A novel method for labelling human immunoglobulin-G with 99Tcm suitable for inflammation scintigraphy

An amount of 1.0 mg human immunoglobulin G (hIgG) treated with ascorbic acid at a molar ratio of 1:5000 for 16 h at 4-7 degrees C was mixed with 250 micrograms GHA and 5 micrograms stannous chloride dihydrate in normal saline. Radiolabelling of hIgG (> 98%) was achieved instantly when mixed with 99Tcm-pertechnetate. The preparation was sufficiently stable in serum at 37 degrees C. The competitive binding assay and gel electrophoresis of the native and reduced hIgG did not show any measurable loss in immunoreactivity and intactness due to its reduction. There was no significant decrease in the radiolabel of labelled hIgG when incubated with diethylenetriaminepentaacetate (DTPA) (50-fold), in contrast with about 9% loss of the radiolabel by treating it with a similar concentration of cysteine. Blood clearance of labelled hIgG in rabbits was biphasic with about 78 min and 8 h as T1/2 of the fast and slow phases. Biodistribution of the radiotracer in mice at 4 h showed its uptake by liver (10.2%), kidneys (5.39%), intestines (7.33%) and muscles (3.9%), which altered to 5.63, 3.20, 4.03 and 6.73%, respectively, at 24 h. The radiotracer was excreted through both renal and hepatobiliary routes. High accumulation of the radiolabelled hIgG in inflammatory lesions of the patients confirmed the clinical usefulness of the method developed.