Liposomal drug delivery Flashcards
WHAT ARE LIPOSOMES?
Microscopic spherical vesicles, self-closed structures composed of one or more concentric phospholipid bilayers with an aqueous phase inside and between the lipid bilayers. Head GROUPS are water soluble and tails are water insoluble.
The exterior: phospholipid bilayer
Interior: aqueous core (That’s why heads face inwards towards the core and outside the core)).
A drug that is hydrophobic can be in the lipid bilayer.
A drug that is hydrophilic can go in the aqueous core.
Phospholipids
Phospholipids are the main constituent of liposomes
Phospholipid molecules are amphipathic:
Hydrocarbon; lipophilic “tails”
Polar; hydrophilic “head” groups
Describe the different classification of liposomes
The size of the liposomes
Size ranges from ~20nm to several microns
The number of bilayers (lamellarity)
- Multilamellar Vesicles (MLV): Liposomes with few bilayers (has a size of >0.5 µm)
- Unilamellar Vesicles: Liposomes with one phosopholipid bilayer. Can classify them according to size:
- Small Unilamellar Vesicles (SUV); 25-100 nm. (Good for drug delivery)
- Large Unilamellar Vesicles (LUV); >100 nm (Good for drug delivery)
- Giant Unilamellar Vesicles (GUV); >1 µm
What are the different types of phospholipid?
Cationic (DOTAP)
Anionic (DOPG)
Neutral (DOPC)
Describe the lipid film hydration method. MLVs
MLVs
Add lipids (hydrophobic) and hydrophobic solvent (chloroformethanol) into flask. Evaporate the solvent out (rotary evaporator), then you want a cake of lipids at the bottom of the flask (dried film of lipids).
Drug molecule or diagnostic molecule (hydrophobic), you add it then.
Once you have this, add an aqueous solvent (such as water) and then put it onto ultrasonic something to give it agitation. Then you form liposomes (large). (Then sonication).
Freeze drying – Freeze liposomes and then dry.
Sonication of MLVs (To make liposome smaller)
SUVs
To make liposomes smaller, you sonicate them to make them smaller vesicles. For drug delivery purposes
Describe the Reverse phase evaporation method to make liposomes
Can be used to give high quantities of liposomes
Aqueous solvent added and lipid at the same time to form an emulsion. Organic solvent evaporated, then lipid forces into aqueous phase. Temperature must be above the phase transition temp to form the liposomes.
MLV formed from thin liposomes
Liposomes have a broad size
High Pressure Extrusion (To make liposome size smaller)
Membrane, push liposomes through filter of a fixed pore size to reduce size with pressure.
Must be carried out above the phase transition temperature of lipids.
What is the
• The temperature at which the membrane changes from a well ordered “solid” gel state to a more “fluid” disordered liquid crystal state (TO FORM LIPOSOME). Must be above transition temperature to form it.
- Tm increases with increase in chain length (longer chain ~ more hydrophobic interaction)
- Tm decreases with increase of unsaturation (cis double bond ~ kink in chain, disrupts orderly packing)
Why is cholesterol present within the liposomal formulation?
To stabilise the bilayers and decrease their permeability in the liquid crystalline state
Not affecting the position of the phase transition
However, when in equimolar proportions with the phospholipid component (same amount of cholesterol and lipids and add them together then you: Totally abolish the heat of transition). Makes liposome unstable.
WHY LIPOSOMES AS DDS?
• Once the drug is encapsulated inside liposomes, this can:
• Alter the pharmacokinetic profile of the drug in several ways
• Protect the drug from fast metabolism
• Increase the systemic circulation time
• Provide sustained release of the drug Increase specificity of drug delivery through passive (without adding anything to the surface) and active targeting (cancer) (adding certain molecules to the surface on the liposomes i.e. antibodies)
Therefore, reduce side effects
Therefore, lower the administered doses
LIPOSOMES: PHARMACOKINETICS
• Nanoparticles and liposomes are uptaken by mononuclear phagocyte system (MPS)
• The MPS is an important part of the immune system. Liposomes are seen as foreign by the immune system.
o Consists of fixed macrophages in the liver, connective tissue, lung
o Free and fixed macrophages also reside in the spleen, lymph nodes, bond marrow, nerve tissue and peritoneal region
• Removal of foreign particles (including liposomes) by the MPS involves opsonisation followed by phagocytosis
• This will lead to a reduction in the circulation time of liposomes and prevent the drug of interest from reaching its target
What is opsonisation?
- Foreign particles in the body are coated by blood components (opsonins)
- Opsonins include antibodies, complement factors, serum albumin and fibronectin
- This will lead to their recognition by phagocytic macrophage cells of the MPS and engulf them so will be removed.
- Once opsonised, the particles adhere to the surface of the macrophages and are subsequently engulfed
WHAT AFFECTS MPS RECOGNITION
Larger liposomes cleared quickly than smaller liposomes (so youd want to make small liposomes so that they remain in the body).
Important for passive targetting
Lipid contents
- Cholesterol-rich liposomes cleared more slowly so they circulate longer
- Liposomes composed of saturated phospholipids cleared more slowly
- Neutral liposomes cleared more slowly than those possessing a negative charge or positive charge (serum albumin can bind to positive charges and can make it bigger and signal to macrophages to remove them from the body).
Surface coating
Liposomes coated with polymer (e.g. PEG) are cleared much more slowly = Stealth® liposomes – distracts body from being recognised as foreign. This will increase circulation half-life.
Active targeting using antibodies
PEGYLATED LIPOSOMES (STEALTH® LIPOSOMES)
ACCUMULATION OF PEG-LIPOSOMES IN SOLID TUMORS VIA LEAKY TUMOR VASCULATURE (EPR EFFECT)
- Presence of endothelial fenestrations (gaps) as large as 200-300nm
- Blocks opsonisation
- Increase of liposome circulation time
- Opportunities for the liposomes to interact with target cells
Introducing PEGylation to increase circulation half-life and hence reach the tumour site
How does a lipoplex get through the membrane?
Cationic liposomes + DNA/siRNA (negatively charged)
Liposomes and nanoparticles go into cells via endocytosis.
Other one: Receptor mediated endocytosis (Antibodies on surface).
Problem: You are delivering a DNA protein – harsh acidic environment in an endosome. More likely to fragment and degrade. Can still design liposome to escape endosome and therefore save it from being degraded.
Depends whether you are delivering DNA or siRNA. DNA needs to get into the nucleus.
siRNA goes into the cytoplasm.
Describe
LIPOSOMES & PULMONARY DRUG DELIVERY
Delivery of drugs to the respiratory tract
Promotes rapid therapeutic responses
Allows lower dosing regimen
Minimises systemic side effects
Avoidance of first pass metabolism
However drugs are still rapidly metabolised and /or rapidly cleared by mucociliary escalator
The need for a drug delivery vector that can reach the lower airways
Liposomes have showed to deposit and control the release of drugs within deep airways. Liposomes are also made of phospholipids endogenous to the lungs e.g. DPPC and PG
