Therapeutic Targeting of CNS Tumours Flashcards
Understand brain tumour resistance to current treatments Appreciate the challenge of the blood-brain barrier Know strategies used in clinic to overcome the blood-brain barrier Appreciate new drug delivery systems for brain tumour treatment Appreciate how drug delivery systems can make treatment safer and more selective Evaluate the use of anti-angiogenic agents against glioblastoma
Define anoikosis
Detachment-induced apoptosis due to removal of a cell from its normal microenvironment
Describe the metastatic cascade
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
Why must chemotherapy be delivered systemically?
So that is can also affect any secondary tumours or cell deposits
Why is systemic chemotherapy an issue in treating brain tumours?
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
State 5 challenges, other than the blood-brain barrier, in treating brain tumours
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
Name 3 cells which form the blood brain barrier
Endothelial cells, pericytes, and astrocytes
Name 4 things which cause slight opening of the blood brain barrier’s tight junctions
Infection, inflammation, trauma, tumours
What characteristics must a drug have to cross the blood brain barrier?
It must be lipid soluble (lipophilic) and below 600Da molecular weight
How does P-glycoprotein inhibit the actions of chemotherapeutic drugs?
It functions as an efflux pump to remove them from brain tumour cells
Name the most commonly used chemotherapeutic drug in treating brain tumours
Temozolomide (TMZ)
How could arabinose or mannitol facilitate chemotherapeutic treatment of brain tumours?
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
Name two methods developed for local delivery of drugs into the brain
Convection-enhanced delivery (CED) and polymeric vesicles
What is the median survival time after diagnosis of glioblastoma multiforme?
14.6 months with aggressive treatment
Give 3 reasons why the vasculature of solid tumours is an attractive target for intervention
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
After reaching what diameter must tumours generate their own blood supply to survive?
2-3mm
What do tumours subsist on before generating their own vasculature?
The diffusion of nutrients from the existing host microvasculature
What is the difference between vessels created by physiological angiogenesis and tumour angiogenesis?
Tumour angiogenesis results in chaotic, inefficient, and permeable vessels
Name three vascular targets in angiogenic blood vessels
VEGF, VEGF receptors, alphavbeta3 integrin, alphavbeta5 integrin, MMP2, MMP9, EGFR
Name three vascular targets in angiogenic perivascular cells
APA aminopeptidase, APN aminopeptidase, NG2 proteoglycan, PDGFRs
Name three targets in tumour cells
VEGF, VEGF receptors, MMP2, MMP9, EGFR
Name two anti-angiogenic VEGF inhibitors in clinical trials for adult malignant glioma
Bevacizumab - monoclonal antibody
Afibercept - decoy receptor
Give 5 potential limitations of anti-angiogenic agents
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
Give 3 advantages of targeted drug delivery systems
1) Increased efficacy
2) Increased safety
3) Control of rate of drug release
State the aim of targeted therapies
To discriminate between tumour and non-malignant cells and to achieve therapies that are non-invasive, systemic, safer, and more efficient
State 5 challenges for targeted drug delivery systems
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
Name four viruses used as gene therapy vectors
Retroviruses, lentiviruses, adenoviruses, adeno-associated viruses, and Herpes simplex viruses
How are viruses adapted to be used as vectors?
Their replication genes are removed to avoid uncontrolled replication in the patient, and replaced with therapeutic genes
State 5 reasons eukaryotic viral vectors have not been successful in systemic gene therapy
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
What are bacteriophages?
Viruses which only infect bacteria
State the two main groups of bacteriophages
Tailed and filamentous
Give 4 advantages of the use of bacteriophages as vectors for cancer gene therapy
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
Give two examples of trials of AAVPhage (AAVP) in cancer gene therapy
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
What is the most common primary brain tumour in children?
Medulloblastoma
Why is paediatric diffuse intrinsic pontine glioma fatal in 99.9% of cases?
The malignant cells entwine with healthy cells in the brainstem so cannot be removed
Describe the results of in vitro trials of RGD4C/PAAV-TNFalpha bacteriophage gene therapy for diffuse intrinsic pontine glioma
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
Describe how the MGMT enzyme confers tumour resistance to temozolomide
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
State at least 3 mechanisms of resistance to temozolomide
MGMT enzyme expression, inactivation of mismatch repair proteins (e.g. MSH6 deficiency), base excision repair protein APNG expression, cancer stem-like cells
Describe the mechanism of action of temozolomide
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
Describe the mechanism of action of cisplatin
Cisplatin induces the formation of reactive oxygen species, disrupts calcium homeostasis, and directly causes cell apoptosis
