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Jan 15, 2011 (Vol. 31, No. 2)

RNAi Delivery Proves Vexing

Emerging Field Simultaneously Holds Both Promise and Problems

  • siRNA-Based Screening

    Scientists at Merck & Co. have utilized siRNA screening to identify novel drug targets. “Recent advances have allowed our scientists to test the effects of knocking down not only each individual gene separately but also multiple genes simultaneously,” noted Jeremy Caldwell, Ph.D., vp, RNA therapeutics.

    Merck is applying siRNA-based screening across several disease states. In cancer, for example, its researchers have used siRNA screening to search for genes that enhance the activity of chemotherapy and eradicate tumor cells—but not normal cells—when they are knocked down.

    Dr. Caldwell agrees that the two biggest challenges with developing siRNA-based therapeutics are delivery and establishing safety. “Delivery of siRNA is currently possible for some solid tumors and some accessible organs, such as the liver and the eyes. However, systemic delivery to other cells, tissues, and organs is more challenging and will likely require multiple approaches optimized for each target.”

    Merck is focusing on optimizing lipid nanoparticles (LNP) for systemic delivery of RNAi therapeutics to the liver. “The safety profile using the LNP delivery approach remains a key challenge. Chemically modifying siRNA can minimize off-target activity; however, as with conventional targeted therapies, inhibiting the properties of one particular target may have difficult-to-predict effects on the activity of other genes in a pathway or network.

    “Another potential concern is that introducing siRNA into the bloodstream can trigger the body’s innate immune response. This effect also can be minimized by chemical modifications to siRNA, but this may not be equally effective in every case.”

    Dr. Caldwell reports that Merck has made progress in optimizing lipid nanoparticles for systemic delivery of RNAi-based therapeutics to the liver. “We’ve demonstrated that optimization of cationic lipids in the lipid nanoparticles improves potency by greater than 10-fold. The extent of PEGylation affects particle size, efficacy, and inflammation. We’ve also found that the larger lipid nanoparticles are suboptimal relative to smaller-sized particles with respect to elevation of liver toxicity markers. Finally, the lipid nanoparticles are rapidly absorbed into the liver and maximally induce the silencing complex within six hours.”

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