PROMISE: Working with the CF community to understand emerging clinical and research needs for those treated with highly effective CFTR modulator therapy

Abstract

Highly effective CFTR modulator drug therapy is increasingly available to those with cystic fibrosis. Multiple observational research studies are now being conducted to better understand the impacts of this important therapeutic milestone on long-term outcomes, patient care needs, and future research priorities. PROMISE is a large, multi-disciplinary academic study focused on the broad impacts of starting elexacaftor/tezacaftor/ivacaftor in the US population age 6 years and older. The many areas of investigation and rationale for each are discussed by organ systems, along with recognition of remaining important questions that will not be addressed by this study alone. Knowledge gained through this and multiple complementary studies around the world will help to understand important health outcomes, clinical care priorities, and research needs for a large majority of people treated with these or similarly effective medications targeting the primary cellular impairment in cystic fibrosis.

Background

Clinical care and research in cystic fibrosis (CF) have been heavily influenced by the emergence of highly effective modulator drug therapies (HEMT) that target the CF transmembrane conductance regulator protein (CFTR). These CFTR modulators increase CFTR function by improving the processing, trafficking, and gating of aberrant proteins in the cell. Results show that CFTR modulators significantly, and at times dramatically, improve health outcomes and quality of life^{1, 2}. Herein we highlight a breadth of observational research studies now being conducted to understand the impact of HEMT regimens across a spectrum of disease manifestations. This work is organized within the PROMISE study (NCT04038047) in the United States and sponsored by the Cystic Fibrosis Foundation, independent of the manufacturer. Nearly 700 individuals will ultimately participate to contribute prospective research visits through 2 years of elexacaftor/tezacaftor/ivacaftor (ETI) treatment. This is similar to other recent post-approval trials (i.e. GOAL; NCT01521338, PROSPECT; NCT02477319), but PROMISE will expand upon key questions that have arisen in the modulator era. In the spirit of multi-disciplinary coordination, 8 sub-studies within PROMISE will be conducted by specialty experts. We describe below the rationale, objectives, and deliverables, while highlighting key questions that will remain to be addressed by further research and other investigative efforts (e.g. RECOVER; NCT04602468, BEGIN; NCT04509050) equally important in understanding the future for PwCF treated with modulators. This is a unique and time sensitive opportunity to understand how effective CFTR restoration alters the disease state as a large majority of PwCF begin HEMT. PROMISE, as one of several studies being conducted within the US CF Therapeutics Development Network (TDN), will broadly clarify remaining needs and priorities for clinical care and research in individuals with CF treated with HEMT. As with similar other studies supported by the CF Foundation and conducted in the TDN, upon study completion and publication of primary study results, de-identified datasets will be available through the CFF TDN Data Archive. Researchers may apply to the CF TDN for use of de-identified data from the archive for research purposes, and must receive appropriate IRB approval before data is sent from the Archive. In PROMISE, biospecimens are also being aliquoted and carefully preserved for future investigator-initiated requests through the centralized CF Foundation Biorepository. When approved, these samples can be linked with de-identified data from the Archive.

Current status of highly effective CFTR modulator therapy

CFTR modulators were discovered on the basis of high throughput screening campaigns using cellular models of chloride transport, followed by medicinal chemistry to optimize drug-like properties³. Clinical proof of concept was first established using ivacaftor to potentiate CFTR activity of CFTR gating mutations, such as G551D, and subsequently with other CFTR mutations with retained surface expression but inadequate function⁴. As gating mutations were relatively responsive to small molecule therapy because of retained cell surface expression, activity in vivo was pronounced, helping to establish the potential for CFTR modulators to substantially alter clinical disease⁵. The more common F508del CFTR mutation proved to be a more challenging target, requiring both a corrector to increase CFTR cell surface expression and a CFTR potentiator to augment activity of the rescued channel⁶. While both lumacaftor/ivacaftor and tezacaftor/ivacaftor proved efficacious, bioactivity and clinical effects fell short of the standard established by ivacaftor in G551D, necessitating further research in modulators targeting F508del⁷. Most recently, a combination of tezacaftor and elexacaftor as correctors to more fully restore F508del CFTR expression, and ivacaftor to augment function, produced results in vitro and in clinical trials that met or exceeded the HEMT benchmark⁸. Consequently, phase 3 testing showed improved sweat chloride indicative of CFTR activity that approaches the diagnostic threshold of CF, substantial improvements in lung function -- including those transitioning from antecedent tezacaftor-ivacaftor, improved nutrition, lower rates of pulmonary exacerbation, and better self-reported quality of life¹. Importantly, efficacy was also established in individuals with a single responsive F508del mutation, a population for whom previous CFTR modulator therapy was unsuccessful². The development of ETI has helped establish the era of HEMT for a large majority of PwCF, and an understanding of the long-term effects of ETI and its influence on a broad array of clinical outcome measures is just beginning to emerge⁹.

