Review of Transient Receptor Potential Canonical (TRPC5) Channel

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A review of transient receptor potential channel (TRPC) modulators and diseases Swagat Sharma, and Corey R. Hopkins J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.8b01954 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Journal of Medicinal Chemistry

A review of transient receptor potential channel (TRPC) modulators and diseases Swagat Sharma and Corey R. Hopkins* Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, NE USA 68198-6125 Keywords: transient receptor potential canonical, TRPC, calcium, selective modulators ABSTRACT: Transient receptor potential canonical (TRPC) channels are highly homologous, non-selective cation channels that form many homo- and heterotetrameric channels. These channels are highly abundant in the brain and kidney and have been implicated in numerous diseases, such as depression, addiction, and chronic kidney disease, among others. Historically, there have been very few selective modulators of the TRPC family in order to fully understand their role in disease despite their physiological significance. However, that has changed recently and there has been a significant increase in interest in this family of channels which has led to the emergence of selective tool compounds, and even preclinical drug candidates, over the past few years. This review will cover these new advancements in the discovery of TRPC modulators and the emergence of newly reported structural information which will undoubtedly lead to even greater advancements. 1.

INTRODUCTION Transient receptor potential (TRP) channels act as molecular sensors of extra- and intracellular

environmental variations and preserve homeostasis.1 Throughout the animal kingdom TRPs are

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found engaged in various roles across multiple locations. Mammalian TRP channels are classified into six subfamilies, TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPP (polycystin), and TRPML (mucoliptin), based on their sequence similarities.2 TRPC are most closely homologous to the Drosophila TRP, which was the first channel identified, and responsible for phototransduction.3 Mammalian TRPC are Ca2+ permeable non-selective cation channels, which are activated by downstream stimulation of phospholipase C (PLC) pathways via G protein-coupled receptors (GPCRs).4 Mammalian TRPC channels are a collection of seven different proteins, which are subdivided into four subgroups: TRPC1, TRPC4/5, TRPC3,6,7 and TRPC2 based on sequence similarity. TRPC3, 6 and 7 share ~70-80% amino acid identity as well as functional and pharmacological similarities. TRPC4 and 5 share a similarly close homology as well as functional similarities (Figure 1). The individual channels function as either homo- or heterotetramers consisting of members of TRPC family, other TRP channels, and non-TRP proteins. TRPC2/3/6/7 can be directly activated by diacylglycerols (DAG), its synthetic derivatives and/or inositol 1,4,5-triphosphate receptors.5 Whereas primary activation of TRPC4/5 is achieved due to internal and external Ca+2 levels.6 TRPC4/5 are activated through Gi/o signaling in addition to the Gq/11-PLC pathway.7 TRPC channels are referred to as receptor-operated channels, since PLC activation is primarily achieved by the stimulation of GPCRs or receptor tyrosine kinases,.4


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Figure 1. Phylogenetic of the human TRPC family. The TRPC channels can be activated through the phospholipase C pathway or, in some instances, by depletion of intracellular Ca2+ stores, depending on cell type, expression level or environment. The TRPC3/6/7 subgroup can be activated by synthetic diacylglycerols (DAG) such as oleyl acetyl glycerol (OAG) (Figure 2).8 The PLC mechanism breaks phosphatidylinositol-4,5biphosphate (PIP2) into IP3 and DAG. IP3 is responsible for Ca2+ release from the intracellular store. Ca2+ signals control diverse cellular process, stretching from gene expression to tissue specific responses (e.g., lymphocyte activation).9 Ca2+ TRPC3/6/7

GPCR

Plasmamebrane

Cytoplasm  



DAG

PIP2 PLC IP3

Ca2+

Endoplasmic reticulumn

Figure 2. Activation of the TRPC3/6/7 subgroup. 2.

