<qa>  What is the target concentrations of each antiretroviral drug between the 50th andTwo approaches will be investigated: 1 a regimen that targets concentrations of each antiretroviral drug between the 50th and 75th percentile of expected concentrations in adults; and 2 a novel regimen in which the target concentrations are based upon a desired ratio between phenotypic drug susceptibility IC90 and the concentrations of pharmacologically active moieties, specifically intracellular nucleoside triphosphates and unbound protease and nonnucleoside inhibitors. 