The probes QE and bivQ showed high affinity and specificity to A431/CCK2R cells. (a) Determination of affinity revealed dissociation constants of 5.8 ± 0.9 nM for QE (left) and 5.8 ± 1.6 nM for bivQ (right). (b) Specificity of CCK2R binding was evaluated after blocking with increasing amounts of non-fluorescent CCK2R ligand. A competitor concentration of 16.7 ± 0.2 nM (QE, left) and 16.6 ± 0.2 nM (bivQ, right) led to a reduction of the initial binding by 50%. Displayed are means and standard errors of three individual experiments with two parallels each.