APOL1-mediated monovalent cation transport contributes to APOL1-mediated podocytopathy in kidney disease.

Two coding variants of apolipoprotein L1 (APOL1), called G1 and G2, explain much of the excess risk of kidney disease in African Americans. While various cytotoxic phenotypes have been reported in experimental models, the proximal mechanism by which G1 and G2 cause kidney disease is poorly understood. Here, we leveraged 3 experimental models and a recently reported small molecule blocker of APOL1 protein, VX-147, to identify the upstream mechanism of G1-induced cytotoxicity. In HEK293 cells, we demonstrated that G1-mediated Na+ import/K+ efflux triggered activation of GPCR/IP3-mediated calcium release from the ER, impaired mitochondrial ATP production, and impaired translation, which were all reversed by VX-147. In human urine-derived podocyte-like epithelial cells (HUPECs), we demonstrated that G1 caused cytotoxicity that was again reversible by VX-147. Finally, in podocytes isolated from APOL1 G1 transgenic mice, we showed that IFN-γ-mediated induction of G1 caused K+ efflux, activation of GPCR/IP3 signaling, and inhibition of translation, podocyte injury, and proteinuria, all reversed by VX-147. Together, these results establish APOL1-mediated Na+/K+ transport as the proximal driver of APOL1-mediated kidney disease.

Repurposing Approved Drugs as Inhibitors of Kv7.1 and Nav1.8 to Treat Pitt Hopkins Syndrome.

PURPOSE: Pitt Hopkins Syndrome (PTHS) is a rare genetic disorder caused by mutations of a specific gene, transcription factor 4 (TCF4), located on chromosome 18. PTHS results in individuals that have moderate to severe intellectual disability, with most exhibiting psychomotor delay. PTHS also exhibits features of autistic spectrum disorders, which are characterized by the impaired ability to communicate and socialize. PTHS is comorbid with a higher prevalence of epileptic seizures which can be present from birth or which commonly develop in childhood. Attenuated or absent TCF4 expression results in increased translation of peripheral ion channels Kv7.1 and Nav1.8 which triggers an increase in after-hyperpolarization and altered firing properties. METHODS: We now describe a high throughput screen (HTS) of 1280 approved drugs and machine learning models developed from this data. The ion channels were expressed in either CHO (KV7.1) or HEK293 (Nav1.8) cells and the HTS used either 86Rb+ efflux (KV7.1) or a FLIPR assay (Nav1.8). RESULTS: The HTS delivered 55 inhibitors of Kv7.1 (4.2% hit rate) and 93 inhibitors of Nav1.8 (7.2% hit rate) at a screening concentration of 10 µM. These datasets also enabled us to generate and validate Bayesian machine learning models for these ion channels. We also describe a structure activity relationship for several dihydropyridine compounds as inhibitors of Nav1.8. CONCLUSIONS: This work could lead to the potential repurposing of nicardipine or other dihydropyridine calcium channel antagonists as potential treatments for PTHS acting via Nav1.8, as there are currently no approved treatments for this rare disorder.

TRPA1 ankyrin repeat six interacts with a small molecule inhibitor chemotype.

TRPA1, a member of the transient receptor potential channel (TRP) family, is genetically linked to pain in humans, and small molecule inhibitors are efficacious in preclinical animal models of inflammatory pain. These findings have driven significant interest in development of selective TRPA1 inhibitors as potential analgesics. The majority of TRPA1 inhibitors characterized to date have been reported to interact with the S5 transmembrane helices forming part of the pore region of the channel. However, the development of many of these inhibitors as clinical drug candidates has been prevented by high lipophilicity, low solubility, and poor pharmacokinetic profiles. Identification of alternate compound interacting sites on TRPA1 provides the opportunity to develop structurally distinct modulators with novel structure-activity relationships and more desirable physiochemical properties. In this paper, we have identified a previously undescribed potent and selective small molecule thiadiazole structural class of TRPA1 inhibitor. Using species ortholog chimeric and mutagenesis strategies, we narrowed down the site of interaction to ankyrinR #6 within the distal N-terminal region of TRPA1. To identify the individual amino acid residues involved, we generated a computational model of the ankyrinR domain. This model was used predictively to identify three critical amino acids in human TRPA1, G238, N249, and K270, which were confirmed by mutagenesis to account for compound activity. These findings establish a small molecule interaction region on TRPA1, expanding potential avenues for developing TRPA1 inhibitor analgesics and for probing the mechanism of channel gating.

