Personalized Medicine Based on Nasal Epithelial Cells: Comparative Studies with Rectal Biopsies and Intestinal Organoids

Discovery and Development of CFTR Modulators for the Treatment of Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which regulates ion and fluid transport across epithelial cells. Mutations lead to complications, with life-limiting lung disease being the most severe manifestation. Traditional treatments focused on managing symptoms, but advances in understanding CF's molecular basis led to small-molecule CFTR modulators. Ivacaftor, which is a potentiator, was approved for gating mutations. Dual combinations like ivacaftor/lumacaftor and ivacaftor/tezacaftor brought together a potentiator and a class 1 corrector for F508del homozygous patients. Triple-combination CFTR modulators, including ivacaftor/tezacaftor/elexacaftor with an additional class 2 corrector, are now the standard of care for most CF patients, transforming the outlook for this disease. These drugs stabilize and potentiate the CFTR protein, improving lung function, sweat chloride levels, quality of life, and survival. This Perspective discusses CFTR structure and mutations, biological assays, medicinal chemistry research in identifying CFTR modulators, and clinical data of these agents.

[Elexacaftor, Tezacaftor and Ivacaftor: immediate efficacy evaluation using home spirometry]

INTRODUCTION

Following two weeks of application of the triple combination therapy of Elexacaftor (E), Tezacaftor (T), and Ivacaftor (I) known as ETI, substantial pulmonary improvement in patients with cystic fibrosis is well-documented. However, few detailed data are available on the action of this treatment over the course of these first 14 days.

METHODS

In this prospective study (NCT05599230), 20 patients aged≥12 years, all of them eligible for ETI, were recruited at the initiation of treatment. Home spirometry (MIR Spirobank®) was performed during the three days preceding the start of treatment and then daily for 14 days, while a respiratory symptom score (RSS) was calculated and a log maintained concerning the events experienced by each patient.

RESULTS

Mean age (± SD) of the 20 patients was 29.4 (± 11.1) years, mean FEV1 was 84.2% (± 17.7), and the mean BMI z-score was 0.18 (± 0.82). Thirteen of them were already on modulators. When compared to the average scores recorded for the three days preceding the start of treatment, FEV1 improvement became significant from the 6th day (D6). After having significantly worsened on D1 (P<0.05), the RSS improved from D6 onwards. The quality of home FEV1 measurements was high (grade A: 81.2%).

CONCLUSIONS

Under ETI treatment, respiratory benefits were significant from D6. The side effects most commonly perceived by the patients occurred during the first four days of treatment. While daily monitoring of home spirometry could indeed be a valuable tool in follow-up of patients with cystic fibrosis, its administration requires suitable and sustained training.

Investigation of CFTR Function in Human Nasal Epithelial Cells Informs Personalized Medicine

We broaden the clinical versatility of human nasal epithelial (HNE) cells. HNEs were isolated from 10 participants harboring cystic fibrosis transmembrane conductance regulator (CFTR) variants: 9 with rare variants (Q359R [n = 2], G480S, R334W [n = 5], and R560T) and 1 harboring R117H;7T;TG10/5T;TG12. Cultures were differentiated at the air-liquid interface. CFTR function was measured in Ussing chambers at three conditions: baseline, ivacaftor, and elexacaftor + tezacaftor + ivacaftor (ETI). Four participants initiated modulators. Q359R HNEs had 5.4% (% wild-type) baseline CFTR function and 25.5% with ivacaftor. With therapy, sweat [Cl-] decreased and symptoms resolved. G480S HNEs had 4.1% baseline and 32.1% CFTR function with ETI. Clinically, forced expiratory volume in 1 second increased and sweat [Cl-] decreased (119 to 46 mmol/L) with ETI. In vitro cultures derived from 5 participants harboring R334W showed a moderate increase in CFTR function with exposure to modulators. For one of these participants, ETI was begun in vivo; symptoms and forced expiratory volume in 1 second improved. The c.1679G>C (R560T) HNEs had less than 4% baseline CFTR function and no modulator response. RNA analysis confirmed that c.1679G>C completely missplices. A symptomatic patient harboring R117H;7T;TG10/5T;TG12 exhibited reduced CFTR function (17.5%) in HNEs, facilitating a diagnosis of mild CF. HNEs responded to modulators (ivacaftor: 32.8%, ETI: 55.5%), and, since beginning therapy, lung function improved. We reaffirm HNE use for guiding therapeutic approaches, inform predictions on modulator response (e.g., R334W), and closely assess variants that affect splicing (e.g., c.1679G>C). Notably, functional studies in HNEs harboring R117H;7T;TG10/5T;TG12 facilitated a diagnosis of mild CF, suggesting the use for HNE functional studies as a clinical diagnostic test.

