Combined VLA-4–targeted radionuclide therapy and immunotherapy in a mouse model of melanoma

Very late antigen-4 (VLA-4; also known as integrin a4ß1) is expressed at high levels in aggressive and metastatic melanoma tumors and may provide an ideal target for imaging and targeted radionuclide therapy (TRT).177Lu-DOTA-PEG4-LLP2A (177Lu-LLP2A) is a TRT that shows high affinity for VLA-4 and high uptake in B16F10 mouse melanoma tumors in vivo. Here, we report efficacy studies of177Lu-LLP2A, alone and combined with immune checkpoint inhibitors (ICIs) (anti-PD-1, anti-PD-L1, and anti-CTLA-4 antibodies), in B16F10 tumor–bearing mice. Methods: Tumor cells (1 · 106) were implanted subcutaneously in C57BL/6 mice. After 8–10 d, the mice were randomized into 8 groups.177Lu-LLP2A was injected intravenously on day 8 or 9 (single dose), and ICI antibodies were administered intraperitoneally in 3 doses. Tumor growth was monitored over time via calipers. Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining for apoptosis was performed on fixed tumors. In a separate study, Cy3-LLP2A or Cy3–scrambled LLP2A was injected in tumor-bearing mice, and tumors were collected 4 h after injection and then analyzed by flow cytometry and immunofluorescence microscopy using different immune cell markers. Results: TRT alone showed efficacy comparable to the dual-ICI anti-PD-1 1 anti-CTLA-4 or anti-PD-L1 1 anti-CTLA-4, whereas TRT 1 ICIs significantly enhanced survival. TUNEL staining showed that the highest levels of apoptosis were in the TRT 1 ICI groups. In addition to targeting tumor cells, TRT also bound immune cells in the tumor microenvironment. Flow cytometry data showed that the tumors consisted of about 77% tumor cells and fibroblasts (CD45-negative/ CD49d-positive) and about 23% immune cells (CD45-positive/ CD49d-positive) and that immune cells expressed higher levels of VLA-4. Cy3-LLP2A and CD49d colocalized with macrophages (CD68), T cells (CD8, CD4), and B cells (CD19). Immunohistochemical analysis identified a significant colocalization of Cy3-LLP2A and CD68. Conclusion: Combination treatment with TRT 1 ICIs targets both tumor cells and immune cells and has potential as a therapeutic agent in patients with metastatic melanoma.