Challenged Urine Bicarbonate Excretion as a Measure of Cystic Fibrosis Transmembrane Conductance Regulator Function in Cystic Fibrosis

Abstract

Background: In cystic fibrosis (CF), renal base excretion is impaired. Accordingly, challenged urine bicarbonate excretion may be an in vivo biomarker of cystic fibrosis transmembrane conductance regulator (CFTR) function.

Objective: To evaluate the association between challenged bicarbonate excretion and clinical characteristics at baseline, quantify the CFTR modulator drug elexacaftor/tezacaftor/ivacaftor-induced changes of challenged bicarbonate excretion after 6 months of treatment, and characterize the intraindividual variation in healthy adults.

Design: Prospective observational study.

Setting: Cystic fibrosis clinic, Aarhus University Hospital, Denmark.

Patients: Fifty adult patients with CF starting CFTR modulator therapy with elexacaftor/tezacaftor/ivacaftor between May 2020 and June 2021.

Measurements: Quantification of urine bicarbonate excretion after an acute oral sodium bicarbonate challenge before and 6 months after elexacaftor/tezacaftor/ivacaftor treatment.

Results: At baseline, challenged urine bicarbonate excretion was associated with several CF disease characteristics. Bicarbonate excretion was higher in patients with residual function mutations. A higher bicarbonate excretion was associated with better lung function, pancreatic sufficiency, and lower relative risk for chronic pseudomonas infections. Elexacaftor/tezacaftor/ivacaftor treatment increased bicarbonate excretion by 3.9 mmol/3 h (95% CI, 1.6 to 6.1 mmol/3 h), reaching about 70% of that seen in healthy control participants. In healthy control participants, individual bicarbonate excretion at each visit correlated with the individual mean bicarbonate excretion. The median coefficient of variation was 31%.

Limitation: Single-center study without a placebo-controlled group.

Conclusion: Although further studies are needed to address the performance and sensitivity of this approach, this early-stage evaluation shows that challenged urine bicarbonate excretion may offer a new, simple, and safe quantification of CFTR function and the extent of its pharmacologic improvement. Elexacaftor/tezacaftor/ivacaftor partially restores renal CFTR function in patients with CF, likely resulting in decreased risk for electrolyte disorders and metabolic alkalosis.

Primary funding source: Innovation Fund Denmark.