L7: monoclonal antibody therapeutics

Question 1

What’s the basic structure of a monoclonal antibody?

Answer 1

• Human IgG have 4 subclasses which have differing effector function
• Fc (fragment crystallizing) – bind the Fc gamma receptor (exist on cells) – controls effector function
• Fab (fragment antigen binding) – bind the antigen – provides the antibody its specificity, they also have variable domains, where antibodies are able to bind the epitopes on antigens
• they have heavy and light chains
• disulfide bonds connect the two

Question 2

What is the importance of different isotypes of IgGs?

Answer 2

• IgG1 - bind all FcgammaRs in high affinity
• IgG2 - binds only FcgammaRIIA
• IgG3 - binds all FcgammaRs
• IgG4 - bind all FcgammaRs in low affinity
• Humans have approx. 6 gamma receptors
• They differ in their affinity, how good they are at binding different antibodies
• They are all stimulatory, so when antibody binds them, they exert a stimulatory function towards the cell, apart from FcgammaRIIB, which is inhibitory
• Different immune cells express different FcgammaRs and therefore will bind different IgG subclasses

Question 3

Which of the FcgammaRs exerts inhibitory action?

Answer 3

• They are all stimulatory, so when antibody binds them, they exert a stimulatory function towards the cell, apart from FcgammaRIIB, which is inhibitory

Question 4

What are the 4 possible effector functions of monoclonal antibodies?

Answer 4

1st function – antibody dependent cellular cytotoxicity (ADCC)
- Antibody is bound to a target cell, as well as NK cell through FcgammaRIII, which stimulates NK cell to release its cytotoxic granules, which are then able to kill the target cell.
2nd function – antibody dependent cell-mediated phagocytosis (ADCP)
- Antibody this time can bind different Fcgamma receptors on a macrophage
- Macrophages are good at phagocytosis, which are stimulated by binding of antibody
3rd function – complement dependent cytotoxicity (CDC)
- Fc portion instead of binding Fcgamma receptor is now binding C1q
- C1q is part of a complement pathway, which is a series of proteins within serum that form a cascade that leads to this complex shooting holes in the target cell, which ends up in its lysis
4th function – agonism/blocking/neutralisation
- Fc independent mechanisms of action, bind target but don’t need Fc portion to do anything
- Can stimulate a T cell, or block receptor and stop any activation happening

Question 5

What are the different types of monoclonal antibodies? What are their features?

Answer 5

• Murine
  o Fully mouse IgG
  o Usually used in preclinical development
  o Can trigger human anti-mouse antibody response (HAMA) – injecting another species protein in human
• Chimeric
  o Mouse IgG backbone
  o Variable regions of antibody ‘humanized’
  o Used in some clinical settings
  o Example – rituximab which targets CD20
  o A lot less immunogenic
• Humanized
  o A mouse antibody which has been humanized
  o Still maintains foreign CDRs but 90% human
  o Mainly used in clinical settings
  o Example – bevacizumab which targets VEGF-A
• Human
  o Fully human IgG
  o Example – ipilimumab which targets CTLA-4

Question 6

What’s the mechanism of action of rituximab?

Answer 6

• First monoclonal antibody developed for oncology use
• Targets CD20 which is expressed on B cells (expressed on both normal and malignant cells)
• When rituximab binds CD20 it inhibits all these signalling pathways within the B cell. Can lead to apoptosis, can make it more sensitive to chemotherapy, can be used in combination.
• Approved for use in many different B cell lineage cancers
• As B cell drive many autoimmune conditions it is also approved for diseases such as Rheumatoid Arthritis
• Design of rituximab has been improved over the years with new iterations to improve efficacy in specific ways and further humanizing it

Question 7

What’s the mechanism of action of nivolumab?

Answer 7

• Targets the PD-1 receptor on activated T cells – discovery of this receptor and pathway won Nobel prize in 2018
• Blocking PD-1 prevents exhaustion of T cells within the tumour microenvironment – prevents binding or PD-L1 ligand
• IgG4 isotype – fully human antibody (over 90% human origin)
• First approved for use in refractory and unresectable melanoma