Use of Potentiators and Correctors to Rescue the Various Effects of Mutations in Cystic Fibrosis

USE OF POTENTIATORS AND CORRECTORS TO RESCUE THE VARIOUS EFFECTS OF MUTATIONS IN CYSTIC FIBROSIS

ABSTRACT

Cystic fibrosis, a severe autosomal recessive disorders due to presence of mutations in cystic fibrosis trans membrane conductance regulator (CFTR) gene. More than 2000 mutations have been identified in CFTR gene world wide. All the mutations have been caegorized into six categories on the basis of their effects on CFTR protein expression, structure and functions. The most common cystic fibrosis F508 del in the nucleotide binding domains which impairs CFTR coupled- domain folding, plasma membrane expression, chloride gating channel and stability. The molecules that target the underlying defect in CFTR protein which are categorized into three main types of modulators viz correctors, potentiators and amplifiers. These modulators are able to restore chloride flow, there by they can improve the CF manifestations enough to relieve symptoms associated with CF. Henceforth, it is  most important to distinguish CFTR mutations according to their penetrance for an abnormal phenotype for clinical management, structure, the molecular and cellular mechanism underlying cystic fibrosis. Correctors are principally  target at F508 del cellular misprocessing, while potenticitors are aimed to restone cAMP dependent chloride channel activity of mutant CFTR at the plasma membranes, special emphasis is given on the molecular basis underlying these new therapies and clinically improving result from the clinical trial studies.

Recent studies are strongly suggesting that triple combination for instance Tezacaftors, ivacaftor and third generation modulator therapy are more potent than previously approved modulators.

Keywords: Cystic Fibrosis, CFTR gene,

∆

F508 mutation Corrector, Potenitator.

INTRODUCTION

Cystic fibrosis (CF< MIM# 219700). a severe autosomal recessive disorder of chloride (Cl^–) conductance across epithelial cell is produced by generation of mutations in the cystic fiorosis transmembran conductance regulator gene (CFTR or ABCG₂ MIM # 602421) which are associated with a wide spectrum of clinical phenotypes viz respiratory distress, chronic pancreatitis and male infertility) due to congenital bilateral absence of the vas deferens (CBAVD),  MIM # 22180) (Rosenstein and cutting 1998; welsh et al, 2001; Kerem, 2006; castellani et al 2008). CF is the most common lethal autosomal recessive mono-genic disease in caucassian population with an average prevalence of one in 2500 live birth with carrier frequency 1 in 25 individual (castellani et al, 2009). Now it is well recognized among non- cancassians in the united states with a prevalence of 11 h 9,200 in Hispanies, (1 in 10, 900 in native Americans, 1 in 15,000 in African Americans and in 30,000 in Asians (Hamosh et al 1998). Pronounced improvement in predicted survival for CF patients over two decades is due to immense progress in understanding of the biochemistry and molecular biology of the cystic fibrosis trnasmemberane  conductance regulators (CFTR) gene responsible  for CF. (Cystic fibrosis foundation 2014 registry). Because of this fact, over the past decade, better understanding of the molecular basis of CF and various manifestation which led to the development of molecular therapies that could be to the immense impact on the course of disease in individuals with specific mutational classes. This review will oscillate around the use of two important  therapeutic approaches, termed potentiator and correctors” which governed in modulation and repairing the functions of the CFTR protein resulted from some classes of CFTR mutations. Henceforth, this structural guided therapeutic would be effective approach for CF treatment.

Therefore, it is utmost important to understand  the structural and functional aspects of human CFTR transporter as well as structural and functional alterations in CFTR protein due to presence of mutations in CFTR gene.

