Effect of concomitant rifampin and clarithromycin on elexacaftor-tezacaftor-ivacaftor using a physiologically based pharmacokinetic (PBPK) modeling approach.

BACKGROUND

Rifampin (RIF) is a first-line therapy for Mycobacterium avium complex (MAC) in people with cystic fibrosis (CF); but it induces CYP3A, leading to significant drug-drug interactions (DDIs) with elexacaftor/tezacaftor/ivacaftor (ETI). Clarithromycin (CLR), a strong CYP3A inhibitor, may counteract rifamycin-mediated induction, potentially enabling co-administration with ETI.

RESEARCH DESIGN AND

METHODS

DDIs between RIF, CLR, and ETI were evaluated using physiologically based pharmacokinetic (PBPK) modeling. Two-drug (e.g., RIF-CLR, RIF-ivacaftor, CLR-ivacaftor) and three-drug combinations (e.g., RIF-CLR-midazolam) were validated against published data. The models were then used to predict interactions between RIF, CLR, and ETI.

RESULTS

Predicted AUC0-24h ratios for ETI with RIF and CLR administration were 0.29 (95% CI 0.26-0.32), 0.30 (95% CI 0.26-0.35), and 0.19 (95% CI 0.16-0.23) for elexacaftor, tezacaftor, and ivacaftor, respectively. Steady-state concentrations exceeded the EC50 targets in 99%, 94%, and 43% of the simulated population, respectively. Lower RIF or higher CLR doses modestly improved ivacaftor target attainment.

CONCLUSIONS

Concomitant RIF and CLR reduced ETI plasma exposures. Elexacaftor and tezacaftor concentrations remained above EC50 targets, while ivacaftor was slightly below. Variability in CLR pharmacokinetics during RIF co-administration may contribute to interindividual differences in ETI exposure.