PO-428 Radiotherapy, immunocytokines and immune checkpoint inhibitors: finding the optimal combination

Abstract

IntroductionAlthough immunotherapy is currently changing cancer treatment practice, primary resistance is still seen partly due to insufficient immunogenic priming. Radiotherapy (RT) induces the release of tumour antigens and thereby increases tumour immunogenicity. Therefore, combination of these therapies has high potential. Here, we investigated the therapeutic outcome of combining a single dose RT with either immunocytokines (L19-IL2) or immune checkpoints inhibitors (ICIs) (aCTLA-4, aPD-L1 and aPD-1), aiming to further activate and/or prolong the RT-elicited immune response.Material and methodsBalb/c or C57BL/6 mice were injected in the right flank with either C51 and CT26 colon carcinoma or Lewis lung carcinoma (LLC) cells. Upon an average volume of 200mm3, animals were randomised in different treatment groups: RT+vehicle/L19-IL2+IgG or RT+vehicle/L19-IL2+aPD L1/aPD1/aCTLA-4. Tumours were irradiated with 5Gy (C51 and CT26) or 10Gy (LLC), as the latter is known to be poorly immunogenic. Vehicle/L19-IL2 (1 mg/kg), aCTLA-4 and IgG (both 10 mg/kg) were given i.v. on day 1, 3 and 5 after RT; aPD-1, aPD-L1 and IgG (all 10 mg/kg) were given i.p. 1, 3, 5, 7 and 9 days after RT. Blood was collected before and after treatment for immune-profiling. Tumour response was quantified as time to reach 4 times starting tumour volume (T4xSV).Results and discussionsIn the CT26 model, RT +L19-IL2, RT +aCTLA-4 and RT +aPD L1 resulted in higher T4xSV compared to RT only (p<0.001, p<0.01 and p<0.05). In the C51 model solely RT +L19-IL2 was better (p<0.001) compared with RT only. RT +L19-IL2 efficacy was significantly (p<0.001) better compared to RT +ICIs in the C51, but not in the CT26 and LLC models. Adding ICIs to 5Gy+L19-IL2 did not result in improved outcome in either of the two models. Conversely, in the LLC model, the triple combination of RT +L19-IL2+aPD L1 yielded a better outcome compared to RT +L19-IL2 (p<0.05) and RT +aPD L1 (p<0.001). Similar results were observed for aCTLA-4 and aPD-1. Overall, we found an ICI-driven increase and decrease of circulating CD4 +and CD8+T cells respectively and an increase of PD-L1 associated with L19-IL2 therapy in all three models.ConclusionOur results show a larger therapeutic effect when RT is combined with L19-IL2 as compared to ICIs in the majority of the models, yet trimodal therapy was also beneficial in one model, revealing a tumor-model dependency of treatment. Immunological profiling of tumours is undergoing to explore the mechanisms underlying differential responses.