Liposomes vs Lipid Nanoparticles

Liposomes have garnered attention as preferred drug carriers. They boast biocompatibility, often eliciting minimal adverse reactions. With a dual-component structure, liposomes can encapsulate both water-soluble and hydrophobic active pharmaceutical ingredients, making them excellent vehicles for drug delivery. Similarly, lipid nanoparticles, akin to liposomes, offer effective delivery of therapeutic payloads, including DNA and mRNA for vaccines, representing a novel colloidal drug delivery system. While liposomes feature an encapsulated aqueous phase, lipid nanoparticles lack this feature, resulting in slight differences in form, composition, and function.

What are Liposomes?

Because of their biocompatibility and ability to degrade naturally, liposomes are extensively studied as drug carriers. They possess a unique structure composed of lipid bilayers primarily made of amphiphilic phospholipids, with an aqueous interior. This allows liposomes to encapsulate both water-soluble/hydrophilic compounds in their aqueous core and lipid-soluble/hydrophobic compounds in their lipid bilayers. Liposomes have become essential tools for delivering drugs and medications. Currently, many liposomal formulations are clinically used for various purposes, including anticancer, anti-inflammatory, antimicrobial, antifungal, anesthetic, and gene therapy treatments.

Features of Liposomes

• Targeting: Liposomes can selectively enter certain tissues or organs of the body, such as the liver and spleen. The concentration of liposomal drugs in the liver is 200 to 700 times higher than that of ordinary drugs.
• Sustained Release: As the drug is wrapped by liposomes, the diffusion speed is reduced, and liposome preparation can delay the excretion and metabolism in the kidney, prolonging the action time.
• Reduced Drug Toxicity: Liposome phospholipid bilayer membrane is similar to mammalian cell membrane, which can reduce the body's immune response and not easy to cause allergic and other immune reactions. For example, amphotericin B liposome can reduce cardiotoxicity.
• Improved Stability: Long-term storage of drugs is easy to deteriorate, but under the protection of the liposome molecular layer, the possibility of oxidative degradation of the drug is greatly reduced, thus prolonging the preservation time of the drug.
• Multiple Routes of Drug Delivery: Liposomes can be formulated for various routes of administration, not only for intravenous administration but also for subcutaneous, intramuscular injection, and mucosal administration. They can also be formulated as creams, oral solutions, etc.
• Controllable Drug Distribution: Due to their targeting capabilities, liposomes can have their surface properties altered during preparation, changing their targeting characteristics and controlling the distribution of drugs within organs and tissues in the body.

Limitations of Liposomes

• Difficulty constraints in large-scale production: Large-scale preparation of liposomes may involve complex processes and high costs, which to some extent affects their wide application in the pharmaceutical industry.
• Stability problems under bumpy and crushed environments: Liposomes may become unstable during prolonged storage and transportation, leading to structural rupture of the liposome or drug leakage, which may affect the efficacy of the drug.
• Limitation of poorly water-soluble drugs: Liposomes have better carrying capacity for water-soluble drugs, while for some poorly water-soluble drugs, the drug loading capacity of liposomes may be limited.
• Limitations of Liposome Drug Encapsulation Rate: Liposomes have a limited drug loading capacity, which may limit the ability to deliver large amounts of drug at once. This may be a challenge for treatments requiring high doses.

What are Lipid Nanoparticles?

Lipid Nanoparticle (LNP) technology can be considered the most advanced non-viral gene delivery system in clinical practice. LNPs are stable nanoparticles composed of functional lipids, polyethylene glycol-modified lipids (PEGylated lipids), saturated phospholipids (DSPC), and cholesterol. Functional lipids are ionizable cationic lipids that play a crucial role in cellular uptake and release. LNPs are typically spherical, with an average diameter ranging from 10 to 1000 nanometers. They consist of a lipid core that dissolves lipophilic molecules and a surfactant layer that stabilizes the particles and protects the nucleic acid payload. The lipid core can be solid or liquid, depending on the type and composition of lipids used. The surfactant layer may include various biological membrane lipids such as phospholipids, cholesterol, bile salts, or sterols. Selectively, LNPs may also have targeting molecules, such as antibodies or peptides, attached to their surface to enhance their specificity and uptake by certain cells.

