Exploiting the DNA damage response pathway to treat cancer
Many cancers have defects in the way in which cells monitor and repair damaged DNA, collectively termed the DNA damage response, or DDR. This renders the cells more susceptible to DNA damage and more dependent on remaining pathways. Traditional cancer treatments, such as radiotherapy and DNA-damaging chemotherapy, are based on this premise. However, such treatments are often accompanied by significant collateral damage and unwanted side effects. Developing treatments that target cancer-specific DDR dependencies aims to preferentially kill cancer cells, while minimising the impact on normal cells. This has the potential for delivering more selective, better tolerated medicines to improve survival in multiple cancers.1
DDR is one of AstraZeneca’s four key platforms in oncology, in addition to immuno-oncology (IO), antibody drug conjugates and tumour drivers and resistance mechanisms. At AstraZeneca, cross-functional groups work to advance our understanding of the role of DDR in cancer, to drive the development of targeted DDR therapies accompanied by diagnostics to enable precision medicine.
We are committed to continuing to investigate potential DDR targets and to harnessing the potential of this science to benefit patients.
The DNA in our cells undergo tens of thousands of damage events every day. If left unrepaired, this can compromise the genome and even result in cell death. The DDR comprises at least 450 proteins that collectively recognise DNA damage, initiate repair when possible or, in the event of overwhelming DNA damage, instruct the cell to stop growing or even die.1,2
Two factors influence the DDR – the type of DNA damage and when during the cell cycle it occurs. Multiple repair pathways exist to deal with specific types of DNA damage. While some types of damage are repaired relatively rapidly, complex DNA damage – such as breaks in both strands of the DNA double helix or damage occurring during DNA replication – requires longer to repair. In this scenario, pathways are activated to pause cells to allow time for repair. These ‘DNA damage dependent cell cycle checkpoints’ ensure cells do not progress through the cell cycle with compromised DNA. They also ensure the most appropriate repair pathway available is used. As such, DNA repair and cell-cycle checkpoint regulators are inherently interlinked in the DDR process.1
Human cells have evolved multiple pathways to try and deal with all the different types of damage that the DNA in our cells experiences; these are DDR pathways. In a normal scenario, there may be an optimal DDR pathway to deal with a specific type of damage. However, if this pathway is lost during the development of the cancer, another DDR pathway may compensate to allow the survival of the cancer cell. This can represent a dependency that can be exploited, providing a potential new therapeutic opportunity.1
Frequent features of cancer include high levels of DNA damage, loss of one or more DDR pathway and increased DNA replication stress. These properties can lead to cancer-specific DDR dependencies that can be exploited as potential therapeutic targets.
- High levels of DNA damage: A high DNA damage burden renders cells more susceptible to the effects of agents that increase DNA damage (the basis for historical cancer treatment using radiation and chemotherapy). Once DNA damage reaches a critical level, cells can die through different mechanisms, including replication catastrophe or mitotic catastrophe.1 The use of DDR inhibitors can further enhance these DNA damaging agents. The challenge is to ensure the enhanced DNA damage is tumour specific.
- Loss of one or more DDR pathways: When a particular DDR pathway is lost in cancer cells, it can result in an associated DDR dependency that has the potential to be targeted with specific inhibitors. Since the DDR dependency is tumour specific, there will not be the same effect on healthy cells as they retain alternative pathways to repair the damage. This is the concept behind the term ‘synthetic lethality’.1
- Increased replication stress: DNA replication is a highly regulated process that guarantees the duplication of the genome once per cell cycle; the compendium of issues that interfere with the normal DNA replication programme is referred to as ‘ DNA replication stress’.3 Replication stress is a major cause of genome instability and a type of DDR, named the replication stress response (RSR), specifically deals with it. Cancers that are characterised by high replication stress will have an increased dependency on DDR proteins associated with the RSR, providing opportunities for therapeutic intervention by targeting them.
We aim to develop targeted therapies that selectively kill cancer cells by: (1) maximising DNA damage in the cancer cells by blocking DDR; (2) exploiting DDR dependencies that result from one or more pathways; (3) targeting DDR proteins associated with the RSR.
We have a world-leading DDR portfolio at AstraZeneca, of which the backbone is a poly (ADP-ribose) polymerase 1 (PARP1) inhibitor. PARP1 is a protein that has historically been known to play a key role in the repair of DNA single strand breaks, but emerging data suggests a number of additional roles in DDR. When PARP1 is inhibited and trapped onto DNA, this can lead to ‘replication fork stalling’ and the formation of DNA double strand breaks that have to be repaired through a pathway known as homologous recombination repair (HRR). Cancer cells that have deficiencies in HRR, for example through mutations in the tumour suppressor genes BRCA1 or BRCA2, are unable to accurately repair such DNA damage when PARP1 is inhibited. The cancer cell then accumulates DNA errors resulting in genomic instability to an extent that it can no longer survive. Normal cells, because they retain their HRR capability, are not affected in this way.
