What are lipid nanoparticles?
Lipid nanoparticles (LNPs) are nanoparticles composed of lipids. They have been developed as vehicles for small molecule delivery by the nanomedicine and materials communities and are now a key component of COVID-19 mRNA vaccines. At present, LNP is a relatively mature technology platform that can be used to deliver RNA drugs, vaccines or gene editing tools.
Lipid nanoparticles (LNPs) are nanoscale route of administration devices mostly composed of lipids. The nanoparticles provide the function of encapsulating and conveying therapeutic substances to particular cells or tissues in the body, such as medicines, nucleic acids (including RNA and DNA), peptides, and other bioactive molecules. LNPs provide several advantages as drug carriers, including biocompatibility, stability, and the ability to encapsulate both hydrophobic and hydrophilic molecules. Lignum nanoparticles typically consist of a lipid bilayer or matrix that acts as a protective enclosure for the medicinal drug payload. LNPs can have varying compositions, including phospholipids, cholesterol, and other lipid derivatives. The self-assembling nanoparticles (lipids) usually have a diameter ranging from 10 to 200 nanometers.
Lipids belong to the category of amphiphilic molecules that are distinguished by the existence of three domains, including a linker that connects the hydrophobic tail region to the polar head group. Different types of lipids, including cationic and ionizable lipids, have been studied for their potential use in mRNA delivery. Lipid nanoparticles (LNPs), which are predominantly constituted of lipids, are utilized to encapsulate and transport bioactive molecules to specific cells or tissues within the body. This includes drugs, nucleic acids (including RNA and DNA), and other therapeutic compounds. In the last twenty years, there has been a proliferation of LNPs, the most notable among which are solid lipid nanoparticles (SLNs), nanostructured lipid carriers (NLCs), lipid–drug conjugates (LDCs), and lipid nanocapsules (LNCs).
Raw Materials for mRNA Vaccine
Cap Analogs
Modified ribo-Nucleoside triphosphates
Polynucleotide tail
ribo-Nucleoside diphosphates (NDPs)
ribo-Nucleoside monophosphates (NMPs)
ribo-Nucleoside triphosphates (NTPs)
What are solid lipid nanoparticles?
With the intention of serving as a substitute for liposomes, Solid Lipid Nanoparticles (SLN) were introduced as the first type of lipid-based solid carrier systems in the nanometer range. Colloidal dispersions in water are known as SLN, and its matrix is composed of solid biodegradable lipids.
The utilization of solid lipids, which significantly diminishes the mobility of drugs incorporated into the lipid matrix, is the primary advantage of SLN. Additionally, this process hinders the aggregation of particles, thereby enhancing stability, limiting the penetration of the drug into the emulsifier film, and promoting sustained drug release. An additional crucial characteristic of SLNs is their uptake by the reticuloendothelial system, which allows them to circumvent the first-pass metabolism upon oral administration. This mechanism effectively enhances the drugs’ bioavailability. Additionally, they enable surface modification of the carrier, which improves the pharmacokinetic profile of the medications and extends the time of blood circulation. An additional benefit of employing SLNs is that the carriers themselves are non-toxic (both acute and chronic), as they are composed of “generally regarded as safe” (GRAS) excipients. The majority of SLN preparation techniques involve the formation of an emulsion, which is also referred to as a pre-emulsion, between the surfactant and the solid lipid. Subsequently, size reduction is achieved through the implementation of techniques like homogenization and ultrasonication. The pre-emulsion can be generated in the presence or absence of organic solvents. After dispersing the medications directly into the molten solid lipids at a temperature 5–10 °C higher than their respective melting points, emulsification and size reduction can occur. To process heat-labile drugs, suitable solvents such as ethanol, butyl alcohol, or diethyl ether are utilized to dissolve them. Subsequently, the dissolved drugs are emulsified, which is followed by size reduction and solvent evaporation. Primarily, the size and polydispersity index of the molecule being incorporated determine the method of preparation that is selected. Due to the fact that SLNs are composed primarily of solid lipids, they have a greater affinity for lipophilic pharmaceuticals than hydrophilic ones. Therefore, it has been reported that SLNs containing hydrophilic drugs have poor entrapment efficiency or stability, both of which are attributable to the drugs’ low affinity for the lipid matrix.
