Wednesday, May 2, 2007
In Focus: FDA allows Nucleonics to start phase I trial for HBV RNAi
Unless most other companies that develop RNAi therapies using synthetic small interfering RNAs (siRNAs), Nucleonics employs DNA-based vectors that direct the expression of so called hairpin RNAs that are then further processed by the endogenous RNAi machinery into small RNAs that are functionally identical to the synthetic siRNAs. This approach, also known as DNA-directed RNAi (ddRNAi), may be advantageous in that it potentially allows for a longer treatment effect due to the potentially longer activity of a DNA vector. RNA, by contrast, is a more short-lived molecule. In the case of Nucleonics, the "naked" DNA is delivered to the liver formulated with cationic lipids. Since hairpins are very short in gene-terms, a plasmid may harbor multiple hairpins and Nucleonics' has 4 of them. As each hairpin targets a different RNA of HBV, the multipronged approach should help minimise drug-resistance which is often seen with viral therapies based on inhibiting a single target. Indeed, other RNAi companies are likely to pursue similar multi-target approaches in their viral programs, Alnylam's flu pre-clinical program being one example.
So far the theory looks promising. However, I have a number of concerns with Nucleonics' program (these were also recently highlighted during a pre-IND meeting with an FDA advisory panel). The major problem is that the company did not have convincing data about in vivo efficacy in animals. They argued that this is due to a lack of appropriate pre-clinical animal models, but I would argue that those, e.g. mice that carry HBV in their genome, exist and should have been used for this purpose. Efficacy studies were consequently limited to tissue culture experiments and in a co-transfection "in vivo" model which really is nothing more than a glorified in vitro system. This means that although HBV silencing in such a model may approach 100%, this is simply because the HBV and ddRNAi vectors tend to go into the same cells during co-transfection. It appeared from their data, however, that only a small fraction of the liver cells received the ddRNAi (and HBV) plasmid, unlikely to be enough to have a therapeutic impact in a patient that carries HBV in a much larger and non-overlapping fraction of cells in the liver. It is likely that this is because in order to be active the DNA needs not only to get into the cell but also the nucleus, which is generally inefficient with non-viral DNA vectors. SiRNAs, however, have the added delivery advantage in that they are active in the cytoplasm and do not have to reach the nucleus. Their smaller size compared to DNA may also help.
In summary, while a lot of the scientific rationale for the trial appears sound, I see delivery as a huge hurdle for this particular RNAi program and am therefore quite skeptical.
For those interested in learning more about Nucleonics' strategy, please visit http://www.nucleonicsinc.com/products/hepb.html
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