Discovery of Arylsulfonamide Nav1.7 Inhibitors: IVIVC, MPO Methods, and Optimization of Selectivity Profile

Reducing agents facilitate membrane patch seal integrity and longevity

The patch clamp method is a widely applied electrophysiological technique used to understand ion channel activity and cellular excitation. The formation of a high resistance giga-ohm seal is required to obtain high-quality recordings but can be challenging due to variables including operator experience and cell preparation. Therefore, the identification of methods to promote the formation and longevity of giga-ohm seals may be beneficial. In this report, we describe our observation that the application of reducing agents (DTT and TCEP) to the external bath solution during whole-cell patch clamp recordings of heterologous cells (HEK and LM) and cultured primary cells (DRG neurons) enhanced the success of giga-ohm seal formation. Reducing agents also maintained the integrity of the seal for longer periods of time at strong hyperpolarizing voltages, whereas an oxidizing agent (H2O2) appeared to have the opposite effect. In summary, we report a useful tool to improve the quality of patch clamp recordings that may be helpful in certain experimental contexts.

New aryl and acylsulfonamides as state-dependent inhibitors of Nav1.3 voltage-gated sodium channel

Voltage-gated sodium channels (Na_vs) play an essential role in neurotransmission, and their dysfunction is often a cause of various neurological disorders. The Na_v1.3 isoform is found in the CNS and upregulated after injury in the periphery, but its role in human physiology has not yet been fully elucidated. Reports suggest that selective Na_v1.3 inhibitors could be used as novel therapeutics to treat pain or neurodevelopmental disorders. Few selective inhibitors of this channel are known in the literature. In this work, we report the discovery of a new series of aryl and acylsulfonamides as state-dependent inhibitors of Na_v1.3 channels. Using a ligand-based 3D similarity search and subsequent hit optimization, we identified and prepared a series of 47 novel compounds and tested them on Na_v1.3, Na_v1.5, and a selected subset also on Na_v1.7 channels in a QPatch patch-clamp electrophysiology assay. Eight compounds had an IC₅₀ value of less than 1 μM against the Na_v1.3 channel inactivated state, with one compound displaying an IC₅₀ value of 20 nM, whereas activity against the inactivated state of the Na_v1.5 channel and Na_v1.7 channel was approximately 20-fold weaker. None of the compounds showed use-dependent inhibition of the cardiac isoform Na_v1.5 at a concentration of 30 μM. Further selectivity testing of the most promising hits was measured using the two-electrode voltage-clamp method against the closed state of the Na_v1.1-Na_v1.8 channels, and compound 15b displayed small, yet selective, effects against the Na_v1.3 channel, with no activity against the other isoforms. Additional selectivity testing of promising hits against the inactivated state of the Na_v1.3, Na_v1.7, and Na_v1.8 channels revealed several compounds with robust and selective activity against the inactivated state of the Na_v1.3 channel among the three isoforms tested. Moreover, the compounds were not cytotoxic at a concentration of 50 μM, as demonstrated by the assay in human HepG2 cells (hepatocellular carcinoma cells). The novel state-dependent inhibitors of Na_v1.3 discovered in this work provide a valuable tool to better evaluate this channel as a potential drug target.

