RNA Interference (RNAi) is an evolutionary conserved cellular mechanism to inhibit gene expression by targeting the mRNA of a given gene in the cytoplasm prior to its translation by ribosomes into a protein. In 2006, the Nobel prize for Medicine was awarded to Drs. Fire and Mello for their discovery of RNAi. At it's core, the RNAi machinery uses a small double stranded cellular RNA, called a micro-RNA (miRNA), as its guide to select the specific mRNAs to degrade. The double stranded miRNA is loaded by an enzyme called TRBP onto one of four a catalytic enzymes called Argonaut (Ago) where the sense or passenger strand is removed, leaving only the anti-sense or guide strand on Ago. The guide strand-Ago is then assembled into a large multi-protein complex call the RNA Interference Silencing Complex (RISC). The guide strand load RISC then selectively base pairs to the target mRNA resulting in the degradation of that mRNA species, but no other mRNAs. Once the mRNA is degraded, the guide strand loaded RISC is free to carry out another degradation of more target mRNA present in the cytoplasm. There are over 1,000 miRNAs that selectively regulate genetic pathways in every cell of our bodies every single day.
In 2001, synthetic double stranded small interfering RNAs (siRNAs) that are 21 nucleotides long were shown to be loaded by TRBP into Ago and induced an RNAi response. However, the strength of the RNAi response with siRNAs was much stronger than with miRNAs. This is due to two reasons. First, siRNAs are synthesized to be a perfect match to the target mRNA for their entire length, whereas miRNAs have mismatches (bulges) in the middle. Second, because of the perfect match, when the siRNA is loaded into Ago2, the only Ago with direct slicer catalytic activity, the mRNA is cleaved by Ago2 (vs. accessory RNAses used for Ago1,3,4). After mRNA cleavage, the guide strand loaded Ago2 is ready to perform another targeted mRNA cleavage. Guide strand loaded Ago2 are a very long-lived protein complex in cells. In fact, in none dividing tissues, a single exposure of siRNAs can result in an RNAi response for upwards of three months. Collectively, these properties opened the door for the development of RNAi therapeutics with unprecedented selectivity to treat a wide variety of human genetic diseases from cancer to pandemic flu to Alzheimer’s disease. Moreover, RNA therapeutics is the only therapeutic modality where the drug (siRNA) can be evolved to keep pace with the genetic moving targets of cancer and viral infections to deliver personalized medicine. However, delivery of siRNAs into cells remains the rate-limiting step for development of RNAi therapeutics.
Solstice Biologics is developing the next generation of RNAi triggers, called siRNNs. siRNNs directly and uniquely address both the delivery and stability problems of RNAi therapeutics.