Stay "Stereocurious": What The Future Holds For Oligo Quality

By Anna Rose Welch, Editorial & Community Director, Advancing RNA

I grew up as a violinist. I was a fearless performer through the age of 10, capable of performing on stages in front of thousands of people without so much as a quiver of my bow. However, my teenage years proved to be a different story altogether. There were several reasons for this awakening of anxiety — turning into an angsty teenager being the predominant one. But I’ll never forget the conversation I had with one of my instructors as we discussed my increasing anxiety about my playing abilities. He explained: As we get better at something, our understanding of what “good” looks like becomes increasingly more developed. In turn, the bar for achieving “good” — and the feeling of how far away we may be from reaching that bar — also increases.

As I sat taking stock of the numerous technical discussions from TIDES a few weeks ago, my violin teacher’s words came rushing back to me. We are in what I would call a particularly disruptive stage for the industry. Yes, we have a well-established, well-understood manufacturing paradigm that has met our needs for decades — and it isn’t going anywhere. We have more than 20 approved oligo products on the market. Based on these facts alone, it may seem that we’re set in our ways and approaching maturity/already are mature. However, the past few months of discussions in this space have convinced me otherwise. I’d argue we are an industry recognizing that what we’ve built will only take us so far. As such, we have and continue to raise the bar of what is possible for us and our products.

Overall, there were two areas of discussion at TIDES that demonstrate the amount of growth we have yet to do as a space — particularly in the realms of regulatory development and quality. As I intend to showcase here in part 1 of this muti-part article, this year’s TIDES was a great reflection of the tension(s) that can arise as we realize how much space exists between where we are and where we’re going next.

From “Stereorandom” to “Stereopure”: Anticipating The Future of Oligo CMC

The introduction of phosphorothioate chemistry was an essential step toward improving our products’ stability and deliverability in vivo. But as we all know, this improvement also brought with it big questions about the role stereochemistry plays in a product’s efficacy.

While we’ve aligned around the fact that controlling our products’ stereochemistry (the Rp/Sp diastereomer ratio) matters for potency, stability, distribution, and toxicity, etc., the past few years have brought debates around how much control of the phosporothioate linkages is truly necessary to produce the most efficacious drugs. Clinical successes for approved “stereorandom” drugs (i.e., comprising a mixture of Rp/Sp diastereomers), like Spinraza, butt up against examples like Mongersen, whose high-profile clinical failure was linked to a lack of control over the product’s chirality, leading to batches boasting different diastereomeric compositions.

As such, we’re an industry conflicted over whether a mixture of Rp/Sp configurations is good enough, or whether we should be pursuing stereo-defined or “stereopure” molecules (i.e., comprising a single diastereomer). A handful of companies like Wave, Alynylam, and Solstice Biologics have set out to answer this question in the literature, with some — namely Wave — joining “Team Stereopure” after seeing enhanced clinical benefits from greater control. However, there has also been research that hasn’t established a clear link between a stereopure product and increased activity/potency.

“It’s fair to say there are still a lot of unanswered questions, and perhaps just the ability to make stereopure forms more easily might help us answer some of these questions,” Alison Moore, CEO of Codexis, said during a panel on stereochemistry at TIDES, pointing to the benefits of future enzymatic manufacturing paradigms in helping us better control stereochemistry. (SPOS isn’t just limiting in terms of scale, but as many of us know well, it also leaves a lot to be desired in controlling the chirality of the phosphorothioate linkages.)

There are several potential benefits to producing stereopure products — increased durability, heightened potency, lower doses, and, in turn, an overall lower COGS being a few discussed throughout the panel. However, as we can also anticipate, establishing the capabilities to produce stereopure products is an investment to be made early on in development, a factor that may make companies with later-stage products less keen to join “Team Stereopure.” Such an investment increases costs of raw materials and demands innovation on the analytical front, seeing as many of the assays necessary to demonstrate a stereopure compound are not standard today. Likewise, as the mixed research suggests, the benefits of going stereopure may be context/tissues/target and sequence dependent.

Though this panel set out to debate the need for stereopure products, all the panel speakers — even those who profess to be part of “Team Stereocurious” rather than “Team Stereopure” — were aligned around the importance of establishing and demonstrating consistency. While companies like Wave are frontrunners in developing stereopure oligos, the current “state of the art” is to make well-controlled mixtures and to develop an analytical method demonstrating the same ratio/pattern of diastereomers.

“You want to ensure that you’re making a consistent pattern, whether it’s a single diastereomer or a mix all the way through,” Fred Fleitz, VP of CMC and development chemistry for Arrowhead Pharmaceuticals explained. “The worst thing for a completely synthetic molecule would be to have some kind of activity assay on the backside to release because you have too much variability in your lot.” (Worth noting, the latest guidance from NMPA on oligos is the first guidance to outline methods for evaluating stereochemistry. While the agency is not currently recommending a release test for stereochemistry, it does recommend a bioassay when demonstrating comparability to ensure that changes do not impact stereochemical composition/biological effect.)

Consistency may be our goal today. However, even more notably: The panel — Team Stereopure and Team Stereocurious both — agreed that stereopure is likely to be our future.

As several experts pointed out, our manufacturing advancements to increase scale and reach larger patient populations will necessitate improvements in COGS and sustainability. Obviously, one of the ways to do this is to increase a drug’s durability/lower doses. Similarly, as more companies continue engaging regulators and our quality expectations become clearer, we likely will start to see economies of scale work in our favor; a larger number of companies targeting stereopure molecules means lower costs of raw materials and manufacturing overall.

As Jason Costigan, head of CMC for Coursera articulated, “We have to go where the puck is going, and I think that the puck is going toward stereopure products. I foresee in the not-too-distant future that, as more successes are had and regulators see that we can control this, they’re going to start asking why everyone is not manufacturing stereopure molecules.”

“I think it’s highly, highly likely that the regulatory expectation will be that you define a single diastereomer down the road,” Fleitz agreed with Costigan. “It’s not going to be next week; it’s not going to be next year. But that’s likely where we’re going.”