mRNA therapeutics: from pandemic response to personalised medicine

Ionisable lipids are the engine. They are neutral at physiological pH, which keeps the circulating particle tolerably non-toxic, and become protonated in the acidifying endosome, where they fuse with the endosomal membrane and release mRNA into the cytosol. The apparent pKa of the ionisable lipid — typically targeted to 6.2–6.5 — is the single most predictive design parameter.

Cholesterol provides structural rigidity and stability in circulation. The helper lipid, often DOPE or DSPC, shapes the packing and sets the phase behaviour of the bilayer. The PEG-lipid sits on the surface, controlling particle size and preventing aggregation and opsonisation, at the cost of reducing cellular uptake if over-used.

LNPs are assembled by rapid mixing of an ethanol phase containing the lipids with an aqueous phase containing mRNA at acidic pH, followed by buffer exchange. Microfluidic mixing gives tighter size distributions than bulk methods, and downstream tangential-flow filtration removes the ethanol and concentrates the product.

Quality control is what separates a laboratory formulation from a therapeutic: particle size and polydispersity by dynamic light scattering, zeta potential for surface charge, encapsulation efficiency by the RiboGreen assay, mRNA integrity by capillary electrophoresis, and potency in a relevant cell-based assay.

The remaining problem is targeting. After intravenous administration, LNPs accumulate mainly in the liver because of ApoE adsorption and hepatocyte uptake — which is ideal for protein replacement in the liver and for vaccines delivered intramuscularly, and unhelpful for almost everything else. Selective organ targeting, added anionic or cationic components and surface ligands are all being used to redirect particles to lung, spleen or specific cell types.

Personalised cancer vaccines produce a patient-specific mRNA encoding neoantigens and deliver it in the same LNP architecture — the manufacturing timeline, not the biology, is the limiting factor. In vivo gene editing delivers Cas9 mRNA and sgRNA in a single particle, turning the liver into a transient editing compartment and avoiding ex vivo cell handling entirely.

Inhaled and nebulised LNPs extend the platform to the lung; targeted LNPs carrying CAR mRNA are being evaluated for in situ T cell engineering in cancer immunotherapy. Each of these applications re-opens the same delivery question: getting the particle to the right cell, not just getting the mRNA into a cell.