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

Introduction: Cystic fibrosis is an autosomal recessive disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that codes for the CFTR anion channel. In the absence of functional CFTR, the epithelial Na⁺ channel is also dysregulated. Airway surface liquid (ASL) hydration is maintained by a balance between epithelial sodium channel (ENaC)-led Na⁺ absorption and CFTR-dependent anion secretion. This finely tuned homeostatic mechanism is required to maintain sufficient airway hydration to permit the efficient mucus clearance necessary for a sterile lung environment. In CF airways, the lack of CFTR and increased ENaC activity lead to ASL/mucus dehydration that causes mucus obstruction, neutrophilic infiltration, and chronic bacterial infection. Rehydration of ASL/mucus in CF airways can be achieved by inhibiting Na⁺ absorption with pharmacological inhibitors of ENaC.

Areas covered: In this review, we discuss ENaC structure and function and its role in CF lung disease and focus on ENaC inhibition as a potential therapeutic target to rehydrate CF mucus. We also discuss the failure of the first generation of pharmacological inhibitors of ENaC and recent alternate strategies to attenuate ENaC activity in the CF lung.

Expert opinion: ENaC is an attractive therapeutic target to rehydrate CF ASL that may serve as a monotherapy or function in parallel with other treatments. Given the increased number of strategies being employed to inhibit ENaC, this is an exciting and optimistic time to be in this field.     

The safety of lumacaftor and ivacaftor for the treatment of cystic fibrosis

Introduction: Lumacaftor-ivacaftor is indicated for treatment of cystic fibrosis (CF) in patients homozygous for the Phe-508del cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations. In clinical trials, treated patients showed improved pulmonary function, reduced pulmonary exacerbations, and other benefits. This article reviews safety of this therapy.

Areas covered: Safety findings in ivacaftor, lumacaftor and combined therapy trials, and reported subsequently through post-approval evaluation, were accessed by PubMed and Google searches using key words ‘VX-770’, ‘ivacaftor’, ‘VX-809’, and ‘lumacaftor’. Transaminitis was seen in ivacaftor and combination trials. Non-congenital cataracts were seen in pre-clinical animal studies and in children taking ivacaftor and combined therapy. Dyspnea occurs in some patients taking lumacaftor and combined therapy and usually resolves without stopping treatment. Lumacaftor is a strong inducer of CYP3A while ivacaftor is a CYP3A sensitive substrate. Combination therapy can decrease systemic exposure of medications that are substrates of CYP3A, decreasing therapeutic effect. Co-administration of lumacaftor-ivacaftor with sensitive CYP3A substrates or CYP3A substrates with narrow therapeutic index is not recommended.

Expert opinion: Lumacaftor-ivacaftor therapy may be associated with ocular and hepatic side effects. Specific recommendations for monitoring are available. Dyspnea occurs, especially during initiation of treatment. Potential drug interactions should be evaluated in patients taking combination therapy. The risk benefit ratio of lumacaftor-ivacaftor favors therapy.     

Lumacaftor/ivacaftor,a novel agent for the treatment of cystic fibrosis patients who are homozygous for the F580del CFTR mutation

Introduction: Cystic Fibrosis (CF) is an autosomal recessive disease affecting up to 90,000 people worldwide. Approximately 73% of patients are homozygous for the F508del cystic fibrosis transmembrane conductance regulator [CFTR] mutation. Traditionally treatment has only included supportive care. Therefore, there is a need for safe and effective novel therapies targeting the underlying molecular defects seen with CF.

Areas covered: In 2016, the Food and Drug Administration and the European Commission approved LUM/IVA (Orkambi), a CFTR modulator that includes both a CFTR corrector and potentiator, for CF patients homozygous for the F508del CFTR mutation. This article reviews the pharmacologic features, clinical efficacy, and safety of LUM/IVA and summarize the available pre-clinical and clinical data of LUM/IVA use.

Expert commentary: LUM/IVA showed modest, but significant improvements from baseline in percent predicted FEV₁ (ppFEV₁) as well as a reduction in pulmonary exacerbations by 35% It was shown to be safe for short- and long-term use. Currently, LUM/IVA is the only oral agent in its class available and represents a milestone the development of therapies for the management of CF. Nonetheless, pharmacoeconomic data are necessary to justify its high cost before is use becomes standard of care.     

