Therapeutic Targeting of CNS Tumours

Question 1

Define anoikosis

Answer 1

Detachment-induced apoptosis due to removal of a cell from its normal microenvironment

Question 2

Describe the metastatic cascade

Answer 2

A premetastatic niche is induced by a distant tumour and mediated by cells derived from the bone marrow. Cells in the primary tumour undergo epithelial-mesenchymal transition and acquire invasive properties. The basement membrane is degraded by proteinases and extracellular matrix is remodelled, allowing tumour cells to invade. These invading cells intravasate into newly formed blood vessels and are transported, eventially extravasating , where they can undergo macrometastasis and grow into a secondary tumour, with the aid of ECM remodelling and angiogenesis

Question 3

Why must chemotherapy be delivered systemically?

Answer 3

So that is can also affect any secondary tumours or cell deposits

Question 4

Why is systemic chemotherapy an issue in treating brain tumours?

Answer 4

Systemic chemotherapy struggles to cross the blood-brain barrier - only a small proportion may cross and reach the primary tumour, requiring large doses with unpleasant side effects

Question 5

State 5 challenges, other than the blood-brain barrier, in treating brain tumours

Answer 5

Resistance (e.g. DNA repair enzymes), heterogeneity, the diffusing nature of some tumours, invasiveness of local delivery into the brain, lack of efficacy systemically, and limitations of the medicines themselves

Question 6

Name 3 cells which form the blood brain barrier

Answer 6

Endothelial cells, pericytes, and astrocytes

Question 7

Name 4 things which cause slight opening of the blood brain barrier’s tight junctions

Answer 7

Infection, inflammation, trauma, tumours

Question 8

What characteristics must a drug have to cross the blood brain barrier?

Answer 8

It must be lipid soluble (lipophilic) and below 600Da molecular weight

Question 9

How does P-glycoprotein inhibit the actions of chemotherapeutic drugs?

Answer 9

It functions as an efflux pump to remove them from brain tumour cells

Question 10

Name the most commonly used chemotherapeutic drug in treating brain tumours

Answer 10

Temozolomide (TMZ)

Question 11

How could arabinose or mannitol facilitate chemotherapeutic treatment of brain tumours?

Answer 11

These hypertonic solutions could osmotically open the blood-brain barrier, dilating cerebral blood vessels and shrinking endothelial cells to widen endothelial tight junctions, allowing delivery of chemotherapeutic drugs from the systemic circulation

Question 12

Name two methods developed for local delivery of drugs into the brain

Answer 12

Convection-enhanced delivery (CED) and polymeric vesicles

Question 13

What is the median survival time after diagnosis of glioblastoma multiforme?

Answer 13

14.6 months with aggressive treatment

Question 14

Give 3 reasons why the vasculature of solid tumours is an attractive target for intervention

Answer 14

1) Endothelial cells lining the blood vessels are directly accessible to systemic drugs, with no need to cross the blood brain barrier
2) It is estimated up to 100 tumour cells are sustained by each endothelial cell
3) Endothelial cells are genetically more stable, therefore unlikely to acquire resistance to therapy
4) The tumour endothelium expresses markers absent or barely detectable in normal blood vessels, providing targets

Question 15

After reaching what diameter must tumours generate their own blood supply to survive?

Answer 15

2-3mm

Question 16

What do tumours subsist on before generating their own vasculature?

Answer 16

The diffusion of nutrients from the existing host microvasculature

Question 17

What is the difference between vessels created by physiological angiogenesis and tumour angiogenesis?

Answer 17

Tumour angiogenesis results in chaotic, inefficient, and permeable vessels

Question 18

Name three vascular targets in angiogenic blood vessels

Answer 18

VEGF, VEGF receptors, alphavbeta3 integrin, alphavbeta5 integrin, MMP2, MMP9, EGFR

Question 19

Name three vascular targets in angiogenic perivascular cells

Answer 19

APA aminopeptidase, APN aminopeptidase, NG2 proteoglycan, PDGFRs

Question 20

Name three targets in tumour cells

Answer 20

VEGF, VEGF receptors, MMP2, MMP9, EGFR

Question 21

Name two anti-angiogenic VEGF inhibitors in clinical trials for adult malignant glioma

