Illustrating how the basic concept of liposomal RNAi delivery can be modified to target specific cell types at low dosages, a study by Sato and colleagues published last week in the online version of Nature Biotech shows how the addition of vitamin A to cationic liposomes carrying siRNAs against Hsp47 can not only efficiently knock down the gene in hepatic stellate (HS) cells and block disease progression, but that this can actually reverse cirrhosis in a number of rat models for this disease [Sato et al. (2008). Resolution of liver cirrhosis using vitamin A-coupled liposomes to deliver siRNA against a collagen-specific chaperone. Nature Biotech doi:10.1038/nbt1396].
It is known that by adjusting their stability and therefore pharmacokinetics, liposomes can be adjusted to shift their relative accumulation in tissues such as liver, tumour, or lung. For example, recent improvements in formulation methods now allow for the delivery of around 90% of the injected dose to the liver. While for liver applications, this already minimizes the exposure of non-target organs to the RNAi therapeutic, the ability to knock down genes in specific cell types at low dosages should further improve its therapeutic index.
Adding small conjugates such as sugars (e.g. Mirus Bio’s DynamicPolyConjugates to target RNAi to either hepatocytes or Kupffer cells) to a basic delivery formulation appears to be particularly promising. The authors of the present paper reasoned that since the HS cell which is central to liver cirrhosis efficiently takes up vitamin A, adding this vitamin to liposomally formulated siRNAs may facilitate their uptake in HS cells, in addition to endowing the nanoparticle with favorable pharmacokinetics by binding to retinol binding protein in circulation.
Indeed, while some non-specific uptake was observed into phagocytic cells, the modified A-liposomes efficiently entered HS cells in a vitamin A-dependent manner while largely avoiding other cell types of the liver, including hepatocytes. Actually, delivery was truly targeted since overall drug uptake was greatly enhanced in cirrhotic rats versus non-cirrhotic rats, likely a reflection of the HS cell activation/proliferation state.
Importantly, knockdown of Hsp47, required for collagen production and therefore a potential target for treating cirrhosis, was achieved at the clinically relevant low dosages of 0.1mg/kg to 0.75mg/kg, resulting in the survival of rats concomitant with resolution of the cirrhosis both by histology and normalization of liver enzymes.
The paper also shows that as we are learning more about the molecular biology and pharmacological properties of synthetic RNAs, the scientific standards for publishing papers on therapeutic RNAi have risen. Consequently, the sequence-specific nature of the therapeutic effect was demonstrated by the use of two additional Hsp47-targeted siRNAs. Cytokines and interferon-alpha were also looked at, including IL-12 which has recently gained notoriety
as a TLR3-related pro-inflammatory cytokine. Reassuringly, no IFN-alpha, TNF-alpha, or IL-12 induction was detected at the reasonably early 7 hour time-point with this unmodified Dicer-substrate (Dicer substrates are longer and therefore should be more prone to immune recognition). However, some non-specific elevation in the apoptosis of rat liver HS cells was observed following treatment with VA-liposomes carrying a control siRNA. Albeit small, it remains to be determined whether this was related to sequence-specific cytokine induction, or liposomally or vitamine A-induced cytotoxicity.
It is likely that the systematic screening for improved VA-liposome chemistries and optimized siRNAs should considerably improve the therapeutic index of this interesting variation on the cationic liposome for siRNA delivery. Similarly, vitamin A derivatives that are still taken up by HS cells, but do not cause potential vitamin A-related toxicities may be warranted for actual clinical use.
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