Isolation, Characterization, and Biological Evaluation of Syn and Anti Diastereomers of [99mTc]Technetium Depreotide: a Somatostatin Receptor Binding Tumor Imaging Agent

The early and later eluting [ 99m TcO]depreotide products on RP-HPLC were confirmed to be the anti and syn diastereomers, respectively, based on proton NMR and circular dichroism spectroscopy. NMR provided evidence of a folded, conformationally constrained structure for the syn diastereomer. The syn diastereomer is predominant (antilsyn ∼ 10:90) in the [ 99m TcO]depreotide preparation and shows a slightly higher affinity (IC 50 = 0.15 nM) for the somatostatin receptor than the anti diastereomer (IC 50 = 0.89 nM). Both diastereomers showed higher binding affinities than the free peptide (IC 50 = 7.4 nM). Biodistribution studies in AR42J tumor xenograft nude mice also showed higher tumor uptake for syn [ 99m TcO]depreotide (6.58% ID/g) than for the anti [ 99m TcO]depreotide (3.38% ID/g). Despite the differences in biological efficacy, the favorable binding affinity, tumor uptake, and tumor-to-background ratio results for both diastereomeric species predict that both are effective for imaging somatostatin receptor-positive tumors in vivo.