Nanomolar CFTR inhibition by pore-occluding divalent polyethylene glycol-malonic acid hydrazides

Potent, Gut-Restricted Inhibitors of Divalent Metal Transporter 1: Preclinical Efficacy against Iron Overload and Safety Evaluation

Divalent metal transporter 1 (DMT1) cotransports ferrous iron and protons and is the primary mechanism for uptake of nonheme iron by enterocytes. Inhibitors are potentially useful as therapeutic agents to treat iron overload disorders such as hereditary hemochromatosis or β-thalassemia intermedia, provided that inhibition can be restricted to the duodenum. We used a calcein quench assay to identify human DMT1 inhibitors. Dimeric compounds were made to generate more potent compounds with low systemic exposure. Direct block of DMT1 was confirmed by voltage clamp measurements. The lead compound, XEN602, strongly inhibits dietary nonheme iron uptake in both rats and pigs yet has negligible systemic exposure. Efficacy is maintained for >2 weeks in a rat subchronic dosing assay. Doses that lowered iron content in the spleen and liver by >50% had no effect on the tissue content of other divalent cations except for cobalt. XEN602 represents a powerful pharmacological tool for understanding the physiologic function of DMT1 in the gut. SIGNIFICANCE STATEMENT: This report introduces methodology to develop potent, gut-restricted inhibitors of divalent metal transporter 1 (DMT1) and identifies XEN602 as a suitable compound for in vivo studies. We also report novel animal models to quantify the inhibition of dietary uptake of iron in both rodents and pigs. This research shows that inhibition of DMT1 is a promising means to treat iron overload disorders.

Small molecule inhibitors of intestinal epithelial anion exchanger SLC26A3 (DRA) with a luminal, extracellular site of action

The anion exchanger protein SLC26A3 (down-regulated in adenoma, DRA) is expressed in the luminal membrane of intestinal epithelial cells in colon, where it facilitates the absorption of Cl^- and oxalate. We previously identified a 4,8-dimethylcoumarin class of SLC26A3 inhibitors that act from the SLC26A3 cytoplasmic surface, and demonstrated their efficacy in mouse models of constipation and hyperoxaluria. Here, screening of 50,000 new compounds and 1740 chemical analogs of active compounds from the primary screen produced five novel classes of SLC26A3-selective inhibitors (1,3-dioxoisoindoline-amides; N-(5-sulfamoyl-1,3,4-thiadiazol-2-yl)acetamides; thiazolo-pyrimidin-5-ones; 3-carboxy-2-phenylbenzofurans and benzoxazin-4-ones) with IC₅₀ down to 100 nM. Kinetic washout and onset of action studies revealed an extracellular site of action for the thiazolo-pyrimidin-5-one and 3-carboxy-2-phenylbenzofuran inhibitors. Molecular docking computations revealed putative binding sites for these inhibitors. In a loperamide model of constipation in mice, orally administered 7-(2-chloro-phenoxymethyl)-3-phenyl-thiazolo [3,2-a]pyrimidin-5-one (3a) significantly increased stool weight, pellet number and water content. SLC26A3 inhibitors with an extracellular site of action offer the possibility of creating non-absorbable, luminally acting inhibitors with minimal systemic exposure following oral administration. Our findings also suggest that inhibitors of related SLC26 anion transporters with an extracellular site of action might be identified for pharmacological modulation of selected epithelial ion transport processes.

Chloride transport modulators as drug candidates

Chloride transport across cell membranes is broadly involved in epithelial fluid transport, cell volume and pH regulation, muscle contraction, membrane excitability, and organellar acidification. The human genome encodes at least 53 chloride-transporting proteins with expression in cell plasma or intracellular membranes, which include chloride channels, exchangers, and cotransporters, some having broad anion specificity. Loss-of-function mutations in chloride transporters cause a wide variety of human diseases, including cystic fibrosis, secretory diarrhea, kidney stones, salt-wasting nephropathy, myotonia, osteopetrosis, hearing loss, and goiter. Although impactful advances have been made in the past decade in drug treatment of cystic fibrosis using small molecule modulators of the defective cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, other chloride channels and solute carrier proteins (SLCs) represent relatively underexplored target classes for drug discovery. New opportunities have emerged for the development of chloride transport modulators as potential therapeutics for secretory diarrheas, constipation, dry eye disorders, kidney stones, polycystic kidney disease, hypertension, and osteoporosis. Approaches to chloride transport-targeted drug discovery are reviewed herein, with focus on chloride channel and exchanger classes in which recent preclinical advances have been made in the identification of small molecule modulators and in proof of concept testing in experimental animal models.

