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Biodistribution and dosimetry study of 123I‐rh‐annexin V in mice and humans

Summary This study reports on the optimization of the labelling procedure of clinical grade 123I‐rh‐annexin V and on the investigation of the biodistribution and dosimetry of 123I‐rh‐annexin V, a tracer proposed for the study of apoptosis in mice and humans. Research grade 123I‐rh‐annexin V was prepared as described previously, whereas clinical grade 123I‐rh‐annexin V was prepared according to a modified IodoGen method. NMRI mice, 3–4 weeks of age, received research grade 123I‐rh‐annexin V (74.0 + 3.7 kBq/mouse) by intravenous (i.v.) injection and killed at preset time points. Afterwards, the collected organs, blood, urine and faeces were counted for radioactivity and determined as %ID/g tissue or %ID over time. Secondly, six volunteers with normal liver and kidney function underwent whole‐body scans up to 21 h after i.v. injection of clinical grade 123I‐rh‐annexin V (345 ±38 MBq). Time‐activity curves were generated for the organs of interest, e.g., thyroid, heart, liver, kidneys and whole body, by fitting the organ specific geometric mean counts, obtained from region of interest analysis of acquired images in humans. The MIRD formulation was applied to calculate the absorbed radiation doses for various organs. Clinical grade 123I‐rh‐annexin V was obtained in radiochemical yields of 87.0 + 6.5% and radiochemical purities >98%. In mice, research grade 123I‐rh‐annexin V accumulated primarily in liver, kidney, stomach and lung tissue, limiting its usefulness for imaging of ongoing apoptosis in the abdominal and thoracic region. Clearance was predominantly urinary. In humans, acquired images with the clinical grade radioligand showed low lung uptake, resulting in good imaging conditions for the thoracic region. On the other hand, delayed imaging of the abdominal region was impeded due to extensive bowel activity. The highest absorbed doses were received by the thyroid, the kidneys, the heart wall, the liver and bone surfaces. The average effective dose of 123I‐rh‐annexin V was estimated to be 0.02 mSv·MBq‐1. The amount of 123I‐rh‐annexin V required for in vivo imaging, results in an acceptable effective dose to the patient.