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Dosimetry of Copper-64-Labeled Monoclonal Antibody 1A3 as Determined by PET Imaging of the Torso

Divisions of Radiation Sciences and Nuclear Medicine, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri
Department of Surgery, The Jewish Hospital of St.Louis, St. Louis, Missouri

Correspondence: For correspondence or reprints contact: P. Duffy Cutler, PhD, Division of Radiation Sciences, Mallinckrodt Institute of Radiology, 510 S. Kingshighway Blvd., St. Louis, MO 63110.

ABSTRACT

We present biodistribution and dosimetry results for ⁶⁴Cu-benzyl-TETA-MAb 1A3 from 15 human subjects injected with this tracer as determined by serial PET imaging of the torso. Methods: PET imaging was used to quantify in vivo tracer biodistribution at two time points after injection. Absorbed dosimetry calculated using MIRD-11 and the updated MIRDOSE3 was compared with estimates obtained using rat biodistribution data. Results: By measuring activity concentrations in the torso, and extrapolating for the whole body using standard organ and tissue volumes, we were able to account for 93% of the injected radiopharmaceutical over a range of imaging times from 0 to 36hr postinjection. Based on PET imaging and the MIRD-11 schema, the liver and spleen are the critical organs with average absorbed doses of 0.12 and 0.10 mGy/MBq (0.44 and 0.39 rad/mCi). The revised MIRDOSE3 scheme yields similar values for these and other organs but also results in a dose of 0.14 mGy/MBq (0.53 rad/mCi) to the heart wall. In the rat, the large intestine is the critical organ at 0.14 mGy/MBq (0.52 rad/mCi) while liver and kidneys each receive 0.11 mGy/MBq (0.41 rad/mCi). Some disparities in absorbed doses determined by these methods are evident but are a result of dissimilar biodistributions in rats and humans. For most organs, rat extrapolated values are higher than the human measurements with PET. Conclusion: This study shows that torso PET imaging can quantitatively measure the whole-body biodistribution of a radiopharmaceutical as long as it has relatively slow pharmacokinetics.

Key Words: copper-64-antibody • positron emission tomography • torso

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