99mTc-labeling and in vivo studies of a bombesin analogue with a novel water-soluble dithiadiphosphine-based bifunctional chelating agent

Recent progress in the synthesis of water-soluble phosphine ligand systems and their corresponding Tc-99m complexes prompted the development of a new bifunctional chelating agent (BFCA) based on a tetradentate dithiadiphosphine framework (P2S2-COOH). The detailed synthesis of this new BFCA is described here. The corresponding Tc-99m complex, Tc-99m-P2S2-COOH, can be formed in >95% yield. To demonstrate the potential of this chelate to efficiently label peptides, Tc-99m-P2S2-COOH was coupled to the N-terminal region of the truncated nine-amino acid bombesin analogue, 5-Ava-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2 [BBN(7-14)], to form (TC)-T-99m-P2S2-BBN(7-14). Conjugation to the peptide was performed in berate buffer (pH 8.5) by applying the prelabeling approach in yields of > 60%. In competitive binding assays, using Swiss 3T3 mouse fibroblast cells against [I-125-Tyr(4)]bombesin, Re-P2S2-BBN(7-14) exhibited an IC50 value of 0.8 +/- 0.4 nM. The pharmacokinetic studies of Tc-99m-P2S2-P2S2-BBN(7-14) its ability to target tissue expressing gastrin-releasing peptide (GRP) receptors were performed in normal mice. The Tc-99m-P2S2-BBN(7-14) exhibit-ed fast and efficient clearance from the blood pool (0.6 +/- 0.1% ID, 4 h postinjection) and excretion through the renal and hepatobiliary pathways (56.4 +/- 8.2 and 28.1 +/- 7.9% ID, 4 h postinjection, respectively). Significant uptake in the pancreas was observed (pancreatic acini cells express bombesin/GRP receptors), producing pancreas:blood and pancreas:muscle ratios of ca. 22 and 80, respectively, at 4 h postinjection.