New and improved compounds for use in diagnostic imaging or therapy having the formula M-N—O—P-G, wherein M is a metal chelator having the structure: wherein R1-R5 and FG are as defined herein (in the form complexed with a metal radionuclide or not), N—O—P is the linker containing at least one non-alpha amino acid with a cyclic group, at least one substituted bile acid or at least one non-alpha amino acid, and G is the GRP receptor targeting peptide. In the preferred embodiment, M is an Aazta metal chelator or a derivative thereof. Methods for imaging a patient and/or providing radiotherapy or phototherapy to a patient using the compounds of the invention are also provided. Methods and kits for preparing a diagnostic imaging agent from the compound is further provided. Methods and kits for preparing a radiotherapeutic agent are further provided.