Cystic fibrosis transmembrane conductance regulator (CFTR) potentiators protect G551D but not ΔF508 CFTR from thermal instability

Cystic fibrosis management in pediatric population-from clinical features to personalized therapy

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations of the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). In 1949, it's been identified as a monogenic disease and was thought to primarily affect individuals of Northern European descent. It was the most prevalent autosomal recessive disease that shortens life. With the availability of multiple testing methodologies nowadays, there is a chance to create novel and enhanced treatment options. Even in the absence of a high sweat chloride test (SCT) result, the discovery of two causal mutations is diagnostic for cystic fibrosis (CF). For a CF diagnosis, however, at least two positive E sweat chloride tests are still required. In order to achieve early and active intervention to manage cystic fibrosis (CF) and its comorbidities, treatment regimens for pediatric patients should be evaluated, improved, and closely monitored. New developments in the treatment of cystic fibrosis (CF) have led to the development of medications derived from molecules that target the pathogenetic pathway of the illness. These options are very efficient and allow pediatric patients to receive individualized care. However, in order to better direct patient care and enhance patient outcomes, it is crucial to research uncommon CF mutations, which can provide crucial information about the prognosis of the disease and the relationships between genotype and phenotype. To ensure the success of creating novel, safer, and more efficient treatment approaches, a deeper understanding of the pathogeny of the illness is required. In the age of customized medicine, genetic research will be essential to improving patient care and quality of life for those with uncommon mutations.

ABC transporter-mediated MXR mechanism in fish embryos and its potential role in the efflux of nanoparticles

ATP-binding cassette (ABC) transporters are believed to protect aquatic organisms by pumping xenobiotics out, and recent investigation has suggested their involvement in the detoxification and efflux of nanoparticles (NPs), but their roles in fish embryos are poorly understood. In this regard, this paper summarizes the recent advances in research pertaining to the development of ABC transporter-mediated multi-xenobiotic resistance (MXR) mechanism in fish embryos and the potential interaction between ABC transporters and NPs. The paper focuses on: (1) Expression, function, and modulation mechanism of ABC proteins in fish embryos; (2) Potential interaction between ABC transporters and NPs in cell models and fish embryos. ABC transporters could be maternally transferred to fish embryos and thus play an important role in the detoxification of various chemical pollutants and NPs. There is a need to understand the specific mechanism to benefit the protection of aquatic resources.

The rescue of F508del-CFTR by elexacaftor/tezacaftor/ivacaftor (Trikafta) in human airway epithelial cells is underestimated due to the presence of ivacaftor

Trikafta, currently the leading therapeutic in Cystic Fibrosis (CF), has demonstrated a real clinical benefit. This treatment is the triple combination therapy of two folding correctors elexacaftor/tezacaftor (VX445/VX661) plus the gating potentiator ivacaftor (VX770). In this study, our aim was to compare the properties of F508del-CFTR in cells treated with either lumacaftor (VX809), tezacaftor, elexacaftor, elexacaftor/tezacaftor with or without ivacaftor. We studied F508del-CFTR function, maturation and membrane localisation by Ussing chamber and whole-cell patch clamp recordings, Western blot and immunolocalization experiments. With human primary airway epithelial cells and the cell lines CFBE and BHK expressing F508del, we found that, whereas the combination elexacaftor/tezacaftor/ivacaftor was efficient in rescuing F508del-CFTR abnormal maturation, apical membrane location and function, the presence of ivacaftor limits these effects. The basal F508del-CFTR short-circuit current was significantly increased by elexacaftor/tezacaftor/ivacaftor and elexacaftor/tezacaftor compared to other correctors and non-treated cells, an effect dependent on ivacaftor and cAMP. These results suggest that the level of the basal F508del-CFTR current might be a marker for correction efficacy in CF cells. When cells were treated with ivacaftor combined to any correctors, the F508del-CFTR current was unresponsive to the subsequently acute addition of ivacaftor unlike the CFTR potentiators genistein and Cact-A1 which increased elexacaftor/tezacaftor/ivacaftor and elexacaftor/tezacaftor-corrected F508del-CFTR currents. These findings show that ivacaftor reduces the correction efficacy of Trikafta. Thus, combining elexacaftor/tezacaftor with a different potentiator might improve the therapeutic efficacy for treating CF patients.

