Isoxazole derivatives as potent transient receptor potential melastatin type 8 (TRPM8) agonists

TRPM8 and TRPA1 ideal targets for treating cold-induced pain

TRP channels are essential for detecting variations in external temperature and are ubiquitously expressed in both the peripheral and central nervous systems as integral channel proteins. They primarily mediate a range of sensory responses, including thermal sensations, nociception, mechanosensation, vision, and gustation, thus playing a critical role in regulating various physiological functions. In colder climates, individuals often experience pain associated with low temperatures, leading to significant discomfort. Within the TRP channel family, TRPM8 and TRPA1 ion channels serve as the primary sensors for cold temperature fluctuations and are integral to both cold nociception and neuropathic pain pathways. Recent advancements in the biosynthesis of inhibitors targeting TRPM8 and TRPA1 have prompted the need for a comprehensive review of their structural characteristics, biological activities, biosynthetic pathways, and chemical synthesis. This paper aims to delineate the distinct roles of TRPM8 and TRPA1 in pain perception, elucidate their respective protein structures, and compile various combinations of TRPM8 and TRPA1 antagonists and agonists. The discussion encompasses their chemical structures, structure-activity relationships (SARs), biological activities, selectivity, and therapeutic potential, with a particular focus on the conformational relationships between antagonists and the channels. This review seeks to provide in-depth insights into pharmacological strategies for managing pain associated with TRPM8 and TRPA1 activation and will pave the way for future investigations into pharmacotherapeutic approaches for alleviating cold-induced pain.

Implications of TRPM3 and TRPM8 for sensory neuron sensitisation

Sensory neurons serve to receive and transmit a wide range of information about the conditions of the world around us as well as the external and internal state of our body. Sensitisation of these nerve cells, i.e. becoming more sensitive to stimuli or the emergence or intensification of spontaneous activity, for example in the context of inflammation or nerve injury, can lead to chronic diseases such as neuropathic pain. For many of these disorders there are only very limited treatment options and in order to find and establish new therapeutic approaches, research into the exact causes of sensitisation with the elucidation of the underlying mechanisms and the identification of the molecular components is therefore essential. These components include plasma membrane receptors and ion channels that are involved in signal reception and transmission. Members of the transient receptor potential (TRP) channel family are also expressed in sensory neurons and some of them play a crucial role in temperature perception. This review article focuses on the heat-sensitive TRPM3 and the cold-sensitive TRPM8 (and TRPA1) channels and their importance in sensitisation of dorsal root ganglion sensory neurons is discussed based on studies related to inflammation and injury- as well as chemotherapy-induced neuropathy.

Therapeutic potential of TRPM8 antagonists in prostate cancer

Transient receptor potential melastatin-8 (TRPM8) represents an emerging target in prostate cancer, although its mechanism of action remains unclear. Here, we have characterized and investigated the effects of TRPM8 modulators in prostate cancer aggressiveness disclosing the molecular mechanism underlying their biological activity. Patch-clamp and calcium fluorometric assays were used to characterize the synthesized compounds. Androgen-stimulated prostate cancer-derived cells were challenged with the compounds and the DNA synthesis was investigated in a preliminary screening. The most effective compounds were then employed to inhibit the pro-metastatic behavior of in various PC-derived cells, at different degree of malignancy. The effect of the compounds was then assayed in prostate cancer cell-derived 3D model and the molecular targets of selected compounds were lastly identified using transcriptional and non-transcriptional reporter assays. TRPM8 antagonists inhibit the androgen-dependent prostate cancer cell proliferation, migration and invasiveness. They are highly effective in reverting the androgen-induced increase in prostate cancer cell spheroid size. The compounds also revert the proliferation of castrate-resistant prostate cancer cells, provided they express the androgen receptor. In contrast, no effects were recorded in prostate cancer cells devoid of the receptor. Selected antagonists interfere in non-genomic androgen action and abolish the androgen-induced androgen receptor/TRPM8 complex assembly as well as the increase in intracellular calcium levels in prostate cancer cells. Our results shed light in the processes controlling prostate cancer progression and make the transient receptor potential melastatin-8 as a ‘druggable’ target in the androgen receptor-expressing prostate cancers.

