Discovery of a Highly Selective and Potent TRPC3 Inhibitor with High Metabolic Stability and Low Toxicity

Inhibition of TRPC3 channels suppresses seizure susceptibility in the genetically-epilepsy prone rats

Transient receptor potential canonical 3 (TRPC3) channels are important in regulating Ca2+ homeostasis and have been implicated in the pathophysiology of chemically induced seizures. Inherited seizure susceptibility in genetically epilepsy-prone rats (GEPR-3s) has been linked to increased voltage-gated Ca2+ channel currents in the inferior colliculus neurons, which can affect intraneuronal Ca2+ homeostasis. However, whether TRPC3 channels also contribute to inherited seizure susceptibility in GEPR-3s is unclear. This study investigated the effects of JW-65, a potent and selective inhibitor of TRPC3 channels, on acoustically evoked seizure susceptibility in adult male and female GEPR-3s. These seizures consisted of wild running seizures (WRSs) that evolved into generalized tonic-clonic seizures (GTCSs). The results showed that acute administration of low doses of JW-65 significantly decreased by 55-89% the occurrence of WRSs and GTCSs and the seizure severity in both male and female GEPR-3s. This antiseizure effect was accompanied by increased seizure latency and decreased seizure duration. Additionally, female GEPR-3s were more responsive to JW-65's antiseizure effects than males. Moreover, JW-65 treatment for five consecutive days completely suppressed acoustically evoked seizures in male and female GEPR-3s. These findings suggest that inhibiting TRPC3 channels could be a promising antiseizure strategy targeting Ca2+ signaling mechanisms in inherited generalized tonic-clonic epilepsy.

Pathophysiological significance and modulation of the transient receptor potential canonical 3 ion channel

Transient receptor potential canonical 3 (TRPC3) protein belongs to the TRP family of nonselective cation channels. Its activation occurs by signaling through a G protein-coupled receptor (GPCR) and a phospholipase C-dependent (PLC) pathway. Perturbations in the expression of TRPC3 are associated with a plethora of pathophysiological conditions responsible for disorders of the cardiovascular, immune, and central nervous systems. The recently solved cryo-EM structure of TRPC3 provides detailed inputs about the underlying mechanistic aspects of the channel, which in turn enables more efficient ways of designing small-molecule modulators. Pharmacologically targeting TRPC3 in animal models has demonstrated great efficacy in treating diseases including cancers, neurological disorders, and cardiovascular diseases. Despite extensive scientific evidence supporting some strong correlations between the expression and activity of TRPC3 and various pathophysiological conditions, therapeutic strategies based on its pharmacological modulations have not led to clinical trials. The development of small-molecule TRPC3 modulators with high safety, sufficient brain penetration, and acceptable drug-like profiles remains in progress. Determining the pathological mechanisms for TRPC3 involvement in human diseases and understanding the requirements for a drug-like TRPC3 modulator will be valuable in advancing small-molecule therapeutics to future clinical trials. In this review, we provide an overview of the origin and activation mechanism of TRPC3 channels, diseases associated with irregularities in their expression, and new development in small-molecule modulators as potential therapeutic interventions for treating TRPC3 channelopathies.

Modulation and Regulation of Canonical Transient Receptor Potential 3 (TRPC3) Channels

Canonical transient receptor potential 3 (TRPC3) channel is a non-selective cation permeable channel that plays an essential role in calcium signalling. TRPC3 is highly expressed in the brain and also found in endocrine tissues and smooth muscle cells. The channel is activated directly by binding of diacylglycerol downstream of G-protein coupled receptor activation. In addition, TRPC3 is regulated by endogenous factors including Ca2+ ions, other endogenous lipids, and interacting proteins. The molecular and structural mechanisms underlying activation and regulation of TRPC3 are incompletely understood. Recently, several high-resolution cryogenic electron microscopy structures of TRPC3 and the closely related channel TRPC6 have been resolved in different functional states and in the presence of modulators, coupled with mutagenesis studies and electrophysiological characterisation. Here, we review the recent literature which has advanced our understanding of the complex mechanisms underlying modulation of TRPC3 by both endogenous and exogenous factors. TRPC3 plays an important role in Ca2+ homeostasis and entry into cells throughout the body, and both pathological variants and downstream dysregulation of TRPC3 channels have been associated with a number of diseases. As such, TRPC3 may be a valuable therapeutic target, and understanding its regulatory mechanisms will aid future development of pharmacological modulators of the channel.

