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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 [ 177 Lu][Lu(DO3Apic-NO2)] − and [ 86 Y][Y(DO3Apic-NO2)] − , followed by post-labeling conjugation to a cysteine-functionalized peptide targeting the prostate specific membrane antigen. The 86 Y-labeled construct efficiently localizes in target tumors with no significant off-target accumulation as evidenced by positron emission tomography, biodistribution studies and metabolite analysis.