Best minimally modified antisense oligonucleotides according to cell nuclease activity.

Minimally modified oligonucleotides belong to the second-generation antisense class. They are phosphodiester oligonucleotides with a minimum of phosphorothioate linkages in order to be protected against serum and cellular exonucleases and endonucleases. They activate RNase H, have weak interactions with proteins, and have thus a better antisense efficiency. Two of them have been designed from an all-phosphorothioate antisense oligonucleotide directed against mdrl-expressing cells. They are protected against serum and cellular enzymatic degradation by the self-forming hairpin d(GCGAAGC) at their 3'-end and by judiciously located phosphorothioate residues, depending on the cellular composition in exonucleases or endonucleases. Besides their already demonstrated ability to cleave pyrimidine sites, endonucleases show some specificity for CpG sites. Their activity is hindered if specific sites are involved in secondary structure as hairpin.