> delivery mechanism: you need to take a very unstable molecule, protect it from the environment - both external, and when inside the patient - and insert it into a human cell. (This is called the "platform", and is usually developed independently from the specific payload.)
Of note, the immune system is pretty good at destroying foreign mRNA so you also need to evade it.
I wouldn't even say the immune system, your body has a ton of nonspecific RNA-digesting enzymes floating around to patrol for exactly this sort of thing happening, even by accident, as cells can sometimes rupture. It's a problem enough that good RNA researchers have a reputation of being clean freaks. Some RNA labs I've been in had a lingering, slightly sweet smell, that's the nonspecific RNAase inhibitor that gets sprayed on everything.
RNA is also just generally fantastically unstable and reactive. You don't want any surface to be too alkaline, for example. There's a reason that basically every life form switched to DNA.
(Though RNA may have been more stable under the high-UV-exposure conditions the early Earth.)
Of note, the immune system is pretty good at destroying foreign mRNA so you also need to evade it.
This article is pretty good: https://berthub.eu/articles/posts/reverse-engineering-source...