2011 Van Goor F, Hadida S, Grootenhuis PD, Burton B, Stack JH, Straley KS, Decker CJ, Miller M, McCartney J, Olson ER, Wine JJ, Frizzell RA, Ashlock M, Negulescu PA. Correction of the F508del-CFTR protein processing defect in vitro by the investigational drug VX-809. Proc Nat Acad Sci USA 2011; 108:18843-84. [PubMed]

The authors describe the in vitro pharmacology of VX-809, a CFTR corrector. In cultured human bronchial epithelial cells isolated from patients with CF homozygous for F508del, VX-809 improved F508del-CFTR processing in the endoplasmic reticulum and enhanced chloride secretion to approximately 14% of non-CF human bronchial epithelial cells, a level associated with mild CF. F508del-CFTR corrected by VX-809 exhibited biochemical and functional characteristics similar to normal CFTR. VX-809 represents a class of CFTR corrector that specifically addresses the underlying processing defect in F508del-CFTR.

–    As DF508 is the most frequent CF mutation it is hoped that this corrector will be as therapeutically active as indicated in these studies as it is likely that even 14% of normal CFTR activity will ameliorate the main symptoms associated with complete absent CFTR activity.  However, the corrector does not seem to have the same dramatic effect on DF508 as VX-770 has on the G551D mutation. However, the results of VX-770 in patients with the GF551D mutation added great impetus to the small molecule corrector research and there is now major investment in this area by pharmaceutical companies and the CF Foundation.

2012 Clancy JP. Rowe SM. Accurso FJ. Aitken ML. Amin RS. Ashlock MA. Ballmann M. Boyle MP. Bronsveld I. Campbell PW. De Boeck K. Donaldson SH. Dorkin HL. Dunitz JM. Durie PR. Jain M. Leonard A. McCoy KS. Moss RB. Pilewski JM. Rosenbluth DB. Rubenstein RC. Schechter MS. Botfield M. Ordonez CL. Spencer-Green GT. Vernillet L. Wisseh S. Yen K. Konstan MW. Results of a phase IIa study of VX-809, an investigational CFTR corrector compound, in subjects with cystic fibrosis homozygous for the F508del-CFTR mutation.  Thorax 2012; 67:12-18. [PubMed]
VX-809,(lumacaftor), a cystic fibrosis transmembrane conductance regulator (CFTR) modulator, has been shown to increase the cell surface density of functional F508del-CFTR in vitro. This randomised, double-blind, placebo-controlled study evaluated the safety, tolerability and pharmacodynamics of VX-809 in adult patients with cystic fibrosis (n=89) who were homozygous for the F508del-CFTR mutation. Subjects were randomised to one of four VX-809 28 day dose groups (25, 50, 100 and 200 mg) or matching placebo. The type and incidence of adverse events were similar among VX-809- and placebo-treated subjects. Respiratory events were the most commonly reported and led to discontinuation by one subject in each active treatment arm. Pharmacokinetic data supported a once-daily oral dosing regimen. Pharmacodynamic data suggested that VX-809 improved CFTR function in at least one organ (sweat gland). VX-809 reduced elevated sweat chloride values in a dose-dependent manner (p=0.0013) that was statistically significant in the 100 and 200 mg dose groups.

There was no statistically significant improvement in CFTR function in the nasal epithelium as measured by nasal potential difference, nor were there statistically significant changes in lung function or patient-reported outcomes. No maturation of immature F508del-CFTR was detected in the subgroup that provided rectal biopsy specimens.

The authors concluded that VX-809 had a similar adverse event profile to placebo for 28 days in F508del-CFTR homozygous patients, and demonstrated biological activity with positive impact on CFTR function in the sweat gland. Additional data are needed to determine how improvements detected in CFTR function secondary to VX-809 in the sweat gland relate to those measurable in the respiratory tract and to long-term measures of clinical benefit.

