Imaging DNA damage repair during radionuclide therapy

Neuroendocrine tumours (NETs) are a mixed group of relatively rare cancers. Although they do not occur very often, NETs are particularly challenging to diagnose and treat. This is mostly because they are hard to find using traditional radiology imaging techniques, and are hard to keep under control using chemotherapy if surgical removal is not possible, and if tumours bacome resistant to chemotherapy, only one therapeutic option is left. This is called Peptide Receptor Radionuclide Therapy (or PRRT in short). For this treatment, a small strand of amino acids, a peptide, is tagged with a radioisotope that emits therapeutic beta-particle radiation. The tagged peptide, when injected intravenously, will track to and bind to signalling beacons ('receptors') on the tumour. Nearly all NET tumours express these beacons, called somatostatin receptors. The radioisotope will track with the peptide to the receptors on the tumour, and irradiate the tumour locally, thereby causing the DNA in the tumour to get irreparably damaged, and causing the tumour cells to die, and the tumour to shrink. In up to 80% of patients, treatment with PRRT causes very good responses (the disease becomes stable, the tumour shrinks significantly or dissapears alltogether).
Despite the therapeutic successes of PRRT, not much is known about the exact biology of how these radiolabelled peptides cause the tumour cells to die, especially knowledge about exactly how much radiation is needed to cure a tumour is critically lacking. For this reason, standard doses are administered to patients that could either be too low, meaning the tumour will survive; or too high, causing unwanted side effects, such as too much irradiation of the kidneys. Different tumours in different patients will also react differently to the treatment.
Our proposal, which combines the expertise of four research groups from three institutions in two countries (UK and the Netherlands), is to use the power of medical imaging (radiology) to measure the amount of DNA damage caused by PRRT therapy, using molecular imaging. First, we will measure the effects of PRRT using the isotope 177Lu on a panel of NET cancer cells, growing in plastic dishes in the laboratory. We have developed a method to visualise DNA damage using a special scanner called a SPECT scanner, used routinely in cancer care. We propose to simultaneously visualise the whereabouts of the therapeutic radioisotope 177Lu, and the location and extent of the DNA damage it is causing. We will test this simultaneous imaging using human NET tumours grown in mice, and image them using our dedicated small-animal SPECT scanners, smaller versions of the patient scanners. Because we can do this non-invasively (without surgical intervention, only a simple injection is needed), and in the whole body all at the same time, and we acquire information in three dimensions, we will have the ability to adjust further 177Lu PRRT therapy to the reaction of each individual patient's tumour and normal tissue. The similarity of the techniques used in mice and patients will allow us to translate our findings from the laboratory to the clinic faster.
In a related set of studies, we will combine 177Lu PRRT therapy with another type of radionuclide therapy, called Auger electrons, emitted by the radioisotope 111In. This particular combination has never before been tested. First, we will test this in NET cancer cells growing in plastic dishes. Then, we will evaluate the combination using human tumours growing in mice, checking whether the combination of the two treatments reduces tumour growth more than using either treatment alone.

Peptide Receptor Radionuclide Therapy (PRRT) is a most effective therapy for the treatment of somatostatin receptor expressing neuroendocrine tumours (NETs). NETs, although rare, present a significant clinical challenge, especially where surgery is impossible. PRRT, often administered as 177Lu- or 90Y-labelled somatostatin peptide analogues dotatate or dotatoc, directs these beta-emitting radionuclides to receptors on tumour cells, irradiating the tumour locally, causing DNA damage and tumour cell death. Since these radiolabelled peptides do not distribute significantly to other parts of the body, radiotoxicity to non-target tissues is limited to the kidneys, because of renal clearance, and some bone marrow toxicity. PRRT has demonstrably good obective response rates of up to 80%.
Despite its frequent and successful use in the clinic, little or no dosimetry or radiobiological considerations are taken into account at the time of PRRT treatment planning or delivery, and treatment is usually administered as a standard dose and time scale, without any considerations for peptide uptake in the tumour, radiosensitivity, DNA damage effects, or dosimetry.
Here, we propose to combine SPECT imaging of 177Lu-dotatoc distribution with DNA damage imaging, developed in our group. We showed that low amounts of 111In-anti-yH2AX-TAT, targeting the DNA damage repair protein yH2AX allows to visualise and quantify the extent of DNA double strand break damage. Combining dosimetry based on 177Lu imaging with visualisation of its radiobiological effects with 111In-anti-yH2AX-TAT will provide information about PRRT efficacy, at the time of treatment administration, allowing for an adaptive treatment regimen, which we hypothesise will increase complete response rates.
Additionally, we will evaluate combination of 177Lu-dotatate PRRT with Auger electron treatment with 111In-anti-yH2AX-TAT, using larger therapeutic doses of 111In. We hypothesise that the combination will be superadditive.

If successful, the research in this proposal will benefit:

Radionuclide therapy researchers:
- By providing an increased understanding of the radiobiological effects of PRRT in vitro and vivo
- Providing a new tool to visualise the DNA damaging effects of PRRT
The radiobiology research community, by:
- Studying the radiobiological effects of radionuclide therapy
- Suggesting new combinations of radionuclide therapies
The research laboratories of the collaborators in this proposal, because of:
- An expansion of the scope of the imaging agent, 111In-anti-yH2AX-TAT
- An expansion of the scope of 177Lu-dotatate
- Application of computer code for micro-dosimetry
- Application of radiobiology to radionuclide therapy
Clinicians and neuroendocrine tumour patients, by:
- Providing optimisation of PRRT regimens, based on DNA damage measurements
- Provide novel, more effective combination therapies to combat neuroendocrine tumours

If successful, the research proposed in this application will lead to more treatment options for patients diagnosed with inoperable neuroendocrine cancer. Our research will also lead to a better understanding of those treatments, allowing further optimisation, and adaptation of the treatment regime to an individual patient's tumour response. We will develop several combination therapies that will improve complete response rates in NET patients, and that will improve overall survival.
The success rate of PRRT using 177Lu-or 90Y- labelled somatostatin analogues is already quite good. A recently successfully completed phase three clinical trial, and the possibility of molecular imaging of NETs using PET/CT imaging with 68Ga-analogues, recently approved by the FDA, further prove the impact and clinical relevance of NET imaging and PRRT. We are convinced that these previous successes can be further improved upon, by including molecular imaging of the therapeutic effects of PRRT.

Our collaborations with NET treating physicians, and the similarity of the techniques used in our preclinical in vivo studies in tumour bearing mice to those used in the clinic for NET patients will expedite the translation of our findings to the clinic, maximising clinical impact.

We also refer to the Academic beneficiaries section above, and the pathways to impact section in appendix.