Effective incorporation of 2'-O-methyl-oligoribonucleotides into liposomes and enhanced cell association through modification with thiocholesterol.

Cholesterol was linked to 2'-O-methyl-oligoribonucleotides (2'-OMe-RNA) via a disulfide bond by reacting the 3'-(pyridyldithio)-modified 2'-OMe-RNA with thiocholesterol in dichloromethane-methanol solution. This ligation reaction was made possible by a novel strategy in which the highly charged oligonucleotide was rendered soluble in nonaqueous solvent through conversion to a lipophilic amidinium salt. The biodegradable lipophilic modification of 2'-OMe-RNA resulted in a large increase in incorporation of such oligonucleotides into liposomes prepared by reversephase evaporation. Furthermore, association of these modified oligonucleotides with cultured TIB 73 cells was 100-fold higher than that seen with unmodified 2'-OMe-RNA in serum-free medium and about 10 to 30-fold higher in the presence of 10% calf serum. During incubation with cells, release of the internalized oligonucleotide from the thiocholesteryl moiety can be demonstrated.