Optimizing Nasal Potential Difference Analysis for CFTR Modulator Development: Assessment of Ivacaftor in CF Subjects with the G551D-CFTR Mutation

Abstract

Nasal potential difference (NPD) is used as a biomarker of the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium channel (ENaC) activity. We evaluated methods to detect changes in chloride and sodium transport by NPD based on a secondary analysis of a Phase II CFTR-modulator study. Thirty-nine subjects with CF who also had the G551D-CFTR mutation were randomized to receive ivacaftor (Kalydeco™; also known as VX-770) in four doses or placebo twice daily for at least 14 days. All data were analyzed by a single investigator who was blinded to treatment assignment. We compared three analysis methods to determine the best approach to quantify changes in chloride and sodium transport: (1) the average of both nostrils; (2) the most-polarized nostril at each visit; and (3) the most-polarized nostril at screening carried forward. Parameters of ion transport included the PD change with zero chloride plus isoproterenol (CFTR activity), the basal PD, Ringer's PD, and change in PD with amiloride (measurements of ENaC activity), and the delta NPD (measuring CFTR and ENaC activity). The average and most-polarized nostril at each visit were most sensitive to changes in chloride and sodium transport, whereas the most-polarized nostril at screening carried forward was less discriminatory. Based on our findings, NPD studies should assess both nostrils rather than a single nostril. We also found that changes in CFTR activity were more readily detected than changes in ENaC activity, and that rigorous standardization was associated with relatively good within-subject reproducibility in placebo-treated subjects (±2.8 mV). Therefore, we have confirmed an assay of reasonable reproducibility for detecting chloride-transport improvements in response to CFTR modulation.

Nasal potential difference: Best or average result for CFTR function as diagnostic criteria for cystic fibrosis?

“…This approach is widely accepted for clinical research [24,25] and diagnostic applications [12]. However, our results demonstrate that this approach underestimates the individual CFTR function in a considerable number, mainly Query-CF patients.…”

Activation of the Endogenous Renin-Angiotensin-Aldosterone System or Aldosterone Administration Increases Urinary Exosomal Sodium Channel Excretion

“…Nasal potential difference has also been used to estimate ENaC activity, although it has not been proven to reflect renal ENaC activity. 26 In animal studies, Western blot analysis of ENaC subunits in kidney tissue has been extensively used to measure expression, but obtaining adequate renal tissue for this measurement in clinical studies is impractical. Western blot analysis for ENaC has been performed on urinary exosomal and total urinary protein in prior clinical studies, 27,28 but these have reported low sensitivity and none have demonstrated a significant response during RAAS activation.…”

Moderate intensity exercise mediates comparable increases in exhaled chloride as albuterol in individuals with cystic fibrosis

“…Since previous research attempting to describe ion transport during exercise and following administration of pharmacological therapies in CF has primarily used measurement of NPD 28–32 , NPD was also measured in this study to compare with EBC measurements. NPD was conducted according to the Cystic Fibrosis Foundation Therapeutics Development Network Standard Operating Procedure 33 .…”