonstrated that binding to the voltage sensor domain 4 site of Na V 1.7 leads to an analgesic effect in vivo. Our findings corroborate the importance of hNa V 1.7 as a drug target for the treatment of pain. KEYWORDS: Sodium channel, Na V 1.7, Na V 1.5, pain, aryl sulfonamide, formalin model, cold allodynia T he sodium channel Na V 1.7 belongs to a family of transmembrane voltage gated sodium channels, which consists of nine isoforms in mammals (Na V 1.1 to Na V 1.9).1−4 Na V 1.7 plays a crucial role in pain sensation, and there is strong genetic evidence linking Na V 1.7 and its encoding SCN9A gene to painful disorders in humans. Gain-of-function mutations in the SCN9A gene result in painful conditions such as inherited erythromelalgia, paroxysmal extreme pain disorder, and idiopathic small fiber neuropathies. In contrast, loss-of-function mutations in the SCN9A gene were found to be the genetic cause of a rare disorder called congenital insensitivity to pain, characterized by a complete loss of the ability to sense painful stimuli. It is noteworthy that no significant side effects have been reported in people lacking Na V 1.7, such as cognitive, motor, or non-nociceptive sensory impairments other than anosmia, giving further support to the concept of Na V 1.7 antagonists as analgesics.1−4 The predominant expression of the Na V 1.7 isoform in the PNS may offer a pathway to limit CNS-related adverse effects by developing compounds that do not cross the blood−brain barrier. Combined, these observations and findings have made Na V 1.7 a promising target for drug development for the treatment of pain. Indeed, there has been tremendous interest in the development of small molecule Na V 1.7 inhibitors as analgesics, particularly isoform-selective inhibitors, and coverage of the progress has been the subject of several excellent reviews. 1−7 In recent years, a series of aryl sulfonamides as Na V inhibitors have been reported that appear to be highly selective for Na V 1.7 over the cardiac ion channel Na V 1.5. [4][5][6]8 Since block of the Na V 1.5 channel may lead to arrhythmia and thus limit the therapeutic potential of nonselective Na V 1.7 inhibitors, isoform-selective inhibitors have attracted considerable interest due to their potential to avoid these adverse events.3,5 An example is aryl sulfonamide PF-04856264 ( Figure 1), which selectively blocks Na V 1.7 over Na V 1.5 and Na V 1.3.

Herein, we report the discovery and optimization of a series of orally bioavailable acyl sulfonamide NaV1.7 inhibitors that are selective for NaV1.7 over NaV1.5 and highly efficacious in in vivo models of pain and hNaV1.7 target engagement. An analysis of the physicochemical properties of literature NaV1.7 inhibitors suggested that acyl sulfonamides with high fsp3 could overcome some of the pharmacokinetic (PK) and efficacy challenges seen with existing series. Parallel library syntheses lead to the identification of analogue 7, which exhibited moderate potency against NaV1.7 and an acceptable PK profile in rodents, but relatively poor stability in human liver microsomes. Further, design strategy then focused on the optimization of potency against hNaV1.7 and improvement of human metabolic stability, utilizing induced fit docking in our previously disclosed X-ray cocrystal of the NaV1.7 voltage sensing domain. These investigations culminated in the discovery of tool compound 33, one of the most potent and efficacious NaV1.7 inhibitors reported to date.

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 V 1.6 and Na V 1.2. However, these currently marketed drugs require high drug exposure and suffer from narrow therapeutic indices. Selective inhibition of Na V 1.6, while sparing Na V 1.1, is anticipated to provide a more effective and better tolerated treatment for epilepsies. In addition, block of Na V 1.2 may complement the anticonvulsant activity of Na V 1.6 inhibition. We discovered a novel series of aryl sulfonamides as CNS-penetrant, isoform-selective Na V 1.6 inhibitors, which also displayed potent block of Na V 1.2. Optimization focused on increasing selectivity over Na V 1.1, improving metabolic stability, reducing active efflux, and addressing a pregnane Xreceptor liability. We obtained compounds 30−32, which produced potent anticonvulsant activity in mouse seizure models, including a direct current maximal electroshock seizure assay.

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