A comprehensive approach to identifying repurposed drugs to treat SCN8A epilepsy.

OBJECTIVE: Many previous studies of drug repurposing have relied on literature review followed by evaluation of a limited number of candidate compounds. Here, we demonstrate the feasibility of a more comprehensive approach using high-throughput screening to identify inhibitors of a gain-of-function mutation in the SCN8A gene associated with severe pediatric epilepsy. METHODS: We developed cellular models expressing wild-type or an R1872Q mutation in the Nav 1.6 sodium channel encoded by SCN8A. Voltage clamp experiments in HEK-293 cells expressing the SCN8A R1872Q mutation demonstrated a leftward shift in sodium channel activation as well as delayed inactivation; both changes are consistent with a gain-of-function mutation. We next developed a fluorescence-based, sodium flux assay and used it to assess an extensive library of approved drugs, including a panel of antiepileptic drugs, for inhibitory activity in the mutated cell line. Lead candidates were evaluated in follow-on studies to generate concentration-response curves for inhibiting sodium influx. Select compounds of clinical interest were evaluated by electrophysiology to further characterize drug effects on wild-type and mutant sodium channel functions. RESULTS: The screen identified 90 drugs that significantly inhibited sodium influx in the R1872Q cell line. Four drugs of potential clinical interest-amitriptyline, carvedilol, nilvadipine, and carbamazepine-were further investigated and demonstrated concentration-dependent inhibition of sodium channel currents. SIGNIFICANCE: A comprehensive drug repurposing screen identified potential new candidates for the treatment of epilepsy caused by the R1872Q mutation in the SCN8A gene.

Discovery and in Vitro Optimization of 3-Sulfamoylbenzamides as ROMK Inhibitors.

Inhibitors of the renal outer medullary potassium channel (ROMK) show promise as novel mechanism diuretics, with potentially lower risk of diuretic-induced hypokalemia relative to current thiazide and loop diuretics. Here, we report the identification of a novel series of 3-sulfamoylbenzamide ROMK inhibitors. Starting from HTS hit 4, this series was optimized to provide ROMK inhibitors with good in vitro potencies and well-balanced ADME profiles. In contrast to previously reported small-molecule ROMK inhibitors, members of this series were demonstrated to be highly selective for inhibition of human over rat ROMK and to be insensitive to the N171D pore mutation that abolishes inhibitory activity of previously reported ROMK inhibitors.

GI-530159, a novel, selective, mechanosensitive two-pore-domain potassium (K2P ) channel opener, reduces rat dorsal root ganglion neuron excitability.

BACKGROUND AND PURPOSE: TREK two-pore-domain potassium (K2P ) channels play a critical role in regulating the excitability of somatosensory nociceptive neurons and are important mediators of pain perception. An understanding of the roles of TREK channels in pain perception and, indeed, in other pathophysiological conditions, has been severely hampered by the lack of potent and/or selective activators and inhibitors. In this study, we describe a new, selective opener of TREK channels, GI-530159. EXPERIMENTAL APPROACH: The effect of GI-530159 on TREK channels was demonstrated using 86 Rb efflux assays, whole-cell and single-channel patch-clamp recordings from recombinant TREK channels. The expression of K2P 2.1 (TREK1), K2P 10.1 (TREK2) and K2P 4.1 (TRAAK) channels was determined using transcriptome analysis from single dorsal root ganglion (DRG) cells. Current-clamp recordings from cultured rat DRG neurons were used to measure the effect of GI-530159 on neuronal excitability. KEY RESULTS: For recombinant human TREK1 channels, GI-530159 had similar low EC50 values in Rb efflux experiments and electrophysiological recordings. It activated TREK2 channels, but it had no detectable action on TRAAK channels nor any significant effect on other K channels tested. Current-clamp recordings from cultured rat DRG neurones showed that application of GI-530159 at 1 µM resulted in a significant reduction in firing frequency and a small hyperpolarization of resting membrane potential. CONCLUSIONS AND IMPLICATIONS: This study provides pharmacological evidence for the presence of mechanosensitive TREK K2P channels in sensory neurones and suggests that development of selective K2P channel openers like GI-530159 could aid in the development of novel analgesic agents. LINKED ARTICLES: This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Validation of ion channel targets.

