^99mTc-HYNIC-Gastrin Peptides: Assisted Coordination of ^99mTc by Amino Acid Side Chains Results in Improved Performance Both In Vitro and In Vivo

Abstract

The aim of this study was to determine the effects of assisted coordination 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 combinations of histidine, glutamic acid, and glycine were synthesized, radiolabeled with 99m Tc/ 99 Tc using either tricine or ethylenediaminediacetic acid as a coligand, and analyzed by the high-performance liquid chromatography and liquid chromatography-mass spectrometric techniques. Stability, receptor binding, and internalization and in vivo targeting in AR42J-bearing mice were assessed. Results: When radiolabeling was performed using tricine as a coligand, the insertion of a histidine residue near the HYNIC residue resulted in the displacement of one molecule of tricine from the coordination 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 significant 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. Conclusion: The insertion of histidine into the sequence of peptide-HYNIC conjugates can result in more stable, more homogeneous complexes that show improvements in tumor-targeting performance both in vitro and in vivo.

Technetium-binding in labelled HYNIC-peptide conjugates: Role of coordinating amino acids

“…This description contrasts with literature depictions of HYNIC as a non-chelating ligand [3,10,31,32], which are commonplace but unsubstantiated.…”

“…The patterns seen here accord with the mass spectrometric observation of single-tricine adduct in a HYNIC-salmon calcitonin conjugate (in which a histidine is located next to the HYNIC) [9] and in a series of new HYNIC-''nanogastrin" analogues. These include two with a glutamic acid residue [19,26,31], [and R. King, M.B. Surfraz, S.C.G.…”

“…Mol. Imaging 34 (2007) S180 (abstract)] and one with a histidine [31] located close to the HYNIC, all of which show only one tricine bound in the conjugate, and another with neither glutamic acid nor histidine, which shows two tricines bound [31]. These gastrin-related peptides illustrate the importance of understanding the possible participation of amino acid side chains in technetium coordination, because the glutamic acid residue is part of the minimal required consensus sequence and is believed to affect kidney retention in vivo [34].…”

“…These factors must be taken into account both when designing the peptide sequence and when devising the appropriate co-ligand to maximise stability and produce a single, non-isomeric, stable labelled species. In vivo evaluation of a ''nanogastrin" analogue designed according to these principles is described elsewhere [31]. …”

Diagnosis and therapy are walking together on radiopeptides’ avenue

“…Slight differences in kinetics and sensitivity have been observed changing both the chelate or the peptide's molecule, also including lanreotide [14]. New proposals, with a possible clinical interest, are also connected with 99m Tc-labelled radiocompounds, using gastrin peptide-hydrazinonicotinamide (HYNIC) as chelator [15].…”