A radiogallium-DOTA-based bivalent peptidic ligand targeting a chemokine receptor, CXCR4, for imaging tumors

1536 Objectives Since expression of CXC chemokine receptor-4 (CXCR4) in tumors is closely related to tumor metastasis and growth, CXCR4 should be a potential target of nuclear imaging probes for evaluating the malignancy. In the present study, based on our previous finding that two functional moieties can be conjugated to radiogallium-DOTA chelate without reducing the complex stability, we designed and synthesized a two CXCR4 antagonist peptides (Ac-TZ14011)-conjugated Ga-DOTA chelate, Ga-DOTA-TZ2. This bivalent derivative was evaluated in vitro as a CXCR4 ligand for nuclear imaging in comparison with monovalent ligand, Ga-DOTA-TZ1. Methods Coupling of DOTA-bis(tert-butyl)ester with Ac-TZ14011, followed by deprotection, afforded DOTA-TZ2. After coordination with nonradioactive Ga3+, the affinity to CXCR4 was evaluated by competitive binding assay with 125I-SDF-1α. 67Ga-labeling was carried out by the reaction of DOTA-TZ2 with 67GaCl3 at 95°C for 30 min. In vitro cellular uptake of 67Ga-DOTA-TZ2 was investigated using CXCR4-overexpressing CHO cells and control CHO cells. Results In competitive binding assays, the IC50 values for Ga-DOTA-TZ2 and Ga-DOTA-TZ1 were 0.27 and 45 nM, respectively, suggesting a multivalent effect for CXCR4 binding. 67Ga-DOTA-TZ2 was synthesized in about 60% radiochemical yield and >99% radiochemical purity. In cellular uptake studies, 67Ga-DOTA-TZ2 exhibited a higher accumulation in CXCR4-expressing cells than control cells. The accumulation of 67Ga-DOTA-TZ2 in CXCR4-expressing cells was significantly higher than that of 67Ga-DOTA-TZ1. Furthermore, a dose-dependent inhibition was observed by addition of Ac-TZ14011, indicating CXCR4-mediated cellular uptake. Conclusions The newly developed radiogallium-DOTA-TZ2 would become a potential sensitive probe for nuclear imaging of CXCR4 in tumors