Definition

An imagining agent composed of granzyme targeted restricted interaction peptide (RIP) specific to family member B (GRIP B) and radiolabeled with the radioisotope copper Cu 64, that can potentially be used to detect granzyme B (GZMB) activity upon positron emission tomography (PET). RIP consists of three domains, including a) Temporin L, an antimicrobial peptide (AMP) and membrane binding domain that is coupled to copper-64, b) a specific GZMB substrate and endoprotease cleavage binding domain, and c) a peptide masking domain that prevents the AMP from adopting its preferred helical conformation. Upon administration of copper Cu 64-GRIP B, the protease cleavage binding domain of RIP is specifically cleaved by extracellular GZMB, and upon cleavage, the AMP changes conformation and the radiolabeled activated AMP targets and irreversibly binds to adjacent phospholipid bilayers within any nearby membranes, thereby sequestering the radioisotope to nearby tumor cells. Upon PET, tissue biodistribution of the radioisotope can be assessed and the accumulated radioisotope in tissues reflects the proteolytic activity of secreted GZMB. GZMB secretion and activity is correlated with immune cell activation and can be enhanced by certain immune checkpoint inhibitors.