Modeling combinations of antiretroviral agents in vitro with integration of pharmacokinetics: guidance in regimen choice for clinical trial evaluation.

We propose a method for the selection of doses and dosing schedule for drugs to be used in combination. This approach uses the simulation of steady-state concentrations of the drugs in the combination and overlays these concentrations onto a three-dimensional effect surface. The MacSynergy II program is used to construct the three-dimensional drug interaction surface from the direct evaluation of drug combination effect in vitro. The study examined the combination of an inhibitor of the human immunodeficiency virus protease, A-77003, and the nucleoside analog zidovudine. Zidovudine concentrations from a steady-state interval were simulated on the basis of the administration of 100 mg every 12 h by mouth, while for A-77003 simulation profiles were for intravenous administration of 800 mg every 4 h as well as a continuous infusion of 200 mg/h. The average percentage of the maximal effect was taken as a measure of regimen effectiveness. Three different schedules of administration were examined. If both drugs were to be administered simultaneously, the model predicts a mean maximal effect of a steady-state interval (12 h) of 67%. If the drug doses were offset by 2 h, the mean maximal effect predicted was 71%. If A-77003 was to be given by continuous infusion, the mean maximal effect predicted was 90%. This method holds promise as a way of quickly evaluating potential combinations of agents that takes into account the drug interaction in a mathematically robust way and that allows the evaluation of the effect of each drug's pharmacokinetic profile.