TRPC and Disease. The TRPC1/4/5 channels have been identified in the potential treatment of diseases of the

central nervous system (anxiety, depression, epilepsy, memory, addiction) and pain. TRPC410 and 511 are expressed in the brain regions associated with anxiety and fear and knockout mice (Trpc4/-

and Trpc5-/-) show decreased fear behavior compared to wild-type mice. In addition, knock-out

mice showed increased exploratory tendencies in both open field test10 and the elevated plus maze.11 Further studies have shown that TRPC4/5 antagonists are active in the tail suspension test



and forced swim test, both preclinical animal models of depression.12 TRPC5 channels have been associated with epilepsy as they are expressed in rat hippocampal CA1 neurons.13 In addition, human patients with focal cortical dysplasia show an upregulation of both TRPC1 and C4 proteins.14,15 TRPC channels (TRPC4 and 5) have been implicated in addiction; however, the direct role has yet to be determined.16 The role of TRPC4/5 have been investigated for roles in different types of pain. Trpc4-/- mice have been shown to have resistance to mustard-oil induced pain and also show an increase in pain thresholds compared to wild-type animals.17 Studies using the TRPC4/5 antagonist, ML204, have also shown positive results (similar to KO animal) in models of neuropathic pain.18 The role of TRPC5 in chronic kidney disease (CKD) and focal segmental glomerulosclerosis (FSGS) has been shown recently. TRPC5 channels are expressed in the kidney podocytes, cells that form the kidney filter, and inhibition of TRPC5 was thought to provide protection for the podocytes in disease.19,20 Hopkins and Greka et al. have shown that use of TRPC5 inhibitors offers protection of the podocytes in animals with advanced kidney disease.21 Both the TRPC4/5 inhibitor ML20420, and the recently disclosed selective TRPC5 inhibitor, AC1903 have shown to prevent podocyte death and decreased the proteinuria caused by kidney damage.21 In addition to kidney disease, TRPC4/5 have been implicated in cancer22 and cardiovascular disease.23

3.

Modulators of TRP channels

3.1. TRPC 1 TRPC1 is a multifunctional protein and is expressed as a homotetramer calcium release channel in the endoplasmic reticulum (ER), and as a heterotetramer with TRPC 4 and 5 to form Ca2+ permeable non-selective cation channels in the plasma membrane.24,25 In the endoplasmic


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reticulum, it forms ternary complexes with STIM1 and Orai1 for regulation of store-operated calcium entry process (SOCE).26 In particular, TRPC1 has been purported to be responsible for SOCE in salivary gland cells, A549 cells, pulmonary artery cells, Chinese hamster ovary (CHO) cells as well as vascular smooth muscle cells.27 Due to cell cycle arrest in the G1 phase, RNAi-mediated genetic ablation of KCa3.1 and/or TRPC1 leads to a significant decrease in cell proliferation.28 Heteromeric TRPC1/4/5 channels are found in mouse brain, predominantly in the hippocampus. A mouse model of Trpc1/4/5-/- showed significant reduction of action potential-triggered excitatory postsynaptic currents (EPSCs) and loss of working memory and relearning competence.29 (–)-Englerin A (EA,1) a natural product obtained from Phyllanthus engleri, was found to have cytotoxic effects against many types of cancer cells (Figure 3).30 Recently many of the TRPC channels have been implicated for the treatment of cancer. Amongst the TRPC1/4/5 cluster, TRPC1 inhibition is believed to be responsible for the death of A498 and Hs578T triple negative breast cancer cells evoked by 1,31 which is a potent, non-selective agonist of TRPC1/4/5 channels.32 The cytotoxic effect of 1 in human synovial sarcoma cells is attributed to Na+ entry mediated by heteromeric TRPC1/4 channels.33 Excessive influx of Ca+2 can also cause similar cytotoxicity.34 Efficient chemical synthesis of 1 has been achieved, as well as analog synthesis in order to better understand its mechanism of activation.35 One of the analogs thus obtained from derivatization, A54, 2, showed an unusual reversal in activity. Instead of agonist activities similar to the parent compound, 2 was shown to antagonize the current produced by 1. This antagonism is curious as it inhibits 1 evoked Ca2+ entry in heteromeric TRPC1/4 channels with an IC50 = 62 nM. In a similar manner, 2 inhibits 1 evoked Ca2+ entry in homomeric TRPC5, but not in homomeric TRPC4 channels. In addition, in TRPC5 2 inhibits 1 mediated activity but potentiates



Gd3+ activation of the channel. These results show that minor modifications of the side chain can lead to drastic changes in activity. One of the reasons given for the activity switch is the change in conformation of the ion channels upon binding of activators. Compound 1 induced change might open up a secondary pocket where 1 and 2 can bind, and this secondary pocket might be an antagonist binding site.36

Figure 3. Structure of englerin A, 1, and its analog A54, 2.

