Targeted knockdown
NATuRE REVIEWS | drug discovery VOLumE 6 | OCTOBER 2007
Achieving targeted and efficient delivery of small-interfering RNAs (siRNAs) in vivo is a key challenge in the development of siRNAbased therapies. Now, Rozema and colleagues, writing in PNAS, demonstrate the ability of nanoparticles — dubbed siRNA Dynamic PolyConjugates — to specifically target hepatocytes and deliver the payload of siRNA to effectively knockdown two endogenous genes in the mouse liver: apolipoprotein B (Apob) and peroxisome proliferatoractivated receptor-a (Ppara).
The liver is an attractive target for siRNA delivery because of its important role in numerous diseases and because of the ease at which hepatocytes take up nanoparticles after simple injection. Indeed, the passive targeting of cholesterol–siRNA conjugates to the liver has recently been demonstrated. However, nonspecific uptake of nanoparticles by non-target
cells in the liver increases the risk of toxicity.
With the aim of achieving specific cell targeting, the authors designed an siRNA polyconjugate that works through the gradual unveiling of its components as it travels to the cytoplasm of the targeted cells. The siRNA is linked by a disulphide bond
to a multifunctional polymer backbone, poly(vinyl ether) (PBAVE).
To this backbone, N-acetylgalactosamine (NAG), a hepatic targeting ligand, and a shielding agent, polyethylene
glycol (PEG), are attached. Following intravenous injection,
PEG protects the complex from nonspecific binding until it reaches the liver, where the NAG ligand engages
the asialoglycoprotein receptor of hepatocytes and receptor-mediated endocytosis ensues. Once in the endosome, the decrease in pH releases the PEG and NAG, revealing
positively charged amine groups on PBAVE, which leads to endosomolysis and release of siRNA–PBAVE into the cytoplasm. Last, the reducing environment of the cytoplasm promotes disulphide-bond cleavage to release the siRNA, allowing it to
silence the target gene.
Effective knockdown of ApoB and Ppara expression in mice using
a single intravenous injection of the respective siRNA polyconjugates was demonstrated by the authors. The
level of RNAi was dose-dependent with lowering of siRNA amount leading to decreases in the phenotypic response that is characteristic of Apob knockdown. Effects of RNAi lasted up to 10 days, and the effects were reversible, with Apob expression
levels returning to near control levels by day 15. Toxicity was also minimal, with insignificant increases in levels of ALT and AST liver enzymes, and also minimal increases in cytokines
indicative of adverse inflammatory responses. As demonstrated by the authors, a further potential advantage of this polyconjugate system is the ease with which the targeting ligand can be changed so that a different cell type can be targeted, therefore expanding its general utility.
Man Tsuey Tse
ORIGINAL RESEARCH PAPER Rozema, D.
B. et al. Dynamic PolyConjugates for targeted in
vivo delivery of siRNA to hepatocytes. Proc. Natl
Acad. Sci. USA 104, 12982–12987 (2007)
FURTHER READING de Fougerelles, A. et al.
Interfering with disease: a progress report on
siRNA-based therapeutics. Nature Rev. Drug Discov. 6, 443–453 (2007) |
