Concise site-specific synthesis of DTPA-peptide conjugates: application to imaging probes for the chemokine receptor CXCR4.

Diethylenetriaminepentaacetic acid (DTPA) is a useful chelating agent for radionuclides such as (68)Ga, (99m)Tc and (111)In, which are applicable to nuclear medicine imaging. In this study, we established a facile synthetic protocol for the production of mono-DTPA-conjugated peptide probes. A novel monoreactive DTPA precursor reagent was synthesized in two steps using the chemistry of the o-nitrobenzenesulfonyl (Ns) protecting group, and under mild conditions this DTPA precursor was incorporated onto an N(ε)-bromoacetylated Lys of a protected peptide resin. The site-specific DTPA conjugation was facilitated by using a highly acid-labile 4-methyltrityl (Mtt) protecting group for the target site of the bioactive peptide during the solid-phase synthesis. A combination of both techniques yielded peptides with disulfide bonds, such as octreotide and polyphemusin II-derived CXCR4 antagonists. DTPA-peptide conjugates were purified in a single step following cleavage from the resin and disulfide bond formation. This site-specific on-resin construction strategy was used for the design and synthesis of a novel In-DTPA-labeled CXCR4 antagonist, which exhibited highly potent inhibitory activity against SDF-1-CXCR4 binding.