Tekmira Partner to Initiate Phase I Study for the Treatment of Severe Hypercholesterolemia

Tekmira’s partner and licensee Alnylam Pharmaceuticals announced today the submission of files to a European regulatory agency in anticipation of a phase I study with a candidate targeting PCSK9 for the treatment of Severe Hypercholesterolemia. The primary aim of this early-stage study is naturally the safety and tolerability of ALN-PCS. An important secondary aim, not just for Alnylam but also for the entire field of RNAi Therapeutics, will be assessing drug activity as measured by target protein levels in serum. Initial data are expected by year end.

ALN-PCS represents Alnylam’s 4^th clinical candidate, three of which are based on Tekmira’s SNALP technology (VSP, TTR, PCS). It also represents the 2^nd candidate under its 5x15(TM)program which aims to advance five RNAi Therapeutics candidates into late-stage clinical development by 2015. All disclosed candidates under that program (TTR, PCS, HPN) are based on SNALP technology.

Despite the success of statin in lowering cholesterol, many patients are still considered to be in need of additional treatment options. PCSK9 has emerged as a very attractive target for such uses based on human genetics which suggest PCSK9 knockdown to reduce ‘bad’ LDL cholesterol in addition to being well tolerated. While ALN-PCS is the first RNAi Therapeutics candidate to target PCSK9, other companies have already started clinical development of PCSK9-targeting hypercholesterolemia candidates.

These competitive efforts are based on monoclonal antibodies and antisense approaches and include a recently initiated program by Santaris (LNA antisense) and monoclonal antibody programs by Amgen, Pfizer, and Regeneron amongst others (mostly phase I and II). ISIS with partner BMS together are also developing a PCSK9 antisense candidate. This candidate, however, still appears to be in late preclinical studies following some delays.

The pre-clinical data show that RNAi, antisense, and monoclonal antibodies can all potently down-regulate or bind and inhibit PCSK9. It seems, however, that one advantage of ALN-PCS could turn out to be that it does not simultaneously down-regulate ‘good’ HDL cholesterol as was observed e.g. in a rodent study by ISIS (antisense; significant 50% reductions; Graham et al 2007) or in a non-human primate monoclonal antibody study by Amgen (moderate 20% reductions; Chan et al 2009).

Obviously, it is still early days to speculate on the eventual competitive profile of the various candidates. With regard to safety and tolerability it is notable, however, that the anticipated highest dose with ALN-PCS in the study is 0.25mg/kg. This means that an average Caucasian may receive only about 10-20mg siRNA per week (assuming bi-weekly or monthly administration). While there is still some more uncertainty about the safety profile of non-DLinDMA SNALPs such as ALN-PCS (this candidate uses MC3 while all other SNALP candidates so far made use of DLinDMA), such low doses make me optimistic that the safety profile should be quite competitive.

Adding Tekmira’s efforts, this program is the 5^th SNALP candidate in 3 years to enter clinical development. At least two more are anticipated this year. ALN-PCS therefore further illustrates one of the advantages of RNAi Therapeutics development, namely that once there is a suitable delivery technology for a given target organ, in this case SNALP for liver delivery, the indications can be rapidly expanded to multiple drug targets. This also means that the inventor behind and manufacturer of ALN-PCS, Tekmira, should receive a milestone from its partner and licensee upon initiation of dosing, adding to an increasing royalty stream.

Targeted Delivery Strategies Coming to the Fore

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.