Evaluation of Second Generation Amiloride Analogs as Therapy for Cystic Fibrosis Lung Disease

Structure-Activity Relationship Study Identifies a Novel Lipophilic Amiloride Derivative that Efficiently Kills Chemoresistant Breast Cancer Cells

Derivatives of the potassium-sparing diuretic amiloride are preferentially cytotoxic toward tumor cells relative to normal cells, and have the capacity to target tumor cell populations resistant to currently employed therapeutic agents. However, a major barrier to clinical translation of the amilorides is their modest cytotoxic potency, with estimated IC 50 values in the high micromolar range. Here we report the synthesis of ten novel amiloride derivatives and the characterization of their cytotoxic potency toward MCF7 (ER/PR-positive), SKBR3 (HER2-positive) and MDA-MB-231 (triple negative) cell line models of breast cancer. Comparisons of derivative structure with cytotoxic potency toward these cell lines underscore the importance of an intact guanidine group, and uncover a strong link between drug-induced cytotoxicity and drug lipophilicity. We demonstrate that our most potent derivative called LLC1 is preferentially cytotoxic toward mouse mammary tumor over normal epithelial organoids, acts in the single digit micromolar range on breast cancer cell line models representing all major subtypes, acts on cell lines that exhibit both transient and sustained resistance to chemotherapeutic agents, but exhibits limited anti-tumor effects in a mouse model of metastatic breast cancer. Nonetheless, our observations offer a roadmap for the future optimization of amiloride-based compounds with preferential cytotoxicity toward breast tumor cells.

Preclinical evaluation of the epithelial sodium channel inhibitor AZD5634 and implications on human translation

Airway dehydration causes mucus stasis and bacterial overgrowth in cystic fibrosis (CF), resulting in recurrent respiratory infections and exacerbations. Strategies to rehydrate airway mucus including inhibition of the epithelial sodium channel (ENaC) have the potential to improve mucosal defense by enhancing mucociliary clearance (MCC) and reducing the risk of progressive decline in lung function. In the current work, we evaluated the effects of AZD5634, an ENaC inhibitor that shows extended lung retention and safety profile as compared with previously evaluated candidate drugs, in healthy and CF preclinical model systems. We found that AZD5634 elicited a potent inhibition of amiloride-sensitive current in non-CF airway cells and airway cells derived from F508del-homozygous individuals with CF that effectively increased airway surface liquid volume and improved mucociliary transport (MCT) rate. AZD5634 also demonstrated efficacious inhibition of ENaC in sheep bronchial epithelial cells, translating to dose-dependent improvement of mucus clearance in healthy sheep in vivo. Conversely, nebulization of AZD5634 did not notably improve airway hydration or MCT in CF rats that exhibit an MCC defect, consistent with findings from a first single-dose evaluation of AZD5634 on MCC in people with CF. Overall, these findings suggest that CF animal models demonstrating impaired mucus clearance translatable to the human situation may help to successfully predict and promote the successful translation of ENaC-directed therapies to the clinic.

Biphasic regulation of CFTR expression by ENaC in epithelial cells: The involvement of Ca2+-modulated cAMP production

The regulatory interaction between two typical epithelial ion channels, cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial sodium channel (ENaC), for epithelial homeostasis has been noted, although the underlying mechanisms remain unclear. Here, we report that in a human endometrial epithelial cell line (ISK), shRNA-based stable knockdown of ENaC produced a biphasic effect: a low (∼23%) degree of ENaC knockdown resulted in significant increases in CFTR mRNA and protein levels, CFTR-mediated Cl⁻ transport activity as well as intracellular cAMP concentration, while a higher degree (∼50%) of ENaC knockdown did not further increase but restored CFTR expression and cAMP levels. The basal intracellular Ca²⁺ level of ISK cells was lowered by ENaC knockdown or inhibition in a degree-dependent manner. BAPTA-AM, an intracellular Ca²⁺ chelator that lowers free Ca²⁺ concentration, elevated cAMP level and CFTR mRNA expression at a low (5 µM) but not a high (50 µM) dose, mimicking the biphasic effect of ENaC knockdown. Moreover, KH-7, a selective inhibitor of soluble adenylyl cyclase (sAC), abolished the CFTR upregulation induced by low-degree ENaC knockdown or Ca²⁺ chelation, suggesting the involvement of sAC-driven cAMP production in the positive regulation. A luciferase reporter to indicate CFTR transcription revealed that all tested degrees of ENaC knockdown/inhibition stimulated CFTR transcription in ISK cells, suggesting that the negative regulation on CFTR expression by the high-degree ENaC deficiency might occur at post-transcription stages. Additionally, similar biphasic effect of ENaC knockdown on CFTR expression was observed in a human bronchial epithelial cell line. Taken together, these results have revealed a previously unidentified biphasic regulatory role of ENaC in tuning CFTR expression involving Ca²⁺-modulated cAMP production, which may provide an efficient mechanism for dynamics and plasticity of the epithelial tissues in various physiological or pathological contexts.

High-Throughput Functional Analysis of CFTR and Other Apically Localized Proteins in iPSC-Derived Human Intestinal Organoids

Induced Pluripotent Stem Cells (iPSCs) can be differentiated into epithelial organoids that recapitulate the relevant context for CFTR and enable testing of therapies targeting Cystic Fibrosis (CF)-causing mutant proteins. However, to date, CF-iPSC-derived organoids have only been used to study pharmacological modulation of mutant CFTR channel activity and not the activity of other disease-relevant membrane protein constituents. In the current work, we describe a high-throughput, fluorescence-based assay of CFTR channel activity in iPSC-derived intestinal organoids and describe how this method can be adapted to study other apical membrane proteins. Specifically, we show how this assay can be employed to study CFTR and ENaC channels and an electrogenic acid transporter in the same iPSC-derived intestinal tissue. This phenotypic platform promises to expand CF therapy discovery to include strategies that target multiple determinants of epithelial fluid transport.

Small-molecule drugs for cystic fibrosis: Where are we now?

The cystic fibrosis (CF) lung disease is due to the lack/dysfunction of the CF Transmembrane Conductance Regulator (CFTR), a chloride channel expressed by epithelial cells as the main regulator of ion and fluid homeostasis. More than 2000 genetic variation in the CFTR gene are known, among which those with identified pathomechanism have been divided into six VI mutation classes. A major advancement in the pharmacotherapy of CF has been the development of small-molecule drugs hitting the root of the disease, i.e. the altered ion and fluid transport through the airway epithelium. These drugs, called CFTR modulators, have been advanced to the clinics to treat nearly 90% of CF patients, including the CFTR potentiator ivacaftor, approved for residual function mutations (Classes III and IV), and combinations of correctors (lumacaftor, tezacaftor, elexacaftor) and ivacaftor for patients bearing at least one the F508del mutation, the most frequent mutation belonging to class II. To cover the 10% of CF patients without etiological therapies, other novel small-molecule CFTR modulators are in evaluation of their effectiveness in all the CFTR mutation classes: read-through agents for Class I, correctors, potentiators and amplifiers from different companies for Class II-V, stabilizers for Class VI. In alternative, other solute carriers, such as SLC26A9 and SLC6A14, are the focus of intensive investigation. Finally, other molecular targets are being evaluated for patients with no approved CFTR modulator therapy or as means of enhancing CFTR modulatory therapy, including small molecules forming ion channels, inhibitors of the ENaC sodium channel and potentiators of the calcium-activated chloride channel TMEM16A. This paper aims to give an up-to-date overview of old and novel CFTR modulators as well as of novel strategies based on small-molecule drugs. Further investigations in in-vivo and cell-based models as well as carrying out large prospective studies will be required to determine if novel CFTR modulators, stabilizers, amplifiers, and the ENaC inhibitors or TMEM16A potentiators will further improve the clinical outcomes in CF management.

First clinical trials of the inhaled epithelial sodium channel inhibitor BI 1265162 in healthy volunteers

Background

Inhibition of the epithelial sodium channel (ENaC) represents a mutation-agnostic therapeutic approach to restore airway surface liquid hydration and mucociliary clearance in patients with cystic fibrosis. BI 1265162 is an inhaled ENaC inhibitor with demonstrated preclinical efficacy.

