What are Lipid Nanoparticles and Their Applications?

Lipid nanoparticles (LNPs) are a type of nanoscale drug delivery system that has been widely studied and applied. People are increasingly interested in their research, mainly because they are significantly superior to traditional drug delivery methods in terms of steady-state delivery, drug release control, and biocompatibility. LNPs can be used to deliver a variety of drugs, including small molecule drugs, proteins, nucleic acids, etc., and have shown great potential in the development of mRNA vaccines.

Lipid Nanoparticles

Lipid nanoparticles (LNPs) are an important nanoscale drug delivery system. They are widely used in drug delivery, gene therapy and vaccine development due to their superior performance in biocompatibility, safety and drug loading capacity. Lipid nanoparticles are usually between 50 nanometers and 1000 nanometers in size and are mainly composed of lipid molecules such as phospholipids, cholesterol and other lipid substances.

Fig. 1. Lipid nanoparticle structure.

The structure of lipid nanoparticles is similar to that of cell membranes, with a double-layer phospholipid membrane, which enables them to effectively fuse into cells and deliver drugs or genetic materials to specific cells or tissues. The core of LNPs can encapsulate hydrophilic (such as small interfering RNA, siRNA) or hydrophobic (such as certain anticancer drugs) substances, while its outer phospholipid membrane provides protection and stability. In in vitro and in vivo tests, LNPs have demonstrated excellent drug release control capabilities and can optimize therapeutic effects by changing solubility, drug loading and release rate.

Lipid Nanoparticle Formulation

The main components of LNPs include lipids, cholesterol, surfactants and adjuvants. Lipids are usually phospholipids, which not only provide structural stability, but also participate in the encapsulation and protection of drug molecules. Cholesterol can enhance the robustness and flexibility of particles, thereby improving delivery efficiency. Surfactants such as PEG-phospholipids can increase the stability and circulation time of nanoparticles and reduce immune responses. In addition, in order to adapt to different drugs and application requirements, other excipients such as peptides or targeting molecules can be added to enhance the specificity and efficiency of delivery.

Cationic Lipids

The earliest lipid nanoparticles used cationic lipids, which readily bind to negatively charged nucleic acids. However, this cationic lipid-based delivery system is toxic and immunogenic both in vivo and in vitro. DOTAP and DOTMA, for example, can be neutralized by negatively charged serum proteins, resulting in toxicity and reduced efficacy. As an alternative, ionizable cationic lipids have been developed, which reduce the toxicity caused by cationic lipid nanoparticles while retaining efficient transfection properties. The ionizable tertiary amine moiety of ionizable cationic lipids such as DLin-KC2-DMA and D-Lin-MC3-DMA has a net positive charge at acidic pH but remains neutral in the blood circulation system (pH 7.4) sex. This pH sensitivity facilitates the delivery of nucleic acid drugs in the body because lipid nanoparticles that remain neutral interact less with the anionic membranes of blood cells, thereby improving the biocompatibility of lipid nanoparticles. Normally, cells take up lipid nanoparticles through endocytosis. When lipid nanoparticles enter the endosome with low pH value, the ionizable cationic lipid is protonated and becomes positively charged, which promotes the instability of the endosomal membrane, causes the membrane to rupture, and the lipid nanoparticles undergo endosomal escape. Currently, there are five major ionizable lipid types widely used for RNA delivery, including branched-tail lipids, biodegradable lipids, polymeric lipids, multi-tail lipids, and unsaturated lipids.

PEGylated Lipids

PEGylated lipids are another important component of lipid nanoparticles. PEGylated lipids, such as DMG-PEG 2000 and DSPE-MPEG-2000, are composed of hydrophilic PEG bound to hydrophobic alkyl chains via phosphate, glycerol, or other linkers. PEGylated lipids are located on the surface of lipid nanoparticles, with lipid domains buried deep within the particles and PEG domains protruding from the surface. PEGylated lipids can prolong the circulation time of liposomes because PEG forms a steric barrier that prevents binding of plasma proteins that would otherwise lead to their rapid clearance by reticuloendothelial cells. Additionally, PEGylated lipids can control the size of nanoparticles. This is because the low pH and ethanol environment will promote aggregation and fusion of lipid nanoparticles during the manufacturing process. The steric barrier of PEGylated lipids prevents this and helps produce a uniform population of particles with narrow polydispersity and small particle size (typically 50-100 nM). Formulations completely lacking PEG lipids produced unstable, polydisperse lipid nanoparticles.

Cholesterol

Cholesterol is hydrophobic and rigid, filling the gaps between lipids within the liposome membrane and promoting vesicle stability. The molecular geometry of cholesterol derivatives can further influence the delivery efficacy and biodistribution of lipid nanoparticles. Auxiliary lipids are mostly phospholipids, such as DSPC and DOPE, which promote cellular uptake and endosomal release by promoting fusion with cell and endosomal membranes.

