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 99mTc 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 99mTc complex, 99mTc-P2S2-COOH, can be formed in >95% yield. To demonstrate the potential of this chelate to efficiently label peptides, 99mTc-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 99mTc-P2S2-BBN(7-14). Conjugation to the peptide was performed in borate buffer (pH 8.5) by applying the prelabeling approach in yields of >60%. In competitive binding assays, using Swiss 3T3 mouse fibroblast cells against [125I-Tyr4]bombesin, Re-P2S2-BBN(7-14) exhibited an IC50 value of 0.8 +/- 0.4 nM. The pharmacokinetic studies of 99mTc-P2S2-BBN(7-14) and its ability to target tissue expressing gastrin-releasing peptide (GRP) receptors were performed in normal mice. The 99mTc-P2S2-BBN(7-14) exhibited 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.