Liposome in Drug Delivery

Liposomes are closed spherical structures formed by the self-assembly of lipids into a bilayer (monolayer) and/or multiple concentric bilayers (multilayer) closing a central water cavity. The particle size of liposomes ranges from 30 nm to micron level, and the thickness of the phospholipid bilayer is approximately 4 to 5 nm. Liposomes originated from the accidental discovery by British scientist Alec Bangham and his colleagues in 1961. They discovered that when phospholipids are dispersed in an aqueous medium, they will spontaneously form closed vesicles.

Fig. 1. Simple schematic diagram of liposomes.

The basic components of liposomes are usually amphipathic phospholipids and cholesterol. Amphipathic phospholipids form a bilayer structure, and cholesterol supports and maintains the bilayer structure. Commonly used phospholipids are sphingomyelin and glycerophospholipids, both of which have a hydrophilic head and a hydrophobic tail region. In an aqueous environment, phospholipid molecules spontaneously arrange into liposomes driven by hydrophobic interactions and other intermolecular interactions. The role of cholesterol is to promote the accumulation of lipid chains and the formation of a bilayer, reduce the fluidity of the bilayer, and reduce the transmembrane transport of water-soluble drugs. Not only that, cholesterol can also reduce the interaction between liposomes and proteins in the body and reduce the loss of phospholipids, thus improving the stability of liposomes.

Advantages of Liposomes

1) The structure of liposomes is very similar to that of cell membranes, making them highly biocompatible and biodegradable. Therefore, liposomes can protect the drug from being degraded by enzymes before reaching the lesion site. At the same time, the drug is hidden inside the liposome in the form of physical encapsulation, which can improve drug stability, reduce drug toxicity, and increase the dosage, to achieve better therapeutic effect.

2) The bilayer surface formed by amphiphilic phospholipids can be physically or chemically modified with ligands or other functional groups to make liposomes have tissue targeting, prolong the effective residence time of liposomes in the lesion site, and even achieve efficient targeted drug delivery.

3) By changing the charge on the surface of the bilayer, it can be used to entrap and deliver DNA and RNA drugs, such as cationic liposomes.

Liposome Delivery Technology

According to the size of the liposome particle size and the number of lipid bilayer membranes, liposomes can be divided into unilamellar vesicles (ULV), oligolamellar vesicles (OLV, 100-1000nm), multilamellar vesicles (MLV, >500nm) and multivesicular liposomes (MVL, >1000nm). Unilamellar liposomes can be divided into small unilamellar vesicles (SUV, 30~100nm) according to particle size, large unilamellar vesicles (LUV, >100nm) and giant unilamellar vesicles (GUV, >1000nm). Oligolamellar liposomes and multilamellar liposomes have an onion-like structure with 2~5 and >5 concentric lipid bilayer membranes, respectively. Different from multilamellar liposomes, multivesicular liposomes include hundreds of non-concentric aqueous chambers surrounded by a single-chamber bilayer lipid membrane, forming a honeycomb structure.

Fig. 2. Classification of liposomes.

Unilamellar Vesicles (ULV)

Doxorubicin hydrochloride liposome (Doxil): currently the most popular variety on the market, and also the one with the most mature technology. Doxorubicin hydrochloride liposomes add distearoylphosphatidylethanolamine PEGylated derivative (DSPE-mPEG 2000) to the formulation, so that they overlap and stagger on the surface of the liposomes to form steric hindrance. Its high hydrophilicity and flexibility will interfere with the hydrophobic interaction between liposomes and plasma proteins, reduce the absorption of liposomes by the reticuloendothelial system (RES), exert a stealth effect, and realize the long circulation function of liposomes in vivo. Therefore, such liposomes are also called long-circulating liposomes or stealth liposomes.

Paclitaxel liposome: Luye Pharma independently developed and marketed a new formulation of paclitaxel liposome for injection (trade name Lipusu) with lecithin and cholesterol as solubilizers. The preparation also contains excipients threonine and glucose. Compared with ordinary paclitaxel injection, Lipusu solves the problem that paclitaxel is insoluble in water, and does not contain a composite solvent of anhydrous ethanol and polyoxyethylene castor oil, but anti-allergic pretreatment is still required before administration. Lipusu encapsulates paclitaxel with phospholipids without changing the activity of the drug. When the liposome enters the blood, the drug is slowly released from the liposome, delaying the rate of drug metabolism, prolonging the action time of the drug in the body, and reducing the side effects of the drug.

