Viruses, Beware! This time, RNAi Therapeutics Mean Business

Viral infections have long been thought of as an attractive therapeutic area for RNAi Therapeutics.  Unfortunately, with the exception of Tekmira’s Ebola biodefense effort funded by the US Department of Defense, this area has had trouble taking off: Nucleonic’s ddRNAi-based HBV program should never have gone into the clinic (and as expected was soon terminated thereafter), and there is considerable concern that the main mechanism of action of Alnylam’s ALN-RSV01 for respiratory viral infection is due to innate immune stimulation of the unmodified RNAi trigger, not RNAi-mediated gene knockdown.

As RNAi Therapeutics as a whole has turned the corner in 2012, so has antiviral RNAi Therapeutics. 

This assessment is based on two quality programs that have either made it into the clinic recently, Calimmune’s ddRNAi candidate for HIV (LVsh5/C46), or is close to it (Arrowhead’s DPC-delivered anti-HBV candidate ARC520 for which an IND is planned in Q2 2013).  In addition, there is expectation that Tekmira’s Ebola program will be able to take advantage of the significant improvements in SNALP delivery technology, thereby considerably increasing the odds for an FDA approval under the Animal Rule (note the recentapproval of a second drug under this rule).

Suppressing Immune Suppression

Antiviral RNAi Therapeutics have to overcome the important theoretical limitation that even a potent, e.g. 99% knockdown of a viral transcript or particles may not be sufficient as in theory a single infected cell may fuel viral rebound.  It turns out that rather than blindly aiming at knockdown potency, RNAi Therapeutics are likely to be more successful when targeting an important mechanism employed by virtually all viruses: avoiding detection or removal by the immune system.

In the case of HBV, a disease affecting North of 200 million patients worldwide, the virus produces large amounts of the Hepatitis B Surface antigen (HBsAg).  This is thought to suppress, by acting as a decoy, the development of a productive anti-HBsAg immune reponse.   It is thus widely believed in the industry that reducing HBsAg is required to finally generate a drug that can achieve a functional cure, essentially paralleling the recent developments in HCV.  Interferon-based treatment regimens may actually partially work via this mechanism, but cure rates are rather low and come with considerable side effects in the form of severe flu-like symptoms.  Moreover, protein-targeting anti-HBV agents such as small molecule-based polymerase inhibitors do not seem to reduce HBsAg.  This leaves RNAi Therapeutics as the most promising mechanism of action.

A recent article in PLOS Pathogen suggests that the Ebola virus similarly churns out decoy viral proteins so as to subvert the immune system into making antibody duds that do not effectively remove the real viral particles.  It is therefore intriguing that an Ebola drug candidate by Tekmira should not only aim at providing the immune system with more time, but also that it would facilitate it mount a more effective antibody response.

No Escape

Another attraction of the RNAi Therapeutics approach for viral diseases is the fact that such agents may be more successful in prohibiting the virus to mutate around the drug and thereby escape its actions (viral escape).  Consequently, all antiviral RNAi trigger selection strategies focus on sites that are conserved in the various genotypes and quasispecies.  Even if the virus is successful at mutating around conserved sites, it is then relatively simple to include a second (such as in Tekmira’s Ebola program) or third RNAi trigger targeting a conserved site such that the virus would have to mutate around two sites at the same time- a highly unlikely event.

In addition to these general antiviral mechanisms, RNAi Therapeutics may also work through more virus-specific mechanisms.  Calimmune’s ddRNAi-based HIV candidate LVsh5/C46 for example down-regulates the cellular receptor for viral entry, CCR5, such that HIV particles cannot enter cells and integrate into their genomes in the first place.  As an ddRNAi gene therapy approach, LVsh5/C46 further takes advantage of the fact that you can express a therapeutic protein along with the RNAi trigger, thus uniquely combining mechanisms of actions in a single drug.    

Smooth Sailing Ahead

Of course, it is impossible to tell whether an RNAi Therapeutic will actually overcome a virus in each case and receive regulatory approval.  Nevertheless, I believe that the above candidates for Ebola, HBV, and HIV stand a real chance. 

The Ebola program by Tekmira is arguably the most advanced, and it is difficult for me to see how based on the non-human primate data and the lower dosages required for SNALP delivery, which should widen the therapeutic window, approval can be denied under the Animal Rule.

For the HBV and HIV candidates that are being developed along more conventional regulatory pathways, I  am similarly optimistic that they will generate some excitement in the near-term.  This is because viral load is a powerful biomarker, often also an approvable endpoint, and even early clinical studies should be able to generate such outcome data (if Arrowhead could help it, they should go straight into patients with ARC520). 

After orphan diseases involving the liver and oncology, antiviral applications are therefore poised to become the third major support of the RNAi Therapeutics platform.