Cystic fibrosis (CF) is the most common lethal genetic disease affecting Caucasians. CF is an autosomal recessive disease with an incidence of between 1 in 2000 and 1 in 3000 live births (Cutting, G. R., Accurso, F., Ramsey, B. W., and Welsh, M. J., Online Metabolic & Molecular Bases of Inherited Disease, McGraw-Hill, 2013). There are over 70,000 people affected worldwide, of which approximately 33,000 are in the United States (www.cff.org/What-is-CF/About-Cystic-Fibrosis/). The hallmarks of CF are excessive mucus secretion and defective mucus clearance resulting in obstruction, infection and inflammation in the airways; pancreatic insufficiency; and elevated sweat chloride concentration. CF is a multisystem disease affecting the lungs, pancreas, and gastrointestinal, hepatobiliary, and reproductive tracts (R. D. Coakley et al., in Cystic Fibrosis, Eds. Hodson, M., Geddes, D., and Bush, A., Edward Arnold, Third Ed., 2007, pp. 59-68).
For most patients, there is a high burden of care for supportive therapies that do not address the root cause of the disease. Supportive therapies include physical airway clearance techniques, inhaled medications (mucolytics, antibiotics, and hypertonic saline), oral anti-inflammatory drugs, pancreatic enzyme replacements, and nutritional supplements (Cystic Fibrosis Foundation Patient Registry 2011 Annual Data Report to the Center Directors, Cystic Fibrosis Foundation, Bethesda, Md., 2012). The median age of survival for patients with cystic fibrosis is into the fourth decade of life.
Cystic fibrosis is caused by mutations in the gene for CFTR (Cystic Fibrosis Transmembrane conductance Regulator), an ion channel found in epithelia as well as other tissues. CFTR is found at the apical membrane of epithelial cells in the airways, intestine, pancreas, and sweat glands (G. R. Cutting, Accurso, F., Ramsey, B. W., and Welsh, M. J., Online Metabolic & Molecular Bases of Inherited Disease, McGraw-Hill, 2013). Mutations in CFTR have been classified into six types (Welsh, M. J., and Smith, A. E., Cell, 1993, 73, 1251-1254 and Sloane, P. A., and Rowe, S. M., Curr. Opin. Pulm. Med., 2010, 16, 591-597): 1) premature termination due to deletion, nonsense, or frameshift mutations, 2) defective trafficking out of the endoplasmic reticulum due to improper folding, 3) improper gating, 4) reduced conductance due to changes in the channel pore, 5) reduced production of channel due to altered splicing, and 6) increased endocytosis from the plasma membrane.
Nearly 2,000 different mutations in CFTR are known to cause CF. Deletion of Phe508 of CFTR (F508del) occurs in approximately 70% of CFTR alleles (Bobadilla, J. L. et al., Human Mutation, 2002, 19, 575-606). Approximately 50% of patients are F508del homozygotes and ca. 40% are heterozygotes so that at least one copy of F508del is present in about 90% of patients. G551D is the third most common mutation and is present in about 4% of patients (Cystic Fibrosis Foundation Patient Registry 2011 Annual Data Report to the Center Directors, Cystic Fibrosis Foundation, Bethesda, Md., 2012).
The F508del mutation causes loss of CFTR function due to both reduced channel density and impaired channel gating. Channel density at the apical membrane is reduced due to protein misfolding. Misfolded CFTR is recognized by cellular quality control mechanisms and degraded (Ward, C. L. and Kopito, R. R., J. Biol. Chem., 1994, 269, 25710-25718). F508del function is further reduced because it has a significantly reduced channel open probability (gating defect) (Dalemans, W. et al., Nature, 1991, 354, 526-528). The G551D mutation results in a protein with normal folding but impaired gating (Illek, B. et al., Am. J. Physiol., 1999, 277, C833-C839).