Molecular Genetics of Cystic fibrosis:

Primary cause of cystic fibrosis is due to presence  of mutations in cystic fibrosis conductance regulatory gene. The CFTR gene identified and cloned about three decades ago (Riordan et al 1989, Rommens et  al 1989, Kerem et al 1989). The CFTR gene spans an approximately 240 kb region on chromosome 7 q 31 3 and consists of 27 exons. The gene is transcribed into a nature messenger RNA of 6.5 kb. The encoded protein contains 1480 amino acid and has a calculated molecular mass of 168  KD (Fig-1A). CFTR protein form an integral membranes glycoprotein. A comparison of the nucleotide sequences across available databases places this protein as a member of the ABC transporter super family. In the linear amino acid structure of CFTR sequence is subjected to hydropathy plot analysis which disclosed that CFTR protein composed of two motifs, each containing a membrane  spanning domain (MSD) that is compromised  of six transmembrane helices and nucleotide binding domain (NBD) that contain sequence predicted to interact with ATP (Bear et al 1992, Hyde et al. 1990) MSD NBD motifs are linked by a unique domain termed as regulatory domain which contains multiple phosphorylation sites and many charged amino acids used  by protein kinase A and C. The carbonyl terminal, consisting of threonine, arginine and leucine (TRL) which is anchored through a PDZ type- binding interaction with the cytoskeleton (Short et al. 1998).   The NBDs, R domain and the N and C- termini are intracellular, while MSD spans the membrane of epithelial cells (Fig. 1B). CFTR transporter functions as a cAMP regulated chloride channel in exocrine epithelia  (Bear et al 1992).

Biogenesis:

The CFTR  polypeptide is translated from the mRNA on the endoplasmic reticulum (ER) as a linear chain of amiono acids with hydrophilic and hydrophobic domains. The nascent polypeptide is assisted by interactions with chapperon proteins with and outside the ER to assume a tertiary folded structure that buries the membrane spanning domains in the ER membrane the process is relatively slow, taking approximately 10 to 15 minutes to complete, and is followed by two different fates. Depending on the cell type making the CFTR, the wild type chain could be  ubiquinated and sent to the proteosome  for degradation  (Handreson et al 2010). Or it could continue along the trafficking pathaway to the Golgi apparatus. The successful CFTR is packaged into vesicles for transport.  Adequate cl transport through CFTR at the cell surface depends not only the characteristic  of the channel pore or degree of phosphorylation and ATP hydrolysis, but also on member of CFTR proteins and turnover number at cell surface (Riordan 2005). CFTR is actively recycled from the plasma membrane, and some mutants are much less stalbe than WT-CFTR.

CFTR function underlying mechanism of a compelling evidence demonstrates that CFTR is a chloride channel in addition to itself playing a role as a conductor of chloride CFTR is bonafied conductance regulator CFTR also regulates other ionic conductance such as outwardly rectifying chloride channel (ORCC) and epithelial sodium channel (ENAC) and renal outer medullary poteassium channel (ROMK ) (Pilewski et al 1999).

However, CFTR is unique in that a cytosolic regulatory (R)  domain controls channel gating. Phosphorylation of the R- domain by protein Kinase A (PKA) and beinding of ATP to both NBDs are necessary for pore opening (Berger et al 2005, Vergani et al 2005, Aleksandrov et al 2001). ATP hydrolysis at the consensus site, formed by the walker A/B matifs in NBD2 and the signature motif in NBDI, disrupts the NBD dimer and closes the pore (Vergani et al 2005) The degenerate site formed by the walker A/B motifs in NBD1and signature motif in NBD2, binds but does not hydrolyee  ATP (Basso et al 2003). Phosphroylation of CFTR leads to structural transitions that promote channel widening while ahead of extracellular gate opening (Zhang et al 2017). Recent  study disclosed that the ion conduction pathway consists of a large cytosolic vestibule, a narrow transmembrane tunnel (TM), and a single gate near the extracellular  surface. The dephroylated regulatory domain, found in the intracellular opening between the two cytosolic halves of the molecule, is positioned to prevent NBD dimerization (Zhang et al 2016. Therefore, phosphorylation and de phosphorylation  led to alteration in the conformation of CFTR transporter which was determined by cryoelectron microscopy (Zhang et al 2017) The phorsphorylated regulatory domain is disengaged  from its inhibitory position,, as a result. NBDs form an asymmetrical “head to tail’ dimer upon binding ATP. Subsequently, the pore is open to the cytosol but remains closed to the extracellular  space, indicating that local movments of the transmemberane helices can control ion access to the pore even in the NBD dimerized conformation (Sorum et al 2018, Liu et al 2017) (Fig-2)

Spectrum of mutations in CFTR gene and their classification

More than 2000 sequence variants have been identified in the CFTR gene (http//www.genet.sickids.on.cafchrome.hml). and many of them have been implicated in a variety of CFTR related pathologic conditions such as respiratory distress, pancreatic inefficiency, meconeum ileus and congnital absence of vas deferens ( castellani et l 2008).  The world wide prevalence  of 24 relatively common mutations identified in more than 50 chromosomes.

