Synthesis and evaluation of a radiolabeled bis-zinc(II)-cyclen complex as a potential probe for in vivo imaging of cell death.

The exposition of phosphatidylserine (PS) from the cell membrane is associated with most cell death programs (apoptosis, necrosis, autophagy, mitotic catastrophe, etc.), which makes PS an attractive target for overall cell death imaging. To this end, zinc(II) macrocycle coordination complexes with cyclic polyamine units as low-molecular-weight annexin mimics have a selective affinity for biomembrane surfaces enriched with PS, and are therefore useful for detection of cell death. In the present study, a C-labeled zinc(II)-bis(cyclen) complex (C-CyclenZn2) was prepared and evaluated as a new positron emission tomography (PET) probe for cell death imaging. C-CyclenZn2 was synthesized by methylation of its precursor, 4-methoxy-2,5-di-[10-methyl-1,4,7,10-tetraazacyclododecane-1,4,7-tricarboxylic acid tri-tert-butyl ester] phenol (Boc-Cyclen2) with C-methyl triflate as a prosthetic group in acetone, deprotection by hydrolysis in aqueous HCl solution, and chelation with zinc nitrate. The cell death imaging capability of C-CyclenZn2 was evaluated using in vitro cell uptake assays with camptothecin-treated PC-3 cells, biodistribution studies, and in vivo PET imaging in Kunming mice bearing S-180 fibrosarcoma. Starting from C-methyl triflate, the total preparation time for C-CyclenZn2 was ~40 min, with an uncorrected radiochemical yield of 12 ± 3% (based on C-CHOTf, n = 10), a radiochemical purity of greater than 95%, and the specific activity of 0.75-1.01 GBq/μmol. The cell death binding specificity of C-CyclenZn2 was demonstrated by significantly different uptake rates in camptothecin-treated and control PC-3 cells in vitro. Inhibition experiments for F-radiofluorinated Annexin V binding to apoptotic/necrotic cells illustrated the necessity of zinc ions for zinc(II)-bis(cyclen) complexation in binding cell death, and zinc(II)-bis(cyclen) complexe and Annexin V had not identical binding pattern with apoptosis/necrosis cells. Biodistribution studies of C-CyclenZn2 revealed a fast clearance from blood, low uptake rates in brain and muscle tissue, and high uptake rates in liver and kidney, which provide the main metabolic route. PET imaging using C-CyclenZn2 revealed that cyclophosphamide-treated mice (CP-treated group) exhibited a significant increase of uptake rate in the tumor at 60 min postinjection, compared with control mice (Control group). The results indicate that the ability of C-CyclenZn2 to detect cell death is comparable to Annexin V, and it has potential as a PET tracer for noninvasive evaluation and monitoring of anti-tumor chemotherapy.