Laboratory Tools to Predict CFTR Modulator Therapy Effectiveness and to Monitor Disease Severity in Cystic Fibrosis

The implementation of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) modulator drugs into clinical practice has been attaining remarkable therapeutic outcomes for CF, a life-threatening autosomal recessive genetic disease. However, there is elevated CFTR allelic heterogeneity, and various individuals carrying (ultra)rare CF genotypes remain without any approved modulator therapy. Novel translational model systems based on individuals' own cells/tissue are now available and can be used to interrogate in vitro CFTR modulator responses and establish correlations of these assessments with clinical features, aiming to provide prediction of therapeutic effectiveness. Furthermore, because CF is a progressive disease, assessment of biomarkers in routine care is fundamental in monitoring treatment effectiveness and disease severity. In the first part of this review, we aimed to focus on the utility of individual-derived in vitro models (such as bronchial/nasal epithelial cells and airway/intestinal organoids) to identify potential responders and expand personalized CF care. Thereafter, we discussed the usage of CF inflammatory biomarkers derived from blood, bronchoalveolar lavage fluid, and sputum to routinely monitor treatment effectiveness and disease progression. Finally, we summarized the progress in investigating extracellular vesicles as a robust and reliable source of biomarkers and the identification of microRNAs related to CFTR regulation and CF inflammation as novel biomarkers, which may provide valuable information for disease prognosis.

In silico analysis and theratyping of an ultra-rare CFTR genotype (W57G/A234D) in primary human rectal and nasal epithelial cells

Mutation targeted therapy in cystic fibrosis (CF) is still not eligible for all CF subjects, especially for cases carrying rare variants such as the CFTR genotype W57G/A234D (c.169T>G/c.701C>A). We performed in silico analysis of the effects of these variants on protein stability, which we functionally characterized using colonoids and reprogrammed nasal epithelial cells. The effect of mutations on cystic fibrosis transmembrane conductance regulator (CFTR) protein was analyzed by western blotting, forskolin-induced swelling (FIS), and Ussing chamber analysis. We detected a residual CFTR function that increases following treatment with the CFTR modulators VX661±VX445±VX770, correlates among models, and is associated with increased CFTR protein levels following treatment with CFTR correctors. In vivo treatment with VX770 reduced sweat chloride concentration to non-CF levels, increased the number of CFTR-dependent sweat droplets, and induced a 6% absolute increase in predicted FEV1% after 27 weeks of treatment indicating the relevance of theratyping with patient-derived cells in CF.

An innovative strategy for personalised medicine in a CFSPID case that evolved with time

We present a challenging case that illustrates how the clinical manifestations in children with CFTR mutations of uncertain significance may change over time. This case highlights the evolution of confirming a diagnosis of CF and emphasises the importance of regular review and monitoring of this patient cohort.

Air-Liquid interface cultures to model drug delivery through the mucociliary epithelial barrier

Epithelial cells from mucociliary portions of the airways can be readily grown and expanded in vitro. When grown on a porous membrane at an air-liquid interface (ALI) the cells form a confluent, electrically resistive barrier separating the apical and basolateral compartments. ALI cultures replicate key morphological, molecular and functional features of the in vivo epithelium, including mucus secretion and mucociliary transport. Apical secretions contain secreted gel-forming mucins, shed cell-associated tethered mucins, and hundreds of additional molecules involved in host defense and homeostasis. The respiratory epithelial cell ALI model is a time-proven workhorse that has been employed in various studies elucidating the structure and function of the mucociliary apparatus and disease pathogenesis. It serves as a critical milestone test for small molecule and genetic therapies targeting airway diseases. To fully exploit the potential of this important tool, numerous technical variables must be thoughtfully considered and carefully executed.

