Solid cancers are considered by many the most important near-term value driver for RNAi Therapeutics. The fact that Calando’s RONDEL nanoparticle delivery technology has now been shown to accumulate in solid cancer tissues is not only an important de-risking event for CALAA-01 itself, but it immediately suggest that RONDEL, and also quite likely other nanoparticle-siRNA delivery technologies, can facilitate the targeting of essentially any gene in solid cancers. It is this flexibility of the platform that makes RNAi Therapeutics ideally suited to satisfy the demands of a genetically such heterogeneous disease group as cancer.
Nanoparticle-siRNA delivery such as RONDEL is currently the most promising approach for treating solid cancers with RNAi Therapeutics. All of these rely on the particles being able to circulate sufficiently long in the blood stream and being small enough so that they can passively accumulate through EPR in the solid tumors as these lack an effective way of draining them through lymphatics. While the absolute amount of RONDEL particles that accumulated in the tumors remains to be determined, the Nature study clearly showed that a decent amount of them does accumulate there at ~0.6mg/kg.
While particle stability and ability to accumulate through EPR is a critical first step where we should also expect to see important differentiation between the technologies, once there, the nanoparticles then have to be taken up by the cancer cells and moreover mediate the cytoplasmic release of the RNAi trigger into the cytoplasm. Once again, RONDEL meets this challenge as it was possible to detect cleaved target mRNA characteristic of an RNAi mechanism of action. Although the so called RACE assay used for this analysis is not quantitative, in my experience, you have to get sufficiently robust RNAi to cleanly detect such products. Consistent with this notion, well over 50% target knockdown was detected for CALAA-01, although the limited number of samples (3) and the reliance on often only historical negative controls leaves open the question of the exact knockdown potency of CALAA-01.
It is in fact the pioneering work with RONDEL-siRNA delivery from which this pharmacokinetic model has been derived and that is now informing quite a bit our thinking of how to design siRNA-nanoparticles for solid cancer applications. For example, it was work on CALAA-01 that showed that the utility of the (transferrin) targeting ligand is in enhancing the cellular uptake of the nanoparticles, not so much in concentrating them in the tumors in the first place, and should be influential in the selection of targeting ligands being contemplated now for all types of delivery technologies, including SNALPs.
Summary of CALAA-01 results
CALAA-01 was also the first serious RNAi Therapeutics candidate for solid cancers that had entered the clinic in 2008. For a more thorough molecular background on this program, please refer to a previous entry (here) that was part of a 3-part collaborative series with Tobias Wolfram on the first RNAi Therapeutics solid cancer candidates. Briefly, CALAA-01 is a self-assembled nanoparticle system based on a cationic co-polymer containing cyclodextrin complexed with siRNA targeting RRM2, a gene involved in DNA replication, and that like most of the advanced nanoparticle-siRNA systems is stabilized through pegylation (some of these carry the transferrin targeting ligand). While Tobias and I had cautioned about a relative lack of preclinical in vivo characterization of CALAA-01, particularly not having directly demonstrated that CALAA-01 can knock down genes via RNAi in solid tumors following intravenous administration, it seems that the gamble of skipping some of the pre-clinical proof-of-concept has paid off for the investigators and Arrowhead Research (of which Calando is a subsidiary).
It was also a clever move that, amongst other tumor types, a number of melanoma patients were enrolled as such tumors can be easily accessed through biopsy and thus represent good material for molecular analysis of a clinical candidate. 3 of these volunteered to be biopsied. Despite the limited sample size, a number of conclusions can be drawn from the data: 1) RONDEL-siRNAs accumulated in the tumor tissues in a dose-dependent manner; 2) consistent with this, in the patient that received the highest dose, an RNAi mechanism of action was unambiguously confirmed by 5’ RACE; 3) this effect was long-lasting (>4 weeks) and accompanied by mRNA knockdown and disease stabilization; 4) RONDEL was safe and well tolerated in a study that involved repeat administrations and dose-limited toxicity has yet to be reached (right now at ~0.6mg/kg with dose escalation ongoing).
Clearly, these are highly encouraging results and call for the continuation of the phase I studies and further investigation of the modular RONDEL delivery platform. Importantly for shareholders, it should put Arrowhead in a better position to fulfill their stated goal of monetizing RONDEL either through licensing or outright sale. Part of the value would derive from the pioneering position of the RONDEL technology and from representing a complementary approach to lipid-based delivery which is currently dominating RNAi Therapeutics development programs.
Implications beyond CALAA-01 and RONDEL
Taking a step back, the CALAA-01 results add to the increasing amount of clinical data that first of all lay to rest concerns that there is a prohibitive fundamental (acute) toxicity issue for RNAi Therapeutics per se, and secondly provide evidence that therapeutic reality is within the reach of current systemic RNAi delivery technologies. They come on the heels of SNALP-ApoB results that showed preliminary signs of RNAi efficacy at a similar dose level while ongoing improvements in SNALP efficacy continue to widen its therapeutic index, and continuing dose escalations for both Alnylam’s VSP-02 and Silence Therapeutics’ Atu-027 solid cancer programs which can be interpreted as confirming the tolerability of some of the important delivery technologies in the space (SNALP and Atuplex).
With such progress, urgency to invest in RNAi Therapeutics, particularly in enabling delivery technologies should return. As the transition into the clinic is fraught with uncertainties for new chemistries (safety, efficacy, and manufacturing scale-up), it is particularly the less than a handful of delivery technologies that have made it there for which a land-grab might erupt as is also indicated by the fact that Tekmira now counts 8 RNAi-related pharmaceutical collaborators (both early and advanced stage), mdRNA at least 3 such early-stage collaborators, and a rumor-driven rise in the shares of RXi Pharmaceuticals possibly due to their mysterious claims on ‘self-delivering’ siRNAs. With the publication of the results that are about to garner widespread attention, Arrowhead Research/Calando should, of course, also get a few enquiries for RONDEL as it resumes its search for partners. Considering the generally weak share prices of RNAi Therapeutics companies, all the ingredients seem to be in place to see related deal activities, with a significant portion of the value being driven by the solid cancer opportunity for RNAi Therapeutics.