Abstract
The immune checkpoint programmed death 1 receptor (PD-1) expressed on some tumor-infiltrating lymphocytes, and its ligand (PD-L1) expressed on tumor cells, enable cancers to evade the immune system. Blocking PD-1 with the monoclonal antibody pembrolizumab is a promising immunotherapy strategy. Thus, noninvasively quantifying the presence of PD-1 expression in the tumor microenvironment prior to initiation of immune checkpoint blockade may identify the patients likely to respond to therapy. We have developed a 64Cu-pembrolizumab radiotracer and evaluated human dosimetry. The tracer was utilized to image hPD-1 levels in two subcutaneous mouse models: (a) 293 T/hPD-1 cells xenografted into NOD-scid IL-2Rγnull mice (NSG/293 T/hPD-1) and (b) human peripheral blood mononuclear cells engrafted into NSG bearing A375 human melanoma tumors (hNSG/A375). In each mouse model two cohorts were evaluated (hPD-1 blockade with pembrolizumab [blk] and non-blocked [nblk]), for a total of four groups (n = 3-5/group). The xenograft-to-muscle ratio in the NSG/293 T/hPD-1 model at 24 h was significantly increased in the nblk group (7.0 ± 0.5) compared to the blk group (3.4 ± 0.9), p = 0.01. The radiotracer dosimetry evaluation (PET/CT ROI-based and ex vivo) in the hNSG/A375 model revealed the highest radiation burden to the liver. In summary, we validated the 64Cu-pembrolizumab tracer's specific hPD-1 receptor targeting and predicted human dosimetry.