STRUCTURE, GATING, AND REGULATION OF THE CFTR ANION CHANNEL

The cystic fibrosis transmembrane conductance regulator (CFTR) belongs to the ATP binding cassette (ABC) transporter superfamily but functions as an anion channel crucial for salt and water transport across epithelial cells. CFTR dysfunction, because of mutations, causes cystic fibrosis (CF). The anion-selective pore of the CFTR protein is formed by its two transmembrane domains (TMDs) and regulated by its cytosolic domains: two nucleotide binding domains (NBDs) and a regulatory (R) domain. Channel activation requires phosphorylation of the R domain by cAMP-dependent protein kinase (PKA), and pore opening and closing (gating) of phosphorylated channels is driven by ATP binding and hydrolysis at the NBDs. This review summarizes available information on structure and mechanism of the CFTR protein, with a particular focus on atomic-level insight gained from recent cryo-electron microscopic structures and on the molecular mechanisms of channel gating and its regulation. The pharmacological mechanisms of small molecules targeting CFTR's ion channel function, aimed at treating patients suffering from CF and other diseases, are briefly discussed.

CFTR pharmacology

CFTR protein is an ion channel regulated by cAMP-dependent phosphorylation and expressed in many types of epithelial cells. CFTR-mediated chloride and bicarbonate secretion play an important role in the respiratory and gastrointestinal systems. Pharmacological modulators of CFTR represent promising drugs for a variety of diseases. In particular, correctors and potentiators may restore the activity of CFTR in cystic fibrosis patients. Potentiators are also potentially useful to improve mucociliary clearance in patients with chronic obstructive pulmonary disease. On the other hand, CFTR inhibitors may be useful to block fluid and electrolyte loss in secretory diarrhea and slow down the progression of polycystic kidney disease.

Benzopyrimido-pyrrolo-oxazine-dione CFTR inhibitor (R)-BPO-27 for antisecretory therapy of diarrheas caused by bacterial enterotoxins

Secretory diarrheas caused by bacterial enterotoxins, including cholera and traveler's diarrhea, remain a major global health problem. Inappropriate activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel occurs in these diarrheas. We previously reported that the benzopyrimido-pyrrolo-oxazinedione (R)-BPO-27 inhibits CFTR chloride conductance with low-nanomolar potency. Here, we demonstrate using experimental mouse models and human enterocyte cultures the potential utility of (R)-BPO-27 for treatment of secretory diarrheas caused by cholera and Escherichia coli enterotoxins. (R)-BPO-27 fully blocked CFTR chloride conductance in epithelial cell cultures and intestine after cAMP agonists, cholera toxin, or heat-stable enterotoxin of E. coli (STa toxin), with IC₅₀ down to ∼5 nM. (R)-BPO-27 prevented cholera toxin and STa toxin-induced fluid accumulation in small intestinal loops, with IC₅₀ down to 0.1 mg/kg. (R)-BPO-27 did not impair intestinal fluid absorption or inhibit other major intestinal transporters. Pharmacokinetics in mice showed >90% oral bioavailability with sustained therapeutic serum levels for >4 h without the significant toxicity seen with 7-d administration at 5 mg/kg/d. As evidence to support efficacy in human diarrheas, (R)-BPO-27 blocked fluid secretion in primary cultures of enteroids from human small intestine and anion current in enteroid monolayers. These studies support the potential utility of (R)-BPO-27 for therapy of CFTR-mediated secretory diarrheas.-Cil, O., Phuan, P.-W., Gillespie, A. M., Lee, S., Tradtrantip, L., Yin, J., Tse, M., Zachos, N. C., Lin, R., Donowitz, M., Verkman, A. S. Benzopyrimido-pyrrolo-oxazine-dione CFTR inhibitor (R)-BPO-27 for antisecretory therapy of diarrheas caused by bacterial enterotoxins.

