Physiologically Based Pharmacokinetic Modeling to Evaluate Drug‐Drug Interactions of Tacrolimus With Ritonavir, a CYP3A Irreversible Inhibitor: Applications for Dosing Optimization in Transplant Patients

Immunosuppressant tacrolimus is frequently coadministered with other drugs in clinical practice. Ritonavir is a CYP3A irreversible inhibitor that is used in combination with nirmatrelvir for the treatment of COVID-19 in nirmatrelvir/ritonavir. We aimed to apply physiologically based pharmacokinetic (PBPK) modeling to investigate the dose adjustment strategy for tacrolimus during short-term coadministration with ritonavir. PBPK models for tacrolimus and ritonavir were successfully developed based on in vitro and clinical data, and were used to predict drug-drug interaction levels via metabolic enzyme inhibition mechanisms. The recommended dose strategy based on our model simulation is to hold tacrolimus during nirmatrelvir/ritonavir treatment and restart half of the initial dose on day 3 after the 5-day ritonavir course, followed by resuming the full dose on day 5. This administration strategy can maintain trough concentrations of tacrolimus within the therapeutic window during and after ritonavir treatment.