Sunday, March 21, 2010

CALAA-01 Results Indicate RNAi Therapeutics to Revolutionize Solid Cancer Treatment

In yet another important proof point that RNAi Therapeutics is slowly but surely morphing into a therapeutic reality, Calando just reported in the highly prestigious journal Nature an interim analysis of their phase I clinical study of CALAA-01 demonstrating an RNAi mechanism of action in solid cancer patients. The results are the most direct demonstration of RNAi in Man thus far, and, at least equally significant, show that the Enhanced Permeability and Retention (EPR) effect is indeed about to turn into the Achilles' Heel of solid cancers in the era of nanoparticle siRNA delivery.

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.

Update May 29, 2010: More details about the study, including the safety profile was published recently in an abstract for the upcoming 2010 ASCO cancer conference:

Systemic delivery of siRNA via targeted nanoparticles in patients with cancer: Results from a first-in-class phase I clinical trial

Author(s): A. Ribas and colleagues

Background: Systemically delivered small interfering RNA (siRNA) would allow targeting oncogenic molecules beyond current approaches. We report on the first siRNA trial with a targeted nanoparticle delivery system. Methods: Open-label, dose- escalation trial in pts with solid refractory cancers receiving 4 i.v. infusions (30 min) on d 1, 3, 8, and 10 of 21-d cycles. CALAA-01 nanoparticles consisted of a cyclodextrin-based polymer, transferrin protein (hTf) targeting ligand, polyethylene glycol (PEG) for stability, and siRNA against ribonucleotide reductase M2 (RRM2). The 70 nm particles were designed to minimize renal clearance and allow tumor vasculature permeation with binding to tumor hTf receptors (TfR). Primary endpoints (safety, MTD determination) were based on the first cycle. Results: 15 pts accrued to 5 dose levels (3, 9, 18, 24, 30 mg/m2), median age 62 (range 53-85). Most common histologies: GI (4), melanoma (3). Dose escalation progressed with no DLTs. Most common treatment-related AEs: fatigue (47%), fever/chills (33%), allergic (33%), constipation (33%), nausea/vomiting (20%), all g1-2. 1 pt had g3 anemia and 2 pts had g2 thrombocytopenia, all shortly after CALAA-01 infusions with rapid recovery. 1 pt had possibly-related sinus bradycardia (g2). No objective tumor responses were seen; 1 pt at highest dose had stable metastatic melanoma for 4 mo, a change from prior course. Biopsies in 3 pts (melanoma) showed particles in tumors. At 30 mg/m2 RRM2 knockdown (mRNA and protein) was seen with confirmation of mechanism by specific cleavage sequence (RACE-PCR). TfR was not downregulated in cancer cells.Conclusions: Systemic delivery of siRNA via targeted nanoparticles is safe and can induce specific, siRNA-mediated gene silencing. This approach could be expanded to any currently undruggable cancer therapy target.


Nick said...

Hi Dirk,
Thanks for the commentary. What do you think: the fact that only 1.5 of the patients (patient C, and a little bit of patient B) showed CDP update by fluorescent imaging, only one of the 3 patients (patient C only) showed intended mRNA cleavage by RACE, and the TEM lacked some negative controls.....
As much as I personally want siRNA therapeutics to succeed, I might be a little hesitant to call that "highly encouraging." But, at least there doesn't seem to be much toxicity.

Dirk Haussecker said...

Hi Nick-
your point about the limited number of patients and the need for more is well taken. On the other hand, I believe the imaging data are consistent with the dose-dependent RNAi mechanism of action (5' RACE results). Beyond the qualitative data, it would be interesting to know how much of injected amount of RONDEL actually made it intact into the solid tumors and, related to this, the precise pharmacokinetics of the formulation. These types of questions is where Calando and potential collaborators may want to address in a collaboration first.

Showing safety and RNAi mechanism of action for the first experience of RONDEL in Man is what I consider a 'highly encouraging' start, also for nanoparticle-mediated RNAi delivery to solid tumors in general (EPR effect). Platforms that can make it stably into solid tumors, then be taken up by the tumor cells (non-specific uptake or ligand-mediated uptake) and then release the siRNA in a (pH-)triggered manner should have a very good shot at having a significant impact on such diseases. Progress in the field supports that some technologies may be there already or are very close.

Anonymous said...

Hi Dirk,

Do you have an update on the clinical trials of siRNAs from different companies? (Phases, timelines, etc.) Just curious where the technology is at.


Nick said...

Thanks for your clarification, Dirk. I think I understand what you were trying to say about "highly encouraging" (that, at least, there wasn't extreme toxicity and that there was consistency between microscopy and RACE for each of the 3 patients, even if the consistency was a negative result).

In terms of "RNAi mechanism of action for the first experience of RONDEL in Man" being "highly encouraging" in 1 of the 3 patients: my personal choice of words would have been "curiously motivating".

Thx again. I am learning a lot from your blog!

Dirk Haussecker said...


I believe a year from now will be a good time for such a review as I am still looking for important data from Alnylam’s VSP and TTR and Tekmira’s ApoB2. Silence and Calando may also have interesting updates from their cancer trials.

Pete Shuster said...

Hi Dirk,

We are showing promising results using nanoparticles to deliver siRNA to tumors via intravenous injection in mice and rats. Hopefully, these techniques will continue to yield increasing efficacy in humans.

By Dirk Haussecker. All rights reserved.

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