∆

F508 is responsible for approximately two thirds (66%) of all CF Chromosomes; however there is great mutational heterogeneity in the remaining one-third of alleles depending on populations and geographical locations. A group of CFTR alleles also exists in non- Caucasian populations. Few mutations have been found with an unusually high frequency in specific population, indicating founder effective genetic drift. However, there is a broad apectrum of  all types of mutations represented by a large numbers of rare allcles and are distributed throughout  the entire gene IV notably, CF is probably a more common in people of Indian origion than previously thought. Recently our group identified and characterized CFTR mutations in CF subjects presented at PGIMER Chandigarh  India. (Sharma et al 2009, Sharma et al 2009, Sharma et al 2014, Prasad et al 2010.   The mutation

∆

F 508 is the most common. However, it represents only 27% of all analyzed CF alleles. The other  mutations viz S549N, C. 1161 del C, R 709X, R792 X, and (3849+10kbC>T are other common mutations  found among CF subjects. Equal numbers of novel mutations compared to already known mutations reflects heterogonous spectrum of CFTR mutations in India. It is noteworthy here that growing awareness of CFTR mutations that do not always causes CF, and individuals with mild or single organs system manifestations of CFTR related disease have made this Mendelian relationship  more  complex (Sosnay et al. 2016).

CLASSIFICATION OF CFTR MUTATIONS

More than 2,000 mutations have been identified in CF population world wide which are classified  into six classes based on their consequences on protein expression, structure,  traffic and CFTR functions (Fig-3). (Mac Donald et al 2007, welsh et al 2004, Stanke and Tummler 2016).

Class-1 Mutations causes alterations in the synthesis of CFTR protein by affecting  CFTR transcription. These mutations explicit their effect within the nucleus and usually involve the processing of transcript in most cases, the introduction of a premature  termination codon, whether by a change of a codon to a non sense mutation or by a frameshift, results in the degradation of the transcript by a nonsense medicated mRNA decay mechanism (Frischmeyer et al 2007). Major examples of this type of mutation include G, 542X, T1282X, R553X, 621+ 1G →T etc.

Class II mutations lead to misfolding or improper processing of CFTR protein resulting in degradation before it is able to reach the apical membrane i.e.. Phe 508 del, Asn 1303 Lys,Ile 507 del, Arg 560 Thr etc.  (Egan 2016).

The most prevalent mutation is the  Phe 508 del ( C1521 1523 del CTT) which led to a 3 base pair deletion resulting in omission of phenylalanine  position 508. Notably, this mutaton accounts of 70% of all CFTR alleles, and as high as 86% in Northern European Cucasians (Sosnay et al 2013). Phe 508 del CFTR causes protein misfolding resulting in efficient endoplasmic reticulum associated degradation (ERAD) and minimal protein expression at the plasma membrane. Loss of phenylalanine residue within NBDI leads to misfolding of CFTR. It is recognized by chepesones that shunt the misfolded protein to degradation pathway (Jansen et al 1995).

Class III Disease – associated mutations in  this class causes defective channel regulation and gating due to  diminished ATP binding and hydrolysis. However, CFTR properly folded and inserted in the cell memberane, yet not albe to be activated (Anderson et al 1992, Logan et al 1994). Class III mutations include Gly 551 Asp, Gly 178 Arg, Gly 551 Ser, Ser 549 Asn. etc.

Class IV mutations

This class of mutations  led to defective  chloride conductance and in some cases, affect the magnitude and ion selectivity of the channel pore.  Notwithstanding, some of the class IV mutations may reduce flow of ions, some CFTR functions is preserved  which in turn leads to mild from of CF (Sheppard et al 1993, Tabcharani et al 1993). Prevalent mutations of this class includes Arg 117 His, Arg 347 Pro, Arg 117 Cys, Arg 334 Trp etc.

