New Assay Test Predicts That 50% of Ovarian Cancers Will Respond To In Vitro PARP Inhibition
Posted by Paul Cacciatore on November 11, 2010
U.K. researchers develop a new test that could be used to select ovarian cancer patients who will benefit from a new class of drugs called “PARP inhibitors.”
U.K. researchers have developed a new test that could be used to select which patients with ovarian cancer will benefit from a new class of drugs called “PARP (poly (ADP-ribose) polymerase) inhibitors,” according to preclinical research presented at the National Cancer Research Institute (NCRI) Cancer Conference held in Liverpool on November 8th. According to the test results, approximately 50 percent of all patients with ovarian cancer may benefit from PARP inhibitors.
Dr. Asima Mukhopadhyay Discusses Her Research Into A More Tailored Treatment For Ovarian Cancer
PARP Inhibition & BRCA Gene Mutations: Exploiting Ovarian Cancer’s Inherent Defects
- Genetics 101
DNA (deoxyribonucleic acid) is the genetic material that contains the instructions used in the development and functioning of our cells. DNA is generally stored in the nucleus of our cells. The primary purpose of DNA molecules is the long-term storage of information. Often compared to a recipe or a code, DNA is a set of blueprints that contains the instructions our cells require to construct other cell components, such as proteins and RNA (ribonucleic acid) molecules. The DNA segments that carry this genetic information are called “genes.”
A gene is essentially a sentence made up of the bases A (adenine), T (thymine), G (guanine), and C (cytosine) that describes how to make a protein. Any change in the sequence of bases — and therefore in the protein instructions — is a mutation. Just like changing a letter in a sentence can change the sentence’s meaning, a mutation can change the instruction contained in the gene. Any changes to those instructions can alter the gene’s meaning and change the protein that is made, or how or when a cell makes that protein.
Gene mutations can (i) result in a protein that cannot carry out its normal function in the cell, (ii) prevent the protein from being made at all, or (iii) cause too much or too little of a normal protein to be made.
- Targeting DNA Repair Through PARP Inhibition
Normally functioning BRCA1 and BRCA2 genes are necessary for DNA repair through a process known as “homologous recombination” (HR). HR is a form of genetic recombination in which two similar DNA strands exchange genetic material. This process is critical to a cell’s ability to repair its DNA in the event that it becomes damaged, so the cell can continue to function.
A cell’s DNA structure can be damaged by a wide variety of intentional (i.e., select cancer treatments) or unintentional (ultraviolet light, ionizing radiation, man-made chemicals, etc.) factors. For example, chemotherapy regimens used in the treatment of cancer, including alkylating agents, topoisomerase inhibitors, and platinum drugs, are designed to damage DNA and prevent cancer cells from reproducing.
In approximately 10 percent of inherited ovarian cancers, the BRCA 1 or BRCA2 gene is damaged or mutated. When the BRCA1 or BRCA2 gene is mutated, a backup type of DNA repair mechanism called “base-excision repair” usually compensates for the lack of DNA repair by HR. Base-excision repair represents a DNA “emergency repair kit.” DNA repair enzymes such as PARP, whose activity and expression are upregulated in tumor cells, are believed to dampen the intended effect of chemotherapy and generate drug resistance.
When the PARP1 protein – which is necessary for base-excision repair – is inhibited in ovarian cancer cells possessing a BRCA gene mutation, DNA repair is drastically reduced, and the cancer cell dies through so-called “synthetic lethality.” In sum, PARP inhibitors enhance the potential of chemotherapy (and radiation therapy) to induce cell death. Healthy cells are unaffected if PARP is blocked because they either contain one or two working BRCA1 or BRCA2 genes which do an effective DNA repair job through use of HR.
- PARP Inhibitors: A New Class of Targeted Therapy
PARP inhibitors represent a new, targeted approach to treating certain types of cancers. PARP inhibition has the potential to overwhelm cancer cells with lethal DNA damage by exploiting impaired DNA repair function inherent in some cancers, including breast and ovarian cancers with defects in the BRCA1 gene or BRCA 2 gene, and other DNA repair molecules. Inhibition of PARP leads to the cell’s failure to repair single strand DNA breaks, which, in turn, causes double strand DNA breaks. These effects are particularly detrimental to cancer cells that are deficient in repairing double strand DNA breaks and ultimately lead to cancer cell death.
