To investigate the therapeutic efficacy and potential mechanisms of integrin α _vβ₃-targeted radionuclide therapy (TRT) in combination with anti-programmed cell death protein ligand 1 (PD-L1) immunotherapy.

Integrin α _vβ₃-targeted molecule Arg-Gly-Asp (RGD) was conjugated with Evans blue (EB) and then labeled with ¹⁷⁷Lu to obtain ¹⁷⁷Lu-EB-RGD. The radioactivity and radiochemical purity were determined. MicroSPECT imaging, biodistribution, and in vivo therapeutic efficacy were subsequently performed in MC38 murine colon cancer models. Volume of tumor and body mass of mice were observed to assess the therapeutic efficacy and safety (n=9 in each group). Flow cytometry was used to evaluate therapy response of saline-treated (control, group A), 18.5 MBq ¹⁷⁷Lu-EB-RGD-treated (group B), 10 mg/kg PD-L1 antibody-treated (group C), TRT combined with immunotherapy-treated (group D, 18.5 MBq ¹⁷⁷Lu-EB-RGD and 10 mg/kg PD-L1 antibody) mice and alterations in tumor microenvironment (PD-L1⁺ immune cells, CD8⁺ T cells and regulatory T cells). Independent-sample t test and repeated measures analysis of variance were used for data analysis.

The radioactivity of ¹⁷⁷Lu-EB-RGD was (55.85±14.00) GBq/μmol. SPECT imaging clearly visualized the MC38 tumors in mice models with high uptake and long retention time, the tumor/muscle ratio reached 14.87±0.88 at 24 h postinjection, while less uptake and retention in normal tissues. Tumor uptake of¹⁷⁷Lu-EB-RGD was significantly higher than that of ¹⁷⁷Lu-RGD 4 h post-injection ((12.00±1.60) vs (3.69±0.37) %ID/g; t=8.63, P<0.01). The efficacy results between each treatment group was significantly different (F=7.32, P=0.03) at day 6 post-treatment. The combination therapy showed the most outstanding anti-tumor efficacy with 7/9 mice showed complete response. Flow cytometry results showed that TRT up-regulated the PD-L1 expression significantly, namely, PD-L1⁺ immune cells in group B and group A were significantly different (CD45⁺ /PD-L1: 2.34% vs 0.95%, CD11b⁺ /PD-L1: 2.41% vs 0.66%; t values: 11.17 and 8.70, both P<0.01); immunotherapy and combination therapy dramatically stimulated the infiltration of CD8⁺ T cells (2.07% vs 0.26%, 2.71% vs 0.26%; t values: 4.10 and 6.03, both P<0.05).

TRT in combination with immunotherapy synergistically enhance anti-tumor efficacy, which is expected to play a role in the treatment of patients with advanced tumor where TRT can be applied.

Colonic neoplasms; Evans blue; Arginine-glycine-aspartic acid; Lutetium; Programmed cell death 1 receptor; Mice