Lightning Fast Wave Life Sciences Demonstrates High ADAR Editing Rates Across Targets

There was a time, not that long ago, when 1-3% ADAR editing rates in tissue culture cells were typically reported in the field.  The hope then was that with further chemical optimization, editing rates could be increased high enough to have a clinically relevant impact in the setting of a gain-of-function approach.  Mathematically speaking, go from nothing to something is an immeasurable relative increase.

In the realm of biology, this is pertinent to diseases like Duchenne Muscular Dystrophy or Spinal Muscular Atrophy where relatively small, 10-20% target engagement by RNA Therapeutics have been demonstrated (splice modulator SPINRAZA) or are expected (exon skippers) to have big disease modifying activity when given to patients essentially genetically null (=not expressing) dystrophin and SMN1, respectively.

In theory, the upper limit of ADAR Editing should be extremely high since near-complete editing of ion channel and neurotransmitter receptor pre-mRNAs are seen in neurobiology.

The perception that ADAR Editing mediated by oligonucleotides could be generally low in clinical applications started to shift when Monian and colleagues at Wave Life Sciences reported in a groundbreaking Nature Biotech paper a year ago robust 50%+ editing rates of the alpha-1 antitrypsin Z allele.  It suggested that this can be achieved by painstaking chemical optimization at a ‘lucky' target site.  This is not much different from how small molecules get chemically matured following an initial low-affinity, low-specificity hit.

For the type of blockbuster market opportunity like alpha-1-antitrypsin this effort is well worth it.  Still, finding potent editing-enabling oligos in an efficient manner would open many doors such as testing scientific hypotheses faster, especially as the ADAR Editing pioneers sift through their list of candidate targets and indications.

Enter Wave Life Sciences and their PRISM platform.  I have never quite understood what exactly is behind this platform for oligonucleotide discovery (RNAi, exon skipping and ADAR Editing).  It does appear, however, to test many combinations of chemical modifications at the base, sugar, and internucleotide linker while considering their chemical neighborhoods and stereochemistry, ultimately coming up with design principles for potent oligonucleotide therapeutics candidates.

Certainly, given that modern lab automation allows for increased throughputs (see biotechtv tour of Aera Therapeutics), such a Big Data, Artificial Intelligence approach to oligonucleotide drug development makes a lot of sense and can give biotech companies crucial competitive advantages in terms of time and better molecules.  

Two chemistry insights regarding ADAR technology that have emerged from PRISM stand out so far.  Firstly, the zwitterionic phosphorylguanidine (PN) backbone linker, preferably in a stereopure format.  The PN chemistry is shown to allow for improved unassisted cellular uptake into various cell types in mice (immune cells, various liver cell types, renal cells etc) while stereopurity brings advantages in terms of ADAR recognition of the duplex substrate.  This sounds similar to the morpholino chemistry that has proven to be quite safe following systemic administration (e.g. eteplirsen by Sarepta), but is very rapidly eliminated from the circulation into urine.  It will therefore be important to show in future studies that the dose demands for oligos with predominantly PN backbone are not as high.

With regard to the orphan base, the base opposite the adenosine to be edited, Wave is honing in on deoxy-N3 uridine as its preferred chemistry.

In addition to alpha-1-antitrypsin (now partnered with GSK), robust 50-90% editing is demonstrated for a number of clinically relevant genes such as UGP2 (à epileptic encephalopathy 83), and Nrf2-Keap signaling (stress regulation in chronic disease).  For the latter two targets, not just one, but several highly potent editing oligos could be identified.  This reflects increased emphasis by Wave in applying RNA Editing to targets that are not for correcting specific mutations, but where the goal is to increase expression of a protein where it could be helpful, but without necessarily changing its inherent function or sequence. 

This allows Wave to scan for potent editing oligos often along the entire target (pre-)mRNA instead of being limited to just one site!  Also, a number of genetic diseases are caused by a various mutations dispersed throughout a gene so that a mutation-correction approach may require the development of multiple editing oligos and some sites will not be amenable to AàI approaches at all.  In the end, this increases the market potential of a given oligonucleotide.

