Oligonucleotide Strategies beyond the Liver

As the first major wave of pivotal trial successes involving gene knockdown in the liver has reached the shores, oligonucleotide therapeutics are quickly establishing themselves as the dominant modality in this important target organ for new drug development.  While the industry is milking this organ for therapeutic applications, my thoughts are directed towards the next tissue opportunities that should further feed this revolution in drug development we are witnessing now.

After not having been part of the conference circus for 2 years, this week’s Oligonucleotide Therapeutics Society annual meeting in Bordeaux, France, was just what the doctor ordered for me to get a clear perspective on this issue.  Importantly, lessons learned from GalNAc-targeted oligonucleotide delivery to hepatocytes, but also LNP delivery to the liver prior to this, now allow the field to take the next step in extending delivery beyond the liver.

Chemical stability

All these efforts essentially share their use of highly modified oligonucleotides which have particularly changed the philosophy around the RNAi modality.  This allows the oligonucleotide to not only reach the target tissue intact, but also to remain trapped in endosomal compartments which serve as slow-release depots for long duration of action.

Chemical modification also has greatly reduced the immunogenicity of RNAi triggers and obviated the need for protective nanoparticle formulations.  These often came with the added liability of amplifying the immunostimulatory potential of these molecules.  Consequently, decade-old approaches are now being revisited with more fully modified RNAi trigger versions (e.g. self-delivering RNAi trigger structures as pursued by RXi Pharmaceuticals and the Khvorova group at UMass).

Ironically, after all the song and dance by RNAi bellwether Alnylam about the utility of exotic modifications in their conference presentations (one can also call it willful misleading of the field- not really the purpose of scientific conferences), the strong trend is towards maximizing 2’-O-methyl content, in addition to some 2’-F and phosphorothioation at the RNAi trigger termini.

PK enhancers

One of the reasons why the liver became the first major oligonucleotide target organ is that it is readily accessible from the blood.  This allows it to soak up oligonucleotides before they get removed by renal filtration.  In an effort to fight this tendency, the use of PK enhancers, in particular lipophilic groups is frequently seen.  Ionis and Alnylam are testing these for example for getting better distribution to the muscle and potentially also better functional uptake.

Receptors

PK enhancers, however, are largely about shifting around biodistribution, but it is hepatocyte ASGPR-type receptors that are the most valuable assets the industry is striving to identify.  My highlight of the conference therefore was a talk on a collaboration by AstraZeneca and Ionis demonstrating strikingly selective and effective targeting of beta cells in pancreatic islets. Think diabetes! 

It is the GLP1-receptor that does the magic here and which can be targeted by GLP1-peptides for effective oligonucleotide uptake.  While the in vivo validation was limited to rodent models, including an elegant GLP1-receptor knockout mouse model, I am convinced that the findings will translate to larger animals and humans.  It is one of those things you just know when seeing such data.

This example illustrates the value of knowing your target cell type really well, as this may allow you to identify additional ASGPR-type receptors which had been thought of elusive.  But even if they are lacking in some tissues, a nice, yet simple strategy to overcome this was illustrated by MPEG LA and Axolabs: by linking more than one RNAi trigger to a small scaffold, they were able to show that cellular oligonucleotide uptake capacity can be increased beyond the limits of receptor amount on the cell surface.

 

While the liver certainly does not need this strategy, it should definitely be applied to new targets like the beta cells.  Let free market competition do its magic and have oligonucleotide therapeutics solve diabetes now that you can effectively reach hepatocytes, adipocytes, and now also beta cells. Yes, I love the free markets, but I digress… 

After beta cells, it was a collaboration between Alnylam and Johnson & Johnson on overcoming the long-held dream of oligonucleotide therapeutics addressing gene regulation in cancer cells.  Here, small and stable ~2nm peptide scaffolds referred to as centyrins were coupled to the RNAi trigger and directed towards different receptors like PSMA and EGFR.  Perhaps the most striking aspect of centyrin-siRNA conjugates was their effective tumor penetration where prior RNAi delivery attempts like LNPs had fallen short.

Endosomal release

Sometimes getting to the endosomes alone is not enough when the rate of cytosolic release therefrom is insufficient.  So despite of the DPC fiasco last year and despite of aborted arginin-based endosomal release attempts prior to this, active endosomal release is still embraced in some delivery efforts.  Most notably, Sarepta has shown dramatic increases in dystrophin exon skipping in non-human primates with new peptide-PMOs (PPMOs) compared to their unconjugated parent molecules.

Of course, everybody now wants to know what the therapeutic window really is.  While the Sarepta representative at OTS was a bit cagey when asked about it, Sarepta’s CEO noted in a recent investor presentation that the filing of an IND by the end of the year would be a major positive signal in that regard.

Finally, all of the above developments are aided by more general progress in technologies interrogating biology such as single cell technologies (cell type isolation from complex tissues like the kidney), reduced chemistry costs allowing for much larger numbers of oligonucleotides to be screened, and ubiquitous low-cost and high-throughput sequencing.

 

Sometimes I pinch myself asking whether all this is real and not just a figment of my imagination, but at least in my mind the stars just keep aligning allowing for RNAi and oligonucleotide therapeutics to take the next step up the value ladder.

RNAi Therapeutics: Qiagen High-Throughput RNAi User Meeting San Francisco (Part 1 of 2)

An army of RNAi enthusiasts descended today onto the UCSF Mission Bay Campus in San Francisco, just across the street from Sirna Therapeutics/Merck, to discuss the latest in the use of RNAi for high-throughput screening. Illustrating the ever increasing popularity of RNAi, 175 scientists registered for this year’s one-day meeting, some from as far as Japan, compared to 75 when it was held for the first time two years ago in Boston. It was therefore fitting that a number of presenters noted as an aside that the discovery of RNAi and microRNAs not only transformed molecular biology, but also a number of lab personnel from bench-weary scientists to those that live and breathe RNAi every day. In the next two posts, I will try and highlight a few points from the meeting seen through my RNAi Therapeutics filter.

