The initial studies focused on evaluating the spacer chemistry and different chelators for labelling of the nonpeptidic compounds with 99mTc, proving the feasibility of specific tumor targeting in athymic nude mice bearing xenografts based on stably transfected HEK293 cells expressing human CCK2R or the CCK2i4svR splice variant [49]. This evidence concerns the gene CCKBR and neoplasm.