Exploration of TRPM8 Binding Sites by β-Carboline-Based Antagonists and Their In Vitro Characterization and In Vivo Analgesic Activities

Transient receptor potential melastatin 8 (TRPM8) ion channel represents a valuable pharmacological option for several therapeutic areas. Here, a series of conformationally restricted derivatives of the previously described TRPM8 antagonist N,N′-dibenzyl tryptophan 4 were prepared and characterized in vitro by Ca²⁺-imaging and patch-clamp electrophysiology assays. Molecular modeling studies led to identification of a broad and well-defined interaction network of these derivatives inside the TRPM8 binding site, underlying their antagonist activity. The (5R,11aS)-5-(4-chlorophenyl)-2-(4-fluorobenzyl)-5,6,11,11a-tetrahydro-1H-imidazo[1′,5′:1,6]pyrido[3,4-b]indole-1,3(2H)-dione (31a) emerged as a potent (IC₅₀ = 4.10 ± 1.2 nM), selective, and metabolically stable TRPM8 antagonist. In vivo, 31a showed significant target coverage in an icilin-induced WDS (at 11.5 mg/kg ip), an oxaliplatin-induced cold allodynia (at 10–30 μg sc), and CCI-induced thermal hyperalgesia (at 11.5 mg/kg ip) mice models. These results confirm the tryptophan moiety as a solid pharmacophore template for the design of highly potent modulators of TRPM8-mediated activities.

Characterization of New TRPM8 Modulators in Pain Perception

BACKGROUND

Transient Receptor Potential Melastatin-8 (TRPM8) is a non-selective cation channel activated by cold temperature and by cooling agents. Several studies have proved that this channel is involved in pain perception. Although some studies indicate that TRPM8 inhibition is necessary to reduce acute and chronic pain, it is also reported that TRPM8 activation produces analgesia. These conflicting results could be explained by extracellular Ca²⁺-dependent desensitization that is induced by an excessive activation. Likely, this effect is due to phosphatidylinositol 4,5-bisphosphate (PIP2) depletion that leads to modification of TRPM8 channel activity, shifting voltage dependence towards more positive potentials. This phenomenon needs further evaluation and confirmation that would allow us to understand better the role of this channel and to develop new therapeutic strategies for controlling pain.

EXPERIMENTAL APPROACH

To understand the role of TRPM8 in pain perception, we tested two specific TRPM8-modulating compounds, an antagonist (IGM-18) and an agonist (IGM-5), in either acute or chronic animal pain models using male Sprague-Dawley rats or CD1 mice, after systemic or topical routes of administration.

RESULTS

IGM-18 and IGM-5 were fully characterized in vivo. The wet-dog shake test and the body temperature measurements highlighted the antagonist activity of IGM-18 on TRPM8 channels. Moreover, IGM-18 exerted an analgesic effect on formalin-induced orofacial pain and chronic constriction injury-induced neuropathic pain, demonstrating the involvement of TRPM8 channels in these two pain models. Finally, the results were consistent with TRPM8 downregulation by agonist IGM-5, due to its excessive activation.

CONCLUSIONS

TRPM8 channels are strongly involved in pain modulation, and their selective antagonist is able to reduce both acute and chronic pain.

Thieno[2,3-d]pyrimidine as a promising scaffold in medicinal chemistry: Recent advances

Thienopyrimidine scaffold is a fused heterocyclic ring system that structurally can be considered as adenine, the purine base that is found in both DNA and RNA-bioisosteres. Thienopyrimidines exist in three distinct isomeric forms. The current review discusses thieno[2,3-d]pyrimidine as a one of the opulent heterocycles in drug discovery. Its broad range of medical applications such as anticancer, anti-inflammatory, anti-microbial, and CNS protective agents has inspired us to study its structure-activity relationship (SAR), along with its relevant synthetic strategies. The present review briefly summarizes synthetic approaches for the preparation of thieno[2,3-d]pyrimidine derivatives. In addition, the promising biological activities of this scaffold are also illustrated with explanatory diagrams for their SAR studies.

