Abstract Book

S23

ESTRO 37

Hyperthermia treatments, using temperatures up to 45°C, can directly kill cells, and also has indirect effects through either a reduction in tumour blood flow or by inducing an immune response. We are currently doing experiments with the combination of a CI and heat treatment. The CI of interest is the monoclonal antibody CDF1 male mice were inoculated on the right rear foot with a C3H mammary carcinoma. Treatments were performed when tumours reached a volume of 200mm 3 . These included sham treated mice (controls), heat (42.5°C for 1 hour) on day 0, intra peritoneal injection with anti-CTLA-4 (10 mg/kg) on day 1, days 1 and 3, or days 1-4, or the combination of heat with anti-CTLA-4. Tumour size was measured daily, and the time to reach five times treatment volume (TGT5) was the endpoint. Mouse body weight was also recorded to assess toxicity. Results are listed as Mean (± Standard Error). One-way ANOVA comparison of group means was performed, and a P<0.05 was considered significant. Results The TGT5 for the control group was 6.6 days (+ 0.2). For the groups treated with anti-CTLA4 on day 1, days 1 and 3, or days 1-4, the TGT5 was 5.8 days (+ 0.4), 5.8 days (+ 0.4) and 6.8 days (+0.3), respectively. None of these values were significantly different from controls. In the group treated with heat the TGT5 was significantly increased to 11.1 days (+ 0.9). When combined with the anti-CTLA-4 this value was increased to 11.6 days (+ 1.2) for treatment on day 1, 12.9 days (+ 2.7) for days 1 and 3, and 15.4 days (+ 0.8) for days 1-4. The values for day 1 and 1 and 3 were not significantly different from heat alone, but when heat and anti-CTLA-4 on days 1-4 were combined we saw a significantly increase in treatment days (p=0.004) One mouse even showed tumour control and remained without signs of recurrence after 3 months. Conclusion The C3H mammary carcinoma is a non-immunogenic tumour, and as such is insensitive to treatment with checkpoint inhibitors. However, when treated with heat the tumour becomes CI sensitive to anti-CTLA-4 administered on days 1-4. Our data thus clearly shows the potential benefit of combining hyperthermia and specific CI’s to improve immunogenicity. Since the clinical potential of hyperthermia is not when used alone but when combined with more conventional cancer therapies such as radiation, our future plan is to investigate the potential of combining hyperthermia, anti-CTLA-4 and radiation. OC-0054 Combination treatment with radiotherapy plus IL-2/anti-IL-2 complexes and its theranostic evaluation H. Jing 1 , M. Hettich 2 , E. Firat 2 , S. Gaedicke 2 , M. Bartholomä 3 , G. Niedermann 1 1 University Clinics, Department of Radiation Oncology and German Cancer Consortium DKTK, Freiburg, Germany 2 University Clinics, Department of Radiation Oncology, Freiburg, Germany 3 University Clinics, Department of Nuclear Medicine, Freiburg, Germany Purpose or Objective Immune checkpoint blockers (ICBs) have revolutionized oncology. Combination with immunogenic radiotherapy (RT) can enhance their efficacy. However, alternatives are needed for non-responding patients and those with pre-existing or ICB-induced autoimmune symptoms. Combinations of IL-2 with RT could be such an alternative. But IL-2 has a short half-life; in addition, depending on binding to its high-affinity receptor containing CD25, it stimulates immunosuppressive CD4+ regulatory T cells (Tregs) and causes potentially life- threatening vascular leakage. To circumvent these that targets CTLA-4. Material and Methods

disadvantages, we tested the combination of hypofractionated RT (hRT) and IL-2/anti-IL-2 complexes (IL-2c); IL-2c bind to the intermediate-affinity receptor (not containing CD25) and have a longer half-life. Material and Methods Mice with established melanomas were treated with local hRT (2 × 12 Gy) and IL-2 or IL-2c. Besides tumor size and survival, the number of tumor-specific T cells were assessed by flow cytometry. In addition, a novel PET tracer was developed by conjugating therapeutically active IL-2c with the chelator NOTA and loading the conjugate with radioactive 64 Cu. Results Treatment of mice bearing established B16 melanomas with hRT + IL-2c was superior to hRT + IL-2 or hRT alone; IL-2c treatment alone was not effective. The better antitumor response correlated with increased tumor- specific T cells and NK cells, but not CD4+ Tregs, in the irradiated tumor and in lymphoid organs. The novel PET tracer allowed visualization of the whole-body distribution of IL-2c and their bound receptors in naïve mice and tumor-bearing mice. Surprisingly, the tumor uptake was non-specific and only moderate. This prompted experiments suggesting that specific IL-2c binding in the tumor is limited by IL-2 secreted by tumor- resident effector cells. Lastly, we show that the IL-2c- induced splenomegaly, due to massive bystander expansion of CD8+ T and NK cells, is only transient. Conclusion We developed a novel combination treatment consisting of hRT and high-molecular-weight IL-2c, which resulted in long-term tumor control and sometimes cures in mice with large, established melanomas. PET imaging and biodistribution studies with the novel IL-2c tracer suggested that IL-2c act, to a considerable extent, outside of the tumor. OC-0055 Immunocytokine, immune checkpoint inhibitor and radiotherapy: finding the right combination D. Marcus 1 , V. Olivo Pimentel 1 , A. Van der Wiel 1 , R. Biemans 1 , N. Lieuwes 1 , D. Neri 2 , J. Theys 1 , A. Yaromina 1 , L. Dubois 1 , P. Lambin 1 1 Department of Radiotherapy, GROW - School for Oncology and Developmental Biology- Maastricht Comprehensive Cancer Centre, Maastricht, The Netherlands 2 Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, Zürich, Switzerland Purpose or Objective Cancer immunotherapy reaches its height and immune cytokines as well as checkpoint inhibitors show a lot of promise. However, results are neither unambiguous nor optimal yet. In this study we aim to optimize therapeutic outcome by combining radiotherapy (RT) and the immunocytokine L19-IL2 with several immune checkpoint inhibitors, namely anti-PD-1, anti-PDL-1 and anti-CTLA-4. L19-IL2 consists of an antibody fragment targeting the tumour neovasculature bound to IL2 and stimulates a cytotoxic T cell response. Checkpoint inhibitors prevent T cell exhaustion. Material and Methods Balb/c mice were injected unilaterally with either C51 or CT26 colon carcinoma cells and randomized at an average tumor volume of 200 mm 3 . Different treatment combinations were applied: RT + vehicle, RT + L19-IL2, RT + anti-PD-1, RT + L19-IL2 + anti-CTLA-4, RT + L19-IL2 + anti-PD-1, RT + L19-IL2 + anti-PDL-1. L19-IL2 (1 ug/g, i.v.), anti-PD-1 and anti-PDL-1 (both 10 mg/kg, i.p.) were administered on day 1, 3, 5, 7 and 9 after a single dose (5 Gy) RT, anti-CTLA-4 (10 mg/kg, i.v.) on day 2. Blood samples were taken from the vena saphena at start of treatment and after treatment ended. Outcome was determined by growth delay and flow cytometry analysis.