Labeling and stability of radiolabeled antibody fragments by a direct 99mTc-labeling method.

The in vitro labeling and stability of 99mTc-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 99mTc-D-glucarate complex. No loss of radioactivity incorporation was observed for all the 99mTc-labeled antibody fragments after 24 h incubation at 37 degrees C. The 99mTc-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 99mTc-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 99mTc and antibody fragments was elucidated by challenging with a diamide ditholate (N2S2) compound. The Fab' with IgG2a isotype displayed tighter binding to 99mTc in comparison to the Fab' from IgG1 and IgG3 isotype in N2S2 challenge and incubation with human plasma. The in vivo biodistribution of five 99mTc-labeled fragments were evaluated in normal mice. In conclusion, the direct labeling method allows stable 99mTc labeling of antibody fragments from three of the major murine isotypes.