Class V Mutations: are also associated with a milder form of CF, because they can also result in the production of a reduced amount of normally functional protein due to a reduced number of CFTR transcripts due to a promoter or splicing abnormality which includes 3840+10 KBC→T, 2789 + 5 G

→

A, 3120+ IG

→

A, 5 T etc. which leads to aberrant splicing which is incomplete, and a small amount of normally spliced CFTR transcripts  is made, leading to the synthesis of some CFTR protein (Highsmith et al, 1997,  Sosnay, et al 2013).

Class VI mutations are also defined as those that exhibit reduced cell surface half life a property shared by many class II alleles. This class of multations include 4326 del TC, Gln 1422 X, 4279 InsA etc.

A quantum Leap in improvement of CF using potentiators and correctors. A novel approach:    

DNA variation in CFTR, each entry includes the predicted effect of a mutation on gene functions. Eighty two percent of the reported mutations in the CF mutations database have putative deleterious effects, whereas 14% appear to be sequence variants of  no functional consequences, and the remaining are of unknown effect (http://www.genet.sickkids.on.ca/cftr/home html). However, symptomatic therapies have increased the life expectancy of CF patients. Recently, considerable efforts have been  made to identify molecules that can increase either the mutant biosynthetic processing efficiency and cell surface density which are termed as “Correctors” or the activity of plama membrane residual mutant  CFTR activity (Hanraban et al 2013). Potontiators are those pharmacologic molecules those enhance PKA regulated chloride channel gating of CFTR which can be assessed by cell based high- throughput screening assay using fluroenscence based assay of membrane potentical (Van Goor et al 2006), halide efflux (Pedemonte et al 2005). On the other hand correctors are pharmacologic agents those act  as chaperons which are able to “rescue” misfold CFTR and allow trafficking to the cell surface.

CFTR modulators including potentiators and correctors i.e. Correctors (Lumacaftor):Vx-809, VRT-640, VRT-325, Genzyme (Compd48) VX-661, KM11057, Dynosore, 4C, and 15Jf etc.: Potentiators (Ivacaftor: VX-770, VRT-532, PGOI, SF-03, CCF-853, 8F508 at C02 and Genistein etc. This nomenclature describes their mechanism of action, which  is determined by the type of mutation they target (Slone-2010).

Mechanism –based corrector combination restores- CFTR fold and function

Deletion of F 508 del is the most common mutation which is present in at least in least one allele in 90% of CF patients, impairs CFTR folding, stability at the endoplasmic reticulum and  plasma membrane and chloride channel gating which can be rectified completely by the ATP analog P-d ATP (Du et al 2005, Denning et al 1992, Miki et al 2010).

The open probability  of F 508 del – CFTR channel is  15 times lower than that of wild type (WT) channels (Miki et al 2010). CFTR domain swapped architecture forms multiple domain interfaces that appears to be critical in the channel structure and functional integrity (Okiyoneda et al 2013). Molecular targets of available correctors: Class 1 stabilizes the NBD1- MSD1/2 interface, class II target NBD2, and only chemical chapersons, surrogates of Class III correctos, stablizes the human ΔF 508-NBD/ while VX-809 can correct primarily destablizing the NBD1- MSD1/2 interface, functional plasma membrane expression of

∆

F508- CFTR also requires that counteract the NBD1 and NBD2 stability defect in CF bronchial epithelial cells and intestinal organoids. Thus structural guided correetor combination represent an effective  approach for CF therapy(Okiyoneda et al 2000).

Recent study reported that combined effect of VX-770 and VX-809 interpreted as an equilibrium shift  toward the  open channel conformation of F 508 del- CFTR channels  (Kopekin et al 2014). However, both CFTR potentiatiors VX-770 and the correctors VX 809 may not sufficient to restore all functional defects caused by the F508 del mutation (Amaral & Farinha 2013). Of course, this mutant also led to CFTR insensitive to cAMP stimulation and less stable in the cell membrane ( Okiyonda et al 2010, Osteogaard et al 2011). Therefore,  other molecules viz amplifiers send inhibitor  along with Potentiators and corrector are needed to restore maximal functional activity of CFTR protein.

