TREK1 channel activation as a new analgesic strategy devoid of opioid adverse effects

Discovery of ONO-2920632 (VU6011887): A Highly Selective and CNS Penetrant TREK-2 (TWIK-Related K+ Channel 2) Preferring Activator In Vivo Tool Compound

Herein we describe our initial work on the K2P family of potassium ion channels with the chemical optimization and characterization of a novel series of TWIK-Related K+ Channel (TREK)-1/2 dual activators and TREK-2 preferring activators derived from a high-throughput screening hit. The exercise provided TREK activators with good CNS penetration and others with low CNS exposure to enable exploration of both central and peripheral TREK activation. From this, ONO-2920632 (VU6011887 = 19b) emerged as a reasonably potent (human Tl+; TREK-1 EC50 = 2.8 μM (95% Emax), TREK-2 EC50 = 0.30 μM (184% Emax)), first-generation CNS penetrant (rat Kp = 0.37) in vivo tool compound with selectivity versus the other K2P channels (>91-fold selective vs TASK1, TASK2, TASK3, TRAAK, TWIK2, and 31-fold selective vs TRESK) and no significant activity in a large ancillary pharmacology panel. ONO-2920632 (VU6011887) displayed robust, dose dependent efficacy when dosed orally in a mouse pain model (acetic acid writhing assay), where it was equipotent at 3 mg/kg to the assay standard indomethacin at 10 mg/kg. The therapeutic potential of TREK channel activation has long been hampered by a lack of selective, small molecule tools, and this work provides a variety of in vivo tool compounds for the community.

The TREK-1 potassium channel is involved in both the analgesic and anti-proliferative effects of riluzole in bone cancer pain

BACKGROUND

The metastasis of tumors into bone tissue typically leads to intractable pain that is both very disabling and particularly difficult to manage. We investigated here whether riluzole could have beneficial effects for the treatment of prostate cancer-induced bone pain and how it could influence the development of bone metastasis.

METHODS

We used a bone pain model induced by intratibial injection of human PC3 prostate cancer cells into male SCID mice treated or not with riluzole administered in drinking water. We also used riluzole in vitro to assess its possible effect on PC3 cell viability and functionality, using patch-clamp.

RESULTS

Riluzole had a significant preventive effect on both evoked and spontaneous pain involving the TREK-1 potassium channel. Riluzole did not interfere with PC3-induced bone loss or bone remodeling in vivo. It also significantly decreased PC3 cell viability in vitro. The antiproliferative effect of riluzole is correlated with a TREK-1-dependent membrane hyperpolarization in these cells.

CONCLUSION

The present data suggest that riluzole could be very useful to manage evoked and spontaneous hypersensitivity in cancer-induced bone pain and has no significant adverse effect on cancer progression.

Extracellular modulation of TREK-2 activity with nanobodies provides insight into the mechanisms of K2P channel regulation

Potassium channels of the Two-Pore Domain (K2P) subfamily, KCNK1-KCNK18, play crucial roles in controlling the electrical activity of many different cell types and represent attractive therapeutic targets. However, the identification of highly selective small molecule drugs against these channels has been challenging due to the high degree of structural and functional conservation that exists not only between K2P channels, but across the whole K+ channel superfamily. To address the issue of selectivity, here we generate camelid antibody fragments (nanobodies) against the TREK-2 (KCNK10) K2P K+ channel and identify selective binders including several that directly modulate channel activity. X-ray crystallography and CryoEM data of these nanobodies in complex with TREK-2 also reveal insights into their mechanisms of activation and inhibition via binding to the extracellular loops and Cap domain, as well as their suitability for immunodetection. These structures facilitate design of a biparatropic inhibitory nanobody with markedly improved sensitivity. Together, these results provide important insights into TREK channel gating and provide an alternative, more selective approach to modulation of K2P channel activity via their extracellular domains.

