Dissociation of indium from indium-111-labelled diethylene triamine penta-acetic acid conjugated non-specific polyclonal human immunoglobulin G in inflammatory foci

Abstract

Several investigators have reported retention of indium-111 in infectious foci after intravenous injection of¹¹¹In-labelled immunoglobulin G (IgG). With this study we intended to test the hypothesis that, upon administration of¹¹¹In-diethylene triamine pentaacetic acid (DTPA-IgG),¹¹¹In is retained in the infectious foci after dissociation from IgG. Therefore we measured the tissue distribution of double-labelled¹¹¹In-DTPA-IgG-(carbon-14) in rats with a focal infection and compared the results with corresponding data for DTPAIgG-(¹⁴C). DTPA-conjugated IgG was labelled with¹¹¹In via citrate transchelation.¹¹¹In-DTPA-IgG and DTPA-IgG were labelled with¹⁴C through methylation. High-performance liquid chromatography (HPLC) and instant thin-layer chromatography analysis were performed to test the in vitro stability of the labelled proteins. Young Wistar rats with aStaphylococcus aureus infection of the left calf muscle were injected intravenously with 0.2 ml of a solution containing either 0.4 MBq¹¹¹In and 30 kBq¹⁴C or 30 kBq¹⁴C labelled to 80 μg IgG. Groups of five rats were sacrificed at 2, 6, 24, and 48 h. p.i. Activity uptake was determined for plasma, urine, abscess, muscle and various other tissues. Averages and standard deviations were calculated for groups of five rats. HPLC analysis was performed on plasma and urine samples taken up to 48 h p.i. The radiochemical purity of the IgG preparations was >95%. The labelled, preparations appeared stable in vitro. The 14C abscess activity decreased from 1.2% to 0.7% of the injected dose per gram (% I.D./g) between 2 and 48 h after injection and was linearly related to the¹⁴C plasma concentration. However, the¹¹¹In concentration in the infectious foci remained constant over time (1.0% I.D./g) despite a decreasing concentration of¹¹¹In in plasma. Labelling with¹⁴C did not influence the abscess uptake of¹¹¹In after administration of¹¹¹In-DTPA-IgG. On the other hand, conjugation with DTPA and labelling with In 111 did not influence the tissue distribution of¹⁴C-IgG either. Assuming that¹⁴C-IgG behaves like native IgG, our results strongly suggest that in abscesses 111In is released from IgG with local retention of the¹¹¹In. The dissociation of¹¹¹In from IgG provides a new explanation for retention of¹¹¹In in sites of inflammation. This phenomenon might also be relevant to the explanation of non-specific tumour uptake of monoclonal antibodies labelled with¹¹¹In through DTPA.