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

Abstract The in vitro labeling and stability of 99m Tc-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 99m Tc- d -glucarate complex. No loss of radioactivity incorporation was observed for all the 99m Tc-labeled antibody fragments after 24 h incubation at 37 °C. The 99m Tc-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 99m Tc-labeled antibody fragments displayed no loss of immunoreactivity after prolonged incubation in phosphate buffer up to 24 h at 37 °C. The bonding between 99m Tc and antibody fragments was elucidated by challenging with a diamide ditholate (N 2 S 2 ) compound. The Fab′ with IgG2a isotype displayed tighter binding to 99m Tc in comparison to the Fab′ from IgG1 and IgG3 isotype in N 2 S 2 challenge and incubation with human plasma. The in vivo biodistribution of five 99m Tc-labeled fragments were evaluated in normal mice. In conclusion, the direct labeling method allows stable 99m Tc labeling of antibody fragments from three of the major murine isotypes.