Cystic fibrosis drug ivacaftor stimulates CFTR channels at picomolar concentrations

The devastating inherited disease cystic fibrosis (CF) is caused by mutations of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) anion channel. The recent approval of the CFTR potentiator drug ivacaftor (Vx-770) for the treatment of CF patients has marked the advent of causative CF therapy. Currently, thousands of patients are being treated with the drug, and its molecular mechanism of action is under intensive investigation. Here we determine the solubility profile and true stimulatory potency of Vx-770 towards wild-type (WT) and mutant human CFTR channels in cell-free patches of membrane. We find that its aqueous solubility is ~200 fold lower (~60 nanomolar), whereas the potency of its stimulatory effect is >100 fold higher, than reported, and is unexpectedly fully reversible. Strong, but greatly delayed, channel activation by picomolar Vx-770 identifies multiple sequential slow steps in the activation pathway. These findings provide solid guidelines for the design of in vitro studies using Vx-770.

Toward inclusive therapy with CFTR modulators: Progress and challenges

Cystic fibrosis is caused by gene mutations that result in an abnormal Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein on the surface of cells. CFTR modulators are a novel class of drugs that directly target the molecular defect. CFTR modulators include potentiators that result in improved activity of the channel; correctors that help the protein traffic to the cell surface properly; and readthrough agents that restore full-length CFTR by suppression of premature termination codons, among other novel classes more recently established. While some of these drugs, CFTR potentiators in particular, have provided remarkable improvements for CF patients, others have yet to achieve profoundly improved outcomes, and many CF patients are not yet impacted by CFTR modulators due to lack of knowledge regarding susceptibility of their mutations to treatment. One limitation to expanding these types of therapies to the maximum number of patients with CF is the lack of rigorously validated clinical biomarkers that can determine efficacy on an individual basis, as well as few pre-clinical tools that can predict whether an individual with a rare combination of mutant alleles will respond to a particular CFTR modulator regimen. In this review, we discuss the various groups of CFTR modulators and their status in clinical development, as well as address the current literature on biomarkers, pre-clinical cell-based tools, and the role of pharmacometrics in creating therapeutic strategies to improve the lives of all patients with cystic fibrosis, regardless of their specific mutation.

Functional defect of variants in the adenosine triphosphate-binding sites of ABCB4 and their rescue by the cystic fibrosis transmembrane conductance regulator potentiator, ivacaftor (VX-770)

ABCB4 (MDR3) is an adenosine triphosphate (ATP)-binding cassette (ABC) transporter expressed at the canalicular membrane of hepatocytes, where it mediates phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene are responsible for several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ABCB4 missense variations that reside in the highly conserved motifs of ABC transporters, involved in ATP binding. Five disease-causing variations in these motifs have been identified in ABCB4 (G535D, G536R, S1076C, S1176L, and G1178S), three of which are homologous to the gating mutations of cystic fibrosis transmembrane conductance regulator (CFTR or ABCC7; i.e., G551D, S1251N, and G1349D), that were previously shown to be function defective and corrected by ivacaftor (VX-770; Kalydeco), a clinically approved CFTR potentiator. Three-dimensional structural modeling predicted that all five ABCB4 variants would disrupt critical interactions in the binding of ATP and thereby impair ATP-induced nucleotide-binding domain dimerization and ABCB4 function. This prediction was confirmed by expression in cell models, which showed that the ABCB4 mutants were normally processed and targeted to the plasma membrane, whereas their PC secretion activity was dramatically decreased. As also hypothesized on the basis of molecular modeling, PC secretion activity of the mutants was rescued by the CFTR potentiator, ivacaftor (VX-770).

CONCLUSION

Disease-causing variations in the ATP-binding sites of ABCB4 cause defects in PC secretion, which can be rescued by ivacaftor. These results provide the first experimental evidence that ivacaftor is a potential therapy for selected patients who harbor mutations in the ATP-binding sites of ABCB4. (Hepatology 2017;65:560-570).

