Particle size stands as a decisive critical parameter throughout the delivery journey of mRNA-LNP therapeutics. Unlike mRNA molecules that carry genetic information or ionizable lipids with stimuli-responsive capabilities, even minor variations in LNP particle size profoundly impact every performance indicator, including colloidal stability, encapsulation efficiency, in vivo biodistribution and cellular uptake. So what is the optimal particle size for LNPs, and what lies behind this scientifically validated range?
Consensus on the Golden Size Range: 60 to 100 Nanometers
Clinical-stage and commercially available mRNA vaccines, alongside extensive academic research, have converged on a 60–100 nm particle size range for LNPs. The two leading COVID-19 mRNA vaccines feature mean particle sizes between 80 nm and 100 nm, with formulations developed by Pfizer/BioNTech and Moderna both falling within this interval. LNPs fabricated via microfluidic mixing of lipid components and mRNA typically yield particles ranging from 60 nm to 100 nm. For instance, two SARS-CoV-2 vaccine candidates, nCoVsaRNA and ARCoV, exhibit average particle sizes of 75 nm and 89 nm respectively.
This size window is not arbitrarily defined, but the optimal solution constrained by multiple physical, chemical and biological factors. The U.S. FDA classifies particle size and particle size distribution as Critical Quality Attributes (CQAs) for liposomal drug products. The industry widely recognizes 60–100 nm as the ideal particle size range, with a polydispersity index (PDI) below 0.2 as a standard benchmark. Systematic reviews demonstrate that current LNP manufacturing platforms can stably produce particles sized from 60 nm to 180 nm, and a PDI no greater than 0.30 is the basic requirement to meet regulatory specifications. Undoubtedly, 60–100 nm has become the golden particle size for mRNA-LNP formulation development.
Drawbacks of Undersized LNPs: Renal Clearance and Reduced Encapsulation Efficiency
LNPs smaller than 60 nm present multiple drawbacks. First, particles of excessively small dimensions have limited loading capacity for mRNA. To deliver a sufficient drug dose with the same lipid input, a greater number of LNP particles are required, which increases formulation volume and production costs.
Second, undersized particles are prone to rapid renal clearance. The glomerular filtration barrier primarily eliminates molecules smaller than 10 nm, while particles below 50 nm face a high risk of being filtered out by the kidneys. This shortens circulation half-life and impairs tissue targeting capability.
More importantly, reduced particle size is strongly correlated with decreased encapsulation efficiency. Tiny LNPs lack sufficient internal volume within the lipid bilayer to encapsulate and protect mRNA, leaving a large portion of RNA exposed to the external environment and vulnerable to nuclease degradation.
Conversely, particles larger than 200 nm trigger another set of issues. Such oversized LNPs are readily recognized and rapidly phagocytosed by macrophages in the liver and spleen. As professional phagocytes, macrophages readily clear large foreign particulates, leading to massive accumulation of LNPs in the reticuloendothelial system rather than delivery to target tissues. Simply put, overly small LNPs offer insufficient protection and undergo rapid metabolism, while oversized ones are eliminated by phagocytes prematurely. The 60–100 nm range strikes a perfect balance between these two extremes.
Physical Mechanisms Governing Size Regulation: Hepatic Sinusoidal Fenestrae and Cellular Uptake
The 0–100 nm size range is also precisely adapted to the anatomical structures of the liver and target cells.
Following intravenous administration, most LNPs must traverse hepatic sinusoidal endothelium to reach hepatocytes. Hepatic sinusoids are lined with numerous fenestrae measuring 100–200 nm in diameter. Particles larger than 200 nm cannot pass through these pores, whereas LNPs sized below 100 nm efficiently penetrate sinusoidal walls and enter the perisinusoidal space. This explains why LNP-siRNA therapeutics such as Onpattro and numerous liver-targeted mRNA therapies adopt a particle size below 100 nm.
At the cellular level, particle size directly dictates cellular uptake efficiency. Most therapeutic LNPs enter target cells via endocytosis, a biological process highly sensitive to particle dimension. The optimal size range for efficient endocytosis is approximately 80–120 nm. Notably, influenza virus (~80 nm) and reovirus (~120 nm) naturally fall within this window, evolving to exploit receptor-mediated endocytosis for efficient cellular entry. Accordingly, LNPs sized between 80 nm and 100 nm can leverage this evolutionarily optimized cellular entry pathway to achieve robust uptake.
Fine-Tuning Particle Size: Formulation and Manufacturing Strategies
LNP particle size is not fixed upon production and can be precisely modulated via multiple approaches.
Microfluidic mixing is the dominant manufacturing technology for LNPs. By adjusting the flow rate ratio and total flow rate of the organic phase (lipids dissolved in ethanol) and aqueous phase (mRNA in acidic buffer), particle size can be controlled within milliseconds. Higher flow rates accelerate mixing and generate smaller particles, while lower flow rates result in larger particles. Combined with design of experiments (DoE), this technique enables reproducible fabrication of LNPs with predefined sizes across the 60–180 nm spectrum.
Lipid formulation is another core lever for size control. PEGylated lipids are typically incorporated at a molar ratio of 1–2 mol%. Excess PEG-lipid increases aqueous interfacial area and lowers encapsulation efficiency, while insufficient PEG-lipid causes particle aggregation and a subsequent rise in particle size. Additional parameters, including the molecular weight of PEG chains and the nitrogen-to-phosphorus (N/P) ratio between ionizable lipids and mRNA, also enable fine particle size tuning.
Innovative modulation methods have emerged in recent years. Studies since the early 2020s have verified that adding salts such as sodium chloride to the buffer serves as a simple and effective strategy to enlarge LNP size to over 200 nm, with the effect following the Hofmeister series and exhibiting concentration dependence. Though LNPs of 200–300 nm are not suitable for intravenous administration, they facilitate uptake by dendritic cells and demonstrate promising potential in cancer vaccine development.
Particle Size as a Determinant of Tissue Distribution
LNPs of different sizes exhibit distinct in vivo biodistribution profiles, a phenomenon known as passive targeting:
Particles smaller than 50 nm tend to accumulate gradually in the bone marrow;
Particles ranging from 50 nm to 100 nm achieve favorable liver targeting;
Particles of 100–200 nm possess strong immunogenicity and are ideal for activating immune cells in vaccine applications;
Particles larger than 200 nm are mechanically trapped in pulmonary capillaries and enriched in lung tissues.
For systemic administration, particle size acts as a key factor governing tissue tropism.
Conclusion
Particle size is far more than a numerical parameter; it serves as a critical physical bridge connecting molecular design to in vivo performance of LNPs. From the threshold of glomerular filtration and the dimension of hepatic sinusoidal fenestrae, to the optimal window for endocytosis and the phagocytic capacity of macrophages, the 60–100 nm golden range represents the most balanced solution against multiple biological barriers.
A thorough understanding of how particle size affects delivery performance not only optimizes existing formulations but also provides scientific guidance for the next generation of organ-targeted and cell-specific delivery systems. When LNPs travel throughout the body with an optimally tuned size, half the success of mRNA therapeutics is secured.
