Preparation of Indium-115-Labeled Diethylenetriaminetetraacetic Acid Monoacetamide Peptides Purified by 8-Hydroxyquinoline

In this communication we describe a novel procedure for the preparation and purification of diethylenetriaminepentaacetic (DTPA)-acylated and 115In3+-labeled oligopeptides using 8-hydroxyquinoline for the removal and quantification of nonbound indium ions. First the Nα- or Nα,Nϵ-DTPA oligopeptides containing C-terminal KDEL signal motif were produced by solid-phase synthesis. For this the free carboxyl group of DTPA dianhydride was activated in situ for a short period of time yielding a major product. Reversed-phase HPLC-purified DTPA oligopeptides were labeled with 115In3+ in aqueous buffer solution at pH 3.8. For the removal as well as for the detection of uncoordinated 115In3+ ions we have utilized the 115In3+ complex-forming ability of 8-hydroxyquinoline in chloroform. Following an optimized extraction procedure the free indium ion content was measured by spectrophotometry in the organic phase. Data obtained by this method and verified by total-reflection X-ray fluorescence spectroscopy and thin-layer chromatography demonstrated that free 115In3+ could be efficiently removed and sensitively detected in the presence of DTPA oligopeptide chelator. No release of 115In3+ from its DTPA complex was observed. This method could be useful for the preparation of indium complexes of peptides and perhaps proteins containing a DTPA moiety and nonradioactive isotope ligand.