Secretory diarrhoea: mechanisms and emerging therapies

Diarrhoeal disease remains a major health burden worldwide. Secretory diarrhoeas are caused by certain bacterial and viral infections, inflammatory processes, drugs and genetic disorders. Fluid secretion across the intestinal epithelium in secretory diarrhoeas involves multiple ion and solute transporters, as well as activation of cyclic nucleotide and Ca(2+) signalling pathways. In many secretory diarrhoeas, activation of Cl(-) channels in the apical membrane of enterocytes, including the cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, increases fluid secretion, while inhibition of Na(+) transport reduces fluid absorption. Current treatment of diarrhoea includes replacement of fluid and electrolyte losses using oral rehydration solutions, and drugs targeting intestinal motility or fluid secretion. Therapeutics in the development pipeline target intestinal ion channels and transporters, regulatory proteins and cell surface receptors. This Review describes pathogenic mechanisms of secretory diarrhoea, current and emerging therapeutics, and the challenges in developing antidiarrhoeal therapeutics.

Benzopyrimido-pyrrolo-oxazine-dione (R)-BPO-27 Inhibits CFTR Chloride Channel Gating by Competition with ATP

We previously reported that benzopyrimido-pyrrolo-oxazinedione BPO-27 [6-(5-bromofuran-2-yl)-7,9-dimethyl-8,10-dioxo-11-phenyl-7,8,9,10-tetrahydro-6H-benzo[b]pyrimido [4',5':3,4]pyrrolo [1,2-d][1,4]oxazine-2-carboxylic acid] inhibits the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel with low nanomolar potency and reduces cystogenesis in a model of polycystic kidney disease. We used computational chemistry and patch-clamp to show that enantiomerically pure (R)-BPO-27 inhibits CFTR by competition with ATP, whereas (S)-BPO-27 is inactive. Docking computations using a homology model of CFTR structure suggested that (R)-BPO-27 binds near the canonical ATP binding site, and these findings were supported by molecular dynamics simulations showing a lower binding energy for the (R) versus (S) stereoisomers. Three additional lower-potency BPO-27 analogs were modeled in a similar fashion, with the binding energies predicted in the correct order. Whole-cell patch-clamp studies showed linear CFTR currents with a voltage-independent (R)-BPO-27 block mechanism. Single-channel recordings in inside-out patches showed reduced CFTR channel open probability and increased channel closed time by (R)-BPO-27 without altered unitary channel conductance. At a concentration of (R)-BPO-27 that inhibited CFTR chloride current by ∼50%, the EC50 for ATP activation of CFTR increased from 0.27 to 1.77 mM but was not changed by CFTRinh-172 [4-[[4-oxo-2-thioxo-3-[3-trifluoromethyl)phenyl]-5-thiazolidinylidene]methyl]benzoic acid], a thiazolidinone CFTR inhibitor that acts at a site distinct from the ATP binding site. Our results suggest that (R)-BPO-27 inhibition of CFTR involves competition with ATP.

Poly(ethylene glycol) as a scaffold for high-affinity open-channel blockers of the mouse nicotinic acetylcholine receptor

High-affinity blockers for an ion channel often have complex molecular structures that are synthetically challenging and/or laborious. Here we show that high-affinity blockers for the mouse nicotinic acetylcholine receptor (AChR) can be prepared from a structurally simple material, poly(ethylene glycol) (PEG). The PEG-based blockers (PQ1–5), comprised of a flexible octa(ethylene glycol) scaffold and two terminal quaternary ammonium groups, exert low- to sub-micromolar affinities for the open AChR pore (measured via single-channel analysis of AChRs expressed in human embryonic kidney cells). PQ1–5 are comparable in pore-binding affinity to the strongest AChR open-channel blockers previously reported, which have complex molecular structures. These results suggest a general approach for designing potent open-channel blockers from a structurally flexible polymer. This design strategy involves simple synthetic procedures and does not require detailed information about the structure of an ion-channel pore.

