Cancer therapy Flashcards
What are the five classes of cytotoxic chemotherapies?
- Alkylating agents – includes the mustard gas
- Antimetabolites
- Anthracyclines
- Vinca alkaloids and taxanes
- Topoisomerase inhibitors
They all act by attacking structures directly in rapidly dividing cells, the majority target DNA while taxanes and vinca alkaloids attack microtubules which are also essential for cell division.
Explain the mechanism of action of alkylating agents.
- Adds an alkyl (CNH2N+1) group to guanine residues in DNA.
- This causes cross-links to be formed in the DNA strands and leads to its distortion.
- Cross links prevent DNA from uncoiling at replication and this triggers apoptosis.
- -The cell detects this and then triggers apoptosis via the checkpoint pathway.
- In some cancerous conditions, the tumour cell will prevent apoptosis and just cut out the distorted section which will promote mispairing.
Pseudo-alkylating agents
- Add platinum instead of the alkyl group to guanine residues in DNA. Same mechanism of cell death as akylating agents.
- – Examples: carboplatin, cisplatin, oxaliplatin – one of main chemotherapies.
Explain the mechanism of action of anti-metabolites.
- Inhibit DNA replication and transcription – causing DNA checkpoints and trigger apoptosis.
- Can be purine or pyrimidine analogues – they masquerade as these residues and stop DNA synthesis since they are distorted nucleotides.
- Can also be folate antagonists - Inhibits the enzyme dihydrofolate reductase to inhibit folic acid synthesis which is essential for nucleic acid formation.
- Examples: methotrexate (blocks folic acid metabolism and hence DNA synthesis), 5-fluorouracil (pyrimidine analogue that blocks transcription/replication)
Explain the mechanism of action of anthracyclines.
- Inhibit transcription and replication and DNA repair – act as intercalating agents and insert themselves between nucleotides in the DNA/RNA strand and cause distortion.
- This leads to DNA checkpoints and trigger apoptosis.
- Cause DNA damage and membrane damage by releasing free radicals. Therefore there’s a maximum dose the patient can take – can cause heart failure if too high a dose.
- Examples: doxorubicin, epirubicin.
Explain the mechanism of vinca alkaloids and taxanes.
- They target the microtubules during cell division – ‘spindle poisons’.
- Vinca alkaloids target assembly of the microtubules while taxanes target disassembly.
- They inhibit polymerisation and cause disassembly of microtubules so that the cells won’t be pulled apart so cells undergo mitotic arrest and are killed.
- Taxanes are naturally derived – from the pacific yew tree. (Once discovered a lot of deforestation occurred which had harmful effects on squirrel populations, but it’s now made artificially).
- Microtubules are very important in resting cells – e.g. nerve cells they have long axons where the cytoskeleton is very important. Therefore a side effect is nerve damage – patients can experience loss of sensation and numbness.
- Usually combined with cisplatin as this gives a much better response.
Explain the mechanism of action of topoisomerase inhibitors.
– Topoisomerase prevents DNA torsional strain during replication – as during replication a fork is formed and if there’s no change in coiling there will be great strain which can be damaging.
– Inhibiting this enzyme will cause the helix to break each time they try to replicate – either temporary single strand breaks (topo1) or double strand breaks (topo2) – there are two classes of drugs.
– These drugs also protect the free ends of DNA from aberrant recombination events.
(Drugs such as anthracyclines have anti-topoisomerase effects through their action on DNA)
– Some inhibitors such as topotecan will alter binding of the topoisomerase complex to DNA which will induce permanent breaks in the DNA strand.
Outline the mechanisms of tumour resistance to cytotoxic chemotherapy.
– Resistance can be either intrinsic (resistant factors pre-existed the treatment) or acquired - developed during treatment due to mutations, over-expression of target or use of alt pathways:
- The cancer cell can repair its own DNA, with a lot of errors but the cell will continue to survive and divide – base excision repair. Happens when cancer cell has been exposed to chemotherapy.
- Or the cancer cell can mutate, making the tubules undergo a change that will prevent it binding to taxanes.
- The drug can also be effluxed from the cell by ATP binding cassette (ABC) transporters.
- There can also be reduced uptake of the drug.
- Inactivation of death signals - P53 mutation can occur – no apoptosis triggered by damage. Or activaton of pro-survival signals.
What are the two types of modern non-cytotoxic cancer therapies? And why should we target growth factor pathways?