2013 Farinha CM. King-Underwood J. Sousa M. Correia AR. Henriques BJ. Roxo-Rosa M. Da Paula AC. Williams J. Hirst S. Gomes CM. Amaral MD. Revertants, Low Temperature, and Correctors Reveal the Mechanism of F508del-CFTR Rescue by VX-809 and Suggest Multiple Agents for Full Correction. Chem Biol 2013; 20:943-55. [PubMed]
Cystic fibrosis is mostly caused by the F508del mutation, which impairs CFTR protein from exiting the endoplasmic reticulum due to misfolding. VX-809 (lumacaftor)  is a small molecule that rescues F508del-CFTR localization, which recently went into clinical trial but with unknown mechanism of action (MoA).  Herein, the authors assessed if VX-809 is additive or synergistic with genetic revertants of F508del-CFTR, other correctors, and low temperature to determine its MoA. They  explored and integrated those various agents in combined treatments, showing how they add to each other to identify their complementary MoA upon correction of F508del-CFTR.

The authors’ experimental and modeling data, while compatible with putative binding of VX-809 to NBD1:ICL4 interface, also indicate scope for further synergistic F508del-CFTR correction by other compounds at distinct conformational sites/cellular checkpoints, thus suggesting requirement of combined therapies to fully rescue F508del-CFTR.

2013 Ren HY. Grove DE. De La Rosa O. Houck SA. Sopha P. Van Goor F. Hoffman BJ. Cyr DM. VX-809 corrects folding defects in cystic fibrosis transmembrane conductance regulator protein through action on membrane-spanning domain 1. Mol Biol Cell 2013; 24:3016-24. [PubMed]
CFTR contains two membrane-spanning domains (MSDs), two nucleotide-binding domains (NBDs), and a regulatory domain, and its channel assembly requires multiple interdomain contacts. The most common CF-causing mutation, F508del, occurs in NBD1 and results in misfolding and premature degradation of F508del-CFTR. VX-809 is an investigational CFTR corrector that partially restores CFTR function in people who are homozygous for F508del-CFTR. To identify the folding defect(s) in F508del-CFTR that must be repaired to treat CF, the authors explored the mechanism of VX-809 action. VX-809 stabilized an N-terminal domain in CFTR that contains only MSD1 and efficaciously restored function to CFTR forms that have missense mutations in MSD1. The action of VX-809 on MSD1 appears to suppress folding defects in F508del-CFTR by enhancing interactions among the NBD1, MSD1, and MSD2 domains. The ability of VX-809 to correct F508del-CFTR is enhanced when combined with mutations that improve F508del-NBD1 interaction with MSD2.

The authors suggest that the use of VX-809 in combination with an additional CFTR corrector that suppresses folding defects downstream of MSD1 may further enhance CFTR function in people with F508del-CFTR.

2014  Eckford PD. Ramjeesingh M. Molinski S. Pasyk S. Dekkers JF. Li C. Ahmadi S. Ip W. Chung TE. Du K. Yeger H. Beekman J. Gonska T. Bear CE. VX-809 and related corrector compounds exhibit secondary activity stabilizing active F508del-CFTR after its partial rescue to the cell surface. Chem Biol 2014; 21(5):666-78. [PubMed]
The most common mutation causing cystic fibrosis (CF), F508del, impairs conformational maturation of CF transmembrane conductance regulator (CFTR), thereby reducing its functional expression on the surface of epithelia. Corrector compounds including C18 (VRT-534) and VX-809 have been shown to partially rescue misfolding of F508del-CFTR and to enhance its maturation and forward trafficking to the cell surface.

The authors show that there is an additional action conferred by these compounds beyond their role in improving the biosynthetic assembly. In vitro studies show that these compounds bind directly to the metastable, full-length F508del-CFTR channel. Cell culture and patient tissue-based assays confirm that in addition to their cotranslational effect on folding, certain corrector compounds bind to the full-length F508del-CFTR after its partial rescue to the cell surface to enhance its function. These findings may inform the development of alternative compounds with improved therapeutic efficacy.

2014 Baroni D. Zegarra-Moran O. Svensson A. Moran O. Direct interaction of a CFTR potentiator and a CFTR corrector with phospholipid bilayers.Euro Biophys J 2014; 43(6-7):341-6.[PubMed]
Cystic fibrosis transmembrane conductance regulator (CFTR) potentiators and correctors are new drugs that target the basic CFTR protein defect and are expected to benefit cystic fibrosis patients. To optimise the substances so far proposed for human use, and to minimise unwanted side effects, it is essential to investigate possible interactions between the drugs and cell components. The authors used small-angle X-ray scattering with synchrotron radiation to analyse the effects of two representative drugs, the potentiator VX-770 (Ivacaftor), approved for human use, and the corrector VX-809 (Lumacaftor), on a model phospholipid membrane.

