DSC in liposome and lipid nanoparticles development

DSC is used to monitor temperature-induced order-disorder transitions, like protein unfolding, gel to liquid-crystalline phase transitions in lipids or structural transitions in nucleic acids. It recently emerged as an important technique for the characterization of liposomes and LNPs based drug carriers, which are components of gene-therapy drugs and third-generation vaccines.

DSC thermogram provides information on the stability effects of different lipid compositions of the carrier, as well as its load – small molecule drugs, proteins, or nucleic acids. The properties of lipids-based vectors are determined by a complex balance of interaction between all their components. DSC profile change can reveal disruption or stabilization of the vector structure and can be used as an indicator of the quality of preparation. Variance of the Tm from its regular value may be an indication of contamination of the sample or improper preparation resulting in heterogeneous mixture of liposomes with different size or composition, like unilamellar and multilamellar vesicles. For example, DPPC liposomes with diameters smaller than about 35 nm have main transition Tm at about 37°C; larger vesicles ‘melt’ at about 41°C. Transformation of liquid-crystalline lipid bilayers into the gel phase is exothermic, which can be attributed to the formation of van der Waals contacts in the gel phase. Fewer such contacts (thus higher enthalpy level) are likely to be formed in curved SUV than in LUVs [1].

With mRNA loaded LNPs, DSC thermogram also reflects the structural interactions between the nucleic acid and the lipidic components. The overlayed DSC thermograms of a free mRNA and the same mRNA encapsulated in two different LNPs are shown in fig.1 

The peak for the free mRNA shows the thermal stability (Tm) of the nucleic acid as well as its enthalpy (kcal/mole – area under the peak). The enthalpy reflects the amount and energy of the intramolecular bonds maintaining the mRNA tertiary structure. The heat effects observed for the main transitions in mRNA-LNP1 and mRNA-LNP2 cannot be accounted for by the heat of transition of the free mRNA only. The substantial positive Tm shift when the mRNA is encapsulated in an LNP points to the addition of intermolecular stabilizing interactions between the mRNA molecule and the cationic lipids. The near 10x increase of the enthalpy is an indication that these interactions are extensive [2].  

1. Chemistry and Physics of Lipids, 64 (1993) 129-142
2. Vaccines (Basel). 2022 Jan; 10(1): 49.