Methods

Three phase I trials of BI 1265162 in healthy male subjects are presented: NCT03349723 (single-rising-dose trial evaluating safety, tolerability and pharmacokinetics (PK)); NCT03576144 (multiple-rising-dose trial evaluating safety, tolerability and PK); and NCT03907280 (absolute bioavailability trial).

Results

BI 1265162 single doses ≤1200 µg and multiple doses of 600 µg were well tolerated. Adverse events were balanced across treatment groups, were of mainly mild or moderate intensity and resolved by trial-end. One subject discontinued from trial medication on day 7 (asymptomatic hyperkalaemia adverse event; recovered day 8). One subject experienced a serious adverse event (neuropathia vestibularis) leading to hospitalisation and missed one of the four dosing periods. Both events were not considered to be drug-related and subjects recovered. BI 1265162 displayed dose-proportional, time-independent PK; maximum accumulation was 1.6-fold; calculated effective elimination half-life was 3.6–8.7 h over the dose ranges tested. Renal excretion was not a major drug elimination route. Oral and inhaled dosing (±activated oral charcoal) absolute bioavailability was 0.50% and ∼40%, respectively.

Conclusion

BI 1265162 single or multiple doses up to 6.5 days were well tolerated. Systemic exposures mainly represent drug absorbed through the lungs and not the gastrointestinal tract, with ∼40% of the inhaled dose reaching the systemic circulation. Accumulation was minimal. Twice-daily dosing is supported for future development.

Cell and animal studies have demonstrated that BI 1265162 is a potent ENaC inhibitor. Three phase I trials show that single- and multiple-dose BI 1265162 is safe. BI 1265162 is being tested in phase II studies, using twice-daily dosing, in people with CF.https://bit.ly/3nPUkrO

ENaC inhibition in cystic fibrosis: potential role in the new era of CFTR modulator therapies

Small-molecule cystic fibrosis transmembrane conductance regulator (CFTR) modulator drugs for cystic fibrosis are the first therapies since the disease was initially described by Fanconiet al. [1] in 1936 to target and partially restore the function of the CFTR Cl⁻ channel. CFTR modulator therapy is expected to have significant clinical benefits for many, but it does not result in a cure and is not appropriate or available for all patients with cystic fibrosis [2, 3]. In this review, evidence is described suggesting that inhibiting the epithelial Na⁺ channel (ENaC) responsible for the Na⁺/fluid absorption that contributes to airway surface dehydration and impaired mucociliary clearance (MCC) observed in cystic fibrosis airways may significantly improve clinical outcomes irrespective of the CFTR genotype, and may synergise with currently approved CFTR modulators to further improve clinical outcomes.

ENaC inhibition with BI 1265162 is a promising strategy to optimise outcomes in patients with CF either eligible, or ineligible, for CFTR modulator therapy. Phase II clinical trials of BI 1265162 must now show this translates into clinical benefit.https://bit.ly/2OQ1IUI

Lack of Kcnn4 improves mucociliary clearance in muco-obstructive lung disease

Airway mucociliary clearance (MCC) is the main mechanism of lung defense keeping airways free of infection and mucus obstruction. Airway surface liquid volume, ciliary beating, and mucus are central for proper MCC and critically regulated by sodium absorption and anion secretion. Impaired MCC is a key feature of muco-obstructive diseases. The calcium-activated potassium channel KCa.3.1, encoded by Kcnn4, participates in ion secretion, and studies showed that its activation increases Na⁺ absorption in airway epithelia, suggesting that KCa3.1-induced hyperpolarization was sufficient to drive Na⁺ absorption. However, its role in airway epithelium is not fully understood. We aimed to elucidate the role of KCa3.1 in MCC using a genetically engineered mouse. KCa3.1 inhibition reduced Na⁺ absorption in mouse and human airway epithelium. Furthermore, the genetic deletion of Kcnn4 enhanced cilia beating frequency and MCC ex vivo and in vivo. Kcnn4 silencing in the Scnn1b-transgenic mouse (Scnn1b^tg/+), a model of muco-obstructive lung disease triggered by increased epithelial Na⁺ absorption, improved MCC, reduced Na⁺ absorption, and did not change the amount of mucus but did reduce mucus adhesion, neutrophil infiltration, and emphysema. Our data support that KCa3.1 inhibition attenuated muco-obstructive disease in the Scnn1b^tg/+ mice. K⁺ channel modulation may be a therapeutic strategy to treat muco-obstructive lung diseases.

Silencing the calcium-activated potassium channel KCa.3.1 improves mucociliary clearance in muco-obstructive lung disease by decreasing sodium absorption in the airways.

Cystic Fibrosis: Overview of the Current Development Trends and Innovative Therapeutic Strategies

Cystic Fibrosis (CF), an autosomal recessive genetic disease, is caused by a mutation in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). This mutation reduces the release of chloride ions (Cl-) in epithelial tissues, and hyperactivates the epithelial sodium channels (ENaC) which aid in the absorption of sodium ions (Na+). Consequently, the mucus becomes dehydrated and thickened, making it a suitable medium for microbial growth. CF causes several chronic lung complications like thickened mucus, bacterial infection and inflammation, progressive loss of lung function, and ultimately, death. Until recently, the standard of clinical care in CF treatment had focused on preventing and treating the disease complications. In this review, we have summarized the current knowledge on CF pathogenesis and provided an outlook on the current therapeutic approaches relevant to CF (i.e., CFTR modulators and ENaC inhibitors). The enormous potential in targeting bacterial biofilms using antibiofilm peptides, and the innovative therapeutic strategies in using the CRISPR/Cas approach as a gene-editing tool to repair the CFTR mutation have been reviewed. Finally, we have discussed the wide range of drug delivery systems available, particularly non-viral vectors, and the optimal properties of nanocarriers which are essential for successful drug delivery to the lungs.

Losartan Rescues Inflammation-related Mucociliary Dysfunction in Relevant Models of Cystic Fibrosis

Rationale: Despite therapeutic progress in treating cystic fibrosis (CF) airway disease, airway inflammation with associated mucociliary dysfunction remains largely unaddressed. Inflammation reduces the activity of apically expressed large-conductance Ca2+-activated and voltage-dependent K+ (BK) channels, critical for mucociliary function in the absence of CFTR (CF transmembrane conductance regulator).Objectives: To test losartan as an antiinflammatory therapy in CF using CF human bronchial epithelial cells and an ovine model of CF-like airway disease.Methods: Losartan's antiinflammatory effectiveness to rescue BK activity and thus mucociliary function was tested in vitro using primary, fully redifferentiated human airway epithelial cells homozygous for F508del and in vivo using a previously validated, now expanded pharmacologic sheep model of CF-like, inflammation-associated mucociliary dysfunction.Measurements and Main Results: Nasal scrapings from patients with CF showed that neutrophilic inflammation correlated with reduced expression of LRRC26 (leucine rich repeat containing 26), the γ subunit mandatory for BK function in the airways. TGF-β1 (transforming growth factor β1), downstream of neutrophil elastase, decreased mucociliary parameters in vitro. These were rescued by losartan at concentrations achieved by nebulization in the airway and oral application in the bloodstream: BK dysfunction recovered acutely and over time (the latter via an increase in LRRC26 expression), ciliary beat frequency and airway surface liquid volume improved, and mucus hyperconcentration and cellular inflammation decreased. These effects did not depend on angiotensin receptor blockade. Expanding on a validated and published nongenetic, CF-like sheep model, ewes inhaled CFTRinh172 and neutrophil elastase for 3 days, which resulted in prolonged tracheal mucus velocity reduction, mucus hyperconcentration, and increased TGF-β1. Nebulized losartan rescued both mucus transport and mucus hyperconcentration and reduced TGF-β1.Conclusions: Losartan effectively reversed CF- and inflammation-associated mucociliary dysfunction, independent of its angiotensin receptor blockade.