In addition, there are many other single-chamber liposome products, such as iribidate hydrochloride liposomes, amphotericin B liposomes, vincristine sulfate liposomes, daunorubicin liposomes, etc. At present, there are no major difficulties in the development of single-chamber liposomes. The technology and prescriptions are relatively mature, and they are not restricted by excipients and equipment.

Multivesicular Liposomes (MVL)

The interior of multivesicular liposomes is composed of multiple vesicles arranged in a honeycomb pattern, and each vesicle has a phospholipid bilayer. The internal vesicles are tightly cross-linked and stacked to form a vesicle-like structure. The place where multiple vesicle chambers meet and accumulate is composed of neutral lipids. The commonly used neutral lipid is triglyceride. Neutral lipids are crucial to the stability of multivesicle structures, forming membranes between multiple vesicle chambers, maintaining the internal shape of the chambers and stabilizing the entire vesicle structure. In addition, the neutral lipid-stabilized internal hydrophilic cavity, which accounts for 95% of the volume of the entire vesicle structure, is suitable for entrapping aqueous drugs, while the lipid phospholipid bilayer of the cavity can entrap fat-soluble drugs.

Table 1. Marketed multivesicular liposomes.

Multilamellar Vesicles (MLV)

TLC has developed multiple multilamellar liposome products utilizing its exclusive BioSeizer® and NanoX™ drug delivery systems. The researchers likened the structure of multivesicular liposomes to a pomegranate, while the structure of multilamellar liposomes was like an onion. It can be seen from the electron microscope images that it is precisely because of the cystic or multi-layered concentric structure that multivesicular liposomes and multilamellar liposomes have sustained release effects.

Other Liposome Delivery Technologies

Environment-responsive liposome delivery technology: that is, the drug encapsulated in liposomes can act pharmacodynamically only under a specific physiological environment. Currently, the most researched ones include pH-sensitive liposome technology, temperature-sensitive liposome technology, and magnetic liposome technology. They all achieve controlled release or targeted delivery of drugs through changes in the environment (pH, temperature, magnetic field).

Long circulation liposome delivery technology: After the liposomes enter the circulation system, the powerful mononuclear phagocyte system (MPS) in the body will phagocytose and eliminate the liposomes as foreign invaders in a short period of time. At the same time, phospholipase in the blood hydrolyzes phospholipids, reducing the half-life of the drug in the body. There are currently two long-circulating liposome technologies: red blood cell-mimicking liposome technology and liposome technology modified with PEG or PEG derivatives.

Deformable liposome delivery technology: Deformable liposomes are also called flexible liposomes. By adding surfactants such as bile salts, the liposomes are highly hydrophilic, extremely flexible and permeable, and can pass through intercellular pores that are several times smaller than their own particle size. Such characteristics may allow such liposomes to shine in the skin drug delivery system. Liposomes pass through the stratum corneum through sweat glands and hair follicles to achieve transdermal administration.

Membrane fusion liposome delivery technology: Based on traditional liposomes, viruses with fusion properties are added to form a new type of carrier with high efficiency and low toxicity. The vector can be fused to a specific virus to deliver the virus specifically into target cells. Membrane fusion liposome delivery technology is mainly suitable for the encapsulation and delivery of biological macromolecules, such as protein drugs, genetic drugs, vaccines, etc.

Immunoliposome delivery technology: surface modification of traditional liposomes with antibodies or receptors. Because the surface modification of the liposome can specifically bind to cells, it is concentrated in the lesion site, and the encapsulated drug is released after adsorption, fusion, and phagocytosis, thereby achieving targeted drug delivery. The biggest advantage of immunoliposome delivery technology is its specificity. Cells without specific binding sites will not be exposed to the drug, reducing toxic side effects on normal tissues.

The special structure of liposomes makes stability an important factor limiting drug delivery. The stability of liposomes can be divided into physical stability, chemical stability and biological stability. In order to maintain the integrity of the liposome vesicle structure, it is necessary to maintain the balance of various interactions within the liposomes and between liposomes. The biological stability of liposomes is more due to the formulation composition and preparation process. For example, PEGylated phospholipids in liposomes can form a hydrated film on the surface of liposomes to prevent liposomes from being quickly recognized and cleared by MPS. The addition of an appropriate amount of sphingomyelin can also improve the stability of liposomes.