Azithromycin and tezacaftor/ivacaftor is associated with first-degree heart block in an adult with cystic fibrosis.

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene is expressed in the heart, but this is not known to cause myocardial dysfunction or abnormalities in the ECG in people with CF. CFTR modulators such as tezacaftor/ivacaftor improve lung function and overall health in people with CF. Minor adverse events have been reported in the early clinical trials, but no consistent changes in the ECGs have been reported. This case highlights an unusual side effect of first degree heart block that occurred after more than 8 months of azithromycin and tezacaftor/ivacaftor in combination. Drug withdrawal and reintroduction confirmed that neither drug alone, but only the combination, caused this change. As tezacaftor/ivacaftor is also present in elexacaftor/tezacaftor/ivacaftor, care may be needed to exclude this delayed interaction with azithromycin.