In recent years, liposome-based drug delivery systems have garnered significant attention and acclaim within the pharmaceutical realm. These versatile platforms offer unparalleled opportunities for precise and controlled release of therapeutic agents, revolutionizing the landscape of targeted therapy. This comprehensive review aims to navigate the intricate domain of liposome technology, providing a detailed exploration of its formulation methodologies, diverse application areas, and cutting-edge advancements. At the heart of liposome technology lies the principle of encapsulating drugs within lipid-based vesicles, offering protection and targeted delivery to specific tissues or cells. By modulating the composition and structure of liposomes, researchers can tailor their properties to optimize drug encapsulation efficiency, enhance stability, and improve pharmacokinetic profiles. Such versatility has propelled liposomes to the forefront of drug delivery research, promising breakthroughs in treating a myriad of medical conditions. This review delves into the underlying principles governing liposome design and synthesis, elucidating the diverse strategies employed to overcome challenges and maximize therapeutic efficacy. From innovative formulation methodologies to novel encapsulation techniques, each aspect contributes to the evolution of liposome-based drug delivery systems. Furthermore, we explore the vast application domains of liposomes, ranging from oncology and infectious diseases to inflammatory disorders. Through meticulous scrutiny of preclinical and clinical studies, we highlight the therapeutic potential of liposomes across diverse medical conditions. This comprehensive analysis equips healthcare professionals with a nuanced understanding of liposome-based drug delivery, empowering them to make informed decisions and optimize treatment strategies in clinical practice.

As we embark on this journey through the landscape of liposome-based drug delivery systems, we aim to provide valuable insights that not only enhance current therapeutic approaches but also pave the way for future innovations in patient care and pharmaceutical science.

LIPOSOMES

• Lipid Bilayer:

At the core of liposome structure is the lipid bilayer. Lipids are molecules that have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. When lipids are dispersed in an aqueous solution, they spontaneously form bilayers, with the hydrophilic "heads" oriented towards the surrounding water and the hydrophobic "tails" facing inward, shielded from water.

• Aqueous Core:

Within this lipid bilayer structure lies an aqueous core. This core is essentially a watery compartment enclosed by the lipid bilayer. It provides an environment suitable for encapsulating hydrophilic molecules such as drugs, enzymes, or nucleic acids.

• Size and Morphology:

Liposomes can vary in size from tens to hundreds of nanometers in diameter. They can adopt different morphologies, including spherical (unilamellar vesicles) or multilamellar structures (multiple concentric bilayers).

• Lamellarity:

Liposomes can be unilamellar (single bilayer) or multilamellar (multiple concentric bilayers). Unilamellar liposomes are generally preferred for drug delivery applications due to their homogeneity and controlled release properties.

• Surface Properties:

The outer surface of liposomes can be modified to alter their properties, such as stability, circulation time in the bloodstream, and targeting specificity. Surface modifications can involve attaching targeting ligands, polymers (e.g., PEGylation for enhanced circulation), or other functional molecules.

• Flexibility:

Liposomes possess a degree of flexibility due to the fluid nature of lipid bilayers. This flexibility allows them to deform and adapt to their environment, facilitating interactions with biological membranes and cellular uptake.

History of Liposomes:

The history of liposomes dates back to the 1960s when Alec D. Bangham, a British hematologist, first discovered them while studying blood clotting. In 1961, Bangham and his colleagues observed that lipids extracted from egg yolks formed closed bilayer structures when suspended in water. These structures resembled the phospholipid bilayer of biological membranes and were later termed "liposomes." In 1964, Bangham published a seminal paper describing the properties and potential applications of liposomes. His work laid the foundation for further research into liposome technology, particularly in the field of drug delivery. Throughout the 1970s and 1980s, scientists explored various methods for preparing and characterizing liposomes. This period saw significant advancements in liposome formulation techniques, including the development of sonication, extrusion, and reverse-phase evaporation methods. In 1981, liposomal doxorubicin (Doxil®) became the first liposome-based drug to receive regulatory approval from the United States Food and Drug Administration (FDA) for the treatment of Kaposi's sarcoma, a type of cancer commonly associated with AIDS.