Antidiarrheal efficacy and cellular mechanisms of a Thai herbal remedy

Screening of herbal remedies for Cl(-) channel inhibition identified Krisanaklan, a herbal extract used in Thailand for treatment of diarrhea, as an effective antidiarrheal in mouse models of secretory diarrheas with inhibition activity against three Cl(-) channel targets. Krisanaklan fully inhibited cholera toxin-induced intestinal fluid secretion in a closed-loop mouse model with ∼50% inhibition at a 1 ∶ 50 dilution of the extract. Orally administered Krisanaklan (5 µL/g) prevented rotavirus-induced diarrhea in neonatal mice. Short-circuit current measurements showed full inhibition of cAMP and Ca(2+) agonist-induced Cl(-) conductance in human colonic epithelial T84 cells, with ∼ 50% inhibition at a 1 ∶ 5,000 dilution of the extract. Krisanaklan also strongly inhibited intestinal smooth muscle contraction in an ex vivo preparation. Together with measurements using specific inhibitors, we conclude that the antidiarrheal actions of Krisanaklan include inhibition of luminal CFTR and Ca(2+)-activated Cl(-) channels in enterocytes. HPLC fractionation indicated that the three Cl(-) inhibition actions of Krisanaklan are produced by different components in the herbal extract. Testing of individual herbs comprising Krisanaklan indicated that agarwood and clove extracts as primarily responsible for Cl(-) channel inhibition. The low cost, broad antidiarrheal efficacy, and defined cellular mechanisms of Krisanaklan suggests its potential application for antisecretory therapy of cholera and other enterotoxin-mediated secretory diarrheas in developing countries.

CFTR inhibitors

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cAMP-regulated Cl- channel whose major function is to facilitate epithelial fluid secretion. Loss-of-function mutations in CFTR cause the genetic disease cystic fibrosis. CFTR is required for transepithelial fluid transport in certain secretory diarrheas, such as cholera, and for cyst expansion in autosomal dominant polycystic kidney disease. High-throughput screening has yielded CFTR inhibitors of the thiazolidinone, glycine hydrazide and quinoxalinedione chemical classes. The glycine hydrazides target the extracellular CFTR pore, whereas the thiazolidinones and quinoxalinediones act at the cytoplasmic surface. These inhibitors have been widely used in cystic fibrosis research to study CFTR function at the cell and organ levels. The most potent CFTR inhibitor has IC50 of approximately 4 nM. Studies in animal models support the development of CFTR inhibitors for antisecretory therapy of enterotoxin-mediated diarrheas and polycystic kidney disease.

Cholera: pathophysiology and emerging therapeutic targets

Cholera is a diarrheal disease that remains an important global health problem with several hundreds of thousands of reported cases each year. This disease is caused by intestinal infection with Vibrio cholerae, which is a highly motile gram-negative bacterium with a single-sheathed flagellum. In the course of cholera pathogenesis, V. cholerae expresses a transcriptional activator ToxT, which subsequently transactivates expressions of two crucial virulence factors: toxin-coregulated pilus and cholera toxin (CT). These factors are responsible for intestinal colonization of V. cholerae and induction of fluid secretion, respectively. In intestinal epithelial cells, CT binds to GM1 ganglioside receptors on the apical membrane and undergoes retrograde vesicular trafficking to endoplasmic reticulum, where it exploits endoplasmic reticulum-associated protein degradation systems to release a catalytic A1 subunit of CT (CT A1) into cytoplasm. CT A1, in turn, catalyzes ADP ribosylation of α subunits of stimulatory G proteins, leading to a persistent activation of adenylate cyclase and an elevation of intracellular cAMP. Increased intracellular cAMP in human intestinal epithelial cells accounts for pathogenesis of profuse diarrhea and severe fluid loss in cholera. This review provides an overview of the pathophysiology of cholera diarrhea and discusses emerging drug targets for cholera, which include V. cholerae virulence factors, V. cholerae motility, CT binding to GM1 receptor, CT internalization and intoxication, as well as cAMP metabolism and transport proteins involved in cAMP-activated Cl(-) secretion. Future directions and perspectives of research on drug discovery and development for cholera are discussed.

Chloride channel-targeted therapy for secretory diarrheas

Secretory diarrheas caused by bacterial and viral enterotoxins remain a significant cause of morbidity and mortality. Enterocyte Cl(-) channels represent an attractive class of targets for diarrhea therapy, as they are the final, rate-limiting step in enterotoxin-induced fluid secretion in the intestine. Activation of cyclic nucleotide and/or Ca(2+) signaling pathways in secretory diarrheas increases the conductance of Cl(-) channels at the enterocyte luminal membrane, which include the cystic fibrosis transmembrane conductance regulator (CFTR) and Ca(2+)-activated Cl(-) channels (CaCCs). High-throughput screens have yielded several chemical classes of small molecule CFTR and CaCC inhibitors that show efficacy in animal models of diarrheas. Natural-product diarrhea remedies with Cl(-) channel inhibition activity have also been identified, with one product recently receiving FDA approval for HIV-associated diarrhea.

