Cystic Fibrosis: Emergence of Highly Effective Targeted Therapeutics and Potential Clinical Implications

Abstract

Cystic fibrosis (CF) remains the most common life-shortening hereditary disease in white populations, with high morbidity and mortality related to chronic airway mucus obstruction, inflammation, infection, and progressive lung damage. In 1989, the discovery that CF is caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene that encodes a cAMP-dependent anion channel vital for proper Cl⁻ and HCO₃⁻ transport across epithelial surfaces provided a solid foundation for unraveling underlying disease mechanisms and the development of therapeutics targeting the basic defect in people with CF. In this review, we focus on recent advances in our understanding of the molecular defects caused by different classes of CFTR mutations, implications for pharmacological rescue of mutant CFTR, and insights into how CFTR dysfunction impairs key host defense mechanisms, such as mucociliary clearance and bacterial killing in CF airways. Furthermore, we review the path that led to the recent breakthrough in the development of highly effective CFTR-directed therapeutics, now applicable for up to 90% of people with CF who carry responsive CFTR mutations, including those with just a single copy of the most common F508del mutation. Finally, we discuss the remaining challenges and strategies to develop highly effective targeted therapies for all patients and the unprecedented potential of these novel therapies to transform CF from a fatal to a treatable chronic condition.

Contents

Evolution of CF Pathogenesis

 Mechanisms of CFTR Dysfunction

 Alternative Targets to Compensate for CFTR Dysfunction

 Consequences of CFTR Dysfunction in the Airways

Transformation of CF Therapy

 CFTR Modulators

 Circumventing CFTR

Transformation of CF Therapeutic Development

 Understanding the Impact of New Therapies

 Changing Drug Development Landscape and the Need for Innovative Trials

Conclusions

The care and prognosis of people with cystic fibrosis (CF) is in the midst of a transformation. This has been heralded by the successful development of CFTR modulators that address the basic defect, leading to substantial clinical improvement, and has advanced in application from a small and genetically focused subset of the CF population to a broad population that includes those with one or two copies of the most common F508del CF mutation, addressing up to 90% of individuals with CF (1). The basis of this transformation ultimately stems from groundbreaking work that led to the discovery of the CFTR gene and its underlying cell biology, which began to usher in an era of mutation-based therapy. In tandem, foundational studies have broadened our understanding of underlying pathophysiology, particularly related to how the CF ion transport defect leads to organ pathophysiology. The result has been novel therapies closely tied to the underlying pathogenesis of CF that have led to meaningful improvement in outcomes over the last decade, as well as the identification of treatment opportunities currently in development that will continue to advance the care for all individuals with CF.

Evolution of CF Pathogenesis

Mechanisms of CFTR Dysfunction

The CFTR anion channel

The discovery of the CFTR gene 30 years ago was a key milestone in the elucidation of the underlying cause of CF and set the stage for the development of CFTR-directed therapeutics (2–4). Biochemical and functional studies showed that despite the structure of CFTR featuring two transmembrane domains, two nucleotide-binding domains, and a unique regulatory (R) domain resembling that of adenine nucleotide-binding cassette transporters (5), CFTR functions as an anion channel (6). CFTR channels are expressed at the apical membrane of epithelial cells lining the surface and submucosal glands of the conducting airways and the lumens of the exocrine pancreas, gastrointestinal tract, sweat glands, and some other epithelial and nonepithelial tissues. CFTR channels conduct Cl⁻ and HCO₃⁻ on activation by PKA (protein kinase A)-dependent phosphorylation and play a central role in transepithelial ion/fluid transport and the regulation of the volume, salt content, and pH of the airway surface liquid (ASL) and other epithelial secretions (7) (Figure 1).

Figure 1.

Role of CFTR in healthy airways and molecular mechanisms causing CFTR dysfunction in cystic fibrosis airways. (A) In healthy airways, CFTR is expressed at the apical surface of airway epithelial cells together with the epithelial Na⁺ channel ENaC. CFTR plays a central role in cAMP-mediated anion (Cl⁻ and HCO₃⁻) secretion, and ENaC is limiting for the absorption of sodium and fluid across the airway epithelium. Coordinated regulation of CFTR and ENaC enables proper airway surface hydration and effective mucociliary clearance. (B–D) In cystic fibrosis (CF), a spectrum of CFTR mutations causes CFTR dysfunction via different molecular mechanisms. (B) CFTR nonsense, frameshift, or canonical splicing mutations (class I) abrogate CFTR synthesis. (C) Many missense mutations, including the most common F508del mutation, impair CFTR folding (class II) and lead to retention in the endoplasmic reticulum and degradation by the proteasome. (D) Some missense and noncanonical splicing mutations produce CFTR anion channels that reach the cell surface but are not fully functional because of a spectrum of defects, such as altered regulation reducing their open probability (class III), diminished ion conductance (class IV), reduced amount of functional CFTR (class V), or decreased membrane residence time of CFTR at the apical surface (class VI). A common consequence of CFTR dysfunction and unbalanced ENaC-mediated sodium/fluid absorption is airway surface dehydration and impaired mucociliary clearance, setting the stage for airway mucus plugging, chronic infection, and inflammation in patients with CF. CFTR = cystic fibrosis transmembrane conductance regulator; ENaC = epithelial sodium channel. Adapted by permission from Reference 19.

Traditional classification of CFTR mutations

Genetic studies have identified a large spectrum of more than 2,000 variants in the CFTR gene (http://www.genet.sickkids.on.ca), of which ∼400 mutations have been confirmed to be disease causing so far. CFTR mutations have been grouped into six classes (I–VI) according to the dominant mechanism through which they cause CFTR dysfunction (8–10) (Figure 1). Class I mutations, including nonsense, frameshift, and canonical splicing mutations, lead to premature termination codons (PTCs) and severe defects in CFTR synthesis. Class II mutations lead to impaired folding and trafficking, causing a severe reduction in the number of CFTR expressed at the cell surface; these mutations include F508del, which is the most common CF-causing mutation and is present on at least one allele in up to 90% of people with CF worldwide. Class III mutations cause impaired channel gating with severely reduced open probability of CFTR channels expressed at the apical cell membrane. Class IV mutations impair channel conductance. Class V mutations reduce the abundance of functionally normal channels because of abnormalities in promoter activity or noncanonical splicing defects. Class VI mutations reduce the stability of CFTR channels at the cell surface. Studies of the relationship between mutation class, CFTR function, and clinical CF phenotype demonstrated that class I, II, and III mutations generally result in little to no CFTR activity and severe CF multiorgan disease; class IV, V, and VI mutations retain residual CFTR function that is associated with a less-severe disease phenotype best evidenced by exocrine pancreatic sufficiency, noting there are frequent exceptions when missense alleles cause molecular abnormalities that are incomplete. This classification scheme predicted that fundamentally different strategies will be required to overcome these distinct molecular defects (i.e., correction of intracellular CFTR synthesis and processing for class I and II mutations vs. activation, augmentation, and stabilization of mutant CFTR channels that are expressed at the cell surface for class III–VI mutations). This paradigm has therefore been generally useful for the understanding of the molecular pathogenesis and clinical classification, as well as for the development of CFTR-directed therapeutics.

