Why CQAs Matter More for LNPs Than for Conventional Drug Products
For a conventional small molecule tablet, the critical quality attributes that determine in vivo performance, dissolution rate, disintegration time, content uniformity, are relatively straightforward to measure and control. For a lipid nanoparticle (LNP) system, the relationship between physicochemical properties and in vivo behaviour is more intricate, and the analytical tools needed to characterise it are more demanding.
An LNP that looks identical to a passing batch in a visual inspection can behave very differently in vivo if its particle size distribution has shifted, its encapsulation efficiency has dropped, or its surface charge has changed. The four critical quality attributes discussed in this article, particle size, polydispersity index (PDI), encapsulation efficiency, and zeta potential, are the core measurements that define the identity and performance of an LNP product, and they must be reliably measured and controlled throughout development and into GMP manufacturing.
The 4 Critical Quality Attributes
| CQA | What It Measures | Typical Target Range | Why It Matters |
| Particle size (Z-average) | Mean hydrodynamic diameter of the particle population, measured by DLS | 50-200 nm for most therapeutic applications | Size determines circulation time, tissue distribution, and cellular uptake. Particles above 200 nm clear rapidly; below 50 nm may pass through kidney filtration. |
| Polydispersity index (PDI) | Width of the particle size distribution (0 = monodisperse; 1 = polydisperse) | Below 0.2 for clinical LNP products | High PDI indicates a heterogeneous population. Broader distributions correlate with variable in vivo performance and reduced reproducibility between batches. |
| Encapsulation efficiency (EE%) | Proportion of the nucleic acid or small molecule payload that is encapsulated within the LNP rather than free in solution | Greater than 85% for mRNA LNPs; target depends on payload and indication | Free (unencapsulated) payload is bioavailable but unprotected, degrades rapidly, and contributes to off-target effects. Low EE% reduces effective dose and increases immunostimulation risk for nucleic acid payloads. |
| Zeta potential | Net surface charge of the particle at the slipping plane, measured by electrophoretic light scattering | Near-neutral for ionisable LNPs in physiological conditions; negative in buffer (typically minus 5 to minus 20 mV) | Surface charge affects colloidal stability, protein adsorption (corona formation), and cellular uptake. Highly charged particles aggregate more readily and may trigger immune activation. |
Target ranges above are representative of typical clinical-stage LNP products based on published literature. Programme-specific specifications must be established based on the molecule and intended clinical use.
Measuring Particle Size and PDI: Dynamic Light Scattering
Dynamic light scattering (DLS) is the standard technique for measuring LNP particle size and PDI. DLS measures the Brownian motion of particles in suspension and uses the diffusion coefficient to calculate the hydrodynamic diameter via the Stokes-Einstein equation. It is fast, non-destructive, and highly sensitive to small shifts in particle size, making it well suited for both development screening and GMP release testing.
DLS has important limitations. It is an intensity-weighted measurement and is therefore disproportionately sensitive to larger particles, which scatter more light. A small number of aggregates can significantly shift the reported Z-average upward even when the bulk of the population is within specification. For LNP batches where aggregation is a concern, complementary techniques such as nanoparticle tracking analysis (NTA) or asymmetric flow field-flow fractionation (AF4) provide a number-weighted or separated view of the size distribution.
Measuring Encapsulation Efficiency: The RiboGreen Assay
For nucleic acid payloads including mRNA and siRNA, encapsulation efficiency is most commonly measured using the RiboGreen fluorescence assay. The assay compares the fluorescence signal from the nucleic acid in an intact LNP sample (where encapsulated nucleic acid is inaccessible to the dye) with the signal from a sample disrupted with a detergent such as Triton X-100 (where all nucleic acid is accessible). The ratio of the two signals gives the encapsulation efficiency.
For small molecule payloads, encapsulation efficiency is typically measured by separating the encapsulated and free fractions using size exclusion chromatography or ultracentrifugation and then quantifying each fraction by LC-UV or LC-MS/MS.
Integrating CQA Measurement into the Development and GMP Lifecycle
Development Phase
During formulation development, CQA measurements are used to screen formulation variables including lipid ratios, drug-to-lipid ratios, and processing conditions. A design of experiments (DoE) approach linking formulation parameters to CQA outcomes allows the formulation space to be explored efficiently and a robust formulation to be identified before GMP scale-up.
GMP Manufacturing
At GMP manufacturing scale, CQA measurements are used as in-process controls during manufacture and as release tests that every batch must pass before it can be used in a clinical study. The specification ranges for each CQA are derived from the development data, with limits set to ensure that batches released for clinical use are within the range of material that has demonstrated acceptable in vivo performance.
LNP Characterisation at Ardena Oss
Ardena’s GMP nanomedicine facility in Oss is equipped with DLS instruments, RiboGreen assay capability, and the analytical infrastructure needed to characterise LNP products across all four critical quality attributes during development and GMP manufacturing. The analytical team works alongside the formulation scientists to integrate CQA monitoring into the development workflow from early screening batches through to GMP release testing.