Plasma esterases in cystic fibrosis: the impact of a respiratory exacerbation and its treatment

Objectives: To determine the effect of an exacerbation of respiratory symptoms in cystic fibrosis (CF) on the activities of plasma benzoylcholinesterase and butyrylcholinesterase. Methods: Twenty-nine patients with CF in a respiratory exacerbation and 27 healthy volunteers matched for age and sex were recruited. Blood was obtained from the patients when commencing antibiotic treatment and 14 days later on completion of treatment. One blood sample was taken from the healthy volunteers. The activities of benzoylcholinesterase and butyrylcholinesterase were determined by spectrophotometric assay. The circulating inflammatory markers, C-reactive protein and neutrophil elastase-α₁antiproteinase complex were also measured. Results: Benzoylcholinesterase activity was significantly (P = 0.001) lower in patients at the start of a respiratory exacerbation, compared with healthy controls [mean (SD): 917 (274) versus 1191(298) nmol · ml⁻¹ · min⁻¹]. Benzoylcholinesterase activity increased significantly in patients to 1013 (237) nmol · ml⁻¹ · min⁻¹, following a course of antibiotic treatment (P = 0.006). Butyrylcholinesterase activity was also lower (P = 0.001) in patients at the start of a respiratory exacerbation, compared with healthy controls [5.54 (1.64) versus 7.01 (1.79) μmol · ml⁻¹ · min⁻¹], and increased significantly in the patients to 6.31 (1.58) μmol · ml⁻¹ · min⁻¹ following treatment (P = 0.006). Conclusion: We demonstrated significant suppression of plasma esterase activities during an exacerbation of respiratory symptoms in CF, which was only partially reversed after antibiotic treatment. Further studies are needed to examine other pathways of drug metabolism in this group of chronically infected patients. Received: 8 June 1998 / Accepted in revised form: 18 September 1998     

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.     

The guinea-pig tracheal potential difference as an in vivo model for the study of epithelial sodium channel function in the airways

Background and purpose:

The epithelial sodium channel (ENaC) is a key regulator of airway mucosal hydration and mucus clearance. Negative regulation of airway ENaC function is predicted to be of clinical benefit in the cystic fibrosis lung. The aim of this study was to develop a small animal model to enable the direct assessment of airway ENaC function in vivo.

Experimental approach:

Tracheal potential difference (TPD) was utilized as a measure of airway epithelial ion transport in the guinea-pig. ENaC activity in the trachea was established with a dose–response assessment to a panel of well-characterized direct and indirect pharmacological modulators of ENaC function, delivered by intra-tracheal (i.t.) instillation.

Key results:

The TPD in anaesthetized guinea-pigs was attenuated by the direct ENaC blockers: amiloride, benzamil and CF552 with ED₅₀ values of 16, 14 and 0.2 μg kg⁻¹ (i.t.), respectively. 5-(N-Ethyl-N-isopropyl) amiloride, a structurally related compound but devoid of activity on ENaC, was without effect on the TPD. Intra-tracheal dosing of the Kunitz-type serine protease inhibitors aprotinin and placental bikunin, which have previously been demonstrated to inhibit proteolytic activation of ENaC, likewise potently attenuated TPD in guinea-pigs, whereas α₁-antitrypsin and soya bean trypsin inhibitor were without effect.

Conclusions and implications:

The pharmacological sensitivity of the TPD to amiloride analogues and also to serine protease inhibitors are both consistent with that of ENaC activity in the guinea-pig trachea. The guinea-pig TPD therefore represents a suitable in vivo model of human airway epithelial ion transport.     

Activation of human CIC-2 Cl- channels: implications for cystic fibrosis

1. The CIC-2 Cl- channels are present in the adult human lung epithelia and, therefore, are a potential target for therapy in cystic fibrosis. 2. Activators of CIC-2 Cl- channels that may have physiological relevance include activation by reduced external pH, protein kinase A and arachidonic acid. 3. Activators of CIC-2 Cl- channels that have therapeutic potential include amidation and omeprazole and, perhaps, effectors of arachidonic acid metabolism.     

