Tetraamine-Derived Bifunctional Chelators for Technetium-99m Labelling: Synthesis, Bioconjugation and Evaluation as Targeted SPECT Imaging Probes for GRP-Receptor-Positive Tumours

Owing to its optimal nuclear properties, ready availability, low cost and favourable dosimetry, 99mTc continues to be the ideal radioisotope for medical‐imaging applications. Bifunctional chelators based on a tetraamine framework exhibit facile complexation with Tc(V)O2 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 99mTc is described herein. The chelator 01 was used as a starting material for the facile synthesis of chelators functionalised with OH (02), N3 (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 99mTc afforded the radiotracer 99mTc‐N4‐BB‐ANT, with radiolabelling yields of >97 % at a specific activity of 37 GBq μmol−1. An IC50 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 99mTc‐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 99mTc‐N4‐BB‐ANT warrant its potential candidature for clinical translation. Right on target: The facile synthesis of different tetraamine (N4)‐based chelators for the conjugation of biomolecules and subsequent labelling with 99mTc is described (see scheme). A bombesin‐antagonist peptide conjugated to one of the N4 chelators showed excellent properties as a probe for single‐photon‐emission computed tomography (SPECT) imaging for gastrin‐releasing peptide (GRP)‐receptor‐positive tumours as the targets.