Evaluation of 99Tc(m)-ECD as a white blood cell labelling agent

99Tc(m)-ECD is a new agent for perfusion brain imaging. Its brain retention is attributed to the enzymatic conversion of lipophilic 99Tc(m)-ECD to polar monoacid and diacid derivatives. Based on its proposed mechanism of retention in the brain, labelling of white blood cells (WBC) with 99Tc(m)-ECD has been studied at our laboratory. A labelling efficiency of 42% was achieved by incubating WBC with 99Tc(m)-ECD in phosphate buffered saline medium for 30 min. There was a washout of 50% of the radioactivity from the cells in 1 h, which might contribute to increased background in potential human studies. However, rapid urinary elimination of the radioactivity is expected to deal with this problem due to the rapid in vivo conversion of the parent compound to polar metabolites. 99Tc(m)-ECD appears to be a promising agent for labelling WBC. Furthermore, already prepared multidose 99Tc(m)-ECD may be used for either brain perfusion imaging or WBC labelling.