Whole body periodic acceleration in normal and reduced mucociliary clearance of conscious sheep

The purpose of this investigation was to ascertain whether nitric oxide (NO) released into the circulation by a noninvasive technology called whole body periodic acceleration (WBPA) could increase mucociliary clearance (MCC). It was based on observations by others that nitric oxide donor drugs increase ciliary beat frequency of nasal epithelium without increasing mucociliary clearance. Tracheal mucous velocity (TMV), a reflection of MCC, was measured in sheep after 1-hour treatment of WBPA and repeated after pretreatment with the NO synthase inhibitor, L-NAME to demonstrated action of NO. Aerosolized human neutrophil elastase (HNE) was administered to sheep to suppress TMV as might occur in cystic fibrosis and other inflammatory lung diseases. WBPA increased TMV to a peak of 136% of baseline 1h after intervention, an effect blocked by L-NAME. HNE reduced TMV to 55% of baseline but slowing was reversed by WBPA, protection lost in the presence of L-NAME. NO released into the circulation from eNOS by WBPA can acutely access airway epithelium for improving MCC slowed in cystic fibrosis and other inflammatory lung diseases as a means of enhancing host defense against pathogens.

First clinical trials of novel ENaC targeting therapy, SPX-101, in healthy volunteers and adults with cystic fibrosis

BACKGROUND

ENaC inhibition has been investigated as a CF treatment; however, small molecule inhibitors of ENaC lack efficacy and/or have shown dose-limiting hyperkalemia. SPX-101 is a novel, investigational small peptide (SPLUNC1 mimetic) that regulates ENaC density with the potential for efficacy without systemic effects.

METHODS

Two trials are presented: The first was a Phase 1, 2-part, randomized, double-blind, placebo-controlled, ascending-dose study of nebulized SPX-101 in healthy adults. Part 1 evaluated 4 single doses of SPX-101 ranging from 20 to 240 mg. Part 2 evaluated a 14-day regimen of SPX-101 at 4 doses of SPX-101 ranging from 10 to 120 mg BID. Pharmacokinetics, adverse events, spirometry, vital signs, electrocardiograms, pulse oximetry, and clinical laboratory values were assessed. The second trial was a tolerability-confirming, Phase 1b, open-label study conducted in 5 adult subjects with CF. Ascending doses of SPX-101 inhalation solution (10 mg-120 mg BID) were administered for 7 days. Safety was assessed as described above.

RESULTS

All 64 healthy volunteers (32 in each Part) completed the single and multiple dose study. SPX-101 was well tolerated with little/no systemic exposure and with no hyperkalemia. Adverse events were generally mild with reported respiratory events associated with the purported pharmacological activity of SPX-101. Tolerability of SPX-101 was similarly observed in adults with CF; all 5 subjects treated with SPX-101 completed the study.

CONCLUSIONS

SPX-101 was well-tolerated across a range of doses and had little/no systemic exposure in healthy adults and adults with CF, thus supporting further study in patients with CF. CLINICALTRIAL.

GOV REGISTRATION

NCT03056989.

Cystic Fibrosis: Proteostatic correctors of CFTR trafficking and alternative therapeutic targets

Introduction: Cystic fibrosis (CF) is the most frequent lethal orphan disease and is caused by mutations in the CFTR gene. The most frequent mutation F508del-CFTR affects multiple organs; infections and subsequent infections and complications in the lung lead to death. Areas covered: This review focuses on new targets and mechanisms that are attracting interest for the development of CF therapies. The F508del-CFTR protein is retained in the endoplasmic reticulum (ER) but has some function if it can traffic to the plasma membrane. Cell-based assays have been used to screen chemical libraries for small molecule correctors that restore its trafficking. Pharmacological chaperones are correctors that bind directly to the F508del-CFTR mutant and promote its folding and trafficking. Other correctors fall into a heterogeneous class of proteostasis modulators that act indirectly by altering cellular homeostasis. Expert opinion: Pharmacological chaperones have so far been the most successful correctors of F508del-CFTR trafficking, but their level of correction means that more than one corrector is required. Proteostasis modulators have low levels of correction but hold promise because some can correct several different CFTR mutations. Identification of their cellular targets and the potential for development may lead to new therapies for CF.

The epithelial sodium channel (ENaC) as a therapeutic target for cystic fibrosis

Cystic fibrosis (CF) is a monogenic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR dysfunction is characterized by abnormal mucociliary transport due to a dehydrated airway surface liquid (ASL) and hyperviscous mucus, among other pathologies of host defense. ASL depletion is caused by the absence of CFTR mediated chloride secretion along with continued activity of the epithelial sodium channel (ENaC) activity, which can also be affected by CFTR mediated anion conductance. Therefore, ENaC has been proposed as a therapeutic target to ameliorate ASL dehydration and improve mucus transport. Inhibition of ENaC has been shown to restore ASL hydration and enhance mucociliary transport in induced models of CF lung disease. To date, no therapy inhibiting ENaC has successfully translated to clinical efficacy, in part due to concerns regarding off-target effects, systemic exposure, durability of effect, and adverse effects. Recent efforts have been made to develop novel, rationally designed therapeutics to produce-specific, long-lasting inhibition of ENaC activity in the airways while simultaneously minimizing off target fluid transport effects, systemic exposure and side effects. Such approaches comprise next-generation small molecule direct inhibitors, indirect channel-activating protease inhibitors, synthetic peptide analogs, and oligonucleotide-based therapies. These novel therapeutics represent an exciting step forward in the development of ENaC-directed therapies for CF.

Antisense oligonucleotide targeting of mRNAs encoding ENaC subunits α, β, and γ improves cystic fibrosis-like disease in mice

BACKGROUND

The epithelial sodium channel ENaC consists of three subunits encoded by Scnn1a, Scnn1b, and Scnn1g and increased sodium absorption through this channel is hypothesized to lead to mucus dehydration and accumulation in cystic fibrosis (CF) patients.

METHODS

We identified potent and specific antisense oligonucleotides (ASOs) targeting mRNAs encoding the ENaC subunits and evaluated these ASOs in mouse models of CF-like lung disease.

RESULTS

ASOs designed to target mRNAs encoding each ENaC subunit or a control ASO were administered directly into the lungs of mice. The reductions in ENaC subunits correlated well with a reduction in amiloride sensitive channel conductance. In addition, levels of mucus markers Gob5, AGR2, Muc5ac, and Muc5b, periodic acid-Schiff's reagent (PAS) goblet cell staining, and neutrophil recruitment were reduced and lung function was improved when levels of any of the ENaC subunits were decreased.

CONCLUSIONS

Delivery of ASOs targeting mRNAs encoding each of the three ENaC subunits directly into the lung improved disease phenotypes in a mouse model of CF-like lung disease. These findings suggest that targeting ENaC subunits could be an effective approach for the treatment of CF.

Recent advances in developing therapeutics for cystic fibrosis

Despite hope that a cure was imminent when the causative gene was cloned nearly 30 years ago, cystic fibrosis (CF [MIM: 219700]) remains a life-shortening disease affecting more than 70 000 individuals worldwide. However, within the last 6 years the Food and Drug Administration's approval of Ivacaftor, the first drug that corrects the defective cystic fibrosis transmembrane conductance regulator protein [CFTR (MIM: 602421)] in patients with the G551D mutation, marks a watershed in the development of novel therapeutics for this devastating disease. Here we review recent progress in diverse research areas, which all focus on curing CF at the genetic, biochemical or physiological level. In the near future it seems probable that development of mutation-specific therapies will be the focus, since it is unlikely that any one approach will be efficient in correcting the more than 2000 disease-associated variants. We discuss the new drugs and combinations of drugs that either enhance delivery of misfolded CFTR protein to the cell membrane, where it functions as an ion channel, or that activate channel opening. Next we consider approaches to correct the causative genetic lesion at the DNA or RNA level, through repressing stop mutations and nonsense-mediated decay, modulating splice mutations, fixing errors by gene editing or using novel routes to gene replacement. Finally, we explore how modifier genes, loci elsewhere in the genome that modify CF disease severity, may be used to restore a normal phenotype. Progress in all of these areas has been dramatic, generating enthusiasm that CF may soon become a broadly treatable disease.