Small molecules called ‘correctors’ have been shown to reverse the folding/trafficking defect of F508del CFTR and increase the density of CFTR channels at the plasma membrane (Pedemonte, N. et al., J. Clin. Invest., 2005, 115, 2564-2571, Van Goor, F. et al., Am. J. Physiol. Lung Cell. Mol. Physiol., 2006, 290, L1117-1130, Van Goor, F. et al., Proc. Nat. Acad. Sci. USA, 2011, 108, 18843-18848). ‘Potentiators’ are small molecules that increase the channel open probability of mutant CFTR, reversing the gating defect. Pharmacological repair of F508del is thought to require at least a corrector and a potentiator to address the folding and gating defects while G551D may see benefit from a potentiator only.
Kalydeco® (ivacaftor, VX-770) is a marketed potentiator that improves the gating characteristics of G551D. In G551 D patients, it substantially improved lung function (percent predicted FEV1 increased 10-13%), allowed weight gain, and reduced the frequency of pulmonary exacerbations (Ramsey, B. W. et al., New Eng. J. Med., 2011, 365, 1663-1672, Davies, J. C. et al., Am. J. Resp. Crit. Care Med., 2013, 187, 1219-1225). Kalydeco® is also approved for people with G1244E, G1349D, G178R, G551S, S1251N, S1255P, S549N, and S549R mutations and application to other mutations including those with partial function is being investigated.
While monotherapy with Kalydeco® did not lead to any appreciable improvement in F508del homozygote patients (Flume, P. A. et al., Chest, 2012, 142, 718-724), a combination of a corrector (VX-809, lumacaftor or VX-661, tezacaftor) with Kalydeco® resulted in a modest improvement in lung function (percent predicted FEV1 increased 3-4%) (Wainwright, C. E. et al., N. Engl. J. Med., 2015, 373, 220-231, Pilewski, J. M. et al., J. Cystic Fibrosis, 2015, 14, Suppl. 1, S1). The VX-809 plus Kalydeco® combination (called Orkambi®) is a marketed therapy for F508del homozygote patients.
For both the G551D and the F508del patient populations, improved therapies are expected to provide further benefit to patients. Most G551D patients are G551 D/F508del compound heterozygotes and treatment with the combination of the corrector VX-661 plus Kalydeco® resulted in a further increase in lung function over Kalydeco® alone (Pilewski, J. M. et al., J. Cystic Fibrosis, 2015, 14, Suppl. 1, S1).
Mutations in CFTR that are associated with moderate CFTR dysfunction are also evident in patients with conditions that share certain disease manifestations with cystic fibrosis but do not meet the diagnostic criteria for cystic fibrosis. In these patients, CFTR dysfunction at epithelial cell layers can occur and give rise to abnormal mucus and endocrine secretions that are similar to those that characterize cystic fibrosis. CFTR dysfunction may also be acquired. Chronic inhalation of particulate irritants, including cigarette smoke, pollution, and dust can result in reduced CFTR ion-channel activity.
Modulation of CFTR activity may also be beneficial for other diseases not directly caused by mutations in CFTR, such as secretory diseases and other protein folding diseases mediated by CFTR. CFTR regulates chloride and bicarbonate flux across the epithelia of many cells to control fluid movement, protein solubilization, mucus viscosity and enzyme activity. Defects in CFTR can cause blockage of the airway or ducts in many organs, including the liver and pancreas. Potentiators are compounds that enhance the gating activity of CFTR present in the cell membrane. Any disease which involves thickening of the mucus, impaired fluid regulation, impaired mucus clearance or blocked ducts leading to inflammation and tissue destruction could be a candidate for potentiators. Therefore, there exists a significant therapeutic need for novel small molecules that act as potentiators of CFTR.
In addition to cystic fibrosis, CFTR-related diseases or other diseases which may benefit from modulation of CFTR activity include, but are not limited to, asthma, bronchiectasis, chronic obstructive pulmonary disease (COPD), constipation, diabetes mellitus, dry eye disease, pancreatitis, rhinosinusitis and Sjögren's Syndrome.
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