Liposome Components and Structure
Liposomes are composed of natural or synthetic phospholipids that self-assemble in aqueous environments. The phospholipid molecules contain lipophilic fatty acid "tails" and hydrophilic phosphate "heads". When mixed with water, the phospholipids spontaneously form concentric bilayers resembling an onion. An internal aqueous core allows water-soluble molecules to be encapsulated, while the hydrophobic lipid bilayers sequester fat-soluble compounds. Cholesterol is often incorporated into the bilayers to improve stability. Additional surface modifications can functionalize liposomes for targeting moieties.
Loading Strategies
Drugs can be loaded into liposomes via several methods depending on the drug's solubility. Water-soluble drugs dissolve within the inner aqueous core, while lipid-soluble drugs partition into the bilayer membranes. A pH or concentration gradient driving force can actively load drugs across the bilayer. Once loaded, drugs are stably contained yet readily released upon liposome uptake by target cells. Entrapping drugs within liposomes can protect them from degradation and reduce toxicity.
Benefits of Liposome Drug Delivery
Liposome Drug Delivery provide significant benefits over free drug administration. Extensive circulation allows liposomes to accumulate preferentially in tumor tissues via the enhanced permeability and retention effect. This passive targeting improves therapeutic indices for cancer chemotherapy. Modified liposomes can also actively target receptors on specific cell surfaces. Surface ligands like antibodies selectively deliver drugs to pathogens, diseased cells, or desired organs. Controlled release at the target site maintains therapeutic levels while minimizing plasma concentrations and systemic side effects. Liposomes have proven especially useful for cancer, antifungal, antiviral, and anti-inflammatory therapies.
Approved Liposomal Products
Several liposomal drug formulations have gained FDA approval since 1995. Doxil was the first liposomal cancer drug approved for treating HIV-related Kaposi's sarcoma and ovarian cancer. Liposomal amphotericin B formulations like AmBisome revolutionized treatment of severe fungal infections. Onivyde, a liposomal irinotecan, improves outcomes for metastatic pancreatic cancer. Cipla and Cadila Healthcare also market generic liposomal drugs in India. These approvals validated liposomes as an efficient drug delivery system with commercial applications. Many more products are in clinical development.
Novel Surface Modifications
Ongoing research optimizes liposomal targeting ability. "Stealth" liposomes incorporate polymers like PEG to evade reticuloendothelial system uptake, extending circulation half-lives. Positive or negative surface charges can improve cellular binding and uptake. Antibody, peptide, or aptamer conjugation actively delivers liposomes to disease biomarkers for enhanced localization. Stimuli-responsive "smart" liposomes are another active area - they destabilize only in response to triggers like pH, enzymes, or temperature at target sites to control release. These surface-engineered liposomes expand the therapeutic scope and commercial potential of the liposomal platform.
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