FXYD2 antisense oligonucleotide provides an efficient approach for long-lasting relief of chronic peripheral pain

Chronic pain, whether of inflammatory or neuropathic origin, affects about 18% of the population of developed countries, and most current treatments are only moderately effective and/or cause serious side effects. Therefore, the development of novel therapeutic approaches still represents a major challenge. The Na,K-ATPase modulator FXYD2 is critically required for the maintenance of neuropathic pain in rodents. Here, we set up a therapeutic protocol based on the use of chemically modified antisense oligonucleotides (ASOs) to inhibit FXYD2 expression and treat chronic pain. We identified an ASO targeting a 20-nucleotide stretch in the FXYD2 mRNA that is evolutionarily conserved between rats and humans and is a potent inhibitor of FXYD2 expression. We used this sequence to synthesize lipid-modified forms of ASO (FXYD2-LASO) to facilitate their entry into dorsal root ganglia neurons. We established that intrathecal or intravenous injections of FXYD2-LASO in rat models of neuropathic or inflammatory pain led to a virtually complete alleviation of their pain symptoms, without causing obvious side effects. Remarkably, by using 2'-O-2-methoxyethyl chemical stabilization of the ASO (FXYD2-LASO-Gapmer), we could significantly prolong the therapeutic action of a single treatment up to 10 days. This study establishes FXYD2-LASO-Gapmer administration as a promising and efficient therapeutic strategy for long-lasting relief of chronic pain conditions in human patients.

Transition between conformational states of the TREK-1 K2P channel promoted by interaction with PIP2

Members of the TREK family of two-pore domain (K2P) potassium channels are highly sensitive to regulation by membrane lipids, including phosphatidylinositol-4,5-bisphosphate (PIP₂). Previous studies have demonstrated that PIP₂ increases TREK1 channel activity, however, the mechanistic understanding of the conformational transitions induced by PIP₂ remain unclear. Here, we used coarse-grained molecular dynamics (CG-MD) and atomistic MD simulations to model the PIP₂ binding site on both the up and down state conformations of TREK-1. We also calculated the free energy of PIP₂ binding relative to other anionic phospholipids in both conformational states using potential of mean force (PMF) and free energy perturbation (FEP) calculations. Our results identify state-dependent binding of PIP₂ to sites involving the proximal C-terminus and we show that PIP₂ promotes a conformational transition from a down state towards an intermediate that resembles the up state. These results are consistent with functional data for PIP₂ regulation and together provide evidence for a structural mechanism of TREK-1 channel activation by phosphoinositides.

The Prostacyclin Analogue, Treprostinil, Used in the Treatment of Pulmonary Arterial Hypertension, is a Potent Antagonist of TREK-1 and TREK-2 Potassium Channels

Pulmonary arterial hypertension (PAH) is an aggressive vascular remodeling disease that carries a high morbidity and mortality rate. Treprostinil (Remodulin) is a stable prostacyclin analogue with potent vasodilatory and anti-proliferative activity, approved by the FDA and WHO as a treatment for PAH. A limitation of this therapy is the severe subcutaneous site pain and other forms of pain experienced by some patients, which can lead to significant non-compliance. TWIK-related potassium channels (TREK-1 and TREK-2) are highly expressed in sensory neurons, where they play a role in regulating sensory neuron excitability. Downregulation, inhibition or mutation of these channels leads to enhanced pain sensitivity. Using whole-cell patch-clamp electrophysiological recordings, we show, for the first time, that treprostinil is a potent antagonist of human TREK-1 and TREK-2 channels but not of TASK-1 channels. An increase in TASK-1 channel current was observed with prolonged incubation, consistent with its therapeutic role in PAH. To investigate treprostinil-induced inhibition of TREK, site-directed mutagenesis of a number of amino acids, identified as important for the action of other regulatory compounds, was carried out. We found that a gain of function mutation of TREK-1 (Y284A) attenuated treprostinil inhibition, while a selective activator of TREK channels, BL-1249, overcame the inhibitory effect of treprostinil. Our data suggests that subcutaneous site pain experienced during treprostinil therapy may result from inhibition of TREK channels near the injection site and that pre-activation of these channels prior to treatment has the potential to alleviate this nociceptive activity.

