Targeted Drug Delivery

Question 1

What is the problem of non-targeted drug delivery?

Answer 1

• Drugs that are injected IV will have little inherent affinity for a specific site and distribute by normal pharmacokinetic principles to many sites
• Therefore, the drug at these sites are at a very low concentration (not much potency) and with no specificity.
• If potency wants to be increased, then concentration will have to be increased, thus increasing chance of toxicity to unncessary areas.

Question 2

Why is targeted or site specific drug delivery important?

Answer 2

• Targeting to a specific site has the potential to:
  
  • increase specificity
    
    • which reduces need for higher drug dose = potentially saving money
    • also increases chance of drug to target site interactions = higher potency
  • reduce toxicity
    
    • by preventing the spread of drug to unncessary areas

Question 3

What THREE main ways can targeted drug delivery system be achieved by?

Answer 3

1. A carrier that has a passive affinity (e.g. accumulation) for a specific site and that decreases free drug concentrations in general circulation.
   
   1. Microspheres/Nanoparticles
   2. Liposomes
   3. Polymers (Linear & Dendrimer)
   4. ‘Stealth’ Technology with PEG chains or sphingomyelin
2. A drug conjugated to a targeting molecule with high affinity for a particular site.
   
   1. Receptor agonist/antagonists
   2. Lectins
   3. Antibodies
3. A carrier that also contains a targeting group to actively target to a specific site.
   
   1. PEGylated liposome + HER2 antibody fragments

Question 4

Describe the rationale underpinning the enhanced permeability and retention effect.

Answer 4

• Enhanced Permeability and Retention (EPR) is a method of Passive Targeting.
  
  • Enhanced Permeability:
    
    • Large macromolecules/colloids escape vasculature poorly, but preferentially accumulate in tumours via ‘leaky’ vasculature.
  • Retention:
    
    • ​Once in the tumour, these large macromolecules/colloids are also not well drained away via the compressed lymph vessels that becomes ineffective.
      
      • ​Due to the rapid growth of tumour tissues that cause a build-up of pressure b/w cells that squash the lymph vessels.

Question 5

What is the limitation of the Enhanced Permeability and Retention (EPR) Passive Targeting method. What can overcome this limitation?

Answer 5

It needs to avoid the Reticuloendothelial System (RES) & Monocyte-Phagocyte System (MPS). Therefore, the key for EPR is to for the colloid to circulate long enough to accumulate in the tumour.

‘Stealth’ Technology can be applied to avoid the RES/MPS uptake to increase circulation time allowing for targetting to the tumour.

Question 6

Name THREE types of typical drug carriers

Answer 6

1. Microspeheres/Nanoparticles
2. Liposomes
3. Polymers (Linear & Dendritic)

Question 7

What is the difference between a Microsphere and a Nanoparticle?

Answer 7

The size.

Microspheres are > 1000 nm

Nanoparticles are < 1000 nm

Question 8

What is the principle behind Microspheres/Nanoparticles as a drug carrier?

Answer 8

• They are solid polymeric spheres with:
  
  • Drug incorporated into matrix OR
  • Drug adsorbed/grafted to the surface
• Materials:
  
  • albumin, gelatin
  • biodegradable synthetic polymers:
    
    • polyvinylalcohol (PVA) ; polylactic acid (PLA); polyglycolic acid (PGA)
• Advantages:
  
  • ​Enhanced permeability & retention (EPR)
  • Tumours have a receptor for Albumin
• Uses:
  
  • ​Wide application in drug imaging
• Example: Abraxane
  
  • Paclitaxel in nanoparticulate albumin

Question 9

“Microspheres & Nanoparticles are typically more stable than liposomes.” True or False.

Answer 9

True.

Liposomes can auto-oxidise and thus have a shorter shelf life.

Question 10

What are liposomes? And what can they accomodate?

Answer 10

Liposomes are Phospholipid bilayer vesicles.

They can accomodate both lipid and water soluble drugs. The lipid drug can be situated throughout the bilayer and the water-soluble drug can be placed in the core.

Example:

• Liposomal amphoteracin B (to reduce toxicity) for NeXstar
• Liposomal doxorubicin
• Caelyx/ Doxil (PEGylated liposome)

Question 11

What is the difference between a multilamellar vesicle (MLV) and a simple unilamellar vesicle (SUV)?

Answer 11

• Both MLV & SUV are liposomes.
  
  • SUV is around 60 nm
    
    • contains a single BILAYER
  • MLV is between 100 - 1000 nm
    
    • containes multiple BILAYERS

Question 12

Give an example of when linear polymer-drug conjugates have benefited drug delivery.

Answer 12

• Polyglutamate - paclitaxel (Xyotax)
  
  • Polymer drug conjugate for oesophageal cancer
  • Uses a cathepsin sensitive linker that allows specific release in tumour when cleaved by its high conc. of cathepsin enzyme.
  • Thus, reduces side effects in circulation.
• PEG-L-aspariginase (Oncaspar)
  
  • ​L-asparaginase modified by covalently conjugating units of monomethoxypolyethylene glycol (PEG); Molecular weight of 5,000.
  • Used Acute Lymphoblastic Leukaemia (ALL).
  • PEGylation increases molecular weight; reduces renal clearance, decreases immunogenicity, increases half-life, wihtout shielding activity

Question 13

Explain the significiance of PEG-L-asparaginase (Oncaspar). And list FIVE benefits using a PEG in its targeted delivery formulation.