Since then, liposomes have been extensively studied for their potential applications in drug delivery, diagnostics, gene therapy, and vaccines. Researchers have continued to refine liposome formulations, exploring novel lipid compositions, surface modifications, and targeting strategies to enhance their efficacy and safety profiles. Today, liposomes represent one of the most widely investigated drug delivery systems, with numerous liposomal formulations approved for clinical use and many more in various stages of preclinical and clinical development. Their versatility and biocompatibility make them promising tools for addressing diverse therapeutic challenges and advancing medical treatment options.

Types of Liposomes:

1. Based on their Structure

1. Multilamellar Vesicles (MLVs):

 These are large, onion-like structures composed of multiple lipid bilayers separated by aqueous compartments. MLVs are formed by the hydration of dry lipid films and are characterized by their concentric layers of lipid bilayers.

2. Small Unilamellar Vesicles (SUVs):

SUVs are small, spherical vesicles consisting of a single lipid bilayer encapsulating an aqueous core. They are typically formed by sonication or extrusion techniques and have diameters ranging from 20 to 100 nanometers.

3. Large Unilamellar Vesicles (LUVs):

LUVs are similar to SUVs but have larger diameters, typically ranging from 100 to 1000 nanometers. They are also formed by sonication or extrusion methods and are commonly used in drug delivery and research applications.

4. Giant Unilamellar Vesicles (GUVs):

GUVs are very large vesicles with diameters exceeding 1 micrometer. They are formed by various techniques such as electroformation or gentle hydration of lipid films. GUVs are often used in biophysical studies and cell membrane mimicking experiments.

5. Oligolamellar Vesicles (OLVs):

OLVs are intermediate structures between MLVs and SUVs, consisting of a few concentric lipid bilayers with relatively larger diameters compared to SUVs. They are formed by controlled hydration of lipid films and offer a compromise between the stability of MLVs and the size uniformity of SUVs.

2. Based on their Method of preparation

1. Conventional Liposomes:

These liposomes are typically prepared by the thin-film hydration method. In this process, lipid components are dissolved in an organic solvent, and then the solvent is evaporated to form a thin lipid film. Subsequent hydration of the lipid film with an aqueous solution results in the formation of multilamellar vesicles (MLVs), which can then be sonicated or extruded to obtain smaller unilamellar vesicles (SUVs) or large unilamellar vesicles (LUVs), respectively.

2. Sonication Method:

This method involves the direct dispersion of lipids in an aqueous medium followed by sonication using high-frequency sound waves. The mechanical agitation leads to the formation of small unilamellar vesicles (SUVs) or large unilamellar vesicles (LUVs) depending on the sonication parameters.

3. Extrusion Method:

Liposomes prepared by the extrusion method are produced by forcing a lipid suspension through a series of polycarbonate membranes with defined pore sizes. This process results in the formation of liposomes with uniform size distribution, typically small unilamellar vesicles (SUVs).

4. Reverse-Phase Evaporation Method:

In this method, lipids and an aqueous phase containing the drug are mixed in an organic solvent. The solvent is then evaporated under reduced pressure, resulting in the formation of a lipid film. Subsequent hydration of the lipid film leads to the formation of multilamellar vesicles (MLVs), which are then sonicated to obtain small unilamellar vesicles (SUVs) or large unilamellar vesicles (LUVs).

5. Freeze-Thawing Method:

Liposomes can also be prepared by the freeze-thawing method, which involves freezing and thawing lipid dispersions multiple times. This process promotes the formation of multilamellar vesicles (MLVs) with a high encapsulation efficiency for hydrophilic drugs.

6. Hydration of Lipid Films:

Liposomes can be formed by hydrating pre-formed lipid films with an aqueous solution containing the drug of interest. This method is often used for the preparation of large unilamellar vesicles (LUVs) and allows for precise control over the lipid composition and drug encapsulation.

3. Based on their Application:

1. Drug Delivery:

Liposomes are extensively used as carriers for delivering drugs, including chemotherapy agents, antibiotics, and antifungals. They can encapsulate both hydrophilic and hydrophobic drugs, protecting them from degradation and enhancing their bioavailability.

2. Vaccine Delivery

 Liposomes can encapsulate antigens and adjuvants, improving the immune response to vaccines. They facilitate antigen presentation to immune cells and can be tailored to target specific cells or tissues, enhancing vaccine efficacy.

3. Gene Delivery:

Cationic liposomes are employed for delivering nucleic acids, such as DNA, siRNA, and mRNA, into cells for gene therapy, gene silencing, or genetic vaccination. They can form stable complexes with nucleic acids, promoting their cellular uptake and intracellular delivery.

