Phospholipids

Phospholipids are an integral part of drug delivery systems today and offer multipurpose solutions to capture, stabilize, and dispense medications. They are the kind of molecules that make it possible to create lipid bilayers, vesicles, and micelles that are essential to the synthesis of many different drug delivery systems.

So What Are Phospholipids and What Do They Do?

Phospholipids are organic molecules with a hydrophilic phosphate head and hydrophobic fatty acid tails. This amphiphilic aspect is their essential role in drug delivery. In water, the phospholipids simply form bilayers or micelles, with the hydrophilic heads exposed to the water and the hydrophobic tails confined inside. It is this self-assembling capacity that allows phospholipids to create stable drug carriers such as liposomes or micelles that contain both hydrophobic and hydrophilic drugs.

How Do Phospholipids Deliver Drugs?

• Protection and Stabilization: Phospholipid bilayers shield encapsulated drugs from enzymatic breakdown and oxidative degradation, prolonging expiration dates.
• Targeted delivery: Targeted delivery of drugs is possible through modifications to phospholipid molecules. By adding ligands or antibodies, for instance, phospholipid liposomes can target and attach to the right cells or tissues.
• Greater biocompatibility: Phospholipids are biocompatible and toxicity-free, so they are the perfect drug carrier compounds. Moreover, phospholipids can be optimised for drug delivery by altering their chemical composition and surface modification.
• Controlled release: Different membrane thickness and pore sizes are possible in phospholipid liposomes to control drug release, whether it's prolonged or pulsatile.

How Are Phospholipids Used in Drug Delivery Systems?

Liposomes: Targeted Drug Delivery Systems

Liposomes are globular vesicles containing phospholipid bilayers that can hold both hydrophilic and hydrophobic molecules. They have some benefits, such as shielding the drugs from enzymatic breakdown, decreasing toxicity, and allowing selective, controlled release. Liposomes can be tailored for improved targeting capability by size alone, active ligand, or antibody surface attachment. This is why liposomes are the perfect vehicle for cancer therapy, gene therapy, and vaccines.

Among the most important applications of liposomes are chemotherapeutics. Antibodies that are harmful to healthy cells could be packed into liposomes and then directed at the tumor cells with minimal side effects. Liposomal doxorubicin, for instance, proved more effective and less cardiotoxic than free drug formulations.

Figure 1. Different phospholipid-based vesicles for drug delivery^[1].

Micelles: Overcoming Poor Drug Solubility

Micelles are nanosized molecular structures of amphiphilic molecules such as phospholipids that coalesce into spheres in water. These structures have hydrophobic centers that dissolve poorly soluble pharmaceuticals and render them bioavailable. Micelles are particularly useful for water-residual drugs like most anticancer medications, antibiotics, and anti-inflammatory medicines. By packing them, micelles keep these medications stable in solution and transport them to the endpoint.

Micellar formulations have found uses in cancer and chronic inflammation, where hydrophobic drugs must be brought to the point of action as quickly as possible for therapeutic effect.

Figure 2. Illustration of contact-facilitated drug delivery mechanism where lipid membrane of phospholipid-fumagillin containing nanoparticles^[2].

Phospholipid Complexes: Drug Stability and Solubility Enhancer

Phospholipids can also be associated with other medications to increase stability, solubleness, and bioavailability. A classic case is in phosphatidylcholine (lecithin) used to make intravenous emulsions. These phospholipid complexes are excellent for hydrophobic drugs and allow for the administration of hydrophobic medications in solutions, so that they remain stable and absorbable during treatment.

Figure 3. Drug-phospholipid complexes are considered ideal for oral administration^[3].

Applications in Tumor Treatment, Gene Therapy, and Beyond

Phospholipids have applications for drug delivery in a variety of therapeutic fields, proving to be multidisciplinary in the management of complex pathologies. Similarly in oncology, phospholipid liposomes have revolutionised chemotherapy drug delivery with targeted treatment that minimizes systemic side effects while improving the drug's potency in the tumor. As for gene therapy, phospholipid vesicles such as sphingosomes, which inject nucleic acids into the cells in question, represent a promising option for treating genetic disease.

Phospholipids enable drugs to be delivered into the inflammatory tissue, where they reduce systemic side effects and improve the local response. This has proved especially useful in the management of rheumatoid arthritis and inflammatory bowel disease (IBD).

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