Liposomes Detailed Notes

Introduction to Liposomes

• Liposomes are microscopic vesicles.
• Composed of one or several lipid membranes.
• Surround a discrete aqueous space.
• Contain lipid-soluble drugs within the membrane itself.

Chemistry of Phospholipids (Amphipathic)

• Phospholipids have a polar head group and two hydrocarbon tails.
• The presence of polar and nonpolar groups gives them amphipathic character.
• This amphipathic nature causes phospholipids to form closed bilayers in water.
• Tails associate, and polar head groups orient toward the bulk water phase, forming lipid bilayers.

Phospholipid Bilayer Structure

• Polar (Hydrophilic) head: Attracted to water.
• Non-polar (Hydrophobic) tail: Repelled by water.
• Aqueous Interior: Contains water-soluble drugs.
• Lipid Bilayer: Contains lipid-soluble drugs.

Types of Liposomes

• Liposomes are classified according to:
  • Size
  • Methods of Preparation

A- Size:

• 1 . Multi lamellar vesicles (ML.Vs):
  • Have more than one lamella.
  • Size varies between 100 to $1000 nm$.
• 2 . Small unilamellar vesicles (SUVs):
  • Smallest possible size for phospholipid vesicles.
  • Have just a single lamella.
  • Diameters less than $50 nm$.
• 3. Large unilamellar vesicles (LUVs):
  • Diameters greater than SUVs (order of $50$ to $10,000 nm$).
  • Close to the size of living cells.
  • Have just a single lamella.

B- Methods of Preparation:

• 1. Multilamellar vesicles
  • Produced by raising the temperature of the lipid in contact with water.
  • Gentle manual shaking, also called 'handshaken liposomes'.
• 2 . Small unilamellar vesicles
  • Originally produced using ultrasonic irradiation to break up MLV suspensions.
  • MLV suspension is completely broken down in the process.
• 3 . Large unilamellar vesicles
  • Prepared from SUVs and ether injection.
  • Involves introducing an ether solution into a hot aqueous buffer to form large planar sheets of bilayer membrane.

Cholesterol's Role in the Membrane

• Cholesterol molecules have several functions:
  • a ) Immobilize the first few hydrocarbon groups of the phospholipids molecules.
    • Makes the lipid bilayer less deformable.
    • Decreases its permeability to small water-soluble molecules.
  • b ) Cholesterol prevents crystallization of hydrocarbons and phase shifts in the membrane.

Liposomal Targeting

• Several methods exist for targeting liposomes to cellular systems.
• Classifications:
  • (1) Physical targeting
  • (2) Active targeting

(1) Physical targeting:

• Depends on directing a liposome to a location where the environment induces content release.
  • a ) Temperature-sensitive targeting
    • Lipid chain length is chosen such that the agent leaks slowly at body temperature.
    • Heating the target site above the chain-melting temperature releases the agent.
  • b ) PH-sensitive liposomes
    • Release contents in regions of low pH such as infection, inflammation, and tumors.
  • c ) Light sensitivity
    • Contents are released upon exposure to light of appropriate wavelength.
    • Mechanisms:
      • ( 1 ) Light-induced phase transitions in bilayer lipids.
      • ( 2 ) Lipids undergoing photodegradation.
      • ( 3) Lipids undergoing photoisomerization (conformational changes).
  • d ) Magnetic field sensitivity
    • Uses a magnetic field to direct liposomes encapsulating magnetic and iron particles.

(2) Active targeting:

• Involves modifying the liposome surface to bind specifically to receptor sites on the target cell.
• Cell surfaces are coated with carbohydrate residues linked to glycolipids and glycoproteins.
• Active targeting generally involves carbohydrate-protein interaction.
• Immunoliposomes: specific antibodies attached to the liposome surface for targeting.
• Attaching proteins (antibodies) to the liposomal surface forms "proteoliposomes".

Liposome for Drug Delivery

• Water-insoluble drugs are carried in the hydrophobic region.
• Protective layer against immune destruction.
• Water-soluble drugs are carried in the hydrophilic region.
• Proteins bonded onto surface can target the liposome.
• Lipid soluble drug in bilayer.

Interactions of Liposomes with Cells

• Four principle ways liposomes interact with cells:
  • (1) Adsorption
  • (2) Endocytosis
  • (3) Lipid exchange
  • (4) Fusion

Applications of Liposomes

• (1) Cancer phototherapy and radiotherapy using liposomes
  • (A) Phototherapy
  • (B) Radiotherapy
• (2) Successful carrier systems for targeted delivery of drugs

(1) Cancer phototherapy and radiotherapy using liposomes:

• Most phototherapy and radiotherapy act primarily on dividing cells.
• Large fraction of tumor (because of the fast growth), can be deprived of $O_2$.
• These cells are therefore much more resistance to phototherapy and radiotherapy because $O_2$ increase the effects of both radiotherapy and phototherapy.

Phototherapy:

• Liposomes accumulate photo sensitizer in tumor cell.
• The body is transparent for wavelengths above $650 nm$.
• Typical penetration depths are up to $2 cm$, after illumination, the excited photosensitizer generates single oxygen which is toxic and by photo oxidation kills the cell.

Radiotherapy:

• ( 1 ) Classical Radiotherapy normally with $β$ or $γ$ radiation. These rays require $O_2$ to increase their biological affectivity.
• Tumor cells have normally very low $O_2$ levels and are more resistant to radiotherapy than healthy cells.
• alpha particles cannot be used because their path length through tissue is too short. The advantages of using alpha particles would be that they do not require $O_2$ for their effectiveness and they destroy all tumor cells.
• Because of this short path length, however, appropriate targeting it required. Boron has a very large cross-section for capturing neutrons upon irradiation with low energy neutrons. Producing alpha particle which has a path length of $10 μm$. The treatment includes administration of boron atoms by liposomes and irradiation of the tissue with neutrons and liposomes offer a way of delivering high concentration of Boron atoms into tumor sites.
• ( 2 ) Radiotherapy can be enhanced by the presence of $O_2$ or radio sensitizers by using liposomes containing hemoglobin enhance oxygen concentration in tumors and increase the efficacy of radiotherapy at no additional risk.

(2) Successful carrier systems for targeted delivery of drugs:

• Liposomes are used as carriers for drugs because they can serve several different purposes, liposomes can direct a drug to a certain target.
• Secondly, liposomes can prolong the duration of drug exposure. Liposomes can protect a drug against degradation ( e.g. metabolic degradation).
• Liposomes can protect the patient against side effects of the encapsulated drugs. For example, liposome encapsulation greatly reduces exposure of the heart to doxorubicin and thereby its cardio toxicity.