To understand why timing matters so much in cystic fibrosis, it helps to review the biology of the disease. Cystic fibrosis is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator, or CFTR, a protein that functions as a chloride and bicarbonate channel on the surface of epithelial cells lining the airways, pancreas, sweat glands, and other organs. When CFTR is absent or dysfunctional, salt and water transport across these epithelia is disrupted, dehydrating the airway surface liquid that normally allows cilia to sweep inhaled pathogens and debris out of the lungs. The result is thick, sticky mucus, chronic infection, relentless inflammation, and, over years, structural lung damage. In the pancreas, the same mechanism blocks digestive enzyme delivery, producing exocrine pancreatic insufficiency in the majority of patients, which impairs growth and nutrition from infancy onward.

The F508del mutation, a deletion of the amino acid phenylalanine at position 508 of the CFTR protein, is carried on at least one allele by roughly ninety percent of people with cystic fibrosis worldwide, and many patients, like those in the new study, are homozygous for it. F508del-CFTR is a doubly defective protein: it folds poorly and is largely retained and degraded inside the cell before it ever reaches the cell membrane, and the small fraction that does reach the surface has an abbreviated residence time and impaired channel gating. This dual defect motivated the development of combination therapy. Lumacaftor acts as a corrector, improving the folding and trafficking of F508del-CFTR so that more protein reaches the cell surface, while ivacaftor acts as a potentiator, increasing the open probability of the channels that do arrive. Together, the two drugs restore a meaningful fraction of CFTR function in patients homozygous for F508del.

CFTR modulators were first tested and approved in adolescents and adults, and the extension of this therapeutic class to younger children has proceeded through carefully staged clinical trials. Phase 3 open-label studies established the safety, pharmacokinetics, and pharmacodynamics of lumacaftor/ivacaftor in children aged two to five years and, subsequently, in children aged one to less than two years, all homozygous for F508del-CFTR. A phase 2 placebo-controlled trial in children two through five years of age further examined whether the drug combination could alter the trajectory of early disease. These trials were essential regulatory stepping stones, but they involved relatively small numbers of children followed under the controlled conditions of a clinical trial. What has been missing is evidence from routine clinical practice, where adherence, dosing, comorbidities, and monitoring differ from the trial setting. The Italian multicenter study addressed precisely this gap by evaluating real-life effectiveness and safety of lumacaftor/ivacaftor in preschool children.

The findings, as summarized in the commentary, provide important real-world evidence that early CFTR modulation improves biomarkers of CFTR function, pulmonary outcomes, and early lung physiology while maintaining an acceptable safety profile. The most direct biomarker of CFTR activity is the sweat chloride concentration. Because defective chloride transport in sweat glands is the biochemical hallmark of cystic fibrosis, elevated sweat chloride is the diagnostic gold standard, and reductions in sweat chloride following modulator therapy serve as a pharmacodynamic readout of restored channel function. Proof-of-concept work with ivacaftor established sweat chloride as a responsive biomarker, and subsequent studies have documented that modulator-induced reductions vary considerably between individuals, a heterogeneity that clinicians must keep in mind when interpreting individual responses. The Italian study’s demonstration that sweat chloride improves in treated preschoolers confirms that the drugs are engaging their biological target even in very young children.

Perhaps more consequential than the biomarker data are the pulmonary findings. Lung disease in cystic fibrosis begins far earlier than symptoms suggest, and by the preschool years many children already have measurable abnormalities even though standard spirometry remains normal. This is because forced expiratory volume in one second, the workhorse measure of lung function in older patients, is insensitive to the early peripheral airway disease that characterizes the beginning of the cystic fibrosis lung phenotype. The lung clearance index, derived from multiple-breath washout testing, measures the ventilation inhomogeneity that arises when small airways become obstructed, and it can be performed reliably in sleeping preschool children. Normative multiple-breath washout data corrected for sensor error have made standardized interpretation possible, and longitudinal studies have shown that an elevated lung clearance index at four years of age predicts subsequent lung function, and that determinants of progression measured by this index can be identified early in life. In other words, the lung clearance index captures the earliest, still potentially reversible phase of cystic fibrosis lung disease.