The PROMISE Study and Core Assessments

PROMISE enrolls those prescribed ETI according to the FDA-approved label, which included PwCF age 12 years and older at the start of the study in late 2019 and at least 1 F508del mutation. The study is conducted at 56 CF research sites across the US and will follow participants for the first 2 years of ETI use. The study has fully enrolled 487 participants, 98% of whom successfully started ETI. Approximately 50% are homozygous for F508del, the majority of whom transitioned to ETI from a 2-drug modulator combination (i.e. tezacaftor/ivacaftor or lumacaftor/ivacaftor). The remaining study participants have 1 copy of F508del and largely have no prior modulator use, except for 35 who also have a G551D allele and previously used ivacaftor monotherapy (Table 1).

Table 1:

Genotype and prior modulator drug use in the PROMISE Study participants

Genotype	modulator drug use when starting ETI
	None  n=238	IVA n=34	LUM/IVA n=64	TEZ/IVA n=151	Total n=487
F508del homozygous	27 (11%)	0 (0%)	63 (98%)	146 (97%)	236 (48%)
F508del plus minimal function mutation*	192 (81%)	0 (0%)	1 (2%)	2 (1%)	195 (40%)
F508del plus G551D	2 (1%)	33 (97%)	0 (0%)	0 (0%)	35 (7%)
F508del plus other mutation	17 (7%)	1 (3%)	0 (0%)	3 (2%)	21 (4%)

IVA: ivacaftor; LUM: lumacaftor; TEZ: tezacaftor

^*as defined in clinical trials of ETI

Research sites have been organized to focus on a shared group of assessments (i.e. ‘Core’ outcome measures) plus additional targeted outcomes distributed to a subgroup of sites and aligned with organ-system sub-studies (Figure 1). Core measurements include lung function by spirometry (e.g. percent predicted forced expiratory volume in 1 second (ppFEV₁)), body mass index (BMI), respiratory symptom-based quality of life, and change in sweat chloride concentration (SwCl). In anticipation of FDA approval for ETI soon to include children 6 to 11 years old, a large pediatric sub-study was added. A separate but related study (BEGIN) will enroll those in infancy to age 5 years old and will evaluate the natural history of CF in young children prior to modulator use, followed by research measuring the impact of ETI in these children who generally have less established end organ damage.

Figure 1.

Organ systems being evaluated in the PROMISE study.

A wide range of organ systems and disease manifestations are being studied during the multi-year observational period. This will allow us to better understand the impacts of ETI therapy in clinical use and where it may be most beneficial to focus or prioritize ongoing therapeutic and research goals in those treated with highly effective modulator drugs.

The Core analyses of PROMISE are typical outcomes for CFTR modulator studies and will be important to test the clinical effectiveness of ETI in a real-world population. Apropos, only 48% of those enrolled in PROMISE met key eligibility criteria for the pivotal trials, mostly due to an upper limit inclusion criterion for ppFEV₁. Linking these core data to the outcomes measured in the sub-studies allows investigators to test for associations with improvements in SwCl, respiratory symptoms, or lung function, and to explore potential biomarkers of disease status, response to therapy, or progression over time. A central feature of PROMISE is biospecimen collection for immediate use and banking (Table 2) to address unanticipated questions conceived by the broader CF community. SwCl, as a biomarker of CFTR function, will be serially measured during the 2 years in PROMISE. It is particularly interesting when considering those transitioning from other CFTR modulator regimens or those already using HEMT in the form of ivacaftor. A separate study (CHEC-SC NCT03350828) is focusing on SwCl on a population basis in a much larger-scale cross-sectional study of individuals beginning or transitioning between CFTR modulator therapies.