Additive Potentiation of R334W-CFTR Function by Novel Small Molecules

The R334W (c.1000C>T, p.Arg334Trp) is a rare cystic fibrosis (CF)-causing mutation for which no causal therapy is currently approved. This mutation leads to a significant reduction of CF transmembrane conductance regulator (CFTR) channel conductance that still allows for residual function. Potentiators are small molecules that interact with CFTR protein at the plasma membrane to enhance CFTR-dependent chloride secretion, representing thus pharmacotherapies targeting the root cause of the disease. Here, we generated a new CF bronchial epithelial (CFBE) cell line to screen a collection of compounds and identify novel potentiators for R334W-CFTR. The active compounds were then validated by electrophysiological assays and their additive effects in combination with VX-770, genistein, or VX-445 were exploited in this cell line and further confirmed in intestinal organoids. Four compounds (LSO-24, LSO-25, LSO-38, and LSO-77) were active in the functional primary screen and their ability to enhance R334W-CFTR-dependent chloride secretion was confirmed using electrophysiological measurements. In silico ADME analyses demonstrated that these compounds follow Lipinski’s rule of five and are thus suggested to be orally bioavailable. Dose−response relationships revealed nevertheless suboptimal efficacy and weak potency exerted by these compounds. VX-770 and genistein also displayed a small potentiation of R334W-CFTR function, while VX-445 demonstrated no potentiator activity for this mutation. In the R334W-expressing cell line, CFTR function was further enhanced by the combination of LSO-24, LSO-25, LSO-38, or LSO-77 with VX-770, but not with genistein. The efficacy of potentiator VX-770 combined with active LSO compounds was further confirmed in intestinal organoids (R334W/R334W genotype). Taken together, these molecules were demonstrated to potentiate R334W-CFTR function by a different mechanism than that of VX-770. They may provide a feasible starting point for the design of analogs with improved CFTR-potentiator activity.

Standards of care for CFTR variant-specific therapy (including modulators) for people with cystic fibrosis

Cystic fibrosis (CF) has entered the era of variant-specific therapy, tailored to the genetic variants in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. CFTR modulators, the first variant-specific therapy available, have transformed the management of CF. The latest standards of care from the European CF Society (2018) did not include guidance on variant-specific therapy, as CFTR modulators were becoming established as a novel therapy. We have produced interim standards to guide healthcare professionals in the provision of variant-specific therapy for people with CF. Here we provide evidence-based guidance covering the spectrum of care, established using evidence from systematic reviews and expert opinion. Statements were reviewed by key stakeholders using Delphi methodology, with agreement (≥80%) achieved for all statements after one round of consultation. Issues around accessibility are discussed and there is clear consensus that all eligible people with CF should have access to variant-specific therapy.

Organoid Technology and Its Role for Theratyping Applications in Cystic Fibrosis

Cystic fibrosis (CF) is a autosomal recessive, multisystemic disease caused by different mutations in the CFTR gene encoding CF transmembrane conductance regulator. Although symptom management is important to avoid complications, the approval of CFTR modulator drugs in the clinic has demonstrated significant improvements by targeting the primary molecular defect of CF and thereby preventing problems related to CFTR deficiency or dysfunction. CFTR modulator therapies have positively changed the patients' quality of life, especially for those who start their use at the onset of the disease. Due to early diagnosis with the implementation of newborn screening programs and considerable progress in the treatment options, nowadays pediatric mortality was dramatically reduced. In any case, the main obstacle to treat CF is to predict the drug response of patients due to genetic complexity and heterogeneity. Advances in 3D culture systems have led to the extrapolation of disease modeling and individual drug response in vitro by producing mini organs called "organoids" easily obtained from nasal and rectal mucosa biopsies. In this review, we focus primarily on patient-derived intestinal organoids used as in vitro model for CF disease. Organoids combine high-validity of outcomes with a high throughput, thus enabling CF disease classification, drug development and treatment optimization in a personalized manner.

Identification of novel F508del-CFTR traffic correctors among triazole derivatives

