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Gallium Complexation, Stability, and Bioconjugation of 1,4,7-Triazacyclononane Derived Chelators with Azaheterocyclic Arms


Schmidtke, Alexander; Läppchen, Tilman; Weinmann, Christian; Bier-Schorr, Lorenz; Keller, Manfred; Kiefer, Yvonne; Holland, Jason P; Bartholomä, Mark D (2017). Gallium Complexation, Stability, and Bioconjugation of 1,4,7-Triazacyclononane Derived Chelators with Azaheterocyclic Arms. Inorganic Chemistry, 56(15):9097-9110.

Abstract

We have recently introduced a 1,4,7-triazacyclononane (TACN) based chelating system with additional five-membered azaheterocyclic substituents for complexation of radioactive Cu2+ ions. In this work, we investigated the complexation properties of these novel chelators with Ga3+. In labeling experiments, we could show that the penta- and hexadentate imidazole derivatives NODIA-Me 4 and NOTI-Me 1 can be labeled with 68Ga in specific activities up to ∼30 MBq nmol–1, while the corresponding thiazole derivative NOTThia 2 did not label satisfactorily under identical conditions. NMR studies on the Ga complexes of 1 and the model compound NODIA-Me-NH-Me 5 revealed formation of rigid 1:1 chelates with a slow macrocyclic interconversion and inert Ga–N bonds to the methylimidazole residues on the NMR time scale. The TACN-derived bifunctional chelator NODIA-Me was furthermore conjugated to a prostate-specific membrane antigen (PSMA) targeting moiety to give the corresponding bioconjugate NODIA-Me-PSMA 7. Serum stability and ligand challenge experiments of 68Ga-7 confirmed formation of a stable complex for up to 4 h. The remaining coordination site of five-coordinate Ga complexes was found to be occupied by monodentate ligands including hydroxide and chloride anions depending on the conditions. According to density functional theory calculations, coordination of monodentate ligands as well as of the amide group for the bioconjugated ligand are energetically plausible. Finally, the labeled bioconjugate 68Ga-7 exhibited rapid renal clearance in biodistribution studies performed by small animal PET imaging with no indication of transchelation/demetalation in vivo. Altogether, our results provide strong evidence for a stable Ga complexation of our novel TACN-based chelators bearing imidazole arms. Despite the formation of two complexes incorporating different monodentate ligands in vitro, the imidazole type ligands show promise as chelating agents for the future development of gallium based radiopharmaceuticals.

Abstract

We have recently introduced a 1,4,7-triazacyclononane (TACN) based chelating system with additional five-membered azaheterocyclic substituents for complexation of radioactive Cu2+ ions. In this work, we investigated the complexation properties of these novel chelators with Ga3+. In labeling experiments, we could show that the penta- and hexadentate imidazole derivatives NODIA-Me 4 and NOTI-Me 1 can be labeled with 68Ga in specific activities up to ∼30 MBq nmol–1, while the corresponding thiazole derivative NOTThia 2 did not label satisfactorily under identical conditions. NMR studies on the Ga complexes of 1 and the model compound NODIA-Me-NH-Me 5 revealed formation of rigid 1:1 chelates with a slow macrocyclic interconversion and inert Ga–N bonds to the methylimidazole residues on the NMR time scale. The TACN-derived bifunctional chelator NODIA-Me was furthermore conjugated to a prostate-specific membrane antigen (PSMA) targeting moiety to give the corresponding bioconjugate NODIA-Me-PSMA 7. Serum stability and ligand challenge experiments of 68Ga-7 confirmed formation of a stable complex for up to 4 h. The remaining coordination site of five-coordinate Ga complexes was found to be occupied by monodentate ligands including hydroxide and chloride anions depending on the conditions. According to density functional theory calculations, coordination of monodentate ligands as well as of the amide group for the bioconjugated ligand are energetically plausible. Finally, the labeled bioconjugate 68Ga-7 exhibited rapid renal clearance in biodistribution studies performed by small animal PET imaging with no indication of transchelation/demetalation in vivo. Altogether, our results provide strong evidence for a stable Ga complexation of our novel TACN-based chelators bearing imidazole arms. Despite the formation of two complexes incorporating different monodentate ligands in vitro, the imidazole type ligands show promise as chelating agents for the future development of gallium based radiopharmaceuticals.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:2017
Deposited On:08 Feb 2018 11:31
Last Modified:18 Apr 2018 11:49
Publisher:American Chemical Society (ACS)
ISSN:0020-1669
Funders:SNSF Professorship PP00P2_163683, ERC-StG-2015, NanoSCAN − 676904
OA Status:Closed
Publisher DOI:https://doi.org/10.1021/acs.inorgchem.7b01129
Project Information:
  • : FunderSNSF
  • : Grant ID
  • : Project TitleSNSF Professorship PP00P2_163683
  • : Funder
  • : Grant ID
  • : Project TitleERC-StG-2015, NanoSCAN − 676904

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