Hand in hand with a rapidly expanding understanding of the biological mechanisms of RNAi Therapeutics delivery, we are hearing more and more about ligand-guided targeted delivery. Just today, mdRNA announced notice of allowance for a patent application on the identification of a peptide specifically binding to the cell surface of hepatocellular carcinoma cells, and Alnylam has recently started to talk about their discoveries on ApoE-dependent and Apo-E independent SNALP cellular uptake pathways paving the way towards new targeted SNALP delivery strategies (Systemic RNAi Delivery Roundtable). As it increasingly looks like Calando’s transferrin-targeted CALAA-01 will not remain the only targeted RNAi Therapeutics formulation in the clinic for long as targeted delivery is set to provide the next push towards more potent and safer RNAi Therapeutics delivery, I will try and briefly explain the rationale behind targeted delivery and some of the challenges that need to be overcome.
Success in targeted delivery is measured by either preferential uptake of the siRNA in the target tissue compared to non-target tissues or achieving lower efficacious dosages by taking advantage of particularly productive receptor-mediated uptake pathways, preferably both. The detailed pharmacological effects are not only determined by the ligand, but also by where it is attached to. Steps that may be affected can include biodistribution, cellular uptake once at the target tissue, or the avoidance of certain cells and tissues.
SNALP delivery for example may benefit from targeting ligands by reducing uptake by macrophages or relying on non-specific charge-charge interactions for cellular uptake, both of which can be safety liabilities. For the liver, de-targeting from macrophages may be even more important than active targeting. The DPCs by Mirus (now Roche) were particularly exciting here in that the data suggested that the right presentation of ligands (simple sugars in this case) on the particle surface may eliminate the unspecific uptake of a delivery platform and instead re-target it to new cell types. Equally exciting data by Alnylam suggests that it should be possible to greatly limit the ‘non-specific’, mainly ApoE-mediated uptake of ionizable SNALPs by shielding their surface and then re-direct them by adding new ligands on their surface. By then further increasing their circulation times through creating very stable particles (e.g. by increasing the stability of the stealth shield), a delivery platform may then also be applicable to new therapeutic application fields by increasing the chances that a ligand recognizes receptors in distal tissues.
Lipoplexes such as Silence Therapeutics’ Atuplexes may also benefit from targeting ligands. Since the interaction of immune cells with blood endothelia is very well studied this may e.g. allow it to be targeted to specific endothelia such as the blood-brain-barrier.
Targeting ligands are already part of many siRNA-conjugate approaches. Achieving endosomal release in addition to cellular uptake is a big challenge for this area of delivery, and it will be interesting to see whether in fact those receptors that prove effective for nanoparticle delivery may be the types of receptors to be avoided for siRNA-conjugates in favor of channeling them into more non-specific pathways.
There are, of course, also challenges associated with targeted delivery. One is to identify suitable ligand-receptor interactions as endosomal maturation processes can differ greatly, e.g. in the degree and rate of acidification and receptor recycling, which imposes new types of pharmacokinetic demands on a delivery system.
A systematic effort to discover the best receptors may be to screen a panel of siRNA-nanoparticles containing (single-chain/nanobody-type) antibodies on their surface and that are targeted to a wide array of cell surface receptors and then select those with the best silencing results. The most promising receptors, hopefully patentable, may be pursued then either with the antibodies themselves or alternative, smaller ligands. New ligands to given receptors may be discovered through panning peptide display libraries against that receptor, something e.g. that mdRNA does with their trp-cage peptide libraries.
Avoiding adaptive immunity, especially to novel designer ligands is another added challenge for targeted delivery. And finally, when all these questions have been answered, the not-so-trivial task is to find formulation methods that allow for clinical and commercial scale-up of the more complex particles. At the end of the day, however, it is those platforms for which a detailed mechanistic basis has been established and those teams that have turned formulation into an art that will succeed. It can be done.