Effective silencing of ENaC by siRNA delivered with epithelial-targeted nanocomplexes in human cystic fibrosis cells and in mouse lung

INTRODUCTION

Loss of the cystic fibrosis transmembrane conductance regulator in cystic fibrosis (CF) leads to hyperabsorption of sodium and fluid from the airway due to upregulation of the epithelial sodium channel (ENaC). Thickened mucus and depleted airway surface liquid (ASL) then lead to impaired mucociliary clearance. ENaC regulation is thus a promising target for CF therapy. Our aim was to develop siRNA nanocomplexes that mediate effective silencing of airway epithelial ENaC in vitro and in vivo with functional correction of epithelial ion and fluid transport.

METHODS

We investigated translocation of nanocomplexes through mucus and their transfection efficiency in primary CF epithelial cells grown at air-liquid interface (ALI).Short interfering RNA (SiRNA)-mediated silencing was examined by quantitative RT-PCR and western analysis of ENaC. Transepithelial potential (V_t), short circuit current (I_sc), ASL depth and ciliary beat frequency (CBF) were measured for functional analysis. Inflammation was analysed by histological analysis of normal mouse lung tissue sections.

RESULTS

Nanocomplexes translocated more rapidly than siRNA alone through mucus. Transfections of primary CF epithelial cells with nanocomplexes targeting αENaC siRNA, reduced αENaC and βENaC mRNA by 30%. Transfections reduced V_t, the amiloride-sensitive I_sc and mucus protein concentration while increasing ASL depth and CBF to normal levels. A single dose of siRNA in mouse lung silenced ENaC by approximately 30%, which persisted for at least 7 days. Three doses of siRNA increased silencing to approximately 50%.

CONCLUSION

Nanoparticle-mediated delivery of ENaCsiRNA to ALI cultures corrected aspects of the mucociliary defect in human CF cells and offers effective delivery and silencing in vivo.

Chitosan in Non-Viral Gene Delivery: Role of Structure, Characterization Methods, and Insights in Cancer and Rare Diseases Therapies

Non-viral gene delivery vectors have lagged far behind viral ones in the current pipeline of clinical trials of gene therapy nanomedicines. Even when non-viral nanovectors pose less safety risks than do viruses, their efficacy is much lower. Since the early studies to deliver pDNA, chitosan has been regarded as a highly attractive biopolymer to deliver nucleic acids intracellularly and induce a transgenic response resulting in either upregulation of protein expression (for pDNA, mRNA) or its downregulation (for siRNA or microRNA). This is explained as the consequence of a multi-step process involving condensation of nucleic acids, protection against degradation, stabilization in physiological conditions, cellular internalization, release from the endolysosome ("proton sponge" effect), unpacking and enabling the trafficking of pDNA to the nucleus or the siRNA to the RNA interference silencing complex (RISC). Given the multiple steps and complexity involved in the gene transfection process, there is a dearth of understanding of the role of chitosan's structural features (Mw and degree of acetylation, DA%) on each step that dictates the net transfection efficiency and its kinetics. The use of fully characterized chitosan samples along with the utilization of complementary biophysical and biological techniques is key to bridging this gap of knowledge and identifying the optimal chitosans for delivering a specific gene. Other aspects such as cell type and administration route are also at play. At the same time, the role of chitosan structural features on the morphology, size and surface composition of synthetic virus-like particles has barely been addressed. The ongoing revolution brought about by the recent discovery of CRISPR-Cas9 technology will undoubtedly be a game changer in this field in the short term. In the field of rare diseases, gene therapy is perhaps where the greatest potential lies and we anticipate that chitosans will be key players in the translation of research to the clinic.

The Epithelial Sodium Channel Is a Modifier of the Long-Term Nonprogressive Phenotype Associated with F508del CFTR Mutations

Cystic fibrosis (CF) remains the most lethal genetic disease in the Caucasian population. However, there is great variability in clinical phenotypes and survival times, even among patients harboring the same genotype. We identified five patients with CF and a homozygous F508del mutation in the CFTR gene who were in their fifth or sixth decade of life and had shown minimal changes in lung function over a longitudinal period of more than 20 years. Because of the rarity of this long-term nonprogressive phenotype, we hypothesized these individuals may carry rare genetic variants in modifier genes that ameliorate disease severity. Individuals at the extremes of survival time and lung-function trajectory underwent whole-exome sequencing, and the sequencing data were filtered to include rare missense, stopgain, indel, and splicing variants present with a mean allele frequency of <0.2% in general population databases. Epithelial sodium channel (ENaC) mutants were generated via site-directed mutagenesis and expressed for Xenopus oocyte assays. Four of the five individuals carried extremely rare or never reported variants in the SCNN1D and SCNN1B genes of the ENaC. Separately, an independently enriched rare variant in SCNN1D was identified in the Exome Variant Server database associated with a milder pulmonary disease phenotype. Functional analysis using Xenopus oocytes revealed that two of the three variants in δ-ENaC encoded by SCNN1D exhibited hypomorphic channel activity. Our data suggest a potential role for δ-ENaC in controlling sodium reabsorption in the airways, and advance the plausibility of ENaC as a therapeutic target in CF.

Nasospheroids permit measurements of CFTR-dependent fluid transport

Expansion of novel therapeutics to all patients with cystic fibrosis (CF) requires personalized CFTR modulator therapy. We have developed nasospheroids, a primary cell culture-based model derived from individual CF patients and healthy subjects by a minimally invasive nasal biopsy. Confocal microscopy was utilized to measure CFTR activity by analyzing changes in cross-sectional area over time that resulted from CFTR-mediated ion and fluid movement. Both the rate of change over time and AUC were calculated. Non-CF nasospheroids with active CFTR-mediated ion and fluid movement showed a reduction in cross-sectional area, whereas no changes were observed in CF spheroids. Non-CF spheroids treated with CFTR inhibitor lost responsiveness for CFTR activation. However, nasospheroids from F508del CF homozygotes that were treated with lumacaftor and ivacaftor showed a significant reduction in cross-sectional area, indicating pharmacologic rescue of CFTR function. This model employs a simple measurement of size corresponding to changes in CFTR activity and is applicable for detection of small changes in CFTR activity from individual patients in vitro. Advancements of this technique will provide a robust model for individualized prediction of CFTR modulator efficacy.

Nonclinical safety assessment of SPX-101, a novel peptide promoter of epithelial sodium channel internalization for the treatment of cystic fibrosis

BACKGROUND

ENaC inhibition has long been an attractive therapeutic target for the treatment of cystic fibrosis. However, previous attempts at developing ENaC inhibitors have been unsuccessful due to complications arising from systemic circulation of the compounds. Here, we describe the preclinical toxicology assessment of a new inhaled peptide promoter of ENaC internalization delivered as a nebulized aerosol.

METHODS

Preclinical assessment of SPX-101 safety was determined using an in vitro hERG assay, bolus injection of SPX-101 in a canine cardiovascular and respiratory safety pharmacology model and 28-day inhalation toxicology studies of nebulized drug in rats and dogs.

RESULTS

SPX101 had no effects on the respiratory, cardiac or central nervous systems. The 28-day inhalation toxicology studies of nebulized SPX-101 in rats and dogs revealed no drug-related adverse events. Plasma levels of SPX-101 peaked less than 1 h after the end of treatment in rats and were below the limit of detection in canine models.

CONCLUSIONS

SPX-101, a novel peptide promoter of ENaC internalization, elicited no adverse effects at doses up to the MFD and in excess of the highest preclinical efficacious and expected clinical doses. In contrast to channel blockers like amiloride and derivative small molecules, SPX-101 does not achieve significant systemic circulation, thus doses are not limited due to toxic side effects like hyperkalemia and weight loss.