The most prevalent and well known Class III mutation, G 551 D, involves a substitution of glyine for aspartic acid at amino acid 551 which  was founded in 4.5% CF patunts (cystic Fibrosis Foundation Patent Registry 2013). This substitution present at a critical point in the NBD1, the interface with NBD2 (Bompadre et al 2007) However, G551D- CFTR mutant reaches  the plasma membrane of epithelial cells, notwithstanding, protein has a gating defect that abolishes ATP dependent Channel opening resulting in defective channel functioning with very less or no ion transport Ivacaftor (VX770), an investigational drug correct defective channel gating in the CF-causing mutant G551D-CFTR, a CFTR gating mutant whose primary defect is reduced channel open probability after  maximal cAMP stimulation. However VX-770- treated G5511) Channels Po still less than 10% of that of WT channels (Jih and Hwang 2013). Class III potentiator viz (5- nitro-2- (3- phenylpropylamino NPPB) which appears to promote gate opening with unsettled mechanism (Csanady and Torocsik 2014, Lin et al 2016) This channel activation is severely impaired in class III mutation which are associated with more severe disease including pancreatic insufficiency and more rapid lung progression.

Premature termination Codon (PTCs) or nonsenses mutations in the cystic fibrosis trans membrane conductance regulator gene are found in nearly 7% of patients with CF (Watson et al 2004). This class of mutation include frame shift, non sense and splicing mutations are expected to be deleterious. PTCs led to anythesis of truncated protein generally without normal function, as well as reduces  transcript caused by nonsense- mediated mRNA decay (NMD) (Kervestin and Jacobson 2012) Popp and Maquat 2013). The W1282X mutation which truncates CFTR to remove 60% of nucleotide of nucleotide binding domain 2 ( NBD2), is the fifth most common CF – Causing mutation world wide with a prevalence of 50% in Ashkenazi Jewish subject with CF (Watson et al 2004) W1282 X mutation is predicted to generate a  truncated protein (CFTR 1280) to that of wild type CFTR (1480 aa) and thought that CFTR 1281 has defective cellular processing and gating, Several lines of evidence suggest multiple defects in W1282X-CFTR and support corrector, potentiators and read through studies (Veit et al 2016).

Corrector scaffolds of 1- arylpyrazole-4- arylsulfonyl-piperazine and spiro- piperi dine-quinazolinone classes were identified with up to 5 fold greater efficacy than VX-809 (Haggie et al 2017). Remarkably, a phenylsulfonamide pyrrolpyridine  acted asynergi stically with VX- 770 to increase CFTR 1280 function  8 fold over that of VX-770 alone (Haggie et al 2017. These findings suggest  that corrector and potentiator combination may have therapeutic efficacy in cystic fibrosis in cystic fibrosis caused by the W1282X mutation. It is notworthy here that several PTC mutation, including W1274X, Q 1281 Q, 01291X, Y 1307 X, Q131X, Q 1412X and S1455 X and frame shift mutations such as c. 3855 del C, 3884-3885 ins, C 3890-3891 ins T, C. 3884-3885 ins, C,3890-3891 ins T, C. 3908 Dup A, C.4139 del C and C. 4147 – 4148 Ins A, led to detetion of various lengths of the carboxyl terminus of CFTR within NBD2 may be subjected to corrector/ potentiator strategy to reseue the deleterious effects caused by these mutations.

Class IV mutations Viz D1152H, R374D R117H etc. which led to alter the conductivity of the chloride channel  CFTR. In some cases, these mutations are intimate to the ion conduction pore of the CFTR channel (Sheppard et al 1993), whereas others appear to affect conductivity through allosteric mechanisms (Siebert et al 1996).

Class V Mutations reduce the level of wild type CFTR that is produced by splicing mutations. A key example is the 3849+ 10 kb (c. 3717 + 1219C-T) mutation (Highsmith  et al 1994) This mutation activates a cryptic splice site in intron 19 of CFTR that causes mis-splicing of exon on 19. However, the canonical splic site are albe to complete with the cryptic site resulting in the production of 5%-10% of wild type transcript  which is sufficient to escape the pulmonary complications of CF) kermet al 1997). In these mutations, potentiators such as ivacaftor open the CFTR channel and also increase the function of normal CFTR.