The most prevalent cystic fibrosis (CF)-causing mutation - F508del - impairs the folding of CFTR protein, resulting in its defective trafficking and premature degradation. Small molecules termed correctors may rescue F508del-CFTR and therefore constitute promising pharmacotherapies acting on the fundamental cause of the disease. Here, we screened a collection of triazole compounds to identify novel F508del-CFTR correctors. The functional primary screen identified four hit compounds (LSO-18, LSO-24, LSO-28, and LSO-39), which were further validated and demonstrated to rescue F508del-CFTR processing, plasma membrane trafficking, and function. To interrogate their mechanism of action (MoA), we examined their additivity to the clinically approved drugs VX-661 and VX-445, low temperature, and genetic revertants of F508del-CFTR. Rescue of F508del-CFTR processing and function by LSO-18, LSO-24, and LSO-28, but not by LSO-39, was additive to VX-661, whereas LSO-28 and LSO-39, but not LSO-18 nor LSO-24, were additive to VX-445. All compounds under investigation demonstrated additive rescue of F508del-CFTR processing and function to low temperature as well as to rescue by genetic revertants G550E and 4RK. Nevertheless, none of these compounds was able to rescue processing nor function of DD/AA-CFTR, and LSO-39 (similarly to VX-661) exhibited no additivity to genetic revertant R1070W. From these findings, we suggest that LSO-39 (like VX-661) has a putative binding site at the NBD1:ICL4 interface, LSO-18 and LSO-24 seem to share the MoA with VX-445, and LSO-28 appears to act by a different MoA. Altogether, these findings represent an encouraging starting point to further exploit this chemical series for the development of novel CFTR correctors.

Advances in Preclinical In Vitro Models for the Translation of Precision Medicine for Cystic Fibrosis

The development of preclinical in vitro models has provided significant progress to the studies of cystic fibrosis (CF), a frequently fatal monogenic disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein. Numerous cell lines were generated over the last 30 years and they have been instrumental not only in enhancing the understanding of CF pathological mechanisms but also in developing therapies targeting the underlying defects in CFTR mutations with further validation in patient-derived samples. Furthermore, recent advances toward precision medicine in CF have been made possible by optimizing protocols and establishing novel assays using human bronchial, nasal and rectal tissues, and by progressing from two-dimensional monocultures to more complex three-dimensional culture platforms. These models also enable to potentially predict clinical efficacy and responsiveness to CFTR modulator therapies at an individual level. In parallel, advanced systems, such as induced pluripotent stem cells and organ-on-a-chip, continue to be developed in order to more closely recapitulate human physiology for disease modeling and drug testing. In this review, we have highlighted novel and optimized cell models that are being used in CF research to develop novel CFTR-directed therapies (or alternative therapeutic interventions) and to expand the usage of existing modulator drugs to common and rare CF-causing mutations.

Absence of EPAC1 Signaling to Stabilize CFTR in Intestinal Organoids

The plasma membrane (PM) stability of the cystic fibrosis transmembrane conductance regulator (CFTR), the protein which when mutated causes Cystic Fibrosis (CF), relies on multiple interaction partners that connect CFTR to signaling pathways, including cAMP signaling. It was previously shown that activation of exchange protein directly activated by cAMP 1 (EPAC1) by cAMP promotes an increase in CFTR PM levels in airway epithelial cells. However, the relevance of this pathway in other tissues, particularly the intestinal tissue, remains uncharacterized. Here, we used Western blot and forskolin-induced swelling assay to demonstrate that the EPAC1 protein is not expressed in the intestinal organoid model, and consequently the EPAC1 stabilization pathway is not in place. On the other hand, using cell surface biotinylation, EPAC1-mediated stabilization of PM CFTR is observed in intestinal cell lines. These results indicate that the EPAC1 stabilization pathway also occurs in intestinal cells and is a potential target for the development of novel combinatorial therapies for treatment of CF.

Left behind: The potential impact of CFTR modulators on racial and ethnic disparities in cystic fibrosis

The advent of CFTR modulators, a genomic specific medication, revolutionized the treatment of CF for many patients. However, given that these therapeutics were only developed for specific CFTR mutations, not all people with CF have access to such disease-modifying drugs. Racial and ethnic minority groups are less likely to have CFTR mutations that are approved for CFTR modulators. This exclusion has the potential to widen existing health disparities.

New drugs in cystic fibrosis: what has changed in the last decade?

Cystic fibrosis (CF), a life-limiting chronic disease caused by mutations in the cystic fibrosis transmembrane regulator (CFTR) gene, affects more than 90,000 people worldwide. Until recently, the only available treatments were directed to symptom control, but they failed to change the course of the disease. New drugs developed in the last decade have the potential to change the expression, function, and stability of CFTR protein, targeting the basic molecular defect. The authors seek to provide an update on the new drugs, with a special focus on the most promising clinical trials that have been carried out to date. These newly approved drugs that target specific CFTR mutations are mainly divided into two main groups of CFTR modulators: potentiators and correctors. New therapies have opened the door for potentially disease-modifying, personalized treatments for patients with CF.