Early antibiotic treatment of pseudomonas aeruginosa colonisation in cystic fibrosis: a critical review of the literature

Aim To assess the evidence supporting early antibiotic treatment in asymptomatic cystic fibrosis (CF) patients colonised by Pseudomonas aeruginosa (PA).Methods We carried out a computerised (Medline, Embase) and hand search of journals for suitable publications. All English-language clinical studies regarding the efficacy of early antibiotic treatment on PA colonisation in asymptomatic patients were considered. Each eligible publications fitting these criteria were assessed for the following outcome measures: frequency of positive PA cultures; serum level of precipitating antibodies; lung function; survival; number of hospitalisations; adverse effects and resistance to antibiotics.Results Of the 11 studies eventually considered, 3 were randomised—2 versus placebo— and 8 were cohort studies—2 of which had historical controls. Overall, 309 patients (population range 7–91 patients) were recruited. There was a high variability between the individual studies for age, outcome measures, duration of follow-up (1 to 44 months) and treatment (three studies used only aerosol tobramycin, one colistin, four aerosol colistin plus oral ciprofloxacin, one used intravenous treatment and two miscellaneous therapy). An overall evaluation indicated that early antibiotic treatment can reduce the number of positive cultures and the anti-PA antibody titre. In one study, FEV1 was better in the treated group (oral ciprofloxacin and nebulised colistin) than in historical controls, while in one placebo-controlled trial, no effect on lung function was shown after 1 year of tobramycin inhalation. Collateral effects and bacterial resistance were not increased. The short follow-up did not allow definite conclusions with regard to the long-term progression of respiratory insufficiency or survival.Conclusions Evidence was found that antibiotic treatment can reduce the rate of positive cultures and of anti-PA antibody titres in asymptomatic CF patients with newly isolated PA. Different therapeutic options have not been directly compared: a multi-centre comparative study needs to be carried out.     

F508del-cystic fibrosis transmembrane regulator correctors for treatment of cystic fibrosis: a patent review

Introduction: Cystic fibrosis (CF) is an autosomal recessive genetic disease caused by malfunction of CF transmembrane regulator (CFTR). The deletion of a phenylalanine at residue 508 (F508del) is the most common mutation that causes cellular processing, chloride channel gating and protein stability defects in CFTR. Pharmacological modulators of F508del-CFTR, aimed at correcting the cellular processing defect (correctors) and the gating defect (potentiators) in CFTR protein, are regarded as promising therapeutic agents for CF disease. Endeavors in searching F508del-CFTR modulators have shown encouraging results, with several small-molecule compounds having entered clinical trials or even represented clinical options.

Areas covered: This review covers the discovery of F508del-CFTR correctors described in both patents (2005 – present) and scientific literatures.

Expert opinion: Cyclopropane carboxamide derivatives of CFTR correctors continue to dominate in this area, among which lumacaftor (a NBD1-MSD1/2 interface stabilizer) is the most promising compound and is now under the priority review by US FDA. However, the abrogation effect of ivacaftor (potentiator) on lumacaftor suggests the requirement of discovering new correctors and potentiators that can cooperate well. Integration screening for simultaneously identifying combinations of correctors (particularly NBD1 stabilizer) and potentiators should provide an alternative strategy. A recently reported natural product fraction library may be useful for the integration screening.     

Design, synthesis, and structure-activity relationships of novel 2-substituted pyrazinoylguanidine epithelial sodium channel blockers: drugs for cystic fibrosis and chronic bronchitis

Amiloride (1), the prototypical epithelial sodium channel (ENaC) blocker, has been administered with limited success as aerosol therapy for improving pulmonary function in patients with the genetic disorder cystic fibrosis. This study was conducted to synthesize and identify more potent, less reversible ENaC blockers, targeted for aerosol therapy and possessing minimal systemic renal activity. A series of novel 2-substituted acylguanidine analogues of amiloride were synthesized and evaluated for potency and reversibility on bronchial ENaC. All compounds tested were more potent and less reversible at blocking sodium-dependent short-circuit current than amiloride. Compounds 30-34 showed the greatest potency on ENaC with IC(50) values below 10 nM. A regioselective difference in potency was found (compounds 30, 39, and 40), whereas no stereospecific (compounds 33, 34) difference in potency on ENaC was displayed. Lead compound 32 was 102-fold more potent and 5-fold less reversible than amiloride and displayed the lowest IC(50) value ever reported for an ENaC blocker.     

