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Development of a 68Gallium-Labeled D-Peptide PET Tracer for Imaging Programmed Death-Ligand 1 Expression
Published on: February 3, 2023
99mTc-Labeled Cyclic Peptide Targeting PD-L1 as a Novel Nuclear Imaging Probe
Guillermina Ferro-Flores1, Blanca Ocampo-García1, Pedro Cruz-Nova1
1Department of Radioactive Materials, Instituto Nacional de Investigaciones Nucleares, Ocoyoacac 52750, Mexico.
Abstract:
Recent cancer therapies have focused on reducing immune suppression in the tumor microenvironment to prevent cancer progression and metastasis. PD-1 is a checkpoint protein that stops the immune response and is expressed on immune T cells. Cancer cells express a PD-1 ligand (PD-L1) to bind to the T-cell surface and activate immunosuppressive pathways. This study aimed to design, synthesize, and evaluate a 99mTc-labeled PD-L1-targeting cyclic peptide inhibitor (99mTc-iPD-L1) as a novel SPECT radiopharmaceutical for PD-L1 expression imaging. AutoDock software (version 1.5) was used to perform molecular docking for affinity calculations. The chemical synthesis was based on the coupling reaction of 6-hydrazinylpyridine-3-carboxylic acid with a 14-amino-acid cyclic peptide. iPD-L1 was prepared for 99mTc labeling. Radio-HPLC was used to verify radiochemical purity. The stability of the radiopeptide in human serum was evaluated by HPLC. iPD-L1 specificity was assessed by SDS-PAGE. [99mTc]Tc-iPD-L1 cellular uptake in PD-L1-positive cancer cells (HCC827 and HCT116) and biodistribution in mice with induced tumors were also performed. One patient with advanced plantar malignant melanoma received [99mTc]Tc-iPD-L1. The iPD-L1 ligand (AutoDock affinity: -6.7 kcal/mol), characterized by UPLC mass, FT-IR, and UV-Vis spectroscopy, was obtained with a chemical purity of 97%. The [99mTc]Tc-iPD-L1 was prepared with a radiochemical purity of >90%. In vitro and in vivo analyses demonstrated [99mTc]Tc-iPD-L1 stability (>90% at 24 h) in human serum, specific recognition for PD-L1, high uptake by the tumor (6.98 ± 0.89% ID/g at 1 h), and rapid hepatobiliary and kidney elimination. [99mTc]Tc-iPD-L1 successfully detected PD-L1-positive lesions in a patient with plantar malignant melanoma. The results obtained in this study warrant further dosimetric and clinical studies to determine the sensitivity and specificity of [99mTc]Tc-iPD-L1/SPECT for PD-L1 expression imaging.
Insights
A novel technetium-99m-labeled peptide inhibitor targeting programmed death-ligand 1 (PD-L1) was developed for SPECT imaging. This radiopharmaceutical successfully detected PD-L1-positive tumors in preclinical models and a patient, showing potential for cancer diagnostics.
Area of Science:
- Nuclear Medicine
- Oncology
- Radiopharmaceutical Chemistry
Background:
- Cancer therapies increasingly target tumor microenvironment immunosuppression, particularly the programmed death-1 (PD-1) pathway.
- Cancer cells evade immune surveillance by expressing PD-1 ligand (PD-L1), which inhibits immune T cells.
- Accurate imaging of PD-L1 expression is crucial for guiding targeted immunotherapies.
Purpose of the Study:
- To design, synthesize, and evaluate a novel 99mTc-labeled cyclic peptide inhibitor targeting PD-L1 (99mTc-iPD-L1).
- To assess 99mTc-iPD-L1 as a SPECT radiopharmaceutical for imaging PD-L1 expression in cancer.
- To investigate the preclinical and initial clinical utility of 99mTc-iPD-L1.
Main Methods:
- Molecular docking (AutoDock) was used for affinity calculations of the iPD-L1 ligand.
- Chemical synthesis involved coupling a cyclic peptide with a hydrazinylpyridine derivative, followed by 99mTc labeling.
- Radiochemical purity (Radio-HPLC), stability (HPLC), specificity (SDS-PAGE), cellular uptake, biodistribution in tumor-bearing mice, and a human SPECT imaging study were performed.
Main Results:
- The iPD-L1 ligand exhibited high binding affinity (-6.7 kcal/mol) and was synthesized with 97% purity.
- 99mTc-iPD-L1 was prepared with >90% radiochemical purity and demonstrated excellent stability (>90% at 24 h) in human serum.
- In vitro and in vivo studies showed specific PD-L1 recognition, high tumor uptake (6.98 ± 0.89% ID/g at 1 h), and rapid clearance.
- SPECT imaging successfully visualized PD-L1-positive lesions in a patient with malignant melanoma.
Conclusions:
- 99mTc-iPD-L1 is a promising SPECT radiopharmaceutical for imaging PD-L1 expression.
- The developed agent demonstrated favorable preclinical and initial clinical performance in detecting PD-L1-positive tumors.
- Further dosimetric and clinical studies are warranted to establish the sensitivity and specificity of 99mTc-iPD-L1/SPECT imaging.
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