3.2

TRPC 4 TRPC4 occurs in homomeric and heteromeric forms and is found throughout various

locations in the body and mediates various physiological functions, thus making it an interesting target for various disorders. In most of the functional channels identified, TRPC4 forms heterotetramers with TRPC5 and TRPC1. But, unlike TRPC1, homotetramer of TRPC4 are functionally active in vitro. Recently, a cryo-electron microscopy (cryo-EM) structure of TRPC4 in its unliganded (apo) state has been reported (vida infra).37 TRPC4 is highly expressed in brain,38 as well as in peripheral sensory neurons.39 TRPC4 is hypothesized to play a role in the functional neurobiology of the enteric nervous system, which includes calcium homeostasis, membrane excitability, synaptic transmission, axon guidance and sensory functions.17 Pharmacological blockade of the TRPC4/5 channel in the amygdala attenuates sensory- and affective-like pain in


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neuropathy.18 Knockdown of both TRPC4 and TRPC5 inhibited tube formation in an endothelial cell line.22 TRPC4 is also regulated in hypoxia and intravitreal injection of TRPC4 short interfering RNA (siRNA) reduced VEGF-induced retinal neovascularization in oxygen-induced retinopathy.40 In addition, both TRPC4 and TRPC5 have been shown to be involved in anxiety-like behavior in a mouse model (fear conditioning test). Genetic removal of TRPC4 or TRPC5 reduces anxiolytic tendencies in mice.10 In addition, these channels are involved in various physiological and pathophysiological processes, including but not limited to: vascular smooth muscle, endothelial function, adiponectin regulation, and oxidative stress.41,42 However, due to the dearth of selective modulators, the true potential TRPC channels have remained unexplored. In most of the cases, genetic ablation studies have validated the physiological role of this ion channel. In order to find potent and selective modulators of TRPC4, a high-throughput screen (HTS) of 305,000 compounds of the Molecular Libraries Small Molecule Repository (MLSMR) was carried out, which is available to the Molecular Libraries Probe Production Centers Network (MLPCN). For screening, TRPC4 channels were activated using µopioid receptor agonist DAMGO. The hits from the HTS were remade and their SAR was explored to obtain one of the first selective molecular probe for TRPC channel, ML204, 3 (Figure 4). Compound 3 blocked TRPC4 channels in both fluorescent and electrophysiological assays and in the presence of different activators, confirming it to be a channel blocker and not affecting channel activation mechanism. Compound 3 was moderately potent with an IC50 = 0.96 µM when TRPC4β channels are activated using µ-opioid receptor agonist DAMGO and 2.6 µM in electrophysiological assay. It exhibits 19-fold selectivity against TRPC6 (IC50 =17.9 µM). However, it was shown that 3 also inhibits TRPC5, activated through co-stimulation of Gi/o and



Gq/11 signaling by µ-opioid and M3-like muscarinic receptors due to very close similarity between TRPC4 and C5. A larger selectivity screen using EuroFins Lead profiling screen consisting of binding assay panel of various GPCRs, ion channels, and transporters was performed and 3, at a concentration of 10 µM, was found to not significantly interact with 61 of the 68 assays conducted. SAR study around 3 led to compounds with better potency; however, these compounds were not selective against TRPC6.43,44 As with many small molecular probes, 3 was later found to inhibit acetylcholinesterase (AChE) with an IC50 = 0.84 µM.45 Compound 3 is still used to validate many findings and genetic knockout results. Its first of its kind selectivity towards TRPC4/5 made it a valuable probe to establish the physiological role of TRPC channels.

