PEG-Lipid derivatives have the advantages of increasing the stability of liposomes, extending the half-life of blood circulation, improving tumor targeting efficiency and enhancing drug efficacy. In-depth study of the effects of different PEG-lipid derivative modifications on the physical, chemical and biological stability of liposomes will help solve the current problems of PEGylated liposomes. For example, the phenomenon of accelerated blood clearance (ABC) caused by repeated intravenous injections lays the foundation for the development of new targeted agents.
Introduction to PEG Modifiers
Liposomes are spherical closed membrane vesicles with lipid as the main structure, generally used as drug delivery carriers. Liposomes have received widespread attention because they can better change drug absorption, reduce metabolism, extend biological half-life or reduce toxicity. Drug distribution is mainly controlled by the properties of the carrier, not just by the physical and chemical properties of the API. Ordinary liposomes are prone to drug encapsulation leakage and liposome cleavage, and are rapidly cleared by the macrophage system (RES), thus preventing the drug from achieving its target target. Therefore, improving the liposome structure to overcome existing defects and obtain the intended application has become a process that cannot be ignored from drug delivery theory to widespread clinical application.
Factors Affecting the Properties of Liposomes
- Types and Properties of Liposomes
The lipid system is composed of phospholipids as membrane material and additives. Phospholipids are amphiphilic substances with hydrophilic and lipophilic groups in their structure. Currently used to prepare liposomes are natural phospholipids (such as lecithin and soybean phospholipids) and synthetic phospholipids (such as dipalmitoylphosphatidylcholine and distearylphosphatidylcholine). Commonly used additives include cholesterol, stearylamine, phosphatidic acid, etc. Cholesterol can adjust the fluidity and permeability of the bilayer, and stearylamine and stearic acid can change the surface charge properties of liposomes.
Lipid is the main part of the drug carrier, while the other parts are modifications and improvements. For example, Geng et al. injected a trace amount of mRNA-LNP solution into mice and found that replacing DMG-PEG with DSG-PEG could slightly increase protein expression; while using DOPE instead of DSPC could increase protein expression by an order of magnitude.
- Types and Properties of PEG
PEG polymers are polymerized from ethylene oxide and can form linear or branched structures. The molecular weight, terminal functional groups, chain composition and shape of PEG will all affect the pharmacokinetic performance of PEG-lipid in vivo. The molecular formula of linear PEG is H-(O-CH2-CH2)n-OH, which has two ends and can be used to modify new conjugated drugs, with a low drug loading capacity. The branched chain structure has functional groups on one or more ends, such as branched PEG, forked PEG or multi-arm PEG with branched terminal structures, which have multiple conjugation possibilities and increase the drug loading capacity. At the same time, many laboratory data also show that forked PEG and multi-arm PEG can effectively reduce the accelerated blood clearance (ABC) phenomenon caused by PEG modification.
- Chemical Bonding of PEG-Lipid
The main connecting bonds of PEG modified biomolecules include amide bonds, ether bonds, ester bonds and disulfide bonds. Different linkage properties will also have a corresponding impact on the stability of PEG-Lipid in the body, the concentration within the target, and the release rate. For example, researchers target the acidic environment of tumor cells and develop PEG-Lipid linkages (hemiacetals, pH-sensitive benzoates, etc.) that can be cleaved in acidic environments. Or insert acid-sensitive chemical modifications (such as -S-S-, boronic acid esters, etc.) into the PEG backbone to increase drug-loaded liposomes to release drugs in the target.
- PEG-Lipid Mole Ratio/ PEG Concentration
The lifetime of liposomes in circulation and drug delivery rate are affected by the molecular weight and concentration of PEG in the bilayer. A quantitative understanding of how PEG lipid incorporation conditions affect PEG density on a single liposome can optimize liposome circulation kinetics and therapeutic efficacy. Here, a fluorescence-based sensitive single liposome determination method was used, which realized the quantitative analysis of PEG lipid density on a single liposome for the first time. Generally speaking, the concentration of PEG on the liposome surface needs to be appropriate, and the PEG branch chain should be optimized to increase drug loading, stability and cope with the ABC phenomenon.
At present, PEG-Lipid research has developed from simple long circulation to multifunctional long circulation, such as long circulation nanoliposomes, long circulation temperature-sensitive liposomes, pH-sensitive liposomes, etc. More in-depth study of the effects of different PEG-lipid derivative modifications on the physical, chemical and biological stability of liposomes will help solve the current problems of PEGylated liposomes.
References
- Nogueira, E. et al. Design of liposomes as drug delivery system for therapeutic applications. International Journal of Pharmaceutics. 2021, 601: 120571.
- Yu, P. et al. Poly ethylene glycol (PEG)-Related controllable and sustainable antidiabetic drug delivery systems. European journal of Medicinal Chemistry. 2021, 217: 113372.