The Synergistic Anti-tumor Effect Of Iodine-125 Low-dose-rate Brachytherapy And Anti-PD-1 Therapy On Lung Cancer In Mice

Objectives:Radiotherapy（RT）when combined with anti-PD-1 therapy can have a significant anti-tumor effect on non small cell lung cancer,but RT fractionation and dose impact the effects of this combined therapy.Iodine-125 particle implantation(¹²⁵I RPI)is a hyperfractionated low-dose-rate brachytherapy.Its impact on the tumor immune microenvironment and the efficacy of ¹²⁵I RPI combined with PD-1 therapy are unknown.In this study,we evaluated the effectiveness of ¹²⁵I RPI combined with anti-PD-1 antibody in the treatment of lung cancer and their impact on tumor immunity.Methods:A Lewis lung cancer（LLC）mouse model was established in the right hindlimb and radioactive iodine-125 particles were implanted into the tumors.Tumor tissues were obtained six and 12 days after particle implantation,and the expression of PD-1/PD-L1 was detected by flow cytometry.LLC cells were injected subcutaneously into the right hindlimb（primary tumor）on day 0 and into the left flank（secondary tumor）on day 3of mice.On day 10,the mice were randomly divided into PBS,α-PD-1,¹²⁵I RPI,andα-PD-1+¹²⁵I RPI groups.On day 22,primary tumor were extracted from the mice.The proportion of CD4+T cell,CD8+T cell and T regulatory cells（Treg）in the tumor immune microenvironment was detected by flow cytometry,and the primary and secondary tumor volumes were monitored.we examined three different combination schedules:¹²⁵I RPI followed after three days byα-PD-1 m Ab（schedule A）;concurrent administration ofα-PD-1 m Ab and ¹²⁵I RPI（schedule B）;andα-PD-1 m Ab followed after three days by ¹²⁵I RPI（schedule C）.Results:On day 6 after ¹²⁵I RPI,the expression of PD-1 on CD8+T cell was not significantly different from that in the control group（P=0.135）,the expression of PD-L1 was upregulated on day 6（P=0.005）,On day 12 after¹²⁵I RPI,the expression of PD-1（P<0.001）and PD-L1（P<0.0001）was upregulated.¹²⁵I RPI combined with anti-PD-1 therapy synergistically inhibited primary（P=0.003）and secondary（P=0.016）tumor growth,and significantly improve the survival time of mice（P=0.002）.The flow cytometry results showed that the combination therapy could increase the proportion of CD8+T cells（P=0.006）and decrease the proportion of Treg（P<0.001）in the tumor microenvironment.Survival analysis shows that compared with schedule A,schedule B and schedule C can significantly improve the survival time of mice（both P<0.05）Conclusions:¹²⁵I RPI combined with anti-PD-1 therapy can activate anti-tumor immunity and significantly inhibit primary tumor and secondary tumor growth in mice and which presents a promising approach for the treatment of cancer.In addition,the sequence of ¹²⁵I RPI and anti-PD-1 therapy is the key factor affecting the efficacy of combined therapy.