Site and Mechanism of ML252 Inhibition of Kv7 Voltage-Gated Potassium Channels

Kv7 (KCNQ) voltage-gated potassium channels are critical regulators of neuronal excitability and are candidate targets for development of antiseizure medications. Drug discovery efforts have identified small molecules that modulate channel function and reveal mechanistic insights into Kv7 channel physiological roles. While Kv7 channel activators have therapeutic benefits, inhibitors are useful for understanding channel function and mechanistic validation of candidate drugs. In this study, we reveal the mechanism of a Kv7.2/Kv7.3 inhibitor, ML252. We used docking and electrophysiology to identify critical residues involved in ML252 sensitivity. Most notably, Kv7.2[W236F] or Kv7.3[W265F] mutations strongly attenuate ML252 sensitivity. This tryptophan residue in the pore is also required for sensitivity to certain activators, including retigabine and ML213. We used automated planar patch clamp electrophysiology to assess competitive interactions between ML252 and different Kv7 activator subtypes. A pore-targeted activator (ML213) weakens the inhibitory effects of ML252, whereas a distinct activator subtype (ICA-069673) that targets the voltage sensor does not prevent ML252 inhibition. Using transgenic zebrafish larvae expressing an optical reporter (CaMPARI) to measure neural activity in-vivo, we demonstrate that Kv7 inhibition by ML252 increases neuronal excitability. Consistent with in-vitro data, ML213 suppresses ML252 induced neuronal activity, while the voltage-sensor targeted activator ICA-069673 does not prevent ML252 actions. In summary, this study establishes a binding site and mechanism of action of ML252, classifying this poorly understood drug as a pore-targeted Kv7 channel inhibitor that binds to the same tryptophan residue as commonly used pore-targeted Kv7 activators. ML213 and ML252 likely have overlapping sites of interaction in the pore Kv7.2 and Kv7.3 channels, resulting in competitive interactions. In contrast, the VSD-targeted activator ICA-069673 does not prevent channel inhibition by ML252.

Association of respiratory failure with inhibition of NaV1.6 in the phrenic nerve

As part of a drug discovery effort to identify potent inhibitors of NaV1.7 for the treatment of pain, we observed that inhibitors produced unexpected cardiovascular and respiratory effects in vivo. Specifically, inhibitors administered to rodents produced changes in cardiovascular parameters and respiratory cessation. We sought to determine the mechanism of the in vivo adverse effects by studying the selectivity of the compounds on NaV1.5, NaV1.4, and NaV1.6 in in vitro and ex vivo assays. Inhibitors lacking sufficient NaV1.7 selectivity over NaV1.6 were associated with respiratory cessation after in vivo administration to rodents. Effects on respiratory rate in rats were consistent with effects in an ex vivo hemisected rat diaphragm model and in vitro NaV1.6 potency. Furthermore, direct blockade of the phrenic nerve signaling was observed at exposures known to cause respiratory cessation in rats. Collectively, these results support a significant role for NaV1.6 in phrenic nerve signaling and respiratory function.

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.

Translational Pharmacokinetic–Pharmacodynamic Modeling of NaV1.7 Inhibitor MK-2075 to Inform Human Efficacious Dose

MK-2075 is a small-molecule selective inhibitor of the NaV1.7 channel investigated for the treatment of postoperative pain. A translational strategy was developed for MK-2075 to quantitatively interrelate drug exposure, target modulation, and the desired pharmacological response in preclinical animal models for the purpose of human translation. Analgesics used as a standard of care in postoperative pain were evaluated in preclinical animal models of nociceptive behavior (mouse tail flick latency and rhesus thermode heat withdrawal) to determine the magnitude of pharmacodynamic (PD) response at plasma concentrations associated with efficacy in the clinic. MK-2075 was evaluated in those same animal models to determine the concentration of MK-2075 required to achieve the desired level of response. Translation of MK-2075 efficacious concentrations in preclinical animal models to a clinical PKPD target in humans was achieved by accounting for species differences in plasma protein binding and in vitro potency against the NaV1.7 channel. Estimates of human pharmacokinetic (PK) parameters were obtained from allometric scaling of a PK model from preclinical species and used to predict the dose required to achieve the clinical exposure. MK-2075 exposure–response in a preclinical target modulation assay (rhesus olfaction) was characterized using a computational PKPD model which included a biophase compartment to account for the observed hysteresis. Translation of this model to humans was accomplished by correcting for species differences in PK NaV1.7 potency, and plasma protein binding while assuming that the kinetics of distribution to the target site is the same between humans and rhesus monkeys. This enabled prediction of the level of target modulation anticipated to be achieved over the dosing interval at the projected clinical efficacious human dose. Integration of these efforts into the early development plan informed clinical study design and decision criteria.