Expanded classification of CFTR mutations and theratyping

Despite a well-defined classification system for CFTR defects, it is increasingly recognized that single mutations can impair CFTR function by multiple molecular defects, thus preventing a one-to-one mapping between mutations and classification. For example, F508del, the most common class II mutation, not only impairs protein folding and plasma membrane expression but also exhibits a gating defect characteristic of class III mutations, as well as reduced stability at the plasma membrane pathognomonic for class VI mutations (10). Similar defects spanning multiple mutation classes have been described for other common mutations, such as W1282X, R334W, A455E, R347P, N1303K, and R117H (10). These findings may explain at least in part the heterogeneity in responsiveness to pharmacological treatment that has been observed among different CFTR mutations within the same class in vitro and has led to an expanded classification system that accounts for the multiple molecular defects found for some specific CFTR mutations (10). However, for many rare CFTR mutations, particularly for missense variants in which consequences cannot be readily predicted, the molecular and functional consequences and their disease liability remain unknown (11). Efforts are underway to close this knowledge gap by detailed functional and clinical characterization of an increasing number of rare CFTR mutations (www.cftr2.org). In this context, the concept of “theratyping” has recently been introduced, where unclassified CFTR variants are assigned to theratype groups on the basis of their impact on CFTR quantity and function (independent of mutation class), as well as their responsiveness to CFTR-directed therapeutics in vitro (12).

With the emergence of patient-derived model systems, such as intestinal organoids and cultures of conditionally reprogrammed nasal or bronchial cells from patients with rare or even unique CFTR genotypes, this theratyping approach may provide a unique opportunity to realize the potential of personalized medicine for people with CF (13–15). In vitro testing of response to therapy in these patient-derived models may be complemented by n-of-1 studies to determine clinical efficacy in individual patients with rare mutations (12). All these efforts will likely be facilitated by the recent breakthrough in obtaining the three-dimensional structure of full-length human CFTR by cryoelectron microscopy. This advance provides insights into the impact of specific mutations on CFTR channel structure and function at unprecedented resolution, as well as potential opportunities for structure-based drug design and optimization (16–18). Precision-based approaches that use an individual’s primary nasal airway cells, rectal cells to derive intestinal organoids, or induced pluripotent stem cells may provide the potential to predict efficacy, particularly for ultrarare mutations in which clinical trials are not feasible. Alternatively, they may offer a pathway to optimize therapy on an individual basis (12).

Alternative Targets to Compensate for CFTR Dysfunction

CFTR is expressed in concert with other ion channels and transporters that participate in the homeostatic regulation of the volume and pH of the ASL and other epithelial secretions. In particular, the amiloride-sensitive epithelial Na⁺ channel ENaC, the alternative Cl⁻ channels TMEM16A and SLC26A9, and the proton pump ATP12A have been implicated in CF pathogenesis or as alternative targets to compensate for deficient CFTR-mediated Cl⁻ and/or HCO₃⁻ transport in the airways and other organs affected by CF (19, 20). Therapeutic strategies that are based on these alternative targets and their current stage of clinical development are provided in the section on CF therapies, below.

The epithelial Na⁺ channel ENaC

The epithelial Na⁺ channel ENaC is a highly selective small conductance (∼4–5 pS) Na⁺ channel composed of three homologous subunits (α, β, and γ) that constitutes the limiting pathway for active absorption of Na⁺ and fluid from airway surfaces (Figure 1). Based on bioelectric measurements showing increased ENaC activity in addition to deficient cAMP-dependent Cl⁻ conductance in CF airway epithelia, ENaC has long been implicated in the pathogenesis of ASL depletion in CF (19). A series of studies suggested that ENaC is hyperactive in CF airways because of its dysregulation by mutant CFTR (21, 22). More recently, it was found that ENaC activity can also be increased by proteases such as neutrophil elastase that are released from immune cells in inflamed CF airways and activate otherwise “silent” ENaC channels by proteolytic cleavage (23). Via both mechanisms, increased ENaC activity will aggravate the imbalance between fluid secretion and absorption and is thus a key player in the pathogenesis of airway surface dehydration in CF.

The Ca²⁺-activated Cl⁻ channel TMEM16A

Early functional studies detected an alternative CaCC (Ca²⁺-activated Cl⁻ channel) that retains its activity in CF airways and has thus obtained attention as an alternative target to compensate for deficient CFTR-mediated anion transport. However, endogenous activation of this alternate Cl⁻ channel using ATP or denufosol, which act to elevate intracellular Ca²⁺ via triggering purinergic (P2Y2) signaling, was found to be rather transient. Furthermore, more detailed studies on the role of CaCC in health and CF lung disease have been hampered because its molecular identity was unknown (19). Therefore, the identification of TMEM16A, also known as ANO1 (anoctamin-1), as the protein that is responsible for CaCC activity in the airways was a major breakthrough that has led to novel insights into the regulation and function of this alternative anion channel in CF (24). Interestingly, it was found that TMEM16A expression in the airways is strongly induced by cytokines that stimulate mucus production and goblet cell metaplasia (IL-4 and IL-13) and is mainly localized to the apical membrane of mucin-producing goblet cells that express little or no CFTR (25). These observations suggest that TMEM16A-mediated anion secretion may have an important role in the formation and clearance of mucus released from goblet cells in chronically inflamed CF airways.

