CFTR modulation with elexacaftor-tezacaftor-ivacaftor in people with cystic fibrosis assessed by the β-adrenergic sweat rate assay

Understanding CFTR Functionality: A Comprehensive Review of Tests and Modulator Therapy in Cystic Fibrosis

Cystic fibrosis is a genetic disorder inherited in an autosomal recessive manner. It is caused by a mutation in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene on chromosome 7, which leads to abnormal regulation of chloride and bicarbonate ions in cells that line organs like the lungs and pancreas. The CFTR protein plays a crucial role in regulating chloride ion flow, and its absence or malfunction causes the production of thick mucus that affects several organs. There are more than 2000 identified mutations that are classified into seven categories based on their dysfunction mechanisms. In this article, we have conducted a thorough examination and consolidation of the diverse array of tests essential for the quantification of CFTR functionality. Furthermore, we have engaged in a comprehensive discourse regarding the recent advancements in CFTR modulator therapy, a pivotal approach utilized for the management of cystic fibrosis, alongside its concomitant relevance in evaluating CFTR functionality.

Differential effects of ELX/TEZ/IVA on organ-specific CFTR function in two patients with the rare CFTR splice mutations c.273+1G>A and c.165-2A>G

Introduction: Evidence for the efficiency of highly-effective triple-CFTR-modulatory therapy with elexacaftor/tezacaftor/ivacaftor (ETI), either demonstrated in clinical trials or by in vitro testing, is lacking for about 10% of people with cystic fibrosis (pwCF) with rare mutations. Comprehensive assessment of CFTR function can provide critical information on the impact of ETI on CFTR function gains for such rare mutations, lending argument of the prescription of ETI. The mutation c.165-2A>G is a rare acceptor splice mutation that has not yet been functionally characterized. We here describe the functional changes induced by ETI in two brothers who are compound heterozygous for the splice mutations c.273+1G>C and c.165-2A>G.

Methods: We assessed the effects of ETI on CFTR function by quantitative pilocarpine iontophoresis (QPIT), nasal potential difference measurements (nPD), intestinal current measurements (ICM), β-adrenergic sweat secretion tests (SST) and multiple breath washout (MBW) prior to and 4 months after the initiation of ETI.

Results: Functional CFTR analysis prior to ETI showed no CFTR function in the respiratory and intestinal epithelia and in the sweat gland reabsorptive duct in either brother. In contrast, β-adrenergic stimulated, CFTR-mediated sweat secretion was detectable in the CF range. Under ETI, both brothers continued to exhibit high sweat chloride concentration in QPIT, evidence of low residual CFTR function in the respiratory epithelia, but normalized β-adrenergically stimulated production of primary sweat.

Discussion: Our results are the first to demonstrate that the c.165-2A>G/c.273+1G>C mutation genotype permits mutant CFTR protein expression. We showed organ-specific differences in the expression of CFTR and consecutive responses to ETI of the c.165-2A>G/c.273+1G>C CFTR mutants that are probably accomplished by non-canonical CFTR mRNA isoforms. This showcase tells us that the individual response of rare CFTR mutations to highly-effective CFTR modulation cannot be predicted from assays in standard cell cultures, but requires the personalized multi-organ assessment by CFTR biomarkers.

Post-approval studies with the CFTR modulators Elexacaftor-Tezacaftor—Ivacaftor

Triple combination therapy with the CFTR modulators elexacaftor (ELX), tezacaftor (TEZ) and ivacaftor (IVA) has been qualified as a game changer in cystic fibrosis (CF). We provide an overview of the body of literature on ELX/TEZ/IVA published between November 2019 and February 2023 after approval by the regulators. Recombinant ELX/TEZ/IVA-bound Phe508del CFTR exhibits a wild type conformationin vitro, but in patient’s tissue a CFTR glyoisoform is synthesized that is distinct from the wild type and Phe508del isoforms. ELX/TEZ/IVA therapy improved the quality of life of people with CF in the real-life setting irrespective of their anthropometry and lung function at baseline. ELX/TEZ/IVA improved sinonasal and abdominal disease, lung function and morphology, airway microbiology and the basic defect of impaired epithelial chloride and bicarbonate transport. Pregnancy rates were increasing in women with CF. Side effects of mental status changes deserve particular attention in the future.

β-adrenergic sweat test in children with inconclusive cystic fibrosis diagnosis: Do we need new reference ranges?

BACKGROUND

Investigating inconclusive cystic fibrosis (CF) diagnosis in children is difficult without advanced cystic fibrosis transmembrane conductance regulator (CFTR) function tests. This study investigated the utility of beta (β)-adrenergic sweat test to exclude CF in participants with inconclusive diagnosis (CF suspects) in South Africa.

METHODS

β-adrenergic sweat test and sweat chloride tests (SCT) were performed simultaneously in CF suspects and adult controls (healthy, CFTR heterozygotes and CF). Cholinergic and β-adrenergic induced sweat rate was measured by evaporimetry (transepithelial water loss [TEWL]: g H₂ O/m² /h) following intradermal injections. Next-generation sequencing of CFTR was performed in CF suspects. CF diagnosis was defined by genotype.

RESULTS

Thirty-seven controls (10 healthy, 14 CF, 13 CFTR heterozygotes) and 32 CF suspects (26 children; 6 adults) were enrolled. Six were excluded from formal analyses due to β-adrenergic sweat test failure. In adults, evaporimetry was superior to SCT for diagnosis of CF with β-adrenergic:cholinergic ratio TEWL ≤ 0.05 achieving 100% sensitivity and specificity. Twenty-two CF suspect children (age range: 3.4-15.6 years) completed β-adrenergic sweat testing of which none had CF confirmed by genotyping: β-adrenergic:cholinergic ratio > 0.05 successfully excluded CF in all but one child who was CFTR heterozygous. Median peak β-adrenergic TEWL and β-adrenergic:cholinergic ratio in CFTR negative and CFTR heterozygous children was significantly lower than adult controls.

CONCLUSION

β-adrenergic sweat test is more accurate than SCT for excluding CF in children with inconclusive diagnosis. Established reference ranges for β-adrenergic sweat test may not be suitable for children due to lower β-adrenergic sweat secretion compared to adults.