The striking characteristics were the presence of severe rare mutations in Indian  populations which are classified as familiar or so called orphan mutation because of their very low incidence. However, rare familial mutations cannot provide sufficient information on the phenotype induced by these mutations (Sharma et al 2014). Notably the cellular and functional information’s in these mutation can improve CF genetic counseling. Functional and cellular consequences of eleven rare missonse mutations Viz L69H, F87L S 118P, G 126S, H139 Q, F 157C, F 494 L, E 543 A, S 549 N, Y852 F and D1270 E present in CFTR gene  from both classical CF patients and CBAVD patients. CFTR maturation process and the corresponding  ion transport  activity were studied by western blot analysis and automated iodide efflux assay respectively. First striking finding was with fact that L69H belongs as a novel class II CF mutation. The trafficking to the plasma membrane of L69H-CFTR was abnormal as corroborated by western blot analyses which revealed only present of a b-band and abudance of L 659 H mutated CFTR protein in the ER on confocal microcopy imaging (Shama et al 2014) Heneforth, the processing path way of this variant is similar to that of F 508 del CFTR mutation. This study also clearly showed that the L69H mutant was responsive to VX 809 as well as to rescue following low temperature incubation  of cells: interestingly, 3D structure of CFTR (Fig-4), L69H is situated in a short cytosolic α- helix in the N-terminus of CFTR preceding the first transmembrane  helix TMI of MSDI which is also called elbow helix and is conserved feature among the ABC exporter family L69H makes hydrophobic face of this helix and is in close contact with hydrophobic side chains of cytosolic extension of MSD1 TM helices, located just below the membrans plane,  (F191), just before TM3 and especially 1368 just after TM61. The network formed by these hydrophobic amino acids in the cytosolie N- terminus extension (L69), in the intracellular loop ICLI (F191) and in the MSDI- NB1), Linker (1368) might thus play an important role for MSDI folding and stabilization of this domain, thereby, L69H may  perturb these mechanisms, which is support hypothesis with the fact that VX 809 has been shown to act on MSD folding and that it also rescued functional defects in CFTR caused by disease related  mutations in the vicinity of three amino is acids highlighted (P67L, (C206 W).

(Ren et al 2013).

Another important  finding concerns the 5549, located in the LSGGFQ signature motif of NBD1 and may alter the hydrolysis of ATP to regulate channel actively was evidenced from reduction in it channel activity, therefore this mutation classified as class IV. This channel activity was potentiated after incubation of cells at 27⁰C or VX 809 (Shama et al 2014).

CLINICAL DEVELOPMENT OF CFTR MODULATORS

The development of a CFTR targeted molecules are of great significance in the treatment and managements of CF disease. Therefore, the progress in the clinical testing of potentiators and correctors of CFTR function in CF patients is utmost exciting developments in CF therapeutics. A phase 2 clinical trial of ivacaftor was assessed its safety profile over 14-28 days of treatment (Accurso et al 2010). Thirty nine adults with CF having at least one Gly551Asp- CFTR allele involved in randomized placebo- controlled, double- blind, multicenter and multidose study. In this study a significant improvements were noticed in reference to two biomarkers of CFTR activity Viz respiratory nasal potential difference and a gold standard diagnostic marker (Sweat chloride concentration) which was also corroborated with a significant alteration  in FEVI (Forced expiratory volume/sec). at a dose of 150 mg ivacaftor/day). These exciting results in phase II trial study, further attempts were made for phase III trial of ivacaftor, (Ramsey et al 2011, (Davies et al 2011) were undergone for long term randomized placebo controlled clinical trials. There was a signifinat improvement in FEV1% (10.5%) at 24 weeks, weight gain of 2.8 kg and or marked reeduction in sweat  chloride as well as a 55% reduction in the probability of experiencing a pulmonary exacerbation during the course of study. Ivacaftor was also found to be both safe and effective in CF patient 6 yrs of age and over subjects participating in both the STRIVE and ENVISION studies were invited to enroll in the PERSIST study. The adults and adolesents who switched from placebo to ivacaftor showed improvemnts in FEV, weight gain and pulmonary exacerbation rate (Mc Kone et al 2013).

