Synthesis and Evaluation of New Ligands for Gallium Radiolabelling

Abstract

A series of N3O3 triaza-tricarboxylate hexadentate ligands, based on the 6-amino-1,4-diazepine scaffold, has been designed and synthesised. An investigation of the suitability of these ligands towards application in 68Ga-PET has been conducted, with particular focus on the efficiency and efficacy of radiolabelling. Subsequently, selectivity for Ga(III) over other relevant cations has been studied, along with exploration of the kinetics of radiolabelling, stability and in vivo behaviour of the radiolabelled complexes in healthy rats. The four best candidates quantitatively radiolabel within three minutes at room temperature over the pH range 4 – 7, to give a single 68Ga-radiolabelled species which is sufficiently stable for in vivo application. As a result, these compounds are very promising candidates for use as ligands in 68Ga-PET.

Decoration of the AMPED scaffold amine functionalities yielded a series of chelators, which allowed the 68Ga-radiolabelling properties of this family of ligands to be investigated. The ligands have a mixture of acyclic and cyclic properties, which facilitated rapid binding of the metal and formation of a stable complex. Synthetic details and 1H NMR solution state properties are described. With respect to radiolabelling, the best ligands were those with the least sterically demanding exocyclic amine substituents. However, ligands featuring a mono-alkylated exocyclic amine were prone to internal lactamisation under acidic conditions, which rendered these compounds ineffectual as ligands for 68Ga.

The synthesis and radiochemical evaluation of an additional series of ligands, featuring a modified core structure, designed to inhibit the lactamisation reaction and promote ‘pre-organisation’ of the ligand donors were undertaken. The synthetic methodology developed allows for selective functionalisation of the different amine groups of the ring. Modification of the AMPED core by substitution of the tertiary methyl group for a phenyl moiety increased the steric bulk close to exocyclic amine, which inhibited internal lactamisation. Critically, the methyl-for-phenyl substitution does not have a detrimental effect on the complexation properties.

Finally a study was undertaken of the radiolabelled complex speciation, using NMR spectroscopy and X-ray crystallography. The presence of multiple radiolabelled complex species was attributed to the formation of kinetically trapped complexes of differing stability. The relative population of the two major ligand conformations is controlled by the relative steric demand of the substituents at the quaternary site. The exocyclic amine is subject to protonation (pKa 5.7), and therefore the relative population of the ligand conformers is pH dependent over the range 3 – 7. By substituting the methyl group for a more sterically demanding phenyl group, the population of the ligand conformation favouring fast and stable gallium binding was increased.