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Published on: February 3, 2023
Development of Novel ImmunoPET Tracers to Image Human PD-1 Checkpoint Expression on Tumor-Infiltrating Lymphocytes in
Arutselvan Natarajan1, Aaron T Mayer1,2, Robert E Reeves1
1Department of Radiology, School of Medicine, Stanford University, Stanford, CA, USA.
Purpose:
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.
Procedures:
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.
Results:
[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.
Conclusions:
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.
Insights
New immune-PET tracers can visualize human PD-1 expressing tumor-infiltrating lymphocytes (TILs) in melanoma. This breakthrough allows for potential monitoring of cancer treatment response to immune checkpoint blockade therapy.
Area of Science:
- Oncology
- Immunology
- Radiochemistry
- Medical Imaging
Background:
- Cancers evade immune responses by exploiting immune checkpoints like PD-1 and PD-L1.
- Immune checkpoint (IC) blockade therapy shows promise but lacks universal patient response.
- Imaging of tumor-infiltrating lymphocytes (TILs), which express PD-1, may predict therapeutic response.
Purpose of the Study:
- To develop novel immune-tracers for positron emission tomography (PET) to visualize human PD-1 status.
- To assess the ability of these tracers to image adoptively transferred human peripheral blood mononuclear cells (hPBMCs) in a human melanoma xenograft model.
- To evaluate the potential of these tracers for predicting response to IC blockade therapy.
Main Methods:
- Human peripheral blood mononuclear cells (hPBMCs) were adoptively transferred into NSG mice bearing human melanoma (A375).
- An anti-human PD-1 antibody (Keytruda) was radiolabeled with either Zirconium-89 ([89Zr]) or Copper-64 ([64Cu]).
- PET-CT scans were performed at various time points post-injection to image PD-1-expressing hPBMCs in vivo.
Main Results:
- Both [89Zr] and [64Cu] labeled Keytruda successfully imaged human PD-1-expressing TILs in tumors and lymphoid organs.
- High tracer uptake was observed in tumors of mice receiving hPBMCs, significantly higher than control groups.
- Biodistribution studies revealed high tumor-to-muscle ratios, indicating specific tumor targeting.
Conclusions:
- The developed anti-human PD-1 immunoPET tracers specifically image human PD-1-expressing TILs in vivo.
- These tracers demonstrate potential for clinical translation in monitoring cancer treatment response to IC blockade.
- This imaging approach could aid in patient selection and therapeutic strategy optimization for cancer immunotherapy.
