Tuning the properties of tris(hydroxypyridinone) ligands: efficient (68)Ga chelators for PET imaging

The prototype tris(1,6-dimethyl-3-hydroxypyridin-4-one) chelator for gallium-68, THP(Me), has shown great promise for rapid and efficient kit-based (68)Ga labelling of PET radiopharmaceuticals. Peptide derivatives of THP(Me) have been used to image expression of their target receptors in vivo in preclinical and clinical studies. Herein we describe new synthetic routes to the THP platform including replacing the 1,6-dimethyl-3-hydroxypyridin-4-one N(1)–CH(3) group of THP(Me) with O (tris(6-methyl-3-hydroxypyran-4-one, THPO) and N(1)–H (tris(6-methyl-3-hydroxypyridin-4-one), THP(H)) groups. The effect of these structural modifications on lipophilicity, gallium binding and metal ion selectivity was investigated. THP(H) was able to bind (68)Ga in extremely mild conditions (5 min, room temperature, pH 6, 1 μM ligand concentration) and, notably, in vivo, when administered to a mouse previously injected with (68)Ga acetate. The (67)Ga radiolabelled complex was stable in serum for more than 7 days. [(68)Ga(THP(H))] displayed a log P value of –2.40 ± 0.02, less negative than the log P = –3.33 ± 0.02 measured for [(68)Ga(THP(Me))], potentially due to an increase in intramolecular hydrogen bonding attributable to the N(1)–H pyridinone units. Spectrophotometric determination of the Ga(3+)/Fe(3+) complex formation constants for both THP(Me) and THP(H) revealed their preference for binding Ga(3+) over Fe(3+), which enabled selective labelling with (68)Ga(3+) in the presence of a large excess of Fe(3+) in both cases. Compared to THP(Me), THP(H) showed significantly reduced affinity for Fe(3+), increased affinity for Ga(3+) and improved radiolabelling efficiency. THPO was inferior to both THP(H) and THP(Me) in terms of labelling efficiency, but its benzylated precursor Bn-THPO (tris(6-methyl-3-benzyloxypyran-4-one)) provides a potential platform for the synthesis of a library of THP compounds with tunable chemical properties and metal preferences.