title: Tetraamine-derived bifunctional chelators for technetium-99m labelling: synthesis, bioconjugation and evaluation as targeted SPECT imaging probes for GRP-receptor-positive tumours.
abstract: Owing to its optimal nuclear properties, ready availability, low cost and favourable dosimetry, (99m)Tc continues to be the ideal radioisotope for medical-imaging applications. Bifunctional chelators based on a tetraamine framework exhibit facile complexation with Tc(V)O(2) to form monocationic species with high in vivo stability and significant hydrophilicity, which leads to favourable pharmacokinetics. The synthesis of a series of 1,4,8,11-tetraazaundecane derivatives (01-06) containing different functional groups at the 6-position for the conjugation of biomolecules and subsequent labelling with (99m)Tc is described herein. The chelator 01 was used as a starting material for the facile synthesis of chelators functionalised with OH (02), N(3) (04) and O-succinyl ester (05) groups. A straightforward and easy synthesis of carboxyl-functionalised tetraamine-based chelator 06 was achieved by using inexpensive and commercially available starting materials. Conjugation of 06 to a potent bombesin-antagonist peptide and subsequent labelling with (99m)Tc afforded the radiotracer (99m)Tc-N4-BB-ANT, with radiolabelling yields of >97% at a specific activity of 37 GBq micromol(-1). An IC(50) value of (3.7+/-1.3) nM was obtained, which confirmed the high affinity of the conjugate to the gastrin-releasing-peptide receptor (GRPr). Immunofluorescence and calcium mobilisation assays confirmed the strong antagonist properties of the conjugate. In vivo pharmacokinetic studies of (99m)Tc-N4-BB-ANT showed high and specific uptake in PC3 xenografts and in other GRPr-positive organs. The tumour uptake was (22.5+/-2.6)% injected activity per gram (% IA g(-1)) at 1 h post injection (p.i.). and increased to (29.9+/-4.0)% IA g(-1) at 4 h p.i. The SPECT/computed tomography (CT) images showed high tumour uptake, clear background and negligible radioactivity in the abdomen. The promising preclinical results of (99m)Tc-N4-BB-ANT warrant its potential candidature for clinical translation.
authors.affiliation: Division of Radiological Chemistry, University Hospital of Basel, 4031 Basel, Switzerland.
authors.forename: Keelara
authors.initials: K
authors.lastname: Abiraj
authors.forename: Rosalba
authors.initials: R
authors.lastname: Mansi
authors.forename: Maria-Luisa
authors.initials: ML
authors.lastname: Tamma
authors.forename: Flavio
authors.initials: F
authors.lastname: Forrer
authors.forename: Renzo
authors.initials: R
authors.lastname: Cescato
authors.forename: Jean Claude
authors.initials: JC
authors.lastname: Reubi
authors.forename: Kayhan G
authors.initials: KG
authors.lastname: Akyel
authors.forename: Helmut R
authors.initials: HR
authors.lastname: Maecke
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000970:Antineoplastic Agents / Q000138:chemical synthesis* / Q000737:chemistry*; D001839:Bombesin / Q000737:chemistry*; D045744:Cell Line, Tumor; D002614:Chelating Agents / Q000138:chemical synthesis* / Q000737:chemistry*; D019886:Gastrin-Releasing Peptide / Q000378:metabolism*; D006801:Humans; D007553:Isotope Labeling / Q000379:methods; D051379:Mice; D008819:Mice, Nude; D015394:Molecular Structure; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism; D015609:Organotechnetium Compounds / Q000138:chemical synthesis* / Q000737:chemistry* / Q000493:pharmacokinetics; D011073:Polyamines / Q000737:chemistry*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000493:pharmacokinetics*; D013667:Technetium / Q000737:chemistry*; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: D000970:Antineoplastic Agents; D002614:Chelating Agents; D015609:Organotechnetium Compounds; D011073:Polyamines; D019275:Radiopharmaceuticals; D013667:Technetium; D019886:Gastrin-Releasing Peptide; D001839:Bombesin