Our current work has involved development of a pharmacokinetic model for the drug delavirdien (DLV), a recently-approved reverse transcriptase inhibitor, used in combination regimens to treat AIDS. During development, the drug was dosed, in several trials, using adaptive feedback control of 'trough' plasma concentrations. A typical starting regimen is 876.1 (moles (400 mg), given orally, every 8 hours. Both parent DLV (also called U90) and metabolite (N-DLV or U96) concentrations were assayed as feedback. The kinetic model has bolus inputs into an absorptive site, then first order absorption (ka) after a lag time. DLV distributes into central and peripheral compartments with rate of equilibration governed by Cld (Cld = Vc(kcp = Vp(kpc). DLV is eliminated, from the central compartment, by parallel capacity-limited (saturable) and first order clearance pathways (CL90). We parameterize the saturable pathway by Km and intrinsic clearance (CLi = Vmax/Km). DLV which is cleared by the saturable pathway is metabolized to N-DLV, which we modeled as distributing into a single compartment from which drug is cleared by a first order process (CL96). This model is currently being used to analyze data from 40 patients as part of a Phase 2 clinical trial of DLV in HIV infected patients.