The alternative Cl⁻ channel SLC26A9

More recently, SLC26A9 (solute carrier family 26, member 9) has emerged as another alternative Cl⁻ channel of interest for CF. SLC26A9 forms constitutively active Cl⁻ channels that are coexpressed with CFTR in the airways and several other organs affected by CF, including the pancreas and the intestine (26, 27). Similar to TMEM16A, SLC26A9-mediated Cl⁻ secretion is induced by inflammatory stimuli and was shown to prevent airway mucus obstruction in a mouse model of IL-13–induced mucus hypersecretion (28). Emerging evidence suggests molecular and functional interactions between SLC26A9 and CFTR that may be implicated in epithelial Cl⁻ transport in health, as well as deficient Cl⁻/fluid secretion in CF (27). Consistent with this notion, genetic studies found associations between SLC26A9 variants and the severity of CF lung disease as well as the risk for other organ manifestations, such as meconium ileus and CF-related diabetes (27). These studies support SLC26A9 as an important disease modifier and potential alternative target to compensate the Cl⁻ transport defect in multiple organs affected by CF.

The proton pump ATP12A

Recent studies identified the nongastric proton pump ATP12A as a potential modifier of abnormal pH regulation on CF airway surfaces (20). These studies showed that ATP12A is expressed in the apical membrane of human epithelial cells lining the airway surface and submucosal glands. In the absence of CFTR-dependent HCO₃⁻ secretion, imbalanced proton secretion by ATP12A acidified ASL, which impaired host defenses in CF airways by inactivating antibacterial proteins resident on the airway surface (20). These results provided novel insights into the pathophysiology of abnormal pH regulation on CF airway surfaces and suggest ATP12A as potential therapeutic target, both to augment host defense and to accelerate mucus clearance.

Consequences of CFTR Dysfunction in the Airways

Over the past two decades, the emergence of patient-derived airway models—foremost, primary bronchial epithelial cultures, as well a growing number of animal models featuring CF-like lung disease (29–31)—has continuously improved our understanding of how CFTR dysfunction leads to accumulation of highly viscoelastic mucus and impaired host defense in the airways and how that sets the stage for chronic inflammation and infection, which in turn drive progressive bronchiectasis in people with CF.

Impaired mucociliary clearance

Mucociliary clearance (MCC) of inhaled irritants and pathogens is an important defense mechanism of the airways that is prone to fail in CF, probably via different mechanisms. First, studies on the impact of CFTR dysfunction in the superficial airway epithelium demonstrated that an imbalance between impaired CFTR-dependent secretion and intact or even hyperactive ENaC-mediated absorption of ions and fluid in CF leads to ASL depletion. In primary airway epithelial cultures, it was found that this ASL depletion leads to a collapse of cilia and impaired mucociliary transport (32) (Figure 1). The importance of airway surface dehydration in the pathogenesis of CF lung disease is buttressed by the phenotype of the βENaC-overexpression mouse. In this model, ASL depletion was mimicked by airway-specific overexpression of ENaC and caused impaired MCC and chronic lung disease with key features of CF, including mucus plugging, chronic airway inflammation, and structural lung damage (29). Furthermore, biophysical studies provided mechanistic insights into the link between airway surface dehydration and mucociliary dysfunction. These studies showed that dehydration (i.e., hyperconcentration) of the mucus gel on CF airway surfaces results in an increase in osmotic pressure to levels that exceed the osmotic pressure of the subjacent periciliary layer, leading to compression of cilia, slowing of mucus transport, and ultimately mucus stasis and adhesion (33, 34).

Second, CFTR dysfunction in submucosal glands has long been implicated in the pathogenesis of CF lung disease (35). With the development of the CF pig model that contains submucosal glands along the cartilaginous airways similar to humans, it became possible to study the impact of glandular CFTR malfunction on mucus transport in vivo. Using computed tomography–based particle tracking in the airways of newborn pigs, it was found that activation of mucus secretion from submucosal glands by cholinergic stimulation led to a heterogeneous clearance defect in CF pigs, with some particles being stuck while others were cleared normally from the airways (36). Studies in freshly excised airway tissues showed that this clearance defect was associated with the secretion of mucus strands from submucosal glands that remained attached to the gland ducts, thus hindering MCC in CF pigs, whereas strands were detached and rapidly cleared in wild-type pigs (36). These studies also indicated that both CFTR-mediated Cl⁻ and HCO₃⁻ secretion are required for normal detachment and transport of mucus strands from submucosal glands (37). These findings are also consistent with the concept that the reduced availability of HCO₃⁻ plays and important role in abnormal mucus formation and function in CF airways (38, 39), including chelation of Ca²⁺ by HCO₃⁻ to facilitate mucin unfolding to its transportable form, consistent with mucus abnormalities in organs (pancreas, intestine), where dehydration plays less of a role (40).

Third, once chronic inflammation and infection are established, mucus properties in CF airways may be further altered by secondary events such as oxidation. Emerging evidence suggests that oxidative stress in neutrophilic inflammation facilitates cross-linking of mucins via the generation of disulfide bonds, thereby leading to a marked increase in viscoelasticity that contributes to pathologic mucus gel formation in CF (41). Detection of permanent mucus flakes in BAL fluid from young children with CF that correlated with the severity of inflammation and were already present before the onset of computed tomography–defined structural lung disease and/or bacterial infection suggests that this process already occurs in early CF lung disease (42). In this context, it is noteworthy that studies in CF ferrets and βENaC-overexpressing mice showed that mucus plugging per se causes neutrophilic inflammation in the absence of bacterial infection (43, 44) and suggested that this proinflammatory process is at least in part mediated by airway hypoxia leading to the release of IL-1α from hypoxic epithelial cells and triggering of airway neutrophilia via IL-1 receptor signaling (45).

Taken together, these data support that CFTR dysfunction in submucosal glands and the surface epithelium leads to multiple abnormalities in mucus formation and concentration; perhaps coupled with intrinsic HCO₃⁻-dependent abnormalities that alter mucus viscosity, this impairs mucus detachment and transportability and thus contributes to impaired mucus clearance, which in turn paves the way for inflammation and infection in CF airways. Once inflammation is established, abnormalities in viscoelastic properties of CF mucus are aggravated by secondary hits such as oxidation and cross-linking. All these mucus defects occur early in CF lung disease and precede structural lung damage and infection.

Impaired bacterial killing

In addition to impaired MCC, studies in CF pigs have linked CFTR dysfunction to impaired bacterial killing as an independent host defense defect rendering the airways susceptible for chronic infection with CF pathogens such as Staphylococcus aureus and Pseudomonas aeruginosa. These studies showed that newborn CF pigs fail to eradicate bacteria from their lungs after birth (30). It was found that lack of CFTR-mediated HCO₃⁻ secretion rendered airway ASL pH in CF pigs acidic, which led to reduced activities of antimicrobial peptides such as lactoferrin and lysozyme (46). Conversely, an increase in ASL pH by inhibition of the proton pump ATP12A reversed this host defense abnormality in CF pigs, supporting an important role of reduced ASL pH in bacterial killing and as a potential therapeutic target (20). However, evidence of reduced ASL pH in patients with CF is limited and conflicting, and recent in vivo measurements reported that ASL pH in children with CF is similar to that of children without CF (47).

