LABELING AND STABILITY OF RADIOLABELED ANTIBODY FRAGMENTS BY A DIRECT TC-99M-LABELING METHOD

The in vitro labeling and stability of Tc-99m-labeled antibody Fab' fragments prepared by a direct labeling technique were evaluated. Eight antibody fragments derived from murine IgG1 (N = 5), IgG2a (N = 2) and IgG3 (N = 1) isotypes were labeled with a preformed Tc-99m-D-glucarate complex. No loss of radioactivity incorporation was observed for all the Tc-99m-labeled antibody fragments after 24 h incubation at 37-degrees-C. The Tc-99m-labeled antibody fragments (IgG1, N = 2; IgG2a, N = 2; IgG3, N = 1) were stable upon challenge with DTPA, EDTA or acidic pH. Furthermore, using the affinity chromatography technique, two of the Tc-99m-labeled antibody fragments displayed no loss of immunoreactivity after prolonged incubation in phosphate buffer up to 24 h at 37-degrees-C. The bonding between Tc-99m and antibody fragments was elucidated by challenging with a diamide ditholate (N2S2) compound. The Fab' with IgG2a isotype displayed tighter binding to Tc-99m in comparison to the Fab' from IgG1 and IgG3 isotype in N2S2 challenge and incubation with human plasma. The in vivo biodistribution of five Tc-99m-labeled fragments were evaluated in normal mice. In conclusion, the direct labeling method allows stable Tc-99m labeling of antibody fragments from three of the major murine isotypes.