Alnylam’s ALN-RSV01 Clears the Human Proof-of-Concept Bar

[Update following this morning’s conference call: 1) Importantly, statistical analysis showed that the reduced infection rate following ALN-RSV01 administration was independent of the level of pro-inflammatory cytokines, and therefore further indicates an RNAi-based mechanism of action, instead of an siRNA-triggered non-specific cytokine response; 2) The dosages were at the upper end based on previous phase I nasal administration safety studies. There were two dosages used, a 75mg group (8/88 subjects) and a 150mg group (80/88). This number was too small, and probably too close together, to determine dose-response. 3) while I labeled the virus in my summary as a “laboratory strain”, it was stressed that the virus was a relatively fresh clinical, wild-type isolate from an RSV patient and therefore not attenuated; 4) the slides from John De Vincenzo’s presentation in Singapore will be available on the company’s website this weekend.]

Concurrent with a presentation at the respiratory disease conference in Singapore, Alnylam has just released more detailed data from the phase II experimental infection study of ALN-RSV01, an unmodified siRNA for the treatment of RSV infection via RNAi. The study, termed GEMINI, was designed to demonstrate the safety and antiviral activity of intranasally administered ALN-RSV01 in adult volunteers artificially infected with a laboratory strain of RSV virus. As such, it could therefore represent statistically validated proof-of-concept for RNAi activity in humans.

According to the press release, there were no obvious adverse effects attributable to ALN-RSV01. Although this may have been expected based on the previous phase I intranasal safety study results, considering the inflammatory potential of some siRNAs (which were apparently excluded during the pre-clinical siRNA screening process) , an siRNA in the context of a viral infection could have conceivably triggered unforeseen safety issues, even worsening rather than treating the viral infection.

On the efficacy side, ALN-RSV01 statistically reduced the infection rate across a range of laboratory parameters. When given 5 times daily, 2 days before and 3 days after viral administration, the number of volunteers remaining infection free almost doubled, from 12/42 to 24/43 treated with placebo and ALN-RSV01, respectively. Measures of viral dynamics in those patients in which viral infection took hold showed a trend towards improvement with ALN-RSV01, although they did not reach statistical significance. Similarly, symptom scores were not much different between ALN-RSV01 and placebo (at least they were not worse as may have been expected for a siRNA-triggered inflammatory response!).

It therefore appears that at least in this setting efficacy was largely an all-or-none and once viral infection took hold there was little stopping it. For a drug candidate that is designed to treat, but not prevent RSV infection (note that there are very effective preventive neutralizing antibodies for RSV on the market), this may appear disappointing at first glance.

However, there are a number of factors that complicate how predictive these results are for naturally infected patients. One unknown to me is the dose used in the study and whether they were on the conservative or aggressive side, which could have made a big difference in antiviral efficacy (but also safety, of course). Another factor is that in order to achieve reliable experimental infection, the nasal epithelium was probably overwhelmed with viral loads that would not be encountered at early stages of a natural infection. Moreover, although the experimental infection should have initially been largely restricted to the nasal epithelium, the odd survivors may be able to establish reservoirs in areas of the respiratory tract not reached by the nasal administration of the drug, at which point the route of administration became limiting in the ability of ALN-RSV01 to stop RSV replication. This may be addressed with the use of aerosolized versions of ALN-RSV01, or even in conjunction with nasal administration, in future studies.

It will also be interesting to find out whether the all-or-none response was due to viral escape mutants that have changed their sequence at the siRNA target site, as had been observed in a number of pre-clinical antiviral RNAi studies before. In that case, co-administering two different siRNAs, similar to Benitec’s HIV and HCV RNAi strategies, should considerably lower the likelihood of such an event.