Evaluation of a SPLUNC1-derived peptide for the treatment of cystic fibrosis lung disease

In cystic fibrosis (CF) lungs, epithelial Na⁺ channel (ENaC) hyperactivity causes a reduction in airway surface liquid volume, leading to decreased mucocilliary clearance, chronic bacterial infection, and lung damage. Inhibition of ENaC is an attractive therapeutic option. However, ENaC antagonists have failed clinically because of off-target effects in the kidney. The S18 peptide is a naturally occurring short palate lung and nasal epithelial clone 1 (SPLUNC1)-derived ENaC antagonist that restores airway surface liquid height for up to 24 h in CF human bronchial epithelial cultures. However, its efficacy and safety in vivo are unknown. To interrogate the potential clinical efficacy of S18, we assessed its safety and efficacy using human airway cultures and animal models. S18-mucus interactions were tested using superresolution microscopy, quartz crystal microbalance with dissipation, and confocal microscopy. Human and murine airway cultures were used to measure airway surface liquid height. Off-target effects were assessed in conscious mice and anesthetized rats. Morbidity and mortality were assessed in the β-ENaC-transgenic (Tg) mouse model. Restoration of normal mucus clearance was measured in cystic fibrosis transmembrane conductance regulator inhibitor 172 [CFTR(inh)-172]-challenged sheep. We found that S18 does not interact with mucus and rapidly penetrated dehydrated CF mucus. Compared with amiloride, an early generation ENaC antagonist, S18 displayed a superior ability to slow airway surface liquid absorption, reverse CFTR(inh)-172-induced reduction of mucus transport, and reduce morbidity and mortality in the β-ENaC-Tg mouse, all without inducing any detectable signs of renal toxicity. These data suggest that S18 is the first naturally occurring ENaC antagonist to show improved preclinical efficacy in animal models of CF with no signs of renal toxicity.

SPX-101 Is a Novel Epithelial Sodium Channel-targeted Therapeutic for Cystic Fibrosis That Restores Mucus Transport

RATIONALE

Cystic fibrosis (CF) lung disease is caused by the loss of function of the cystic fibrosis transmembrane conductance regulator (CFTR) combined with hyperactivation of the epithelial sodium channel (ENaC). In the lung, ENaC is responsible for movement of sodium. Hyperactivation of ENaC, which creates an osmotic gradient that pulls fluid out of the airway, contributes to reduced airway hydration, causing mucus dehydration, decreased mucociliary clearance, and recurrent acute bacterial infections. ENaC represents a therapeutic target to treat all patients with CF independent of their underlying CFTR mutation.

OBJECTIVES

To investigate the in vitro and in vivo efficacy of SPX-101, a peptide mimetic of the natural regulation of ENaC activity by short palate, lung, and nasal epithelial clone 1, known as SPLUNC1.

METHODS

ENaC internalization by SPX-101 in primary human bronchial epithelial cells from healthy and CF donors was assessed by surface biotinylation and subsequent Western blot analysis. SPX-101's in vivo therapeutic effect was assessed by survival of β-ENaC-transgenic mice, mucus transport in these mice, and mucus transport in a sheep model of CF.

MEASUREMENTS AND MAIN RESULTS

SPX-101 binds selectively to ENaC and promotes internalization of the α-, β-, and γ-subunits. Removing ENaC from the membrane with SPX-101 causes a significant decrease in amiloride-sensitive current. The peptide increases survival of β-ENaC-transgenic mice to greater than 90% with once-daily dosing by inhalation. SPX-101 increased mucus transport in the β-ENaC mouse model as well as the sheep model of CF.

CONCLUSIONS

These data demonstrate that SPX-101 promotes durable reduction of ENaC membrane concentration, leading to significant improvements in mucus transport.

Fluorescent aminoglycosides reveal intracellular trafficking routes in mechanosensory hair cells

Aminoglycosides (AGs) are broad-spectrum antibiotics that are associated with kidney damage, balance disorders, and permanent hearing loss. This damage occurs primarily by killing of proximal tubule kidney cells and mechanosensory hair cells, though the mechanisms underlying cell death are not clear. Imaging molecules of interest in living cells can elucidate how molecules enter cells, traverse intracellular compartments, and interact with sites of activity. Here, we have imaged fluorescently labeled AGs in live zebrafish mechanosensory hair cells. We determined that AGs enter hair cells via both nonendocytic and endocytic pathways. Both routes deliver AGs from the extracellular space to lysosomes, and structural differences between AGs alter the efficiency of this delivery. AGs with slower delivery to lysosomes were immediately toxic to hair cells, and impeding lysosome delivery increased AG-induced death. Therefore, pro-death cascades induced at early time points of AG exposure do not appear to derive from the lysosome. Our findings help clarify how AGs induce hair cell death and reveal properties that predict toxicity. Establishing signatures for AG toxicity may enable more efficient evaluation of AG treatment paradigms and structural modifications to reduce hair cell damage. Further, this work demonstrates how following fluorescently labeled drugs at high resolution in living cells can reveal important details about how drugs of interest behave.

Marked increases in mucociliary clearance produced by synergistic secretory agonists or inhibition of the epithelial sodium channel

Mucociliary clearance (MCC) is a critical host innate defense mechanism in airways, and it is impaired in cystic fibrosis (CF) and other obstructive lung diseases. Epithelial fluid secretion and absorption modify MCC velocity (MCCV). We tested the hypotheses that inhibiting fluid absorption accelerates MCCV, whereas inhibiting fluid secretion decelerates it. In airways, ENaC is mainly responsible for fluid absorption, while anion channels, including CFTR and Ca²⁺-activated chloride channels mediate anion/fluid secretion. MCCV was increased by the cAMP-elevating agonists, forskolin or isoproterenol (10 μM) and by the Ca²⁺-elevating agonist, carbachol (0.3 μM). The CFTR-selective inhibitor, CFTR_inh-172, modestly reduced MCCV-increases induced by forskolin or isoproterenol but not increases induced by carbachol. The ENaC inhibitor benzamil increased basal MCCV as well as MCCV increases produced by forskolin or carbachol. MCC velocity was most dramatically accelerated by the synergistic combination of forskolin and carbachol, which produced near-maximal clearance rates regardless of prior treatment with CFTR or ENaC inhibitors. In CF airways, where CFTR-mediated secretion (and possibly synergistic MCC) is lost, ENaC inhibition via exogenous agents may provide therapeutic benefit, as has long been proposed.

CNS sites activated by renal pelvic epithelial sodium channels (ENaCs) in response to hypertonic saline in awake rats

In some patients, renal nerve denervation has been reported to be an effective treatment for essential hypertension. Considerable evidence suggests that afferent renal nerves (ARN) and sodium balance play important roles in the development and maintenance of high blood pressure. ARN are sensitive to sodium concentrations in the renal pelvis. To better understand the role of ARN, we infused isotonic or hypertonic NaCl (308 or 500mOsm) into the left renal pelvis of conscious rats for two 2hours while recording arterial pressure and heart rate. Subsequently, brain tissue was analyzed for immunohistochemical detection of the protein Fos, a marker for neuronal activation. Fos-immunoreactive neurons were identified in numerous sites in the forebrain and brainstem. These areas included the nucleus tractus solitarius (NTS), the lateral parabrachial nucleus, the paraventricular nucleus of the hypothalamus (PVH) and the supraoptic nucleus (SON). The most effective stimulus was 500mOsm NaCl. Activation of these sites was attenuated or prevented by administration of benzamil (1μM) or amiloride (10μM) into the renal pelvis concomitantly with hypertonic saline. In anesthetized rats, infusion of hypertonic saline but not isotonic saline into the renal pelvis elevated ARN activity and this increase was attenuated by simultaneous infusion of benzamil or amiloride. We propose that renal pelvic epithelial sodium channels (ENaCs) play a role in activation of ARN and, via central visceral afferent circuits, this system modulates fluid volume and peripheral blood pressure. These pathways may contribute to the development of hypertension.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Evaluating the Functionality of Conjunctiva Using a Rabbit Dry Eye Model

Purpose. To assess the conjunctival functionality in a rabbit dry eye (DE) model. Methods. Nictitating membrane, lacrimal and Harderian glands were surgically excised from male New Zealand white rabbits using minimally invasive surgery. Fluorescein/rose Bengal staining of ocular surface (OS) and Schirmer test were done before (BE) and after excision (AE). The expression of interleukin- (IL-) 1β, tumor necrosis factor- (TNF-) α, and MUC5AC proteins were estimated by immunoblotting from conjunctival impression cytology specimens. MUC5AC mRNA was quantified as well. The effect of epithelial sodium channel (ENaC) blockers on tear production and potential differences (PD) of OS were assessed under anesthesia in rabbits with and without surgery. Results. Increase in corneal and conjunctival staining was observed 1 month AE compared to BE. Schirmer tests failed to show decrease in tear production. Elevated IL-1β, and TNF-α, 1 month AE indicated inflammation. MUC5AC expression was elevated 1 month AE. ENaC blockers did not improve tear production in rabbit eyes AE but characteristic changes in PD were observed in rabbits with surgery. Conclusions. DE biomarkers are important tools for OS assessment and MUC5AC expression is elevated in rabbit DE. PD measurement revealed significant electrophysiological changes in rabbits with surgery.