Carl Novina (Harvard and scientific advisor to mdRNA/Nastech) gave the keynote address with an introduction to microRNAs and their molecular mechanism of action. In a paper earlier this year, his lab had developed a biochemical model system to study microRNA repression that led them to conclude that microRNAs repress translation through the inhibition of the joining of the large 60S ribosomal subunit to the mRNA-bound 40S subunit of the protein translation apparatus. In today’s presentation, he expanded on that work by describing an RNAi-based screen for the discovery of genes involved in microRNA function. For this, his lab created luciferase cell lines in which one of the reporters contained a target site for an endogenously expressed microRNA and was therefore repressed. Knocking down a gene contributing to microRNA-mediated repression should therefore lead to an increase in the signal intensity from that reporter gene.

Encouragingly and attesting to the high specificity of RNAi, the screen uncovered many of the known microRNA pathway components such as Drosha, Dicer, and Argonautes 1 and 2. In addition, the screen making use of an siRNA library targeting the “druggable genome”, that is the ~7000 genes in our genome encoding for kinases, cell surface receptors etc. that are amenable to small molecule and/or monoclonal antibody inhibition, uncovered a number of proteasomal components. This raises the intriguing question whether an old hypothesis, namely that microRNAs may induce co-translational protein degradation may indeed be part of the complicated reality of the molecular microRNA repression process.

As an aside, Carl Novina noted that one problem with working on RNAi-related aspects in Boston is the considerable demand for talent there by the many RNAi companies located there. A nice problem to have for the many hungry post-docs at a time when public funding for basic research has stalled to pay for apparently more important projects overseas.

One recurrent theme of the conference was the role of small RNAs in the new era of personalized medicine, particularly for cancer, which not coincidentally also was the loud message heard from last week’s ASCO mega meeting. The case was made by Lynne Bemis (University of Colorado) as she compared the costs of today’s monoclonal antibody cancer therapeutics that run at about $30k per annum with clinical responses in only a subset of patients. To avoid patients wasting precious time on ineffective therapies and also to reduce healthcare costs, it is therefore important that these treatments are accompanied by diagnostics that can predict the response of a patient to the therapy, by the way an area first pioneered by the local Genentech.

The striking conclusion from her research was that microRNAs were by far emerged the best biomarkers for this purpose, far better than single protein or mRNA diagnostics. When compared to proteins this is likely due to the fact that microRNAs function as master regulators, often with direct and critical functions in cancer itself, while a single protein may only reflect a much smaller aspect of a much larger picture. When compared to mRNAs, it is the relatively high stability in most tissue preservation methods and their compatibility with formalin fixation that adds to the competitive advantage of microRNA diagnostics.

As an example, EGFR tyrosine kinase inhibitors (TKI) are now widely used for cancer therapy, yet only 10% respond to treatment. Lynne’s team therefore looked at whether EGFR protein levels, EGFR mutations, or genomic amplification levels of the EGFR gene would be good predictors of drug response. Unfortunately, all of them either failed to do so (EGFR levels) or were not practical (EGFR mutations). Running out of options, they then turned to microRNAs. First they made use of microRNA target prediction software to discover two microRNAs that might target the EGF receptor mRNA. They then hypothesized that in cancers where EGFR had been aberrantly activated one of these microRNAs was lost. As an interesting technical aspect, unlike most other microRNA diagnostics currently being developed by Asuragen, Rosetta Genomics, Exiqon and others, they did not make use of qRT-PCR or microRNA microarray for the detection of microRNA expression levels, but the even simpler pPCR from DNA isolated from tumor tissue microdissected from formalin-fixed samples.

Amazingly, from a sample of 60 tumors of the lung, the miR-128b gene dosage was able to nicely segregate those patients that responded to TKI treatment versus those that did not. None of the other methods tested came anywhere close. As, if not even more spectacularly, it turns out that miR-128b is located on a region of the short arm of chromosome 3 that had been previously implicated and was frequently deleted in lung cancer (around 90% of late-stage lung cancers), but of which the identity of the underlying gene had long been a mystery.

In another elegant study, her group employed mRNA microarray analysis to look for genes either over- or under-expressed in a panel of cancer cell lines. Once identified, microRNA target prediction was used to come up with potential clusters of genes being targeted by the same microRNA. Demonstrating yet another promising method of discovering microRNA diagnostic candidates, 5 out of the top 7 overexpressed genes shared a common microRNA signature, and sure enough, this microRNA alone was more predictive of drug responsiveness than any of the proteins alone.

Eric Lader from the host Qiagen rounded up the session on microRNAs with an overview of products for the quantitation and inhibition of microRNAs sold by Qiagen. Qiagen’s qRT-PCR quantitation method allows for the one-step conversion of all microRNAs into cDNA so that many individual microRNAs can be measured from this one cDNA sample thereby reducing the use of precious clinical material. As to the microRNA inhibition technology, a number of nucleotide modifications were tested in the antagomirs and it was interesting that while a number of them were able to inhibit microRNA function, depending on the modification, they probably did so at various steps of microRNA biogenesis and function and included both microRNA degradation and competitive binding. LNAs and other chemistries with high melting temperature generally worked best while RNase H-dependent antisense did not appear to be an effective microRNA inhibition technology.

In my next post, I will summarize the rest of the meeting including more RNAi screens, cancer, and systems biology. Stay tuned…