Easy and versatile preparation of 68Ga- and Al18F-labelled arylphosphonium salts from a common precursor: stability and preliminary in vivo PET/CT evaluation

The primary aim of this work was to develop and optimise a facile synthesis of radiolabelled arylphosphonium compounds [ 68 Ga] 2 and [ 18 F] 3 from the common macrocyclic precursor 1-(4-(((triphenylphosphonium)yl)methyl)benzyl)-4,7-bis(carboxymethyl)-1,4,7-triazacyclononane acetate ( 1 ). A series of reaction conditions, including temperature, time, ethanol content and reagent concentration were tested to arrive at optimised conditions for labelling with either 68 Ga or Al 18 F. Preclinical evaluations including stability in formulation and in serum, and preliminary imaging in healthy wild type C57BL/6 mice were completed. The target compounds [ 68 Ga] 2 and [ 18 F] 3 could be prepared in near quantitative yields using the optimised conditions. Although the stability of both tracers was acceptable in formulation containing 10% ethanol and 0.5 mg/mL sodium ascorbate, and the stability of [ 18 F] 3 in serum was ideal, the stability of [ 68 Ga] 2 in serum was sub-optimal. Dynamic PET/CT imaging in mice over 60 min revealed no uptake in bone, rapid renal excretion and more prolonged uptake in the liver and intestine for [ 18 F] 3 compared to [ 68 Ga] 2 . The simple synthesis of [ 68 Ga] 2 and [ 18 F] 3 from a common precursor was achieved and was easily translatable. Although the utility of [ 68 Ga] 2 and [ 18 F] 3 for myocardium or cancer imaging is limited, this successful labelling strategy may be applied to a library of other compounds for use in imaging other targets in the body.