OBJECTIVETo synthesize the complex fac-[⁹⁹(m)Tc(CO)₃(H₂O)₃](+) for labeling IgG and investigate the in vitro stability of ⁹⁹(m)Tc(CO)₃(H₂O)₃-IgG and its biodistribution in mice.

METHODSfac-[⁹⁹(m)Tc(CO)₃(H₂O)₃](+) was synthesized and its radiochemical purity determined using polyamide membrane chromatography. IgG was directly labeled with fac-[⁹⁹(m)Tc(CO)₃(H₂O)₃](+) and the labeling ratio was determined using chromatography. The stability of ⁹⁹(m)Tc(CO)₃(H₂O)₃-IgG in human serum albumin and normal saline was evaluated. ⁹⁹(m)Tc(CO)₃(H₂O)₃-IgG was injected via the tail vein into 9 mice at the dose of 3.7×10⁴ Bq/100 µl, and SPECT image was obtained at 2, 4 and 12 h after the injection. The mice were sacrificed at these time points to measure the radioactivity and calculate the %ID/g in each organ.

RESULTSFac-[⁹⁹(m)Tc(CO)₃(H₂O)₃](+) had a radiochemical purity of 82.48% and remained stable in vitro at room temperature within 4 h. The labeling ratio of ⁹⁹(m)Tc(CO)₃(H₂O)₃-IgG was 57.04% with a radiochemical purity exceeding 90%. In the solution of human serum albumin, the labeled IgG maintained a stable radiochemical purity, but in normal saline, its radiochemical purity was lowered to 20% at 24 h. After injection in mice, the labeled IgG was deposited mainly in the liver, spleen, kidneys, and the blood pool showed a sustained radioactivity.

CONCLUSION⁹⁹(m)Tc(CO)₃(H₂O)₃-IgG prepared in this study has good stability in vitro and in vivo in 24 h and shows a biodistribution pattern similar to that of IgG protein in vivo. The intermediate fac-[⁹⁹(m)Tc(CO)₃(H₂O)₃](+) can meet the experimental requirement for labeling monoclonal antibodies and polypeptides.