PARP inhibitors are the first targeted treatment to be developed for women with inherited forms of breast and ovarian cancer carrying faults or mutations in a BRCA gene. Early results from clinical trials are showing promise for patients with the rare inherited forms of these cancers.
Study Hypothesis: PARP Inhibitors May Be Effective Against a Large Proportion of Non-Inherited Ovarian Cancers
As noted above, PARP inhibitors selectively target HR–defective cells and have shown good clinical activity in hereditary breast and ovarian cancers associated with BRCA1 or BRCA2 mutations. The U.K. researchers hypothesized that a high proportion (up to 50%) of sporadic (non-inherited) epithelial ovarian cancers could be deficient in HR due to genetic or epigenetic inactivation of the BRCA1, BRCA2, or other HR-related genes, which occur during a woman’s lifetime. Therefore, PARP inhibitors could prove beneficial to a larger group of ovarian cancer patients, assuming a patient’s HR status can be properly identified.
To test this hypothesis, the U.K. researchers developed a functional assay to test the HR status of primary ovarian cancer cultures derived from patients’ ascitic fluid. The test, referred to as the “RAD51 assay,” scans the cancer cells and identifies which tumor samples contain defective DNA repair ability (i.e., HR-deficient) which can be targeted by the PARP inhibitor. The researchers tested the HR status of each culture, and then subjected each one to in vitro cytotoxicity testing using the potent PARP inhibitor PF-01367338 (formerly known as AG-14699).
Study Results: 90% of HR-Deficient Ovarian Cancer Cultures Respond to PARP Inhibition
Upon testing completion, the U.K. researchers discovered that out of 50 primary cultures evaluated for HR status and cytotoxicity to the PARP inhibitor, approximately 40% of the cultures evidenced normal HR activity, while 60 percent of the cultures evidenced deficient HR activity. Cytotoxicity to PARP inhibitors was observed in approximately 90 percent of the HR deficient cultures, while no cytotoxicity was seen in the cultures that evidenced normal HR activity. Specifically, the PARP inhibitor PF-01367338 was found to selectively block the spread of ovarian tumor cells with low RAD51 expression.
Based upon the findings above, the U.K. researchers concluded that HR-deficient status can be determined in primary ovarian cancer, and that such status correlates with in vitro response to PARP inhibition. Accordingly, the researchers concluded that potentially 50 to 60 percent of ovarian cancers could benefit from PARP inhibitors, but they note that use of the RAD51 assay as a biomarker requires additional clinical trial testing. Although the RAD51 assay test that was used by the U.K. researchers to examine tumor samples in the laboratory is not yet suitable for routine clinical practice, the U.K. research team hopes to refine it for use in patients.
Upon presentation of the testing results, Dr. Asima Mukhopadhyay said:
“Our results show that this new test is almost 100 percent effective in identifying which ovarian cancer patients could benefit from these promising new drugs. We have only been able to carry out this work because of the great team we have here which includes both doctors and scientists.”
The team based at Queen Elizabeth Hospital, Gateshead and the Newcastle Cancer Centre at the NICR, Newcastle University collaborated with Pfizer to develop the new assay to test tumor samples taken from ovarian cancer patients when they had surgery.
Dr. Mukhopadhyay added:
“Now we hope to hone the test to be used directly with patients and then carry out clinical trials. If the trials are successful we hope it will help doctors treat patients in a personalised and targeted way based on their individual tumour. It is also now hoped that PARP inhibitors will be useful for a broad range of cancers and we hope this test can be extended to other cancer types.”
Dr. Lesley Walker, Cancer Research UK’s director of cancer information, said:
“It’s exciting to see the development of promising new ‘smart’ drugs such as PARP inhibitors. But equally important is the need to identify exactly which sub-groups of patients will benefit from these new treatments. Tests like this will become invaluable in helping doctors get the most effective treatments quickly to patients, sparing them from unnecessary treatments and side effects.”
- Test Selects the 60 Per Cent of Women With Inherited Ovarian Cancer Who Could Benefit From “Smart” Drug, Press Release, National Cancer Research Institute Cancer Conference, Liverpool, U.K., November 8, 2010.
