Stearic acid is denoted as n-octadecanoic acid or as 18:0, and it is composed of 18 carbons with no double bonds. Although it is found in small quantities in seed and marine oils, major sources of this long-chain saturated fat include milk fats, which can contain 5 to 15% stearate, as well as lard and cocoa, and shea butter, which can contain from 10-35% stearate. Stearic acid is also a large constituent of hydrogenated fats and oils.
Fig.1 Synthesis and fate of stearic acid. (Sampath, 2005)
Cationic micelles, which were based on hydrophobic modification of hydrophilic polymers, had emerged to be an efficient gene and peptide delivery system. The micelles can self-assemble in aqueous phase and offer unique advantages including protection of nucleic acids, enhanced cell interaction, higher gene transfection, and less toxicity. Scientists had previously reported that stearic acid and branched PEI-2k conjugates (PSA) could form self-assemble cationic micelles. The micelles were able to encapsulate tyrosine's-related protein to peptide and accumulate in the draining lymph nodes after subcutaneous injection.
Fig.2 Schematic representation of how PSA micelles loaded with mRNA was delivered to APCs. (Zhao, 2016)
Consistent with previous findings on siRNA condensation, PEI-OA1 and PEI-StA2 (Polyethylenimine-stearic acid) demonstrated a significant improvement in delivering a range of siRNA doses as compared to parent PEI. Others have also shown that lipid components in polymers positively influenced siRNA delivery. Furthermore, the superior ability to condense, protect, and deliver siRNA that was obtained with PEI-StA2 might be related to the chemical structure of stearic acid. The free-rotation property of the saturated carbon atoms in stearic acid was expected to give the molecule more flexibility to move inward or project outward the complex. PEI-StA2 showing the highest siRNA binding, protection, and delivery among the PEI derivatives. This may be due to the higher stability of the formulation provided by PEI-StA2 and better protection from nuclease degradation. Therefore, it is feasible that PEI-StA2 may exert this better efficiency through mediating siRNA protection and stability in the endosomal compartment, which allows for a higher amount of intact siRNA to reach the cytoplasm.
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