Abstract
Objective A recombinant annexin A5 with the N-terminal extension of six histidine residues was labeled with 99m Tc(I)-tricarbonyl ion to produce the 99m Tc-labeled annexin A5, referred to 99m Tc(I)-his6-annexin A5. We have explored the agent as an effective imaging probe for in vivo detecting the apoptosis of internal tissue subjected with high radiation doses in a γ-irradiated mouse model. Methods [ 99m Tc(CO) 3 (OH 2 ) 3 ] + was prepared and taken to directly label his6-annexin A5. The radiochemical purity of 99m Tc(I)-his6-annexin A5 after size-exclusion separation was measured by HPLC. The binding affinity of 99m Tc(I)- his6-annexin A5 to apoptotic cells was assessed using 20 Gy-irradiated Jurkat T cells. The effectiveness of 99m Tc(I)-his6-annexin A5 as an imaging probe to detect the internal tissue apoptosis was assessed by biodistribution study and nanoSPECT/CT using the animal model of C57BL/6J mice conducted with 10 Gy γ irradiation. Results The radiochemical purity of 99m Tc(I)- his6- annexin A5 could attain ≥95%. The binding affinity of 99m Tc(I)-his 6 -annexin A5 to the 20 Gy-irradiated Jurkat cells was found to be ca. 20-fold higher than that to the sham-irradiated cells. In the animal imaging study, the splenic uptake of 99m Tc(I)-his6- annexin A5 for the 10 Gy-irradiated mice showed from ca. 3-fold to 5-fold higher than those of the sham-irradiated mice from 45 to 165 min postinjection. The corresponding intestinal uptake showed from ca. 2-fold to 3-fold higher during the same period of time postinjection. The biodistribution study demonstrated the organ uptakes comparable with the imaging results. The apoptotic extents of the spleen and the intestine from the SPECT/CT imaging were correlated with an immunohistochemical staining assay for caspase 3 active form fragment. Conclusion This work is the first study to demonstrate that 99m Tc(I)- his6-annexin A5 is a potential clinical imaging agent for detecting radiation-induced tissue apoptosis in an animal model. © The Japanese Society of Nuclear Medicine 2012.