N2- and C8-substituted oligodeoxynucleotides with enhanced thrombin inhibitory activity in vitro and in vivo

2'-Deoxyguanosine (G) analogues carrying various hydrophobic substituents in the N-2 and C-8 positions were synthesized and introduced through solid-phase synthesis into 15-mer oligodeoxynucleotide, GGTTGGTGTGGTTGG, which forms a chairlike structure consisting of two G-tetrads and is a potent thrombin inhibitor. The effects of the substitutions at N-2 and C-8 of the G-tetrad-forming G residues on the thrombin inhibitory activity are relatively small, suggesting that these substitutions cause relatively small, perturbations on the chairlike structure formed by the oligodeoxynucleotide. Introduction of a benzyl group into N-2 of G(6) and G(11) and naphthylmethyl groups into N-2 of G(6) increased the thrombin inhibitory activity, whereas other substituents in these positions had almost no effect or decreased the activity. Particularly, the oligodeoxynucleotide carrying a 1-naphthylmethyl group in the N-2 position of G(6) showed an increase in activity by about 60% both in vitro and in vivo. Substitutions on the N-2 position of other G residues had little effect or decreased the activity. Introduction of a relatively small group, such as methyl and propynyl, into the C-8 positions of G(1), G(5), G(10), and G(14) increased the activity, presumably due to the stabilization of a chairlike structure, whereas introduction of a large substituent group, phenylethynyl, decreased the activity, probably due to the steric hindrance.