Convective washout reduces the antidiarrheal efficacy of enterocyte surface-targeted antisecretory drugs

Secretory diarrheas such as cholera are a major cause of morbidity and mortality in developing countries. We previously introduced the concept of antisecretory therapy for diarrhea using chloride channel inhibitors targeting the cystic fibrosis transmembrane conductance regulator channel pore on the extracellular surface of enterocytes. However, a concern with this strategy is that rapid fluid secretion could cause convective drug washout that would limit the efficacy of extracellularly targeted inhibitors. Here, we developed a convection-diffusion model of washout in an anatomically accurate three-dimensional model of human intestine comprising cylindrical crypts and villi secreting fluid into a central lumen. Input parameters included initial lumen flow and inhibitor concentration, inhibitor dissociation constant (K(d)), crypt/villus secretion, and inhibitor diffusion. We modeled both membrane-impermeant and permeable inhibitors. The model predicted greatly reduced inhibitor efficacy for high crypt fluid secretion as occurs in cholera. We conclude that the antisecretory efficacy of an orally administered membrane-impermeant, surface-targeted inhibitor requires both (a) high inhibitor affinity (low nanomolar K(d)) to obtain sufficiently high luminal inhibitor concentration (>100-fold K(d)), and (b) sustained high luminal inhibitor concentration or slow inhibitor dissociation compared with oral administration frequency. Efficacy of a surface-targeted permeable inhibitor delivered from the blood requires high inhibitor permeability and blood concentration (relative to K(d)).

Cystic fibrosis transmembrane regulator correctors and potentiators

Cystic fibrosis (CF) is caused by loss-of-function mutations in the CF transmembrane conductance regulator (CFTR) protein, a cAMP-regulated anion channel expressed primarily at the apical plasma membrane of secretory epithelia. Nearly 2000 mutations in the CFTR gene have been identified that cause disease by impairing its translation, cellular processing, and/or chloride channel gating. The fundamental premise of CFTR corrector and potentiator therapy for CF is that addressing the underlying defects in the cellular processing and chloride channel function of CF-causing mutant CFTR alleles will result in clinical benefit by addressing the basic defect underlying CF. Correctors are principally targeted at F508del cellular misprocessing, whereas potentiators are intended to restore cAMP-dependent chloride channel activity to mutant CFTRs at the cell surface. This article reviews the discovery of CFTR potentiators and correctors, what is known regarding their mechanistic basis, and encouraging results achieved in clinical testing.

ABSOLUTE CONFIGURATION AND BIOLOGICAL PROPERTIES OF ENANTIOMERS OF CFTR INHIBITOR BPO-27

We previously reported benzopyrimido-pyrrolo-oxazinedione (BPO) inhibitors of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel and showed their efficacy in a model of polycystic kidney disease. Here, we separated the enantiomers of lead compound BPO-27, (1), which contains a single chiral center, and determined their absolute configuration, activity and metabolic stability. Following separation by chiral supercritical fluid chromatography, the R enantiomer, as determined by x-ray crystallography, inhibited CFTR chloride conductance with IC₅₀ ~ 4 nM, while S enantiomer was inactive. In vitro metabolic stability in hepatic microsomes showed both enantiomers as stable, with <5 % metabolism in 4 h. Following bolus interperitoneal administration in mice, serum (R)-1 decayed with t_1/2 ~ 1.6 h and gave sustained therapeutic concentrations in kidney.

Recent advances and new perspectives in targeting CFTR for therapy of cystic fibrosis and enterotoxin-induced secretory diarrheas

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel localized primarily at the apical surfaces of epithelial cells lining airway, gut and exocrine glands, where it is responsible for transepithelial salt and water transport. Several human diseases are associated with an altered channel function of CFTR. Cystic fibrosis (CF) is caused by the loss or dysfunction of CFTR-channel activity resulting from the mutations on the gene; whereas enterotoxin-induced secretory diarrheas are caused by the hyperactivation of CFTR channel function. CFTR is a validated target for drug development to treat these diseases. Significant progress has been made in developing CFTR modulator therapy by means of high-throughput screening followed by hit-to-lead optimization. Several oral administrated investigational drugs are currently being evaluated in clinical trials for CF. Also importantly, new ideas and methodologies are emerging. Targeting CFTR-containing macromolecular complexes is one such novel approach.

