Advanced Drug Delivery 2 - liposomes

Question 1

What are liposomes and their size

Answer 1

• Microparticulate or colloidal drug carriers
• Vesicular structures composed of one or more lipid bilayers encapsulating a central aqueous core
• The lipid molecules are normally phospholipids
• Size: 0.05 – 5 µm

Question 2

Chemical structure of phospholipids

Answer 2

• Two fatty acid tails (hydrophobic)
• Phosphate group head (hydrophilic)
• This structural arrangement allows them to form lipid bilayers

Question 3

Hydrophilic head

Answer 3

Phosphate moiety joined together by an alcohol or glycerol molecule

Question 4

Characteristics of fatty acids

Answer 4

• fatty = lipophilic
• acid = COOH carboxylic group

Question 5

What charge is phosphate

Answer 5

Negative

Question 6

Phosphatidylcholine (PC) derivatives

Answer 6

• Phosphate group is -
• PC is +
• So overall neutral

Question 7

Phosphatidylethanolamine (PE) derivatives

Answer 7

• Phosphate group is -
• PE is +
• So overall neutral

Question 8

Phosphatidylserine (PS) derivatives

Answer 8

• Phosphate group is -
• PS has one + and one -
• So overall negative

Question 9

Phosphatidylglycerol (PG) derivatives

Answer 9

• Phosphate group is -
• PG is -
• So overall negative

Question 10

Discuss cholesterol in the context of liposomes

Answer 10

• Usually added to liposomes
• Amphiphilic:
  Mostly hydrophobic
  OH group makes is slightly hydrophilic
• This allows cholesterol to insert itself in the bilayer with the OH group close to polar head and hydrophobic part next to lipophilic chains

Question 11

The presence of OH group and the properties of liposomes

Answer 11

• The OH groups can form hydrogen bonds with other molecules in the liposome membrane
• Increases its stability and rigidity.
• Can impact the solubility and permeability of the liposome

Question 12

Main transition temperature (TM)

Answer 12

Phospholipid membranes have a parameter called TM

Below the TM:
- cholesterol has a fluidising, disorganising action
- phospholipids bilayer is less fluid, more gel like

Above TM:
- cholesterol has a condensing action
- a lot of energy in system
- bilayer very fluid

Question 13

Conformation of a lisaphospholipid

Answer 13

• e.g. detergents
• Geometry: like a cone
• Therefore most thermodynamically stable conformation is a micelle

Question 14

Conformation of double chain phospholipids with LARGE head groups

Answer 14

• e.g. PC
• Large polar head and smaller tails that occupy smaller volume
• Geometry: like truncated cone
• Therefore, most thermodynamically stable conformation is bilayer vesicle

Question 15

Conformation of double chain phospholipid with SMALL head groups

Answer 15

• e.g. PE
• Geometry: like a cyclinder
• Therefore, most thermodynamically stable conformation is planar bilayer

Question 16

Conformation of phospholipid with two chains and unsaturation

Answer 16

• e.g. Phosphatidylethanol amine (unsaturated)
• Double bonds = tails spreads wider
• Geometry: Inverted truncated cone
• Therefore, most thermodynamically stable conformation is inverted micelle

Question 17

Classification of Liposomes

Answer 17

• SUV - small unilamellar vesicles
• LUV - large unilamellar vesicles
• MLV - multilamellar vesicles
• MVV - multi vesicular vesicles

Question 18

SUV

Answer 18

small unilamellar vesicle (SUV)

Question 19

LUV

Answer 19

large unilameller vesicles (LUV)

Question 20

MLV

Answer 20

multilamellar vesicles

Question 21

MVV

Answer 21

multi vesicular vesicles

Question 22

• How can liposomes be used as drug carriers?

Answer 22

• Can carry both hydrophobic and hydrophilic drugs
• Encapsulate the drug within the liposome; which protects from degradation and improves solubility in water
• Target specific cells/tissues; improves efficiency of drug and reduce SE
• Liposomes can be designed to release the drug in a controlled manner

Question 23

Where hydrophobic and hydrophilic drugs inserts themselves in the liposomes

Answer 23

• hydrophobic drugs = hydrophobic tails (middle of bilayer)
• hydrophilic drugs = aqueous core

Question 24

4 Characteristics of liposomes

Answer 24

1. Drug:lipid ratio
2. Encapsulation efficiency
3. Size
4. Lamellarity

Question 25

Challenge of liposomes

Answer 25

- Bilayers form spontaneously after addition of water to phospholipids. - The challenge is forming stable vesicles with the desired: 1. Size 2. Physio-chemical properties 3. A high drug encapsulation frequency

Question 26

What is high drug encapsulation efficiency

Answer 26

- Measure of how efficiently the drug is incorporated into the liposome - Allowing for less liposomes to be administered (based on efficacy)

Question 27

Typical methodology of preparing liposomes

Answer 27

1. Lipid hydration 2. Selection of liposomes based on size 3. Remove the non-encapsulated drug

Question 28

Thin layer evaporation method of preparing liposomes

Answer 28

1. Mixture of phospholipids dissolved in organic solvent 2. Solvent evaporation leads to phospholipid film in flask 3. Add water-based buffer solution into phospholipid film for lipid hydration 4. Stirring; temperature > transition temperature 5. This causes film hydration under stirring 6. Liposome will start forming

Question 29

Additional methods to form desired lysosome

Answer 29

e.g. Sonification and extrusion

Question 30

What is sonification

Answer 30

1. Prepare a solution of PC in chloroform 2. Place it in a round bottom flask and evaporate solvent 3. Add an aqueous solution of the drug stirring 4. Put liposome mixture in an ultrasound bath to reduce particle size

Question 31

What is extrusion

Answer 31

- Process carried out through polycarbonate filters - to control size under pressure by controlling size of pores in filter

Question 32

Administration routes of liposomes

Answer 32

- Parenteral - Pulmonary - Oral - Topical

Question 33

Biodistribution of liposomes and PK of the API depend on

Answer 33

- size of vesicles - composition of bilayer - fluidity of bilayer

Question 34

Stability of issues before administration and solution

Answer 34

- Oxidation of lipophilic chains - Hydrolysis and formation of lysophospholipids - Tendency to form aggregates - Solution: freeze drying

Question 35

Stabilities issues of liposomes after administration and solution

Answer 35

- Captured by macrophages - Solution: long circulating liposomes (LCL) aka sterically stabilised liposomes (stealth) - attach PEG chains, macrophages will not recognise them, allowing them to circulate for longer

Question 36

4 types of liposomes based on composition and application

Answer 36

1. Conventional liposomes 2. Sterically stabilised (stealth) liposomes 3. Immunoliposomes (antibody targeted) 4. Cationic liposomes (for gene delivery)

Question 37

Clinical applications of liposomes

Answer 37

1. Abelcet 2. AmBisome 3. Doxil

Question 38

What is Abelcet

Answer 38

- Contains amphotericin B - Anti fungal with good activity BUT nephrotoxic - Sodium chloride added for tonicity

Question 39

What is AmBisome

Answer 39

- Amphotericin B encapsulated in conventional liposomes - Parenteral use - SUV (small unilamellar vesicles) 50-100nm - Lower toxicity than normal amphotericin B

Question 40

What is Doxil

Answer 40

- Liposomal encapsulated doxorubicin - Anticancer agent that is very cardiotoxic - Liposomes reduce toxicity - Contain PEGlyated liposomes (long circulating liposomes)