Scheme 1. Strategies for peptide-based radiotracer development. (A) Previous studies focused on using radiotracer-integrated peptide ligands to track protein targets. (B) Conceptual diagram of interface peptide-based tracer design. The core component of the interface peptide-based tracer design is represented by the key sequence in protein B at the binding interface of a PPI, which is a potential ligand (inhibitor) for protein A. When truncated from the parental protein, the peptide sequence from protein B can be developed as radiotracers for tracking protein A. (C) Representative co-crystal structure of PD-1 and PD-L1. Two interface peptides (mpep-1 and hpep-1) were derived from mPD-L1 and hPD-L1, respectively. The black triangles indicate the “hot spots” that mediate the interactions between PD-1 and PD-L1. (D) The study plan of this work. Two kinds of tumors were used for PET study. Two tracers, mPep-1-[⁶⁴Cu] and hPep-1-[⁶⁴Cu], were evaluated for their capacity in imaging of PD-1.