Lopinavir/ritonavir, originally developed for treating the human immunodeficiency virus (HIV), is currently undergoing clinical studies for treating the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although recent reports suggest that lopinavir exhibits in vitro efficacy against SARS-CoV-2, it is a highly protein bound drug and it remains unknown if it reaches adequate in vivo unbound (free) concentrations in lung tissue. We built a physiologically-based pharmacokinetic (PBPK) model of lopinavir/ritonavir in Caucasian and Chinese populations. Our aim was to perform pharmacokinetic/pharmacodynamic correlations by comparing simulated free plasma and lung concentration values achieved using different dosing regimens of lopinavir/ritonavir with EC_50,unbound and EC_90,unbound values of lopinavir against SARS-CoV-2. The model was validated against multiple observed clinical datasets for single and repeated dosing of lopinavir/ritonavir. Predicted pharmacokinetic parameters such as the maximum plasma concentration, area under the plasma concentration-time profile, oral clearance, half-life and minimum plasma concentration at steady state were within two-fold of clinical values for both populations. Using the current lopinavir/ritonavir regimen of 400/100 mg twice daily, lopinavir does not achieve sufficient free lung concentrations for efficacy against SARS-CoV-2. Although the Chinese population reaches greater plasma and lung concentrations as compared to Caucasians, our simulations suggest that a significant dose increase from the current clinically used dosing regimen is necessary to reach the EC_50,unbound value for both populations. Based on safety data, higher doses would likely lead to QT prolongation and gastrointestinal disorders (nausea, vomiting and diarrhea), thus, any dose adjustment must be carefully weighed alongside these safety concerns.

Keywords: Covid-19; Physiologically-based pharmacokinetic modeling; SARS-CoV-2; drug-drug interaction; free drug hypothesis; in vitro-to-in vivo extrapolation; infectious disease; lopinavir; pharmacokinetic-pharmacodynamic correlation; ritonavir.