Decreased siRNA performance at high concentrations: feasible? - (Jul/27/2011 )

Hello. I have been working with siRNAs from Applied Biosystems in HeLa cells, and I have consistently found that at very high concentrations of siRNA transfected (around 100nM) that the level of knockdown is not as high as in conditions where lower concentrations of siRNA are used. This has been shown around 4 times using serial titrations of siRNA from 1e-7M to 1e-11M (half-log titrations). Is this at all unusual or is it sensible that some decrease in performance might occur with excessive siRNA concentrations (perhaps due to anti-viral cellular responses)? Thank you.

EC50 for siRNA is at pM. 100 nM for RNAi is too high and may cause many non specific effects such as sequence-independent cytotoxicity, and sequence dependent immunostimulation and off-target effects. It may also saturate Ago2 leading to low efficiency in knocking down target genes.