title: Scavenging strategy for specific activity improvement: application to a new CXCR4-specific cyclopentapeptide positron emission tomography tracer.
abstract: Huisgen cycloaddition is attractive to label peptide because of its rapidity and bioorthogonality. However, for larger tracers, the physico-chemical differences between the precursor and the tracer are usually insufficient to allow their separation by HPLC, reducing the specific activity. This is of importance for peptidic tracers because the combination of their high-affinity receptor with low specific activity results in the precursor saturating the receptors, causing non-specific tracer binding. Here, we report a fast, one-pot, general strategy to circumvent this issue, yielding a tracer of improved specific activity. It consists in adding a lipophilic azide after the labeling step to scavenge unreacted precursor into a more lipophilic species that does not co-elute with the tracer. We applied this strategy to a new fluorinated cyclopentapeptidic CXCR4 antagonist for the PET imaging of cancer, CCIC15, for which we managed to reduce the apparent peptide concentration by a factor of 34 in 10 min. This tracer was radiolabeled by click chemistry with 2-[(18) F]fluoroethylazide, yielding the tracer in 18 ± 6% (n = 5) end-of-synthesis radiochemical yields (EOS-RCY) in 1.5 h from [(18) F]fluoride with a specific activity of 19.4 GBq µmol(-1) . Preliminary biological evaluation of the probe confirmed potency and specificity for CXCR4; further biological evaluation is underway.
authors.affiliation: Comprehensive Cancer Imaging Centre, Faculty of Medicine, Hammersmith Hospital Campus, Imperial College London, Du Cane Road, London, W12 0NN, United Kingdom; Department of Chemistry, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom.
authors.forename: Guillaume P C
authors.initials: GP
authors.lastname: George
authors.forename: Federica
authors.initials: F
authors.lastname: Pisaneschi
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Stevens
authors.forename: Quang-Dé
authors.initials: QD
authors.lastname: Nguyen
authors.forename: Ola
authors.initials: O
authors.lastname: Åberg
authors.forename: Alan C
authors.initials: AC
authors.lastname: Spivey
authors.forename: Eric O
authors.initials: EO
authors.lastname: Aboagye
mesh_terms: D001386:Azides / Q000737:chemistry; D045744:Cell Line, Tumor; D006801:Humans; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry* / Q000494:pharmacology; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D019718:Receptors, CXCR4 / Q000378:metabolism*
chemical_list: C522266:2-fluoroethylazide; D001386:Azides; D010456:Peptides, Cyclic; D019275:Radiopharmaceuticals; D019718:Receptors, CXCR4; C000598336:cyclo(glycyl-tyrosyl-3-(1-(2-fluoroethyl)-1H-1,2,3-triazol-4-yl)propanoic acid-arginyl-2-naphthyalanyl); C496951:cyclo(tyrosyl-arginyl-arginyl-3-(2-naphthyl)alanyl-glycyl)
keywords: CXCR4; FC131; cancer; scavenging; specific activity