Reassessment of Diethylenetriaminepentaacetic Acid (DTPA) as a Chelating Agent for Indium-111 Labeling of Polypeptides Using a Newly Synthesized Monoreactive DTPA Derivative

Previous studies on indium-111 (¹¹¹In) labeling of polypeptides and peptides using cyclic diethylenetriaminepentaacetic dianhydride (cDTPA) as a bifunctional chelating agent (BCA) have indicated that DTPA might be a useful BCA for ¹¹¹In labeling of polypeptides at high specific activities when DTPA can be incorporated without inducing intra- or intermolecular cross-linking. To investigate this hypothesis, a monoreactive DTPA derivative with a maleimide group as the peptide binding site (MDTPA) was designed and synthesized. A monoclonal antibody (OST7, IgG<INF>1</INF>) was used as a model polypeptide, and conjugation of MDTPA with OST7, ¹¹¹In radiolabeling of MDTPA−OST7, and the stability of ¹¹¹In−MDTPA−OST7 were investigated using cDTPA and benzyl-EDTA derivatives as references. SDS−PAGE analysis demonstrated that while cDTPA induced intramolecular cross-linking, no such undesirable side reactions were observed with MDTPA. MDTPA generated ¹¹¹In-labeled OST7 with high radiochemical yields at higher specific activities than those produced using cDTPA and benzyl-EDTA derivatives as the BCAs. Incubation of each ¹¹¹In-labeled OST7 in human serum indicated that MDTPA generated ¹¹¹In-labeled OST7 of much higher and a little lower stability than those derived from cDTPA and benzyl-EDTA derivatives, respectively. These findings indicated that the low in vivo stability of cDTPA-conjugated antibody reported previously is not attributable to low stability of ¹¹¹In−DTPA but to formation of intramolecular cross-linking during cDTPA conjugation reactions. The present study also indicated that MDTPA and its precursor, the tetra-tert-butyl derivative of DTPA, would be useful BCAs for ¹¹¹In radiolabeling of polypeptides that have rapid blood clearance with high specific activities.