Validation of a post-radiolabeling bioconjugation strategy for radioactive rare earth complexes with minimal...

The nine-coordinate aza-macrocycle DO3Apic-NO2 and its kinetically inert rare earth complexes [M(DO3A-pic-NO2)]- (M = La, Tb, Eu, Lu, Y) can be readily bioconjugated to surface accessible thioles on peptides and proteins with a minimal structural footprint. All complexes express thioconjugation rate constants in the same order of magnitude (k = 0.3 h^-1) with the exception of Sc (k = 0.89 h^-1). Coupling to peptides and biologics with accessible cysteines also enables post-radiochelation bioconjugation at room temperature to afford injection-ready radiopharmaceuticals as demonstrated by formation of [¹⁷⁷Lu][Lu(DO3Apic-NO2)]^- and [⁸⁶Y][Y(DO3Apic-NO2)]^-, followed by post-labeling conjugation to a cysteine-functionalized peptide targeting the prostate specific membrane antigen. The ⁸⁶Y-labeled construct efficiently localizes in target tumors with no significant off-target accumulation as evidenced by positron emission tomography, biodistribution studies and metabolite analysis.