In summary, being able to consistently achieve 50%+ editing rates will be sufficient for most therapeutic editing approaches.  Going from say 2% of something to 50% is a 25-fold increase, but maxing out at 100% for another 2x may not give that much of additive benefit.  Of course, biological pathways can sometimes be complex and the responses may not be as linear.  At 80%+ editing, diseases caused by dominant-negative mutations would also come within the realm of therapeutic possibilities of ADAR editing and this includes liver-related diseases of piZZ alpha-1-antitrypsin, but here ADAR Editing may face inherently more potent knockdown mechanisms such as RNAi.

Disclosure: I am long WVE, as I am impressed by the speed with which they have chemically matured editing oligos and their dystrophin exon skipper is showing intriguing early clinical results (RNA, not protein level).  However, I have not taken a full position yet as I want to see the company first demonstrate robust target engagement in the clinic (including for the dystrophin exon skipper).  So far, all the clinical results have greatly disappointed with claims of 10-20%-type target knockdowns

Korro Bio To Become Third Publicly Listed ADAR Editing Company

Today, pure-play ADAR Editing Korro Bio announced that it will reverse merge into biotech shell Frequency Therapeutics (current ticker: FREQ, to be changed to KRRO).  It will thus become the 3^rd publicly traded RNA Editing company following ProQR (pure-play) and Wave Life Sciences, the latter entertaining a broader mix of oligonucleotide therapeutics modalities (ADAR editing, exon skipping, RNAi).

Following two private founding rounds of ~$210M in 2020 and 2022, the transition into the public markets which is being accompanied by another $117M cash injection from mainly existing venture backers led by Surveyor Capital and Cormorant Asset Management, is to prepare the company making the transition to the clinic.  Its lead candidate is to address both the lung and liver manifestations of alpha-1-antitrypsin disease (AATD) caused by the prevalent piZZ genotype.

Today’s development explains their surprising announcement earlier this year to adopt liposomal delivery instead of GalNAc-targeted chemically modified oligonucleotides as is practiced by industry leaders ProQR and Wave Life Sciences.  Without the prospect of a clinical candidate, such an ‘IPO’ would not have been possible. 

As I had noted in an earlier blog entry though, such a development candidate is likely to fail both from a clinical and commercial point of view.  Firstly, for AATD patients at high risk of liver disease or actually manifesting liver disease, a chronically administered LNP seems like a bad idea. Indeed, Korro Bio today revealed significant liver enzyme elevations in animal models at doses (2mg/kg) that are likely required for robust SERPINA1 editing and are substantially higher than what is used for the clinically approved MC3-based LNP formulation Patisiran in ATTR amyloidosis by RNAi Therapeutics company Alnylam.

From a competitive point of view, the LNP approach suffers from the need of frequent, possibly weekly intravenous infusions whereas less frequent (I expect monthly) subcutaneous administration schedules should be feasible with Wave’s first clinical GalNAc editing oligo and possibly less frequently as oligo chemistry advances (similar to RNAi).  As Wave is likely to be a year ahead of Korro in the clinic, this alone makes it a head-scratcher approach for a fast-follower.

As we have learned from the RNAi Therapeutics field, further stabilizing Korro’s oligonucleotide is unlikely to extend dosing frequency as LNPs release most of their cargo into the cytoplasm almost instantaneously whereas the long duration of action by oligo-conjugates is explained by their gradual release from endosomes.

There is one scenario, however, where I can see Korro Bio’s candidate to have staying power, namely in being the only approach among the ADAR Editing and CRISPR genome editing and gene therapy candidates that can successfully treat both the lung and liver manifestations of AATD by achieving >90%-type editing levels.  As we have learned from Arrowhead Pharmaceuticals' liver AATD program (partnered with Takeda), even with near complete removal of toxic alpha-1-antitrypsin expression in the liver, prolonged treatment will likely be necessary to see a robust clinical response (phase 3 involves >3 years of dosing).

It is for this reason that I view the ADAR and genome editing approaches mainly aimed at those suffering from lung disease (which RNAi cannot address), including those with mixed phenotypes.  I will discuss clinical development landscape further in my next blog entry.

What I most like about Korro Bio, a prolific IP filer, is their mix of ADAR Editing programs encompassing genetic correction for AATD and Parkinson’s (LRRK2), anti-protein aggregation (TDP43 in ALS), modulating ion channel (NAV1.7 in pain), disrupting protein-protein interaction in alcoholic hepatitis and activating kinases.  But to extract the full value from applying RNA Editing to these attractive disease areas, Korro Bio needs to catch up on oligonucleotide chemistry and designs.