Dynamic PolyConjugates™ — siRNA Delivery System
In the past decade, the discovery of the gene silencing mechanism known as RNA interference (“RNAi”) exploded onto the scientific scene and has been hailed as one of the greatest molecular biology discoveries of our generation. This newly found ability to suppress the production of disease causing proteins has become a powerful discovery tool used to investigate the function and role of individual genes, and may ultimately form the basis for a whole new class of medicines. The significance of this biological mechanism was reinforced in October 2006 when the two researchers credited with discovering this powerful biological phenomenon, Drs. Andrew Fire and Craig Mello, were jointly awarded “The Nobel Prize in Physiology or Medicine for 2006.”
While the tremendous research, medical and commercial potential of RNAi has fueled investment in this field, the main impediment holding back its clinical application has been the lack of an effective method to systemically deliver therapeutic nucleic acids. Drawing upon 12 years of research focused exclusively on nucleic acid chemistries and delivery, researchers at Mirus Bio recently made a key breakthrough that consolidates several independent discoveries into a potent new delivery platform – one that establishes a new performance standard for delivery of RNAi-inducing therapeutics.
Dynamic PolyConjugates™ (“DPC”) are the first rationally designed synthetic formulation for nucleic acid delivery that mimics the natural viral targeting and disassembly process. The DPC complex is chemically synthesized to include the following functional elements:
- Endosomolytic Polymer
- Charge Masking Agents
- Environmental Responsive Linkage Chemistry
- Targeting Ligand (specific to the target cell or tissue)
- Short interfering RNA Sequence (siRNA specific to a disease target gene)
Fully assembled DPCs form nanoscale structures of an optimal size for facilitating bioavailability. DPCs circulate as a protected complex when injected into the bloodstream. Once the complexes reach and attach to a target cell, the DPCs are internalized via the natural process of receptor-mediated endocytosis. The complex dissociates in the endosome due to the acidity within this compartment and releases the siRNA molecule into the cytoplasm, whereupon it can silence gene activity via the RNAi mechanism. Keys to the success of this formulation are (1) the endosomolytic potency of the polymer, (2) the unique ability of the complex to respond to location-specific environmental cues, and (3) efficient cellular targeting.
Unmodified and unprotected siRNAs are rapidly degraded when injected into the bloodstream. Several strategies have been attempted to increase circulation half-life and functionality, including incorporation of stabilizing chemical modifications, encapsulating the siRNA in liposomes, and attaching either a cholesterol group or common polymers to the siRNA. While these modifications improved performance over naked siRNAs, reported animal model data to date continues to disappoint and highlight the need for a better delivery solution. Researchers at Mirus Bio have discovered and patented a new class of polymers that provide a unique dual benefit. These polymers both impart superior functionality to the nucleic acid while in the bloodstream and promote highly efficient transport of the siRNA from the endosome into the surrounding cytoplasm in the target cell. Without this latter membrane-active trait, the siRNA would remain sequestered in the endosomal compartment, unable to trigger gene silencing.
A second basis for the efficacy of DPCs is the use of proprietary labile conjugation chemistry to (a) attach targeting molecules that enable the DPC to preferentially attach to target cells of interest, and (b) attach interaction-inhibiting agents to the polymer to prevent unwanted interactions with non-target cells and help prolong circulation times. When standard non-labile chemistries were evaluated, the resulting complexes were found to possess extended circulation times, but were unable to release their siRNA cargo from the endosome in order to induce RNAi. In contrast, with Mirus Bio’s conjugation chemistry, the fully assembled complex sequentially attaches to and passes through the target cell’s membranes. The DPC linkage chemistry responds to the changing environment, causing the complex to disassemble in a stepwise fashion in order to free the active therapeutic siRNA sequence.
Finally, the ability to actively target specific cells is a powerful attribute of this formulation. Various targeting molecules including sugar or peptide based receptor-targeting ligands, monoclonal antibodies, or heavy-chain antibody fragments can be incorporated into the DPCs to target specific cell types. Proof-ofconcept studies to date using cell-specific ligands have confirmed specific targeting to liver hepatocytes, a useful target for a wide range of metabolic and infectious diseases, as well as to implanted solid tumors.
While numerous investigators have published data showing a range of siRNA delivery and/or targeting efficiencies, few have shown the ability to combine efficient targeted delivery, low toxicity, effective gene silencing, and the ability to generate the predicted physiologic effect. Dynamic PolyConjugates stand alone in their ability to meet these criteria. Data to date in mouse and rat models are highly encouraging. In one model, we reproducibly achieved >80% silencing of the disease target apolipoprotein B and observed concordant reductions in serum cholesterol levels. Silencing levels were dose dependent and the durability of silencing from a single dose extended for up to two weeks. In a second model, we demonstrated consistent knockdown of pparα, a well characterized gene that is critically important in fatty acid metabolism, and triggered concordant phenotypic changes in blood triglyceride levels. The data from these experiments have been submitted for publication.
The rational design strategy and chemical synthesis process utilized to create Dynamic PolyConjugates offers unmatched versatility for tailoring this platform to a wide variety of applications. Ongoing work is currently focused on extending initial research findings into larger species, targeting additional tissue types (including cancer) through the use of selective targeting ligands, and systematically refining and optimizing this exciting new RNAi platform for both research and clinical use.
© 2008 Mirus Bio Corporation. All Rights Reserved.