Nirmatrelvir/ritonavir treatment of patients with COVID-19 taking tacrolimus: case series describing the results of drug–drug interactions

Nirmatrelvir/ritonavir is a novel drug combination that is authorized by the Food and Drug Administration for the treatment of coronavirus disease 2019 (COVID-19). Ritonavir is a cytochrome P450 3A inhibitor and a P-glycoprotein inhibitor that increases the plasma concentration of tacrolimus and other medications. We describe the cases of two patients treated with nirmatrelvir/ritonavir: a patient who had undergone kidney transplantation and another with a history of hematopoietic stem cell transplantation. Toxic concentrations of tacrolimus were induced in both. This case series highlights the risk associated with the concomitant administration of tacrolimus and nirmatrelvir/ritonavir.