Figure 1.

Schematic depiction of the functional principle of radiolabeled NPY(Y₁)R- and gastrin-releasing peptide receptor (GRPR)-binding heterobivalent peptidic ligands (HBPLs) for tumor imaging: Monomeric peptide radiotracers can only bind to one receptor type and thus miss tumor tissues or lesions that do not express the respective receptor due to tumor heterogeneity or disease progression whereas the use of radiolabeled HBPLs, which can bind to more than one target receptor type, results in a higher probability of target visualization.