title: Development of Novel ImmunoPET Tracers to Image Human PD-1 Checkpoint Expression on Tumor-Infiltrating Lymphocytes in a Humanized Mouse Model.
abstract: It is well known that cancers exploit immune checkpoints (programmed death 1 receptor (PD-1) and its ligand (PD-L1)) to evade anti-tumor immune responses. Although immune checkpoint (IC) blockade is a promising approach, not all patients respond. Hence, imaging of tumor-infiltrating lymphocytes (TILs) is of high specific interest, as they are known to express PD-1 during activation and subsequent exhaustion in the tumor microenvironment and are thought to be potentially predictive of therapeutic responses to IC blockade.
  
  
  
  We developed immune-tracers for positron emission tomography (PET) to image hPD-1 status of human peripheral blood mononuclear cells (hPBMCs) adoptively transferred to NOD-scid IL-2Rγnull(NSG) mice (hNSG) bearing A375 human skin melanoma tumors. The anti-PD-1 human antibody (IgG; keytruda) was labeled with either Zr-89 or Cu-64 radiometals to image PD-1-expressing human TILs in vivo.
  
  
  
  [89Zr] Keytruda (groups = 2; NSG-ctl (control) and hNSG-nblk (non-blocking), n = 3-5, 3.2 ± 0.4 MBq/15-16 μg/200 μl) and [64Cu] Keytruda (groups = 3; NSG-ctl, NSG-blk (blocking), and hNSG-nblk; n = 4, 7.4 ± 0.4 MBq /20-25 μg/200 μl) were administered in mice. PET-CT scans were performed over 1-144 h ([89Zr] Keytruda) and 1-48 h ([64Cu] Keytruda) on mice. hNSG mice exhibited a high tracer uptake in the spleen, lymphoid organs and tumors. At 24 h, human TILs homing into melanoma of hNSG-nblk mice exhibited high signal (mean %ID/g ± SD) of 3.8 ± 0.4 ([89Zr] Keytruda), and 6.4 ± 0.7 ([64Cu] Keytruda), which was 1.5- and 3-fold higher uptake compared to NSG-ctl mice (p = 0.01), respectively. Biodistribution measurements of hNSG-nblk mice performed at 144 h ([89Zr] Keytruda) and 48 h ([64Cu] Keytruda) p.i. revealed tumor to muscle ratios as high as 45- and 12-fold, respectively.
  
  
  
  Our immunoPET study clearly demonstrates specific imaging of human PD-1-expressing TILs within the tumor and lymphoid tissues. This suggests these anti-human-PD-1 tracers could be clinically translatable to monitor cancer treatment response to IC blockade therapy.
authors.affiliation: Department of Radiology, School of Medicine, Stanford University, Stanford, CA, USA.
authors.forename: Arutselvan
authors.initials: A
authors.lastname: Natarajan
authors.affiliation: Department of Radiology, School of Medicine, Stanford University, Stanford, CA, USA.|Department of Bioengineering, Stanford University, Stanford, CA, USA.
authors.forename: Aaron T
authors.initials: AT
authors.lastname: Mayer
authors.affiliation: Department of Radiology, School of Medicine, Stanford University, Stanford, CA, USA.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Reeves
authors.affiliation: Department of Comparative Medicine, Stanford University, Stanford, CA, USA.
authors.forename: Claude M
authors.initials: CM
authors.lastname: Nagamine
authors.affiliation: Department of Radiology, School of Medicine, Stanford University, Stanford, CA, USA. sgambhir@stanford.edu.|Department of Bioengineering, Stanford University, Stanford, CA, USA. sgambhir@stanford.edu.|Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. sgambhir@stanford.edu.|Molecular Imaging Program at Stanford, Department of Radiology, Stanford University, James H. Clark Center, 318 Campus Drive, E153, Stanford, CA, 94305, USA. sgambhir@stanford.edu.
authors.forename: Sanjiv Sam
authors.initials: SS
authors.lastname: Gambhir
mesh_terms: D000818:Animals; D061067:Antibodies, Monoclonal, Humanized / Q000737:chemistry*; D060890:B7-H1 Antigen / Q000378:metabolism*; D045744:Cell Line, Tumor; D002851:Chromatography, High Pressure Liquid; D004195:Disease Models, Animal; D006801:Humans; D016246:Lymphocytes, Tumor-Infiltrating / Q000378:metabolism* / Q000473:pathology*; D051379:Mice; D000072078:Positron Emission Tomography Computed Tomography; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals; D014018:Tissue Distribution
chemical_list: D061067:Antibodies, Monoclonal, Humanized; D060890:B7-H1 Antigen; C423236:CD274 protein, human; D019275:Radiopharmaceuticals; C582435:pembrolizumab
keywords: 64-Cu; 89-Zr; ImmunoPET; Keytruda; PD-1; Tumor-infiltrating lymphocytes