The pro-oligonucleotide approach: solid phase synthesis and preliminary evaluation of model pro-dodecathymidylates.

A modified phosphoramidite method has been designed for the solid-phase synthesis of two dodecathymidine phosphotriesters and two dodecathymidine thionophosphotriesters. In these analogs, each internucleoside link bears an S -acyl-2-thioethyl (Me-SATE or tBu-SATE) group removable upon esterase activation. Efficient synthesis of these lipophilic analogs was achieved thanks to the use of a photolabile linker anchored to the solid support in combination with thymidine-3'- O -phosphoramidites having a SATE group in place of the regular 2-cyanoethyl one. Both dodecathymidine phosphotriester and thionophosphotriester having S -acetyl-2-thioethyl groups were found to be stable in the presence of snake venom and calf spleen phosphodiesterases whereas, upon incubation in CEM cell extracts, they were selectively hydrolyzed to the anionic parent dodecathymidylate and dodecathymidine phosphorothioate, respectively. In addition, Me-SATE-protected dodecathymidine thionophosphotriester was stable in mouse and human sera as well as in human gastric juice. These results depict the potential of SATE-protected oligonucleotides as prodrugs of antisense oligonucleotides.