The synthetic and therapeutic expedition of isoxazole and its analogs

Isoxazole, constituting an important family of five-membered heterocycles with one oxygen atom and one nitrogen atom at adjacent positions is of immense importance because of its wide spectrum of biological activities and therapeutic potential. It is, therefore, of prime importance that the development of new synthetic strategies and designing of new isoxazole derivatives should be based on the most recent knowledge emerging from the latest research. This review is an endeavor to highlight the progress in the chemistry and biological activity of isoxazole derivatives which could provide a low-height flying bird's eye view of isoxazole derivatives to the medicinal chemists for the development of clinically viable drugs using this information.

Ultrasound-assisted facile one-pot sequential synthesis of novel sulfonamide-isoxazoles using cerium (IV) ammonium nitrate (CAN) as an efficient oxidant in aqueous medium

A series of novel 3,5-disubstituted isoxazoles have been synthesized, using a new, green, and versatile "one-pot three-steps" methodology. The key step is an oxidative 1,3-dipolar cycloaddition under ultrasonic irradiation, occurring in aqueous media, and mediated by cerium (IV) ammonium nitrate (CAN). CAN is a one-electron oxidant, highly soluble in water, slightly toxic and inexpensive, that allows the in situ conversion of the intermediate aldoximes into nitrile oxide. The syntheses are highly regioselective, as illustrated by the structures of the final compounds, which have been fully assessed by spectral analyses (¹H and ¹³C NMR, MS). This study illustrates the potency of the ultrasound activation to synthesize a set of highly functionalized heterocycles, with potential applications in biology, in short reaction times and following an eco-friendly process.

Novel selective, potent naphthyl TRPM8 antagonists identified through a combined ligand- and structure-based virtual screening approach

Transient receptor potential melastatin 8 (TRPM8), a nonselective cation channel, is the predominant mammalian cold temperature thermosensor and it is activated by cold temperatures and cooling compounds, such as menthol and icilin. Because of its role in cold allodynia, cold hyperalgesia and painful syndromes TRPM8 antagonists are currently being pursued as potential therapeutic agents for the treatment of pain hypersensitivity. Recently TRPM8 has been found in subsets of bladder sensory nerve fibres, providing an opportunity to understand and treat chronic hypersensitivity. However, most of the known TRPM8 inhibitors lack selectivity, and only three selective compounds have reached clinical trials to date. Here, we applied two virtual screening strategies to find new, clinics suitable, TRPM8 inhibitors. This strategy enabled us to identify naphthyl derivatives as a novel class of potent and selective TRPM8 inhibitors. Further characterization of the pharmacologic properties of the most potent compound identified, compound 1, confirmed that it is a selective, competitive antagonist inhibitor of TRPM8. Compound 1 also proved itself active in a overreactive bladder model in vivo. Thus, the novel naphthyl derivative compound identified here could be optimized for clinical treatment of pain hypersensitivity in bladder disorders but also in different other pathologies.

Synthesis, high-throughput screening and pharmacological characterization of β-lactam derivatives as TRPM8 antagonists