Answer 13

• It is classified as a Linear polymer-drug conjugate.
• L-asparaginase modified by covalently conjugating units of monomethoxypolyethylene glycol (PEG) to the enzyme.
• Used in Acute Lymphoblastic Leukaemia (ALL).
• PEGylation:
  
  • Increases molecular weight (MW = 5,000)
  • Reduces renal clearance
    
    • ​b/c kidneys have weight limit on clearance
  • Decreases immunogenicity
    
    • ​prevents recognition by macrophages
  • Increases half-life​ (circulation time)
  • Doesn’t shield activity

Question 14

Briefly explain what a Dendrimer is, and oppurtunities it might bring to targeted delivery.

Answer 14

• Dendritic polymer-drug conjugate
• Multiple layers (Generations) of polymer built around a central core.
  
  • size 1-100nm diameter, commonly 1 - 15nm
• Increasing interests in:
  
  • Drug loading via core interaction or surface conjugation
  • Opportunity to modify biopharmaceutics via surface derivatisation.

Question 15

Explain the Stealth effect conferred by PEGylation of proteins, nanoparticles, liposomes or polymers.

Answer 15

• Stealth involves conjugating PEG chains to the surface of a colloidal carrier.
• PEG presents a water-like (hydrophilic) exterior to immune system
  
  • This renders the colloid invisible and;
  • Prevents binding of opsonic factors which promote phagocytosis
• Therefore, this increases the circulation time allowing for targeting to specific site (e.g. tumour).

Question 16

What THREE factors are altered by increasing the Molecular Weight of PEGs

Answer 16

Use of higher MW PEGs:

• Increases size
• Decreases Clearance (Cl)
• Increases Half-life (t_1/2) [circulation time]

Question 17

How is Caelyx a product of targeted drug delivery?

Answer 17

• Caelyx is Doxorubicin in PEGylated liposome formulation
• It was originally developed for Kaposi’s sarcoma (KS) in AIDS, and now is also used in advanced ovarian and breast cancer.
• The PEGylation resulted in much longer t_(1/2) (up to 50 fold) than simple doxorubicin.
• The EPR effect is up to 20 fold increase in [dox] inKS leisions compared with normal skin, since the vascular endothelium of tumour is more permeable than normal endothelium.
• This reduces exposure of free doxorubicin to systemic circulation, thus reducing side effects such as;​ decrease in hair loss, nausea/vomiting, caridac tox and kidney tox.
• BUT, there is also an accumulation of PEGylated doxorubicin in soles of feet and hands leading to palmar plantar erythrodysethesia or hand-foot syndrome.
• Led to use of Myocet (non-PEGylated liposomal doxirubicin) as an alternative to reduce side effects, but not as potent as Caelyx.

Question 18

List THREE types of Targeting agents.

Answer 18

1. Receptor agonist/antagonist
   (Receptors at specific locations)
2. Lectins
   (bind carbohydrate residues, many of which act as cell surface markers)
3. Antibodies

Question 19

List THREE types of antibodies used as targeting agents in drug delivery, and briefly describe each one.

Answer 19

1. Monoclonal antibodies
   
   1. ​Specific antibody-antigen interaction
   2. Usually made in mice. Thus, potential immune rejection response
2. Chimeric antibodies
   
   1. ​Humanised antibodies with mouse-related protein sequences
3. Antibody fragments
   
   1. ​Diabodies.
   2. Bi-specific antibody fragments that are smaller and easier to work with.

Question 20

Using Brentuximab vedotin (Adcetris) as an example, explain the rationale behind Antibody-Drug Conjugates (‘ADCs’).

Answer 20

• Monoclonal antibody targeting CD30 linked to the anti-microtuble agent monomethyl auristatin E (MMAE)
• The linker is a dipeptide cleavable by cathepsin (an enzyme overexpressed in tumours)
• The antibody brings cytotoxic drug to target area & drops it there on the cell.

Question 21

What is the one major limitation to the Antibody-Drug Conjugate (ADC)

Answer 21

• 1 targeting group = 1 drug molecule
  
  • ADC doesn’t have the ability to carry more than 1 drug molecule.
  • Therefore, the drug molecule will have to be very potent to be effective
• Hence, the delivery system involving Active Targeting of Drug Carriers, where conjugation of targeting group (e.g. antibody) to surface of nanoparticle, liposome, or polymer allows you to load more than one drug molecule.

Question 22

What is an advantage of Active Targeting of Drug Carriers as a drug delivery system?

Answer 22

• Conjugation of targeting group (e.g. antibody) to surface of carrier (e.g. nanoparticle, liposome or polymer)
• This allows a larger ‘payload’
  
  • i.e. more drug molecules per targeting group

Question 23

What is meant by the term ‘immunoliposome’?

Answer 23

• Combination of stealth technology (e.g. PEGylated liposome) with monoclonal antibodies
• E.g.
  
  • PEGylated liposome + HER2 Antibody fragments + quantum dot for tumour targeting