A prerequisite for a successful target-based drug discovery program is a robust data set that increases confidence in the validation of the molecular target and the therapeutic approach. Given the significant time and resource investment required to carry a drug to market, early selection of targets that can be modulated safely and effectively forms the basis for a strong portfolio and pipeline. In this article we present some of the more useful scientific approaches that can be applied toward the validation of ion channel targets, a molecular family with a history of clinical success in therapeutic areas such as cardiovascular, respiratory, pain and neuroscience.

A novel selective and orally bioavailable Nav 1.8 channel blocker, PF-01247324, attenuates nociception and sensory neuron excitability.

BACKGROUND AND PURPOSE: NaV 1.8 ion channels have been highlighted as important molecular targets for the design of low MW blockers for the treatment of chronic pain. Here, we describe the effects of PF-01247324, a new generation, selective, orally bioavailable Nav 1.8 channel blocker of novel chemotype. EXPERIMENTAL APPROACH: The inhibition of Nav 1.8 channels by PF-01247324 was studied using in vitro patch-clamp electrophysiology and the oral bioavailability and antinociceptive effects demonstrated using in vivo rodent models of inflammatory and neuropathic pain. KEY RESULTS: PF-01247324 inhibited native tetrodotoxin-resistant (TTX-R) currents in human dorsal root ganglion (DRG) neurons (IC50 : 331 nM) and in recombinantly expressed h Nav 1.8 channels (IC50 : 196 nM), with 50-fold selectivity over recombinantly expressed TTX-R hNav 1.5 channels (IC50 : ∼10 µM) and 65-100-fold selectivity over TTX-sensitive (TTX-S) channels (IC50 : ∼10-18 µM). Native TTX-R currents in small-diameter rodent DRG neurons were inhibited with an IC50 448 nM, and the block of both human recombinant Nav 1.8 channels and TTX-R from rat DRG neurons was both frequency and state dependent. In vitro current clamp showed that PF-01247324 reduced excitability in both rat and human DRG neurons and also altered the waveform of the action potential. In vivo experiments n rodents demonstrated efficacy in both inflammatory and neuropathic pain models. CONCLUSIONS AND IMPLICATIONS: Using PF-01247324, we have confirmed a role for Nav 1.8 channels in both inflammatory and neuropathic pain. We have also demonstrated a key role for Nav 1.8 channels in action potential upstroke and repetitive firing of rat and human DRG neurons.

Voltage sensor interaction site for selective small molecule inhibitors of voltage-gated sodium channels.

Voltage-gated sodium (Nav) channels play a fundamental role in the generation and propagation of electrical impulses in excitable cells. Here we describe two unique structurally related nanomolar potent small molecule Nav channel inhibitors that exhibit up to 1,000-fold selectivity for human Nav1.3/Nav1.1 (ICA-121431, IC50, 19 nM) or Nav1.7 (PF-04856264, IC50, 28 nM) vs. other TTX-sensitive or resistant (i.e., Nav1.5) sodium channels. Using both chimeras and single point mutations, we demonstrate that this unique class of sodium channel inhibitor interacts with the S1-S4 voltage sensor segment of homologous Domain 4. Amino acid residues in the "extracellular" facing regions of the S2 and S3 transmembrane segments of Nav1.3 and Nav1.7 seem to be major determinants of Nav subtype selectivity and to confer differences in species sensitivity to these inhibitors. The unique interaction region on the Domain 4 voltage sensor segment is distinct from the structural domains forming the channel pore, as well as previously characterized interaction sites for other small molecule inhibitors, including local anesthetics and TTX. However, this interaction region does include at least one amino acid residue [E1559 (Nav1.3)/D1586 (Nav1.7)] that is important for Site 3 α-scorpion and anemone polypeptide toxin modulators of Nav channel inactivation. The present study provides a potential framework for identifying subtype selective small molecule sodium channel inhibitors targeting interaction sites away from the pore region.