Synthesis and characterization of two 111In-labeled DTPA-peptide conjugates

This report describes the synthesis and characterization of two In-111-labeled DTPA-peptide conjugates (DTPA-MA and DTPA-BA). It is surprising to find that In-111(DTPA-MA) and 111In(DTPA-BA) are more hydrophilic than their corresponding Y-90 analogues, suggesting a different coordination sphere in In-111 and Y-90 complexes of the same DTPA-peptide conjugate. By a reversed phase HPLC method, both In-111(DTPA-MA) and In-111(DTPA-BA) showed only one radiometric peak in their respective HPLC chromatogram due to a rapid interconversion of different isomers (particularly cis and trans isomers for In-111(DTPA-MA); cis-cis, cis-trans, trans-cis, and trans-trans isomers for In-111(DTPA-BA)). The interconversion of different isomers involves the wagging of the diethylenetriamine backbone, shuffling of the NO or NO2 donor sets, and a rapid inversion at the terminal amine-nitrogen atoms.