Nontuberculous mycobacteria (NTM) are the pathogens of concern in people with cystic fibrosis (pwCF) due to their association with deterioration of lung function. Treatment requires the use of a multidrug combination regimen, creating the potential for drug-drug interactions (DDIs) with cystic fibrosis transmembrane conductance regulator (CFTR)-modulating therapies, including elexacaftor, tezacaftor, and ivacaftor (ETI), which are eliminated mainly through cytochrome P450 (CYP) 3A-mediated metabolism. An assessment of the DDI risk for ETI coadministered with NTM treatments, including rifabutin, clofazimine, and clarithromycin, is needed to provide appropriate guidance on dosing. The CYP3A-mediated DDIs between ETI and the NTM therapies rifabutin, clarithromycin, and clofazimine were evaluated using physiologically based pharmacokinetic (PBPK) modeling by incorporating demographic and physiological "system" data with drug physicochemical and in vitro parameters. Models were verified and then applied to predict untested scenarios to guide continuation of ETI during antibiotic treatment, using ivacaftor as the most sensitive CYP3A4 substrate. The predicted area under the concentration-time curve (AUC) ratios of ivacaftor when coadministered with rifabutin, clofazimine, or clarithromycin were 0.31, 2.98, and 9.64, respectively, suggesting moderate and strong interactions. The simulation predicted adjusted dosing regimens of ETI administered concomitantly with NTM treatments, which required delayed resumption of the standard dose of ETI once the NTM treatments were completed. The dosing transitions were determined based on the characteristics of the perpetrator drugs, including the mechanism of CYP3A modulation and their elimination half-lives. This study suggests increased doses of elexacaftor/tezacaftor/ivacaftor 200/100/450 mg in the morning and 100/50/375 mg in the evening when ETI is coadministered with rifabutin and reduced doses of elexacaftor/tezacaftor 200/100 mg every 48 h (q48h) and ivacaftor 150 mg daily or a dose of elexacaftor/tezacaftor/ivacaftor 200/100/150 mg q72h when coadministered with clofazimine or clarithromycin, respectively. Importantly, the PBPK simulations provide evidence in support of the use of treatments for NTM in pwCF receiving concomitant dose-adjusted ETI therapy.
Keywords: PBPK modeling; cystic fibrosis; drug interactions; nontuberculosis mycobacteria.