3. Biological basis of cancer therapy Flashcards
Most common cancers worldwide
The most common cancers worldwide – lung, breast, bowel, prostate and stomach.
- Most common by gender – Lung in men, breast in women.
- “Western cancers” include – breast, colorectal, lung and prostate.
Types of genetic mutation causing cancer:
- Chromosome translocation.
- Gene amplification (from copy number variations).
- Point mutations – in promotor/enhancer regions.
- Deletions/insertions.
- Epigenetic alterations.
- Heritable mutations.
Cancer treatment: explain the main chemotherapeutic and targeted approaches to treating cancer
Main Anti-‐Cancer Modalities
- Surgery
- Radiotherapy
- Chemotherapy
- Immunotherapy
Cancers are genetically “messy” – so attacking their DNA is a good idea
Systemic therapy: Cytotoxic Chemotherapy
- Alkylating agents
- Antimetabolites
- Anthracyclines
- Vinca alkaloids and taxanes
- Topoisomerase inhibitors
Targeted Therapies
- Small molecule inhibitors
- Monoclonal antibodies
Cytotoxic drugs ‘select’ rapidly dividing cells by targeting their structures (mostly DNA)
Cytotoxic Chemotherapy
- Given IV or occasionally orally
- It works systemically
- Non-‐targeted -‐ it affects all rapidly dividing cells in the body e.g. hair and intestinal epithelium
Different times at which it can be used:
- Post-‐operatively = adjuvant – ensure that all residual cells are neutralized as a means of improving prognosis
- Pre-‐operatively = neoadjuvant – way to avoid mastectomy
- As a monotherapy or in combination
- With curative or palliative intent
Alkylating and pseudoalkylating agents
Alkylating Agents
- These add alkyl (CNH2N+1) groups to guanine residues in DNA
- It then cross-‐links DNA strands and prevents DNA from uncoiling at replication
- It then triggers apoptosis (via a DNA checkpoint pathway)
- It encourages miss-‐pairing (oncogenic)
Pseudo-‐alkylating Agents
- These add platinum to guanine residues in DNA
- It triggers the same mechanism of death as alkylating agents
Alkylating and Pseudo-‐alkylating Agents
Alkylating agents:
- Chlorambucil
- Cyclophosphamide
- Dacarbazine
- Temozolomide
Pseudo-‐alkylating agents:
- Carboplatin
- Cisplatin
- Oxaliplatin
Side effects:
- Hair loss (not carboplatin)
- Nephrotoxicity
- Neurotoxicity
- Ototoxicity (platins)
- Nausea
- Vomiting
- Diarrhoea
- Immunosuppression
- Tiredness
Model of DNA after alkylating agent treatment:
The crosslinking between the guanine residues that have been alkylated means that the DNA strands can’t separate at replication
Anti-‐metabolites
- Analogues of purine or pyrimidine residues.
- Inhibit DNA synthesis, breaking of the double strand of DNA and apoptosis
- They block DNA replication and DNA transcription
- Anti-‐metabolites can be purine analogues (adenine or guanine) or pyrimidine analogues (thymine/uracil and cytosine)
- They can also be folate antagonists (these inhibit dihydrofolate reductase, which is required to make folic acid, an important building block for all nucleic acids (especially thymine))
Examples of anti-‐metabolites:
- Folate – Methotrexate.
- Purine – 6-mercaptopurine, decarbazine, fludarabine.
- Pyrimidine – 5-fluorouracil, capecitabine, gemcitabine.
Side effects of anti-‐metabolites:
- Hair loss (alopecia) -‐ not 5-‐fluorouracil or capecitabine
- Bone marrow suppression causing anaemia, neutropenia and thrombocytopenia
- Increased risk of neutropenic sepsis (and death) or bleeding
- Nausea and vomiting (leading to dehydration)
- Mucositis and diarrhoea
- Palmar-‐plantar erythrodysesthesia (PPE) – red limbs and skin begins to peel
- Fatigue
Anthracyclines
- Inhibit transcription and replication by intercalating (i.e. inserting between) nucleotides within the DNA/RNA strand
inhibitis transcritpions and replication threrefore blocks dna repair and create free radical that will damage the cels and caue apoptosis - mm
- They also block DNA repair (mutagenic)
- They create DNA-‐damaging and cell membrane damaging oxygen free radicals
Examples of anthracyclines:
- Doxorubicin
- Epirubicin
Side effects of anthracyclines:
- Cardiac toxicity (arrhythmias, heart failure) -‐ probably due to damage induced by free radicals
- Alopecia
- Neutropenia
- Nausea
- Vomiting
- Fatigue
- Skin changes
- Red urine (doxorubicin = ‘the red devil’)
Vinca Alkaloids and Taxanes
Originally derived from natural sources
They work by inhibiting the assembly (vinca alkaloids) or disassembly (taxanes) of MITOTIC MICROTUBULES causing dividing cells to undergo mitotic arrest
- Vinblastine
- Vincriste
Side effects of microtubule targeting drugs:
- Nerve damage: peripheral neuropathy (limb numbness), autonomic neuropathy (BP and Gi sidturbance)
- Hair loss
- Nausea
- Vomiting
- Bone marrow suppression (neutropenia, anaemia etc.)
