Combined VLA-4-Targeted Radionuclide Therapy and Immunotherapy in a Mouse Model of Melanoma

Abstract

Very late antigen-4 (VLA-4; also known as integrin α₄β₁) 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 of ¹⁷⁷Lu-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 + anti-CTLA-4 or anti-PD-L1 + anti-CTLA-4, whereas TRT + ICIs significantly enhanced survival. TUNEL staining showed that the highest levels of apoptosis were in the TRT + 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 + ICIs targets both tumor cells and immune cells and has potential as a therapeutic agent in patients with metastatic melanoma.

Keywords: PET imaging; VLA-4; immune checkpoint inhibitors; melanoma; targeted radionuclide therapy.