Biodistribution and radiation dosimetry of 99mTc-HMPAO-labeled monocytes in patients with rheumatoid arthritis

Rheumatoid arthritis (RA) involves the accumulation of monocyte-derived macrophages in the affected synovial tissue. This process of cell migration could be portrayed scintigraphically to monitor noninvasively the effects of therapy on the progress of the disease. For this purpose, labeling of purified autologous monocytes with Tc-99m-hexamethylpropyleneamine oxime (Tc-99m-HMPAO) at very high specific radioactivity has recently been developed. The aim of this study was to assess the biodistribution and radiation dosimetry of Tc-99m-HMPAO-labeled monocytes in adult patients with RA. Methods: In 8 patients with RA, monocytes were isolated from 100 mL of blood and labeled with Tc-99m-HMPAO to a yield of 10 Bq/cell. Multiple whole-body scans were performed up to 20 h after reinjection of an average of 200 MBq of Tc-99m-HMPAO-labeled monocytes. Urine and blood samples were collected. The fraction of administered activity in 7 source organs was quantified from the attenuation-corrected geometric mean counts in conjugate views. Radiation-absorbed doses were estimated with OLINDA/EXM software. Results: Autologous monocytes labeled with Tc-99m-HMPAO at high intracellular yields showed in vivo kinetics comparable with labeled leukocytes, with initial trapping in the lungs followed by distribution into the liver, spleen, and bone marrow. The radiation-absorbed estimates for 99mTc-HMPAO-labeled monocytes were comparable with those for 99mTc-HMPAO-labeled mixed white blood cells, with an effective dose of 0.011 mSv/MBq. Conclusion: 99mTc-HMPAO-labeled monocytes have biodistribution and radiation dosimetry similar to those of Tc-99m-HMPAO- labeled mixed white blood cells and might therefore be used for in vivo monitoring of immunomodulating therapy in patients with RA.