**5. CFTR modulator agents**

CFTR modulator agents are small molecules which 'modulate' the function of the abnormal CFTR protein. Unlike gene therapy, they do not alter the CFTR gene. However, these agents do manipulate the underlying genetic consequence of CF mutations. Currently two different classes of modulator agents have been developed;


#### **5.1 CFTR modulator drug design**

High-throughput drug discovery programs enabled the development of such agents. These discovery programs were established to identify active compounds ("hits") from large chemical libraries suitable for industrial-scale screening. High-throughput screening (HTS) assays need to be robust, have high throughput using small sample volumes together with adequate sensitivity, reproducibility and accuracy to ensure differentiation between a very large amount of compounds [35]. Ion channels are key targets for drug design and thus HTS have been an important part of such drug discovery processes, including for CF [36, 37].

The two classes of small molecules for CFTR protein modulation were identified via HTS techniques from libraries that consisted of chemically diverse drug-like and lead-like compounds acquired from both commercial vendors and internal medicine chemistry programs. If compounds had an activity >2.5 standard deviations (SD) from the mean, then they received further testing. For example, from

~164,000 synthetic compounds initial screened, approximately 100 were suitable for further study in one study [38]. The molecules identified were optimized and evaluated in terms of pharmacokinetics and toxicology [39, 40].

#### **5.2 Potentiator therapy**

The first small molecule clinically available for individuals with CF following HTS was Ivacaftor (Kalydeco®). It is an oral CFTR potentiator agent, which can be given to CF individuals who have gating, residual function, splice or conduction mutations [41–44]. It was originally developed for the Gly551Asp-CFTR mutation (G551D; a class III mutation), which results in defective cAMP CFTR channel gating. The gating of the channel reflects the opening and closed states of the CFTR protein. If gating is defective, then a low probability of CFTR channel opening occurs and in turn reduced overall CFTR function. Ivacaftor treatment results in increased chloride transportation across the cell membrane by improving channel gating and thus the time that activated CFTR channels remain open.

The initial phase 3 studies in individuals aged 12 years and older (STRIVE) and those aged between 6 and 12 years of age (ENVISION) evaluated ivacaftor or placebo in patients with at least one Gly551Asp-CFTR mutation. STRIVE identified a significant improvement in percentage predicted (pp) FEV1 in the treatment arm of 10% at 24 weeks (primary endpoint) that was maintained at 48 weeks. This was together with a 3 kg weight gain, an 8-point increase in the Cystic Fibrosis Questionnaire Revised (CFQ-R) score (an increase in the score out of 100 reflects an improvement in quality of life with a 4-point change being clinically relevant) alongside a reduction in sweat chloride to below the definite diagnostic threshold for CF to a mean of 47.8 mmmol/l [41]. Similar results were demonstrated in children in ENVISION [45]. Participants from both of these studies were then enrolled into the open-labeled extension study (PERSIST) where all individuals received ivacaftor therapy. These individuals maintained the improvements in lung function, weight and exacerbation rates at 144 weeks [46].

#### **5.3 Monotherapy for Phe508del mutations**

Such exceptional clinical outcomes were a major advancement in the treatment options for individuals with CF. However, initially modulator therapy was only suitable for approximately 5% of CF individuals as it was only available for gated mutations. Agents that could alter abnormal protein trafficking together with CFTR channel gating and cell membrane surface stability that results from the Phe508del-CFTR mutation (class II mutation) would have a far greater impact upon the CF community. As multiple stages in the CFTR conformational maturation process are affected with the Phe508del-CFTR mutation, different treatment approaches were needed.

HTS therefore progressed to evaluating agents that would be suitable for other mutation classes, focusing on agents could have an impact on dysfunctional protein trafficking [38]. Lumacaftor is an oral corrector agent, which in vitro studies have demonstrated can corrects protein misfolding [47]. However, monotherapy with either ivacaftor or lumacaftor did not lead to clinically relevant improvements in individuals homozygous for the p.Phe508del-CFTR mutation [48, 49].

#### **5.4 Combination therapy: potentiator and corrector agents**

As monotherapy only lead to minimal clinically relevant outcomes for Phe508del-homozygotes, the argument strengthened for the use of lumacaftor in

**45**

*Overview of CFTR Modulators and Gene Therapy DOI: http://dx.doi.org/10.5772/intechopen.91022*

and mortality, this is still a significant outcome [52, 53].

therapies were associated with significant improvements in CFQ-R.