Table 2:

Primary and secondary investigational areas of focus sub-studies within PROMISE

Focus Area	Key Outcomes of Interest	Additional Biomarkers or Assays
Clinical Changes	lung function (FEV1), respiratory symptoms, sweat chloride, BMI
Mucus Biology	mucociliary clearance, percent solids in sputum	Sputum viscosity, exhaled breath condensate pH and sialic acid:urea ratio
Microbiology	P. aeruginosa and S. aureus sputum density, stool and respiratory microbiota, stool metagenome	S. maltophilia, A. xylosoxidans, and Burkholderia species, PCR vs. culture-based results, bacterial lineage stability
Inflammation and Host Response	Sputum free neutrophil elastase, serum hsCRP	Sputum IL-1B, IL-6, IL-8, IL-10, TNF-a, IFN-g, calprotectin, serum HMGB-1, calprotectin
Gastroenterology	GI symptoms (PAGI-SYM, PAC-SYM, PAC-QOL), intestinal pH and transit time	Bristol stool score, fecal calprotectin, elastase, steatocrit. Serum IRT, intestinal fatty acid binding protein, 7-hydroxy-4-cholesten-3-one, FGF-19. Urine prostaglandin E2 metabolites
Endocrinology	Effect on glucose metabolism/control (glucose excursion, peak, glucose tolerance category), insulin use and dosage. Lumbar and hip bone mineral density. Whole body lean body mass	Insulin, glucagon, incretin activities, free fatty acid flux and glucose disposal. Additional measures of bone mass, mineral content, and density. Additional measures of body composition, IGF-1, and IGF-BP3.
Liver disease	Transient elastography, serum C4, and FGF-19	Liver disease classification, GGTP, FIB4, and APRI
Nasal airway epithelial cell function	Forskolin-stimulated Isc	Airway surface liquid depth, in vitro ciliary functioning and mucus viscosity
Pediatric (age 6–11y) participants	Lung clearance index, sweat chloride, FEV1, airway microbiology from sputum and oropharyngeal swab collection, respiratory and GI symptoms, BMI
Biospecimen collection for future research	Historical and prospective data, serum, plasma, blood leukocytes, urine, bacterial colonies and bacterial DNA, stool, processed sputum, expanded nasal airway epithelial cells

Several important research questions will be left unanswered by PROMISE and are broadly relevant to HEMT, including the impact of HEMT on structural lung disease (e.g. CT or other imaging modalities), adherence to prescribed medications over time and its relationship to effectiveness, and the impact of ETI on CF-related sinus disease. Impacts on mental health and female fertility are not systematically collected. Additionally, while this study will record hospitalizations and outpatient antibiotic use, it is not designed to systematically capture all acute pulmonary exacerbation events. PROMISE does not include an untreated comparison group, which complicates estimation of impact on long-term outcomes such as lung function decline. Instead, analyses will be performed comparing participants to their historical data in the CF Foundation National Patient Registry¹⁰.

Mucus Biology and Mucociliary Clearance

CFTR dysfunction leads to progressive changes in airway mucus that result in its retention. Although restoration of CFTR activity with ivacaftor significantly improved mucociliary clearance (MCC)¹¹, the combination of lumacaftor and ivacaftor in F508del homozygous individuals did not have meaningful effects on MCC. While this likely reflects different levels of CFTR restoration, little work has been done to investigate the biochemical and biophysical links between CFTR activation and MCC in humans. Understanding how restoration of CFTR improves MCC, whether due to improvements in mucus hydration, pH, viscosity/rheology, or a combination of these factors, could drive future drug development decisions.

In the PROMISE Mucus/MCC Sub-study, MCC will be measured in people with CF in those with mild to moderate pulmonary disease (FEV₁ ≧ 40%) using a standardized gamma scintigraphy method¹². We expect to observe substantial improvements in MCC measurements, which will provide a key functional endpoint against which novel and mechanistic outcome measures will be compared. Using induced sputum sampling, we will evaluate the effects of ETI on mucus hydration (percent solids) and rheologic properties. Exhaled breath condensate (EBC) collections will also be performed to measure pH, given the role of CFTR role in bicarbonate secretion, and mucus sialic acid:urea ratio as an index of mucin concentration¹³. Coupling these outcomes could yield insights into the mechanism by which MCC improves and its relationship to severity of underlying airway obstruction or degree of lung function improvement. Further, thresholds of CFTR activity sufficient to alter mucus viscosity and transport may be established.

