Immuno-radiotherapy

Aims

To define radiotherapy and checkpoint inhibitor combinations for potential clinical benefit; to establish relevant tumor types for clinical applications of the optimal combinations, and to evaluate these clinically.

Background

Cancer immunotherapy has resulted in unprecedented improvements in outcome in patients with a spectrum of solid tumors. This is primarily the result of developing agents that target immune regulatory checkpoints, namely the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), or programmed death 1 (PD-1) and its ligand programmed death 1 ligand (PD-L1). Despite positive results, many patients show little or no response to checkpoint inhibitors. While considerable effort is currently underway to understand the mechanisms for resistance that limit the activity of these immune therapies, at the same time combination approaches are being investigated to try and improve outcomes. Radiation therapy is one such approach since there is evidence, especially at higher doses per fraction, that radiation alone can also induce an immune response.

Methods

We will use a variety of murine tumor models that range from poorly to highly immunogenic. These will include a C3H mammary carcinoma, EMT6 breast cancer, CT26 colorectal carcinoma, Lewis lung carcinoma, Pan02 pancreatic and B16 melanoma. The initial studies will be focused on determining the optimal combination of radiation and checkpoint inhibitors. Radiation will include single doses, a simplified fractionated schedule (2 Gy/fraction delivered over a two week period), a stereotactic schedule (3 x 15 Gy), and proton irradiation. The checkpoint inhibitors will involve both established agents (i.e., anti-CTLA-4/PD-1/PD-L1 antibodies) and experimental agents in development. For these we will investigate the combination with the various radiation treatments with the checkpoint inhibitors administered before, during, or after irradiation. Once optimal treatment schedules are established, we will apply these to relevant patient-derived xenografts (PDX) that represent those cancer types in which radiation and checkpoint inhibitors are likely to be used clinically (i.e., head & neck, prostate, lung, pancreatic, anal, and sarcomas). These will involve established models and new ones taken directly from patients. Since PDX tumors can only be established in immune-suppressed mice, we will use immune-compromised NSG animals which have been immunologically humanized by engraftment with PBMCs or CD34-positive hematopoietic stem cells. The endpoints for tumor response will be a tumor growth delay assay (i.e., time taken to reach 3-5 times the treatment volume). Throughout the tumor studies, mouse body weight will be continually monitored to observe any system toxicity.

The tumor data will be supported by measurements of various biomarkers to help us both predict and assess response. These will include checkpoint inhibitory receptor expression and infiltration of host cells that include T-cells, macrophages and fibroblasts. Assessment will involve both immunostaining and flow cytometry methods. In addition, we will apply imaging techniques (i.e., positron emission tomography) to allow us to non-invasively predict and monitor response.

In addition to the pre-clinical experiments, we will also undertake clinical studies. These will be started early and run in parallel to the pre-clinical studies with the initial investigations being based on currently adopted combinations of checkpoint inhibitors and conventional radiation treatment. However, we will also include modifications based on the results from the pre-clinical studies. These clinical studies will involve Phase I/II investigations in cancer of, for example, the head & neck, lung, pancreas, prostate, anal region and sarcomas. The focus will be on the effects in the primary tumors but also possible abscopal effects that can influence metastatic disease. Tumor and liquid biopsies will also be taken during treatment to investigate changes in immunity and the tumor microenvironment. Clinical studies will be planned in collaboration with relevant clinicians.

Expected results

The preclinical models and clinical studies will allow us to predict potential effective combinations of radiation (conventional fractionation, stereotactic treatments, and proton therapy) and checkpoint inhibitors (anti-CTLA-4/PD-1/PD-L1 and novel experimental agents in development), identifying feasible tumor types for this combined treatment and establishing novel imaging/assessment parameters to predict response.

Impact/Relevance/Ethics

The results of this study will contribute significantly to the foundation for future clinical application of radiotherapy in combination with checkpoint inhibitors.