Challenges of LNP-based Drug Delivery

• Formulation Processes and Optimization: LNPs require complex and precise processes to obtain the desired size, shape, charge, stability, encapsulation rate, and release kinetics, and also need to be compatible with different nucleic acid types and applications.
• Toxicity and Immunogenicity: lipid nanoparticles can trigger inflammatory reactions, immune responses or adverse effects in host cells or organs, depending on the type and composition of the lipid or surfactant used, and can also interfere with endogenous lipid metabolism or clearance mechanisms.
• Targeting and Penetration: LNP needs to overcome various biological barriers, such as the blood-brain barrier, the mucosal barrier, or the tumor microenvironment, in order to reach the target cells or tissues, and it also needs to avoid uptake by the reticuloendothelial system or other non-target cells.
• Transformation and Scale-up: LNPs need to be manufactured at an industrial level with high quality, safety and reproducibility.LNPs also need to meet regulatory requirements and clinical standards for different therapeutic indications.

Improving Strategies for LNP Drug Delivery

• Modification of Lipid or Surfactant Composition: LNPs can be designed with different types and ratios of lipids or surfactants to optimize their physicochemical properties such as size, charge, stability, encapsulation rate and release kinetics. They can also be modified with biocompatible or biodegradable lipids or surfactants to reduce their toxicity or immunogenicity.
• Attachment to Targeting Molecules: LNPs can be functionalized with ligands, such as antibodies, peptides, aptamers, or carbohydrates, that bind to specific receptors or antigens on the surface of target cells or tissues. This can enhance the specificity, uptake and intracellular delivery of LNPs.
• Use of External Stimuli: LNPs can respond to a variety of external stimuli such as light, temperature, pH, magnetic fields or ultrasound, which can trigger the release of the nucleic acid payload at the desired site, which can enhance the spatiotemporal control and efficacy of the LNP.
• Perform Systematic Preformulation Studies: In preclinical experiments, the physicochemical properties, biological interactions, pharmacokinetic profiles, and safety issues of LNPs can be evaluated. This helps to identify optimal formulation parameters, predict potential challenges and optimize the scale-up process.

Comparison of Liposomes and Lipid Nanoparticles

The key difference between liposomes and lipid nanoparticles lies not only in their respective applications, but also in their own morphology, structure, composition and production processes.

• Broadly speaking, lipid nanoparticles are nanoparticles formed by lipids, so liposomes are also a type of lipid nanoparticle. However, in specific scientific research, lipid nanoparticles are used to describe a type of nanoparticle distinct from liposomes. The main difference between liposomes and lipid nanoparticles lies in their morphology. Liposomes are organized into bilayered lipid structures, forming closed vesicles with a hydrophilic interior cavity. On the other hand, lipid nanoparticles lack a hydrophilic cavity. Instead, lipid nanoparticles form multilayered cores dispersed between lipid layers due to electrostatic interactions between cationic lipids and negatively charged nucleic acids present internally.
• As for their makeup, liposomes and lipid nanoparticles share common components, primarily lipids and cholesterol. However, while lipid nanoparticles require the presence of ionizable lipids in their formulation, liposomes do not impose such strict requirements on lipid types. Nevertheless, notable distinctions exist between liposomes and lipid nanoparticles in the proportion of each constituent, particularly the amount of cholesterol incorporated. For example, the classic liposome product DOXIL typically consists of HSPC:CHOL:DSPE-PEG2000 = 3:1:1. Conversely, the components of the two mRNA COVID-19 vaccines currently in circulation exhibit a considerably higher cholesterol content, with values of 42.7% and 38.5%, respectively, surpassing that found in liposomes.
• During the production process, the upstream processes and steps for liposomes and lipid nanoparticles differ, but the downstream production processes are nearly identical. In traditional liposome preparation, coarse liposomes are first formed by combining the lipid phase and aqueous phase, followed by controlling the particle size through homogenization or extrusion processes. In contrast, lipid nanoparticles are rapidly mixed with lipid ethanol solution and nucleic acid acidic aqueous solution through a microfluidic mixing system, which has a connector to control particle size during the mixing and collision process. The downstream buffer exchange process for both liposomes and lipid nanoparticles is essentially the same, employing Tangential Flow Filtration (TFF) technology for buffer exchange or purification, followed by terminal 0.22 μm membrane filtration to remove bacteria.

Liposomes and lipid nanoparticles stem from a shared origin, with liposomes having already established a significant presence in drug delivery across various indications. In contrast, lipid nanoparticles are primarily utilized for nucleic acid delivery at present, showing promising potential for further development and application.