Figure 4. Structure of ML204, 3, and the SAR observations. The role of TRPC4 in sensory function such as visceral pain was established using knockout mice in a mustard oil induced pain like behavioral model. TRPC4 antagonist 3 showed dose dependent relief in pain, without any adverse cardiovascular or other side effects. This established TRPC4 as a novel target for pain.17 Microinjection of 3 at doses 5 and 10 µg into the right amygdala of neuropathic animals produced a dose-related mechanical antihypersensitivity effect in the injured limb.18 In addition to 3 from the HTS of the MLSMR library, another lead M084, 4, was identified as a TRPC4/5 inhibitor (Table 1). Although 4 was less potent than 3, it had better pharmacokinetic


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properties and provided an alternate structural class. TRPC4 channels stably co-expressed with μ receptors in a Ca2+ assay gave an IC50 = 3.7 ± 0.5 μM when DAMGO was used at 0.1 μM, Similarly, in cells that co-expressed TRPC4 with 5-HT1A receptors, the fluorescence increase evoked by 5-HT (1 μM) was inhibited by preincubation with 4, and it also inhibits catechol-evoked (CCh, 1 μM) membrane depolarization in cells that co-expressed TRPC1, TRPC4, and M2 receptors in a concentration-dependent manner with an estimated IC50 of 8.3 ± 1.7 μM. This confirms the inhibition of 4 on TRPC1/C4 heteromeric channels. Overall IC50 values for TRPC4 and TRPC5 are 8.2 μM and 10.3 μM, respectively, as determined by the FMP assay using DAMGO to stimulate Gi/o via the co-expressed μ receptor. Interestingly, 4 was not active on TRPC3, with an IC50 of ∼50 μM. SAR studies around 4 reveals position 2-amino group of the benzimidazole is required for activity. The amine group can be either primary or secondary in the form of a ring structure (e.g., piperidine or pyrrolidine). Any other changes at position 2 such as aromatic group, extra amine, alcohol or ether are not tolerated. Out of the SAR studies 10 comes out to be the most potent compound (IC50 = 4.3 μM) and had excellent selectivity towards TRPC6 and C3. (Table 1)46 Table 1. SAR studies around M084, 4.

Cmpd

Structure

TRPC4

TRPC5 TRPC6 IC50 (M)

TRPC3

N NH

M084, 4

N H Cl

N

5

N N H

10.3

8.2

59.6

48.6

4.1

3.1

57.1

30.4

5.2

6.6

63.8

19.3

11.0

11.1

>100

>100

N

6 7

N N H N

N N H



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N N

8

N H N

9

H N

N H

10

N N N H

5.5

8.0

>100

>100

10.2

12.4

>100

>100

4.3

3.5

>100

>100

Intraperitoneal (IP) administration of 4 at 10 mg/kg to C57BL/6 male mice reduces the immobility time in forced swim test and tail suspension test (TST) after administration within as short as 2 hours. In a model of chronic unpredictable stress (CUS), treatment with 4 reversed the enhanced immobility time in forced swim test and decreased the latency to feed in novelty suppressed feeding test. These results show 4 to have antidepressant and anxiolytic-like effects similar to the known antidepressant, amitriptyline. Acute treatment with 4 at 10, 20 and 40 mg/kg, reduces the immobility time in TST. 4 is brain penetrant after a single dose (IP, 10 mg/kg, 2 h), with the brain concentration of 4 (2256 ± 384 ng/mL, 1105 ± 65 ng/g, and 335 ± 55 ng/mL in serum, brain, and CSF, respectively).47 Acute treatment of 4 led to a rise in the level of brainderived neurotrophic factor (BDNF) and c-fos in peripheral cortex of normal mice, a similar phenomenon is observed with chronic treatment of other anti-depressants.48 But, in a similar fashion to 3, many of the analogs of 4, having 2-cyclicamines also inhibit butyrylcholinesterase at low micromolar concentrations.49 Hydra Biosciences has performed a high-throughput screen of ~65,000 compounds and subsequent modifications of the lead molecule have identified various low nanomolar, and in some cases, picomolar inhibitors of TRPC4 and 5 homo- and heteroteramers. Based on the results, >600 xanthine analogs have been synthesized and characterized for their inhibitory activity on TRPC4/5.50 HC-070, 11 and HC-608, 12 were among the best compounds out of the study (Figure


