Isomerism and solution dynamics of (90)Y-labeled DTPA--biomolecule conjugates

This report describes the synthesis of two DTPA-conjugated cyclic peptides, cyclo(Arg-Gly-Asp-D-Phe-Lys)DTPA (SQ169) and [cyclo(Arg-Gly-Asp-D-Phe-Lys)](2)DTPA (SQ170), and a chromatographic study of their (90)Y complexes (RP762 and RP763, respectively). The goal is to study the solution structure and the possible isomerism of (90)Y-labeled DTPA-biomolecule conjugates at the tracer level (approximately 10(-10) M). RP762 was prepared in high radiochemical purity (RCP > 95%) by reacting 2 microg of SQ169 with 20 mCi of (90)YCl(3) (corresponding to a SQ169:Y ratio of approximately 4:1) in the 0.5 M ammonium acetate buffer (pH 8.0) at room temperature. RP763 was prepared in a similar fashion using SQ170. In both cases, the (90)Y-chelation was instantaneous. By a reversed-phase HPLC method, it was found that RP762 exists in solution as a mixture of two detectable isomers (most likely cis and trans isomers), which interconvert at room temperature. The interconversion of different isomeric forms of RP762 involves a rapid exchange of "wrapping isomers" via the "wagging" of the diethylenetriamine backbone, "shuffling" of the two NO(2) donor sets, and inversion at the ternimal amine-nitrogen atom. The inversion at a terminal nitrogen atom requires simultaneous dissociation of the NO(2) donor set. For RP763, the interconversion of different isomers is much faster than that for RP762 due to the weak bonding of two carbonyl-oxygen donors. Therefore, RP763 shows only one broad radiometric peak in its HPLC chromatogram. The rapid interconversion of different isomers is intramolecular via a partial dissociative mechanism. The results obtained in this study are consistent with the lack of kinetic inertness of (90)Y- and (111)In-labeled DTPA-biomolecule conjugates. Thus, the design of new BFCs should be focused on those which form lanthanide complexes with high thermodynamic stability and more importantly kinetic inertness.