background:The effect of current therapies in improving the survival of lung cancer patients remains far from satisfactory. It is consequently desirable to find more appropriate therapeutic opportunities based on informed insights. A molecular pharmacological analysis was undertaken to design an improved chemotherapeutic strategy for advanced lung cancer.

Herein we describe the synthesis of 4‘-thio-dTTP (1) and 4‘-thio-dCTP (2) and their susceptibility for PCR amplification. Double stranded 4‘-thioDNAs were amplified sufficiently by KOD dash DNA polymerase under appropriate conditions. Through this PCR study, it was shown that not only DNA-directed 4‘-thioDNA synthesis but also 4‘-thioDNA-directed 4‘-thioDNA synthesis occurred in the presence of 1 and/or 2. We then tested the ability of the resulting 4‘-thioDNAs to act as templates for transcription. In this experiment, the 4‘-thioDNAs as templates afforded RNA in 38−49% yield relative to the case of the natural DNA template. We further investigated whether transcription occurred in mammalian cells. As a result, not only natural DNA but also 4‘-thioDNAs were transcribed to afford a shRNA which showed gene-silencing effects via RNAi machinery.

A detailed study of the modification pattern-RNAi activity relationships by using siRNAs that are modified with 4'-thioribonucleosides has been carried out against photinus luciferase and renilla luciferase genes in cultured mammalian NIH/3T3, HeLa, and MIA PaCa-2 cell lines. When the photinus luciferase gene was targeted, all of the modified siRNAs showed activity equal to, or less than the unmodifed siRNA. In contrast, all modified siRNAs that have a similar modification pattern showed activity equal to or much higher than the unmodified siRNA when tested with the renilla luciferase gene. These results indicated that siRNAs such as RNA33 and RNA53, which each have four residues of the 4'-thioribonucleoside unit on both ends of the sense strand and four residues on the 3'-end of the antisense strand, were the most effective. Accordingly, we succeeded in developing modified siRNAs that have the greatest number of 4'-thioribonucleosides without loss of RNAi activity, and that exhibit potent RNAi activity against two target genes in three different cell lines. Our findings also indicate the significance of target sequences and cell lines when RNAi activity is compared with that of the unmodified siRNA.

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