The mammalian transient receptor potential melastatin channel 8 (TRPM8), highly expressed in trigeminal and dorsal root ganglia, mediates the cooling sensation and plays an important role in the cold hypersensitivity characteristic of some types of neuropathic pain, as well as in cancer. Consequently, the identification of selective and potent ligands for TRPM8 is of great interest. Here, a series of compounds, having a β-lactam central scaffold, were prepared to explore the pharmacophore requirements for TRPM8 modulation. Structure-activity studies indicate that the minimal requirements for potent β-lactam-based TRPM8 blockers are hydrophobic groups (benzyl preferentially or ^tBu) on R¹, R², R³ and R⁵ and a short N-alkyl chain (≤3 carbons). The best compounds in the focused library (41 and 45) showed IC₅₀ values of 46 nM and 83 nM, respectively, in electrophysiology assays. These compounds selectively blocked all modalities of TRPM8 activation, i.e. menthol, voltage, and temperature. Molecular modelling studies using a homology model of TRPM8 identified two putative binding sites, involving networks of hydrophobic interactions, and suggesting a negative allosteric modulation through the stabilization of the closed state. Thus, these β-lactams provide a novel pharmacophore scaffold to evolve TRPM8 allosteric modulators to treat TRPM8 channel dysfunction.

Isoxazole ring as a useful scaffold in a search for new therapeutic agents

Due to its relatively easy synthesis, isoxazole ring has been as an object of interest for chemists and pharmacologists from research groups all over the world. Its chemical modifications include both connection of isoxazole with other aromatic, heteroaromatic or non aromatic rings and substitution with different alkyl groups. Thanks to their usually low cytotoxicity, isoxazole derivatives are still popular scaffolds for the development of new agents with variable biological activities, such as antimicrobial, antiviral, anticancer, anti-inflammatory, immunomodulatory, anticonvulsant or anti-diabetic properties. This review discusses the chemical structure of recently developed isoxazole derivatives with regards to their activity and potential therapeutic use.

Transient receptor potential melastatin 8 ion channel in macrophages modulates colitis through a balance-shift in TNF-alpha and interleukin-10 production

The transient receptor potential (TRP) ion channel family is well characterized in sensory neurons; however, little is known about its role in the immune system. Here we show that the cold-sensing TRPM8 has an unexpected role in innate immunity. TRPM8 expression and function in macrophages were demonstrated in vitro using molecular techniques and calcium imaging. In addition, adoptive macrophage transfer and systemic interleukin (IL)-10 overexpression were performed in experimental colitis. TRPM8 activation induced calcium-transients in murine peritoneal macrophages (PM) and bone marrow-derived macrophages of wild-type (WT) but not TRPM8-deficient mice. TRPM8-deficient PM exhibited defective phagocytosis and increased motility compared with those in WT, whereas the opposite effects of TRPM8 activation were induced in WT PM. TRPM8 activation or blockage/genetic deletion induced a anti- or pro-inflammatory macrophage cytokine profile, respectively. WT mice treated with repeated menthol (TRPM8 agonist) enemas were consistently protected from experimental colitis, whereas TRPM8-deficient mice showed increased colitis susceptibility. Adoptive transfer of TRPM8-deficient macrophages aggravated colitis, whereas systemic IL-10 overexpression rescued this phenotype. TRPM8 activation in peptidergic sensory neurons did not affect neuropeptide release from the inflamed colon. TRPM8 in macrophages determines pro- or anti-inflammatory actions by regulating tumor necrosis factor-α and interleukin-10 production. These findings suggest novel TRPM8-based options for immunomodulatory intervention.

Transient Receptor Potential Melastatin 8 Channel (TRPM8) Modulation: Cool Entryway for Treating Pain and Cancer

TRPM8 ion channels, the primary cold sensors in humans, are activated by innocuous cooling (<28 °C) and cooling compounds (menthol, icilin) and are implicated in sensing unpleasant cold stimuli as well as in mammalian thermoregulation. Overexpression of these thermoregulators in prostate cancer and in other life-threatening tumors, along with their contribution to an increasing number of pathological conditions, opens a plethora of medicinal chemistry opportunities to develop receptor modulators. This Perspective seeks to describe current known modulators for this ion channel because both agonists and antagonists may be useful for the treatment of most TRPM8-mediated pathologies. We primarily focus on SAR data for the different families of compounds and the pharmacological properties of the most promising ligands. Furthermore, we also address the knowledge about the channel structure, although still in its infancy, and the role of the TRPM8 protein signalplex to channel function and dysfunction. We finally outline the potential future prospects of the challenging TRPM8 drug discovery field.

