Acute administration of ivacaftor to people with cystic fibrosis and a G551D-CFTR mutation reveals smooth muscle abnormalities

Abstract

Background: Airflow obstruction is common in cystic fibrosis (CF), yet the underlying pathogenesis remains incompletely understood. People with CF often exhibit airway hyperresponsiveness, CF transmembrane conductance regulator (CFTR) is present in airway smooth muscle (ASM), and ASM from newborn CF pigs has increased contractile tone, suggesting that loss of CFTR causes a primary defect in ASM function. We hypothesized that restoring CFTR activity would decrease smooth muscle tone in people with CF.

Methods: To increase or potentiate CFTR function, we administered ivacaftor to 12 adults with CF with the G551D-CFTR mutation; ivacaftor stimulates G551D-CFTR function. We studied people before and immediately after initiation of ivacaftor (48 hours) to minimize secondary consequences of CFTR restoration. We tested smooth muscle function by investigating spirometry, airway distensibility, and vascular tone.

Results: Ivacaftor rapidly restored CFTR function, indicated by reduced sweat chloride concentration. Airflow obstruction and air trapping also improved. Airway distensibility increased in airways less than 4.5 mm but not in larger-sized airways. To assess smooth muscle function in a tissue outside the lung, we measured vascular pulse wave velocity (PWV) and augmentation index, which both decreased following CFTR potentiation. Finally, change in distensibility of <4.5-mm airways correlated with changes in PWV.

Conclusions: Acute CFTR potentiation provided a unique opportunity to investigate CFTR-dependent mechanisms of CF pathogenesis. The rapid effects of ivacaftor on airway distensibility and vascular tone suggest that CFTR dysfunction may directly cause increased smooth muscle tone in people with CF and that ivacaftor may relax smooth muscle.

Funding: This work was funded in part from an unrestricted grant from the Vertex Investigator-Initiated Studies Program.

Figure 1. Ivacaftor treatment rapidly improves sweat chloride concentration and airflow obstruction.

(A) Sweat chloride concentration (mmol/l). Dashed line indicates diagnostic threshold for cystic fibrosis (60 mmol/l). (B) Forced expiratory volume in 1 second, percent predicted (FEV₁ [% pred.]). (C) Forced vital capacity, percent predicted [FVC (% pred.)]. (D) Forced expiratory flow rate 25%–75% (FEF_25%–75%). Studies were performed day 0 (before ivacaftor) and following 2 days of ivacaftor treatment. In individual panels, each symbol represents a different subject. Connected symbols represent individual subjects before and after ivacaftor. Horizontal bars represent mean ±SEM, and a paired t test was performed. *P ≤ 0.01.

Figure 2. Ivacaftor improves CT-based air-trapping assessment.

CT-derived 3-dimensional rendering of the lung and airways from a study participant before (day 0) and after ivacaftor treatment (day 2). Regions of air trapping (Hounsfield units [HU] < –856) are highlighted in light blue.

Figure 3. Ivacaftor rapidly improves air trapping.

Studies were performed on day 0 (before ivacaftor) and following 2 days of ivacaftor treatment. (A) Total lung air trapping. Air trapping was defined as the percentage of voxels below –856 Hounsfield units (HU) on the RV (expiratory) CT scan. (B) Regional lung air trapping. (C) Correlation between air trapping (day 0) defined by the percentage of total lung voxels below –856 HU vs. air trapping defined by the RV/TLC ratio. The RV (expiratory) and TLC (inspiratory) lung volumes were determined from CT datasets (day 0). In individual panels, each symbol represents a different subject. In A and B, connected symbols represent individual subjects before and after ivacaftor. n = 12 subjects. Horizontal bars represent mean ±SEM. A Wilcoxon matched pairs signed-rank test (A) or a paired t test (B) was performed. A Pearson’s correlation coefficient (r) was determined for data in C. *P < 0.05. RV, residual volume; TLC, total lung capacity.

Figure 4. Less air trapping correlates with improved spirometry.

(A) Total inspiratory lung volume and (B) Total expiratory lung volume from CT datasets on day 0 (before ivacaftor) and following 2 days of ivacaftor treatment (day 2). (C) Correlation between changes in total lung air trapping (percentage of lung voxels below –856 Hounsfield units [HU]) and changes in forced expiratory volume in 1 second (FEV₁) (% predicted) before and after ivacaftor. (D) Forced vital capacity (FVC, liters) from spirometry and vital capacity (VC, liters) from CT-based quantification after ivacaftor. For individual panels, each symbol represents a different subject. In A and B, connected symbols represent individual subjects before and after ivacaftor. Horizontal bars represent mean ±SEM. A paired t test was performed for data in A and B. A Pearson’s correlation coefficient (r) was determined for data in C and D. *P < 0.05.

Figure 5. Airway effects of ivacaftor.

(A) Forced expiratory volume in 1 second (FEV₁) (% of pred.) on day 0 (before ivacaftor) and following 2 days of ivacaftor treatment (day 2). The light bar represents the average spirometric value prior to bronchodilator, and the dark bar represents the bronchodilator response. n = 12 subjects. (B) Airway distensibility grouped by airway size. (C) Airway lumen area, grouped by airway size, from inspiratory (insp.) and expiratory (exp.) CT scans. (D) Lack of a correlation between change in distensibility of airways <4.5 mm and change in air trapping after ivacaftor treatment. Each symbol represents a different subject. (E) Airway wall thickness was quantified from inspiratory CT scans on day 0 and day 2. For panels B, C, and E, airways were grouped by lumen diameter obtained from day 0 inspiratory scans. A total of 396 airways were analyzed (33 average airways/subject), ranging in size from 2.4–20.8 mm. Eighty-four airways were measured in the <4.5 mm group (average of 7 airways/subject, n = 11 subjects), 167 airways in the 4.5–6.5 mm group (average of 14 airways/subject, n = 12 subjects), and 145 airways in the >6.5 mm group (average of 12 airways/subject, n = 12 subjects). In individual panels, each symbol represents a different subject, and connected symbols represent individual subjects before and after ivacaftor. Bars represent mean ±SEM. *P < 0.05 using a generalized linear mixed model to adjust for repeated measures on individual subjects. For D, the correlation measure (r) was calculated using a linear mixed model to accommodate for a differing number of airway distensibility measurements for each subject.

Figure 6. Ivacaftor decreases vascular tone.

Pulse wave analysis (PWA) was performed on day 0 (before ivacaftor) and following 2 days of ivacaftor treatment (day 2). (A) Heart rate-corrected (75 bpm) augmentation index (AIx@75). (B) Pulse wave velocity. (C) Correlation between baseline pulse wave velocity and baseline airway distensibility (airways < 4.5 mm diameter). (D) Correlation between change in pulse wave velocity and change in airway distensibility (airways < 4.5 mm diameter) after ivacaftor treatment. For individual panels, each symbol represents a different subject. In A and B, connected symbols represent individual subjects before and after ivacaftor. Values are mean ±SEM. A paired t test (A) or a Wilcoxon matched pairs signed-rank test (B) was performed. *P < 0.05. For C and D, the correlation measure (r) was calculated using a linear mixed model to accommodate for a differing number of airway distensibility measurements for each subject.