Phase 4 Gly 551 Asp observational study (GOAL) in which the patients having one Gly 551 Asp mutation participated and assessed their clinical status before and after  initiation of ivacaftor approved by US-FDA (Rowe et al 2014). Ivacftor had a remarkable effect on body weight and body mass index (BMI) as well as have direct effect on gartnointestinal (GI) PH, led to improved absorption associated with  duodenal alkalization due to involvement of CFTR as a bicarbonate transporter suggesting the normalization of GPH via CFTR protonation may improve pancreatic enzyme function, nutrient digestion and absorption (Rowe et al 2014).

Use of modulators in homonzygous Phe 508del patients

F 508 del has its deleterious effects on CFTR protein at different levels which include misfoldig and loss of activity of chloride channels thereby to restore the activity of CFTR protein is most difficult task (Mendoza et al 2012), Rabeh et al 2012) In view of in vitro evidence a combination of corrector and potentiators possibly more potent, therapy, in F

∆

508 del homogygous mutants, the higher dose of ivacaftor in combination with lumacaftor in CF patients with either homozygous or heterozygous F

∆

508 del demonstrated significant improvement  in sweat chloride level as well as in FEV, In phase II and subsequently in phase III trial (Marigowada et al 2014, Boyle et al 2014). Based on the inspiring lung function results, a combination therapy of both corrector and polentiator was executed in two large multicenter for 24 weak Phase II rendomized control trials termed TRAFFIC and TRANSPORT (Wainwright et al 2015). The rigrous analysis revealed a reduction of CF pulmonary exacerbation (pooled reduction of 35%) increase BMI and a modest improvements in a validated symptoms score (CFQ-R)                                                                         

Intersetingly, it is noteworthy here that U.S. food and drug administration  (FDA) approved a new drug that treats the underlying cause of cystic fibrosis. Tezacaftor ivacator is approved for people with CF ages 12 and older who have two copies of most common CF mutation, F 508 del as well as for individuals who have a single copy of one of 26 specified mutations regardless of their other mutation. This is exciting report for the cystic fibrosis community and a big step forward ongoing efforts to find new and better treatments to address the underlying cause of the disease, said P. W. Campbell, President and CEO of the CF foundation.  We are optimistic that next generation CFTR modulators that blued on tis advance could bring transformative treatments to nearly 90 percent of people with CF, and we remain committed to finding effective new treatments  for every individual living with the disease. It is noteworthy that early studies strongly suggest that triple combination therapies are potentially more effective and could treat significantly more people than previously approved modulators. Additionally, the trials that tested tezacaftor or, ivacaftor  and third modulators therapy showed  positive results in individuals with only one F 508 del mutation, regardless of then beyond mutation. i.e.

711+3A

→(>)

G, 2789+5G

→

A, 3772-26A

→

G, 3849+10kbC

→

T,  A455E,  A1067E, D11OE, D579G, D1152H, D1270N, E56K, E193K, E831X, F1052V, F1074L, K1060T, L206W, P67L, R 74W,  R117C, R347H, R352Q, R1070W, S945L, S977F

Future perspectives

Remarkable developments  over the past decade in small molecules viz (correctors and potentiators therapy) for CF is most potent and efficient advances in this field. Applications of these molecules target underlying molecular defects in CFTR mutant proteins. Notwithstanding limited efficacy of first generation corrector molecules as well as inadequate understanding of their molecular actions, warrants the further study for basic and clinical research. Furthermore, high throughput screening is needed to identify the more efficient and safe molecules for the treatment of CF patients. It is noteworthy here that the thermodynamics of bolding of CFTR protein in relation to its physiochemical interaction as well as moleculer  interactions responsible for channel gating  process is utmost important to develop the third generation correctors and potentiators molecules, F

∆508

is most common mutation  with 80% frequency which  led to the folding and gating defects. New generation molecules would be used to correct the underlying molecular defect in F

∆

508 to maximal level in both in cell study as well as in vivo clinical trials.

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