Th1 adjuvant ARNAX, in combination with radiation therapy, enhances tumor regression in mouse tumor-implant models

Radiation therapy (RT) rarely induces tumor regression at untreated metastatic sites, the so-called abscopal effect. A syngeneic tumor (EG7) transplanted into a Th1-dominant mouse strain (C57BL/6) regressed significantly on the treated side and less on the contralateral side after RT. Additional subcutaneous administration of ARNAX, a non-inflammatory adjuvant, further accelerated tumor regression on the untreated side. This suggests that ARNAX after RT significantly enhances the tumor regression effect on the irrelevant tumor. Based on this setting, we next observed similar tumor shrinkage after RT and ARNAX by transplanting syngeneic breast cancer tumors (4T1) into a Th2-dominant mouse strain (BALB/c). The results were as follows: 1. ARNAX enhanced RT-mediated tumor shrinkage comparable to polyI:C; 2. In the Th2 mouse strain, little tumor regression occurred on the untreated side compared to tumor regression on the treated side after RT alone; 3. RT+ARNAX treatment caused additive regression on the treated side and induced slight tumor regression on the untreated side; 4. PD-L1 antibody + RT combination therapy caused tumor regression and further induced additive regression with ARNAX; 5. The combination of RT and ARNAX reduced the number and volume of lung metastases compared to RT alone. However, tumor regression was not always accompanied by a significant prolongation of survival in the mice receiving our regimen and protocol (one 10Gy radiation and a single ARNAX treatment). In conclusion, RT therapy promoted abscopal tumor regression in both Th2 and Th1 models with the addition of the non-inflammatory adjuvant ARNAX.