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  2. Targeting fibroblast activation protein (FAP): next generation PET radiotracers using squaramide coupled bifunctional DOTA and DATA5m chelators

Targeting fibroblast activation protein (FAP): next generation PET radiotracers using squaramide coupled bifunctional DOTA and DATA5m chelators

  • EJNMMI Radiopharm Chem. 2020 Jul 29;5(1):19. doi: 10.1186/s41181-020-00102-z.
Euy Sung Moon 1 Filipe Elvas 2 Gwendolyn Vliegen 3 Stef De Lombaerde 2 Christel Vangestel 2 Sven De Bruycker 4 An Bracke 3 Elisabeth Eppard 5 Lukas Greifenstein 1 Benedikt Klasen 1 Vasko Kramer 5 Steven Staelens 4 Ingrid De Meester 3 Pieter Van der Veken 3 Frank Rösch 6
Affiliations

Affiliations

  • 1 Department of Chemistry - TRIGA Site, Johannes Gutenberg University Mainz, 55128, Mainz, Germany.
  • 2 Department of Nuclear Medicine, Antwerp University Hospital (UZA), 2650, Edegem, Belgium.
  • 3 Department of Pharmaceutical Sciences, Laboratory of Medical Biochemistry, University of Antwerp, 2610, Wilrijk, Belgium.
  • 4 Molecular Imaging Center Antwerp (MICA), University of Antwerp, 2610, Wilrijk, Belgium.
  • 5 Positronpharma SA, 7500921 Providencia, Santiago, Chile.
  • 6 Department of Chemistry - TRIGA Site, Johannes Gutenberg University Mainz, 55128, Mainz, Germany. frank.roesch@uni-mainz.de.
Abstract

Background: Fibroblast activation protein (FAP) is a proline selective serine Protease that is overexpressed in tumor stroma and in lesions of many Other Diseases that are characterized by tissue remodeling. In 2014, a most potent FAP-inhibitor (referred to as UAMC1110) with low nanomolar FAP-affinity and high selectivity toward related Enzymes such as prolyl oligopeptidase (PREP) and the dipeptidyl-peptidases (DPPs): DPP4, DPP8/9 and DPP2 were developed. This inhibitor has been adopted recently by other groups to create radiopharmaceuticals by coupling bifunctional chelator-linker systems. Here, we report squaric acid (SA) containing bifunctional DATA5m and DOTA Chelators based on UAMC1110 as pharmacophor. The novel radiopharmaceuticals DOTA.SA.FAPi and DATA5m.SA.FAPi with their non-radioactive derivatives were characterized for in vitro inhibitory efficiency to FAP and PREP, respectively and radiochemical investigated with gallium-68. Further, first proof-of-concept in vivo animal study followed by ex vivo biodistribution were determined with [68Ga]Ga-DOTA.SA.FAPi.

Results: [68Ga]Ga-DOTA.SA.FAPi and [68Ga]Ga-DATA5m.SA.FAPi showed high complexation > 97% radiochemical yields after already 10 min and high stability over a period of 2 h. Affinity to FAP of DOTA.SA.FAPi and DATA5m.SA.FAPi and its natGa and natLu-labeled derivatives were excellent resulting in low nanomolar IC50 values of 0.7-1.4 nM. Additionally, all five compounds showed low affinity for the related Protease PREP (high IC50 with 1.7-8.7 μM). First proof-of-principle in vivo PET-imaging animal studies of the [68Ga]Ga-DOTA.SA.FAPi precursor in a HT-29 human colorectal Cancer xenograft mouse model indicated promising results with high accumulation in tumor (SUVmean of 0.75) and low background signal. Ex vivo biodistribution showed highest uptake in tumor (5.2%ID/g) at 60 min post injection with overall low uptake in healthy tissues.

Conclusion: In this work, novel PET radiotracers targeting fibroblast activation protein were synthesized and biochemically investigated. Critical substructures of the novel compounds are a squaramide linker unit derived from the basic motif of squaric acid, DOTA and DATA5m bifunctional Chelators and a FAP-targeting moiety. In conclusion, these new FAP-ligands appear promising, both for further research and development as well as for first human application.

Keywords

DATA5m; DOTA; FAP; Gallium-68; HT-29; PREP; Squaramide; Squaric acid.

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