- Mukhopadhya A., Soohoo S., Uzir B., et. al. Functional evaluation of homologous recombination pathway in epithelial ovarian cancer and the potential for targeted chemotherapy with PARP inhibitors.
- Mukhopadhyay A., Elattar A., Cerbinskaite A., et. al. Development of a functional assay for homologous recombination status in primary cultures of epithelial ovarian tumor and correlation with sensitivity to poly(ADP-ribose) polymerase inhibitors. Clin Cancer Res 2010; 16(8):2344-51.
- Targeting DNA Repair Through PARP Inhibition, Investigational Research Slide Kit, AstraZeneca Oncology [Adobe Reader PDF document].
- A list of open ovarian cancer clinical trials involving PARP inhibitors.
- A list of open solid tumor clinical trials involving PARP inhibitors.
About The Researchers
Dr. Asima Mukhopadhyay is a doctor and clinical research fellow working at the Queen Elizabeth Hospital, Gateshead and the Northern Institute for Cancer Research at Newcastle University. Queen Elizabeth Hospital is run by Gateshead Health NHS Foundation Trust and is the home for gynecological oncology for the North East of England and Cumbria. She received a bursary to attend the conference, which was awarded on the merit of her work.
Key researchers on the study included Dr. Richard Edmondson, who was funded by the NHS, and Professor Nicola Curtin, who was funded by the Higher Education Funding Council. Dr Asima Mukhopadhyay is funded by the NHS.
Dr Richard Edmondson is a consultant gynecological oncologist at the Northern Gynaecological Oncology Centre, Gateshead and a Senior Lecturer at the Newcastle Cancer Centre at the Northern Institute for Cancer Research, Newcastle University, and is a member of the research team.
Nicola Curtin is Professor of Experimental Cancer Therapeutics at Newcastle University and is the principal investigator of this project.
Current and future work involves working closely with Pfizer. Pfizer developed one of the PARP inhibitors and supported this project.
About The Newcastle Cancer Centre
The Newcastle Cancer Centre at the Northern Institute for Cancer Research is jointly funded by three charities: Cancer Research UK, Leukaemia and Lymphoma Research, and the North of England Children’s Cancer Research Fund. Launched in July 2009, the Centre is based at the Northern Institute for Cancer Research at Newcastle University. The Centre brings together some of the world’s leading figures in cancer research and drug development. They play a crucial role in delivering the new generation of cancer treatments for children and adults by identifying new drug targets, developing new drugs and verifying the effectiveness and safety of new treatments. This collaborative approach makes it easier for researchers to work alongside doctors treating patients, allowing promising new treatments to reach patients quickly.
About the NCRI Cancer Conference
The National Cancer Research Institute (NCRI) Cancer Conference is the UK’s major forum for showcasing the best British and international cancer research. The Conference offers unique opportunities for networking and sharing knowledge by bringing together world leading experts from all cancer research disciplines. The sixth annual NCRI Cancer Conference was held from November 7-10, 2010 at the BT Convention Centre in Liverpool. For more information visit www.ncri.org.uk/ncriconference.
About the NCRI
The National Cancer Research Institute (NCRI) was established in April 2001. It is a UK-wide partnership between the government, charity and industry which promotes cooperation in cancer research among the 21 member organizations for the benefit of patients, the public and the scientific community. For more information visit www.ncri.org.uk.
NCRI members include: the Association of the British Pharmaceutical Industry (ABPI); Association for International Cancer Research; Biotechnology and Biological Sciences Research Council; Breakthrough Breast Cancer; Breast Cancer Campaign; CancerResearch UK; CHILDREN with LEUKAEMIA, Department of Health; Economic and Social Research Council; Leukaemia & Lymphoma Research; Ludwig Institute for Cancer Research; Macmillan Cancer Support; Marie Curie Cancer Care; Medical Research Council; Northern Ireland Health and Social Care (Research & Development Office); Roy Castle Lung Cancer Foundation; Scottish Government Health Directorates (Chief Scientist Office);Tenovus; Welsh Assembly Government (Wales Office of Research and Development for Health & Social Care); The Wellcome Trust; and Yorkshire Cancer Research.