Abstract
CCR8 chemokine receptor is a selective marker of tumor-infiltrating regulatory T cells (ti-Tregs) which interfere with the efficacy of checkpoint inhibitor immunotherapy (ICI) in many types of cancer. Eliminating CCR8+ ti-Tregs dramatically improves the results of subsequent ICI. We have recently reported using 225Actinium-labeled anti-CCR8 IgG for killing CCR8+ ti-Tregs in murine colorectal tumors which synergized with subsequent anti-CTLA4 ICI. Here, we aimed to compare the in vivo behavior of anti-CCR8 full-sized IgG and its Fab fragments to select the best antibody format for the pre-clinical development of this combination modality. Anti-CCR8 Fab fragments were generated by papain digest of the whole IgG. The whole IgG and Fab were conjugated to bifunctional chelating agent DOTA and labeled with 111Indium ((111)In). MC8 and CT6 murine colorectal tumor-bearing C57Bl6 and Balb/c mice, respectively, were administered (111)In-DOTA-IgG or (111)In-DOTA-Fab and imaged with microSPECT/CT at 2-72 h post-injection. A biodistribution was performed after the last imaging time point. Both (111)In-DOTA-IgG and (111)In-DOTA-Fab demonstrated high tumor uptake in both MC38 and CT26 tumors, with (111)In-DOTA-IgG uptake being significantly higher from the 24 h time point and onwards. (111)In-DOTA-Fab also exhibited pronounced kidney uptake which persisted even at 72 h. The kidney clearance and retention of (111)In-DOTA-Fab might represent a problem during therapy employing 225Actimium or other long-lived therapeutic radionuclides by potentially causing a dose-limiting kidney toxicity. This imaging/biodistribution evaluation not only determined that full-size anti-CCR8 IgG is the optimal antibody format for pre-clinical development but also informed on the timing of immunotherapy administration in future radioimmunotherapy and immunotherapy combination studies.