By reconstruction of the electron density profile of unilamellar vesicles treated with VX-770 or VX-809 the authors found that these drugs penetrate the phospholipid bilayer. VX-809 becomes homogeneously distributed throughout the bilayer whereas VX-770 accumulates predominantly in the internal leaflet, behaviour probably favoured by the asymmetry of the bilayer, because of vesicle curvature.
Penetration of the bilayer by these drugs, probably as part of the mechanisms of permeation, causes destabilization of the membrane; this must be taken into account during future drug development.

–     It is still early days in the use of these correctors and potentiators. They have such a profound effect on those treated that it is not surprising they have a variety of other effects and such studies are welcome.

2017 Talamo Guevara M, McColley SA. The safety of lumacaftor and ivacaftor for the treatment of cystic fibrosis. Expert Opin Drug Saf. 2017 Nov;16(11):1305-1311. doi: 10.1080/14740338.2017.1372419. Epub 2017 Sep 21. [Pubmed]
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 favours therapy.

– A comprehensive summary of the potential problems with these drugs.

2017 Barnaby R, Koeppen K, Nymon A, Hampton TH, Berwin B, Ashare A, Stanton B. Lumacaftor (VX-809) restores the ability of CF macrophages to phagocytose and kill Pseudomonas aeruginosa. Am J Physiol Lung Cell Mol Physiol. 2017 Nov 16:ajplung.00461.2017. doi: 10.1152/ajplung.00461.2017. [Epub ahead of print] [Pubmed]
Studies were conducted to examine the effects of lumacaftor alone and in combination with ivacaftor (i.e., Orkambi) on the ability of human CF (del508Phe/del508Phe) monocyte derived macrophages (MDMs) to phagocytose and kill P. aeruginosa. Lumacaftor alone restored the ability of CF MDMs to phagocytose and kill P. aeruginosa to levels observed in MDMs obtained from non-CF donors (WT-CFTR). This effect contrasts with the partial correction (~15%) of del508Phe Cl- secretion of airway epithelial cells by lumacaftor. Ivacaftor reduced the ability of lumacaftor to stimulate phagocytosis and killing of P. aeruginosa. Lumacaftor had no effect on P. aeruginosa-stimulated cytokine secretion by CF MDMs. Ivacaftor (5 µM) alone, or in combination with lumacaftor, reduced the secretion of several pro-inflammatory cytokines.

The authors suggest the clinical efficacy of Orkambi may be related in part to the ability of lumacaftor to stimulate phagocytosis and killing of P. aeruginosa by macrophages.

2017 Loo TW, Clarke DM. Corrector VX-809 promotes interactions between cytoplasmic loop one and the first nucleotide-binding domain of CFTR. Biochem Pharmacol. 2017 Jul 15;136:24-31. doi: 10.1016/j.bcp.2017.03.020. Epub 2017 Mar 31.  [Pubmed]
A large number of correctors have been identified that can partially repair defects in folding, stability and trafficking of CFTR processing mutants that cause cystic fibrosis (CF). The best corrector, VX-809 (Lumacaftor), has shown some promise when used in combination with a potentiator (Ivacaftor). Understanding the mechanism of VX-809 is essential for development of better correctors. Here, the authors tested their prediction that VX-809 repairs folding and processing defects of CFTR by promoting interactions between the first cytoplasmic loop (CL1) of transmembrane domain 1 (TMD1) and the first nucleotide-binding domain (NBD1).

Their results (which are detailed in the full summary) suggest that the mechanism by which VX-809 promotes maturation and stability of CFTR is by promoting CL1/NBD1 interactions.