The combination corrector therapies described, enable individuals with the commonest CF mutation the potential of receiving modulator therapy. However, lumacaftor/ivacaftor and tezacaftor/ivacaftor do not fully restore CFTR protein function. Furthermore, there is still no small molecule therapy for 30% of the individuals with CF who are heterozygotes for Phe508del and have a minimal function (MF) mutation. MF mutations give rise to either the production of defective proteins or no protein production. They include insertion, deletion, nonsense and canonical splice mutations. As up to 90% of CF individuals have one

**5.5 Future modulator therapy**

combination with ivacaftor. Hence, these two therapies were trialed in combination (Orkambi®). Phase 3 multicentre studies (TRAFFIC and TRANSPORT) of this combination versus placebo elicited a modest gain in absolute pp. FEV1 of 3% at 24 weeks (primary endpoint) together with significant increases in body mass index (BMI) [50]. The lung function increase being comparatively small to that seen with ivacaftor for gated mutation. However, importantly the 96-week open label extension study (PROGRESS), where all individuals within the initial trials received lumacaftor in combination with ivacaftor, did demonstrate a 42% reduction in the annual rate of lung function decline when compared with matched US registry controls [51]. As rate of lung function decline is known to correlate with morbidity

Although lumacaftor in combination with ivacaftor is associated with stabilization of lung disease together with weight improvement, patients can experience various side-effects. Respiratory related adverse events were the commonest complications in the trials and up to 7% of patients discontinued treatment in PROGRESS. In real-world experiences, there have been even higher discontinuation rates of up to 30% [54, 55]. Also, lumacaftor is a potent inducer of the CYP3A4 enzymes and can have interactions with various concurrent medications. Development of other corrector agents with an improved side-effect profile and the potential for enhanced correction of the protein trafficking were therefore required. This led to the development of tezacaftor, another small molecule corrector agent. Tezacaftor in combination with ivacaftor (Symdeko®/Symveki®), for individuals homozygous for the Phe508del-CFTR mutation, when compared with placebo resulted in a 4% absolute improvement in ppFEV1, together with a fivepoint improvement in CFQ-R scores but without any significant change in BMI (EVOLVE). Although the increments in lung function were still not as substantial as that seen in ivacaftor use for gating mutations, the adverse events were much lower than with lumacaftor/ivacaftor treatment. The discontinuation rate in the active treatment arm was only 2.9% and none of these were due to respiratory events [56, 57]. It thus appears that the corrector lumacaftor has a poorer side-effect profile than tezacaftor, rather than it being a complete class effect. Tezacaftor/ ivacaftor can also be given to patients who have certain residual function and splice mutations (*E56K, P67L, R74W, D110E, D110H, R117C, E193K, L206W, R347H, R352Q, A455E, D579G, 711+3A*➔*G, E831X, S945L, S977F, F1052V, K1060T, A1067T, R1070W, F1074L, D1152H, D1270N, 2789+5G*➔*A, 3272-26A*➔*G, 3849+10kbC*➔*T*) [58]. Patients with Phe508del and a residual function mutation were studied in the phase III trial EXPAND, which was a crossover study where patients either received tezacaftor-ivacaftor, ivacaftor monotherapy or placebo. The patients were studied for two 8-week intervention periods separated by an 8-week washout period. Change in ppFEV1 (the primary endpoint) was greatest in the treatment arms with a 6.8% increase with tezacaftor-ivacaftor and 4.7% increase with ivacaftor. Both

#### *Overview of CFTR Modulators and Gene Therapy DOI: http://dx.doi.org/10.5772/intechopen.91022*

*Cystic Fibrosis - Heterogeneity and Personalized Treatment*

**5.2 Potentiator therapy**

~164,000 synthetic compounds initial screened, approximately 100 were suitable for further study in one study [38]. The molecules identified were optimized and

The first small molecule clinically available for individuals with CF following HTS was Ivacaftor (Kalydeco®). It is an oral CFTR potentiator agent, which can be given to CF individuals who have gating, residual function, splice or conduction mutations [41–44]. It was originally developed for the Gly551Asp-CFTR mutation (G551D; a class III mutation), which results in defective cAMP CFTR channel gating. The gating of the channel reflects the opening and closed states of the CFTR protein. If gating is defective, then a low probability of CFTR channel opening occurs and in turn reduced overall CFTR function. Ivacaftor treatment results in increased chloride transportation across the cell membrane by improving channel

The initial phase 3 studies in individuals aged 12 years and older (STRIVE) and those aged between 6 and 12 years of age (ENVISION) evaluated ivacaftor or placebo in patients with at least one Gly551Asp-CFTR mutation. STRIVE identified a significant improvement in percentage predicted (pp) FEV1 in the treatment arm of 10% at 24 weeks (primary endpoint) that was maintained at 48 weeks. This was together with a 3 kg weight gain, an 8-point increase in the Cystic Fibrosis Questionnaire Revised (CFQ-R) score (an increase in the score out of 100 reflects an improvement in quality of life with a 4-point change being clinically relevant) alongside a reduction in sweat chloride to below the definite diagnostic threshold for CF to a mean of 47.8 mmmol/l [41]. Similar results were demonstrated in children in ENVISION [45]. Participants from both of these studies were then enrolled into the open-labeled extension study (PERSIST) where all individuals received ivacaftor therapy. These individuals maintained the improvements in lung func-

