Design, synthesis, and initial evaluation of high-affinity technetium bombesin analogues

Potent antagonists of bombesin-like peptides have shown great potential for applications in cancer therapy. A Tc-99m-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-Gln-Asn-Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH2 (Lys(3)-bombesin). We coupled two diaminedithiol (DADT) bifunctional chelating agents (BCAs 1 and 2) to the Lys(3) residue at the N-terminal region that is not required for binding to the receptor. Tc-99m labeling was performed by ligand exchange on addition of [Tc-99m(3)]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 Tc-99m-labeled analogues were 3:1 for products from BCA 1 and 9:1 for the products from BCA 2. Macroscopic amounts of the Tc-99 analogues were similarly prepared using [Tc-99]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 Tc-99 analogues were used in vitro in competitive binding assays in rat brain cortex membranes against [I-125-Tyr(4)]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.