Potentiators exert distinct effects on human, murine, and Xenopus CFTR

VX-770 (Ivacaftor) has been approved for clinical usage in cystic fibrosis patients with several CFTR mutations. Yet the binding site(s) on CFTR for this compound and other small molecule potentiators are unknown. We hypothesize that insight into this question could be gained by comparing the effect of potentiators on CFTR channels from different origins, e.g., human, mouse, and Xenopus (frog). In the present study, we combined this comparative molecular pharmacology approach with that of computer-aided drug discovery to identify and characterize new potentiators of CFTR and to explore possible mechanism of action. Our results demonstrate that 1) VX-770, NPPB, GlyH-101, P1, P2, and P3 all exhibited ortholog-specific behavior in that they potentiated hCFTR, mCFTR, and xCFTR with different efficacies; 2) P1, P2, and P3 potentiated hCFTR in excised macropatches in a manner dependent on the degree of PKA-mediated stimulation; 3) P1 and P2 did not have additive effects, suggesting that these compounds might share binding sites. Also 4) using a pharmacophore modeling approach, we identified three new potentiators (IOWH-032, OSSK-2, and OSSK-3) that have structures similar to GlyH-101 and that also exhibit ortholog-specific potentiation of CFTR. These could potentially serve as lead compounds for development of new drugs for the treatment of cystic fibrosis. The ortholog-specific behavior of these compounds suggest that a comparative pharmacology approach, using cross-ortholog chimeras, may be useful for identification of binding sites on human CFTR.

Potentiator synergy in rectal organoids carrying S1251N, G551D, or F508del CFTR mutations

The potentiator VX-770 (ivacaftor/KALYDECO™) targets defective gating of CFTR and has been approved for treatment of cystic fibrosis (CF) subjects carrying G551D, S1251N or one of 8 other mutations. Still, the current potentiator treatment does not normalize CFTR-dependent biomarkers, indicating the need for development of more effective potentiator strategies. We have recently pioneered a functional CFTR assay in primary rectal organoids and used this model to characterize interactions between VX-770, genistein and curcumin, the latter 2 being natural food components with established CFTR potentiation capacities. Results indicated that all possible combinations of VX-770, genistein and curcumin synergistically repaired CFTR-dependent forskolin-induced swelling of organoids with CFTR-S1251N or CFTR-G551D, even under suboptimal CFTR activation and compounds concentrations, conditions that may predominate in vivo. Genistein and curcumin also enhanced forskolin-induced swelling of F508del homozygous organoids that were treated with VX-770 and the prototypical CFTR corrector VX-809. These results indicate that VX-770, genistein and curcumin in double or triple combinations can synergize in restoring CFTR-dependent fluid secretion in primary CF cells and support the use of multiple potentiators for treatment of CF.

New and emerging targeted therapies for cystic fibrosis

Cystic fibrosis (CF) is a monogenic autosomal recessive disorder that affects about 70,000 people worldwide. The clinical manifestations of the disease are caused by defects in the cystic fibrosis transmembrane conductance regulator (CFTR) protein. The discovery of the CFTR gene in 1989 has led to a sophisticated understanding of how thousands of mutations in the CFTR gene affect the structure and function of the CFTR protein. Much progress has been made over the past decade with the development of orally bioavailable small molecule drugs that target defective CFTR proteins caused by specific mutations. Furthermore, there is considerable optimism about the prospect of gene replacement or editing therapies to correct all mutations in cystic fibrosis. The recent approvals of ivacaftor and lumacaftor represent the genesis of a new era of precision medicine in the treatment of this condition. These drugs are having a positive impact on the lives of people with cystic fibrosis and are potentially disease modifying. This review provides an update on advances in our understanding of the structure and function of the CFTR, with a focus on state of the art targeted drugs that are in development.

Combination therapy with cystic fibrosis transmembrane conductance regulator modulators augment the airway functional microanatomy

Recently approved therapies that modulate CFTR function have shown significant clinical benefit, but recent investigations regarding their molecular mechanism when used in combination have not been consistent with clinical results. We employed micro-optical coherence tomography as a novel means to assess the mechanism of action of CFTR modulators, focusing on the effects on mucociliary clearance. Primary human airway monolayers from patients with a G551D mutation responded to ivacaftor treatment with increased ion transport, airway surface liquid depth, ciliary beat frequency, and mucociliary transport rate, in addition to decreased effective viscosity of the mucus layer, a unique mechanism established by our findings. These endpoints are consistent with the benefit observed in G551D patients treated with ivacaftor, and identify a novel mechanism involving mucus viscosity. In monolayers derived from F508del patients, the situation is more complicated, compounded by disparate effects on CFTR expression and function. However, by combining ion transport measurements with functional imaging, we establish a crucial link between in vitro data and clinical benefit, a finding not explained by ion transport studies alone. We establish that F508del cells exhibit increased mucociliary transport and decreased mucus effective viscosity, but only when ivacaftor is added to the regimen. We further show that improvement in the functional microanatomy in vitro corresponds with lung function benefit observed in the clinical trials, whereas ion transport in vitro corresponds to changes in sweat chloride. Functional imaging reveals insights into clinical efficacy and CFTR biology that significantly impact our understanding of novel therapies.

