RONDEL Study Sheds Light on Fate of Cationic RNAi Delivery Systems

As regular readers may know already, I am not convinced that the polycation-based RONDEL delivery technology, and therefore Arrowhead’s CALAA-01 which is in clinical development for advanced solid cancers have bright prospects. In addition to the immune stimulation concerns which only came to the fore after the human RNAi proof-of-concept Nature paper (Davis et al. 2010) was published and which may be correctable if Arrowhead would modify their siRNAs, it is the limited pharmacokinetic profile with this delivery technology that makes it unlikely that the formulation can effectively take advantage of the enhanced permeability and retention (EPR) effect for tumor delivery. New research (Zuckerman et al., 2012) by the Davis group from Caltech, which is also the inventor of this technology, sheds light on the likely mechanism behind the pharmacokinetic profile. Importantly, this research has implications for the design of all delivery technologies involving complex formation between RNAi triggers and constitutively positively charged polymers and lipids (e.g. AtuPLEX, PEI-based polyplexes).

Previous biodistribution studies (Bartlett et al, 2007) showed that much of the RONDEL-delivered siRNA ended up in the kidney and bladder. At the time, it seemed plausible that this was due to displacement of the short siRNAs from the polycation backbone by competing negatively charged components in the blood. Thus, the pharmacokinetic behavior of the siRNAs would be almost like that of naked siRNAs which are known to be rapidly excreted through the kidneys. The new research, however, looked in more details at the pharmacokinetics of siRNA and delivery components in the kidney-bladder. This revealed that, in fact, the formulated siRNAs followed a slightly different profile than would be expected from that of naked siRNAs. Bringing to bear a number of sophisticated visualization technologies, it was found that the nanoparticles were first deposited intact in and around the glomerular basement membrane (GBM) before falling apart into their constituent components which, in turn, were small enough to escape through the slit pores into the urine (intact nanoparticles would be too large to make it into the urine).

Further adding credence to the notion that the RONDEL particles remain intact in circulation was a report (Oney et al., 2009) that the polycation backbone chemistry in RONDEL can be used to scavenge free oligonucleotides in circulation, which can be useful e.g. to regulate the activity of aptamer therapeutics. Similarly, the research by Zuckerman et al. shows that mixing separately added siRNA and RONDEL components in blood allows for the reconstitution of the nanoparticles.

The reason for the instability in the GBM is thought to be due to the strong negative charge of this extracellular matrix, meaning that the siRNAs are competed off the polymers there. I was quite surprised to learn during my cursory review of GBM biology that cationic polymers such as polyethyleneimines (PEI), a common nucleic acid delivery chemistry, are used to visualize the GBM! It is therefore likely that what was found for RONDEL will have implications for the pharmacokinetics, and safety, of other cation-based delivery systems. I mention safety, too, since it is known that particulate deposits (e.g. antibody-antigen) in the GBM can often cause glomerulonephritis (‘leaky kidney’). Whether the latter will be a problem may also depend on the duration the complexes reside there. At least for RONDEL, the time seems to be quite limited (on the order of minutes).

Preclinical validation of CALAA-01 in tissue culture and rodent studies

In addition to the pharmacokinetic study by Zuckerman and colleagues, another CALAA-01-related study was just published (Rahman et al., 2012). Conducted in Atlanta, with help from Caltech, this research is more optimistic about CALAA-01 by adding data in support of the validity of RRM2 as a cancer target. I myself have had problems understanding why this gene was chosen in particular when there are so many other attractive targets out there for therapeutic cancer RNAi. At least the tissue culture data strongly suggest that there is a target-dependent inhibition of cell proliferation. The animal knockdown data were not as strong, but some target knockdown, and even more pronounced anti-cancer effects were observed. Unfortunately, this being with a non-modified siRNA, one may want to take this with a grain of salt.