Pharmacological interventions for the correction of ion transport defect in cystic fibrosis

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated and ATP-gated Cl^- channel expressed in the apical plasma membrane of epithelial cells in the airways, digestive and reproductive tracts. Cystic fibrosis (CF) caused by mutations in the CFTR gene is characterised by chronic airway obstructions and infections, pancreatic failure, male infertility and elevated levels of salt in sweat. A pharmacological therapy would help to restore the defective transepithelial Cl^- transport observed in CF cells. Therefore, searching for potent and specific small molecules or peptides able to stimulate transepithelial Cl^- transport through direct interaction with CFTR or via CFTR-independent mechanisms has become a crucial end point in the field. With the growing understanding of the pharmacology of CFTR activity and processing, a number of academic investigators and biopharmaceutical companies have developed high-throughput screening assays, and reported active seeking of CFTR activators or modulators of airway functions in order to treat CF. This article provides an updated overview of the new emerging molecules and discusses the corresponding patent literature.     

Cystic fibrosis transmembrane conductance regulator and the outwardly rectifying chloride channel: a relationship between two chloride channels expressed in epithelial cells

1. Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) result in the primary defect observed in patients with cystic fibrosis. 2. The CFTR is a member of the ATPase-binding cassette (ABC) transporter family but, unlike other members of this group, CFTR conducts a chloride current that is activated by cAMP. 3. In epithelial cells, the cAMP-stimulated chloride current is conducted by both CFTR and the outwardly rectifying chloride channel (ORCC). 4. The present review summarizes the current knowledge of the properties of the two channels, as well as their relationship. Because the gene encoding the ORCC has not been identified, a discussion as to possible candidates for this chloride channel is included.     

A safety evaluation of pirfenidone for the treatment of idiopathic pulmonary fibrosis

ABSTRACT

Introduction: Pirfenidone is a novel oral anti-fibrotic agent approved for the treatment of idiopathic pulmonary fibrosis (IPF). Since IPF is a chronic and progressive disease most commonly encountered in an older population, therapeutic options should be not only effective, but also free from drug interactions and as safe and tolerable as possible.

Areas covered: Comprehensive data from randomized controlled trials, meta-analyses, safety studies, and post-marketing data are available to assess the efficacy and safety of pirfenidone in the treatment of IPF. Information on efficacy, adverse events, drug tolerability and discontinuation rates both in clinical trials and real-world clinical experiences are reported.

Expert opinion: Pirfenidone has an abundance of data supporting its use in mild-to-moderate IPF. Observational evidence suggests a similar efficacy in severe IPF. In clinical trials, observational studies and real-world use, adverse events are frequent, though generally mild and well tolerated, especially with adequate patient education. Preventative strategies, along with timely and appropriate management of adverse events are critical in improving patient compliance, thereby ensuring the benefits of long-term treatment with pirfenidone.     

Effects of lacosamide, a novel sodium channel modulator, on dorsal horn neuronal responses in a rat model of neuropathy

Various mechanisms underlie the complexity of neuropathic pain (pain due to disease of the somatosensory system), with each mechanism bearing a different order of relevance from one person and pain state to the next. Successful treatment is contingent on sound knowledge of underlying mechanisms that may occur at peripheral, spinal and/or supraspinal sites. In particular, ion channels throughout the nervous system are known to play an intimate part in neuropathic pain, and thus stand as good targets for analgesic drugs. Agents that modulate voltage-gated sodium channel function can reduce action potential propagation along sensory neurones to reduce the transmission and perception of nociceptive signals. Lacosamide is a functionalised amino acid that affects voltage-gated sodium channels in a novel way by enhancing the slow inactivating ‘braking’ state of these channels. To validate lacosamide's inhibitory efficacy in vivo, we unilaterally ligated spinal nerves L5 and L6 in rats to induce a state of neuropathy, and on post-operative days 14-17 recorded evoked-responses of deep dorsal horn neurones before and after spinal or systemic lacosamide delivery. Lacosamide's effects on various measures in spinal nerve-ligated rats were compared to rats that underwent sham surgery. Our results show that neuropathy induced novel inhibitory effects of lacosamide on mechanical and electrical responses, and enhanced inhibitory effects on thermal responses after systemic or spinal administration, suggesting state-preference actions of lacosamide.     

