Figure 2. Turning siRNA into drugs.

This three-step process begins with in silico design and in vitro screening of target siRNAs, is followed by incorporating stabilizing chemical modifications on lead siRNAs as required, and ends with the selection and in vivo evaluation of delivery technologies that are appropriate for the target cell type/organ and the disease setting. A schematic illustration of the important features of siRNA structure (two base pair overhangs, seed region and mRNA cleavage site) is shown. An example of an optimized siRNA molecule that incorporates chemical modifications to increase nuclease stability and to minimize off-targeting is shown below. The phosphorothioate (P=S) and 2′-base sugar modifications (green circles) are illustrative of exonuclease and endonuclease stabilizing chemistries, respectively; other chemistries that confer similar properties on an siRNA duplex also exist³⁹. The passenger strand and the guide strand are represented in blue and orange, respectively.