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Retroviral integrases are essential for viral replication and represent an attractive chemotherapeutic target. In the current study, we demonstrated the activity of micromolar concentrations of dinucleotides against human immunodeficiency virus type 1 (HIV-1), HIV type 2 (HIV-2), simian immunodeficiency virus, and feline immunodeficiency virus integrases. The structure-activity relationship indicates that 5'-phosphorylation enhances potency and that phosphodiester and sugar modifications affect the inhibition of HIV-1 integrase. Base sequence selectivity was observed: pAC, pAT, and pCT were the most potent inhibitors, whereas pAA, pGA, and pGC showed low activity at 100 microM. The inhibition by pAC is consistent with the interaction of the enzyme with the 5' end of the noncleaved strand (5'-AC-3'). The linear and cyclic dinucleotides released by the 3'-processing reaction did not affect enzymatic activity at physiological concentrations. An increase in the length to trinucleotides or tetranucleotides enhanced potency by only 2-3-fold, suggesting that two neighboring bases may be sufficient for significant interactions. Inhibition of a truncated (50-212) integrase mutant and global inhibition of all nucleophiles in the 3'-processing reaction suggest that dinucleotides bind in the catalytic core. All of the active dinucleotides inhibited enzyme/DNA binding in their respective IC50 range. Although the dinucleotides tested showed no antiviral activity, these observations demonstrate the usefulness of dinucleotides in elucidating enzyme mechanisms and as potential ligands for cocrystallization and as lead structures for development of antivirals.