Other targets we’re exploring in DDR include:1
- WEE1: WEE1 protein kinase activity regulates cell cycle progression allowing tumour cells time to repair DNA damage. WEE1 is overexpressed in many breast, lung and colon cancers, and has a key role in the RSR, suggesting tumour cells rely heavily on this mechanism.
- ATR: ATR is a key protein kinase responsible for regulating the RSR. It plays multiple roles in this respect, including providing a cell cycle checkpoint role as well as facilitating DNA double strand break repair.
- ATM: ATM is a protein kinase involved in activating DNA damage cell cycle checkpoint and coordinating the repair of DNA double-strand breaks through different pathways, including HRR and non-homologous end joining (NHEJ). If ATM is inhibited in combination with DNA damaging agents (e.g. radiation and chemotherapy), this can lead to an increase in DNA double strand breaks above the threshold tolerated by cancer cells.
- Aurora B: Aurora B protein kinase assists in DNA chromosome alignment during cell division. Its inhibition causes either unequal splitting of the chromosomes between the daughter cells or failure of the cell to divide, leading to cell death. Aurora B kinase is known to be over-expressed in liver, colon, breast, renal, lung and thyroid cancers. Inhibition of Aurora B kinase has the potential to increase mitotic stress and therefore be combined with other DDR agents.
- DNA-PK: DNA dependent protein kinase (DNA-PK) is critical in repairing DNA double strand breaks through the NHEJ pathway. DNA-PK has also been linked to the RSR. Opportunities exist for combining an inhibitor of DNA-PK with DNA double strand break inducing agents, as well as other DDR agents that target the RSR.
With a breadth of inhibitors available covering the major DDR pathways, we are exploring combinations with each other and with other anti-cancer agents, with the intent of exploiting cancer-specific DDR deficiencies, leaving no escape route for the cancer cells.1 For example, combining two DDR inhibitors in the right way could simultaneously increase tumour DNA damage in the DNA synthesis (S) phase of the cell cycle and prevent its repair at key checkpoints.
Our aim is to develop combinations to achieve stronger and more durable responses, to tackle emerging resistance and to explore the opportunity to reach patients who are not expected to respond to DDR-based monotherapy. We are also exploring the potential of combining DDR inhibitors with other targeted therapies and with standard-of-care treatments such as radiation and chemotherapy.1
Emerging evidence suggests that inhibition of DDR pathways can prime an anti-tumour immune response, and we have initiated multiple clinical trials to assess the value of combination therapies that target DDR and immune response pathways.1
Development of syngeneic models has enabled pre-clinical assessment of DDR/IO combinations, and we are building a panel of patient-derived xenograft (PDX) models with DDR mutations for preclinical and co-clinical trial studies. Emerging data from external collaborations using PDX models will guide differentiation of DDR inhibitor combinations.
We have established teams dedicated to building pre-clinical capabilities to increase mechanistic understanding of our DDR agents. Comprising state of the art technology and bespoke assays and models, our pre-clinical platform enables detailed mechanism of action and safety studies. These in turn inform patient selection strategies as well as providing dose and schedule insights. Key areas of ongoing research include understanding the role of the RSR in cancer, PARP inhibitor differentiation and new DDR target identification and validation.
In addition, we are performing CRISPR/Cas9 functional genomic screens to identify new markers and mechanisms of sensitivity or resistance to current DDR inhibitors.
Precision medicine is at the heart of our ambition to eliminate cancer as a cause of death. By understanding cancer biology and the molecular drivers of disease, we aim to develop targeted drugs to ensure the right medicine reaches the right patients at the right time, which should result in a greater chance of success in the clinic and ultimately improve patient outcomes.
Developing targeted DDR therapies accompanied by diagnostics for precision medicine is a key focus across all our programmes. To achieve this, we need to be able to identify which patients have tumours with specific DDR defects. This has the potential to allow us to match the right treatment to the right patient.
One way we are doing this is by exploring the use of a panel of HRR genes that are mutated in various cancers. As part of this effort we are investigating the potential of circulating tumour DNA (ctDNA), which is present in the plasma of some patients. For example, exploratory studies have shown it is possible to detect HRR mutations in ctDNA in metastatic prostate cancer patients with greater accuracy compared with DNA derived from a tumour biopsy.
Our translational scientists are also using a range of technologies and exploratory end-points, including genomics, ctDNA, and detection of DDR proteins in nuclear foci, to develop assays to inform patient selection. We are using ctDNA to monitor changes in the DNA of patients who relapse to provide insight into mechanisms of resistance and identify opportunities for developing new therapies to target this.
This is an exciting area of science and we are fully committed to harnessing its potential to benefit patients.
1 O’Connor M (2015). Targeting The DNA Damage Response In Cancer. Molecular Cell. 60(4): 547-560
2 Nature. Eukaryotes and Cell Cycle. Available at: http://www.nature.com/scitable/topicpage/eukaryotes-and-cell-cycle-14046014. Last accessed October 2018.
3 Forment & O’Connor (2018). Targeting the replication stress response in cancer. Pharmacology & Therapeutics 2018: 188; 155-167