There is a common assumption in biotech that early-stage LNP characterization requires either a fully equipped analytical lab or an immediate CRO relationship. Neither is true, at least not for the characterization questions that matter at the stage where you are choosing between candidate formulations rather than generating regulatory submission data. This post is about what we can learn before we have access to a CRO budget, using methods that are accessible at university core facilities or with modest in-house equipment.
We are not suggesting you skip CRO-grade characterization for IND-enabling studies. You cannot, and you should not try. ICH Q1A(R2) and the FDA's guidance on nanoparticle characterization for drug products require a level of analytical rigor that is not achievable without validated instruments and documented methods. What we are describing is the selection-phase characterization that precedes that — the work that tells you which of five candidate formulations is worth advancing to rigorous GLP characterization.
Dynamic Light Scattering: Size Distribution as a First Filter
Dynamic light scattering (DLS) is the first characterization measurement we run on every LNP batch we prepare. The technique measures Brownian motion of particles in solution and converts the autocorrelation function of scattered laser light intensity into a hydrodynamic size distribution using the Stokes-Einstein equation. For LNPs, we are interested in two numbers: the Z-average diameter (intensity-weighted mean diameter) and the polydispersity index (PDI).
Our target range for brain-directed LNP candidates is a Z-average diameter of 80–120 nm with a PDI below 0.15. The size target is set by what the literature suggests is optimal for potential transcytosis at brain endothelium — particles much larger than 150 nm are likely too large for receptor-mediated endocytosis at the BBB, while particles below 50 nm present formulation stability challenges. The PDI criterion filters out polydisperse preparations that likely contain a mixture of aggregates and small particles, which would complicate downstream interpretation.
A DLS instrument that meets our needs costs roughly $30,000–$80,000 for a benchtop Malvern Zetasizer or equivalent (a worthwhile purchase for any lab running LNP work regularly), but access to DLS instrumentation is available at essentially every research university materials characterization core. Turnaround for a 10-sample batch is a few hours including instrument time.
DLS has real limitations that are worth stating. It is intensity-weighted, which means large aggregates (even at low number concentration) can dominate the Z-average and mask a population of correctly sized particles. A preparation that looks acceptable by DLS Z-average might contain a small aggregate population that would be problematic in vivo. Number-weighted distribution from DLS is less reliable than intensity-weighted for LNPs due to the physics of light scattering at this size range. For this reason, DLS size data is necessary but not sufficient — it needs to be supplemented by direct visualization.
Cryo-EM: Morphology and Internal Structure
Cryo-electron microscopy (cryo-EM) provides direct visualization of individual LNP morphology at nanometer resolution. The sample is vitrified (rapidly frozen in liquid ethane at -196°C) in a thin film of buffer, then imaged in the transmission electron microscope at cryogenic temperatures. Unlike negative-stain EM, cryo-EM preserves the native hydrated state of the particles without staining artifacts.
For LNP candidate screening, cryo-EM answers questions that DLS cannot. The internal structure — whether particles are unilamellar (single lipid bilayer encapsulating aqueous core), multilamellar (concentric bilayer shells), or in an inverse hexagonal/cubic phase — matters significantly for encapsulation efficiency and mRNA release kinetics. Ionizable lipid LNPs at neutral pH often adopt an electron-dense core with an inverse hexagonal or disordered lamellar internal organization rather than a classic unilamellar vesicle morphology, and this internal organization is thought to be important for endosomal escape.
We have found cryo-EM particularly useful for detecting batch-to-batch differences that DLS misses. Two LNP batches with similar Z-average diameters (say, 95 nm and 100 nm) and similar PDI can look dramatically different in cryo-EM: one batch producing well-defined discrete particles and another producing fused or aggregated structures that DLS registers as only slightly larger. When an LNP formulation shows good DLS numbers but poor in vitro transfection, cryo-EM is typically the first thing we look at.
Access to cryo-EM at early-stage is the constraint. The instrument is expensive (roughly $3–7M for a modern 200–300 kV cryo-TEM), sample preparation requires a vitrification plunger (Vitrobot, Leica GP2, or equivalent at ~$80,000), and operating the instrument requires trained personnel. However, cryo-EM user facilities are now available at most research universities and many national facilities, and the cost per session (typically $150–400/hour instrument time, plus operator time) is accessible if you are running targeted candidate comparisons rather than routine QC. We typically run cryo-EM on the top 2–3 candidates identified by DLS and encapsulation efficiency, not on every batch.
Encapsulation Efficiency: The RiboGreen Assay
Encapsulation efficiency (EE%) — the fraction of input mRNA successfully encapsulated within LNPs rather than free in the external aqueous phase — is a critical parameter that DLS and cryo-EM do not directly measure. An LNP preparation with perfect size distribution but 40% EE contains a large fraction of naked mRNA that will be rapidly degraded by serum RNases in vivo and contributes nothing to therapeutic efficacy while adding potential immunogenicity.
