Development and Evaluation of a Novel 99mTc-Labeled Annexin A5 for Early Detection of Response to Chemotherapy

99m Tc-HYNIC-annexin A5 can be considered as a benchmark in the field of apoptosis imaging. However, 99m Tc-HYNIC-annexin A5 has characteristics of high uptake and long retention in non-target tissues such as kidney and liver. To minimize this problem, we developed a novel 99m Tc-labeled annexin A5 using a bis(hydroxamamide) derivative [C 3 (BHam) 2 ] as a bifunctional chelating agent, and evaluated its usefulness as an imaging agent for detecting apoptosis. The amino group of C 3 (BHam) 2 was converted to a maleimide group, and was coupled to thiol groups of annexin A5 pretreated with 2-iminothiolane. 99m Tc labeling was performed by a ligand exchange reaction with 99m Tc-glucoheptonate. Biodistribution experiments for both 99m Tc-C 3 (BHam) 2 -annexin A5 and 99m Tc-HYNIC-annexin A5 were performed in normal mice. In addition, in tumor-bearing mice, the relationship between the therapeutic effects of chemotherapy (5-FU) and the tumor accumulation of 99m Tc-C 3 (BHam) 2 -annexin A5 just after the first treatment of 5-FU was evaluated. 99m Tc-C 3 (BHam) 2 -annexin A5 was prepared with a radiochemical purity of over 95%. In biodistribution experiments, 99m Tc-C 3 (BHam) 2 -annexin A5 had a much lower kidney accumulation of radioactivity than 99m Tc-HYNIC-annexin A5. In the organs for metabolism, such as liver and kidney, radioactivity after the injection of 99m Tc-HYNIC-annexin A5 was residual for a long time. On the other hand, radioactivity after the injection of 99m Tc-C 3 (BHam) 2 -annexin A5 gradually decreased. In therapeutic experiments, tumor growth in the mice treated with 5-FU was significantly inhibited. Accumulation of 99m Tc-C 3 (BHam) 2 -annexin A5 in tumors significantly increased after 5-FU treatment. The accumulation of radioactivity in tumor correlated positively with the counts of TUNEL-positive cells. These findings suggest that 99m Tc-C 3 (BHam) 2 -annexin A5 may contribute to the efficient detection of apoptotic tumor response after chemotherapy.