Pharmacologically targeting transient receptor potential channels for seizures and epilepsy: Emerging preclinical evidence of druggability

As one of the most prevalent and disabling brain disorders, epilepsy is characterized by spontaneous seizures that result from aberrant, excessive hyperactivity of a group of highly synchronized brain neurons. Remarkable progress in epilepsy research and treatment over the first two decades of this century led to a dramatical expansion in the third-generation antiseizure drugs (ASDs). However, there are still over 30% of patients suffering from seizures resistant to the current medications, and the broad unbearable adversative effects of ASDs significantly impair the quality of life in about 40% of individuals affected by the disease. Prevention of epilepsy in those who are at high risks is another major unmet medical need, given that up to 40% of epilepsy patients are believed to have acquired causes. Therefore, it is important to identify novel drug targets that can facilitate the discovery and development of new therapies engaging unprecedented mechanisms of action that might overcome these significant limitations. Also over the last two decades, calcium signaling has been increasingly recognized as a key contributory factor in epileptogenesis of many aspects. The intracellular calcium homeostasis involves a variety of calcium-permeable cation channels, the most important of which perhaps are the transient receptor potential (TRP) ion channels. This review focuses on recent exciting advances in understanding of TRP channels in preclinical models of seizure disorders. We also provide emerging insights into the molecular and cellular mechanisms of TRP channels-engaged epileptogenesis that might lead to new antiseizure therapies, epilepsy prevention and modification, and even a cure.

TRPC channels as emerging targets for seizure disorders

Epilepsy is characterized by seizures of diverse types that affect about 1-2% of the population worldwide. Current antiseizure medications are unsatisfactory, as they merely provide symptomatic relief, are ineffective in about one-third of patients, and cause unbearable adverse effects. Transient receptor potential canonical (TRPC) channels are a group of nonselective cation channels involved in many physiological functions. In this review, we provide an overview of recent preclinical studies using both genetic and pharmacological strategies that reveal these receptor-operated calcium-permeable channels may also play fundamental roles in many aspects of epileptic seizures. We also propose that TRPC channels represent appealing targets for epilepsy treatment, with a goal of helping to advance the discovery and development of new antiseizure and/or antiepileptogenic therapies.

Inhibition of TRPC3 channels by a novel pyrazole compound confers antiseizure effects

OBJECTIVE

As a key member of the transient receptor potential (TRP) superfamily, TRP canonical 3 (TRPC3) regulates calcium homeostasis and contributes to neuronal excitability. Ablation of TRPC3 lessens pilocarpine-induced seizures in mice, suggesting that TRPC3 inhibition might represent a novel antiseizure strategy. Among current TRPC3 inhibitors, pyrazole 3 (Pyr3) is most selective and potent. However, Pyr3 only provides limited benefits in pilocarpine-treated mice, likely due to its low metabolic stability and potential toxicity. We recently reported a modified pyrazole compound 20 (or JW-65) that has improved stability and safety. The objective of this study was to explore the effects of TRPC3 inhibition by our current lead compound JW-65 on seizure susceptibility.

METHODS

We first examined the pharmacokinetic properties including plasma half-life and brain to plasma ratio of JW-65 after systemic administration in mice. We then investigated the effects of TRPC3 inhibition by JW-65 on behavioral and electrographic seizures in mice treated with pilocarpine. To ensure our findings are not model specific, we assessed the susceptibility of JW-65-treated mice to pentylenetetrazole (PTZ)-induced seizures with phenytoin as a comparator.

RESULTS

JW-65 showed adequate half-life and brain penetration in mice, justifying its use for central nervous system conditions. Systemic treatment with JW-65 before pilocarpine injection in mice markedly impaired the initiation of behavioral seizures. This antiseizure action was recapitulated when JW-65 was administered after pilocarpine-induced behavioral seizures were well established and was confirmed by time-locked electroencephalographic monitoring and synchronized video. Moreover, JW-65-treated mice showed substantially decreased susceptibility to PTZ-induced seizures in a dose-dependent manner.

SIGNIFICANCE

These results suggest that pharmacological inhibition of the TRPC3 channels by our novel compound JW-65 might represent a new antiseizure strategy engaging a previously undrugged mechanism of action. Hence, this proof-of-concept study establishes TRPC3 as a novel feasible therapeutic target for the treatment of some forms of epilepsy.