The standard method for mRNA EE% measurement is the RiboGreen assay (Quant-iT RiboGreen RNA Reagent, Thermo Fisher). RiboGreen is a fluorescent intercalating dye that binds RNA and dramatically increases fluorescence upon binding — it does not penetrate intact lipid membranes, so intact LNPs yield only background signal from the dye. By measuring fluorescence before and after disruption of the LNP (typically with Triton X-100 at 0.5% final concentration, which solubilizes the lipid membranes and releases encapsulated mRNA), you obtain the ratio of encapsulated to total mRNA.
The calculation is straightforward: EE% = (1 - F_intact/F_disrupted) × 100, where F_intact is the fluorescence of intact LNPs with RiboGreen, and F_disrupted is the fluorescence after Triton X-100 addition. An EE% above 85% is our threshold for advancing a formulation candidate — below that, the free RNA fraction is large enough to complicate interpretation of any subsequent biological assay.
Practical notes from our work: RiboGreen is sensitive to the LNP buffer composition. Residual ethanol from the LNP preparation can affect lipid membrane integrity and give falsely low EE% readings. Dialysis or spin-column purification of the LNP preparation before the RiboGreen assay is important for accurate results. We also run the assay in duplicate for each condition (intact vs. disrupted) and calculate EE% from the mean of at least three independent LNP preparations before concluding anything about formulation performance.
In Vitro Transfection in HEK293T
After size, morphology, and EE% characterization confirms that a formulation is physically acceptable, we run a basic in vitro transfection experiment in HEK293T cells to confirm that the LNP can actually deliver functional mRNA and produce the target protein. HEK293T (Human Embryonic Kidney 293 cells transformed with SV40 Large T antigen) are the standard first-pass cell line for this because they are easy to culture, transfect efficiently, and have a well-characterized baseline transcriptome.
We use eGFP mRNA or firefly luciferase mRNA as a reporter to decouple delivery efficiency from target-protein-specific biology. The assay protocol is straightforward: seed HEK293T at 50,000–100,000 cells per well in a 24-well plate, allow 24h for adherence, add LNP-mRNA at a range of lipid doses (typically spanning 0.01–1.0 µg mRNA per well), and read out fluorescence or luminescence at 24h and 48h post-treatment.
The dose-response curve shape tells us more than the peak expression value. A formulation that shows a flat response at low dose followed by a sharp increase at high dose suggests poor endosomal escape efficiency — you need to overwhelm the endosomal buffering capacity before you see release. A formulation with a gradual dose-response starting at low dose suggests efficient endosomal escape at physiologically relevant LNP concentrations. We also check cell viability at higher LNP doses (CellTiter-Glo or trypan blue, depending on the cell line) because cytotoxicity at therapeutic doses disqualifies a formulation regardless of how well it transfects surviving cells.
We want to be explicit about what HEK293T transfection data does and does not tell us. HEK293T cells do not resemble the cell types that matter for our CNS applications (neurons, choroid plexus epithelial cells, brain endothelium). They are easy to grow and transfect precisely because they have high non-specific endocytic activity and permissive membrane biology that is not representative of primary neuronal cells. A formulation that works well in HEK293T may fail in differentiated neurons because the endocytic pathway activity is fundamentally different. We use HEK293T data as a minimum bar — if a formulation can't transfect HEK293T efficiently, it is not worth advancing. Passing HEK293T does not mean the formulation is adequate for neuronal delivery.
What This Tells You — and What Comes Next
The four-assay package we have described — DLS sizing, cryo-EM morphology, RiboGreen EE%, and HEK293T transfection — tells you whether a candidate LNP formulation is physically acceptable and functionally competent for mRNA delivery. It is not a full characterization package by any regulatory standard, and it does not tell you anything about in vivo behavior.
What it does tell you, at modest cost and with university core facility access, is enough to make principled formulation choices. From five candidate formulations, you can typically eliminate two or three based on DLS and EE% alone (failed size criteria or poor encapsulation), and one or two more based on cryo-EM (poor morphology) or HEK293T (poor transfection). The one or two survivors are worth the resource investment of more rigorous in vitro testing in disease-relevant cell types (for us, iPSC-derived neurons or primary rodent cortical neurons) and eventually in vivo pharmacokinetics.
The honest assessment of this approach: it is resource-constrained science. We are not running the characterization panel we would run with a larger budget — SAXS for internal lipid phase characterization, NTA (nanoparticle tracking analysis) as an orthogonal size method, cryo-electron tomography for three-dimensional particle reconstruction, GPC-based mRNA integrity analysis. Each of those adds information. But the four-assay approach we can do now produces formulation decisions that we are confident are better than no characterization, and it builds the experimental infrastructure that a more complete characterization program can extend later.