Aquaporin 9 phosphorylation mediates membrane localization and neutrophil polarization

Neutrophils are of prime importance in the host innate defense against invading microorganisms by using two primary mechanisms-locomotion toward and phagocytosis of the prey. Recent research points to pivotal roles for water channels known as AQPs in cell motility. Here, we focused on the role of AQP9 in chemoattractant-induced polarization and migration of primary mouse neutrophils and neutrophil-like HL60 cells. We found that AQP9 is phosphorylated downstream of fMLFR or PMA stimulation in primary human neutrophils. The dynamics of AQP9 were assessed using GFP-tagged AQP9 constructs and other fluorescent markers through various live-cell imaging techniques. Expression of WT or the phosphomimic S11D AQP9 changed cell volume regulation as a response to hyperosmotic changes and enhanced neutrophil polarization and chemotaxis. WT AQP9 and S11D AQP9 displayed a very dynamic distribution at the cell membrane, whereas the phosphorylation-deficient S11A AQP9 failed to localize to the plasma membrane. Furthermore, we found that Rac1 regulated the translocation of AQP9 to the plasma membrane. Our results show that AQP9 plays an active role in neutrophil volume regulation and migration. The display of AQP9 at the plasma membrane depends on AQP9 phosphorylation, which appeared to be regulated through a Rac1-dependent pathway.

Potent, metabolically stable benzopyrimido-pyrrolo-oxazine-dione (BPO) CFTR inhibitors for polycystic kidney disease

We previously reported the discovery of pyrimido-pyrrolo-quinoxalinedione (PPQ) inhibitors of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel and showed their efficacy in an organ culture model of polycystic kidney disease (PKD) (J. Med. Chem. 2009, 52, 6447-6455). Here, we report related benzopyrimido-pyrrolo-oxazinedione (BPO) CFTR inhibitors. To establish structure-activity relationships and select lead compound(s) with improved potency, metabolic stability, and aqueous solubility compared to the most potent prior compound 8 (PPQ-102, IC(50) ∼ 90 nM), we synthesized 16 PPQ analogues and 11 BPO analogues. The analogues were efficiently synthesized in 5-6 steps and 11-61% overall yield. Modification of 8 by bromine substitution at the 5-position of the furan ring, replacement of the secondary amine with an ether bridge, and carboxylation, gave 6-(5-bromofuran-2-yl)-7,9-dimethyl-8,10-dioxo-11-phenyl-7,8,9,10-tetrahydro-6H-benzo[b]pyrimido [4',5':3,4]pyrrolo [1,2-d][1,4]oxazine-2-carboxylic acid 42 (BPO-27), which fully inhibited CFTR with IC(50) ∼ 8 nM and, compared to 8, had >10-fold greater metabolic stability and much greater polarity/aqueous solubility. In an embryonic kidney culture model of PKD, 42 prevented cyst growth with IC(50) ∼ 100 nM. Benzopyrimido-pyrrolo-oxazinediones such as 42 are potential development candidates for antisecretory therapy of PKD.

A plant-derived hydrolysable tannin inhibits CFTR chloride channel: a potential treatment of diarrhea

PURPOSE

The present study examined the effects and mechanisms of actions of penta-m-digalloyl-glucose (PDG), a hydrolysable tannin extracted from Chinese gallnut, on cystic fibrosis transmembrane conductance regulator protein (CFTR).

MATERIALS AND METHODS

Fisher rat thyroid cells stably expressing human CFTR (FRT cells) and human intestinal T84 cells were used as cell models to investigate the effects of PDG on chloride secretion using short-circuit current analysis. The mechanisms by which PDG affected chloride secretion were also examined. Finally, in vivo antidiarrheal efficacy and effects of PDG on intestinal fluid absorption were evaluated in mouse closed-loop models.

RESULTS

In FRT cells, apical chloride current induced by forskolin, CPT-cAMP and apigenin were reversibly inhibited by PDG (IC50 approximately 10microM) without effects on intracellular cAMP content and cell viability. Similarly, in T84 cells, PDG effectively inhibited chloride secretion induced by forskolin and cholera toxin. However, it had no effect on calcium-induced chloride secretion. In mice, a single intraluminal injection of PDG (0.6 mg/kg) reduced cholera toxin-induced intestinal fluid secretion by 75% with no effect on intestinal fluid absorption.

CONCLUSIONS

PDG represents a new class of CFTR inhibitors. Further development of this class of compounds may provide a new therapeutic intervention for diarrhea.