Inhibition of airway surface fluid absorption by cholinergic stimulation

In upper airways airway surface liquid (ASL) depth and clearance rates are both increased by fluid secretion. Secretion is opposed by fluid absorption, mainly via the epithelial sodium channel, ENaC. In static systems, increased fluid depth activates ENaC and decreased depth inhibits it, suggesting that secretion indirectly activates ENaC to reduce ASL depth. We propose an alternate mechanism in which cholinergic input, which causes copious airway gland secretion, also inhibits ENaC-mediated absorption. The conjoint action accelerates clearance, and the increased transport of mucus out of the airways restores ASL depth while cleansing the airways. We were intrigued by early reports of cholinergic inhibition of absorption by airways in some species. To reinvestigate this phenomenon, we studied inward short-circuit currents (Isc) in tracheal mucosa from human, sheep, pig, ferret, and rabbit and in two types of cultured cells. Basal Isc was inhibited 20–70% by the ENaC inhibitor, benzamil. Long-lasting inhibition of ENaC-dependent Isc was also produced by basolateral carbachol in all preparations except rabbit and the H441 cell line. Atropine inhibition produced a slow recovery or prevented inhibition if added before carbachol. The mechanism for inhibition was not determined and is most likely multi-factorial. However, its physiological significance is expected to be increased mucus clearance rates in cholinergically stimulated airways.

Duration of action of hypertonic saline on mucociliary clearance in the normal lung

Inhalation of hypertonic saline (HS) acutely enhances mucociliary clearance (MC) in both health and disease. In patients with cystic fibrosis (CF), repeated use of HS causes a sustained improvement in MC as well as clinical benefit. The pharmacodynamic duration of activity on MC may be an important determinant of its therapeutic potential in other airways diseases. Before moving toward testing the clinical benefits of HS for non-CF indications, we sought to assess the duration of pharmacodynamic effects of HS in healthy subjects by performing radiotracer clearance studies at baseline, 30-min post-HS administration, and 4-h post-HS administration. Indeed, acceleration of MC was observed when measured 30 min after HS inhalation. This acceleration was most pronounced in the first 30 min after inhaling the radiotracer in the central lung region (mean Ave30Clr = 15.5 vs. 8.6% for 30-min post-HS treatment vs. mean baseline, respectively, P < 0.005), suggesting that acute HS effects were greatest in the larger bronchial airways. In contrast, when MC was measured 4 h after HS administration, all indices of central lung region MC were slower than at baseline: Ave30Clr = 5.9% vs. 8.6% (P = 0.10); Ave90Clr = 12.4% vs. 16.8% (P < 0.05); clearance through 3 h = 29.4 vs. 43.7% (P < 0.002); and clearance through 6 h = 39.4 vs. 50.2% (P < 0.02). This apparent slowing of MC in healthy subjects 4-h post-HS administration may reflect depletion of airway mucus following acute HS administration.

NVP-QBE170: an inhaled blocker of the epithelial sodium channel with a reduced potential to induce hyperkalaemia

BACKGROUND AND PURPOSE

Inhaled amiloride, a blocker of the epithelial sodium channel (ENaC), enhances mucociliary clearance (MCC) in cystic fibrosis (CF) patients. However, the dose of amiloride is limited by the mechanism-based side effect of hyperkalaemia resulting from renal ENaC blockade. Inhaled ENaC blockers with a reduced potential to induce hyperkalaemia provide a therapeutic strategy to improve mucosal hydration and MCC in the lungs of CF patients. The present study describes the preclinical profile of a novel ENaC blocker, NVP-QBE170, designed for inhaled delivery, with a reduced potential to induce hyperkalaemia.

EXPERIMENTAL APPROACH

The in vitro potency and duration of action of NVP-QBE170 were compared with amiloride and a newer ENaC blocker, P552-02, in primary human bronchial epithelial cells (HBECs) by short-circuit current. In vivo efficacy and safety were assessed in guinea pig (tracheal potential difference/hyperkalaemia), rat (hyperkalaemia) and sheep (MCC).

KEY RESULTS

In vitro, NVP-QBE170 potently inhibited ENaC function in HBEC and showed a longer duration of action to comparator molecules. In vivo, intratracheal (i.t.) instillation of NVP-QBE170 attenuated ENaC activity in the guinea pig airways with greater potency and duration of action than that of amiloride without inducing hyperkalaemia in either guinea pig or rat. Dry powder inhalation of NVP-QBE170 by conscious sheep increased MCC and was better than inhaled hypertonic saline in terms of efficacy and duration of action.

CONCLUSIONS AND IMPLICATIONS

NVP-QBE170 highlights the potential for inhaled ENaC blockers to exhibit efficacy in the airways with a reduced risk of hyperkalaemia, relative to existing compounds.

Roflumilast combined with adenosine increases mucosal hydration in human airway epithelial cultures after cigarette smoke exposure

Chronic obstructive pulmonary disease (COPD) is a growing cause of morbidity and mortality worldwide. Recent studies have shown that cigarette smoke (CS) induces cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction, which leads to airway-surface liquid (ASL) dehydration. This in turn contributes to the mucus dehydration and impaired mucociliary clearance that are seen in the chronic bronchitis form of COPD. Roflumilast is a phosphodiesterase 4 inhibitor that may improve lung function and reduce the frequency of exacerbations in patients with COPD. Although roflumilast can affect cAMP metabolism, little is known about the downstream pharmacological effects in the airways. We hypothesized that roflumilast would increase ASL rehydration in human bronchial epithelial cultures (HBECs) after chronic CS exposure. cAMP production was measured by Förster resonance energy transfer in HEK293T cells and by ELISA in HBECs. ASL height was measured by xz-confocal microscopy after air exposure or following HBEC exposure to freshly produced CS. Roflumilast had little effect on cAMP or ASL height when applied on its own; however, roflumilast significantly potentiated adenosine-induced increases in cAMP and ASL height in CS-exposed HBECs. Roflumilast increased the rate of ASL height recovery in cultures after CS exposure compared with controls. In contrast, the β2-adrenergic receptor agonists isoproterenol and salmeterol failed to increase ASL height after CS exposure. Our data suggest that roflumilast can increase ASL hydration in CS-exposed HBECs, which is predicted to be beneficial for the treatment of mucus dehydration/mucus stasis in patients with COPD chronic bronchitis.

Therapeutic options for hydrating airway mucus in cystic fibrosis

BACKGROUND

In cystic fibrosis (CF), genetic mutations in the CF transmembrane conductance regulator (CFTR) gene cause reduced chloride efflux from ciliated airway epithelial cells. This results in a reduction in periciliary liquid (PCL) depth of the airway surface liquid due to associated reduced water efflux. PCL layer dehydration reduces mucociliary clearance (MCC), leading to airway obstruction (reduced airflow and inflammation due to pathogen invasion) with mucus plug formation.

SUMMARY

Rehydrating mucus increases MCC. Mucus hydration can be achieved by direct hydration (administering osmotic agents to set up an osmotic gradient), using CFTR modulators to correct dysfunctional CFTR, or it can be achieved pharmacologically (targeting other ion channels on airway epithelial cells). Key Messages: The molecular mechanisms of several therapies are discussed in the context of pre-clinical and clinical trial studies. Currently, only the osmotic agent 7% hypertonic saline and the CFTR 'potentiator' VX-770 (ivacaftor) are used clinically to hydrate mucus. Emerging therapies include the osmotic agent mannitol (Bronchitol), the intracellular Ca(2+)-raising agent Moli1901/lancovutide, the CFTR potentiator sildenafil [phosphodiesterase type 5 (PDE5) inhibitor] and the CFTR 'corrector' VX-809 (lumacaftor). Other CFTR correctors (e.g. 'chemical chaperones') are also showing pre-clinical promise.

