Study of Binding Kinetics and Specificity of 99mTc-SSS-Complex and 99mTc-HMPAO to Blood Cells

Nuclear medicine offers several techniques and procedures to image infection, but radiolabelled autologous white blood cells (WBCs) are still the gold standard. These cells are usually labelled with 111In or 99mTc bound to a hydrophobic chelating agent that allows these isotopes to pass through the plasma membrane and enter in the cytoplasm. The most common compound in Europe is HMPAO that efficiently chelates 99mTc. However, up to 20-40% of the complex is released from the cells in the first few hours. The aim of this study was to radiolabel a new compound, (S3CPh)2 (S2CPh)-complex (SSS-complex) with 99mTc and compare its binding kinetics and specificity for WBC with HMPAO. The SSS-complex was labelled with 99mTc and analysed by iTLC and RP-HPLC. In vitro quality controls included a stability assay in serum and saline. Results showed a labelling efficiency of 95 ± 1.2% and 98 ± 1.4% for 99mTc-SSS-complex and 99mTc-HMPAO, respectively (p=ns). 99mTc-SSS-complex was stable in serum and in saline up to 24 h (94 ± 0.1%). Cell labelling experiments showed a higher incorporation of 99mTc-SSS-complex than 99mTc-HMPAO by granulocytes (62.6 ± 17.8% vs 40.5 ± 15%, p=0.05), lymphocytes (59.9 ± 22.2% vs 29.4 ± 13.5%; p=0.03), and platelets (44.4 ± 24% vs 20.5 ± 10.7%; p=ns), but the release of radiopharmaceutical from granulocytes at 1 h was lower for HMPAO than for SSS-complex (10.3 ± 1.9% vs 21.3 ± 1.8%; p=0.001). In conclusion, 99mTc-SSS-complex, although showing high labelling efficiency, radiochemical purity, and stability, is not a valid alternative to 99mTc-HMPAO, for example, in vivo white blood cells labelling because of high lymphocyte and platelet uptake and rapid washout from granulocytes.