Effect of derivatization of ribophosphate backbone and terminal ribophosphate groups in oligoribonucleotides on their stability and interaction with eukaryotic cells.

Various derivatives of oligoribonucleotides were synthesized by the H-phosphonate method. Different modifications of the ribophosphate backbone were designed in order to protect the derivatives against nucleolytic enzymes present in the biological media. These modifications include coupling of fluorescein moiety to 3'-terminal ribose, 2'-O-methylation of ribose, introduction of phosphorothioate internucleotide bonds throughout the molecule, replacement of the two last 3'-terminal phosphodiester bonds by phosphoroamidates and coupling of the last 3'-terminal nucleotide via the 3'-3'-phosphodiester bond. All modifications were tested for their effect on the stability of the derivatives against phosphodiesterase from snake venom and nucleases of the cell culture media. 2'-O-methylated oligoribonucleotides containing either terminal 3'-3'-linkage or two 3'-terminal phosphoroamidate internucleotide bonds appeared to be the most stable under the most severe conditions used. The results demonstrate a possibility to use protected oligoribonucleotide derivatives for experiments in vivo when the use of deoxy-analogues might be ineffective. The uptake of 2'-O-methylated derivatives and their 5'-cholesterol conjugates (coupled via a disulfide bond) by human carcinoma cells did not differ from that of the corresponding oligodeoxyribonucleotides. 85% of the bound derivatives were found in the membrane-cytosolic fraction, while only 15% were found in the nuclear fraction. The oligonucleotide moiety of 2'-O-methyloligoribonucleotide-cholesterol conjugate was not translocated through the cellular membrane. After cleavage of the linkage between cholesterol and oligonucleotide by dithiothreitol the major portion of the oligonucleotide moiety was released into the media. The derivatives, as well as their 5'-cholesterol conjugates, which entered the cells, were stable and protected from action of dithiothreitol dissolved in culture media. These results demonstrate an endocytosis mechanism of penetration as observed in similar experiments using oligodeoxyribonucleotides.