Because MCC measurements require specialized research capabilities, we also hope to determine whether any of the described ex vivo assays might suitably substitute for MCC. This study may also help us understand whether ETI has fully corrected the underlying mechanisms of mucociliary dysfunction, and may elucidate treatment targets that guide alternative therapies that complement CFTR modulation, such as dornase alfa and hypertonic saline.

Airway Microbiology

Questions remain about the effects of modulators on chronic lung infections, a cardinal manifestation of CF. This has not been a focus of development efforts, and most knowledge comes from independent studies that have produced inconsistent results. For example, studies based on registry data showed ivacaftor decreased the prevalence of culture positivity for P. aeruginosa^14–16. However, prospective studies using PCR and 16s rRNA gene sequencing have generally shown minimal changes in sputum microbiota. In contrast, a smaller study using quantitative culture found that ivacaftor rapidly reduced P. aeruginosa density and lung inflammation¹⁷. Interestingly, P. aeruginosa density rebounded in the second year of treatment, and many of the P. aeruginosa strains present before ivacaftor persisted for years after treatment despite sustained reductions in sweat chloride, raising questions about the durabilty of HEMT-related effects that need to be addresssed.

The PROMISE microbiology substudy examines nearly 250 participants with collection at 1, 3, 6, 12, and 24 month time points. Larger numbers will increase the probability of generating robust findings on CF pathogens and help identify target populations most likely to experience HEMT-related effects in microbiology. The serial timepoints will define changes in pathogen density over time. PROMISE collects induced sputum, addressing decrements in sputum production after modulator treatment. The use of a CLIA-certified laboratory for centralized analysis that occurs without freezing is advantagous for reliability and consistency. Work comparing culture and PCR measurements of P. aeruginosa showed that culture detected larger changes after ivacaftor or antibiotic treatment, perhaps because PCR measures both live and dead organisms^{17, 18}. We will measure quantitative qPCR of targeted species, as well as 16S rRNA gene sequencing to measure a broader composition of microbial DNA reflective of both lower and upper airway components in sputum. Genome sequencing studies will determine if pathogen strains present pre-treatment persist. Sequencing may also identify bacterial adaptations to modulator-induced changes in the lung environment, and determine how modulators affect the composition of genetically-diversified pathogen populations.

One future challenge will be defining causal relationships between changes in respiratory microbiology and other important disease manifestations. For example, decreased pathogen density could reduce lung inflammation, increase mucociliary clearance, or both. Likewise, primary improvements in inflammation or mucociliary clearance could significantly alter airway microbiology. PROMISE will not examine residual host defense defects that may be present and impact bacterial survival after ETI. Similarly, testing the effects of combining antibiotics with modulator initiation to improve bacterial eradication will require a separate effort.

Effects of HEMT on Pulmonary and Systemic Inflammation

Inflammation is a hallmark feature of CF and plays a key role in the progression of lung injury. Previous studies that have investigated the effects of ivacaftor on airway inflammation have yielded mixed results. In the GOAL study, there were no significant changes in sputum markers of inflammation (free neutrophil elastase activity, α1-antitrypsin, secretory leukoprotease inhibitor, IL-1β, IL-6, IL-8) following 6 months of ivacaftor therapy in two distinct cohorts: one small group with milder lung disease assessed by sputum induction¹⁶, and a second with more impaired lung function who spontaneously expectorated sputum before and after initiation of therapy¹⁹. In contrast, a separate study found that ivacaftor markedly reduced sputum markers of inflammation (neutrophil elastase activity, IL-1β, IL-8) during the first year of treatment in a cohort of 12 adults with the G551D mutation, most of whom were chronically infected with P. aeruginosa¹⁷. These data suggest that at least in people with established lung disease, HEMT is unlikely to resolve inflammation. The PROMISE study provides an opportunity to determine the effects of restoration of CFTR function on airway and systemic inflammation, critical downstream consequences of impaired CFTR function. Individuals achieving normal or near-normal sweat chloride concentrations, may be a particularly interesting group in which to evaluate the extent to which inflammation is improved.

In PROMISE, induced sputum will be collected from up to 200 participants to detect changes from baseline in two important markers of neutrophilic inflammation (free neutrophil elastase, calprotectin), plus a predetermined cytokine panel will be serially examined through two years of treatment. Since sputum from these participants will also be evaluated for microbiologic outcomes on the same schedule, correlations between airway inflammation and infection can be assessed. This large, paired study population provides a real opportunity to understand key features of CF lung disease after starting HEMT.

