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Background: Sweat chloride (SC) concentrations in people with cystic fibrosis (PwCF) reflect relative CF transmembrane conductance regulator (CFTR) protein function, the primary CF defect. Populations with greater SC concentrations tend to have lesser CFTR function and more severe disease courses. CFTR modulator treatment can improve CFTR function within specific CF genotypes and is commonly associated with reduced SC concentration. However, SC concentrations do not necessarily fall to concentrations seen in the unaffected population, suggesting potential for better CFTR treatment outcomes. We characterized post-modulator SC concentration variability among CHEC-SC study participants by genotype and modulator., Methods: PwCF receiving commercially approved modulators for ≥90 days were enrolled for a single SC measurement. Clinical data were obtained from chart review and the CF Foundation Patient Registry (CFFPR). Variability of post-modulator SC concentrations was assessed by cumulative SC concentration frequencies., Results: Post-modulator SC concentrations (n = 3787) were collected from 3131 PwCF; most (n = 1769, 47 %) were collected after elexacaftor/tezacaftor/ivacaftor (ETI) treatment. Modulator use was associated with lower SC distributions, with post-ETI concentrations the lowest on average. Most post-ETI SC concentrations were <60 mmol/L (79 %); 26 % were <30 mmol/L. Post-ETI distributions varied by genotype. All genotypes containing at least one F508del allele had individuals with post-ETI SC ≥60 mmol/L, with the largest proportion being F508del/minimal function (31 %)., Conclusions: Post-modulator SC concentration heterogeneity was observed among all genotypes and modulators, including ETI. The presence of PwCF with post-modulator SC concentrations within the CF diagnostic range suggests room for additional treatment-associated CFTR restoration in this population., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 European Cystic Fibrosis Society. Published by Elsevier B.V. All rights reserved.)

Background: The Characterizing CFTR Modulated Changes in Sweat Chloride and their Association with Clinical Outcomes (CHEC-SC) study is a large epidemiologic study designed to determine the relationship between sweat chloride response and clinical outcomes in people with cystic fibrosis (CF) on commercially approved CFTR modulators. A challenge to study feasibility was capturing sweat chloride measurements before modulator initiation. We tested the hypothesis that historic sweat chloride approximated contemporary pre-modulator values to estimate CFTR modulator-induced changes, allowing a single-visit study design., Methods: GOAL and PROSPECT were multi-center prospective studies of individuals initiating ivacaftor or lumacaftor-ivacaftor. At enrollment, pre-modulator sweat chloride was measured and historic results recorded. Post-modulator sweat chloride was measured at 1, 3 and 6 months. For this analysis, differences between historic and pre-modulator sweat chloride were estimated. CFTR modulator-induced sweat chloride mean changes were compared using historic and pre-modulator sweat chloride., Results: Paired historic and pre-modulator sweat chloride (n=406 participants) revealed a non-significant mean change of -1.0 mmol/L (95% CI: -2.71, 0.66) over an average of 17.2 years. Calculating sweat response to ivacaftor or lumacaftor-ivacaftor using historic or pre-modulator values resulted in similar estimates of modulator response. Based on these results, the CHEC-SC study was designed with a single, post-modulator sweat chloride measurement., Conclusions: Historic sweat chloride values provide a reliable estimate of pre-modulator sweat chloride for people starting on modulator therapy. The CHEC-SC study anticipates capturing approximately 5,000 sweat chloride values, providing an unprecedented understanding of sweat chloride across the CF population in the era of CFTR modulators., Competing Interests: Declaration of Competing Interest ETZ, NMH, DRV, SMR, MWK: grants or consulting CFF; DRV consults for aMoon, Arrevus, Eloxx, Enbiotix, Felix, Ionis, Matinas, Merck, Polyphor, Respirion, Savara; SMR consults for Vertex; MWK consults for Anthera, AzurRx, Celtaxsys, Chiesi, Ionis, Kala, Laurent, Merck, Paranta, pH Pharma, Santhera, Vertex; JPC, KOD and MS: none., (Copyright © 2021. Published by Elsevier B.V.)

• Modulator-induced sweat chloride (SC) responses across the CF population were consistent with those reported from prior clinical studies. • Variation in SC response to a single highly effective modulator, ivacaftor, was evident across individuals with differing mutation profiles; in vivo SC responses correlated with in vitro ivacaftor modulated CFTR functional responses. • With the exception of the cohort of F508del homozygous participants established on tezacaftor/ivacaftor, there were notable shifts in the proportion of participants with SC levels <60mmol/L across modulators and genotype groups independent of the magnitude of average SC change observed pre- to post- modulator. • Female as compared to male sex at birth was not strongly associated with greater SC response to CFTR modulators as has been identified in prior studies. CHEC-SC is an ongoing epidemiologic study characterizing modulator-induced sweat chloride (SC) responses across the CF population, with interim results available prior to the availability of triple combination modulator therapy. Eligible participants had been prescribed a modulator for ≥90 days with re-enrollment allowed upon establishment of a new modulator. Pre-modulator SC values were obtained from chart review; post-modulator sweat was collected and analyzed locally. SC changes were descriptively summarized with biologic sex effects adjusted for age, weight, and CFTR genotype. Heterogeneity in ivacaftor SC response was characterized in relation to published CFTR functional responses. 1848 participants provided 2004 SC measurements, 26.2% on ivacaftor, 39.1% on lumacaftor/ivacaftor, and 34.7% on tezacaftor/ivacaftor. Average SC changes for all modulators were consistent with those reported in previous clinical studies, with greater variation in SC response observed among rarer mutations and notable shifts in the proportion with SC <60mmol/L independent of the magnitude of SC change. Ivacaftor induced in vitro CFTR functional change was significantly correlated with ivacaftor-modulated SC response (Pearson correlation= ‒0.52, 95% CI: ‒0.773, ‒0.129). Average SC change from ivacaftor to tezacaftor/ivacaftor was ‒4.9 mmol/L (n=17,95% CI:‒9.3, ‒0.5) and differed from those switching from lumacaftor/ivacaftor (10.0 mmol/L, n=139, 95% CI:7.8,12.3). Sex at birth was not associated with SC response. CHEC-SC is the largest study characterizing modulator-induced SC changes across the CF population. There was a strong association between ivacaftor induced in vitro CFTR function and SC response across a genotypically heterogenous cohort. Biological sex was not associated with SC response. [ABSTRACT FROM AUTHOR]