Ebert M, Cook AM, Nowak AK, Gill S.
Funding: Cancer Australia. 2019-2021.
Synopsis (lay): Checkpoint blockade immunotherapy has been used to treat a growing number of cancer types. It can work amazingly well for some people, and not at all for others. There is evidence that radiotherapy may be a useful tool to increase the proportion of people who benefit. Here, we are trying to change structure and function of the blood vessels in the tumour in a way that can help the immune system to kill of the cancer cells.
There is a need to increase the efficacy of immunotherapy with checkpoint inhibitors
Although immune checkpoint inhibitors have proven effective in many cancer types and even offer the promise of a complete response in aggressive cancers like melanoma, they are still only effective in fewer than 40% of patients. Additionally, checkpoint inhibitor drugs are very expensive (up to $150,000 per patient per year). Methods to increase the proportion of people who respond effectively to checkpoint inhibitors are therefore urgently required.
Radiation has a clear role for both stimulating and facilitating immune response
The existence of a synergistic relationship between radiation and immune response has been known for some years. Radiation can kill cells in a particular manner that releases signalling molecules and danger signals and can initiate an anti-cancer immune response – most potently at high radiation doses per fraction. Radiotherapy can also boost the diversity of targets the T cells within the tumour can recognise and react to.
However, radiation can also negatively impact the supply and exhaustion of T cells. Additionally, less radiosensitive, more immunosuppressive lymphocyte subsets (e.g., regulatory T cells) can be present in relatively larger quantities following radiotherapy. These factors can prevent any new anti-cancer immune responses generated by tumour irradiation from being fully functional.
We hope to counteract the negative effects of immunosuppressive cells and T cell exhaustion, through the action of immune checkpoint inhibitor drugs, aiming to combine radiotherapy and immunotherapy to induce a strongly synergistic effect.
Radiation also alters access to the tumour environment
One significant problem for the anti-tumour immune response is the difficulty T cells may encounter in invading the tumour. By their nature, tumours have abnormal vasculature; this can represent a physical barrier that actively hinders T cell infiltration and activation. Normalisation of the tumour vascular environment is therefore a key strategy to enhancing the effect of cancer immunotherapy.
The impact of radiation on tumour vasculature is dependent on the quantity of radiation delivered at each individual irradiation (‘fraction’). At high doses per fraction, the structural cells making up blood vessels are killed off – destroying the tumour vasculature. This can lead to poorer access to the tumour environment for circulating T cells, and an increase in poorly oxygenated (hypoxic) areas. In contrast, lower doses of radiation per fraction can suppress expression of endothelial growth factors – leading to normalisation and increased permeability of tumour vasculature, reoxygenation of the tumour microenvironment, and conversion of immunosuppressive cells (e.g., macrophages) toward anti-tumour types. Conversion of hypoxic areas into normally oxygenated ones is important, since: 1) one of the ways tumour cells are killed by irradiation involves oxidation of tumour DNA; and 2) many metabolic processes involved in the immune reaction require oxygen to operate efficiently.
Hence, optimal choice of radiation doses can effectively turn an immunologically inactive (‘cold’) tumour into an active (‘hot’) one. The type of fractionation needs to be well considered; there is a balance between the promotion and sustenance of vascular normalisation and induction of anti-immune response, and the killing of the vascular cells plus the very T cells that would attack the tumour.
The interplay between tumour microenvironment modulation, immune cell infiltration and radiotherapy fractionation represents a complex and unexplored multi-variate problem
The multiple competing factors associated with radiotherapy fractionation, resulting tumour stromal and immunological effects, and the potential time-points at which checkpoint inhibitors could be introduced, lead to a complex multi-variate problem which needs to be optimised. Such optimisation should be informed according to knowledge of the underlying competing microenvironment effects. All studies to date have focussed on fixed-fractionation delivery of radiation, and it is known that the timing of immunotherapy relative to radiation delivery impacts response. We are proposing a treatment schedule whereby the tumour microenvironment is pre-conditioned or ‘primed’ to allow the best possible immunological response.
WE HYPOTHESISE THAT, IN COMBINATION WITH INTRODUCTION OF AN APPROPRIATELY TIMED CHECKPOINT INHIBITOR DRUG, INFILTRATION OF ANTI-TUMOUR T CELLS CAN BE ENHANCED AND A MORE EFFECTIVE RESPONSE ACHIEVED.
This proposal is aimed at testing that hypothesis and generating the associated understanding of response in order to optimise patient treatment scheduling.