Chemical Engineering of Therapeutic siRNAs for Extrahepatic Delivery
Anastasia Khvorova, PhD
UMass Chan Medical School
Innovations in chemistry fuelling advances in RNA therapeutics
Chemistry innovations are allowing RNA drug developers to look beyond the liver and could one day even yield a cure for the common cold, a leading researcher said.
The field of RNA therapeutics is undergoing a period of advancements, according to Anastasia Khvorova, a professor at the RNA Therapeutics Institute at UMass Medical School. She told delegates at the TIDES USA oligonucleotide and peptides conference about the sector’s expanding pipelines.
“If you think about what is in a clinic or what has been shown a robust efficacy in non-human primate models, there are five classes of conjugates,” Khvorova said.
“There is GalNAc for liver hepatocytes, DC [docosanoic acid] for muscle, fat, heart, and placenta, [and a] C16 conjugate for the lungs and CNS [central nervous system] – Alnylam[i] just announced data [recently] that showed efficacy in CNS indications – multivalent compounds, which, again, are being used in CNS indications.”
Stability
Much of the innovation has been focused on making the drug candidates more stable, so they can be formulated for sustained delivery.
“What is absolutely critical for all those conjugates to work in vivo — specifically in humans — is full chemical stabilization. The reason why it’s absolutely essential … is because upon internalization compounds are entrapped in endosomes and lysosomes,” Khvorova said.
“And this entrapment creates intracellular depos that are slowly released, which is what provides multimonth efficacy. At this point, with a single injection you can get a 12-month duration of effect.”
Khvorova also cited extended nucleic acid chemistry – eXNA[ii] – developed by Ken Yamada, an assistant professor at the RNA Therapeutics Institute at UMass Medical School, as another important innovation in the field.
“In extended nucleic acid chemistry, an extra carbon is introduced in a backbone three prime to the phosphate. And the amazing thing about eXNA chemistry is it’s orthogonal to phosphide modifications, so you combine both chemistries,” she said.
Accumulation
This ability to combine chemistries enhances stability, according to Khvorova, who cited a study that suggested that using eXNA on a modified oligonucleotide can result in a ninefold increase in stability, and enhanced stability impacts tissue accumulation.
“In certain tissues, like the heart, there can be a fifteenfold increase in additional accumulation, and this observed enhancement does result in functional target involvement and the silencing of targets in those hepatic tissues,” Khvorova said.
“There is a lot going on in the chemistry front … and further chemical enhancement of the stability of sRNA, done in a way that doesn’t interfere with assembly, gives you a pass to enhancing extra hepatic delivery, distribution, accumulation, and functional efficacy.”
Distribution control
Lipid head engineering, the practice of modifying the head structures of oligonucleotide conjugates to enhance their characteristics, is another highly active area of research.
Khvorova said, “Internalization of hydrophobic conjugates is not selective, it is driven by a wide range of receptors, which are overrepresented on different cell types.”
While this potentially allows developers to target a wide range of cells, it also makes it harder to control distribution.
This is where lipid engineering can help, Khvorova said.
“I think the major value of this concept of engineering of the conjugates is minimizing liability associated with nonspecific delivery to monocytes and other immune cells.”
SARS-CoV-2
Khvorova also discussed the impact SARS-CoV-2, the virus that started the COVID-19 pandemic, had on RNA therapeutics innovation.
“The whole world was working on SARS-CoV-2, and we did as well. We worked with the Korkin lab[iii] to identify sites that are conservative not only in different patients, but conservative in different families of SARS variants, which have shown up over the last couple of decades,” she said.
The team tested a range of compounds for the ability to block viral replication, in which several showed activity.
“From many dozens of compounds we have screened, only a handful — like three or four — showed profound impact on blocking viral infection in the lungs.”
And the approach has potential to be used more widely, Khvorova said.
“I think this platform right now represent an interesting opportunity and serves as a proof of concept that this sustained silencing in the lung is feasible and can be used as a way to block infection,” she said.
“My dream would be to do something for the common cold because while it’s not life threatening, it impacts quality of life.”
References
[i] https://investors.alnylam.com/press-release?id=27441
[ii] https://www.biorxiv.org/content/biorxiv/early/2023/05/26/2023.05.26.542506.full.pdf
[iii] https://repository.escholarship.umassmed.edu/handle/20.500.14038/51911
