Harnessing scandium chelation chemistry for the development of radiopharmaceuticals

The recent FDA-approval of 68Ga-imaging radiopharmaceuticals and their 177Lu-based companion therapeutics underscores the potential of metallic radioisotopes for clinical applications in diagnostic and therapeutic nuclear medicine. The improvements made to accelerator-based production of radiometals beyond the 68Ga, 177Lu theranostic pair has opened opportunities to produce radiometals with a broad range of half-lifes and emission properties, expanding the scope of imageable disease targets. However, subsequent development of clinically applicable radiopharmaceuticals has been impeded by a significant gap in knowledge of aqueous coordination chemistry of the corresponding metal ions. Scandium(III) aqueous chemistry represents a prime example of this conundrum. 43Sc (Eβ+ avg = 476keV, t1/2= 3.9h) and 44Sc (Eβ+ avg = 632keV, t1/2= 4h) have ideal properties for positron emission tomography (PET) imaging up to 24h post injection. For therapy applications, the emission properties of 47Sc (Eβ− avg = 162keV, t1/2 = 80.4h) are comparable to 177Lu (Eβ− avg = 134keV, t1/2 = 159.6h). The Sc(III) ion is a close chemical match to Lu(III) with respect to ionic radius and chemical hardness; therefore 43Sc/44Sc also represents an ideal diagnostic isotope partner to the already widely accessible 177Lu therapy isotope, producing more directly predictive image-derived pharmacokinetics and dosimetry for radiotherapy. However, the pronounced solution chemistry knowledge deficiency has hampered efficient separation, isolation, and application of Sc(III) isotopes as clinical radiopharmaceuticals. To enable the synthesis of a broad range of diagnostic and therapeutic radiopharmaceuticals based on Sc and Lu isotopes, we propose three aims towards new chemical and in vivo validated strategies that enable high-yielding, high molar activity, low temperature radiochemistry approaches to theranostic radiopharmaceuticals.