Synthesis, characterization, and X-ray crystal structure of In(DOTA-AA) (AA = p-aminoanilide): a model for 111In-labeled DOTA-biomolecule conjugates

This report describes the synthesis and structural characterization of the indium complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono(p-aminoanilide) (DOTA-AA), a model compound for (111)In-labeled DOTA-biomolecule conjugates. In(DOTA-AA) was prepared by reacting DOTA-AA with 1 equiv of InCl(3) in 0.5 M ammonium acetate buffer (pH approximately 6). It was characterized by spectroscopic methods (IR, ES-MS, and (1)H NMR), elemental analysis, and X-ray crystallography. For comparison purposes, we also prepared the complex Y(DOTA-AA). ES-MS and (1)H NMR data are consistent with the proposed structure. HPLC analysis using a reversed phase method shows that the retention time of In(DOTA-AA) is approximately 2.0 min shorter than that of Y(DOTA-AA), demonstrating that In(DOTA-monoamide) is more hydrophilic than Y(DOTA-monoamide). In the solid state, In(DOTA-AA) has a twisted square antiprismatic coordination geometry with all eight donor atoms (N(4)O(4)) bonded to the In center. The average In-N and In-O distances are almost identical to those of Y-N and Y-O bonds found in Y(DOTA-d-Phe-NH(2)) even though the ionic radius of Y(3+) is much longer than that of In(3+). It seems that In(3+) does not fit the coordination cavity of DOTA-AA perfectly. The (1)H NMR data clearly demonstrated that In(DOTA-AA) becomes fluxional at room temperature, most likely due to dissociation of the acetamide-oxygen, rotation of acetate chelating arms, and inversion of ethylenic groups of the macrocyclic ring. Results from this study and our previous studies (Liu, S.; Pietryka, J.; Ellars C. E.; Edwards D. S. Bioconjugate Chem. 2002, 13, 902-913) suggest that the In(3+) complex of DOTA-monoamide in the solid state might be different from that in solution due to dissociation of the carbonyl-oxygen donor. Although Y(3+) and In(3+) complexes of DOTA-monoamide are both eight-coordinate in the solid state, the difference in their solution structures is most likely responsible for their difference in lipophilicity.