BMS-663068 is an oral prodrug of the HIV-1 attachment inhibitor BMS-626529, which prevents viral attachment to host CD4؉ T cells by binding to HIV-1 gp120. To guide dose selection for the phase 3 program, pharmacokinetic/pharmacodynamic modeling was performed using data from two phase 2 studies with HIV-1-infected subjects (n ؍ 244). BMS-626529 population pharmacokinetics were described by a two-compartment model with first-order elimination from the central compartment, zero-order release of prodrug from the extended-release formulation into a hypothetical absorption compartment, and first-order absorption into the central compartment. The covariates of BMS-663068 formulation type, lean body mass, baseline CD8؉ T-cell percentage, and ritonavir coadministration were found to be significant contributors to intersubject variability. Exposure-response analyses showed a relationship between the log e -transformed concentration at the end of a dosing interval (C tau ) normalized for the protein binding-adjusted BMS-626529 half-maximal (50%) inhibitory concentration (PBAIC 50 ) and the change in the HIV-1 RNA level from the baseline level after 7 days of BMS-663068 monotherapy. The probability of achieving a decline in HIV-1 RNA level of >0.5 or >1.0 log 10 copies/ml as a function of the log e -transformed PBAIC 50 -adjusted C tau after 7 days of monotherapy was 99 to 100% and 57 to 73%, respectively, for proposed BMS-663068 doses of 400 mg twice daily (BID), 600 mg BID (not studied in the phase 2b study), 800 mg BID, 600 mg once daily (QD), and 1,200 mg QD. On the basis of a slight advantage in efficacy of BID dosing over QD dosing, similar responses for the 600-and 800-mg BID doses, and prior clinical observations, BMS-663068 at 600 mg BID was predicted to have the optimal benefit-risk profile and selected for further clinical investigation. (The phase 2a proof-of-concept study AI438006 and the phase 2b study AI438011 are registered at ClinicalTrials.gov under numbers NCT01009814 and NCT01384734, respectively.) A s the management of HIV-1 infection requires lifelong treatment with sequential combination antiretroviral therapy (cART), antiretrovirals with a novel mechanism of action that can be used in combination with other agents to form active regimens following virologic failure are needed. This is particularly relevant for heavily treatment-experienced patients who, by definition, have limited remaining treatment options due to viral drug resistance, toxicity, and drug-drug interactions (1). For such patients, treatment guidelines recommend construction of a new regimen containing two or, ideally, three fully active agents where possible, on the basis of treatment history, viral drug resistance, and/or a new mechanism of action (1, 2).BMS-663068 is an oral prodrug of the first-in-class HIV-1 attachment inhibitor BMS-626529, which prevents initial viral attachment to the host CD4 ϩ T cell by binding to the viral envelope protein gp120 (3). BMS-663068 is administered as an extendedrelease (ER) formulation, which...