Changes in systemic measures of inflammation were also measured in a subset of the GOAL cohort treated with ivacaftor²⁰. Circulating HMGB-1 and calprotectin decreased significantly at 1 and 6 months after treatment initiation compared to baseline. In PROMISE, changes in serum high sensitivity C reactive protein, calprotectin, and HMGB-1 will be investigated. Importantly, if inflammation persists in the face of partial restoration of CFTR activity, this finding will support the need for adjunctive anti-inflammatory therapy in pwCF. Future studies will need to investigate how HEMT modulates the function of innate (neutrophils and macrophages) and adaptive (B and T lymphocytes) immune cells and whether HEMT alters the aberrant and dysregulated inflammatory responses by the airway epithelium during viral infections or other stimuli.

Gastrointestinal Disease and Symptoms

Gastrointestinal (GI) disease is another central feature of cystic fibrosis²¹. Malabsorption associated with pancreatic insufficiency, abnormal intestinal secretions, dysmotility, and chronic inflammation are all present in CF, and many individuals experience malnutrition and disordered growth²². Abdominal pain, bloating, abnormal stools, constipation, and gastro-esophogeal reflux are found in as many as 80% of pwCF²³. These problems impact quality of life and survival^24–26. Determining what impact HEMT has on these gastrointestinal problems is critical, especially as supportive care is inadequate for many pwCF.

Previous data demonstrated improvement in small bowel pH with ivacaftor in individuals with the G551D mutation²⁷, but no change in transit time. Data collected on gastrointestinal motility profiles and intestinal pH in participants treated with ETI will be important to understanding its impact on GI ailments, particularly when correlations between symptoms, motility, and luminal pH are discerned.

Five areas of cystic fibrosis-related gastrointestinal disease will be evaluated in the PROMISE study: inflammation, malabsorption, dysmotility, luminal pH and symptomatology. These impact weight maintenance and growth. The target population includes all participants for assessment of symptoms and quality of life, and subgroups for examining inflammation, non-pancreatic malabsorption, exocrine pancreatic function, intestinal pH and dysmotility. Symptomatology and quality of life related to gastrointestinal disease are measured throughout the 2-year study using the Patient Reported Outcomes used in the GALAXY study (NCT03801993), while inflammation and exocrine pancreatic function are measured using fecal testing.

The fecal samples to be collected during PROMISE will also be analyzed for changes in the microbiome. Compared with the lung, the microbiology of the CF gastrointestinal (GI) tract remains relatively undefined; however, a growing number of studies have identified a CF dysbiosis that correlates with measures of GI inflammation, impaired nutrition and growth, and even the risk of respiratory exacerbations^28–31. A study of 16 PwCF with pancreatic exocrine insufficiency³¹ identified concurrent improvements in fecal measures of dysbiosis and inflammation with ivacaftor treatment; another recent study of 12 pancreatic-sufficient PwCF demonstrated no such changes with ivacaftor³². The large size, frequent sample collection, and diversity of functional and symptomatic outcomes planned in the PROMISE study will define the role of the GI microbiome with much greater resolution. Similar to other areas of focus in PROMISE, additional follow up beyond two years may be necessary to understand the full GI effects of HEMT.

CF-Related Liver Disease

Liver involvement beyond laboratory-based safety monitoring has not been systematically studied in clinical trials or prior post-approval studies of CFTR modulator drugs. The primary focus has been on safety as determined by serum transaminases and bilirubin. Recent progress in the understanding of CF liver disease and its relationship to serum and imaging markers of liver fibrosis enabled the inclusion of markers of liver disease in PROMISE. Notably, liver disease is a leading source of morbidity and mortality in PwCF and may become increasingly important as pulmonary complications improve^{33, 34}. Potential targets for therapeutic impact and related biomarkers chosen in PROMISE are shown in Table 2.