4. Cosmetics and Personal Care:

Liposomes are used in skincare products for delivering active ingredients like vitamins, antioxidants, and moisturizers. They enhance the penetration of these ingredients into the skin, improving their efficacy.

5. Diagnostic Imaging:

Liposomes can be loaded with imaging agents, such as fluorescent dyes or contrast agents, for diagnostic purposes. They enable the visualization of specific tissues or organs in imaging techniques like fluorescence microscopy or magnetic resonance imaging (MRI).

D. Based upon specialty characteristics of liposomes:

1. Targeting Liposomes:

Immunoliposomes:

Modified with targeting ligands like antibodies for specific cell or tissue targeting.

Receptor-Targeted Liposomes:

 Engineered to bind to specific receptors overexpressed in diseased tissues, enhancing targeted drug delivery.

2. Stimuli-Responsive Liposomes:

pH-Sensitive Liposomes:

Release drugs in response to changes in pH, useful for targeted delivery to acidic environments like tumor tissues.

Temperature-Sensitive Liposomes:

Release drugs upon changes in temperature, enabling controlled drug release at specific sites.

3. Functional Liposomes:

Theranostic Liposomes:

Combine therapeutic and diagnostic capabilities for simultaneous imaging and treatment.

Radiolabeled Liposomes: Incorporate radioactive isotopes for imaging or targeted radionuclide therapy.

4. Modified Liposomes:

Stealth Liposomes:

Coated with polymers like PEG to evade immune recognition and prolong circulation time.

Long-Circulating Liposomes:

 Engineered for extended circulation to enhance drug delivery efficiency.

5. Specialized Liposomes:

Nanovaccines:

Liposomal formulations designed for vaccination purposes, often tailored to enhance immune responses.

Bioactive Liposomes:

Loaded with bioactive compounds for therapeutic purposes like wound healing or tissue regeneration.

PREPARATION OF LIPOSOMES:

Material :

1. Lipids:

Lipids are the primary building blocks of liposomes and play a crucial role in their formation. Common lipids used in liposomal formulations include phospholipids such as phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and cholesterol. These lipids self-assemble into bilayer structures, forming the membrane of the liposomes.

2. Drugs or Active Ingredients:

Liposomes can encapsulate a wide range of drugs or active ingredients, including hydrophobic and hydrophilic compounds. Examples include anticancer drugs, antibiotics, antifungal agents, vitamins, antioxidants, and nucleic acids (e.g., siRNA, mRNA). Encapsulation within liposomes can improve the solubility, stability, and bioavailability of these compounds.

3. Stabilizers:

Stabilizers are often added to liposomal formulations to enhance their stability and prevent aggregation or fusion of liposomes. Common stabilizers include sugars (e.g., sucrose, trehalose), polymers (e.g., polyethylene glycol, PEG), and proteins (e.g., serum albumin). These stabilizers can also help prevent premature release of encapsulated drugs and improve circulation time in vivo.

4. Targeting Ligands:

In targeted drug delivery applications, targeting ligands such as antibodies, peptides, or aptamers may be incorporated into liposomal formulations. These ligands can selectively bind to receptors or antigens expressed on target cells or tissues, enabling specific delivery of drugs to desired sites while minimizing off-target effects.

5. Buffering Agents:

Buffering agents such as phosphate-buffered saline (PBS) or HEPES may be used to maintain the pH and osmolarity of the liposomal formulation, ensuring stability and compatibility with biological systems.

6. Chelating Agents:

Chelating agents like ethylenediaminetetraacetic acid (EDTA) may be included to chelate metal ions, which can promote lipid oxidation and destabilize liposomal formulations.

7. Antioxidants:

Antioxidants such as ?-tocopherol (vitamin E) or butylated hydroxytoluene (BHT) may be added to liposomal formulations to protect lipids from oxidation and improve stability during storage.

8. Solvents:

Organic solvents like chloroform or ethanol may be used during the preparation of liposomal formulations to dissolve lipids and drugs, although efforts are typically made to remove these solvents before administration.