The History of Lipid Nanoparticle
As early as 1978, studies on LNP-mRNA were carried out by researchers, which was limited by in vitro experiments and the immunogenicity of mRNA, but no breakthrough was made at that time. Until 2005, professor Katalin and Weissman of Pennsylvania State University used the modified pseudouridine to reduce the immunogenicity of mRNA and play the role of mRNA transfer. Then the launch of the onpattro in 2018 confirmed the effectiveness and safety of the LNP delivery system. After a long-term accumulation of technology, the current LNP-encapsulated nucleic acid system has become a new type of therapy for the prevention and treatment of various diseases. Currently, LNP has successfully entered the clinic for delivery of mRNA and siRNA.
Lipid nanoparticles toxicity
The lipid components used in LNPs are biocompatible and non-toxic. Commonly used lipids, such as phospholipids and cholesterol, are generally well-tolerated by the body. Surface modifications of LNPs, such as the addition of polyethylene glycol (PEG), can enhance biocompatibility and reduce immune responses. However, in some studies, higher levels of LNP also have some toxicity.
The body weight analysis demonstrated that nanoparticles containing solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) induced significant cytotoxicity in vitro but minimal toxicity in vivo. There was no in vitro toxicity or alteration in body weight induced by the nanoemulsion (NE). Conversely, it is possible that the SLN and NLC elicit an in vivo inflammatory response. Lipid peroxidation was observed in the livers of the animals under all nanoparticle systems; however, a reduction in antioxidant defenses was only induced by SLN and NLC. This suggests that the induction of oxidative stress in the liver is the primary mechanism of toxicity. The increased toxicity observed with SLN and NLC may be the cause of this consequence.
Liposomes vs lipid nanoparticles
Liposomes possess a specific vesicular arrangement, whereas lipid nanoparticles contain a wider array of morphologies, such as spherical nanoparticles and nanostructured carriers. Liposomes are composed mainly of phospholipids and cholesterol, while lipid nanoparticles can contain solid lipids, liquid lipids, and various lipid derivatives. Lipid nanoparticles have benefits such as greater stability, regulated drug release, and increased drug carrying capacity in comparison to liposomes.Both liposomes and lipid nanoparticles have a wide range of uses in delivering drugs, with some similarities but also some differences in their advantages, depending on the unique needs of the medication and the tissue being targeted.
The Structure of RNA-LNP
A lipid nanoparticle (LNP) contains hundreds of small interfering RNA (siRNA) molecules, each surrounded by cationic lipids, phospholipids, and cholesterol. The outside of the particle is coated in pegylated lipids. LNPs for messenger RNA (mRNA) are made with similar ingredients but contain only a few mRNA strands. Pegylated lipids can improve the stability of nanoparticles and prolong the metabolism time of nanoparticles in blood. Cationic lipid has high efficient RNA encapsulation and is ionizable. Phospholipids are one of the components of the liposome membrane. Cholesterol regulates membrane fluidity and stabilizes the structure of nanoparticle.
1. Cationic lipids and ionizable lipids
Cationic lipids (CLs) and ionizable lipids (ILs) initiate the first step of self-assembly by electrostatic interaction. Lipid complexes containing cationic lipids are still widely used for the delivery of nucleic acid. However, due to the toxicity and lack of in vivo potency, they have been largely replaced by PH-responsive ionizable lipids. The overall structure of cationic and ionizable lipids can be divided into three parts :(1) head, (2) linker, and (3) tail.
2. Cholesterol
Cholesterol is a naturally abundant cell membrane component and is often used as a structural lipid in LNP formulations. LNP formulations contain approximately 20-50% cholesterol.
3. Phospholipids
Phospholipids help encapsulate nucleic acids and stabilize LNPs. The proportion of total lipids is usually between 10% and 20%. Phospholipids are also commonly used as structural lipids in LNP formulations because they spontaneously organize into lipid bilayers and higher phase transition temperatures enhance the membrane stability of LNP. Like cell membranes, phospholipids are located on the periphery of LNP. These lipids are usually semi-synthetic.
