In vitro and in vivo evaluation of [99mTc]-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 [(99m)Tc]-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. [(99m)Tc]-(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. [(99m)Tc]-(CO)(3) His-annexin A5 rapidly cleared from the blood and predominantly accumulated in the kidneys. Absorbed dose (per organ) was found to be 116 ± 64 μGy/MBq for the kidneys and 10.38 ± 0.50 μGy/MBq for the liver. The effective dose was 7.00 ± 0.28 μSv/MBq. Spontaneous apoptosis in Colo205-bearing mice was visualised by [(99m)Tc]-(CO)(3) His-annexin A5 SPECT and correlated well with caspase-3 immunostaining (R=0.867, P<.01).
Conclusion: [(99m)Tc]-(CO)(3) His-annexin A5 may be a useful novel radioligand for the in vivo detection of cell death associated with PS expression. A simple, noninvasive way of detecting apoptosis in vivo could have many applications including a better understanding of the extent and timing of apoptosis in response to cancer therapies and assessment of early tumor response.
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