Gaining the Upper Hand on Pulmonary Drug Delivery

Asthma, Chronic Obstructive Pulmonary Disease (COPD) and Cystic Fibrosis (CF) are all pulmonary diseases which are characterized by chronic inflammation and an increase in mucus production. Excess mucus in the airways correlates with pathophysiology such as a decline in lung function and prolonged bacterial infections. New drugs to treat these chronic respiratory diseases are currently being developed and include both inhaled and orally administered compounds. Whilst oral drugs may be easier to administer, they are more prone to side-effects due to higher bioavailability. Inhaled compounds may show reduced bioavailability, but face their own unique challenges. For example, thick mucus in the respiratory tracts of asthma, CF and COPD patients can act as a physical barrier that impedes drug delivery. Mucus also contains a high number of enzymes and proteases that may degrade compounds before they reach their site of action. Furthermore, some classes of drugs are rapidly absorbed across the respiratory epithelia into systemic circulation, which may limit their duration of action and/or cause off-target effects. This review discusses some of the different treatment options that are currently available and the considerations that need to be taken into account to produce new therapies for the treatment of chronic respiratory diseases.

Lung inflammation in cystic fibrosis: pathogenesis and novel therapies

Despite remarkable progress following the identification of the causing gene, the final outcome of cystic fibrosis (CF) remains determined mainly by the progressive reduction of lung function. Inflammation of the airways is one of the key elements of the pathogenesis of the disease: it is responsible for the destruction of lung architecture, resulting in progressive loss of respiratory function. Bronchial infection induces an intense inflammatory reaction characterized by a massive invasion of neutrophils, the properties of which seems altered in CF. Moreover, the inflammatory process is also marked by a profuse release of soluble pro-inflammatory mediators, such as interleukin (IL)-6, IL-1β and IL-8 cytokines. In contrast, release of the anti-inflammatory mediator IL-10 is reduced, thus reflecting a pro-/anti-inflammatory imbalance. The inflammation/infection pair seems hard to dissociate, and the origin of the baneful consequences of the persisting excessive inflammatory responses remains to be cleared up: does inflammation follow or rather precede infection? Recent data suggest that uncontrolled inflammation is constitutive in CF. Countering it at early stages of the disease in order to prevent irretrievable damages in lungs remains a major priority in treating patients with CF. In this review, we discuss the usefulness and limitations of mouse models of CF to study the pathogenesis of human lung inflammatory disease, and the development of new potential strategies to reduce the inflammatory burden in the airways.

Epithelial sodium channel silencing as a strategy to correct the airway surface fluid deficit in cystic fibrosis

In the respiratory system, Na(+) absorption and Cl(-) secretion are balanced to maintain an appropriate airway surface fluid (ASF) volume and ensure efficient mucociliary clearance. In cystic fibrosis (CF), this equilibrium is disrupted by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, resulting in the absence of functional CFTR-dependent Cl(-) secretion. The consequences of defective Cl(-) transport are worsened by the persistence of Na(+) absorption, which contributes to airway surface dehydration. We asked whether normal ASF can be restored to an equal extent by recovering Cl(-) secretion from mutated CFTR or by reducing Na(+) absorption. This is highly relevant in the selection of the best strategy for the treatment of patients with CF. We analyzed the ASF thickness of primary cultured bronchial CF and non-CF epithelia after silencing the epithelial Na(+) channel (ENaC) with specific short, interfering RNAs (siRNAs) and after the pharmacological stimulation of CFTR. Our results indicate that (1) single siRNAs complementary to ENaC subunits are sufficient to reduce ENaC transcripts, Na(+) channel activity, and fluid transport, but only silencing both the α and β ENaC subunits at the same time leads to an increase of ASF (from nearly 7 µm to more than 9 µm); (2) the ASF thickness obtained in this way is about half that measured after maximal CFTR stimulation in non-CF epithelia (10-14 µm); and (3) the pharmacological rescue of mutant CFTR increases the ASF to the same extent as ENaC silencing. Our results indicate that CFTR rescue and ENaC silencing both produce a significant and long-lasting increase of airway hydration in vitro.

Managing the underlying cause of cystic fibrosis: a future role for potentiators and correctors

Cystic fibrosis (CF), a severe genetic disease, is caused by mutations that alter the structure and function of CFTR, a plasma membrane channel permeable to chloride and bicarbonate. Defective anion transport in CF irreversibly damages the lungs, pancreas, liver, and other organs. CF mutations cause loss of CFTR function in multiple ways. In particular, class 3 mutations such as p.Gly551Asp strongly decrease the time spent by CFTR in the open state (gating defect). Instead, class 2 mutations impair the maturation of CFTR protein and its transport from the endoplasmic reticulum to the plasma membrane (trafficking defect). The deletion of phenylalanine 508 (p.Phe508del), the most frequent mutation among CF patients (70-90 %), destabilizes the CFTR protein, thus causing both a trafficking and a gating defect. These two defects can be overcome with drug-like molecules generically called correctors and potentiators, respectively. The potentiator Kalydeco™ (also known as Ivacaftor or VX-770), developed by Vertex Pharmaceuticals, has been recently approved by the US FDA and the European Medicines Agency (EMA) for the treatment of CF patients carrying at least one CFTR allele with the p.Gly551Asp mutation (2-5 % of all patients). In contrast, the corrector VX-809, which significantly improves p.Phe508del-CFTR trafficking in vitro, is still under study in clinical trials. Because of multiple defects caused by the p.Phe508del mutation, it is probable that rescue of the mutant protein will require combined treatment with correctors having different mechanisms of action. This review evaluates the status of experimental and clinical research in pharmacotherapy for the CF basic defect.

Airway drug pharmacokinetics via analysis of exhaled breath condensate

Although the airway surface is the anatomic target for many lung disease therapies, measuring drug concentrations and activities on these surfaces poses considerable challenges. We tested whether mass spectrometric analysis of exhaled breath condensate (EBC) could be utilized to non-invasively measure airway drug pharmacokinetics and predicted pharmacological activities. Mass spectrometric methods were developed to detect a novel epithelial sodium channel blocker (GS-9411/P-680), two metabolites, a chemically related internal standard, plus naturally occurring solutes including urea as a dilution marker. These methods were then applied to EBC and serum collected from four (Floridian) sheep before, during and after inhalation of nebulized GS-9411/P-680. Electrolyte content of EBC and serum was also assessed as a potential pharmacodynamic marker of drug activity. Airway surface concentrations of drug, metabolites, and electrolytes were calculated from EBC measures using EBC:serum urea based dilution factors. GS-9411/P-680 and its metabolites were quantifiable in the sheep EBC, with peak airway concentrations between 1.9 and 3.4 μM measured 1 h after inhalation. In serum, only Metabolite #1 was quantifiable, with peak concentrations ∼60-fold lower than those in the airway (45 nM at 1 h). EBC electrolyte concentrations suggested a pharmacological effect; but this effect was not statistical significant. Analysis of EBC collected during an inhalation drug study provided a method for quantification of airway drug and metabolites via mass spectrometry. Application of this methodology could provide an important tool in development and testing of drugs for airways diseases.

Acute hyperkalemia associated with inhalation of a potent ENaC antagonist: Phase 1 trial of GS-9411

BACKGROUND

Inhaled epithelial sodium channel (ENaC) blockers are designed to increase airway surface liquid volume, thereby benefiting cystic fibrosis patients. This study evaluated the safety, tolerability, and pharmacokinetics of multiple doses of ENaC blocker GS-9411, in healthy participants.

METHODS

This randomized, double-blind, placebo-controlled, parallel-group, residential, Phase 1 study evaluated inhaled GS-9411 (2.4, 4.8, and 9.6 mg) or placebo, dosed twice daily for 14 days.