Molecular Dynamics and Theratyping in Airway and Gut Organoids Reveal R352Q-CFTR Conductance Defect

A significant challenge to making targeted cystic fibrosis transmembrane conductance regulator (CFTR) modulator therapies accessible to all individuals with cystic fibrosis (CF) are many mutations in the CFTR gene that can cause CF, most of which remain uncharacterized. Here, we characterized the structural and functional defects of the rare CFTR mutation R352Q, with a potential role contributing to intrapore chloride ion permeation, in patient-derived cell models of the airway and gut. CFTR function in differentiated nasal epithelial cultures and matched intestinal organoids was assessed using an ion transport assay and forskolin-induced swelling assay, respectively. CFTR potentiators (VX-770, GLPG1837, and VX-445) and correctors (VX-809, VX-445, with or without VX-661) were tested. Data from R352Q-CFTR were compared with data of 20 participants with mutations with known impact on CFTR function. R352Q-CFTR has residual CFTR function that was restored to functional CFTR activity by CFTR potentiators but not the corrector. Molecular dynamics simulations of R352Q-CFTR were carried out, which indicated the presence of a chloride conductance defect, with little evidence supporting a gating defect. The combination approach of in vitro patient-derived cell models and in silico molecular dynamics simulations to characterize rare CFTR mutations can improve the specificity and sensitivity of modulator response predictions and aid in their translational use for CF precision medicine.

CFTR Modulator Therapy for Rare CFTR Mutants

Cystic fibrosis (CF), the most common genetic disease among the Caucasian population, is caused by mutations in the gene encoding for the CF transmembrane conductance regulator (CFTR), a chloride epithelial channel whose dysfunction results in severe airway obstruction and inflammation, eventually leading to respiratory failure. The discovery of the CFTR gene in 1989 provided new insights into the basic genetic defect of CF and allowed the study of potential therapies targeting the aberrant protein. In recent years, the approval of “CFTR modulators”, the first molecules designed to selectively target the underlying molecular defects caused by specific CF-causing mutations, marked the beginning of a new era in CF treatment. These drugs have been demonstrated to significantly improve lung function and ameliorate the quality of life of many patients, especially those bearing the most common CFTR mutatant F508del. However, a substantial portion of CF subjects, accounting for ~20% of the European CF population, carry rare CFTR mutations and are still not eligible for CFTR modulator therapy, partly due to our limited understanding of the molecular defects associated with these genetic alterations. Thus, the implementation of models to study the phenotype of these rare CFTR mutations and their response to currently approved drugs, as well as to compounds under research and clinical development, is of key importance. The purpose of this review is to summarize the current knowledge on the potential of CFTR modulators in rescuing the function of rare CF-causing CFTR variants, focusing on both investigational and clinically approved molecules.

Assays of CFTR Function In Vitro, Ex Vivo and In Vivo

Cystic fibrosis, a multi-organ genetic disease, is characterized by abnormal function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a chloride channel at the apical membrane of several epithelia. In recent years, therapeutic strategies have been developed to correct the CFTR defect. To evaluate CFTR function at baseline for diagnosis, or the efficacy of CFTR-restoring therapy, reliable tests are needed to measure CFTR function, in vitro, ex vivo and in vivo. In vitro techniques either directly or indirectly measure ion fluxes; direct measurement of ion fluxes and quenching of fluorescence in cell-based assays, change in transmembrane voltage or current in patch clamp or Ussing chamber, swelling of CFTR-containing organoids by secondary water influx upon CFTR activation. Several cell or tissue types can be used. Ex vivo and in vivo assays similarly evaluate current (intestinal current measurement) and membrane potential differences (nasal potential difference), on tissues from individual patients. In the sweat test, the most frequently used in vivo evaluation of CFTR function, chloride concentration or stimulated sweat rate can be directly measured. Here, we will describe the currently available bio-assays for quantitative evaluation of CFTR function, their indications, advantages and disadvantages, and correlation with clinical outcome measures.