- Monoclonal antibodies
- Small molecule inhibitors
Growth factor receptors are a good target –> >50% are associated with human malignancies:-
- Growth factor receptors (especially receptor tyrosine kinases) are over-expressed in many cancers. e.g. HER2 – 25% of breast cancers and EGFR in breast and colorectal cancers.
- Or there’s overexpression of the ligand itself – e.g. VEGF in prostate, kidney and breast cancers. It promotes angiogenesis near tumour.
- Or the receptor is constitutively activated due to mutations – ligand independent activation. e.g. in EGFR in lung cancer, FGFR in head, neck cancers and myelomas.
How do monoclonal antibodies work? What are their limitations?
They target the extracellular component of growth factor receptors – they are too large to cross the membrane.
i. They can bind and neutralise ligand – e.g. Bevacizumab, which binds and neutralises VEGF in colorectal cancer.
ii. Or prevent dimerization of the receptors by binding to it – e.g. Cetuximab which acts on the EGFR.
iii. Or cause internalisation of the receptor leading to its down-regulation.
iv. Indirect mechanism – activate receptor phagocytosis, which induces complement activity or antibody dependent cytoxicity.
- The fact that they can’t enter cell is a limitation –not effective against constitutively activated receptors.
- Originally derived from mouse antibodies, but there was risk of anaphylaxis due to complement activation so chimeric Abs were then developed. Now fully humanised Abs are available.
- The antibody can also be designed (i.e. conjugated with drugs or radio-labelled) in different ways to maximise its effects
How do small molecule inhibitors work? Give an example.
- They work by crossing into the cell and bind to the ATP binding pocket of the kinase domain of the growth factor receptor. As they are ATP mimetics.
- They block its auto-phosphorylation and inactivate downstream cascade, regardless of any upstream signals present.
- Effective against constitutively activated receptors.
- e.g. Imatinib/Glivec – was the first SMI –> 90% response rates were seen, complete clearance of leukemia from blood.
- SMI’s can also act on other enzymes. e.g. poly (ADP) ribose polymerase (PARP) inhibitors which are used in patients with BRCA1.
- SMIs have been developed to target kinases and therefore affect intracellular pathways – e.g. sorafinib which targets Raf kinase.
- SMIs can be developed to block anti-apoptosis mechanisms inhibiting proteins involved in this inhibitory pathway. e.g AKT inhibitors.
What are the disadvantages of small molecule inhibitors?
– While there’s less cytotoxicity with SMIs and MAbs, cancer cells can develop resistance. E.g. in Imatinib, changes occurred in ATP binding pocket could no longer bind drug. Resistance to dasatinib and nilotinib has also emerged.
– SMIs are pleiotropic – one ATP binding domain can look a lot like another protein, can cause unexpected cytoxicity.
SMIs can be promiscuous and bind to several targets.
What are future possible mechanisms for cancer therapy?
i. Antisense oligonucleotides
- - They cross into membrane, they are mRNA and are complementary to gene of interest.
- - Once bound to gene, they activate RNAse H causing degradation of the RNA.
- - Very targeted mechanism – good for undruggable targets.
- - But they are very difficult to make, sometimes they can’t cross membrane or have difficult with breaking the backbone – causing great toxicity.
ii. RNAi - similar process, antisense mRNA but it activates RISC complex instead.
- - Creates cellular memory unlike antisense oligonucleotides which is lasting.
- - Nanotherapeutics has been developed to create packaging to deliver the mRNA so that it can enter cell and then nanomaterial is degraded.
http: //www.sciencedaily.com/releases/2012/02/120227094331.htm
What are two success stories in cancer therapy development?
- Targeting B-Raf in cancer therapy
- - Activating mutations of B-Raf have been identified in >60% melanomas.
- -Substitution of glutamic acid for valine (V600E) = 90% of BRAF mutations ; causes a 500-fold increase in activity
- - B-Raf inhibitor Vemurafenib showed dramatic Phase I activity in melanoma – 80%.
- - Extended life span of mutation holders by 7 months.
- - Side effects include arthralgia, skin rash and photosensitivity. - Immune modulation via programmed cell death 1 (PD-1)
- - PD-L1 is present on the surface of cancer cells.
- - It binds to the PD-1 on T cells causing them to no longer recognise the tumour cells as foreign or induce their apoptosis –> form of immunosuppression.
- - If it’s blocked, the immune system will be activated against the tumour.
- - Nivolumab blocks PD-L1 from binding PD-1.
- - Nivolumab was developed as an anti-PD1 antibody which had survival rate of 31% (usually 5%) in melanoma and survival of 16 months.