Therapeutic potential of cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors in polycystic kidney disease

In the common genetic disorder autosomal dominant polycystic kidney disease (ADPKD), kidney function is disrupted by multiple fluid-filled epithelial cysts. Cyst growth in ADPKD involves fluid accumulation within the cyst lumen driven by cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial Cl- secretion. This suggests that inhibitors of the CFTR Cl- channel might retard cyst growth. This review considers how knowledge of CFTR structure and function and its role in transepithelial salt and water movements provides insight into the mechanism of action of CFTR inhibitors. Some small molecules, termed open-channel blockers, inhibit directly the CFTR Cl- channel by physically obstructing the CFTR pore and preventing Cl- flow. By contrast, other small molecules, termed allosteric inhibitors, bind to CFTR at a site remote from the channel pore and interfere with conformational changes that open the pore. The application of high-throughput screening to CFTR drug discovery has led to the identification of new inhibitors of the CFTR Cl- channel including the thiazolidinone CFTR(inh)-172 and the glycine hydrazide GlyH-101. The demonstration that CFTR inhibitors retard cyst expansion and kidney enlargement in mouse models of ADPKD provides proof of concept for the use of small-molecule CFTR inhibitors in the treatment of ADPKD.

Nanomolar potency pyrimido-pyrrolo-quinoxalinedione CFTR inhibitor reduces cyst size in a polycystic kidney disease model

Inhibitors of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel are predicted to slow cyst enlargement in polycystic kidney disease and reduce intestinal fluid loss in secretory diarrheas. Screening of approximately 110000 small synthetic and natural compounds for inhibition of halide influx in CFTR-expressing epithelial cells yielded a new class of pyrimido-pyrrolo-quinoxalinedione (PPQ) CFTR inhibitors. Testing of 347 analogues established structure-activity relationships. The most potent compound, 7,9-dimethyl-11-phenyl-6-(5-methylfuran-2-yl)-5,6-dihydro-pyrimido[4',5'-3,4]pyrrolo[1,2-a]quinoxaline-8,10-(7H,9H)-dione, PPQ-102, completely inhibited CFTR chloride current with IC(50) approximately 90 nM. The PPQs, unlike prior CFTR inhibitors, are uncharged at physiological pH, and therefore not subject to membrane potential-dependent cellular partitioning or block efficiency. Patch-clamp analysis confirmed voltage-independent CFTR inhibition by PPQ-102 and showed stabilization of the channel closed state. PPQ-102 prevented cyst expansion and reduced the size of preformed cysts in a neonatal kidney organ culture model of polycystic kidney disease. PPQ-102 is the most potent CFTR inhibitor identified to date.

Crofelemer, an antisecretory antidiarrheal proanthocyanidin oligomer extracted from Croton lechleri, targets two distinct intestinal chloride channels

Crofelemer, a purified proanthocyanidin oligomer extracted from the bark latex of Croton lechleri, is in clinical trials for secretory diarrheas of various etiologies. We investigated the antisecretory mechanism of crofelemer by determining its effect on the major apical membrane transport and signaling processes involved in intestinal fluid transport. Using cell lines and measurement procedures to isolate the effects on individual membrane transport proteins, crofelemer at 50 microM had little or no effect on the activity of epithelial Na(+) or K(+) channels or on cAMP or calcium signaling. Crofelemer inhibited the cystic fibrosis transmembrane regulator (CFTR) Cl(-) channel with maximum inhibition of approximately 60% and an IC(50) approximately 7 microM. Crofelemer action at an extracellular site on CFTR produced voltage-independent block with stabilization of the channel closed state. Crofelemer did not affect the potency of glycine hydrazide or thiazolidinone CFTR inhibitors. Crofelemer action resisted washout, with <50% reversal of CFTR inhibition after 4 h. Crofelemer was also found to strongly inhibit the intestinal calcium-activated Cl(-) channel TMEM16A by a voltage-independent inhibition mechanism with maximum inhibition >90% and IC(50) approximately 6.5 microM. The dual inhibitory action of crofelemer on two structurally unrelated prosecretory intestinal Cl(-) channels may account for its intestinal antisecretory activity.