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5). 12 has been reported earlier as Pico-145 due to its picomolar range activity in some assays. 12 has an IC50 of 1.3 nM against tetracycline inducible exogenous human TRPC5, IC50 of 0.34 nM against TRPC4, 0.19 nM against TRPC5-1 and 0.033 nM against TRPC4-1. In a sphingosine 1phosphate (SIP) based activation of TRPC4, IC50 values at -100 and +100 mV were 0.012 nM and 0.030 nM, respectively. Surprisingly, 12 showed unusual behavior in channels activated by Gd3+, where at low concentrations it behaved as an agonist and at higher concentrations (100 nM) it showed inhibitory activity. Gd3+ did not activate TRPC4-1 channel. Apart from TRPC1/4/5, it did not modulate any other channels at micromolar concentrations.51 11 inhibited recombinantlyexpressed human TRPC5 (hTRPC5) with an IC50 = 9.3 nM and TRPC4 with an IC50 = 46.0 nM. In whole-cell manual patch clamp, 11 inhibited lanthanum-activated hTRPC5-mediated currents with an IC50 = 0.52 nM. Increased potency in patch clamp electrophysiology experiment compared to fluorometric assays has often been observed and reported for other TRP channels.52

Figure 5. Structures of Hydra Biosciences TRPC4/5 antagonists. The elimination half-life (T1/2) of 11 was 3.5 h, volume of distribution at steady state (Vss) for 11 was 1.5 L/Kg and clearance of 13 mL/min/kg in mice. The compound is brain penetrant, with a concentration 299 nmol/kg in plasma and 339 nmol/L in brain (1 mg/kg, IV dosing). Plasma protein binding (PPB) of 11 is very high (%Fu = 0.4); however, they were still able to measure the



free fraction in the brain, which was sufficient considering the dosing regimen. IP administration of 11 starting at 0.3 mg/kg in a mouse model reduces the immobility time in forced swim test and tail suspension test (TST) after administration within as short as 60 min.12 In addition to the xanthine scaffold, the Hydra group has reported in the patent literature a number of closely related structures (Figure 6). These patent applications encompass 6,6-systems such as the pyrido[3,4-d]pyrimidine-2,4(1H,3H)-dione, 13, system with compounds ranging in potency using the Fluo4 cell-based fluorescence assay 195 – 9,800 nM53, quinazoline-2,4(1H,3H)dione, 14, (20,000 nM)55 ring systems. Additionally, the group has reported 6,5-ring systems such as pyrrolo[3,2d]pyrimidine-2,4(3H)-dione, 16, (408 – 7,540 nM)56 and the thieno- and furo[2,3-d]pyrimidine2,4(1H,3H)dione, 17, (20

Another collaborative work from the University of Leeds and the Academy of Chinese Medical Sciences, led to the identification of natural and synthetic flavonoid based modulators of TRPC5. Out of a screening of natural products from Chinese medicines, galangin 19, was identified as an inhibitor of Gd3+ activated TRPC5 channel with IC50 of 0.45 µM. 19 also inhibited currents evoked by endogenous activation mechanism such as SIP (sphingosine-1-phosphate), confirming the modulation to be Ca2+ independent (Table 3). It was also shown to be an inhibitor of endogenous TRPC5 containing channels. A small SAR effort was able to identify 20, which came out to be the most potent with an IC50 of 0.28 µM in GD3+ activation assay. The hydroxy group on the chromen-4-one core was shown to be critical for activity. 20 inhibits ~65% of TRPC 1/4/5 channels at a concentration of 5 µM. It was a very weak inhibitor of other TRP channels; however, the most unexpected result comes in the case of TRPC5 activated by endogenous mechanism such as SIP, in this assay, contrary to parent compound 19, 20, acts as an agonist and not antagonist. This limits the use of this series of natural products.63 Table 3. SAR around compound 20 and analogs Cmpd Galangin, 19

Structure

TRPC5 IC50 (µM)

cLogP

0.45

2.76


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AM12, 20 (Galangin derivative)

0.28

3.53

Kaempferol, 21

3.9

2.46

Quercetin, 22

6.5

2.16

Myricetin, 23

>>10

1.85

Apigenin, 24

>>10

2.71

Luteolin, 25

>>10

2.40

Recently, Hopkins and Greka and co-workers have disclosed the most selective TRPC5 antagonist to date, AC1903, 26 (Figure 7).64 The group disclosed 26 as a selective TRPC5 inhibitor and showed its activity against a variety of animal models of chronic kidney disease.21 In a transgenic rat model of FSGS, chronic administration (IP injections) of 26 suppressed severe proteinuria and prevented loss of podocytes. In addition, 26 provided a benefit in a model of hypertensive proteinuric kidney disease.21 The group has published the medicinal chemistry efforts around the identification of 26, which came from the same HTS as 3. The benzimidazole core scaffold is conserved across 4, 18 and 26; however, 26 is the most selective for TRPC5 (IC50 = 4.1 M, patch clamp).64 Modifications of the 2-benzimidazole portion revealed the 5-membered heteroarylmethyl groups were tolerated – phenyl and pyridine were significantly less active.