Development of Non-opioid Analgesics Targeting Two-pore Domain Potassium Channels

Two-pore domain potassium (K2P) channels are a diverse family of potassium channels. K2P channels generate background leak potassium currents to regulate cellular excitability and are thereby involved in a wide range of neurological disorders. K2P channels are modulated by a variety of physicochemical factors such as mechanical stretch, temperature, and pH. In the the peripheral nervous system (PNS), K2P channels are widely expressed in nociceptive neurons and play a critical roles in pain perception. In this review, we summarize the recent advances in the pharmacological properties of K2P channels, with a focus on the exogenous small-molecule activators targeting K2P channels. We emphasize the subtype-selectivity, cellular and in vivo pharmacological properties of all the reported small-molecule activators. The key underlying analgesic mechanisms mediated by K2P are also summarized based on the data in the literature from studies using small-molecule activators and genetic knock-out animals. We discuss advantages and limitations of the translational perspectives of K2P in pain medicine and provide outstanding questions for future studies in the end.

Translational value of non-human primates in opioid research

Preclinical opioid research using animal models not only provides mechanistic insights into the modulation of opioid analgesia and its associated side effects, but also validates drug candidates for improved treatment options for opioid use disorder. Non-human primates (NHPs) have served as a surrogate species for humans in opioid research for more than five decades. The translational value of NHP models is supported by the documented species differences between rodents and primates regarding their behavioral and physiological responses to opioid-related ligands and that NHP studies have provided more concordant results with human studies. This review highlights the utilization of NHP models in five aspects of opioid research, i.e., analgesia, abuse liability, respiratory depression, physical dependence, and pruritus. Recent NHP studies have found that (1) mixed mu opioid and nociceptin/orphanin FQ peptide receptor partial agonists appear to be safe, non-addictive analgesics and (2) mu opioid receptor- and mixed opioid receptor subtype-based medications remain the only two classes of drugs that are effective in alleviating opioid-induced adverse effects. Given the recent advances in pharmaceutical sciences and discoveries of novel targets, NHP studies are posed to identify the translational gap and validate therapeutic targets for the treatment of opioid use disorder. Pharmacological studies using NHPs along with multiple outcome measures (e.g., behavior, physiologic function, and neuroimaging) will continue to facilitate the research and development of improved medications to curb the opioid epidemic.

TREK1 channel activation as a new analgesic strategy devoid of opioid adverse effects

BACKGROUND AND PURPOSE

Opioids are effective painkillers. However, their risk-benefit ratio is dampened by numerous adverse effects and opioid misuse has led to a public health crisis. Safer alternatives are required, but isolating the antinociceptive effect of opioids from their adverse effects is a pharmacological challenge because activation of the μ opioid receptor triggers both the antinociceptive and adverse effects of opioids.

EXPERIMENTAL APPROACH

The TREK1 potassium channel is activated downstream of μ receptor and involved in the antinociceptive activity of morphine but not in its adverse effects. Bypassing the μ opioid receptor to directly activate TREK1 could therefore be a safer analgesic strategy.

KEY RESULTS

We developed a selective TREK1 activator, RNE28, with antinociceptive activity in naive rodents and in models of inflammatory and neuropathic pain. This activity was lost in TREK1 knockout mice or wild-type mice treated with the TREK1 blocker spadin, showing that TREK1 is required for the antinociceptive activity of RNE28. RNE28 did not induce respiratory depression, constipation, rewarding effects, or sedation at the analgesic doses tested.

CONCLUSION AND IMPLICATIONS

This proof-of-concept study shows that TREK1 activators could constitute a novel class of painkillers, inspired by the mechanism of action of opioids but devoid of their adverse effects.