[Part 1 of a 3-part collaborative series with Tobias Wolfram on the first notable attempts at RNAi Therapeutics for solid cancers that have entered the clinic]
When during the company's R&D day the CEO of Alnylam, John Maraganore, highlighted Calando’s cyclodextrin-based siRNA delivery technology (RONDEL) as one of the noteworthy non-SNALP systemic siRNA delivery technologies out there, it certainly piqued my interest. This is not least because any cash infusion and longer-term commitment by a partner like Alnylam would do wonders for the parent company of Calando, Arrowhead Research which is a conglomerate of early-stage, IP-focused business units and has just barely scraped by bankruptcy through a diet of cutbacks. While I always remembered the maturity of RONDEL, developed in the Mark Davis lab at Caltech, to be years behind SNALP, Tobias and I decided to to take a closer look at the development path of CALAA-01, the first clinical RONDEL delivery candidate and also investment focus of reorganized Arrowhead Research.
When Tobias first heard of the technology, it struck him as a very elegant, because simple, and modular method to formulate targeted nanoparticles. In fact, there are not many targeted nanoparticle siRNA delivery approaches where the components supposedly can be assembled by the pharmacist just before patient administration. RONDEL siRNA delivery consists of mixing together siRNA, a short cyclodextrin-containing polycation, and adamantane-coupled PEG stabilizers some of which carry a transferrin ligand, so as to create 60-80nm particles. These particles were rationally conceived to satisfy a range of pharmacologic and formulation considerations. Their suitability for solid cancer relies on the enhanced permeability and retention (EPR) effect of nanoparticles with reasonably long circulation times (here supposedly achieved by PEG stabilization), the ability of the particles to be taken up into cancer cells by transferrin receptor-mediated endocytosis and their subsequent release into the cytoplasm in a pH-dependent manner.
Unfortunately, what we soon came to realize was that while the concept is a very nice one indeed, the particles, particularly CALAA-01, remain to be better characterized both physically (shape, uniformity, storage and biological stability etc) and for their RNAi knockdown ability in vivo. For example, knockdown of RRM2, the target of CALAA-01, and subsequent tumor inhibition have not been demonstrated in a convincing in vivo system. Instead, knockdown efficiencies have largely been limited to in vitro studies. Moreover, these involved siRNAs that were selected with what today would be considered outdated methods and probably as a result were not very potent. The in vivo studies were essentially limited to pharmacological investigations, with the combination of in vitro efficacy and in vivo pharmacology forming the rationale for moving CALAA-01 into the clinic. Moreover, even when considering only the pharmacology, measures such as biodistributions and circulation times did not fit the model which may be explained by nanoparticle instability in vivo, something that really needs to be investigated further. Also, since the siRNAs were unmodified it strikes me as rather strange that no innate immune induction and only moderate adaptive immunity were reported.
What I found to be a valuable take-home message from those studies, although according to Tobias’ liking resting too much on indirect evidence, but not necessarily data obtained with the CALAA-01 clinical candidate, was that the utility of the targeting ligand appeared to be in increasing the cellular uptake of particles with little positive surface charge, less so in skewing the biodistribution towards the solid cancers per se. This could also be because the particles were cleared relatively rapidly from circulation, mostly into the kidney and bladder, which raises further questions about the purity and integrity of the particles. In this light, the mention of nanoparticle assembly by a pharmacist may also be interpreted a necessity due to storage problems of fully formulated particles. Nanoparticle assembly is notoriously sensitive to even slight changes in parameters such as temperature, speed of mixing etc so that it would be preferable for the physician to just administer the drug without the need for prior handling. It is therefore unfortunate that results from long-term storage and robustness of the formulation method were not presented. Nevertheless, an enhanced cellular uptake through the addition of a ligand could critically increase the therapeutic index of a cancer RNAi Therapeutic, and the modular nature of the RONDEL system should easily facilitate such additions.
Our assessment that CALAA-01 was probably entered into the clinic too early naturally rests on the publicly available data only. The publication dates of the relevant data, however, strongly suggest that they indeed represent the relevant data points and considering the financial situation of Calando, it would not appear that much were to be gained by holding back on positive data. This is by no means to belittle what otherwise is very rich science. Unfortunately, it is here that the tension between corporate demands for advancing a pipeline and the need to sufficiently advance the science is most evident and in the end risks harming both objectives. Optimistically, completion and evaluation of the CALAA-01 phase I trial will allow for valuable insights into the performance of RONDEL delivery in man for the benefit of any follow-up programs. Without any such strong data, it is questionable however whether the promise of RONDEL as a differentiated and flexible platform for RNAi Therapeutics delivery alone will be enough to make Arrowhead Research a good RNAi Therapeutics investment.
PS: To expand on the latter point, Tobias and I also discussed that, in general, it is easy to caution against entering RNAi Therapeutics candidates into the clinic early and dismiss such as a move of desperation. On the other hand, the case can be made that, when it comes to RNAi Therapeutics as a broadly applicable platform, clinically evaluating candidates which for example would not be expected to effect large knockdowns can be justified in that the data coming out of these studies may provide timely data invaluable for follow-up programs using very similar delivery approaches. Although investors will rightly fear the costs of a failed trial particularly for small biotech companies, such data may be valued more highly by a potential Big Pharma partner. This argument receives added weight in an environment like now where it is very difficult to raise capital from the public markets, and partnering is the primary means for small biotech to obtain capital at acceptable terms.