Amide replacements were not active, and N-methylation was less active. The left-hand phenyl portion of the benzimidazole was also resistant to change with the azabenzimidazole losing all activity. The southern part of the molecule provided more SAR information. The chain length could be lengthened to three and ethers were also active. The phenyl ring was required; however, substitution was allowed.64

Figure 7. Discovery and SAR of AC1903, 26. In 2018, Goldfinch Bio, a biotechnology company focused on developing therapies for kidney, disease presented data on their lead molecule GFB-887, a selective TRPC5 inhibitor, at the American Society of Nephrology (ASN) Kidney Week 2018 Conference.65 GFB-887 is a TRPC5 inhibitor with nanomolar activity and has selectivity across other TRP channels and TRPC subtype specificity. In addition, it prevented the loss of stress fibers that are critical in maintaining kidney function, including protection against protamine sulfate induced loss of stress fibers and restoration of stress fibers in podocytes after knockdown of synaptopodin. Lastly, GFB-887 improved proteinuria in hypertension-induced FSGS in relevant preclinical models without altering blood pressure. Unfortunately, no structural data for GFB-887 has been disclosed as of this review, but having a molecule in clinical development for chronic kidney disease would be a major development in this area.


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3.4

TRPC2 Unlike its rodent counterpart, TRPC2 is a pseudogene in humans and doesn’t form functional

channels.66 3.5

TRPC3 In B lymphocytes, TRPC1 and TRPC3 regulates B cell receptor (BCR)-mediated Ca2+

oscillations that activates nuclear factor of activated T cells (NFAT).67 TRPC3 is shown to be associated with PLCγ2 controlling amplification of receptor-mediated signals.68 Studies on transgenic mice and RNAi-mediated knockdown or overexpression in cardiac myocytes have shown TRPC3 and TRPC6 to promote cardiac hypertrophy through activation of calcineurin and its downstream effector, NFAT.69-71 2-aminoethoxydiphenylborane (2APB, 27), is shown to be a weak inhibitor of TRPC3/6/7 in HEK293 cells of both wild type and with stably expressed TRPC channels (Figure 8). It was thought to be operating via modulating the 1,4,5-triphosphate (IP3) receptors, but further studies revealed it was inhibiting TRPC channels directly. The inhibitory activities are observed whether the channels are activated by muscarinic agonist or synthetic dicalyglyecroal (DAG), oleyl-acetylglycerol (OAG). DAG is established as an endogenours activator of TRPC3/6/7 channels, formed by the activation of the phospholipase C (PLC) mechanism attached to the membrane GPCR.8,72 27 is not selective and it partially blocks TRPC3/6/7 in concentration dependent manner at high micromolar concentrations (50-100 µM).72 27, like 3, has been extensively used as a molecular probe tool to validate the role of TRPC3/6/7 subfamily. When given along with anticancer drug bortezomib, it has been shown to potentiate the effects of the drug by suppression of calcium mediated autophagy.73



Administration of high D-glucose significantly increased reactive oxygen species in human monocytes. High D-glucose or peroxinitrite significantly increases the expression of TRPC1, TRPC3, TRPC5, TRPC6, TRPM6 and TRPM7 mRNA along with TRPC3 and TRPC6 proteins. Increased oxidative stress by lipopolysaccharide or tumor necrosis factor-α also increases TRP mRNA expression. Increase in TRPC3 and TRPC6 protein expression is accompanied by an increase in the 1-oleoyl-2-acetyl-sn-glycerol-induced calcium influx, which was blocked by 27. TRPC6 mRNA was significantly higher in monocytes from 18 patients with type 2 diabetes mellitus compared to 28 control subjects (p30

