Arrowhead Research Breaks Up Dynamic PolyConjugates into Two

Arrowhead Research announced yesterday that it had received Notice of Allowance from the USPTO for a Dynamic PolyConjugate-related patent application (for systemic RNAi delivery).  Instead of it being your run-of-the-mill patent PR involving known technology that finally received patent protection, it was really about revealing Arrowhead’s fundamentally new approach towards DPC delivery (for my take on the original form of DPC, see here).

The patent, part of a series of patent filings that have come out recently, shows that under Roche’s ownership, the technology has morphed through a number of iterations from the original complex polyconjugate chemistry combining endosomolytic polymer, masking groups, RNAi trigger, PEG, and cell targeting ligand all in one molecule (schematic shows such molecule and presumed mechanism of delivery), into one where a RNAi trigger and the masked polymer, both targeted individually, are administered as separate agents. 

One problem with the original design has been that combining all the functional groups, including negatively charged RNAi triggers and positively charged polymers, into one molecule was not particularly easy.  The tendency to aggregate and poor yields made it a quite expensive and difficult-to-scale proposition.

Turns out that such complicated chemistry wasn’t needed after all.  As long as the RNAi trigger and the masked endosomolytic agent end up in the same place, it does not make much of a difference whether they are getting there as one molecule or separately.  In the example provided, namely for gene knockdown in hepatocytes, the RNAi trigger could be conjugated to either cholesterol or a cluster of galactose sugars, whereas the masked endosomolytic polymer was targeted to the hepatocytes by galactose.  Viewed differently, the polymer allowed the cholesterol-siRNA that apparently gets trapped in the endosomes when alone to be released into the cytoplasm.  Hence, the multi-fold increase in potency (Arrowhead Research says it’s 500-fold) over Alnylam’s original 50mg/kg cholesterol-siRNA report (Soutschek et al., 2004).

Another potential advantage of this separated approach is that it makes each component smaller, perhaps 'one day' enabling subQ dosing.  However, as mipomersen's FDA AdCom meeting briefing docs show, when it comes to subQ dosing, be careful what you wish for.

Manufacturing appears to have been one of the issues delaying the clinical translation of DPCs for quite some time (Arrowhead Research says they are ready to file an IND in Q2 2012 for Arc520 in HepB).  Toxicity, mainly due to premature unmasking in the blood instead of in the target cell endosome, seems to have been the other main reason.  Such premature unmasking also adversely affected circulation times, thereby rendering attempts to get beyond the liver, one of the original promises of DPCs, futile.  We will probably get word from the company soon what solutions it found for this challenge, but it seems that, based on the emerging patent literature (including non-human primate data; e.g. WO 2012/083185), masking the membrane penetrating peptide mellitin with endosomal protease-sensitive groups, is a promising approach.