In vitro and in vivo evaluation of [Tc-99m]-labeled tricarbonyl His-annexin A5 as an imaging agent for the detection of phosphatidylserine-expressing cells

Introduction: Apoptosis is one of the mechanisms behind successful chemotherapy and radiation treatment. Radiolabeled annexin A5 has been demonstrated to be a successful tool in the detection of apoptosis following chemotherapy in vivo. Methods: His-tagged annexin A5 was labeled with [Tc-99m]-tricarbonyl and evaluated as apoptosis imaging radiotracer in vitro and in vivo. The binding of the radiotracer was evaluated in Colo205 cells stimulated with 5-FU (1 mM) for 4 and 24 h, and confirmed by flow cytometry. Biodistribution and dosimetric studies were performed in healthy nude mice (n=5) via planar scintigraphy. [Tc-99m]-(CO)(3) His-annexin A5 was also evaluated for in vivo imaging of spontaneous apoptosis in Colo205-bearing mice (n=12). Results: The labeling procedure yielded a compound with 95-99% radiochemical purity and good in vitro stability. In vitro binding experiments indicated that the radiotracer retained its PS-binding activity. [Tc-99m]-(CO)(3) His-annexin AS rapidly cleared from the blood and predominantly accumulated in the kidneys. Absorbed dose (per organ) was found to be 116 64 mu Gy/MBq for the kidneys and 10.38 +/- 0.50 mu Gy/MBq for the liver. The effective dose was 7.00 +/- 0.28 mu Sv/MBq. Spontaneous apoptosis in Colo205-bearing mice was visualised by [Tc-99m]-(CO)(3) His-annexin AS SPECT and correlated well with caspase-3 immunostaining (R=0.867, P