Effect of Methylation of the Hydrophilic Domain of Tocopheryl Ammonium-Based Lipids on their Nucleic Acid Delivery Properties.

Lipid-enabled nucleic acid delivery has garnered tremendous attention in recent times. Tocopherol among the cationic lipids, 3b-[ N -( N ', N '-dimethylamino-ethane)carbamoyl]-cholesterol hydrochloride (DC-Chol) with a headgroup of dimethylammonium, and cholesterol as a hydrophobic moiety are found to be some of the most successful lipids and are being used in clinical trials. However, limited efficacy is a major limitation for their broader therapeutic application. In our prior studies, we demonstrated tocopherol to be a potential alternative hydrophobic moiety having additional antioxidant properties to develop efficient and safer liposomal formulations. Inspired by DC-Chol applications and taking cues from our own prior findings, herein, we report the design and synthesis of four alpha-tocopherol-based cationic derivatives with varying degrees of methylation, AC-Toc (no methylation), MC-Toc (monomethylation derivative), DC-Toc (dimethylation derivative), and TC-Toc (trimethylation derivative) and the evaluation of their gene delivery properties. The transfection studies showed that AC-Toc liposomes exhibited superior transfection compared to MC-Toc, DC-Toc, TC-Toc, and control DC-Chol, indicating that methylation in the hydrophilic moiety of Toc-lipids reduced their transfection properties. Cellular internalization studies in the presence of different endocytosis blockers revealed that all four tocopherol lipids were internalized through clathrin-mediated endocytosis, whereas control DC-Chol was found to be internalized through both macropinocytosis and clathrin-mediated endocytosis. These novel Toc-lipids exhibited higher antioxidant properties than DC-Chol by generating less reactive oxygen species, indicating lower cytotoxicity. Our present findings suggest that AC-Toc may be considered as an alternative to DC-Chol in liposomal transfections.