Calcium minerals such as hydroxyapatite (HAp) can be detected non-invasively in vivo using nuclear imaging agents such as [ 18 F]NaF (available from cyclotrons), for positron emission tomography (PET) and 99m Tc-radiolabelled bisphosphonates (BP; available from 99m Tc generators for single photon emission computed tomography (SPECT) or scintigraphy). These two types of imaging agents allow detection of bone metastases (based on the presence of HAp) and vascular calcification lesions (that contain HAp and other calcium minerals). With the aim of developing a cyclotron-independent PET radiotracer for these lesions, with broad calcium mineral affinity and simple one-step radiolabelling, we developed [ 68 Ga]Ga-THP-Pam. Radiolabelling with 68 Ga is achieved using a mild single-step kit (5 min, room temperature, pH 7) to high radiochemical yield and purity (>95%). NMR studies demonstrate that Ga binds via the THP chelator, leaving the BP free to bind to its biological target. [ 68 Ga]Ga-THP-Pam shows high stability in human serum. The calcium mineral binding of [ 68 Ga]Ga-THP-Pam was compared in vitro to two other 68 Ga-BPs which have been successfully evaluated in humans, [ 68 Ga]Ga-NO2AP BP and [ 68 Ga]Ga-BPAMD, as well as [ 18 F]NaF. Interestingly, we found that all 68 Ga-BPs have a high affinity for a broad range of calcium minerals implicated in vascular calcification disease, while [ 18 F]NaF is selective for HAp. Using healthy young mice as a model of metabolically active growing calcium mineral in vivo, we compared the pharmacokinetics and biodistribution of [ 68 Ga]Ga-THP-Pam with [ 18 F]NaF as well as [ 68 Ga]NO2AP BP. These studies revealed that [ 68 Ga]Ga-THP-Pam has high in vivo affinity for bone tissue (high bone/muscle and bone/blood ratios) and fast blood clearance (t1/2 <10 min) comparable to both [ 68 Ga]NO2AP BP and [ 18 F]NaF. Overall, [ 68 Ga]Ga-THP-Pam shows high potential for clinical translation as a cyclotron-independent calcium mineral PET radiotracer, with simple and efficient radiochemistry that can be easily implemented in any radiopharmacy.

New DO2A-based lipophilic and cationic chelators for gallium-68 have been synthesised for cardiac PET imaging. These radiotracers show preferential uptake in healthy cardiac tissue over cardiac tissue depolarised by CCCP.

Hexadentate tris(3,4-hydroxypyridinone) ligands (THP) complex Fe3+ at very low iron concentrations and their high affinities for oxophilic trivalent metal ions have led to their development for new applications as bifunctional chelators for the radiometal gallium-68 (68Ga). THP-peptide bioconjugates rapidly and quantitatively complex 68Ga at room temperature, neutral pH, and micromolar ligand concentrations, making them amenable to kit-based radiosynthesis of 68Ga PET radiopharmaceuticals. With the aim to produce an N-hydroxysuccinimide-(NHS)-THP reagent for kit-based 68Ga-labeling and PET imaging, THP-derivatives were designed and synthesized to exploit the advantages of NHS chemistry for coupling with peptides, proteins, and antibodies. The more stable five-carbon atoms linker product was selected for a proof-of-concept conjugation and radiolabeling study with an anti-programmed death ligand 1 (PD-L1) camelid single domain antibody (sdAb) under mild conditions and further evaluated for site-specific amide bond formation with a synthesized glucagon-like peptide-1 (GLP-1) targeting peptide using solid-phase synthesis. The obtained THP-GLP-1 conjugate was tested for its 68Ga chelating ability, demonstrating to be a promising candidate for the detection and monitoring of GLP-1 aberrant malignancies. The obtained sdAb-THP conjugate was radiolabeled with 68Ga under mild conditions, providing sufficient labeling yields after 5 min, demonstrating that the novel NHS-THP bifunctional chelator can be widely used to easily conjugate the THP moiety to different targeting molecules (e.g., antibodies, anticalins, or peptides) under mild conditions, paving the way to the synthesis of different imaging probes with all the advantages of THP radiochemistry.