Dendrimers are branched, nanometer-sized, spherical modular structures that, depending on the monomers, surface and internal modifications, can be engineered to carry out various functions, including nucleic acid delivery. Although dendrimers as a platform for the delivery of siRNAs is still in its late infancy, with maybe another 3-6 years before we will see first clinical trials using such technology, there are a couple of advances that have been reported in the literature that make it seem to be a promising versatile platform for therapeutic siRNA delivery.
Like cationic lipids and polymers (in a way a dendrimer is a highly branched polymer), dendrimers of positive charge can be generated to bind to the nucleic acid. Similar to most first-generation siRNA delivery approaches, siRNA-dendrimer delivery borrows heavily from the experience in delivering plasmid DNA. In addition to the well-known advantage that siRNAs do not have to get into the nucleus in order to be functional, the major hurdle of non-viral DNA delivery, the relatively small size allows that siRNA not only to be positioned as a polyplex on a positively charged dendrimer surface, but also to be well hidden within the sphere. This is an advantage since many polyplexes are prone to aggregation with adverse consequences to the pharmacology of the particle (RES uptake; predisposes to triggering innate immunity; size becomes too big to escape circulation etc).
Internal siRNA packaging, for example by removing the positive charge on the surface and positively charging the interior as demonstrated by Tamara Minko’s group from Rutgers (Patil and colleagues 2008: Surface-Modified and Internally Cationic Polyamidoamine Dendrimers for
Efficient siRNA Delivery), is just one of the strategies to help the dendrimer nanoparticle stay intact in the body. Other approaches include caging the siRNA-dendrimer polyplex via disulfide linkages which at the same time would allow for the timed release of the siRNA presumably in the reducing environment of the cytoplasm. This technology was very neatly demonstrated in yet another paper by the Minko group (Taratula et al 2009: Surface-engineered targeted PPI dendrimer for efficient intracellular and intratumoral siRNA delivery). In addition to caging, the particles were further stabilized by the addition of, you guessed it, PEG.
Since removing the positive surface charge will reduce interaction with the negatively charged cell surface and subsequent uptake of the nanoparticle, the addition of a targeting ligand (e.g. onto the PEG) directed against a cell surface receptor may ensure that the particle is taken up via receptor-mediated endocytosis. This approach, of course, is not exclusive to dendrimer delivery and there appears to still be significant potential for the entire field of RNAi Therapeutics delivery to test the utility of a number of target receptors and targeting ligands (e.g. peptide display libraries as done by mdRNA; MAb display libraries- how about a collaboration with a company like Morphosys?; or also other libraries such as small molecules and sugars). What was curious about Taratula et al’s result of adding a synthetic LHRH peptide analogue for targeting cancer cells is that it also reduced uptake of the particle by LHRH-negative cells with corresponding tissue culture knockdown results. Increasing the ratio of siRNA uptake in target vs normal cells is particularly important for cancer RNAi Therapeutics as these are typically aimed at killing cells.
But what really caught my attention in the Taratula paper was the biodistribution of the caged-PEG siRNA-dendrimer particles in a mouse model of cancer. By elegantly labeling individually siRNA, the dendrimer, and the cancer cells, it was convincingly shown that LHRH-targeted particles were very selectively targeted towards the cancer. It goes without saying that getting the siRNA to the place in the body where you want it to act is a very important first step. What was less convincing, however, was the efficacy of functional siRNA release into the cytoplasm with unfortunately no in vivo knockdown results shown. Also, the tissue culture cell microscopy experiments to me do not demonstrate efficient siRNA release, but mostly show the particles to be stuck in the endosome.
Endosomal release, of course, is a huge problem with a lot of the siRNA delivery technologies, but I am hopeful that the improvements in the pharmacologies as demonstrated by these papers can be combined with technologies imparting the dendrimers with endosomolytic activity. Due to the modularity of the dendrimer system this seems to be a realistic goal as there are now a number of technologies that allow for endosomal release, that incorporated into Mirus’ DPCs being one, and the generation of nanoworms (‘dendriworms’: Agrawal et al 2009: Functional Delivery of siRNA in Mice Using Dendriworms) being another of them. I am also encouraged by what appears to be a quite favorable safety profile of dendrimers.
For the investors among you, you will probably be asking about the commercial landscape for dendrimer-siRNA delivery and how to profit from it. The Australian dendrimer company Starpharma has been on my radar, but besides stating their interest in siRNA delivery, I have seen little material emerge from this. Due to geographic proximity and the fact that Benitec has a few InterfeRx picks, maybe they should be talking to Benitec. Then, of course, it is possible that one of the elephants in the room, particularly Alnylam, Roche, and Merck, will be looking hard to round up the IP and know-how in the field.