RNAi Therapeutics and mRNA Vaccines for COVID19

This is a time where the biotech industry has to mobilize resources to mitigate the impact of pandemic SARS-CoV-2. 

Nucleic acid-based therapeutics are prime candidates in this battle because of the speed with which drug candidates can be designed and their specificity.  This blog provides an overview of efforts in this area with an emphasis on my two favorite (and IMO most promising) nucleic acid approaches: RNAi Therapeutics and mRNA vaccines.

RNAi Therapeutics

RNAi Therapeutics (and by extension the competing antisense, ASO approach), knocking down genes for therapy, can be used in two ways to address the current pandemic.

First, the RNAi triggers could target the virus itself in an existing infection.  The hope would be that by doing so the damage, mainly lung inflammation leading to respiratory distress, can be mitigated sufficiently for the patient’s immune system to gain time and strength to successfully overcome the infection.  The question then would be how early would be early enough.

The second approach would be to target a host factor critical for viral replication.  The cellular entry receptor ACE2 is the most promising candidate target gene here.  This strategy seems particularly promising for prophylactically protecting those at high-risk of an infection, e.g. medical personnel, as protection may last for a month or so based on the very long durations of efficacy seen in human studies targeting genes in the liver and early studies in the lungs of sheep.

Application in a prophylactic setting would probably have the added delivery advantage in that the lung would be less congested and thus more accessible.  Also, delivery may not have to reach so deep into the lung as it would need to during later stages.   

Delivery

Theory is one thing, but getting the RNAi triggers to where they are needed in the body is and has always been the main challenge for RNAi Therapeutics.  Unlike targeting genes in the liver which is now well-established, RNAi in the lung is a re-emerging area of RNAi development.

Given that COVID19 is a respiratory illness where the virus intrudes the body via the respiratory epithelium, a local, inhaled delivery approach should be adequate and also happens to be the most promising route for RNAi Therapeutics in addressing pulmonary disease.

In the early days of RNAi Therapeutics development, the world was facing the SARS outbreak. Seemingly attesting to the promise of speed and specificity against emerging pathogens, Sirnaomics (based in both the US and China) soon published a high-profile paper on the efficacy of RNAi in a monkey model of SARS coronavirus.  

In hindsight and with the benefit of 15 more years of RNAi Therapeutics development up-and-downs, however, these results were probably based on innate immunostimulatory artefacts given the use of unmodified Tuschl-type RNAi triggers and a nasal instillation route of administration making employing sugar water as a carrier.

Today, however, RNAi Therapeutics is much more advanced and an approach that utilizes aerosolized highly modified and thus stabilized RNAi triggers seems most appropriate. 

Arrowhead Pharmaceuticals has emerged as the front-runner in lung RNAi and is close to filing for clinical study approval for addressing cystic fibrosis.  Their approach combines the two most promising elements of today’s RNAi Therapeutics.  In addition to RNAi trigger stabilization, Arrowhead is adding targeting ligands to their agents, in this case small moieties targeting integrin on lung epithelial cells.

While Arrowhead has not announced their entry into the COVID19 race, Sirnaomics and Alnylam Pharmaceuticals (along with partnered Vir Biotechnology) have.  While it is unclear what particular approach Sirnaomics is using 15 years after their SARS work, Alnylam will be using highly modified RNAi triggers.  Whether targeting ligands will be utilized or not is unknown to me (update: the press release refers to 'conjugates of siRNA' so a targeting ligand is likely). 

Of note, Alnylam had suffered an anti-viral innate immunostimulatory fiasco with their first commercial RNAi development program around the same time that Sirnamoics was working on SARS.  That program was for the respiratory syncytial virus (RSV) and Alnylam should be able to capitalize on the the lung RNAi development experience back then.

Other RNAi-related companies with experience in inhaled lung delivery are miRNA Therapeutics company Miragen (miRNAs are structurally quite similar to RNAi triggers), Genevant/Arbutus (descendants of Tekmira which became famous for their Ebola efforts), and Arcturus Therapeutics.

mRNA vaccines

Arcturus Therapeutics and Genevant are now focused on developing messenger RNA (mRNA) therapeutics and vaccines, including for lung disease cystic fibrosis.  The most known name in this area is Moderna Therapeutics which has a high-profile, government-sponsored vaccine effort against SARS-CoV-2.  Ultimately, it is vaccines that will allow the world to fully recover from the COVID19 scare while therapeutics should be used in a much more focused manner.

The delivery challenge for vaccines is also lessened by the fact that it is a gain-of-function approach and that the immune system itself is expert at spotting foreign, in this case viral antigen expression.  The challenge here is that this is has to be achieved in the right immunological context so that a fruitful immune response is formed.  

The fact that Moderna were the first to ship mRNA for imminent trials shows how easy it is for mRNA to go from viral sequence to product candidate and drug material production.  While I used to poke fun at Moderna for building factories and installing robots without having much science behind it to fill the production halls, for emerging biothreats like SARS-CoV-2 this has proven to be prescient.

CureVac, an mRNA competitor of Moderna, has also announced their entree into COVID19.  While they may lag behind Moderna in terms of robotics and manufacturing capacity, they should be more expert in vaccine development (‘a little innate immune stimulation by the mRNA agent itself may go a long way’).

The most intriguing and differentiated entrant in the mRNA vaccine area to me, however, is Arcturus Therapeutics (disclosure: no stock position, but considering taking one) to me.  This is because they are using a self-replicating RNA ('STARR') that they claim to require ~40-fold less RNA to be intramuscularly injected. Formulation into an LNP 'LUNAR' particle may further lower the required dose.  Given that vaccinating the world will in the end also be a manufacturing challenge, being able to start with 40-times or thereabouts less material is a serious practical advantage.

COVID19 shall pass and therapeutics and vaccines will play an important role here.  Even more important, however, is that everybody does their part in minimizing and slowing the spread of the virus ('flatten the curve', #SocialDistancing), particularly to protect those most at risk without having to resort to draconian measures.