Published in Nature in September 2022, Ken Garber explores the hopes of mRNA companies who are setting their sights back on therapeutic proteins. Disrupted by the COVID-19 pandemic, these companies are “doubling down on the goal of converting human cells into protein factories to treat disease”.
The recognised success of the COVID-19 vaccines produced by companies such as Moderna and BioNTech/Pfizer has “validated the mRNA technology platform”. Thus, with renewed enthusiasm these companies return to the “growing pipeline” of therapeutics.
The article suggests that, in comparison with gene therapies, mRNA offer “potential safety advantages” because “there is no genome integration or permanent modification”. Furthermore, Garber identifies manufacturing benefits, and the “opportunity to improve drug characteristics”.
“Unlike recombinant proteins, which are confined to the extracellular space, mRNA can reach any subcellular compartment.”
A “major process advantage” is the fact that it doesn’t need living cells, an improvement on recombinant proteins and lots of vaccines. However, “synthetic mRNA is intrinsically unstable in vivo”. Understanding mRNA in order to develop the COVID-19 vaccines “culminated decades of research”. These vaccines use nucleoside modifications to the sequence to “improve stability”. Intracellular delivery is facilitated by “encapsulation in lipid nanoparticles”.
However, the article suggests that we are facing “several technical obstacles” to furthering the progress of therapeutic pursuits. Trials have been terminated for “lack of efficacy”. Moderna’s advanced molecule was dropped by AstraZeneca. Although mRNA therapeutics “are composed of the same basic package” as the vaccines, it’s far easier to make vaccines work.
“As vaccine euphoria gives way to therapeutic reality, drug developers in the field are crafting solutions to the problems – particularly related to delivery, immunogenicity, and duration of protein expression, which present much greater hurdles for mRNA therapeutics than for vaccines.”
The article emphasises the quantity of protein required to treat disease. This, “implying heavier and more frequent dosing”, increases the risk of toxicity. This toxicity is “mainly immunological”. The immune system identifies unmodified mRNA as viral RNA, “triggering pattern recognition receptors”. This leads to “type 1 interferon and inflammatory cytokine production”.
In 2005, researchers at the University of Pennsylvania reported a “key advance”. They incorporated pseudoridine nucleosides to “dampen immunogenicity and boost translation”. Although mRNA vaccines are supported by “some innate immune system activation”, it’s not the same for therapeutic purposes.
“Immunogenicity is an absolute liability for an mRNA therapeutic”.
Solving this has historically taken two routes; companies “incorporate non-natural nucleosides” or use “sequence optimisation”. The former conceals the mRNA from the immune system. The latter “typically involves switching out uridine nucleosides for guanosines and cytidines in specific codons”.
Another toxicity issue identified in the article is the “accumulation of lipid nanoparticle carriers in the liver”. This might have been addressed by more recent lipid nanoparticles, “engineered for rapid degradation”.
Toxicity is not the only hurdle to overcome. mRNA is “inherently short-lived”, representing another challenge for therapeutics. An enhancement can be enabled through “nucleoside modifications” and “sequence optimisation”. A promising development is Circular RNA (circRNA), which is “very long-lived”. Companies are described as “enthusiastic” at the prospect that this could lengthen the duration.
Some, in attempts to “get around” the issue, are targeting diseases with already long-lived protein expression, or where “durable expression” isn’t a requirement. For example, primary ciliary dyskinesia is the object of Sanofi and Ethris’ interest.
To add to the list of obstacles, the article questions whether mRNA therapeutics can “get beyond the liver”. One solution might be to “engineer lipid nanoparticles with affinity for other tissues”. However, rodent models are “unreliable predictors” of how this might work for humans. Another option incorporates microRNA target sites into mRNA UTRs to achieve “some selective expression”.
The article concludes with a question: “how much further can mRNA engineering take the field?”. For John Androsavich of Pfizer, we should be aspiring to “progress, not perfection”.
“I don’t think there is one discovery that will make mRNA rock solid”.
He believes it requires a “combination of innovations” to produce an optimised product.