- Arthralgia (severe pain in a joint without swelling or other signs of arthritis)
- Allergy
Topoisomerase Inhibitors
- Topoisomerases are responsible for the uncoiling of DNA -‐ they prevent DNA torsional strain during DNA replication and transcription
- They induce temporary single strand (topo1) or double strand (topo2) breaks in the phosphodiester backbone of DNA
- They protect the free ends of DNA from aberrant recombination events
- Drugs, such as anthracyclines, have anti-‐topoisomerase effects through their action on DNA
- Specific topoisomerase inhibitors alter the binding of the complex to DNA and allow permanent DNA breaks
Examples of topoisomerase inhibitors:
- Topotecan (topo 1)
- Irinotecan (topo 1)
- Etoposide (topo 2)
Side effects of topoisomerase inhibitors:
Irinotecan = acute cholinergic type syndrome (diarrhoea, abdominal cramps, diaphoresis (sweating) -‐ they are therefore given atropine)
- Hair loss
- Nausea
- Vomiting
- Fatigue
- Bone marrow suppression
FEC Treatment
FEC treatment:
Side effects:
- Nausea and vomiting
- Marrow suppression
- Thrombocytopenia
- Mucositis
- Diarrhoea
If it decreases the risk of reccurence by 30% definitely worth the toxicity
Discuss cancerous resistance mechanisms
Cells ability to be resistance:
- The cell can survive the damage by the cytotoxic drugs which may lend to the lower chance of relapse/cure.
- DNA repair mechanisms upregulated.
- Base excision repair using PARP.
- DNA efflux by ATP-binding cassettes (ABC) transporters.
Modern, targeted (non-‐cytotoxic) therapies seek to manipulate what we know about cancer cells
- This mainly involved monoclonal antibodies and small molecule inhibitors
- There is a lot of signalling within cancer cells and these signals can be cut in monogenic cancers
- However, for other cancers, parallel pathways and feedback cascades are activated
- We are now in an era of dual kinase inhibitors, which prevent feedback loops but increase toxicities
Explain the rationale for developing new targets and new drugs in cancer therapy
Six Hallmarks of Cancer
6 Hallmarks: SPINAP
- Self-‐sufficient
- Pro-‐invasive and metastatic
- Insensitive to anti-‐growth signals
- Non-‐senescent
- Anti-‐apoptotic
- Pro-‐angiogenic
In other words:
- Survives with minimal stimulation
- Grows regardless of intake
- Doesn’t take the hint to move out
- Spreads rapidly into the surrounding area
- Refuses to grow up
The 6 hallmarks have now been extended to become 10 hallmarks:
In addition to the 6 previously mentioned, these 4 have been added: DIE U
- Dysregulated metabolism
- Evades the immune system
- Unstable DNA
- Inflammation
- Cancer cells are self-‐sufficient in growth signals
- Normal cells need growth signals to move from a quiescent (resting) state to an active proliferating state
- These signals are transmitted to cells via growth factors that bind to transmembrane receptors (tyrosine kinase linked receptors) and activate downstream signalling pathways
NOTE: refer back to Cancer 3 for the full tyrosine kinase mediated signalling pathway
Receptor tyrosine kinase are associated with >50% of human malignancies
Over-‐expression of receptors (HER2, EGFR)
HER2 -‐ amplified and over-‐expressed in 25% of breast cancers
EGFR -‐ over-‐expressed in breast and colorectal cancer
PDGFR -‐ glioma (brain cancer)
These are all growth factor receptors so over-‐expression will lead to an upregulation of the kinase cascade and signal amplification
Over-‐expression of the ligand
VEGF -‐ prostate, kidney and breast cancer
This also leads to upregulation of the kinase cascade and signal amplification
Constitutive (ligand independent) receptor activation
EGFR -‐ lung cancer
FGFR -‐ head and neck cancers, myeloma
Monoclonal antibodies
- -momab (derived from mouse antibodies)
- -ximab (chimeric) e.g cetuximab
- -zumab (humanised) e.g. bevacizumab trastuzumab
- -mumab (fully human) e.g. panitumumab
- Humanized monoclonal antibody: murine regions (black) interspersed within the light (light gray) and heavy (dark gray) chains of the Fab portion.
