Design, Synthesis, and Initial Evaluation of High-Affinity Technetium Bombesin Analogues

• Published in: Bioconjugate chemistry 1998-03, Vol.9 (2), p.218-225
• Main Authors: Baidoo, Kwamena E, Lin, Kuo-Shyan, Zhan, Yougen, Finley, Paige, Scheffel, Ursula, Wagner, Henry N
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Binding Sites; Bombesin - analogs & derivatives; Bombesin - chemical synthesis; Bombesin - chemistry; Bombesin - metabolism; Bombesin - pharmacokinetics; Chelating Agents; Chromatography, High Pressure Liquid; Drug Design; Isotope Labeling; Male; Mice; Molecular Structure; Morpholines - chemistry; Neoplasms - diagnostic imaging; Radionuclide Imaging; Rats; Rats, Sprague-Dawley; Receptors, Bombesin - metabolism; Sulfhydryl Compounds - chemistry; Technetium; Tissue Distribution
• ISSN: 1043-1802; 1520-4812
• DOI: 10.1021/bc9701959

Potent antagonists of bombesin-like peptides have shown great potential for applications in cancer therapy. A 99mTc-labeled agent capable of identifying patients who could benefit from these emerging therapies would have a great impact on patient management. This study involves the synthesis and initial evaluation of technetium diaminedithiolate analogues derived from the potent bombesin analogue Pyr-Gln-Lys-Leu-Gly-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2 (Lys3-bombesin). We coupled two diaminedithiol (DADT) bifunctional chelating agents (BCAs 1 and 2) to the Lys3 residue at the N-terminal region that is not required for binding to the receptor. 99mTc labeling was performed by ligand exchange on addition of [99mTc]glucoheptonate to a solution of the adduct at room temperature. Two products were obtained from each adduct on analysis by HPLC. The major to minor product ratios of the 99mTc-labeled analogues were 3:1 for products from BCA 1 and 9:1 for the products from BCA 2. Macroscopic amounts of the 99Tc analogues were similarly prepared using [99Tc]glucoheptonate. In this case, the major to minor ratios were 2:1 for the products from both BCAs. For initial evaluation of the binding of the Tc-labeled peptides to bombesin receptors, the 99Tc analogues were used in vitro in competitive binding assays in rat brain cortex membranes against [125I-Tyr4]bombesin. Results of the in vitro assays showed that the inhibition constants (K i) of the major and minor products were 3.5 ± 0.7 and 3.9 ± 1.5 nM, respectively, for the products from BCA 1; and 7.4 ± 2.0 and 5.2 ± 1.5 nM for the products derived from BCA 2, respectively. The high affinity exhibited by these technetium analogues is an indication of their potential for use in non-invasive in vivo biochemical characterization of cancers that possess receptors for bombesin.