Structural studies on radiopharmaceutical DOTA-minigastrin analogue (CP04) complexes and their interaction with CCK2 receptor

Abstract

BackgroundThe cholecystokinin receptor subtype 2 (CCK-2R) is an important target for diagnostic imaging and targeted radionuclide therapy (TRNT) due to its overexpression in certain cancers (e.g., medullary thyroid carcinoma (MTC)), thus matching with a theranostic principle. Several peptide conjugates suitable for the TRNT of MTC have been synthesized, including a very promising minigastrin analogue DOTA-(DGlu)6-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2 (CP04). In this contribution, we wanted to see whether CP04 binding affinity for CCK-2R is sensitive to the type of the complexed radiometal, as well as to get insights into the structure of CP04-CCK2R complex by molecular modeling.ResultsIn vitro studies demonstrated that there is no significant difference in CCK-2R binding affinity and specific cellular uptake between the CP04 conjugates complexed with [68Ga]Ga3+ or [177Lu]Lu3+. In order to investigate the background of this observation, we proposed a binding model of CP04 with CCK-2R based on homology modeling and molecular docking. In this model, the C-terminal part of the molecule enters the cavity formed between the receptor helices, while the N-terminus (including DOTA and the metal) is located at the binding site outlet, exposed in large extent to the solvent. The radiometals do not influence the conformation of the molecule except for the direct neighborhood of the chelating moiety.ConclusionsThe model seems to be in agreement with much of structure-activity relationship (SAR) studies reported for cholecystokinin and for CCK-2R-targeting radiopharmaceuticals. It also explains relative insensitivity of CCK-2R affinity for the change of the metal. The proposed model partially fits the reported site-directed mutagenesis data.

Triazolo-Peptidomimetics: Novel Radiolabeled Minigastrin Analogs for Improved Tumor Targeting

“…The assignment was carried out with routine correlation spectroscopy (COSY), rotating frame nuclear Overhauser effect spectroscopy (ROESY), heteronuclear single quantum correlation (HSQC), and heteronuclear multiple bond correlation (HMBC) spectra, and the rotating-frame Overhauser effect (ROE) intensities were converted into distance restraints. Structure calculations using these restraints were performed with the XPLOR-NIH software package , and confirmed random coil conformations in all cases. , …”

“…Despite great recent advances in the structural determination of GPCRs, the three-dimensional structure of the CCK2R and the precise binding mode of cholecystokinin, gastrin, and related peptides to this receptor are not yet known. However, several mutagenesis and modeling studies have identified essential regions and residues for ligand binding. ,,− To get a better understanding of the properties of TZMG 6 , we built a computational model of this ligand bound to the CCK2R (Figure A and SI, p S90). The C-terminal part of the peptide (Trp 14 -Nle 15 -Asp 16 -Phe 17 -NH 2 ) forms a short turn buried in the orthosteric binding site (Figure B), where it can interact with, e.g., residues Arg 57 , His 376 , His 207 , and Arg 356 .…”

“…Structure calculations using these restraints were performed with the XPLOR-NIH software package 45,46 and confirmed random coil conformations in all cases. 39,47 Despite great recent advances in the structural determination of GPCRs, 48 the three-dimensional structure of the CCK2R and the precise binding mode of cholecystokinin, gastrin, and related peptides to this receptor are not yet known. However, several mutagenesis and modeling studies have identified essential regions and residues for ligand binding.…”

DOTA-MGS5, a New Cholecystokinin-2 Receptor-Targeting Peptide Analog with an Optimized Targeting Profile for Theranostic Use

“…This improved stability against enzymatic degradation in vivo was not an unexpected finding. Methylation of the peptide bond between Trp and Nle and introduction of voluminous amino acids at the C terminus were already investigated for CCK4 analogs, achieving compounds that are protected from degradation in rat brain homogenates (23). Higher bioavailability is likely to increment the amount of intact radiopeptide reaching the target site.…”

A Decade’s Progress in the Development of Molecular Imaging Agents Targeting the Growth Hormone Secretagogue Receptor

“…Most often, the imaging portion of a radiopharmaceutical is positioned far from the binding site so as not to have a negative affect on the binding affinity. [54][55][56][57][58] This is particularly common for radiotracers bearing metal chelators due to the relatively large size of the appended metal-complex. However, the authors chose to apply a more compact design that incorporates the imaging moiety in a position crucial for binding.…”