Tryptamine-Based Derivatives as Transient Receptor Potential Melastatin Type 8 (TRPM8) Channel Modulators

Pharmacological modulation of the transient receptor potential melastatin type 8 (TRPM8) is currently under investigation as a new approach for the treatment of pain and other diseases. In this study, a series of N-substituted tryptamines was prepared to explore the structural requirements determining TRPM8 modulation. Using a fluorescence-based screening assay, we identified two compounds acting as an activator (2-(1H-indol-3-yl)-N-(4-phenoxybenzyl)ethanamine, 21) or an inhibitor (N,N-dibenzyl-2-(1H-indol-3-yl)ethanamine, 12) of calcium influx in HEK293 cells. In patch-clamp recordings, compound 21 displayed a significantly higher potency (EC50 = 40 ± 4 μM) and a similar efficacy when compared to menthol; by contrast, compound 12 produced a concentration-dependent inhibition of menthol-induced TRPM8 currents (IC50 = 367 ± 24 nM). Molecular modeling studies using a homology model of a single rat TRPM8 subunit identified a putative binding site located between the VSD and the TRP box, disclosing differences in the binding modes for the agonist and the antagonist.

Structure-activity relationships of the prototypical TRPM8 agonist icilin

A series of structural analogues of the TRPM8 agonist icilin was prepared. The compounds were examined for their ability to exert agonist or antagonist effects in HEK-293 cells expressing the TRPM8 receptor. Most structural modifications of the icilin structure largely met with diminished TRPM8 agonist activity. Cinnamamide 'open-chain' analogs of icilin, however, demonstrated significant antagonistic actions at the TRPM8 receptor. Optimal potency (IC50=73 nM) was observed in the 3-iodo derivative 18l.

Functional and biochemical interaction between PPARα receptors and TRPV1 channels: Potential role in PPARα agonists-mediated analgesia

Transient receptor potential vanilloid type-1 (TRPV1) channels expressed in primary afferent neurons play a critical role in nociception triggered by endogenous and exogenous compounds. In the present study, the functional and biochemical interaction between TRPV1 channels and type-α peroxisome proliferator-activated receptors (PPARα) has been investigated. In TRPV1-expressing CHO cells, patch-clamp studies revealed that acute application of the PPARα agonists clofibrate (CLO; 0.1-100 μM), WY14643 (1-300 μM), or GW7647 (0.1-100 nM) activated TRPV1 currents in a concentration-dependent manner, with EC50s of 5.3 ± 0.8 μM, 13.0 ± 1.2 μM, and 12.7 ± 0.3 nM, respectively. The role of PPARα in these pharmacological responses was confirmed by the ability of the PPARα antagonist GW6471 (10 μM) to block CLO-, WY14643- and GW7647-induced TRPV1 activation, and by the observation that modulation of PPARα levels via siRNA-mediated suppression or PPARα over-expression affected TRPV1 channel activation by PPARα agonists accordingly. In cells cotransfected with PPARα and TRPV1, PPARα receptors were detected in TRPV1-immunoprecipitated fractions. When compared to capsaicin (CAP), TRPV1 currents activated by PPARα agonists showed a higher degree of acute desensitization and tachyphylaxis; moreover, GW7647, when pre-incubated at a concentration (1nM) unable to activate TRPV1 currents per se, desensitized CAP-induced TRPV1 currents. Finally, a sub-effective concentration of each PPARα agonist inhibited TRPV1-dependent bradykinin-induced [Ca(2+)]i transients in sensory neurons. Collectively, these results provide evidence for a PPARα-mediated pathway triggering TRPV1 channel activation and desensitization, and highlight a novel mechanism which might contribute to the analgesic effects shown by PPARα agonists in vivo.