>100

0.68

2.6

57

0.03

13

27

0.68

4.1

58

0.003

8.6

>30

0.59

4.0

59

0.033

25

>30

0.65

3.2

60 (DS88790512)

0.011

>100

>300

0.64

2.2

Cmpd

Structure

Functional TRPC6 channels are found to be overexpressed in various types of cancer and play a vital role in cell-signaling, proliferation, differentiation, and apoptosis. Increase in TRPC6 mediated cytosolic Ca2+ has been implicated in development of gastric cancer.92-94 Previously Qu


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et al. had published a series of potent pyrazolopyrimidines based stimulators of the TRPC 3/6/7 subfamily (Figure 14).95 The lead compound, 61, was the most efficacious for TRPC3 (EC50 = 0.019 µM) followed by TRPC7 (EC50 = 0.090 µM) and TRPC6 (EC50 = 1.385 µM). Further derivatization of the pyrazolo[1,5-a]pyrimidine by Ding et al. surprisingly lead to modifications rendering this class of molecules switch to function as inhibitors rather than stimulators.96 Though out of this SAR campaign only two low micromolar inhibitors 63 and 66 were discovered (Table 9). But both of these inhibitors showed dose dependent activity against in vivo and in vitro models of gastric cancer.

Figure 14. TRPC3/6/7 stimulator Compound 62 was found to be a TRPC6 agonist, and any substitution of the ethyl carbamate on R1 led to loss of agonistic activity. Fascinatingly, substitution of tert-butyl carbamate 63 switched the stimulator to an inhibitor (Table 9). Substitution of fluoride at R2 by chloride or CF3 led to a loss in inhibitory activity. Except for 66, all other modifications led to inactive compounds. Replacing the piperidine also led to complete loss in inhibitory activity.96 Inhibitory activity of 66 was in the order of TRPC6 > C7 > C3. In an in vitro cytotoxicity MTT assay against gastric cancer cell lines, 66 suppressed the proliferation of AGS and MKN45 cells in a concentration-dependent manner with IC50 of 17.1 and 18.5 µM, respectively. Also at a high concentration of 100 µM, no cytotoxicity was found against HK-2 cells. In BALB/c nude mice



implanted with gastric tumor, liposomal preparation of 66 at a dose of 100 mg/kg and 200 mg/kg twice a day led to 38.5% and 61.5 % reduction in tumor growth.96 As observed with previous TRPC modulators small structural changes leads to activity switch or complete loss of activity. Table 9. SAR of pyrazolopyrimidines as TRPC6 inhibitor

Cmpd 62 63 64 65 66

4.

R1 EtOCO Boc Boc Boc EtOCO

R2 F F Cl CF3 CF3

R3 H H H H Me

TRPC6 IC50 (µM) 4.7 µM (EC50) 8.4 NA NA 1.0

Cryo-EM Structures Up until recently, there did not exist any structural information regarding the TRP channels;

however, that changed dramatically in 2018. The explosion of cryo-electron microscopy (cryoEM) techniques has enabled the solving of many, previously unattainable, receptor classes including TRPC.97-101 Cyro-EM is becoming an ever expanding tool with many advantages over traditional X-ray: 1. crystallization is not required, 2. only small amounts of protein/sample are needed, and 3. the technique has the potential to deal with compositional and conformational mixture. Many of these solved structures were first published on the Preprint server BioRxiv; but have since been published in more traditional journals. TRPC4 has been solved for both the mouse


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(EMDB: EMD-6901; PDB: 5Z96)37 and zebrafish (EMDB: EMD-4339; PDB: 6G1K)102 in the unliganded (apo) state with resolutions of 3.3 and 3.6 Å, respectively. In the mouse TRPC4 structure, the authors compared the structure of TRPC4 to toher TRP structures and found the six helices in each transmembrane domain (TMD) are similar.37 They found differences in the intracellular architecture, notably in the S2-S3 linker which has a two-helical turn. In addition, TRPC4 has a disulfide bond between Cys549 and Cys554 which links the S5 and the pore helix. In addition, the human TRPC6 as well as the human TRPC3 have been solved. The hTRPC6 homotetramer