Despite some open issues and remaining controversies regarding the relative roles of these defects caused by CFTR dysfunction and their mechanistic links to chronic inflammation and infection in CF lung disease, the emergence of highly effective CFTR-directed therapeutics provides an unprecedented opportunity to correct all of these defects in people with CF.

Transformation of CF Therapy

The improved understanding of CF pathogenesis and strong motivation to mitigate its manifestations has led to a multifaceted therapeutic approach (1). Supportive therapies including hydrators of the airway surface, mucolytics, inhaled antimicrobials, systemic antiinflammatory treatments, and nutritional support are the mainstays of CF treatment and have contributed to improved life expectancy over the last decade that has now exceeded 47 years in the United States (48). Although these topics have been reviewed elsewhere (49), it is notable that recent progress in the development of CFTR modulators, first approved for specific and rare genotypes, are providing pronounced efficacy, altering disease progression, and even modifying organ manifestations in young patients. As additional CFTR-directed therapies reach the overwhelming majority of people with CF, and other ion channel–directed pathways to circumvent CFTR are in development, we are in the midst of a transformation in the care of patients with CF that promises to alter the face of the disease.

CFTR Modulators

CFTR potentiators

CFTR potentiators activate mutant CFTR by potentiating channel gating that is governed by PKA-mediated phosphorylation and cyclic AMP (50–54), a feature useful for the treatment of many CFTR mutations to augment their function, even those without innate gating abnormalities (Figure 2). The small molecule ivacaftor, as well as several other CFTR potentiators, were discovered as part of high-throughput screening campaigns (50–54) and bind CFTR within the membrane to augment ATP-independent channel opening (55). Ivacaftor induced ∼50% CFTR activity in G551D/F508del-CFTR–expressing primary epithelial cells and native patient tissues assayed by electrophysiologic techniques (53, 56). This dose-dependent effect was confirmed in a phase 2 study among individuals with CF due to G551D CFTR (57) and recapitulated in long-term studies, which demonstrated improved FEV₁% (by 10%) compared with placebo, reduced pulmonary exacerbations by 55%, augmented weight gain, improved respiratory symptoms, and substantially reduced sweat chloride (mean values to ∼55 mEq/L, below the traditional diagnostic threshold of 60 mEq/L). This collection of outcomes has defined the highly effective CFTR modulator therapy (HEMT) benchmark and has served as a standard for assessing new CFTR modulators in development, in vitro, and in proof-of-concept studies. Subsequent studies in children aged 6 to 12 years extended these results to younger age groups (58, 59). Ivacaftor is generally safe and well tolerated (60), noting drug levels can be affected by drug interactions related to CYP3A4 (61). Other CFTR potentiators, including deuterated ivacaftor (62), QBW251, and GLPG1837 (63), have demonstrated biological and clinical activity in short proof-of-concept trials in patients with gating mutations or other related missense alleles. Several others have demonstrated activity in vitro in primary human airway cell models. Overall, these results suggest a class-wide effect of CFTR potentiators.

Figure 2.

Schematic of approach to CFTR restoration with small molecules, by mutation class. Therapeutic approaches with definitive studies in solid boxes; those under investigation with dashed boxes. CFTR mutations can be addressed pharmacologically based on their underlying molecular defect. Nonsense mutations, a form of class I mutation, have the potential to be restored by premature termination codon (PTC)-suppressing drugs that induce full-length functional CFTR protein. As some protein forms generated by translational readthrough are dysfunctional, the addition of correctors and/or potentiators may also be needed. Misfolded mutations including Phe508del require multiagent therapy to first restore processing and trafficking with one or two CFTR correctors, then augmentation of ion channel function by CFTR potentiators. Mutations that are localized to the cell surface but are dysfunctional, as in class III, IV, and V mutations, can be addressed by CFTR potentiators alone. Class VI mutations, not shown, are expected to behave similarly. CFTR amplifiers that increase CFTR transcription and/or translation efficiency would also be expected to have broad ranging effects across mutation classes once efficacy is established. UGA = urinidine-guanosine-adenosine ‘opal’ stop codon. Illustration by Patricia Ferrer Beals. Adapted by permission from Reference 1.

Highly effective CFTR modulation has broadened our original understanding of the role of CFTR in disease pathogenesis. Ivacaftor resolves mucus plugging in airways susceptible to reproducible mucus obstruction events (64, 65), accelerates MCC (66, 67), and increases exhaled nitric oxide levels (68). Other beneficial effects include structural improvements as observed by radiographic imaging, such as reduced bronchiectasis (69–72) and sinus disease severity (73–75). An association between ivacaftor use and reduced frequency of P. aeruginosa–positive cultures obtained by routine clinical sampling has been observed in observational studies (76, 77), suggesting changes in innate defense alter bacterial growth (78). Microbiome analyses have shown log-fold reductions in bacterial counts after initiation of ivacaftor, but they have also shown that bacteria counts can rebound over subsequent years (79). Weight gain observed with ivacaftor treatment in G551D patients may be related to its beneficial effect on intestinal pH via augmented bicarbonate secretion through the proximal intestine and possibly residual pancreatic ducts; these data suggest that mucosal function might also be improved throughout the gut (67, 80). Improved growth (81), reduced hepatic steatosis (82), as well as positive endocrine effects (e.g., improved insulin secretion [83, 84], and control of CF-related diabetes [85]) have also been reported.

Ivacaftor has been evaluated in young children and infants with CF, in the hope that early administration may preserve organ dysfunction and potentially avoid initial inflammation and bacterial colonization. Ivacaftor improved lung function measured by lung clearance index in patients aged 2 to 6 years (86). Among the youngest population, an increase in the number of children who convert from pancreatic insufficient to pancreatic sufficient is inversely proportionate to age. This suggests that pancreatic injury may be sufficiently reversible to avoid permanent exocrine pancreatic insufficiency in some individuals, a concept also supported by in utero therapy in CF ferrets with the G551D mutation treated with ivacaftor (87).