2019 Ferrera L, Baroni D, Moran O. Lumacaftor-rescued F508del-CFTR has a modified bicarbonate permeability.J Cyst Fibros. 2019 Feb 6. pii: S1569-1993(18)30927-5. doi: 10.1016/j.jcf.2019.01.012. [Epub ahead of print] [Pubmed]


Deletion of phenylalanine at position 508, F508del, the most frequent mutation among Cystic fibrosis (CF) patients, destabilizes the protein, thus causing both a folding and a trafficking defect, resulting in a dramatic reduction in expression of CFTR. In vitro treatment with lumacaftor produces an enhancement of anion transport in cells. We studied the permeability properties of the CFTR mutant F508del treated with the corrector lumacaftor, showing that the rescued protein has selectivity properties different than the wild type CFTR, showing an augmented bicarbonate permeability. This difference would indicate a diverse conformation of the rescued F508del-CFTR, that is plausibly reflected on an improper regulation of the airway surface liquid, lessening the efficacy of the corrector. Our findings rather support the idea that a combination of correctors would be required to address the CFTR-dependent bicarbonate permeability.

Loretta Ferrera (figure)  is at the Istituto Giannina Gaslini, U.O.C. Genetica Medica, Genova, Italy. 

The authors suggest these findings indicate that early CF lung disease is characterized by an increased mucus burden and inflammatory markers without infection or structural lung disease and suggest that mucolytic and anti-inflammatory agents should be explored as preventive therapy.

– An interesting combined contribution to the increasing work to understand early lung changes in CF . 

2018 Carlile GWYang QMatthes ELiao JRadinovic SMiyamoto CRobert RHanrahan JWThomas DY.A novel triple combination of pharmacological chaperones improves F508del-CFTR correction.  Sci Rep. 2018 Jul 30;8(1):11404. doi: 10.1038/s41598-018-29276-y  Free PMC Article   [Pubmed]
Pharmacological chaperones (e.g. VX-809, lumacaftor) that bind directly to F508del-CFTR and correct its mislocalization are promising therapeutics for Cystic Fibrosis (CF). However to date, individual correctors provide only ~4% improvement in lung function measured as FEV1, suggesting that multiple drugs will be needed to achieve substantial clinical benefit. Here we examine if multiple sites for pharmacological chaperones exist and can be targeted to enhance the rescue of F508del-CFTR with the premise that additive or synergistic rescue by multiple pharmacological chaperones compared to single correctors indicates that they have different sites of action.                                                                                                                                First, we found that a combination of the pharmacological chaperones VX-809 and RDR1 provide additive correction of F508del-CFTR. Then using cellular thermal stability assays (CETSA) we demonstrated the possibility of a third pharmacologically important site using the novel pharmacological chaperone tool compound 4-methyl-N-[3-(morpholin-4-yl) quinoxalin-2-yl] benzenesulfonamide (MCG1516A).                                                                     All three pharmacological chaperones appear to interact with the first nucleotide-binding domain (NBD1). The triple combination of MCG1516A, RDR1, and VX-809 restored CFTR function to >20% that of non-CF cells in well differentiated HBE cells and to much higher levels in other cell types. Thus the results suggest the presence of at least three distinct sites for pharmacological chaperones on F508del-CFTR NBD1, encouraging the development of triple corrector combinations.

Kessler L. Can lumacaftor-ivacaftor reverse glucose-tolerance abnormalities in cystic fibrosis? J Cyst Fibros. 2020 May 5. pii: S1569-1993(20)30126-0. doi: 10.1016/j.jcf.2020.04.013. [Epub ahead of print] [Pubmed]

     Laurence Kessler

We are thankful to Manfred Ballmann and his colleagues for their interest in our recently published article [1]. By analyzing the effect of one year of lumacaftor–ivacaftor treatment on metabolic status in 40 cystic fibrosis patients affected by early glucose tolerance abnormalities, we observed an improvement in glucose tolerance, together with favourable changes in weight and pulmonary function. Furthermore, both one- and two-hour glycemia decreased in the subgroup, with improved oral glucose tolerance test (OGTT) categories. In their correspondence, Manfred Ballmann et al. propose that the effects of lumacaftor–ivacaftor should be interpreted with caution, due to the selected study population and natural history of glucose-tolerance abnormalities in cystic fibrosis.

