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Nov 1, 2012 (Vol. 32, No. 19)

RNAi’s Enabling Powers

  • Polymer-Based Delivery Technology

    Click Image To Enlarge +
    Arrowhead Research designed its Dynamic Polyconjugates (DPC) technology to deliver siRNA safely and efficiently to target cells.

    Safe and efficient delivery of siRNA to the appropriate target cells is the critical advance needed to enable RNAi therapeutics. The R&D team at Arrowhead Research has been working to achieve this goal with their polymer-based delivery technology, Dynamic Polyconjugates (DPC).

    What makes the DPC technology so unique is that these membrane-active polymers can be reversibly masked so that their ability to disrupt the integrity of membranes is inhibited until they reach the acidic environment of endosomes. Within this environment the acid-labile mask is removed and the polymer reveals its net cationic charge that disrupts the endosomal membrane, effectively dumping the siRNA molecules into the cytoplasm where they can interact with the cells’ RNAi machinery—thereby effecting knockdown of gene expression.

    “While the DPC delivery system is truly dynamic, upon IV injection, the polymer carries a net-negative charged mask to protect cellular membranes it encounters en route to the liver. But it is kind of a misnomer to call the current DPC structure a conjugate. We find that we no longer need to tether the siRNA to the polymer in order to deliver both to the liver hepatocyte target cells,” shared David Rozema, Ph.D., vp of chemistry.

    “This new formulation is much simpler now, and not limited by consideration of the ‘where’ and ‘how many’ siRNA molecules to attach to the polymer to maximize efficacy. This also makes the analytics in CMC manufacturing more straightforward. Current protocols involve simply co-injecting the polymer and the siRNA.”

    The elegance of the DPC technology is in its simplicity. By attaching targeting ligands for binding to cell surface receptors separately to the polymer and the siRNA, both the masked polymer and the siRNA molecule accumulate on hepatocytes in the liver. Cellular uptake of both molecules is facilitated by receptor-mediated endocytosis. Then, within the acidic environment of the endosome, the unmasked polymer displays its membrane lytic activity, thereby dumping the siRNA payload into the cytoplasm where it exerts target gene knockdown. Using siRNA directed against Factor VII in proof-of-concept studies, Dr. Rozema said the team has demonstrated that the polymer and the siRNA can be co-injected separately with the same outcome of specific, high-level gene knockdown.

    “We will be taking this DPC technology to the clinic. The therapeutic siRNA ARC-520 is directed against hepatitis B virus, a small virus that produces RNAs that all overlap. By virtue of this overlap, the therapeutic siRNA acts to cleave all the viral RNA preventing viral protein production. Further, because HBV replicates through an RNA intermediate, treating infected cells with siRNA directed against that RNA blocks formation of new viral particles,” said David Lewis, Ph.D., vp of biology at Arrowhead.

    “We are confident our approach will result in knockdown of viral proteins and block replication. Our goal is to enable robust HbsAg sero conversion in patients. This only occurs in approximately 10% of the cases using current HBV therapies.”

    While there is plenty of work to be done focusing on disease states in the liver, including viral infectious disease, metabolic disorders, and cancer, the DPC technology could conceivably be targeted anywhere in the body based on the attachment of cell-specific ligands on the polymer and therapeutic siRNA. Arrowhead Research is investing in this approach as indicated by the acquisition of Alvos Therapeutics, which has developed a clinically validated technology platform that discovers tissue-specific receptor targets and homing peptides.

  • Self-Delivery RNAi Compounds

    At RXi Pharmaceuticals the R&D effort has led to the development of novel RNAi compounds that have drug-like properties. What makes these small hydrophobically modified, asymmetric RNAs truly unique is that they don’t require an additional delivery vehicle for cellular uptake. Hence their name, self-delivering RNAi compounds, or sd-rxRNA.

    The sd-rxRNA compounds are hybrids between conventional antisense RNA molecules and RNAi compounds with the best features of both. Like antisense RNA they have good PK/PD profiles while maintaining the highly potent intracellular activity typical of RNAi compounds.

    The modifications that have been made to the molecule not only promote cellular uptake (such as hydrophobic modifications) but are also beneficial to protect the molecule from nuclease attack and prevent an immune response. Research has shown that cellular uptake is mediated by endosomal uptake and the RNAi compounds don’t require a receptor-mediated internalization mechanism.

    “In the design of the sd-rxRNA molecules, there is a critical balance between the size of the RNA (<15 nucleotides in duplex region, plus a single-stranded tail) and the amount and location of the hydrophobic modifications. The optimal design was identified following several rounds of medicinal chemistry screening,” shared James Cardia, Ph.D., manager, platform technology at RXi.

    “Our results from testing cellular uptake in over 15 different cell lines in vitro showed that regardless of the cell type, whether the cells are primary or established cell lines, adherent or in suspension, all cells tested readily take up the sd-rxRNA based on their structure and show significant gene silencing, greater than 70%.”

    RXI-109 was developed based on the novel chemistry of the sd-rxRNA platform. RXI-109 targets and reduces connective tissue growth factor (CTGF), a key regulator of fibrosis, through an RNAi mechanism to prevent overexpression of the protein in response to injury.

    “Because CTGF promotes dermal scarring, silencing its overexpression is predicted to minimize scar formation at the site of trauma or surgery,” shared Pamela Pavco, Ph.D., chief development officer. “Our nonclinical toxicology data to date indicates that RXI-109 is tolerated systemically and that reduction of CTGF is not deleterious in the context of wound healing.”

    RXI-109 is RXi’s first sd-rxRNA to be advanced into clinical trials and is currently involved in a Phase I trial, a dose-escalation study designed to evaluate safety and tolerability of RXI-109 in humans following intradermal administration. Following proof of concept studies in dermal scarring, the use of RXI-109 as a treatment for other fibrotic indications will be investigated.

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