Breakthrough therapies: Cystic fibrosis (CF) potentiators and correctors

Cystic Fibrosis is caused by mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) gene resulting in abnormal protein function. Recent advances of targeted molecular therapies and high throughput screening have resulted in multiple drug therapies that target many important mutations in the CFTR protein. In this review, we provide the latest results and current progress of CFTR modulators for the treatment of cystic fibrosis, focusing on potentiators of CFTR channel gating and Phe508del processing correctors for the Phe508del CFTR mutation. Special emphasis is placed on the molecular basis underlying these new therapies and emerging results from the latest clinical trials. The future directions for augmenting the rescue of Phe508del with CFTR modulators are also emphasized.

A synonymous codon change alters the drug sensitivity of ΔF508 cystic fibrosis transmembrane conductance regulator

Synonymous mutations, such as I507-ATC→ATT, in deletion of Phe508 in cystic fibrosis transmembrane conductance regulator (ΔF508 CFTR), the most frequent disease-associated mutant of CFTR, may affect protein biogenesis, structure, and function and contribute to an altered disease phenotype. Small-molecule drugs are being developed to correct ΔF508 CFTR. To understand correction mechanisms and the consequences of synonymous mutations, we analyzed the effect of mechanistically distinct correctors, corrector 4a (C4) and lumacaftor (VX-809), on I507-ATT and I507-ATC ΔF508 CFTR biogenesis and function. C4 stabilized I507-ATT ΔF508 CFTR band B, but without considerable biochemical and functional correction. VX-809 biochemically corrected ∼10% of both of the variants, leading to stable, forskolin+3-isobutyl-1-methylxanthine (IBMX)-activated whole-cell currents in the presence of the corrector. Omitting VX-809 during whole-cell recordings led to a spontaneous decline of the currents, suggesting posttranslational stabilization by VX-809. Treatment of cells with the C4+VX-809 combination resulted in enhanced rescue and 2-fold higher forskolin+IBMX-activated currents of both I507-ATT and I507-ATC ΔF508 CFTR, compared with VX-809 treatment alone. The lack of an effect of C4 on I507-ATC ΔF508 CFTR, but its additive effect in combination with VX-809, implies that C4 acted on VX-809-modified I507-ATC ΔF508 CFTR. Our results suggest that binding of C4 and VX-809 to ΔF508 CFTR is conformation specific and provide evidence that synonymous mutations can alter the drug sensitivity of proteins.

Generation of Distal Airway Epithelium from Multipotent Human Foregut Stem Cells

Collectively, lung diseases are one of the largest causes of premature death worldwide and represent a major focus in the field of regenerative medicine. Despite significant progress, only few stem cell platforms are currently available for cell-based therapy, disease modeling, and drug screening in the context of pulmonary disorders. Human foregut stem cells (hFSCs) represent an advantageous progenitor cell type that can be used to amplify large quantities of cells for regenerative medicine applications and can be derived from any human pluripotent stem cell line. Here, we further demonstrate the application of hFSCs by generating a near homogeneous population of early pulmonary endoderm cells coexpressing NKX2.1 and FOXP2. These progenitors are then able to form cells that are representative of distal airway epithelium that express NKX2.1, GATA6, and cystic fibrosis transmembrane conductance regulator (CFTR) and secrete SFTPC. This culture system can be applied to hFSCs carrying the CFTR mutation Δf508, enabling the development of an in vitro model for cystic fibrosis. This platform is compatible with drug screening and functional validations of small molecules, which can reverse the phenotype associated with CFTR mutation. This is the first demonstration that multipotent endoderm stem cells can differentiate not only into both liver and pancreatic cells but also into lung endoderm. Furthermore, our study establishes a new approach for the generation of functional lung cells that can be used for disease modeling as well as for drug screening and the study of lung development.