TAR RNA represents an attractive target for the intervention of human immunodeficiency virus type 1 (HIV-1) replication by small molecules. We now describe three small molecule inhibitors of the HIV-1 Tat-TAR interaction that target the RNA, not the protein. The chemical structures and RNA binding characteristics of these inhibitors are unique for each molecule. Results from various biochemical and spectroscopic methods reveal that each of the three Tat-TAR inhibitors recognizes a different structural feature at the bulge, lower stem, or loop region of TAR. Furthermore, one of these Tat-TAR inhibitors has been demonstrated, in cellular environments, to inhibit (a) a TAR-dependent, Tat-activated transcription and (b) the replication of HIV-1 in a latently infectious model. Drug discovery has traditionally involved a search for inhibitors of protein complexation to small molecules (e.g., substrates or ligands) or macromolecules (e.g., other proteins, nucleic acids, or polysaccharides). While agonists or antagonists of macromolecular receptors that become useful drugs must display a wide range of other attributes, including the appropriate physicochemical properties, pharmacokinetic and pharmacodynamic properties, stability, etc., they must first be protein ligands. The vast majority of available drugs act by binding noncovalently to a protein, preventing or (less often) stimulating that protein's complexation to a complement (1).Complexes of nucleic acid and proteins are key intermediates in all transcriptional and translational processes. Some nucleic acids (e.g., ribozymes) even form functional complexes with small molecules (2). Nonetheless, nucleic acids are widely viewed as ineffective targets for the discovery of low-molecular weight inhibitors. One reason is that the linear motif in single-stranded DNA and the repetitive motif in double-stranded DNA provide attractive targets for large, linear binding molecules (3), but unattractive targets for the small molecules that lead to orally available medications. Another reason is that the lack of tertiary structure in DNA does not afford the diverse topology associated with folded proteins.However, single-stranded RNA often folds into welldefined tertiary structures (4). Furthermore, such structures serve as docking sites for transcriptional activators (5) and substrates for self-splicing reactions (6, 7). Can such structured nucleic acids display sufficient shape diversity to permit the complexation of a small molecule at one site in a virtual sea of nucleic acid material? This is the essential question whose presumed negative answer hinders drug discovery at the nucleic acid level.Certain cis-acting RNA elements are essential for the gene expression of human immunodeficiency virus type 1 (HIV-1) (8). The functions and sequences of these RNA molecules have been well characterized (9-11). A segment of HIV-1 mRNA (residues 1-59), identified as the transactivation responsive element (TAR), adopts a stem-loop secondary structure consisting of a highly cons...