Revisiting the role of cystic fibrosis transmembrane conductance regulator and counterion permeability in the pH regulation of endocytic organelles

Organellar acidification by the electrogenic vacuolar proton-ATPase is coupled to anion uptake and cation efflux to preserve electroneutrality. The defective organellar pH regulation, caused by impaired counterion conductance of the mutant cystic fibrosis transmembrane conductance regulator (CFTR), remains highly controversial in epithelia and macrophages. Restricting the pH-sensitive probe to CFTR-containing vesicles, the counterion and proton permeability, and the luminal pH of endosomes were measured in various cells, including genetically matched CF and non-CF human respiratory epithelia, as well as cftr(+/+) and cftr(-/-) mouse alveolar macrophages. Passive proton and relative counterion permeabilities, determinants of endosomal, lysosomal, and phagosomal pH-regulation, were probed with FITC-conjugated transferrin, dextran, and Pseudomonas aeruginosa, respectively. Although CFTR function could be documented in recycling endosomes and immature phagosomes, neither channel activation nor inhibition influenced the pH in any of these organelles. CFTR heterologous overexpression also failed to alter endocytic organellar pH. We propose that the relatively large CFTR-independent counterion and small passive proton permeability ensure efficient shunting of the proton-ATPase-generated membrane potential. These results have implications in the regulation of organelle acidification in general and demonstrate that perturbations of the endolysosomal organelles pH homeostasis cannot be linked to the etiology of the CF lung disease.

Stevioside and related compounds: therapeutic benefits beyond sweetness

Stevioside, an abundant component of Stevia rebaudiana leaf, has become well-known for its intense sweetness (250-300 times sweeter than sucrose) and is used as a non-caloric sweetener in several countries. A number of studies have suggested that, beside sweetness, stevioside along with related compounds, which include rebaudioside A (second most abundant component of S. rebaudiana leaf), steviol and isosteviol (metabolic components of stevioside) may also offer therapeutic benefits, as they have anti-hyperglycemic, anti-hypertensive, anti-inflammatory, anti-tumor, anti-diarrheal, diuretic, and immunomodulatory actions. It is of interest to note that their effects on plasma glucose level and blood pressure are only observed when these parameters are higher than normal. As steviol can interact with drug transporters, its role as a drug modulator is proposed. This review summarizes the current knowledge of the pharmacological actions, therapeutic applications, pharmacokinetics and safety of stevioside and related compounds. Although much progress has been made concerning their biological and pharmacological effects, questions regarding chemical purity and safety remain unsolved. These issues are discussed to help guide future research directions.

Thiazolidinone CFTR inhibitors with improved water solubility identified by structure-activity analysis

The thiazolidinone 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone (CFTR(inh)-172) inhibits cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel conductance with submicromolar affinity and blocks cholera toxin-induced intestinal fluid secretion. Fifty-eight CFTR(inh)-172 analogs were synthesized to identify CFTR inhibitors with improved water solubility, exploring modifications in its two phenyl rings, thiazolidinone core, and core-phenyl connectors. Greatest CFTR inhibition potency was found for 3-CF(3) and polar group-substituted-phenyl rings, and a thiazolidinone core. Two compounds with approximately 1muM CFTR inhibition potency and solubility >180 microM (>10-fold more than CFTR(inh)-172) were identified: Tetrazolo-172, containing 4-tetrazolophenyl in place of 4-carboxyphenyl, and Oxo-172, containing thiazolidinedione in place of the thiazolidinone core. These water soluble thiazolidinone analogs had low cellular toxicity. The improved water solubility of Tetrazolo- and Oxo-172 make them potential lead candidates for therapy of secretory diarrheas and polycystic kidney disease.

Chloride channels as drug targets

Chloride channels represent a relatively under-explored target class for drug discovery as elucidation of their identity and physiological roles has lagged behind that of many other drug targets. Chloride channels are involved in a wide range of biological functions, including epithelial fluid secretion, cell-volume regulation, neuroexcitation, smooth-muscle contraction and acidification of intracellular organelles. Mutations in several chloride channels cause human diseases, including cystic fibrosis, macular degeneration, myotonia, kidney stones, renal salt wasting and hyperekplexia. Chloride-channel modulators have potential applications in the treatment of some of these disorders, as well as in secretory diarrhoeas, polycystic kidney disease, osteoporosis and hypertension. Modulators of GABA(A) (gamma-aminobutyric acid A) receptor chloride channels are in clinical use and several small-molecule chloride-channel modulators are in preclinical development and clinical trials. Here, we discuss the broad opportunities that remain in chloride-channel-based drug discovery.