- Chimeric antibody: murine component (black) of the variable region of the Fab section is maintained integrally.
Monoclonal antibodies target the extracellular component of the receptor. They can do the following:
- The antibody can bind to one of the two receptors and prevent receptor dimerization.
- This causes internalisation of the receptor.
- Neutralise the ligand.
Monoclonal antibodies also activate:
- Fcg-receptor-dependant phagocytosis.
- Cytolysis induced complement-dependant cytotoxicity (CDC).
- Antibody-dependant cellular cytotoxicity (ADCC).
Examples of monoclonal antibodies in oncology
Bevacizumab binds and neutralises VEGF -‐ this improved survival in colorectal cancer
Cetuximab targets EGFR
Small molecule inhibitors
- These bind to the kinase domain within the cytoplasm and block autophosphorylation and downstream signalling
- Glivec was the first ‘targeted therapy’ (aka Gleevec, Imatinib)
In 1973, chromosome translocation in patients with CML was discovered
- This translocation was found to create its own unique fusion protein called Bcr-‐ abl -‐ an enzyme that drove over-‐production of white cells
- In 1996, a drug was found that could specifically target Bcr-‐abl (and not affect other proteins)
- This yielded fantastic clinical results and was thought of as the start of the new era of ‘targeted therapies’
Bcr-‐abl translocation in CML
Glivec is a small molecule inhibitor and targets the ATP binding region within the kinase domain
It inhibits the kinase activity of ABL1
Advantages and Disadvantages of Targeted Therapies
- Resistance mechanisms to targeted therapies
- Mutations in ATP-‐binding domain (e.g. BCR-‐Abl fusion gene and ALK gene, targeted by Glivec and crizotinib respectively)
- Intrinsic resistance (herceptin is effective in 85% of HER2+ breast cancers, suggesting other driving pathways)
- Intragenic mutations
- Upregulation of downstream of parallel pathways (that lead to cell proliferation)
Anti-sense oligonucleates
Anti-‐sense Oligonucleotides: new group of drugs that is being developed
Single-‐stranded, chemically modified DNA-‐like molecule 17-‐22 nucleotides in length
This complementary nucleic acid hybridisation to the target gene hinders translation of specific mRNA
It recruits RNase H to cleave target mRNA
This is good for ‘undruggable’ targets
Mechanism of action of anti-‐sense oligonucleotides:
RNA Interference
Single-‐stranded complementary RNA
This has lagged behind anti-‐sense technology -‐ especially in cancer therapy
Compounds have to be packaged to prevent degradation (nanotherapeutics)
CALAA-‐01 targeted to M2 subunit of ribonucleotide reductase (phase 1 clinical trials in cancer at the moment -‐ results are awaited)
NOTE: all these treatments are very expensive for the NHS
Explain why many cancer treatments cause side effects and recall approaches to minimise this
Small molecule inhibitors
Small molecule inhibitors act on receptor protein tyrosine kinases but also on intracellular kinases -‐ therefore they can affect cell signalling pathways (e.g. kinase cascade)
Small molecule inhibitors that inhibit receptors:
Erlotinib (EGFR)
Gefitinib (EGFR)
Lapatinib (EGFR/HER2)
Sorafenib (VEGFR)
Small molecule inhibitors that inhibit intracellular kinases:
Sorafenib (Raf kinase)
Dasatinib (Src kinase)
Torcinibs (mTOR inhibitors)
By acting on receptors (either externally or internally), targeted therapies block cancer hallmarks (e.g. VEGF inhibitors alter blood flow to the tumour, AKT inhibitors block apoptosis resistance mechanisms) without the toxicity observed with cytotoxics