Several studies have demonstrated that AST to platelet ratio (APRI) is a good marker of hepatic fibrosis³⁵. Recently transient elastography, which measures liver stiffness by ultrasound, was also shown to correlate well with hepatic fibrosis in several liver diseases³⁶ and CF³⁷. Thus, we will measure if hepatic fibrosis is a potential, yet unrecognized, therapeutic target when studying the clinical impact of ETI. Improvement in hepatic fibrosis as determined by APRI and transient elastography has been shown with successful treatment of viral hepatitis C³⁸ and B³⁹. One challenge in CF is the anticipated slow improvement in hepatic fibrosis. We chose a 2-year time frame for the evaluation of changes in markers of fibrosis, liver stiffness and liver classification as studies in viral hepatitis suggest that highly effective antiviral therapy can meaningfully impact these measures in 2–4 years. There is a gradual increase in liver stiffness in CF with age. We chose to focus on the rate of increase in liver stiffness, hypothesizing that we will see no increase in liver stiffness over 2 years of treatment. There is a significant need for an earlier (1–3 month) biomarker for changes in hepatic physiology.

Standard phenotypic characterization of CF liver involvement using phenotypic classification⁴⁰ was selected as a target as it is clinically relevant to care in CF. Exploratory biomarkers FGF19 and 7α hydroxycholest-4-en-3-one (C4) have associated with improved bile flow and reduced malabsorption in other liver diseases^41–43. CFTR is integral to bile acid flow, and improvement of CFTR function in the biliary epithelium would be expected to enhance bile flow and decrease bile acid malabsorption.

A key question regarding ETI in CF liver disease is whether it can reduce or prevent the development of advanced liver disease with portal hypertension that affects about 7% of PwCF. Testing this question may require drug initiation at an early age⁴⁴ and better predictive biomarkers or long-term follow up. Information collected in this and other studies will also help guide future studies testing alternative therapeutic strategies.

Endocrine-related Disorders

With improved life-expectancy in individuals with CF, endocrine complications, including diabetes, bone disease, and reduced linear growth, are increasingly important medical considerations. Prevalence of CF-related diabetes (CFRD) increases with age and is present in nearly 50% of adults⁴⁵. CF-related bone disease (CFBD) is also common, manifesting as decreased bone density and increased fracture rates⁴⁶, and can be present even in childhood⁴⁷. Additionally, shorter stature occurs with severe CFTR class mutations¹⁰.

Understanding how HEMT impacts insulin requirements in people with CFRD and progression to CFRD is critically important. Reductions in pancreatic islet mass, local and systemic inflammation, impaired intestinal secretion of incretins, and disrupted insulin secretion may all contribute to developing CFRD^48–50. Improved insulin secretion, lower rates of progression to CFRD, and CFRD resolution were observed with ivacaftor^{5, 51, 52} although the impact of lumacaftor/ivacaftor on glucose tolerance and insulin secretion remains unresolved. PROMISE is prospectively collecting data on insulin requirements, hypoglycemia, and extended, multi-sample oral glucose tolerance tests performed at baseline, 12-, and 24-months to examine glucose tolerance/excursion, insulin secretory rates, hepatic insulin clearance, and incretin and glucagon secretion.

CFBD appears multifactorial and varies upon individuals: nutritional compromise, reduced physical activity, reduced lean body mass, systemic inflammation, hypogonadism, and systemic glucocorticoid therapy all likely have roles⁴⁶. Data implicate both direct and indirect roles of CFTR in bone health and mineralization⁴⁶. Complicating studies of bone health in youth, shorter stature and pubertal lag may inflate deficits in bone mineral density (BMD). Dual-energy X-ray absorptiometry (DXA) will be obtained for BMD and body composition annually. In youth, annualized bone accretion will be adjusted for growth, pubertal status, linear growth, and lean body mass (LBM) accrual, and in both youth and adults, changes in BMD-Z will be examined. Poor linear growth is independently associated with higher mortality⁵³. Linear growth improvements occur with ivacaftor therapy and are anticipated with therapeutic CFTR correction during PROMISE. Age, gender, and pubertal status specific growth velocity Z-scores will be determined and compared with baseline growth velocity and to normative data. Greater LBM is hypothesized to contribute to the positive associations between nutritional status and pulmonary function⁵⁴. CFTR modulator therapy is associated with weight gain and BMI increases⁵. Whole body DXA will be leveraged to assess LBM and fat mass changes in the context of BMI. Additionally, we will evaluate endocrine growth factors throughout the range of the study.

While PROMISE is positioned to provide important data on the impact of HEMT across a range of important endocrine-related CF disease manifestations, the durability of these effects will need to be considered in future studies. Similarly, the extent to which early HEMT initiation can preserve beta-cell mass and function or prevent rather than reverse abnormalities will require long-term studies with HEMT from a young age.