Lipid Nanoparticles Toxicity

Although LNPs have performed well in the field of drug delivery, their potential toxicity issues cannot be ignored. The toxicity of LNPs may come from their constituent materials. Commonly used ingredients such as phospholipids, cholesterol and polyethylene glycol (PEG) may cause cytotoxicity at high concentrations. In addition, residual organic solvents and surfactants during the preparation process may also have adverse effects on biological systems. In particular, PEG-modified LNPs may induce immune responses, including acute allergic reactions and immunogenicity, when used for a long time. In addition, the distribution and degradation products of LNPs in the body may also cause toxicity issues. LNPs reach the target tissues through blood circulation, but some nanoparticles may remain in the body for a long time, especially in organs rich in macrophages such as the liver and spleen. Retained LNPs may trigger local inflammatory responses and tissue damage. If the degradation products of LNPs are not easily metabolized and cleared, they may also have potential toxicity. Therefore, before clinical application, the toxicity characteristics of LNPs must be fully evaluated, and their toxicity risks must be minimized by optimizing their structure-activity relationship and improving the preparation process. At the same time, the development of new lipid materials and their combination with other nanotechnologies, such as polymer nanoparticles and nanoemulsions, also provide new directions and ideas for the development of LNPs.

Solid Lipid Nanoparticles

Solid Lipid Nanoparticles (SLNs) are nanoscale drug delivery systems used in pharmaceutical formulations, cosmetics and other biotechnology fields. The size of these nanoparticles is generally between 50 and 1000 nanometers. With their unique physicochemical properties, SLNs show great potential in improving drug stability, controlling release and improving bioavailability. The matrix of SLNs is mainly composed of biocompatible solid lipids, which are solid at room temperature and maintain their structure at body temperature. Commonly used lipid materials include triglycerides, stearic acid, waxes and lipids, which have good degradability and low toxicity.

In SLNs, drug molecules can be encapsulated inside the lipid matrix or adsorbed on its surface to form a drug carrier system. In terms of drug delivery, SLNs are widely used in the fields of anticancer drugs, antibacterial drugs and vaccines. For example, many chemotherapy drugs are limited in clinical application due to their low water solubility and high toxicity, while SLNs can significantly improve the efficacy of these drugs and reduce their side effects. In addition, SLNs can also be used as a vaccine delivery system to improve the effectiveness of vaccines by improving the stability of antigens and immune responses. In the field of cosmetics, SLNs have also received a lot of attention. Due to its ability to improve the stability and skin absorption of active ingredients, many skin care products and anti-aging products have begun to apply SLNs technology.

Lipid Nanoparticles for Drug Delivery

It is known that more than 40% of small molecule drugs used to treat cancer have low solubility in water, and liposomes as drug delivery systems can encapsulate these drugs and improve their water solubility. Liposome-encapsulated drugs can reduce the toxicity of drugs to normal tissues and prolong the residence time of drugs. Many lipid nanoparticle drug formulations have been widely used in many clinical trials as delivery systems for anticancer, anti-inflammatory, antibiotic, antifungal, anesthetic, and other drugs and gene therapies, especially in the field of delivering nucleic acid drugs. For example, the first approved liposomal drug, Doxil, a lipid nanoparticle formulation of the antitumor drug doxorubicin (liposomal doxorubicin nano-drug), uses nanoparticles to extend circulation time in human plasma while reducing the cardiotoxicity of doxorubicin.

In the field of gene therapy, lipid nanoparticles are particularly suitable for the delivery of nucleic acid drugs, such as mRNA and siRNA. mRNA vaccines are a successful example of the application of LNPs. COVID-19 vaccines (such as Pfizer-BioNTech and Moderna vaccines) use lipid nanoparticles to encapsulate and protect mRNA, ensuring stability in the body and promoting efficient delivery to target cells. This technology significantly improves the stability and effectiveness of vaccines and accelerates the development and promotion of vaccines.

Liposome vs Lipid Nanoparticle

Liposomes are spherical vesicles wrapped by a phospholipid bilayer, and the center usually contains water-soluble substances. LNPs are often designed to deliver nucleic acids, such as mRNA or siRNA, and have played a key role in the development of mRNA vaccines (such as COVID-19 vaccines) in recent years. They can efficiently enter cells and release their contents, providing an important platform for gene therapy and immunotherapy. Whereas liposomes and lipophilic polymers have certain functional and material overlap, liposomes are more suited for encapsulating and delivering water-soluble medicines, whereas lipophilic polymers are better suited for nucleic acid transport and sophisticated gene therapy applications. The specific choice of use should be determined based on the physical and chemical properties of the specific drug and the therapeutic needs.

Lipid Nanoparticles Synthesis

There are many methods for preparing lipid nanoparticles, including thin film hydration, microfluidics, nano-homogenization, etc. These methods have their own advantages and disadvantages. The selection of an appropriate preparation method depends on the required particle size, drug loading, and drug properties. Optimizing the preparation process and improving repeatability are one of the current research hotspots. BOC Sciences is a leading provider of chemical products and services, including expertise in lipid nanoparticles. We have a wide range of lipid materials and can design and optimize formulations based on desired properties and applications. In addition, our R&D scientists have expertise in controlling the size of lipid nanoparticles, which is critical to their stability, drug-loading capacity, and targeted delivery. We can optimize formulation parameters to achieve the particle size distribution required by our customers.