Chains Results in Improved Performance Both In Vitro and In Vivo

The aim of this study was to determine the effects of assisted coor- dination by amino acids such as histidine and glutamic acid on the function of 99m Tc-labeled gastrin peptide-hydrazinonicotinamide (HYNIC) conjugates and their ability to target cholecystokinin-R in small-animal models. Methods: Three peptide-HYNIC conjugates containing the -AYGWMDF-NH2 C-terminal sequence and com- binations of histidine, glutamic acid, and glycine were synthesized, radiolabeled with 99mTc/99Tc using either tricine or ethylenediamine- diacetic acid as a coligand, and analyzed by the high-performance liquid chromatography and liquid chromatography-mass spectro- metric techniques. Stability, receptor binding, and internalization and in vivo targeting in AR42J-bearing mice were assessed. Re- sults: When radiolabeling was performed using tricine as a coli- gand, the insertion of a histidine residue near the HYNIC residue resulted in the displacement of one molecule of tricine from the co- ordination sphere, a reduction in the number of radiolabeled species formed, an improvement in the in vitro stability, an increase in the rate of radiopeptide internalization, and a significant improvement in tumor uptake in vivo. When radiolabeling was performed using ethylenediaminediacetic acid as a coligand, no effect on coligand binding, homogeneity, or in vitro stability was observed but a signif- icant improvement in the internalization in vitro and tumor uptake in vivo was again found. All of the complexes formed showed similar receptor affinity in competitive radioligand binding assays. Conclu- sion: The insertion of histidine into the sequence of peptide-HYNIC conjugates can result in more stable, more homogeneous com- plexes that show improvements in tumor-targeting performance both in vitro and in vivo.