was

solved

with

a

high-affinity

inhibitor

(2-(benzo[d][1,3]dioxol-5-

ylamino)thiazol-4-yl)(3,5-dimethylpiperidin-1-yl)methanone (BTDM) to 3.8 Å (EMDB: EMD6856; PDB: 5YX9).103 Interestingly, the TRPC6 structure shows a two-layer architecture where the bell-shaped cytosolic layer holds the transmemberane layer. The inhibitor, BTDM, sits between the S5-S6 pore domain and the voltage sensor-like domain to inhibit the channel opening.103 The hTRPC3 has been solved by two groups. One in the lipid-occupied closed state at 3.3 Å (EMDB: EMD-7620; PDB: 6CUD)104, and the other at 4.4 Å (EMDB: EMD-6911; PDB: 5ZBG).103 The TRPC3 structure shows the S6 portion apart from the TRP helix – which is in contrast to the TRPM, TRPV and TRPA channels. In addition, the S3 is quite large and may create a cavity in the extracellular domain for binding of small molecules. These structures provide a view of the design of these channels and should pave the way for a better understanding of the mechanism of these channels as well as provide insight into future mutagenesis, structural studies, and ultimately, medicinal chemistry drug design. More potent and selective modulators of these channels will ultimately shed new light into the molecular mechanisms of how these compounds interact.



5.

Conclusion The roles of TRP channels are only now beginning to be more fully understood as selective

small molecule modulators are available to complement the genetic approaches used previously. These genetic alterations can be achieved with excellent selectivity; however, the full effect can lead to secondary and tertiary effects that can affect data analysis. Having more selective inhibitors and activators can help with this understanding by allowing for better data analysis due to their unique properties. The disease implications for the TRP channels are wide and have distinct ramifications for a variety of indications, from kidney disease, depression, addiction, and cancer. With the exciting emergence of Cyro-EM and the ability to gain structural information for these channels, it is expected that more potent and selective tool compounds will be developed. From these studies, the expectation is that highly effective drugs will follow. AUTHOR INFORMATION Corresponding Author *Phone: +1 (402) 559-5729; Email: [email protected] Notes The authors declare no competing financial interest Biographies Swagat Sharma, M.S. is a graduate student in the Department of Pharmaceutical Sciences in the College of Pharmacy at the University of Nebraska Medical Center. Swagat completed his M.S. in Dr. Rahul Jain’s lab at the National Institute of Pharmaceutical Education and Research (NIPER-Mohali), India. After working in industry for a brief period he joined the laboratory of


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Dr. Hopkins in 2016 and has been working towards the design and synthesis of biologically active small molecular probes for modulation of ion channels. Corey R. Hopkins, Ph.D. is Associate Professor in the Department of Pharmaceutical Sciences in the School of Pharmacy at the University of Nebraska Medical Center. Corey completed his doctorate in 2002 at the University of Pittsburgh on the total synthesis of the naphthyridinomycin/bioxalomycin class of compounds. He also developed a novel ring expansion methodology to make pharmacophore analogs of Dnacin. Corey moved to Aventis Pharmaceuticals in 2001 and later became Senior Research Investigator in Medicinal Chemistry. In 2008, Corey served as Associate Director of Medicinal Chemistry for the Vanderbilt Center for Neuroscience Drug Discovery where he led the chemistry efforts on multiple projects and has been at UNMC since 2016 where his research focuses on ion channels, GPCRs and kinases for therapeutic application.

ACKNOWLEDGMENTS This research was supported by a grant from the NIH (NIDDK: R01DK103658) to C.R.H. ABBREVIATIONS USED TRP, transient receptor potential; TRPC, transient receptor potential canonical; TRPV, transient receptor potential vanilloid; TRPM, transient receptor potential melastatin; TRPA, transient receptor potential ankyrin; TRPP, transient receptor potential polycystin; TRPML, transient receptor potential mucoliptin; PLC, phospholipase C; DAG, diacylglycerol; OAG, oleyl acetyl glycerol; CKD, chronic kidney disease; SOCE, store-operated calcium entry process; CHO, Chinese hamster ovary; CSF, cerebrospinal fluid; PPB, plasma protein binding; BTP,



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Table of Contents Graphic:


Review of Transient Receptor Potential Canonical (TRPC5) Channel

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