4. Pegylated lipids
Pegylated lipids are an important component of LNP that regulates half-life and cellular uptake. When LNP is assembled, the PEG chain is located on the shell of the nanoparticle due to its hydrophilicity and large size. Like other nanoccarriers, PEG provides an external polymerization layer for LNP to prevent serum protein adsorption and uptake by the monocyte phagocyte system, extending circulation time in vivo. PEG also prevents nanoparticles from accumulating during storage and in the blood. In addition, the amount of Pegylated lipids may determine particle size. Another potential purpose of Pegylated lipids is surface functionalization of LNP. Functional Pegylated lipids can bind LNP to ligands or biological macromolecules.
Advantages of Lipid Nanoparticle
1. High nucleic acid encapsulation efficiency and potent transfection
2. Improved penetration into tissues to deliver therapeutics
3. Low cytotoxicity and immunogenicity
These characteristics make lipid nanoparticles excellent candidates for nucleic acid delivery.
Application of Lipid Nanoparticle
1. mRNA vaccines
LNP can also be used for vaccine research and development to stimulate the body to generate an immune response, rather than the traditionally purified low or non-infectious viruses.
2. Gene therapy
Lipid nanoparticles have been developed and used extensively as nonviral (or synthetic) vectors to treat genetic and acquired disorders in gene therapy.
3. Lipid nanoparticles for drug delivery
Lipid nanoparticles (LNPs) have the ability to encapsulate a diverse array of therapeutic substances, such as hydrophobic medications, nucleic acids (such as mRNA, siRNA, and DNA), peptides, and proteins. The lipid bilayer or matrix of LNPs offers a safeguarded setting for these chemicals, insulating them from deterioration and improving their durability. LNPs enhance the ability of enclosed medications to be absorbed by the body by aiding their passage through biological barriers, such as the gastrointestinal system or the blood-brain barrier. This heightened bioavailability leads to greater medication absorption and enhanced therapeutic effectiveness. LNPs can be manipulated to selectively bind to particular cells or tissues in the body, reducing unintended effects and improving the effectiveness of treatments. Surface modifications, such as the addition of targeting ligands or antibodies, allow LNPs to specifically bind to receptors or antigens present on the surface of desired cells. LNPs provide the regulated release of enclosed medications, facilitating a consistent and extended pattern of drug release kinetics. This controlled release profile maintains ideal medication levels at the intended location for a prolonged duration, decreasing dosage frequency and enhancing patient adherence. LNPs are highly adaptable carriers that may effectively transport various medicinal ingredients and formulations. These can be customized to accommodate different drug payloads and desired release patterns, making them useful for a wide range of applications in drug delivery.
4. Lipid nanoparticles for mrna delivery
When it comes to delivering messenger RNA (mRNA) to cells, lipid nanoparticles (LNPs) are an essential technology. Additionally, the invention and implementation of mRNA COVID-19 vaccines has brought this method to the forefront of attention. For therapeutic reasons, LNPs protect the messenger RNA (mRNA) from degradation and aid expedite its entry into cells, where it can be used to generate proteins. LNPs are capable of enclosing mRNA molecules within a protective shell that is made up of a variety of lipids. PEGylated lipids, which help to stabilize the nanoparticles and prolong their circulation time in the bloodstream, ionizable lipids, which help the nanoparticles to fuse with cell membranes and release their mRNA contents into the cells, structural lipids, which provide structural integrity to the nanoparticles, and cholesterol, which enhances the flexibility and stability of the nanoparticles are typically the core components of lipopolysaccharides (which are a type of nanoparticles).
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References:
- Xu L., et al., Lipid nanoparticles for drug delivery, Advanced NanoBiomed Research, 2022, 2(2): 2100109.
- Munir M., et al., Solid lipid nanoparticles: a versatile approach for controlled release and targeted drug delivery, Journal of Liposome Research, 2023: 1-14.
- Mirchandani Y., et al., Solid lipid nanoparticles for hydrophilic drugs, Journal of Controlled Release, 2021, 335: 457-464.
- Winter E., et al., Development and evaluation of lipid nanoparticles for drug delivery: Study of toxicity in vitro and in vivo, Journal of nanoscience and nanotechnology, 2016, 16(2): 1321-1330.
- Hou X., et al., Lipid nanoparticles for mRNA delivery, Nature Reviews Materials, 2021, 6(12): 1078-1094.