Dosimetric analysis of Actinium-225 labeled anti-PD-L1 alpha particle radioimmunotherapy of melanoma cancer in an immunocompetent mouse model

Objectives In the immune checkpoint system the Programmed cell Death Ligand 1 (PD-L1) is co-opted by tumor cells to suppress immune recognition of cancer. PD-L1 is expressed on tumor cells, tumor associated macrophages and other cells in the tumor microenvironment, which can inhibit the T-cell effector function. We have recently shown that anti-PD-L1 antibodies can be modified for radionuclide imaging in an immunocompetent pre-clinical model. The aim of this study was to investigate combined anti-PD-L1 immunotherapy and radionuclide therapy using the alpha particle emitting radionuclide Actinium-225 (225Ac) for melanoma cancer in a immunocompetent mouse model and to perform dosimetric analysis of tumor and limiting normal tissues.

Methods Immunocompetent C57BL/6 mice were injected with one million B16F10 melanoma tumor cells in the right flank 10 days prior to treatment. The mice were injected in the tail vein with 18.9 kBq 225Ac labeled anti-PD-L1 antibodies (225Ac-anti-PD-L1) and euthanized 1, 24 and 72 hours post injection (p.i.). Tumors and normal tissues were removed and measured in a gamma well counter for the dosimetric analysis. In addition, tumor and normal tissues were frozen, cryo-sectioned and imaged using the α-Camera a quantitative digital autoradiography image system to determine the distribution and activity concentration of 225Ac-anti-PD-L1 on sub organ/tumor levels.

Results The calculated mean absorbed radiation dose (from alpha-particle emissions) to the tumor was 3.8 Gy from 225Ac-anti-PD-L1. For the kidneys, which probably is the limiting normal tissue the calculated mean absorbed radiation dose (from alpha-particle emissions) was 2.0 Gy, with 1.3 Gy originating from 225Ac-anti-PD-L1 and 0.65 Gy from the unbound daughter radionuclide Bismuth-213 (213Bi). The α-Camera images showed 1 hour p.i. that the majority of 225Ac-anti-PD-L1 was located within the pelvis region of the kidneys, with a small uptake in medially situated proximal tubules. In the tumor a few hot spots were detected 1 hour p.i. with activity concentrations up to 2.5 times higher than compared with the average of the whole tumor. Hematological toxicity was not observed and the maximum tolerable dose (MTD) for tumor-free mice was greater than 18.9 kBq.

Conclusions A relative high mean absorbed radiation dose was calculated to the tumor from 225Ac-anti-PD-L1 and in combination with the anti-PD-L1 immunotherapy could be an effective combination treatment. Concerning normal tissues a low mean absorbed radiation dose was calculated to the kidneys, but a heterogeneous activity distribution in combination with the alpha particles high relative biological effectiveness (RBE) could lead to later toxicity at the high administered activity used in this study.