Retention of technetium-99m in infectious foci in rats after release from technetium-99m labelled human non-specific polyclonal immunoglobulin G: A dual-label study with hydrazinonicotinamido and iminothiolano immunoglobulin

In an effort to contribute to the understanding of the mechanism of uptake of technetium-99m labelled non-specific polyclonal human immunoglobulin G (hIgG) in inflammatory lesions we compared the tissue distribution of double-labelled99mTc-hydrazinonicotinamido (HYNIC) hIgG-14C and99mTc-iminothiolano hIgG-14C in groups of five Wistar rats with aStaphylococcus aureus infection of the left calf muscle between 2 h p.i. and 24 h p.i. The stability of the two double-labelled hIgG preparations was evaluated in vitro and in plasma in vivo by high-performance liquid chromatography (HPLC) analysis. At 24 h after injection of99mTc-HYNIC-hIgG-14C the abscess uptake of99mTc (1.5% ID/g±0.2% ID/g) was significantly higher (P<0.01) than the14C uptake (1.0% ID/g±0.1% ID/g). After injection of99mTc-iminothiolano hIgG-14C no significant difference (P=0.08) was found between the abscess uptake of the two radionuclides at 24 h p.i. (99mTc: 0.8% ID/g±0.1% ID/g;14C: 0.90% ID/g±0.09% ID/g). HPLC analysis of plasma samples revealed release of99mTc from both double-labelled immunoglobulin preparations. This phenomenon was more pronounced for iminothiolano hIgG than for HYNIC hIgG (43% vs 18%). In most tissues other than abscesses significant differences were also found between the99mTc and the corresponding14C uptake. Our results demonstrate that the chemical form in which99mTc is bound to hIgG severely influences its release from hIgG and its retention in infections.