Extending the benefits of ivacaftor to other mutations has proceeded steadily, first among people with CF who have gating mutations other than G551D (88). Patients with the conductance mutation R117H, which also exhibits residual function and gating abnormalities (89), also responded to ivacaftor; yet, acute changes in efficacy were not as substantial as changes seen in people with the G551D mutation, especially in those with minimal lung disease at baseline (90). Other CFTR mutations that have residual activity (e.g., other conductance mutations, mild processing mutations, or noncanonical CFTR splice variants [91]) exhibit ivacaftor responses that are generally proportionate to baseline levels of CFTR function (91). This finding, supported by clinical data (92), led the U.S. Food and Drug Administration (FDA) to accept in vitro testing to extend approval of ivacaftor to 38 mutations, given its known efficacy and safety profile, and a strong understanding of the CFTR biology and the assay system (93). The breadth across genotypes with a similar mechanism and consistency of benefit across outcomes, both near and long term, established ivacaftor therapy as the benchmark for highly effective CFTR modulation (Figure 3).

Figure 3.

Clinical benchmarks for CFTR modulation. (A) Clinical improvement by change in FEV₁ percent predicted versus placebo at the time of the primary endpoint is plotted for each of the phase 3 trials when available (or phase 2 trials when drug development was halted) in the populations shown. Tezacaftor (TEZ)/ivacaftor (IVA) in F508del (F)/residual function mutation (RF) may approach the highly effective benchmark for many patients, as baseline CFTR activity is higher in that group, by definition. (B) CFTR activity measured in vitro using primary human bronchial epithelial cells on stimulation with CFTR agonists, and estimated as a proportion of wild-type (WT) CFTR activity based on published data, are plotted compared with the change in FEV₁ percent predicted for the groups shown in A. WT CFTR activity is estimated from donors without cystic fibrosis and can vary over time. Data are determined from references or public presentations (92, 102, 110, 111, 169, 178, 179) and adapted and updated from a prior review (180). The effect of elexacaftor (ELX)/TEZ/IVA in patients homozygous for F508del includes the added effects of TEZ/IVA and the subsequent addition of ELX to that population. G = G551D; LUM = lumacaftor; MF = minimal function mutation.

Correctors of F508del CFTR

Correcting the folding and function of F508del, the most dominant mutation found among individuals with CF, has been a major therapeutic focus (Figure 2). High-throughput screening campaigns led to the development of lumacaftor, among other novel correctors (54, 94, 95). Lumacaftor restored sufficient protein folding, trafficking, and stability of the F508del allele as to restore ∼15% of normal CFTR activity to cells derived from F508del-homozygous individuals (96). However, in patients, bioactivity was not sufficient to improve spirometry (97), indicating that potentiation of corrected F508del CFTR would also be necessary to impart clinical benefit (98). Lumacaftor in combination with ivacaftor compared with placebo induced a 3.5% improvement in FEV₁, a 35% reduction in CF exacerbations, and improved body mass index in F508del homozygotes. An observational CFTR biomarker study showed that lumacaftor/ivacaftor combination therapy improves CFTR function, assessed by nasal potential difference and intestinal current measurements, to levels of ∼10% to 20% of normal in F508del-homozygous patients (99). Although the rate of progression in FEV₁% was positively affected (100), clinical testing also revealed that lumacaftor had the potential to induce chest tightness, later shown to be an often-transient adverse event unrelated to CFTR (101), but with a higher prevalence in patients with more severe lung dysfunction.

Noting the need to further improve efficacy and avoid drug interactions, the alternative corrector tezacaftor was developed, which has a similar mechanism as lumacaftor but does not stimulate the metabolism of ivacaftor or other drugs metabolized through the CYP3A4 pathway. Tezacaftor/ivacaftor treatment demonstrated dose-dependent benefit in F508del-homozygous individuals, without inducing chest tightness or other related adverse events (102). Six-month testing in F508del homozygotes confirmed improvements in FEV₁ compared with placebo (∼4%) that were similar to those seen with lumacaftor/ivacaftor and a reduction in exacerbations (by 35%). This, together with an improved adverse effect and pharmacologic profile, set the stage for multiagent therapy with novel CFTR correctors (103). In an accompanying crossover trial, tezacaftor/ivacaftor, compared with either placebo or ivacaftor monotherapy, exhibited substantial efficacy (FEV₁ improvement, ∼7%) in individuals heterozygous for F508del and a residual function mutation that was shown to respond to ivacaftor in vitro (92). As with lumacaftor/ivacaftor, tezacaftor/ivacaftor was not sufficiently active among people with CF with only a single responsive F508del allele. Moreover, tezacaftor/ivacaftor did not improve clinical outcomes over ivacaftor monotherapy in patients with an F508del and a gating mutation.

Alternative correctors have been evaluated and have the potential to be advantageous in specific populations or individuals because of their unique clinical properties. The corrector GLPG2222 was well tolerated in phase 2 trials, with few adverse events when combined with the potentiator GLPG2687; efficacy compared with that of tezacaftor/ivacaftor in patients with a gating mutation (104) or homozygous for F508del (105). Similar responses were seen with phase 2 evaluation of the corrector PTI-801 combined with the potentiator PTI-808 (106), emphasizing the global effects of correctors and the potential for multiagent CFTR modulator therapy.

