Biologically active oligodeoxyribonucleotides. Part 11: The least phosphate-modification of quadruplex-forming hexadeoxyribonucleotide TGGGAG, bearing 3′- and 5′-end-modification, with anti-HIV-1 activity Original Research Article

We have found that a hexadeoxyribonucleotide (5′TGGGAG3′, R-95288), Koizumi, M. et al. Bioorganic & Medicinal Chemistry, 1997, 5, 2235, bearing a 3,4-dibenzyloxybenzyl (3,4-DBB) group at the 5′-end and a 2-hydroxyethylphosphate at the 3′-end, has high anti-HIV-1 activity and the least cytotoxicity in vitro and in vivo. In order to synthesize more potent hexadeoxyribonucleotides, we substituted phosphodiester (PO) bonds in the 6-mer with the least phosphorothioate (PS), phosphoramidate (PN), or methylphosphonate (PMe) bonds. When more than two PN or PMe bonds were introduced into a 6-mer, the phosphate-modified 6-mers had weak or no anti-HIV-1 activity, in spite of quadruplex structure formation. However, when PS bonds were substituted for PO bonds, anti-HIV-1 activity of their 6-mers did not dramatically decrease, compared with compounds substituted with PN or PMe bonds. The results suggest that the formation of a quadruplex structure is not always sufficient for anti-HIV-1 activity of the 6-mer, and that net negative charges derived from PO or PS bonds in the quadruplex are important for anti-HIV-1 activity. Moreover, among various phosphate-modified ODNs, we found that the anti-HIV-1 activity of ODN PS7 with only one PS bond was the same as that of R-95288, both having a high stability in human plasma.