Such exceptional clinical outcomes were a major advancement in the treatment options for individuals with CF. However, initially modulator therapy was only suitable for approximately 5% of CF individuals as it was only available for gated mutations. Agents that could alter abnormal protein trafficking together with CFTR channel gating and cell membrane surface stability that results from the Phe508del-CFTR mutation (class II mutation) would have a far greater impact upon the CF community. As multiple stages in the CFTR conformational maturation process are affected with the Phe508del-CFTR mutation, different treatment approaches were

HTS therefore progressed to evaluating agents that would be suitable for other mutation classes, focusing on agents could have an impact on dysfunctional protein trafficking [38]. Lumacaftor is an oral corrector agent, which in vitro studies have demonstrated can corrects protein misfolding [47]. However, monotherapy with either ivacaftor or lumacaftor did not lead to clinically relevant improvements in

individuals homozygous for the p.Phe508del-CFTR mutation [48, 49].

As monotherapy only lead to minimal clinically relevant outcomes for Phe508del-homozygotes, the argument strengthened for the use of lumacaftor in

**5.4 Combination therapy: potentiator and corrector agents**

evaluated in terms of pharmacokinetics and toxicology [39, 40].

gating and thus the time that activated CFTR channels remain open.

tion, weight and exacerbation rates at 144 weeks [46].

**5.3 Monotherapy for Phe508del mutations**

**44**

needed.

combination with ivacaftor. Hence, these two therapies were trialed in combination (Orkambi®). Phase 3 multicentre studies (TRAFFIC and TRANSPORT) of this combination versus placebo elicited a modest gain in absolute pp. FEV1 of 3% at 24 weeks (primary endpoint) together with significant increases in body mass index (BMI) [50]. The lung function increase being comparatively small to that seen with ivacaftor for gated mutation. However, importantly the 96-week open label extension study (PROGRESS), where all individuals within the initial trials received lumacaftor in combination with ivacaftor, did demonstrate a 42% reduction in the annual rate of lung function decline when compared with matched US registry controls [51]. As rate of lung function decline is known to correlate with morbidity and mortality, this is still a significant outcome [52, 53].

Although lumacaftor in combination with ivacaftor is associated with stabilization of lung disease together with weight improvement, patients can experience various side-effects. Respiratory related adverse events were the commonest complications in the trials and up to 7% of patients discontinued treatment in PROGRESS. In real-world experiences, there have been even higher discontinuation rates of up to 30% [54, 55]. Also, lumacaftor is a potent inducer of the CYP3A4 enzymes and can have interactions with various concurrent medications. Development of other corrector agents with an improved side-effect profile and the potential for enhanced correction of the protein trafficking were therefore required.

This led to the development of tezacaftor, another small molecule corrector agent. Tezacaftor in combination with ivacaftor (Symdeko®/Symveki®), for individuals homozygous for the Phe508del-CFTR mutation, when compared with placebo resulted in a 4% absolute improvement in ppFEV1, together with a fivepoint improvement in CFQ-R scores but without any significant change in BMI (EVOLVE). Although the increments in lung function were still not as substantial as that seen in ivacaftor use for gating mutations, the adverse events were much lower than with lumacaftor/ivacaftor treatment. The discontinuation rate in the active treatment arm was only 2.9% and none of these were due to respiratory events [56, 57]. It thus appears that the corrector lumacaftor has a poorer side-effect profile than tezacaftor, rather than it being a complete class effect. Tezacaftor/ ivacaftor can also be given to patients who have certain residual function and splice mutations (*E56K, P67L, R74W, D110E, D110H, R117C, E193K, L206W, R347H, R352Q, A455E, D579G, 711+3A*➔*G, E831X, S945L, S977F, F1052V, K1060T, A1067T, R1070W, F1074L, D1152H, D1270N, 2789+5G*➔*A, 3272-26A*➔*G, 3849+10kbC*➔*T*) [58]. Patients with Phe508del and a residual function mutation were studied in the phase III trial EXPAND, which was a crossover study where patients either received tezacaftor-ivacaftor, ivacaftor monotherapy or placebo. The patients were studied for two 8-week intervention periods separated by an 8-week washout period. Change in ppFEV1 (the primary endpoint) was greatest in the treatment arms with a 6.8% increase with tezacaftor-ivacaftor and 4.7% increase with ivacaftor. Both therapies were associated with significant improvements in CFQ-R.

