Preliminary evidence for sustained efficacy of CFTR modulator therapy with concomitant rifabutin administration

•Three patient cases suggest addition of rifabutin does not compromise ETI efficacy.•PBPK simulated ETI lung concentrations with concomitant rifabutin remain therapeutic.•The study supports the use of rifabutin for the treatment of NTM in CF receiving ETI. The concomitant use of elexacaftor/tezacaftor/ivacaftor (ETI) and strong CYP3A inducers including rifampin and rifabutin is not recommended due to the risk of drug-drug interactions (DDI). This presents a significant challenge to the treatment of non-tuberculous mycobacteria precluding the first line treatment. While rifabutin induces CYP3A activity, its effect appears to be moderate compared to rifampin. In this study, we investigated three cases in which concomitant use of rifabutin and CFTR modulators (ETI or ivacaftor monotherapy) was used, and these cases suggest that addition of rifabutin did not compromise the efficacy of ETI or ivacaftor as evidenced by pulmonary function and sweat chloride testing. A full physiologically based pharmacokinetic model predicted lung concentrations of ETI upon rifabutin coadministration to exceed the half-maximal effective concentrations (EC50) determined from chloride transport in phe508del human bronchial epithelial cells. This study provides preliminary evidence in support of the use of rifabutin in patients receiving ETI.