PBPK-led guidance for the treatment of cystic fibrosis patients taking elexacaftor-tezacaftor-ivacaftor with the newly approved fixed dose combination nirmatrelvir-ritonavir (PAXLOVID) for the treatment of COVID-19

Background: Cystic fibrosis transmembrane conductance regulator (CFTR) modulating therapies including elexacaftor, tezacaftor, and ivacaftor (ETI) are primarily eliminated through cytochrome P450 (CYP) 3A-mediated metabolism. This creates a therapeutic challenge to the treatment of COVID-19 with nirmatrelvir-ritonavir in people with cystic fibrosis (pwCF) due to the potential for significant drug-drug interactions (DDI). However, pwCF are more at risk of serious illness following COVID-19 infection and hence it is important to manage the DDI risk and provide treatment options. Methods: CYP3A-mediated DDI of ETI was evaluated using a physiologically based pharmacokinetic (PBPK) modeling approach. Modeling was performed incorporating physiological information and drug dependent parameters of ETI to predict the effect of ritonavir (the CYP3A4 inhibiting component of the combination) on pharmacokinetics of ETI. The ETI models were verified using independent clinical pharmacokinetic and DDI data of ETI with a range of CYP3A modulators. Results: When ritonavir was administered on day 1 through 5, the predicted AUC ratio of ivacaftor (the most sensitive CYP3A substrate) on day 6 was 9.31, indicating that its metabolism was strongly inhibited. Based on the predicted DDI, the dose of ETI should be reduced when co-administered with nirmatrelvir-ritonavir to elexacaftor 200mg-tezacaftor 100mg-ivacaftor 150mg on days 1 and 5, with resumption of full dose ETI on day 9, considering the residual inhibitory effect of ritonavir as a mechanism-based inhibitor. Conclusions: Coadministration of nirmatrelvir-ritonavir requires a significant reduction in the ETI dosing frequency with delayed resumption of full dose due to the mechanism-based inhibition with ritonavir.