Ninety-seven percent of your LNPs never deliver

A lipid nanoparticle that reaches the right cell managed to complete the easy part. It is taken up into an endosome, from there the cargo has to escape into the cytosol or face lysosomal degradation. Two independent studies put the fraction that actually achieves release at 1 to 2% (electron microscopy of gold-labelled siRNA) and around 3.5% (galectin-8 recruitment imaging). Those numbers describe formulations that work. Thus, successful mRNA delivery is largely a story about the few percent.

This single fact might be well known but nevertheless reorders the field’s priorities. A 10% improvement in cellular uptake moves a number that is already large. In contrast, a 10% improvement in endosomal escape moves the one number everything downstream depends on, it may well be the closest thing LNP delivery has to a rate-limiting step.

The great escape

A 2026 review in Materials Today cross-referenced seven trafficking studies spanning different ionizable lipids, cargos, particle sizes and cell types. It asked which compartment the endosomal escape actually occurs from and proposes a marker signature as the most plausible reading across all seven: EEA1-negative, Rab5-positive, Rab11-negative, Rab7 either way, LAMP1-negative. This is a hypothesis synthesized from heterogeneous studies, not the result of one confirmatory experiment, and the authors leave open whether the Rab7 ambiguity is incidental. In more accessible terms which align with the consensus knowledge: endosomal escape seems to happen mostly after the early endosome has matured and while the compartment still carries the early machinery, before it is diverted into recycling, before lysosomal fusion. A very narrow window in an ever advancing process.

The competing fate turns out to be as interesting as the productive one. Blocking endocytic recycling with small-molecule inhibitors nearly doubled the escape as a substantial share of the LNP seems to be not destroyed but simply routed back out of the cell.

A gap in methodology

This part carries a lesson that goes beyond nanomedicine. The seven studies vary independently in ionizable lipid, cargo, particle size, cell type and observation time, which is a subtler problem than each of them being wholly different. Some overlap: HeLa recurs often enough that the review proposes it as a reference line. But no two studies hold the other four dimensions fixed, so their escape numbers cannot be compared directly. The same phenomenon was measured in seven differently arranged settings and the studies much rather yield a range of values than a comparable series.

The review’s sobering proposal seems unglamorous but necessary: agree on common benchmarks. Named cell lines, fixed time points, reference formulations at defined molar ratios. Doing so would allow for a new ionizable lipid to be reported as a delta against something everyone else has also measured. New quantification methods are arriving too (FRET pairs on the mRNA itself, iron-oxide relaxometry, a barcoded in vivo assay that reads escape in organs rather than in culture). Embedded in a standardized setting, each of them would be even more useful.

My takeaway: when a field’s central metric cannot be compared across labs, standardizing the measurement is worth a lot more than another incremental improvement to the thing being measured. At its core, this is a data problem wearing a lab coat, and it rhymes with what AssayVault also does for formulation data: with a comparable record set, the whole body of work starts adding up.