Structure-Guided Optimization of Small Molecules Inhibiting Human Immunodeficiency Virus 1 Tat Association with the Human Coactivator p300/CREB Binding Protein-Associated Factor Human immunodeficiency virus 1 (HIV-1) transactivator Tat recruits the human transcriptional co-activator PCAF (p300/CREB binding protein-associated factor) to facilitate transcription of the integrated HIV-1 provirus. We report here structure-based lead optimization of small-molecule inhibitors that block selectively Tat and PCAF association in cells. Our lead optimization was guided by grand-canonical ensemble simulation of the receptor/lead complex that leads to definition of chemical modifications with improved lead affinity through displacing weakly bound water molecules at the ligand-receptor interface.