Advancing Personalized Outcome Measures

While it is established that the majority of those treated will garner benefit from ETI therapy, variability is expected in both effects on CFTR function and clinical status. Previously, work in the GOAL/GOALe² (NCT01521338) and additional observational studies^55–57 has demonstrated that respiratory epithelial tissues cultured at air-liquid interface (ALI) may act as an effective surrogate for an individual patient’s response to highly effective modulators. The PROMISE study will test the hypothesis that grouped and individual responses to CFTR restoration in vitro (e.g. short circuit current activity, increased airway surface layer depth, or other functional parameters determined by micro-optical tomography)^{58, 59} in human nasal epithelial (HNE) cells collected and cultured in the laboratory may serve as a biomarker for clinical outcomes. We will also test correlations of groups defined by genetic mutations.

Through this work, we hope to affirm that in vitro measures conducted using cultured primary nasal airway epithelial cells collected from participants correlate with clinical and other outcomes, thereby supporting HNE as a useful biomarker for clinical response to ETI. Second, we intend to establish a robust biorepository of these tissues to perform future investigations, including use of nasal spheroids as an alternative measure to quantify ion transport and fluid secretion⁶⁰, genome-wide association studies for variability in response to ETI therapy, and other investigator-initiated projects. Findings will allow for more robust mechanistic understanding of the interplay of CFTR rescue and mucociliary clearance and serve as a validation for future use of HNE-based analyses as a predictive tool for CFTR modulators

Considerations for development in pediatric populations

Compared to adolescent and adults, pwCF 6 to 11 years of age generally have limited structural lung damage, relatively preserved lung function, and lower prevalence of chronic Pseudomonas infection. Thus, ETI may have greater potential to modify the natural history of CF in younger age groups. The PROMISE Pediatric sub-study (PROMISE PEDS; NCT04613128) will evaluate longitudinal outcomes associated with ETI initiation in children ages 6–11, providing an important step in our understanding of whether HEMT will be sufficient to control CF disease progression.

Similar to the parent PROMISE study, PROMISE PEDS will evaluate the short- and long-term impact of ETI on lung function, microbiology, nutritional status, sweat chloride, and patient-reported outcomes over 2 years. The primary outcome measure will be the lung clearance index (LCI) as measured by nitrogen multiple breath washout (MBW); spirometry will also be performed. LCI has been shown to be more sensitive than FEV₁ to early CF lung disease, with a robust response to modulator therapies in several studies^61–63.

CFTR modulator therapy may alter the lower airway milieu for microbial pathogens in several ways, including improvement in mucus clearance, stabilization or reversal of structural disease, and/or alteration of neutrophilic airways inflammation. As spontaneous sputum production is often not feasible in this age range, both induced sputum and oropharyngeal samples will be obtained for microbiologic analysis. Oropharyngeal cultures will ensure longitudinal microbiologic data in children unable to consistently produce sputum even with sputum induction. Key microbiologic outcomes will include reduction in sputum pathogen density and eradication of specific pathogens, particularly Pseudomonas aeruginosa and Staphylococcus aureus. Changes in the respiratory microbiota will be measured longitudinally. Associations between changes in lung function, airway microbiology and markers of airway inflammation will be explored. Respiratory samples will also be banked for future studies. Effects of modulator therapy on nutritional status and gastrointestinal health will be captured through longitudinal changes in BMI and GI-related patient-reported outcome measures, and ultimately compared with older cohorts.

Conclusions

The approval of ETI has established a new paradigm in care and research for the majority of PwCF. The PROMISE Study will capture important longitudinal data and biospecimens from these individuals, including those with the G551D CFTR mutation transitioning from ivacaftor monotherapy to ETI. This rapidly evolving landscape supports a rationale to study the clinical and translational impact of ETI across several organ systems, including its effects on novel biological assays and emerging biomarkers, fundamentally improving our understanding of future CF care needs and emerging therapeutic priorities. Large in scope, this US-based collaborative research will also complement other critical research programs around the world as ETI approval expands to other countries and younger individuals. Important questions of adherence to modulator drugs or pre-existing therapies⁶⁴ and the impacts of starting HEMT early in life require attention and are being addressed by other post-approval studies like BEGIN, RECOVER, SIMPLIFY (NCT04378153), QUEST (NCT04320381), CF-STORM, HERO, and others. Collectively, we are positioned to comprehensively understand the real-world use and impact of ETI, which is likely to represent a key milestone in the treatment of this disease.