#### **5.5 Future modulator therapy**

The combination corrector therapies described, enable individuals with the commonest CF mutation the potential of receiving modulator therapy. However, lumacaftor/ivacaftor and tezacaftor/ivacaftor do not fully restore CFTR protein function. Furthermore, there is still no small molecule therapy for 30% of the individuals with CF who are heterozygotes for Phe508del and have a minimal function (MF) mutation. MF mutations give rise to either the production of defective proteins or no protein production. They include insertion, deletion, nonsense and canonical splice mutations. As up to 90% of CF individuals have one Phe508del mutation, if small molecule therapy could significantly increase the amount of functional protein for this mutation, a greater range of CF individuals could be treated as then the therapy would be suitable for those individuals with the Phe508del-MF mutations.

Next generation CFTR correctors are under evaluation in combination with tezacaftor/ivacaftor. Phase 2 and 3 trials of these triple therapy agents; VX-659 and VX-445 have provided further exciting results. These corrector agents have a different structure and mechanism of action and provide additive function to the other two agents. For individuals homozygous for Phe508del an increase in absolute ppFEV1 was 9.7% for VX-659 treatment and 11% for VX-445 therapy. Greater increases in lung function were seen for patients with Phe508del-MF mutations; the absolute change in ppFEV1 was 13.3 and 13.8% for VX-659 and VX-445 respectively. These increases were also alongside significant improvements in quality of life and have been maintained in subsequent phase 3 interim report analyses [59–62]. These are incredible outcomes for individuals with more severe mutations and thus who typically have more severe disease phenotypes.

### **6. Implications of modulator therapy**

Important advances in the clinical outcomes for individuals with CF have been possible since the introduction of modulator therapy. Unfortunately these treatments are currently associated with a substantial cost and as a result are not available for all eligible patients. In the United States the Food and Drug Administration (FDA) has approved all four of the currently available modulator therapies. Some European countries and Australia, have access to ivacaftor, lumacaftor/ivacaftor and tezacaftor/ivacaftor [63]. However, worldwide there is significant inequality of access to these agents.

As an increasing number of modulator agents become available, the CF community will need to determine how they can enable patients to receive these expensive therapies. If funding bodies are going to approve them, it is likely that they will require significant clinical outcomes from their use, especially when funding is through a public system.

## **7. Future directions**

The introduction of modulator therapy, particularly when its use becomes widespread, is likely to have an impact upon the range of CF phenotypes. The amount of phenotypic variations should decrease as fewer patients have significant CFTR channel dysfunctional, and it is likely that the disease manifestations will be less severe. It is well known that respiratory related CF disease is associated with less than 10% CFTR channel function and so there is the potential for modulator therapy to have an impact upon this [21]. However, measurements of the degree of change in CFTR channel functionally with the use of modulator therapy is not being undertaken in the clinical setting. The markers being assessed are all surrogate markers of CFTR channel function and include lung function, sweat chloride, weight and quality of life questionnaires. Hence, it will be interesting to see the long-term impact of significant CFTR modulation on the CF cohort when individuals have been on such treatment for many years from birth. Currently a significant improvement is felt to be an increase in ppFEV1 greater than 10%. Time will tell as to whether such changes have a significant impact upon this long-term multi-system disease.

**47**

**Author details**

Catherine Rang1

, Tom Kotsimbos1,2 and John Wilson1,2\*

2 Department of Medicine, Monash University, Melbourne, Australia

\*Address all correspondence to: john.wilson@monash.edu

provided the original work is properly cited.

1 Department of Respiratory Medicine, Alfred Health, Melbourne, Australia

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Overview of CFTR Modulators and Gene Therapy DOI: http://dx.doi.org/10.5772/intechopen.91022*

The advancements in CF care over the last decade have been remarkable. The use of HTS drug discovery programs have been instrumental in enabling the development of the CFTR modulator agents, first the potentiators and subsequently the corrector agents. The fact that such therapies target the underlying consequence of the CFTR mutation has led to exciting clinical outcomes for individuals with certain CFTR mutations because altering the function of the CFTR protein at the molecular level is essential for true disease change to occur. "Genetic medicines" require a significant improvement in their clinical outcomes before they can become a viable option to modulator therapy. They do however have the advantage that they are not specific for individual mutation classes and could be used as treatment for all

The introduction of newer targeted therapies is transforming CF care, although it remains to be seen how these treatments will impact the CF community in the longer term. Nevertheless, a shift is starting to occur whereby treatments are determined based upon an individual's genetic mutations. It is likely that this will lead to a more personalized model of care and it is hoped a step closer to a cure for

CR – no conflict of interest. TK – Clinical Trial Support and Consultancy fees for Vertex Pharmaceuticals, Inc. JW – Clinical Trial Support and Consultancy fees for

**8. Conclusion**

patients.

this life-limiting disease.

**Conflict of interest**

Vertex Pharmaceuticals, Inc.
