Functionalized lipids refer to the synthetic lipids that do not occur naturally in biological systems, obtained by biochemical modification of natural lipids. Natural lipids are numerous and diverse. Due to their diverse structures, lipids have a variety of important biological functions, such as participating in signal transduction, regulating cellular growth, differentiation, aging and other processes, as well as providing energy and participating in the growth of the body. Therefore, functionalized lipids with specific functional properties obtained through the modification of natural lipids are more widely used in many fields than natural lipids .
Liposomes are lipid-based structures comprising one or more bilayer phospholipids which can encapsulate aqueous media. Functionalized liposomes refer to liposomes with specific functions by modifying the hydrophilic outer surface layers or loading functional substances in the hydrophilic cores, which have long-circulating properties. These functionalized liposomes can deliver drugs to cancerous and tumor tissues mainly through both passive targeting and active targeting methods , as shown in Fig. 1. Therefore, the functionalized liposomes can be applied for disease prevention, diagnosis and treatment and due to the mainly include the following types:
Fig. 1 Passive and active targeting methods for surface-functionalized liposomes in drug delivery .
Functionalized lipids are suitable for several related fields, such as drug delivery and gene transfection.
Drugs usually go through stages of absorption, distribution, and elimination after entering the body. The distribution of drugs in vivo is usually related to plasma protein binding rates, pH of body fluids, physicochemical properties of drugs and internal barriers, while elimination includes metabolism and excretion. The premature elimination of drugs will affect the absorption and duration of action of drugs. Notably, functionalized lipids have different effects on various processes of drugs in vivo, such as enhancing drug absorption, improving the distribution of drugs in vivo, and reducing the elimination of drugs. Therefore, functionalized lipids can improve the therapeutic efficiency during the drug delivery.
Fig. 2 Drug delivery of fuctionalized lipids for cancer diagnosis and therapy.
Gene transfection is the process of introducing target genes into cells and maintaining the biological function within the cells by biochemical or physical methods. This technology has been widely used for the analysis of gene structure and function, gene expression and regulation, making gene therapy possible. Cationic lipids are the most common gene transfection reagents, which are easy to penetrate cell membranes and have high efficiency in gene transfection in vitro. However, natural cationic lipids are very few and basically do not exist in biological systems. So it is necessary to obtain the required cationic lipids through functional modification of lipids, mainly by chemically modifying natural lipids to introduce permanent positive charges into lipid membranes. In summary, cationic lipids, as functionalized lipids have a variety of applications in biotechnology and medicine.
Fig. 3 The proposed mechanism of chemical transfection .
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