These important clinical advances established that corrector-potentiator therapy could provide substantial benefit to F508del homozygotes, although activity was not yet sufficient to mimic the experience of ivacaftor in the most responsive mutations (Figure 3). The search for transformative CFTR modulator combination therapy has thus continued, especially for individuals with CF heterozygous for F508del who do not have an allele that responds to current agents, so-called minimal function mutations. The principle target has remained F508del, but the concept of a single efficacious corrector has largely been temporarily abandoned, because misfolded F508del CFTR exhibits two distinct properties that alter its processing and trafficking: namely, stability of the nucleotide binding domain 1 and the capacity to maintain interdomain assembly (107, 108). It was subsequently recognized that compounds that address each of these mechanisms can correct F508del-CFTR folding and trafficking in an additive or synergistic fashion (62, 109). To that end, several next-generation correctors have been devised that augment the activity of F508del-CFTR when added to tezacaftor/ivacaftor. Phase 2 results with two different corrector compounds (VX-445, now known as elexacaftor, and VX-659) were extremely promising and remarkably similar. Marked improvements in spirometry, respiratory symptoms, and sweat chloride were observed in patients heterozygous for F508del and a minimal function allele, patients with CF not previously responsive to tezacaftor/ivacaftor, in addition to F508del homozygotes already using tezacaftor/ivacaftor over 4-week testing periods (62, 109). Phase 3 studies with one of these agents, elexacaftor (VX-445), when used with tezacaftor/ivacaftor, confirmed these findings and achieved the benchmark set by ivacaftor treatment of gating mutations (Figure 3) (110, 111). In patients with only a single F508del allele responsive to CFTR modulator therapy, elexacaftor-tezacaftor-ivacaftor triple-combination therapy improved FEV₁ by 14.3% compared with placebo through 24 weeks, reduced the pulmonary exacerbation rate by 63%, and markedly improved respiratory symptoms (110). A parallel study evaluated patients homozygous for F508del. After a run-in period with tezacaftor-ivacaftor, the addition of elexacaftor increased FEV₁ by 10.0% compared with tezacaftor/ivacaftor over 4 weeks and was accompanied by a large improvement in respiratory symptoms (111). Given the drugs were also safe and well tolerated, the FDA recently approved elexacaftor/tezacaftor/ivacaftor for individuals with CF with at least one F508del allele—a transformative advance that is applicable to nearly 90% of the CF population on a genetic basis and reached (or exceeded) the prior benchmark set by ivacaftor monotherapy for G551D (Figure 3). This increases the overall proportion of patients in whom highly effective CFTR modulations are eligible to just over 90%, because some highly responsive mutations are found in the absence of F508del (G551D, for example). Three-drug regimens building on tezacaftor-based or other CFTR modulator agents could realize further improvements. Further testing of elexacaftor/tezacaftor/ivacaftor in patients with F508del and either G551D or R117H is in progress and could also enhance the degree of CFTR modulation achieved, and likewise clinical benefit, although success is not assured because the clinical impact of this degree of CFTR modulation over and above the effects of potentiating a highly responsive allele (e.g., G551D) has not yet been tested. Given the broad eligibility and profound clinical impact of these novel therapies, it remains imperative that these therapies become globally accessible and that that the complex and diverse issues regarding this access can be overcome (1). We expect this to be major area of emphasis, as CFTR modulators approved in the United States and a minority of European countries are not yet broadly available, and in countries outside of Europe substantial barriers to supportive care remain a substantial concern (1).

CFTR amplifiers

An alternative approach to correcting or potentiating CFTR is to increase CFTR protein expression, thereby amplifying the amount of mutant protein available for correction (112). This tactic has the potential to benefit people with a variety of mutation subtypes (Figure 2). CFTR amplifiers are a new class of modulators that enhance the efficiency of translation initiation of CFTR mRNA. The CFTR amplifier PTI-428 is well tolerated and improved percent predicted (pp) FEV₁ when added to lumacaftor, tezacaftor, or the corrector PTI-801 (113), setting the stage for a variety of combination regimens, including an alternative three-drug combination of a CFTR amplifier, corrector, and potentiator (114, 115).

Translational readthrough

Readthrough of PTCs is a potential approach to restore CFTR function to the nearly 10% of individuals with CF with one or two PTCs (Figure 2). Translational readthrough occurs when small molecules alter translation termination at the site of the PTC, resulting in insertion of a near-cognate amino acid and consequently a full-length functional protein (116). Proof-of-concept aminoglycoside studies showed efficacious suppression of premature stop mutations within CFTR in human subjects, resulting in the synthesis of full-length, functional CFTR protein in vitro (117–121), in mouse models of CF (122, 123) and in small proof-of-concept studies (117–120), noting not all aminoglycoside trials have been successful (124). Regardless, because of the known toxicity and poor bioavailability, more efficacious agents that avoid these undesirable properties are needed. One promising approach used medicinal chemistry to optimize aminoglycosides for readthrough to improve efficacy and reduce toxicity (125) in cell- and animal-based models of CFTR rescue (126). An aminoglycoside derivative improved CFTR function in heterozygous primary human cells with one PTC mutation and does not induce ototoxicity compared with gentamicin in a tissue-based model for ototoxicity (127). Based on promising pharmacology for subcutaneous dosing (128), CF trials are in progress.

Other small molecules to induce readthrough are being evaluated. Ataluren, formerly PTC124 (129), has been extensively studied but did not have consistent beneficial effects on outcomes in clinical trials for CF, likely due to marginal efficacy (130–133). One reason for the poor outcome in vivo may be related to quantitatively low CFTR mRNA expression due to nonsense-mediated decay, resulting in a smaller pool of mRNA available for translational readthrough, a covariate for treatment response (134, 135). This experience, despite early encouraging results, highlighted the need for better preclinical models, such as primary human bronchial epithelial cells with two PTCs and improved animal models with native tissue expression of nonsense mutations (136). Efforts are underway to identify new compounds that exhibit greater readthrough of common CFTR nonsense alleles and that can be augmented by corrector/potentiator therapy, and to incorporate our latest knowledge of CFTR-dependent functional assays (137). Several FDA-approved compounds were identified in an early variant of this study, before medicinal chemistry, but probably do not have sufficient efficacy to warrant further development on their own (138). Another nonsense-specific approach includes transduction of transfer RNAs that insert amino acids into the PTC site, which is natively more efficient but also requires efficient delivery of the transfer RNA coding sequence (139). Antisense oligonucleotides to block nonsense-mediated decay induced by exon junctional complexes represent a further approach applicable to terminal CFTR alleles (140).

Circumventing CFTR

ENaC inhibition

The absorptive function of ENaC contributes to excessive sodium and water uptake from the epithelium, particularly in the absence of CFTR, which exhibits a counteracting secretory function (141). Unopposed ENaC function therefore exacerbates ASL dehydration, representing a therapeutic target that accelerates mucus transport. Nevertheless, despite considerable effort, ENaC approaches have been challenging to translate. This has largely been attributed to challenges posed by ENaC pharmacology. In aggregate, however, the obstacles faced have raised questions about whether affecting airway dehydration alone is sufficient to improve CF outcomes, particularly given limitations of the inhaled route of delivery. The investigational ENaC inhibitor P-1037 (Parion Sciences) was not effective in patients with stable CF during phase 2 testing, either alone (NCT02343445) or in concert with lumacaftor/ivacaftor (NCT02709109). Several other small molecules that augment potency and may allow higher concentrations to be safely delivered are in development, including those developed by AstraZeneca, Boehringer Ingelheim, and others. Many have consistently demonstrated improved MCC in sheep models of CF-induced dysfunction via CFTR inhibitor and/or neutrophil elastase instillation.