Method:

1. Thin-Film Hydration (Film Dispersion) Method:

In the Thin-Film Hydration method, lipids are dissolved in an organic solvent, typically chloroform or methanol, to form a thin lipid film on the walls of a round-bottom flask or a rotary evaporator. The solvent is then removed under reduced pressure, leaving behind a lipid film. Subsequently, an aqueous phase containing the drug of interest is added to the flask, and the mixture is agitated or subjected to sonication. This process rehydrates the lipid film, leading to the formation of liposomes or lipid nanoparticles with the drug encapsulated inside.

Description:

Lipids are dissolved in an organic solvent to form a thin lipid film by evaporation under reduced pressure. The film is then hydrated with an aqueous solution containing the desired drug or payload, followed by sonication or extrusion to form liposomes.

Advantages:

Simple and widely used method, suitable for small-scale preparations.

Disadvantages:

Variability in liposome size, low encapsulation efficiency for hydrophilic drugs.

2. Reverse Phase Evaporation Method:

Lipids, including phospholipids and cholesterol, are dissolved in an organic solvent such as chloroform or ether. Additionally, the drug of interest, often hydrophobic, is added to the organic phase.

Description:

Lipids and an organic solvent are mixed to form a water-in-oil emulsion. The organic solvent is then evaporated under reduced pressure, leading to the formation of a lipid bilayer vesicle in the aqueous phase.

Advantages:

Suitable for encapsulating hydrophilic drugs, higher encapsulation efficiency compared to thin-film hydration.

Disadvantages:

More complex than thin-film hydration, requires specialized equipment.

3. Extrusion Method:

The Extrusion Method is a widely used technique in pharmaceutical science for the preparation of liposomes or lipid nanoparticles, crucial vehicles for drug delivery systems. Initially, lipids, often phospholipids and cholesterol, are dissolved in an organic solvent along with the desired drug compound. This solution is then dried to form a lipid film, typically on the surface of a glass vessel or a solid support like a porous  membrane. Following the formation of the lipid film, hydration occurs as an aqueous solution is introduced, usually a buffer or saline solution. Subsequently, the hydrated lipid suspension undergoes extrusion through a membrane or filter with defined pore sizes. This extrusion process, often facilitated by a syringe extruder or similar device, imparts disruptive forces on the lipid bilayers, prompting their rearrangement into liposomes.

Description:

Lipids are dissolved in an organic solvent, followed by hydration with an aqueous solution. The resulting multilamellar vesicles (MLVs) are then extruded through polycarbonate membranes with defined pore sizes to obtain uniformly sized liposomes.

Advantages:

Allows for precise control of liposome size, suitable for large-scale production.

Disadvantages:

Requires extrusion equipment, may not be suitable for fragile or heat-sensitive drugs.

4. Sonication Method:

The Sonication Method is a common technique for making liposomes or lipid nanoparticles. Lipids are dissolved in a solvent, dried to form a film, then rehydrated in water. Ultrasonic waves are applied to break up the lipids into smaller vesicles. This method is quick, versatile, and suitable for encapsulating various types of drugs.

 Lipids are dissolved in an organic solvent and hydrated with an aqueous solution to form MLVs. The MLVs are then subjected to ultrasonic irradiation, leading to the formation of smaller unilamellar vesicles (SUVs) or large unilamellar vesicles (LUVs).

Advantages:

Simple and rapid method, suitable for small-scale preparations.

Disadvantages:

Limited control over liposome size distribution, may lead to lipid degradation or drug loss.

5. Detergent Removal Method (Dialysis):

The Detergent Removal Method, typically carried out through dialysis, is a technique for preparing liposomes or lipid nanoparticles. Initially, lipids and the drug are dissolved in an organic solvent along with a detergent, forming a lipid-detergent solution. This solution is then mixed with an aqueous phase, leading to the formation of mixed micelles. Dialysis is employed to remove the detergent, allowing lipid bilayers to spontaneously form and encapsulate the drug within liposomes. This method offers control over drug encapsulation, particularly for hydrophobic drugs, and is suitable for large-scale production.

Description:

Lipids and a detergent are mixed with an aqueous solution containing the payload. The detergent is then removed by dialysis against a buffer solution, leading to the spontaneous formation of liposomes.

Advantages:

Suitable for encapsulating hydrophilic drugs, gentle method that preserves drug integrity.

Disadvantages:

Requires careful optimization of detergent concentration and dialysis conditions, may result in lower encapsulation efficiency.

EVALUTION OF LIPOSOME