RESULTS AND CONCLUSIONS

GS-9411 was well tolerated; 86.1% of treated participants completed dosing (n=31/36). Cough and dizziness (27.8% participants each; most of mild severity) were the most commonly reported adverse events and occurred in both placebo and GS-9411 treatment groups. Arrhythmias were not observed for GS-9411-treated participants, and electrocardiographic changes were not considered clinically significant. Serum potassium levels exceeded the upper limit of normal (>5 mmol/L), 4 hr after the morning dose in GS-9411 (n=16/24) and placebo (n=4/12) treatment groups (38 incidences total). Retesting revealed levels had returned to normal within 2-3 hr. In urine electrolyte analyses, obtained 0-6 hr after the Day 1 morning dose, mean sodium/potassium ratios significantly increased from values 0-6 hr before dosing. Increased urine sodium/potassium ratios corresponded with high urine concentrations of active GS-9411 metabolites, which inhibited sodium reabsorption in the kidney, leading to the observed transient hyperkalemia in these participants. Inhaled GS-9411 was well tolerated except for the emergence of transient clinically significant hyperkalemia; this finding resulted in termination of further clinical development of this drug and will necessitate development of a new generation of ENaC blockers, which provide a sustained improvement in mucociliary clearance, while reducing renal exposure to ENaC blockade. Transient increases in mean urine sodium/potassium ratios appeared to be the first signal of electrolyte imbalances resulting from drug-induced block of ENaC in the kidney. The results of this study strongly suggest that clinical trials of novel ENaC blockers will require intensive measurement of plasma and urine electrolyte levels.

The epithelial sodium channel mediates the directionality of galvanotaxis in human keratinocytes

Cellular directional migration in an electric field (galvanotaxis) is one of the mechanisms guiding cell movement in embryogenesis and in skin epidermal repair. The epithelial sodium channel (ENaC), in addition to its function of regulating sodium transport in kidney, has recently been found to modulate cell locomotory speed. Here we tested whether ENaC has an additional function of mediating the directional migration of galvanotaxis in keratinocytes. Genetic depletion of ENaC completely blocks only galvanotaxis and does not decrease migration speed. Overexpression of ENaC is sufficient to drive galvanotaxis in otherwise unresponsive cells. Pharmacologic blockade or maintenance of the open state of ENaC also decreases or increases, respectively, galvanotaxis, suggesting that the channel open state is responsible for the response. Stable lamellipodial extensions formed at the cathodal sides of wild-type cells at the start of galvanotaxis; these were absent in the ENaC knockout keratinocytes, suggesting that ENaC mediates galvanotaxis by generating stable lamellipodia that steer cell migration. We provide evidence that ENaC is required for directional migration of keratinocytes in an electric field, supporting a role for ENaC in skin wound healing.

Quercetin increases cystic fibrosis transmembrane conductance regulator-mediated chloride transport and ciliary beat frequency: therapeutic implications for chronic rhinosinusitis

BACKGROUND

Increasing epithelial chloride (Cl(-)) secretion in the upper airways represents a putative method for promoting mucociliary clearance through augmentation of airway surface liquid depth. Several naturally occurring flavonoid compounds, including quercetin, have shown the capacity to increase transepithelial Cl(-) transport. Quercetin exhibits well-known antioxidant and anti-inflammatory activity and is now recognized as a potent activator of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel activity in a fashion largely independent of cyclic adenosine monophosphate signaling. The present study investigates whether this compound activates Cl(-) secretion and ciliary beat frequency (CBF) in well-characterized culture models of sinonasal epithelium.

METHODS

Cystic fibrosis and non-cystic fibrosis primary human sinonasal epithelial (HSNE) and murine nasal septal epithelial (MNSE) cultures were studied for transepithelial ion transport in Ussing chambers under voltage clamp conditions and CBF was performed using pharmacologic manipulation.

RESULTS

Change in short circuit current (DeltaI(SC), expressed as microamperes per squared centimeter) in response to quercetin were significantly greater than controls in both MNSE (23.23 ± 5.44 versus 2.47 ± 1.62; p < 0.0001) and HSNE (-8.72 ± 1.88 versus -1.88 ± 0.66; p < 0.01) cultures. CBF was significantly increased in quercetin-treated cells (expressed as fold change over baseline) in wild type (1.65 ± 0.13 versus 1.23 ± 0.05 [control]; p < 0.01), but not CFTR(-/-) (1.65 ± 0.29 versus 1.48 ± 0.38; p = 0.23).

CONCLUSION

Quercetin significantly increased transepithelial Cl(-) transport and CBF in MNSE and HSNE cultures. Future studies investigating quercetin as a means to promote mucociliary transport in individuals with rhinosinusitis are warranted.

Animal models and medical countermeasures development for radiation-induced lung damage: report from an NIAID Workshop

Since 9/11, there have been concerns that terrorists may detonate a radiological or nuclear device in an American city. Aside from several decorporation and blocking agents for use against internal radionuclide contamination, there are currently no medications within the Strategic National Stockpile that are approved to treat the immediate or delayed complications resulting from accidental exposure to radiation. Although the majority of research attention has focused on developing countermeasures that target the bone marrow and gastrointestinal tract, since they represent the most acutely radiosensitive organs, individuals who survive early radiation syndromes will likely suffer late effects in the months that follow. Of particular concern are the delayed effects seen in the lung that play a major role in late mortality seen in radiation-exposed patients and accident victims. To address these concerns, the National Institute of Allergy and Infectious Diseases convened a workshop to discuss pulmonary model development, mechanisms of radiation-induced lung injury, targets for medical countermeasures development, and end points to evaluate treatment efficacy. Other topics covered included guidance on the challenges of developing and licensing drugs and treatments specific to a radiation lung damage indication. This report reviews the data presented, as well as key points from the ensuing discussion.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

New horizons in the treatment of cystic fibrosis

Cystic fibrosis (CF) is a lethal, recessive, genetic disease affecting approximately 1 in 2500 live births among Caucasians. The CF gene codes for a cAMP/PKA-dependent, ATP-requiring, membrane chloride ion channel, generally found in the apical membranes of many secreting epithelia and known as CFTR (cystic fibrosis transmembrane conductance regulator). There are currently over 1700 known mutations affecting CFTR, many of which give rise to a disease phenotype. Around 75% of CF alleles contain the ΔF508 mutation in which a triplet codon has been lost, leading to a missing phenylalanine at position 508 in the protein. This altered protein fails to be trafficked to the correct location in the cell and is generally destroyed by the proteasome. The small amount that does reach the correct location functions poorly. Clearly the cohort of patients with at least one ΔF508 allele are a major target for therapeutic intervention. It is now over two decades since the CF gene was discovered and during this time the properties of CFTR have been intensely investigated. At long last there appears to be progress with the pharmaco-therapeutic approach. Ongoing clinical trials have produced fascinating results in which clinical benefit appears to have been achieved. To arrive at this point ingenious ways have been devised to screen very large chemical libraries for one of two properties: (i) agents promoting trafficking of mutant CFTR to, and insertion into the membrane, and known as correctors or (ii) agents which activate appropriately located mutant CFTR, known as potentiators. The best compounds emerging from these programmes are then used as chemical scaffolds to synthesize other compounds with appropriate pharmaceutical properties, hopefully with their pharmacological activity maintained or even enhanced. In summary, this approach attempts to make the mutant CFTR function in place of the real CFTR. A major function of CFTR in healthy airways is to maintain an adequate airway surface liquid (ASL) layer. In CF the position is further confounded since epithelial sodium channels (ENaC) are no longer regulated and transport salt and water out of the airways to exacerbate the lack of ASL. Thus an additional possibility for treatment of CF is to use agents that inhibit ENaC either alone or as adjuncts to CFTR correctors and/or potentiators. Yet a further way in which a pharmacological approach to CF can be considered is to recruit alternative chloride channels, such as calcium-activated chloride channel (CaCC), to act as surrogates for CFTR. A number of P2Y(2) receptor agonists have been investigated that operate by increasing Ca(2+)(i) which in turn activates CaCC. Some of these compounds are currently in clinical trials. The knowledge base surrounding the structure and function of CFTR that has accumulated in the last 20 years is impressive. Translational research feeding from this is now yielding compounds that provide real prospects for a pharmacotherapy for this disease.