L9, Targeted Therapies

Question 1

Targeted therapy: Targeting, Identification, Effect on cells, specificity..

Answer 1

• Target cancer cells specifically
• Designed to interact with specific cancer targets
• Cytostatic effect (blocking signalling pathways)
• Specific to cancer type and subtype (i.e. molecular profile)

Question 2

How are drugs normally obtained in chemotherapy vs targeted therapy?

Answer 2

• A lot of chemotherapy drugs in use today and historically are derived from natural products
• Targeted therapy drugs are more often specifically designed for purpose

Question 3

List the 7 types of targeted therapies

Answer 3

• Hormone therapies
• Signal transduction inhibitors
• Gene or protein expression modulators
• Apoptosis inducers
• Angiogenesis inhibitors
• Toxin delivery molecules
• Immunotherapies

Question 4

What are typical targets?

Answer 4

• Oncogenes (easier to disrupt function than reactivate as in TS’s)
• e.g. BCR-Abl, Her2, BRAF, P53, VEGF

Question 5

Why is molecular subtyping important in breast cancers but not CML

Answer 5

• In breast cancers, Her2 is expressed only 15% of the time
• However, in CML, BCR-Abl is expressed over 95% of the time
• Able to confidently target a particular pathway in CML cases

Question 6

bcr-abl oncogene: Activation, protein encoded, broad effect

Answer 6

• Occurs due to translocation between chromosomes 9 and 22 (producing ‘Philadelphia Chr.’)
• Encodes Bcr-Abl fusion protein (constitutively active tyrosine kinase)
• When present, drives development of cancers due to involvement in various proliferative pathways

Question 7

Key effects of bcr-abl fusion protein

Answer 7

• Altered adhesion and motility (via phosph. of cytoskeletal proteins)
• Proliferation (via JAK/STAT, via Akt, via mTOR) and survival (via JAK/STAT, via Bcl)
• Transcription (via JAK/Myc, via Ras/Raf)

Question 8

Small molecule inhibitor of BCR-ABL

Answer 8

Imatinib…

• Breakthrough paper on feasibility of targeted therapies
• Significantly better outcomes with targeted therapy produced in trial when compared with non-targeted therapy, interferon)
• Significantly less severe side effects
• This targeted therapy works well for CML since it has a relatively simple genetic profile

Question 9

Molecular subtypes of breast cancer

Answer 9

• HR-/HER2- (Triple Negative); 13%, worst prognosis, not well understood mechanism
• HR+/HER2- (Luminal A); 73%, best prognosis
• HR+/HER2+ (Luminal B); 10%
• HR-/HER2+ (HER2-enriched); 5%
  -> subtyping crucial to prognosis as well as application of correct targeted therapy

Question 10

How can RTKs function as oncogenes?

Answer 10

Deregulation of receptor firing (not exhaustive)…
* Truncations
* Overexpression
* Predimerisation
-> Overactivation of pathway (e.g. HER2 oncogene) via independent ligand firing
* Pathway links well to B301

Question 11

Her2 signalling pathway initiation:

Answer 11

• Dimerise upon ligand binding (ligands are receptor specific)
• Homo/heterodimers of Her2 with Her1, 2, 3 or 4
• Once dimerised, signal Ras/Raf pathway for cell growth
• The other component of the dimer may be involved in other pathways (i.e. PI3k/Akt)
  —> VEGF transcription

Question 12

How does Trastuzumab impact Her2 signalling?

Answer 12

• Prevents cleavage of E-C domain which usually produces overactive signalling
• Blocks dimerisation by binding to E-C domain -> prevents activation of signal transduction pathways
• Activates antibody-dependent cell-mediated toxicity (tumour cell lysed by immune effector cell); Fc region of drug is ‘human-like and thus recognised by immune system
• Encourages endocytosis -> Her2 turnover

Question 13

Frequent mutation in melanoma + characterisation:

Answer 13

• Ras/Raf downstream components: B-Raf V600E and V600k mutations
• Intracellular serine/threonine kinases -> abnormal proliferation

Question 14

Novel drug for V600E mutated BRAF; why did it fail in the lecture example

Answer 14

Vemurafenib: V600 Mutated BRAF inhibitor

• Prone to recurrence due to specificity of mechanism
• Case study from lecture: Sequencing revealed activating mutation in MEK, constitutively active regardless of inhibitor since pathway is MAPK-independent

Question 15

How can cancers become resistant to inhibition of a particular oncogenic pathway?

Answer 15

• Secondary mutation, amplification or activation of target
• Bypass of oncogenic pathway (vast network)
  -> Reactivation of oncogenic pathway
  -> Disease progression

Question 16

MAP Kinase pathway-dependent mechanisms: (Resistance)

Answer 16

1. Increased activity or expression of RTKs
2. RAS mutations
3. RAF kinase switch
4. COT kinase overexpression
5. MEK1 mutation

Question 17

MAP kinase pathway-independent mechanisms (Resistance):

Answer 17

Increased activity of PDGFR-beta. IGF-1R or other RKs

Question 18

How may oncogenic signalling evade cell cycle arrest/apoptosis?

Answer 18

• Normally, Mdm2 targets p53 for degradation via ubiquitylation, but stressful conditions upregulates Arf, which sequesters Mdm2, preserving stable, active p53 -> Cell cycle arrest, apoptosis
• Cancers often upregulate Mdm2, to evade p53 effects
• p53 itself is mutated in ~50% of cancers
• Other parts of the pathway can also be mutated to disrupt p53 activity

Question 19

Mdm2 inhibitor:

Answer 19

• Nutlins
• Inhibit binding of Mdm2 to p53
• Result in p53 re-expression and induction of apoptosis -> Tumour Suppressive

Question 20

How does VEGF promote blood vessel growth?

Pathway

Answer 20

Binds to VEGFR-2 (RTK)…

• PI3k/Akt pathway activation -> vascular cell survival and vascular permeability
• p38MAPK activation -> endothelial cell migration
• Raf pathway activation -> MEK -> Erk -> Endothelial cell proliferation

Question 21

Monoclonal Antibody against VEGF:

Answer 21

Avastin…

• Delays progression of lung and colon cancer
• Commonly used in clinical settings

Question 22

Example of targeted therapy agents for resisting cell death:

Answer 22

• Pro-apoptotic BH3 mimetics