Cancer therapy Flashcards
Overview of how we can target cancer
Kill normal tissue as well if not essential (breast, prostate)
Target mutations or hallmarks: growth control, genetic instability, telomerase, others.
Are cancer cells more vulnerable (proliferate more, lack checkpoints, more stressed)
Viruses
Exploit/boost natural defences
Surgery
Typically, primary tumours removed by surgery. Still arguably the most important therapy as it can cure if there are no metastases - followed (ensuring no stray tumour cells are present) or preceded (debulking to make surgery easier) by cytotoxic drugs and/or radiotherapy
Cytotoxic drugs - overview
Most current therapy consists of cytotoxic drusg that are thought to exploit cancer hallmarks - factors that might distinguish cancer cells from most healthy cells, although many healthy cells also undergo these processes at some point in their lifecycle (although might not be an issue if the healthy cells are redundant, non-essential, or are easily replaced). Many cytotoxic drugs are DNA damaging agents - they alkylate bases, intercalate non-covalently between bases of the DNA helix, crosslink strands, or are toxic analogues of bases (eg 5-fluoro-uracil) that interfere with DNA synthesis. Some inhibit topoisomerases. Others (Taxol and other taxanes and the Vinca alkaloids) interfere with mitosis, generally targeting microtubiles and hence the spindle
How does conventional cytotoxic therapy work?
Most cytotoxic cancer drugs were used clinically before their MoA were known. Believed that they exploit strategies that damage DNA. It’s possible that they exploit genetic instability.
eg Cis-platin creates DNA cross-links, which are repaired by components of HR repair including BRCA2, which can be defective and cause genetic instability in some cancers.
eg Taxanes target the mitotic spindle, which also seems to be defective in some cancers (lagging chromosomes)
Cancer cells may also be more vulnerable than normal cells (may be stressed or unable to undergo cell cycle arrest when DNA is damaged)
Experimental model of a deficient checkpoint - Waldmann experiment
Shows that cells can be made sensitive to DNA damage. Made cancer cells that were defective in the p21 checkpoint protein that arrests the cell cycle in response to DNA damage (downstream of p53). Grew them as grafts on mice and X-irradiated them. X-radiation cured several checkpoint-defective tumours but no wild type controls. The checkpoint defective cells did not arrest when irradiated and presumably died of mitotic catastrophes, while the wild-type cells underwent cycle arrest and then recovered. (see image)
Exploitation of genetic instability - PARP inhibitors
If DNA repair pathway is defective, cells may be killed by blocking alternative repair routes, leaving the cancer cell unable to deal even with everyday spontaneous DNA damage. So far, the single strand break repair pathway has been targeted.
It required the enzyme PARP (poly-ADP ribose polymerase). PARP inhibitors block repair: ss breaks accumulate and halt DNA replication, because at the replication fork the ss break becomes a ds break. Normally, replication-fork blockages are rescued by HR ds break repair. But if a cancer has defective HR repair (like BRCA2-null cells), theu cannot repair, and die, while normal cells manage. Therefore, PARP inhibitors are dramatically more toxic for BRCA2-null cells than normal cells.
However, resistance to blocking replication forks may develop in BRCA2-null cells that have frameshift mutations in BRCA2. A reversion mutation may restore BRCA2. An example of how resistance to therapy may develop
a) Overview of targeting oncogenes
b) Drugs that target receptor tyrosine kinase pathways - Imatinib (and resistance)
a) In order to specifically affect cancer cells, one approach is to target proteins that are aberrantly expressed in these cells, reducing the potential for side-effects. However, off-target activity and resistance are common, although increasing conduct of DNA sequencing, specific mutated genes, and consequently the mutant proteins they encode, can be determined and appropriate therapeutic strategies designed. Many drugs (both small molecule drugs and monoclonal antibodies) have been developed to the receptor tyrosine kinases and their downstream signalling pathways.
b) One cancer-specific target are proteins encoded by fusion genes formed at chromosome rearrangement breakpoints. Imatinib inhibits the tyrosine kinase encoded by the BCR-ABL fusion gene created by the t(9:22) chromosome translocation found in chronic myeloid leukaemia (CML). While very successful, imatinib isn’t actually specific for Bcr-Abl, but inhibits other tyrosine kinases including ckit (and can be used to treat cancers with activation of the other kinases). Most CML patients respond to imatinib, but many eventually develop resistance. There are many mechanisms of resistance, on being the development of point mutations that prevent imatinib binding to the target site in the fusion protein. Second-generation drugs that cypass this resistance have been developed.
Drugs that target receptor tyrosine kinase pathways - Herceptin and alternatives (and resistance)
Herceptin is a small molecule tyrosine kinase inhibitor which targets EGFR/ERBB/HER1 and ERBB2/HER2. An alternative approach is the development and use of monoclonal antibodies eg the monoclonal antibody to HER2 which is used to treat HER2-amplified breast cancer. Anti-BRAF and anti-MEK small molecule drugs are also in use
Use of these drugs often leads to resistance. eg treating colorectal cancers with anti-EGFR often results in emergence of variant, resistant tumours that have acquired mutations in downstream genes such as RAS or RAF. eg the BRAF inhibitor vemurafenib targets the ATP-binding domain and works well on BRAF V600E in melanomas, but resistance usually develops (by acquisition of RAS mutations or amplification of mutant BRAF)
Oncolytic viruses
The idea that viruses might kill cancers, by lysis or exciting an immune response has been around for a while and there are some animal viruses that are not pathogenic for humans that replicate in some cancer cells. One oncolytic virus is licensed for clinical use - an engineered adenovirus H101 1. Adenoviruses prepare infected cells for virus replication by expressing proteins that inactivate Rb1 and p53. The engineered adenoviruses have deletions in the E1B_55k gene, which inactivates p53. The idea is that without this E1b protein, the virus will only replicate in p53-mutant cancers. It is injected directly into the tumour. (see image)
The immune system - boosting out natural defences
a) potential new therapies, problems previously faced
b) checkpoint inhibitors (benefits and problems)
a) Harness the immune system to kill tumours, by breaking tolerance of T cells to tumours. Previous studies of immune suppressed humans and mice showed no major increase in cancer, except where cancer has a viral aetiology. This is because mutant protein expression on a cell would probably induce peripheral tolerance. Also, mutant proteins aren’t selected against as normal cells have many mutations that alter proteins, and in cancers there is no detectable selection against mutations that create new peptides
b) These are monoclonal antibodies that block signals that hold back cytotoxic T-cells. Cytotoxic T-cells are down regulated by CTLA4, PDL1 and PD1 (CTLA4 binds to B7, displacing costimulator CD28, whereas PD1 and PDL1 bind to eachother to provide inhibitory signals. Monoclonal antibodies to these molecules are licensed (see diagram)
Benefits - some patients with advanced metastases, who have failed conventional therapy, show dramatic tumour regression and improvement in health, and some go on being healthy for months. Resistance seems to develop more slowly
Problems - only some patients with certain kinds of tumours respond. Treatment is very toxic, giving inflammation and autoimmunity that can be life-threatening. eg in trials of anti-CTLA4 in melanoma, only 11% patients did better at 3 years, while treatment had to be stopped in 50% patients because of toxicity, and 1% died of immune disease. The cancers that respond well are those with the most mutations, hence highest number of mutant peptides (lung and melanoma). These results can be explained by supposing there are T-cells that recognise mutant peptides, by they are mostly tolerised. If there enough such antigens and tolerance is downregulated, an effective anti-cancer response may emerge, but together with inflammation and autoimmunity.