The study population was composed of patients with either initial impaired glucose tolerance (IGT) or newly diagnosed cystic fibrosis-related diabetes (CFRD), since the moderate functional alteration of beta cells is still potentially reversible at this stage of the disease. We agree with Manfred Ballmann because, in our study, we did not consider the potential worsening of glucose tolerance in subjects with NGT after treatment. However, at this stage of CFRD, very early structural alterations of islets can be observed in the pancreata of CF patients. As such, CFRD suggests that the CFTR modulator is of low interest at this stage of the disease. Bogdani et al. [

2] have evaluated the morphology of tissue from very young CF children (<4 years of age), as well as adult patients with CF and CFRD. The relative number of beta cells in young CF tissue was reduced by 50% or more, when compared to age-matched controls. Furthermore, young CF tissue displayed significantly smaller insulin-positive areas. CFRD pancreata exhibited greater islet injury, with further reduction in islet density and a decreased relative number of beta cells. Together, these results strongly suggest that an early deficiency in beta-cell number in CF may contribute to the development of glucose intolerance in the young CF population and, later in life, to CFRD.

Although our analysis could not exclude any impact of the natural disease history, various longitudinal studies have reported spontaneous improvements in glucose tolerance in CF patients, as well as improvements in patients’ nutritional status and respiratory function. This is due to the natural disease history. To illustrate this point, Manfred Ballmann cited data from the Scheuing study [3], which observed substantial variability in glucose-tolerance abnormalities in CF, as a large cohort of 1128 CF patients benefited from 4643 OGTT over 9 years. Scheuing reported regression from CFRD to NGT in 21.7% of cases, which is comparable with our study data (22.2%, including 31 patients with IGT and 9 with CFRD). Interestingly, when we analyze Scheuing’s [3] data for patients with IGT, 40.1% of patients returned to NGT and 14.6% altered their glucose tolerance and developed a CFRD. Conversely, in our study, 58% of patients returned to NGT and no patients presented with diabetes. Obviously, analysis of these data should consider the limited number of patients involved in our study.

In comparison with other types of diabetes, CFRD is characterized by very particular abnormalities of glucose tolerance. Early postprandial hyperglycemia is indicated by one-hour glucose values at OGTT and by continuous glucose measurement, which may impact lung function and nutritional status for several years prior to the development of diabetes. Recently, research has highlighted a decrease in the incretin effect, together with the role of a CFTR chloride-channel defect, in terms of both beta-cell function and also in alpha cells. This facilitates better understanding of these particular glucose-tolerance abnormalities, as impaired suppressibility of the glucagon release has been reported in CF patients, after an OGTT that possibly contributes to the development of glucose intolerance [4]. From our perspective, it is impossible for our study to exclude the influence of the lumacaftor–ivacaftor on the improvement of glucose tolerance.

However, our study suggests that the CFTR modulator plays a positive role at the very early stage of glucose-tolerance abnormalities in CF, without being able to demonstrate whether this is a direct effect that targets CFTR, the consequence of an improvement in nutritional and respiratory status, or both. Currently, there is an overall lack of large studies that explore treatments for CFRD. As such, there is a need for more analysis of metabolic parameters in randomized studies that evaluate more effective CFTR modulators (e.g., the new triple-combination CFTR modulator [5].

  1. Misgault, B., et al., Effect of one-year lumacaftor-ivacaftor treatment on glucose tolerance abnormalities in cystic fibrosis patients. J Cystic Fibrosis. (Abstract included in section 2020B of
  2. Bogdani alStructural abnormalities in islets from very young children with cystic fibrosis may contribute to cystic fibrosis-related diabetes  2017; 7: 17231
  3. Scheuing al.High variability in oral glucose tolerance among 1,128 patients with cystic fibrosis: a multicenter screening study.PLoS ONE. 2014; 9e112578
  4. Edlund al.CFTR is involved in the regulation of glucagon secretion in human and rodent alpha cells.Sci Rep. 2017; 7: 90
  5. Middleton al.Elexacaftor-tezacaftor-ivacaftor for cystic fibrosis with a single Phe508del allele. N Engl J Med. 2019; 381: 1809-1819      32387043

Laurence Kessler is Professor of Endocrinology and Diabetology at Strasbourg University Hospitals, France