Answer 21

Bevacizumab - monoclonal antibody

Afibercept - decoy receptor

Question 22

Give 5 potential limitations of anti-angiogenic agents

Answer 22

Short half lives, rapid renal clearance, non-specific accumulation, inefficient accumulation at diseased site, severe side effects at high doses, poor tissue and cell membrane permeability in vivo, tumour resistance

Question 23

Give 3 advantages of targeted drug delivery systems

Answer 23

1) Increased efficacy
2) Increased safety
3) Control of rate of drug release

Question 24

State the aim of targeted therapies

Answer 24

To discriminate between tumour and non-malignant cells and to achieve therapies that are non-invasive, systemic, safer, and more efficient

Question 25

State 5 challenges for targeted drug delivery systems

Answer 25

Inefficacy of delivery through the systemic route, achieving the right ratio of drug to vector to targeting agent, making them reproducible enough for pharmaceutical applications, safety, cost

Question 26

Name four viruses used as gene therapy vectors

Answer 26

Retroviruses, lentiviruses, adenoviruses, adeno-associated viruses, and Herpes simplex viruses

Question 27

How are viruses adapted to be used as vectors?

Answer 27

Their replication genes are removed to avoid uncontrolled replication in the patient, and replaced with therapeutic genes

Question 28

State 5 reasons eukaryotic viral vectors have not been successful in systemic gene therapy

Answer 28

1) Undesired uptake by the liver
2) Uptake by the reticuloendothelial system
3) Broad tropism for normal tissues causing toxicity
4) Poor penetration into tumour tissues
5) Presence of antiviral neutralising antibodies

Question 29

What are bacteriophages?

Answer 29

Viruses which only infect bacteria

Question 30

State the two main groups of bacteriophages

Answer 30

Tailed and filamentous

Question 31

Give 4 advantages of the use of bacteriophages as vectors for cancer gene therapy

Answer 31

1) Safe - already administered to humans as antibacterial food additives
2) No need to ablate native tropism
3) Ligand-directed targeting well established
4) Cost effective production in bacteria

Question 32

Give two examples of trials of AAVPhage (AAVP) in cancer gene therapy

Answer 32

1) Hajitou et al (2006, Cell) found that IV tumour-targeted AAVP suppressed tumour growth in mice
2) Tandle et al (2009, Cancer) found that AAVP-delivered TNF-alpha induced apoptosis in the tumour vasculature after systemic delivery

Question 33

What is the most common primary brain tumour in children?

Answer 33

Medulloblastoma

Question 34

Why is paediatric diffuse intrinsic pontine glioma fatal in 99.9% of cases?

Answer 34

The malignant cells entwine with healthy cells in the brainstem so cannot be removed

Question 35

Describe the results of in vitro trials of RGD4C/PAAV-TNFalpha bacteriophage gene therapy for diffuse intrinsic pontine glioma

Answer 35

RGD4C/PAAV-TNFalpha has been found to activate caspase activity and induce cell death. Chemovirotherapy, combining this with cisplatin, has eradicated diffuse intrinsic pontine glioma in vitro

Question 36

Describe how the MGMT enzyme confers tumour resistance to temozolomide

Answer 36

Temozolomide reacts with DNA to form cytotoxic methyl adducts including 06-methylguanine. MGMT restores structural integrity by transferring the methyl group of 06-methyguanine to a cysteine residue in its active site

Question 37

State at least 3 mechanisms of resistance to temozolomide

Answer 37

MGMT enzyme expression, inactivation of mismatch repair proteins (e.g. MSH6 deficiency), base excision repair protein APNG expression, cancer stem-like cells

Question 38

Describe the mechanism of action of temozolomide

Answer 38

Temozolomide is a prodrug, converted to the active form MTIC. This reacts with DNA to form cytotoxic methyl adducts, such as O6-methylguanine, which interfere with replication and cause apoptosis

Question 39

Describe the mechanism of action of cisplatin

Answer 39

Cisplatin induces the formation of reactive oxygen species, disrupts calcium homeostasis, and directly causes cell apoptosis