Additional approaches to block ENaC are also prominent. Peptide analogs of SPLUNC1 (short palate, lung, and nasal epithelial clone 1) promote channel internalization (142) and can inhibit ENaC to restore ASL homeostasis in vitro (143, 144). Proof-of-concept studies (NCT03229252) have had mixed results with the peptide fragment SPX101, and development was recently halted. Inhibition of ENaC activation by blocking channel-activating proteases like prostasin showed initial promise in vitro (145) and in vivo (146), but whether this can circumvent the protease-rich environment of the inflamed CF airway is in question. Newer molecules such as MKA 104, a repurposed anticoagulant, or QUB-TL1 may be a more effective because of broad bioactivity against neutrophil elastase and furin (147). Antisense oligonucleotides to inhibit ENaC expression by targeting either the α or β subunit are in development (148–154) and have the potential to address specificity, durability, and efficacy posed by small molecules if adequate delivery can be achieved.

TMEM activation and pH correction

Identification of TMEM16A as the principle calcium-activated chloride channel has elevated its potential as a specific target to circumvent CFTR-mediated fluid secretion (155). Small molecules have demonstrated the capacity to activate chloride transport and improve ASL hydration in vitro; future studies will need to ascertain whether mucin expression can be decoupled from TMEM activation, a potential explanation for the failure of denufosol to impart clinical benefit in patients with CF. The discovery that H+ pumps are key in generating an acidic pH in humans and pigs, as opposed to mice where they are not expressed, has also raised the possibility that inhibition of the ATP12a pump could be targeted effectively as a clinical treatment (156). Activating Cl⁻/HCO₃⁻ exchangers represents the converse approach that could address acidic pH if sufficient Cl⁻ were present. Circumventing CFTR via generation of nonspecific ion channels by amphotericin represents a recent and intriguing approach that can improve airway pH, ASL hydration, and mucus viscosity (157); whether unregulated ion channels can function to normalize airway disease will require prospective evaluation in patients.

Transformation of CF Therapeutic Development

Understanding the Impact of New Therapies

Future therapeutic development efforts will depend not only on an understanding of the changes in clinical status among a population with greater access to HEMT but also on the discovery and validation of biomarkers and outcome measures able to capture these significant changes. The CF community has embarked on numerous efforts to complement clinical trials of new therapies to study the transformation of our CF population as the prevalence of CFTR modulator therapy use and their efficacy increases (158, 159). Collectively, these studies evaluate both the clinical and biological efficacy of the modulators from pre- to postmodulator initiation among cohorts recruited from the broader CF population prescribed these therapies. Key findings from these studies have enabled confirmation of clinical benefit associated with CFTR modulators and extended our understanding of the potential benefits of these therapies on previously unrecognized outcomes in clinical trials, including evidence that lack of short-term response to highly effective modulator therapy, as defined by no or decreased pulmonary function or weight gain, is still associated with long-term clinical benefit in these respective endpoints (160). The inclusion of multiple biomarkers in these studies, including MCC, sputum inflammatory markers, gastrointestinal pH profile, and sweat rate and chloride, enables further validation of these endpoints through the use of an active therapy to assess their sensitivity and potential for use in future therapeutic development.

The association between modulator-induced changes in sweat chloride and clinical outcomes in particular remains elusive yet critical to understand and realize the full potential of this endpoint. Although individual studies have failed to demonstrate a lack of clear association between changes in sweat chloride and endpoints such as ppFEV₁ (161, 162), there are clear associations when results are aggregated across a spectrum of CFTR modulators and specific CFTR mutation cohorts for which they have been tested (163). Recently, McCague and colleagues used a cohort of more than 50,000 patients enrolled in the CFTR2 (Clinical and Functional Translation of CFTR) project and in vitro measures of CFTR function to estimate mean changes in outcomes, including ppFEV₁ and sweat chloride, that may be anticipated in response to CFTR functional correction observed with currently approved CFTR modulator therapies (164). Confirming these changes with reasonable precision from a genotypically diverse population of individuals with CF on CFTR modulators for a longer duration than a typical clinical trial and with significantly larger patient numbers is an ongoing effort relying on the CF National Patient Registry that could more firmly establish these associations (165). Global registries, now capturing data on more than 90,000 individuals with CF, have consistently proven to be the most important sources available for characterizing associations between the introduction of new therapies and long-term morbidity and mortality. These registries have a new role as CFTR modulator therapies emerge to broadly capture significant treatment-associated changes in clinical outcomes and the changing epidemiology of CF.

Changing Drug Development Landscape and the Need for Innovative Trials

Although progress in CF therapeutic development has benefitted from a comprehensive and growing understanding of the underlying genetics, pathophysiology, and clinical progression of CF, it remains critically dependent on the investment of industry sponsors committed to CF, strong and effective patient advocacy groups, and specialized CF clinical trial networks such as the CF Therapeutics Development Network (CF TDN, est. 1998), the European Society CF Clinical Trials Network (ESCF-CTN, est. 2008), the United Kingdom’s CF Clinical Trial Accelerator Program (CTAP, est. 2016), and the CF Canada Accelerating Clinical Trials Network (CF CANACT, est. 2018) engaged in the design and execution of efficient, high-quality clinical trials (166). The availability of HEMT to increasingly larger populations of patients with CF could not have been achieved without this partnership and infrastructure, yet greater use of HEMT also creates new challenges for the development of future CF therapies. Drug development is now faced with complex issues ranging from the ethics and safety of withdrawing individuals with CF from modulators that are standard of care to enable the evaluation of new alternative modulators to understanding interest and enthusiasm for participating in future clinical trials, regardless of therapeutic class, among those who have begun HEMT (1). Navigating these challenges represents an opportunity for the CF community to pave the way for successful rare disease drug development.

Building a foundation for the new drug development roadmap in CF requires understanding the changing composition of the CF population, acknowledging a growing heterogeneity resulting from the differential adoption of HEMT as standard of care and addressing the development issues that emerge (Figure 4). Drug development efforts must evaluate the evolving CF population and the impact that the use of HEMT has on the hypothesized target population, including how efficacy might be affected among people with differential access to HEMT. Across the CF drug development pipeline, two key development scenarios emerge that will require new strategies: 1) development of new, alternative CFTR modulators; and 2) development of new “add-on” therapies to treat symptomatic aspects of the disease.

Figure 4.

Trial design issues across a transitional cystic fibrosis population with access to highly effective CFTR modulator therapy (HEMT). SOC = standard of care. Illustration by Patricia Ferrer Beals.

