What a Difference a Carbon Makes: H4octapa vs H4C3octapa, Ligands for In-111 and Lu-177 Radiochemistry

The acyclic ligands H(4)C3octapa and p-SCN-Bn-H(4)C3octapa were synthesized for the first time, using nosyl protection chemistry. These new ligands were compared to the previously studied ligands H(4)octapa and p-SCN-Bn-H(4)octapa to determine the extent to which the addition of a single carbon atom to the backbone of the ligand would affect metal coordination, complex stability, and, ultimately, utility for in vivo radiopharmaceutical applications. Although only a single carbon atom was added to H(4)C3octapa and the metal donor atoms and denticity were not changed, the solution chemistry and radiochemistry properties were drastically altered, highlighting the importance of careful ligand design and radiometal-ligand matching. It was found that [In(C3octapa)](-) and [Lu(C3octapa)](-) were substantially different from the analogous H(4)octapa complexes, exhibiting fluxional isomerization and a higher number of isomers, as observed by H-1 NMR, VT-NMR, and 2D COSY/HSQC-NMR experiments. Past evaluation of the DFT structures of [In(octapa)](-) and [Lu(octapa)](-) revealed very symmetric complexes; in contrast, the [In(C3octapa)](-) and [Lu(C3octapa)(-) complexes were much less symmetric, suggesting lower symmetry and less rigidity than that of the analogous H(4)octapa complexes. Potentiometric titrations revealed the formation constants (log K-ML, pM) were similar to 2 units lower for the In-3+ and Lu-3+ complexes of H(4)C3octapa when compared to that of the more favorable H(4)octapa ligand (similar to 2 orders of magnitude less thermodynamically stable). The bifunctional ligands p-SCN-Bn-H(4)C3octapa and p-SCN-Bn-H(4)octapa were conjugated to the antibody trastuzumab and radiolabeled with In-111 and Lu-177. Over a 5 day stability challenge experiment in blood serum, In-111-octapa- and In-111-C3octapa-trastuzumab immunoconjugates were determined to be similar to 91 and similar to 24% stable, respectively, and Lu-177-octapa- and Lu-177-C3octapa-trastuzumab, similar to 89% and similar to 4% stable, respectively. This work suggests that 5-membered chelate rings are superior to 6-membered chelate rings for large metal ions like In3+ and Lu3+, which is a crucial consideration for the design of bifunctional chelates for bioconjugation to targeting vectors for in vivo work.