Today’s results are probably almost all one could have hoped for. They are the culmination of a very well-planned and executed scientific program involving challenges such as the not-so-trivial task of establishing the experimental infection model itself. But there will be little time to rest on their laurels. The eyes are now on the design of the phase II natural infection study slated to start in the first half of this year. The all-or-none response seen here strengthens pre-clinical results suggesting that early detection and treatment will be important for the success of ALN-RSV01 of such studies, and ultimately in the clinic. With increasingly rapid nucleic acid-based diagnostics coming online, hospital-acquired cases of RSV may be the low-hanging fruit. Tomorrow’s conference call may give many of the answers, possibly linked to the enigmatic post-hoc naming of the trial as “GEMINI”.

In the larger scheme of things, the results may also have implications for the treatment and prevention of other respiratory viral infections. In terms of human proof-of-concept, with RNAi and viral infections one always has to take into account the possibility that stimulation of innate immune responses rather than specific gene silencing may have caused the antiviral effect. However, based on the preclinical studies and the fact that the drug was apparently well tolerated, one would probably have to give human proof-of-concept a pass.

Sunday, March 2: The Singapore presentation is now available on the company’s website.

Disclosure: I hold stock in Alnylam Pharmaceuticals.

Alnylam Reports “Positive” Data from RSV Experimental Infection Trial: Human Proof-of-concept for an RNAi Therapeutic?

Alnylam got observers scratching their heads today as they announced “statistically significant anti-viral activity” for ALN-RSV01, an RNAi Therapeutic against RSV infection, in an early phase II trial, but without providing further details.

In this experimental infection trial, an siRNA delivered to the nasal epithelium is tested for activity against a non-pathogenic strain of RSV. The trial has two main purposes: firstly, to provide the first demonstration of RNAi activity in man (human proof-of-concept; HPOC); and secondly, to inform the design of late-stage phase II trials in naturally infected adults slated for the first half of this year, although in that trial delivery will be to the lung by nebulizer (see 13Dec07 Blog: Breaking News: “Alnylam Reports on the Safety of Inhaled RSV-01”).

The question then arises why did they not present any hard data? It is possible that, as often the case in biotech, initial press releases and top-line data are exploited for PR purposes while the warts-and-all data turn out to be less than impressive. However, given their usually well executed PR strategy with which they have earned themselves considerable credibility, I doubt that they would start playing around with the meaning of “statistically significant”.

One possibility is that they are still crunching numbers as to viral titers by PCR and plaque assay as well as symptom scores in the different groups, to then comprehensively present the whole data set at the International Symposium on Respiratory Viral Infections meeting next month in Singapore. Some meetings also do not like it when data are presented beforehand in press releases. In any case, I hope the company makes the full data set available on their website once they are presented. I am particularly curious as to whether they allow insights into the mechanism of action of this drug.

Another maybe more intriguing possibility for the strange lack of any concrete numbers may have to do with the rather equally unusual post-hoc christening of the trial as GEMINI. Assuming that they would not want a failed trial to be well remembered by giving it a name, one that could be maliciously interpreted as to mean “ambiguous”, the naming of an early stage trial AFTER the results are announced may have a deeper meaning. If the results hold up, GEMINI could be used in the future to refer to HPOC as another momentous de-risking event in the history of RNAi Therapeutics. Equally tantalizing is the possibility that the full data presentation of GEMINI may coincide with the announcement of a co-development partner. Although the initial adult market for ALN-RSV01 may be limited, this program should attract considerable interest as it will give the partner a pioneering role in RNAi Therapeutics, potentially associated with first dips for future respiratory disease programs.

So stay tuned on GEMINI as we are likely to hear it being referred to quite more often in the future. January 23, 2008, may have been another important milestone in the development of RNAi Therapeutics. In the meantime, the prize goes to he/she who can figure out first what the acronym GEMINI stands for*.

PS: I would guess that today’s results will have triggered milestone payments from the Novartis and Roche collaborations.

[* Update May 2, 2008: It is becoming clear now that GEMINI stands for the intention of the study to both show a) proof-of-concept for RNAi efficacy in man, and b) further evaluate the safety and efficacy of ALN-RSV01 for the RSV antiviral development program.]