A new platform for high-throughput therapy testing on iPSC-derived lung progenitor cells from cystic fibrosis patients

For those people with cystic fibrosis carrying rare CFTR mutations not responding to currently available therapies, there is an unmet need for relevant tissue models for therapy development. Here, we describe a new testing platform that employs patient-specific induced pluripotent stem cells (iPSCs) differentiated to lung progenitor cells that can be studied using a dynamic, high-throughput fluorescence-based assay of CFTR channel activity. Our proof-of-concept studies support the potential use of this platform, together with a Canadian bioresource that contains iPSC lines and matched nasal cultures from people with rare mutations, to advance patient-oriented therapy development. Interventions identified in the high-throughput, stem cell-based model and validated in primary nasal cultures from the same person have the potential to be advanced as therapies.

•

A Canadian resource (CFIT) has CF donor-matched iPSCs and nasal epithelial cells

•

Lung progenitor cells (LPCs) differentiated from iPSCs express CFTR

•

LPCs from people with rare CFTR mutations enable high-throughput therapy testing

•

Matching nasal cultures can validate patient-specific drug responses in LPCs

Perspectives on the translation of in-vitro studies to precision medicine in Cystic Fibrosis

Recent strides towards precision medicine in Cystic Fibrosis (CF) have been made possible by patient-derived in-vitro assays with the potential to predict clinical response to small molecule-based therapies. Here, we discuss the status of primary and stem-cell derived tissues used to evaluate the preclinical efficacy of CFTR modulators highlighting both their potential and limitations. Validation of these assays requires correlation of in-vitro responses to in-vivo measures of clinical biomarkers of disease outcomes. While initial efforts have shown some success, this translation requires methodologies that are sensitive enough to capture treatment responses in a CF population that now predominantly has mild lung disease. Future development of in-vitro and in-vivo biomarkers will facilitate the generation of new therapeutics particularly for those patients with rare mutations where clinical trials are not feasible so that in the future every CF patient will have access to effective targeted therapies.

Three-Dimensional Airway Spheroids and Organoids for Cystic Fibrosis Research

Cystic fibrosis (CF) is an autosomal recessive multi-organ disease caused by mutations in the CF Transmembrane Conductance Regulator (CFTR) gene, with morbidity and mortality primacy related to the lung disease. The CFTR protein, a chloride/bicarbonate channel, is expressed at the apical side of airway epithelial cells and is mainly involved in appropriate ion and fluid transport across the epithelium. Although many animal and cellular models have been developed to study the pathophysiological consequences of the lack/dysfunction of CFTR, only the three-dimensional (3D) structures termed “spheroids” and “organoids” can enable the reconstruction of airway mucosa to model organ development, disease pathophysiology, and drug screening. Airway spheroids and organoids can be derived from different sources, including adult lungs and induced pluripotent stem cells (iPSCs), each with its advantages and limits. Here, we review the major features of airway spheroids and organoids, anticipating that their potential in the CF field has not been fully shown. Further work is mandatory to understand whether they can accomplish better outcomes than other culture conditions of airway epithelial cells for CF personalized therapies and tissue engineering aims.

A Precision Medicine Approach to Optimize Modulator Therapy for Rare CFTR Folding Mutants

Trikafta, a triple-combination drug, consisting of folding correctors VX-661 (tezacaftor), VX-445 (elexacaftor) and the gating potentiator VX-770 (ivacaftor) provided unprecedented clinical benefits for patients with the most common cystic fibrosis (CF) mutation, F508del. Trikafta indications were recently expanded to additional 177 mutations in the CF transmembrane conductance regulator (CFTR). To minimize life-long pharmacological and financial burden of drug administration, if possible, we determined the necessary and sufficient modulator combination that can achieve maximal benefit in preclinical setting for selected mutants. To this end, the biochemical and functional rescue of single corrector-responsive rare mutants were investigated in a bronchial epithelial cell line and patient-derived human primary nasal epithelia (HNE), respectively. The plasma membrane density of P67L-, L206W- or S549R-CFTR corrected by VX-661 or other type I correctors was moderately increased by VX-445. Short-circuit current measurements of HNE, however, uncovered that correction comparable to Trikafta was achieved for S549R-CFTR by VX-661 + VX-770 and for P67L- and L206W-CFTR by the VX-661 + VX-445 combination. Thus, introduction of a third modulator may not provide additional benefit for patients with a subset of rare CFTR missense mutations. These results also underscore that HNE, as a precision medicine model, enable the optimization of mutation-specific modulator combinations to maximize their efficacy and minimize life-long drug exposure of CF patients.