MO1-15-3 Efficacy of Radiotherapy to Primary Tumor After Nivolumab Treatment in Patients with Advanced Gastric Cancer.

Background Although several studies suggested that immune checkpoint inhibitors may enhance an efficacy of radiotherapy (RT), it remains unclear in patients (pts) with advanced gastric cancer (AGC). Methods Total 34 AGC pts treated with RT for the primary gastric lesion from April 2013 to March 2019 were analyzed in this study. Pts were divided into two groups according to whether they had received nivolumab before RT or not. Primary tumor responses were assessed according to volumetry on CT scan with tumor response as > 70% decrease in tumor volume. Endoscopic responses were evaluated in pts who underwent endoscopy after RT. Tumor infiltrating lymphocytes (TILs) were also analyzed to examine the tumor microenvironmental immune status in the pts. Results Among 34 pts, 16 pts had received nivolumab before RT. There were no significant differences in baseline characteristics between nivolumab-exposed and nivolumab-naive groups. 70% tumor reductions by RT were observed in 25% (4/16) in nivolumab-exposed group but no patients (0/18) in nivolumab-naive group (P = 0.039). Among pts who received endoscopy after RT, four of seven pts in nivolumab-exposed group experienced partial response, while no response was observed among 10 nivolumab-naive pts (P = 0.015). TILs were evaluated in pts whose tumor samples were available. CD8+ T cells/effector regulatory T cells (eTreg cells) ratio in TILs was markedly increased after nivolumab in pts who achieved response, but not observed in pts who had no tumor reduction. A significant increase of CD8+ T cells/eTreg cells ratio after nivolumab treatment was observed in nivolumab-exposed pts (n = 15), whereas such a tendency was not observed in nivolumab-naive pts (n = 10). Conclusions RT to primary gastric cancer significantly led to tumor response in nivolumab-exposed pts compared with nivolumab-naive pts. Immune profiling suggested that nivolumab might enhance the sensitivity to RT by immunoactivation via increasing CD8+ T cell/eTreg cell ratio. [ABSTRACT FROM AUTHOR]