Against this background, the observation that early lumacaftor/ivacaftor therapy improves early lung physiology takes on real significance. The phase 2 placebo-controlled trial of lumacaftor/ivacaftor in children two through five years of age had previously suggested effects on disease progression, and the new real-world data extend that signal into routine care. If modulator therapy started in the preschool years can hold the lung clearance index in the normal range, or prevent the rise that typically heralds progressive small airway obstruction, clinicians may be able to defer or prevent the cascade of infection, inflammation, and structural remodeling that has historically defined the disease course. Chest computed tomography scores, which quantify structural lung damage, have also been shown to improve with the newer triple-combination modulator elexacaftor/tezacaftor/ivacaftor in people aged twelve years and older in the RECOVER trial, reinforcing the principle that CFTR restoration can favorably influence both physiology and structure.

The therapeutic landscape has, of course, moved beyond lumacaftor/ivacaftor. The triple combination of elexacaftor, tezacaftor, and ivacaftor, which pairs two correctors with a potentiator, has demonstrated substantially greater clinical benefit than earlier dual combinations and has been tested in an open-label phase 3 trial in children aged two to five years with at least one F508del allele. Many centers now favor triple therapy where it is available and appropriate. Nevertheless, the Italian study of lumacaftor/ivacaftor remains valuable for several reasons. It documents the effectiveness of dual modulator therapy under real-world conditions in a population in which trial data were limited, it adds to the safety database for young children exposed to these drugs during a critical window of lung development, and it demonstrates the feasibility of conducting multicenter prospective effectiveness studies in this age group. Lumacaftor/ivacaftor also continues to be used in parts of the world where access to newer, more expensive modulators is constrained, making real-world evidence about its performance directly relevant to global practice.

Safety considerations are central to any discussion of treating preschool children with chronic medications. Young children are not simply small adults: they metabolize drugs differently, they cannot reliably report symptoms, and their organs are still developing. The phase 3 studies of lumacaftor/ivacaftor in children aged two to five and one to less than two years established age-appropriate dosing based on pharmacokinetic modeling and documented a tolerability profile consistent with that seen in older patients, with the important caveat that liver function monitoring is required because transaminase elevations have been observed with lumacaftor-containing regimens. The commentary emphasizes that the multicenter Italian study maintained an acceptable safety profile in routine practice, which is reassuring for clinicians who must weigh the long-term benefits of early intervention against the burdens of daily therapy, drug interactions, and laboratory surveillance in toddlers and preschoolers.

The larger message of this body of work is a conceptual one about the natural history of cystic fibrosis and the window in which it can be modified. For decades, treatment was largely reactive, responding to infections and decline as they occurred, and the median survival improved slowly through airway clearance, inhaled antibiotics, pancreatic enzyme replacement, and nutritional support. CFTR modulators changed the equation by addressing the primary molecular defect, and the accumulating evidence from infants and preschoolers suggests that the earlier the molecular defect is corrected, the better the chance of preserving near-normal organ function. Newborn screening now identifies most affected children within weeks of birth, and pancreatic insufficiency in early childhood can be managed with enzyme replacement and nutritional strategies in the era of personalized medicine. What remains is to determine, through studies like the Italian multicenter investigation and the trials that preceded it, exactly how early modulator therapy should begin, which biomarkers best guide treatment decisions in individual children, and whether the improvements seen in sweat chloride and lung clearance index translate into the ultimate outcomes that matter: preserved lung structure, freedom from chronic infection, and a life expectancy and quality of life approaching those of children without the disease. The real-world data from preschool children homozygous for F508del, together with the expert commentary they prompted, support the broader concept that intervention during early childhood offers a genuine opportunity to modify disease progression before irreversible organ damage develops, and they provide clinicians and families with evidence that starting treatment early is not merely biologically plausible but demonstrably beneficial.

Subject of Research: Real-world effectiveness and safety of early CFTR modulator therapy in preschool children with cystic fibrosis

Article Title: Early CFTR modulation and clinical outcomes in preschool cystic fibrosis

Article References: Lawandos, L., & Sodhi, C. P. (2026). Early CFTR modulation and clinical outcomes in preschool cystic fibrosis. Pediatric Research. https://doi.org/10.1038/s41390-026-05441-2

Image Credits: AI Generated

DOI: 10.1038/s41390-026-05441-2

Keywords: cystic fibrosis, CFTR modulators, lumacaftor/ivacaftor, F508del, preschool children, lung clearance index, sweat chloride, pediatric research, early intervention, real-world evidence, lung disease progression, Early