Nonclinical safety assessment of a synthetic peptide thrombopoietin agonist: effects on platelets, bone homeostasis, and immunogenicity and the implications for clinical safety monitoring of adverse bone effects

RWJ-800088 is a novel, potent polyethylene glycol (PEG)-conjugated thrombopoietin (TPO) mimetic that increases platelet levels and protects against thrombocytopenia. A nonclinical safety program was customized for this peptide that takes into account its protein-like structure, synthetic chemical nature, agonist pharmacologic activity, and mode of administration. In repeat-dose toxicity studies, the salient findings were dose-related increases in circulating platelet counts, mean platelet volume, and megakaryocytes in the bone marrow with no antibody formation. Reversible myelofibrosis and hyperostosis were observed in rats, but not dogs, when the circulating platelet levels exceeded 3× those of vehicle controls. The bone effects were due to the exaggerated pharmacologic effect and excessive stimulation and elevation of megakaryocytes by TPO, which results in intramedullary proliferation of fibroblasts and mesenchymal cells followed by osseous metaplasia. These findings support the use of platelet elevations of >3× as a stopping criterion to prevent potential adverse bone-related effects in humans.     

Role of mixed ion channel effects in the cardiovascular safety assessment of the novel anti-MRSA fluoroquinolone JNJ-Q2

BACKGROUND AND PURPOSE

JNJ-Q2, a novel broad-spectrum fluoroquinolone with anti-methicillin-resistant Staphylococcus aureus activity, was evaluated in a comprehensive set of non-clinical and clinical cardiovascular safety studies. The effect of JNJ-Q2 on different cardiovascular parameters was compared with that of moxifloxacin, sparfloxacin and ofloxacin. Through comparisons with these well-known fluoroquinolones, the importance of effects on compensatory ion channels to the cardiovascular safety of JNJ-Q2 was investigated.

EXPERIMENTAL APPROACH

JNJ-Q2 and comparator fluoroquinolones were evaluated in the following models/test systems: hERG-transfected HEK293 cells sodium channel-transfected CHO cells, guinea pig right atria, arterially perfused rabbit left ventricular wedge preparations and in vivo studies in anaesthetized guinea pigs, anaesthetized and conscious telemetered dogs, and a thorough QT study in humans.

KEY RESULTS

The trend for effects of JNJ-Q2 on Tp–Te, QT, QRS and PR intervals in the non-clinical models and the plateau in QTc with increasing plasma concentration in humans are consistent with offsetting sodium and calcium channel activities that were observed in the non-clinical studies. These mixed ion channel activities result in the less pronounced or comparable increase in QTc interval for JNJ-Q2 compared with moxifloxacin and sparfloxacin despite its greater in vitro inhibition of I_Kr.

CONCLUSIONS AND IMPLICATIONS

Based on the non-clinical and clinical cardiovascular safety assessment, JNJ-Q2 has a safe cardiovascular profile for administration in humans with comparable or reduced potential to prolong QT intervals, compared with moxifloxacin. The results demonstrate the importance of compensatory sodium and calcium channel activity in offsetting potassium channel activity for compounds with a fluoroquinolone core.     

IKur/Kv1.5 channel blockers for the treatment of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. Anti-arrhythmic drugs remain the mainstay of therapy, but the available class I and III anti-arrhythmic drugs are only moderately effective in long-term restoring/maintaining sinus rhythm (SR) and can produce potentially fatal ventricular pro-arrhythmia. In an attempt to identify safer and more effective anti-arrhythmic drugs, drug discovery efforts have focused on ‘atrial selective drugs’ that target cardiac ion channel(s) that are exclusively or predominantly expressed in the atria. The ultra-rapid activating delayed rectifier K⁺ current (I_Kur), carried by Kv1.5 channels, is a major repolarizing current in human atria, but seems to play no role in the ventricle. This finding offers the possibility of developing selective I_Kur blockers to restore and maintain SR without a risk of ventricular pro-arrhythmia. Several I_Kur blockers are now being developed but clinical data are still limited, so the precise role of these agents in the treatment of AF remains to be defined. In this review we analyze the possible advantages and disadvantages of the developmental I_Kur blockers as they represent the first step for the development of potential atrial selective drugs for a more effective and safer treatment and prevention of AF.