The development of new CFTR modulators that provide comparable or improved efficacy to commercially approved modulators and offer the potential for broader access remains a critical focus of the CF drug development pipeline. Early-phase clinical trials for a new modulator may be more feasible in those regions without access to HEMT, but this strategy must be balanced with the ethical considerations of pursuing development in countries in which the investigational treatment may not become available to the population expeditiously; furthermore, this strategy will be less feasible over time as use of HEMT becomes more widespread. Ultimately, the question of how to include individuals with access to HEMT into the development plans for new CFTR modulators must be tackled to accelerate their evaluation (167). It is critical that future drug development efforts be poised to contemporarily engage the community and formally assess the willingness of patients and their providers to participate in trials necessitating withdrawal of HEMT. A prior survey study demonstrated that the majority of providers (62%) would consider withdrawal of an HEMT, ivacaftor, for up to 4 weeks for a new drug expected to have comparable efficacy, but this willingness was slightly less among individuals with CF or their caregivers (50%) and significantly decreased across both groups for longer durations (168). Recently, the feasibility of short-term withdrawal of ivacaftor was confirmed in a trial of GLPG1837 (a candidate CFTR potentiator) in patients with G551D CF; 24 of 26 (92%) completed the trial, which required a 1-week ivacaftor washout before 4 to 5 weeks of treatment with the candidate potentiator (63). Ivacaftor was reinitiated thereafter. The FDA acceptance of 28-day pulmonary function endpoints in the placebo-controlled randomized pivotal trials for elexacaftor/tezacaftor/ivacaftor, as opposed to 6-month derived endpoints used for prior pivotal programs, demonstrate the shifting flexibility toward more feasible and proximal endpoints for therapies anticipated to have a stable clinical benefit (110, 111), as frequently shown across CFTR modulator programs (169). Ultimately, the path forward will rely on the ability to identify robust and interpretable study designs that minimize or avoid the impact of withdrawal of chronic HEMT.

Active-comparator trials are an alternative to placebo-controlled trials and minimize (but do not avoid) the impact of withdrawal of standard of care. With this strategy, a portion of participants would be randomized to withdraw from current standard-of-care HEMT for a candidate modulator therapy of less-established efficacy and safety. Active-comparator trials in this CF drug development setting are unique, raising additional challenges. The trials would not only need to be run in an open-label fashion but would also likely require use of the participant’s own prescription for the active comparator because of logistics and legal ramifications of drug acquisition for research purposes, cost, and blinding. On the other hand, the derivation of an accepted noninferiority margin for a new modulator expected to be as efficacious as an approved HEMT is not difficult and leads to relatively modest sample sizes for FEV₁ (approximately 60 per group [168]). Ultimately, efforts to design both placebo and active-controlled trials will likely be necessary to address diverse regulatory bodies.

Collectively, the large improvements in lung function, disease-related symptoms, and risk of acute pulmonary exacerbation are evident among those on HEMT, but these improvements are not synonymous with normalization. To what degree early initiation of HEMT may provide even greater health gains is yet to be understood, but the large existing population of those with CF appear likely to both need and benefit from therapies beyond HEMT. Our first charge as we design future clinical trials is to identify the target population for development as a function of the anticipated effect size of the new therapy, factoring in the heterogeneous global population of patients on HEMT (Figure 4). A key question is whether development efforts of new symptomatic therapies, including antiinflammatories and antiinfectives, should enrich their studies with those with more severe lung disease as defined by lower lung function or higher risk of pulmonary exacerbation. These enrichment strategies may prove powerful for maximizing observed treatment benefit (170) but must be balanced by generalizability and feasibility. Future trials must carefully project the anticipated treatment effects among the mixture of patients on HEMT in their trial, potentially recognizing that the minimal clinically important effect that would rationalize the adoption of a new therapy into standard of care may now vary depending on whether the target population has access to HEMT. Larger sample size requirements for trials designed to test for smaller effect sizes in the presence of HEMT encourage use of adaptive trial methodologies, such as group sequential monitoring for early efficacy or futility; this would enable efficient use of patient resources should a trial conclusion be reached early (171). The potential for newer symptomatic therapies to elicit smaller, but clinically meaningful, improvements in traditional clinical endpoints strongly encourages the inclusion of biomarkers to confirm benefit and establish mechanism of action throughout late-phase development. Although not surrogate endpoints (172), these biomarkers may offer critical supportive data toward regulatory approval, particularly when clinical benefit is incremental. As evidenced with the development of lumacaftor/ivacaftor in the younger population with mild CF, sensitive outcome measures, including lung clearance index derived from the multiple-breath washout technique, can offer important confirmatory evidence of efficacy for drugs with modest effects on traditional endpoints (173, 174).

Although the near term may focus on maximizing flexibility in drug development programs through negotiation of the number of required pivotal trials and choice of comparator, endpoints, and study duration, there is an increased opportunity for innovation in trial design. These innovative trial designs include complex adaptive trial designs, such as seamless phase 2/3 studies, incorporation of data from external controls, and the use of master protocols to study multiple diseases, therapies, or patient subgroups (175). Willingness to explore these innovative designs has been hampered by perceived delays, as these trials often require more lengthy design development, agreement with regulatory agencies on study assumptions, and increased operational complexity. However, incentives now exist for sponsors to pursue these innovative designs within programs such as the FDA’s Complex Innovative Designs Pilot Program and with the development of the European Medicines Agency’s regulatory science to 2025 strategy, which promotes the use of novel trial designs (176, 177). “Genotype to phenotype” clinical trials will ultimately be needed to assess in vivo outcomes in patients assigned therapy on the basis of their in vitro response data through theratyping efforts, and these trials may mimic those used in oncology, which use master protocols to study patients with heterogeneous genetic profiles and multiple therapies in a single protocol. These efforts not only pave the path toward the identification of effective therapies for individuals with rare CFTR variants but ultimately form the basis for using precision medicine to optimize CFTR modulator therapy for all individuals with CF.

Conclusions

Although recent advances in the evolution in CF pathogenesis and resulting therapeutic breakthroughs are remarkable, these advances represent a foundational beginning for the next era of scientific discovery and clinical impact. As highly effective CFTR modulators are adopted into standard of care at earlier ages and a broader spectrum of new disease-modifying therapies mature in the therapeutic pipeline, the longer-term impact of these therapies will soon be understood. In parallel, efforts to modify the disease for all continue in addition to the development of robust symptomatic therapies to complement and improve outcomes across the entire CF population. The transformation of CF toward a disease with significantly less morbidity and mortality is clearly on the horizon.