What are Liposomes?
Liposomes are spherical vesicles made of phospholipid bilayers. They can encapsulate both hydrophilic and lipophilic drugs within their aqueous interior and membrane respectively. Liposomes were first developed in 1960s as model systems to study cell membranes. Later in 1970s, scientists realized their potential as drug carriers.

Classification of Liposomes
Based on the number of bilayers, liposomes are classified as:

Unilamellar Vesicles
Unilamellar vesicles have only one lipid bilayer and are of two types - small unilamellar vesicles (SUVs) having size 20-100 nm and large unilamellar vesicles (LUVs) having size 100-1000 nm.

Multilamellar Vesicles
Multilamellar vesicles (MLVs) consist of multiple concentric bilayers separated by aqueous compartments and have larger size ranging from 200 nm to 5 μm.

Advantages of Liposome Drug Delivery Systems
Liposomes offer several advantages over conventional drug formulations:

1. Enhanced Solubility: Both hydrophobic and hydrophilic drugs can be encapsulated within liposomes, thereby enhancing their solubility. This is valuable for poorly soluble drugs.

2. Passive Targeting: Liposome Drug Delivery accumulate preferentially in tumor tissues via enhanced permeability and retention (EPR) effect. They extravasate from leaky vasculature and stay longer at the target site.

3. Protection from Degradation: The liposomal encapsulation protects encapsulated drugs from degradation in extracellular environment and during circulation in bloodstream.

4. Controlled Release: Drug release from liposomes can be modulated by manipulation of liposomal properties such as lamellarity, lipid composition etc. Sustained release over prolonged periods is achieved.

5. Reduced Toxicity: Toxic side effects of encapsulated drugs are reduced by controlling biodistribution and minimizing interaction with healthy tissues.

6. Functionalization for Active Targeting: Surface of liposomes can be modified by attachment of ligands like antibodies, peptides, sugars for active targeting to specific cell/tissue types overexpressing corresponding receptors.

Methods of Liposome Preparation
The commonly used methods for liposome preparation are:

Film Hydration Method
It involves dissolving lipids in an organic solvent like chloroform, forming a thin lipid film on the walls of a round bottom flask by evaporating the solvent, and hydrating the dry lipid film with aqueous buffer containing the drug.

Ether Injection Method
It involves vigorous shaking of aqueous solution containing the drug with diethyl ether solution of lipids. Rapid removal of ether results in formation of MLVs.

Extrusion Method
MLVs prepared by above methods are further sized down to homogenous LUVs of desired size by multiple extrusions through polycarbonate filters.

Characterization of Liposomes
Size and lamellarity are important parameters that determine liposome properties. Common techniques used for liposome characterization include:

Photon Correlation Spectroscopy (PCS): To determine size distribution profile and ζ-potential.

Transmission Electron Microscopy (TEM): To visualize morphology and lamellarity.

Differential Scanning Calorimetry (DSC): Gives information about phase transition behavior of lipid bilayers upon heating.

Entrapment Efficiency
Percentage of encapsulated drug relative to total amount used in preparation is calculated as entrapment efficiency using an appropriate analytical method like HPLC. An efficiency >90% is desirable.

Stability Studies
Stability of liposomal formulations is assessed for any changes in size, drug leakage, aggregation etc. over storage period under desired conditions of temperature and humidity as per ICH guidelines.

Applications of Liposomes in Drug Delivery
Liposomes find applications across various therapeutic areas by overcoming specific challenges:

Cancer Therapy
Passive targeting using EPR effect and active targeting via ligand modification enhances drug accumulation at tumor sites. Doxil®, Myocet® are FDA approved liposomal anti-cancer drugs showing reduced toxicities.

Gene Delivery
Cationic liposomes condense and deliver genes intracellularly for gene therapy. BGTC, NGT, VICAL are applying this approach in clinical trials.

Ocular Drug Delivery
Sustained ocular drug levels are achieved by prolonging pre-corneal residence using liposomes. Visudyne®, Triamcinolone liposomes are approved for ocular diseases.

Antimicrobial Therapy
Encapsulation protects drugs from bacterial degradation. Liposomal AmBisome® showed improved efficacy against life-threatening fungal infections with reduced nephrotoxicity.

Topical and Transdermal Delivery
Higher skin permeation and retention helps treat dermatological conditions. Liposomal formulations of tretinoin, isotretinoin, 5-fluorouracil among others offer efficacy benefits.

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