Site-Specific Immuno-PET Tracer to Image PD-L1

Area of Science:

• Biomedical Imaging
• Molecular Imaging
• Immunotherapy Research

Background:

• Immune checkpoint blockade therapies necessitate noninvasive imaging of immune checkpoint proteins.
• Current immunoconjugate-based positron emission tomography (immuno-PET) methods face limitations due to heterogeneous, full-length antibodies.
• There is a need for robust, site-specific imaging tools for accurate PD-L1 evaluation.

Purpose of the Study:

• To develop a homogeneous, site-specific immuno-PET probe for imaging immune checkpoint proteins.
• To create a novel imaging agent targeting programmed death-ligand 1 (PD-L1) using unnatural amino acid technology.
• To demonstrate the utility of this approach for visualizing PD-L1 expression in vivo.

Main Methods:

• Engineered a site-specific antibody fragment (Fab) conjugate targeting PD-L1.
• Utilized amber suppression-mediated genetic incorporation of unnatural amino acids (UAAs) for site-specific conjugation of a chelator (NOTA).
• Radiolabeled the conjugate with 64Cu for positron emission tomography (PET) imaging and validated in mouse models.

Main Results:

• Developed a homogeneous, site-specific αPD-L1 Fab conjugate with tight antigen binding.
• Successfully visualized and mapped PD-L1 biodistribution in normal mice using 64Cu-NOTA-αPD-L1 PET imaging.
• Demonstrated differential PD-L1 expression in spleen tissues and confirmed specificity through in vivo blocking and ex vivo staining.

Conclusions:

• Site-specific Fab conjugation using UAAs provides a generalizable strategy for developing sensitive PET imaging probes.
• This approach overcomes limitations of traditional immuno-PET agents, enabling better visualization of immune checkpoint proteins.
• The developed imaging agent facilitates the study of PD-L1 roles in immunotherapy and other immune checkpoint proteins.