Straightforward thiol-mediated protein labelling with DTPA: Synthesis of a highly active 111In-annexin A5-DTPA tracer

BACKGROUND Annexin A5 (anxA5) has been found useful for molecular imaging of apoptosis and other biological processes. METHODS Here, we report an optimised two-step synthesis of annexin A5-diethylene triamine pentaacetic acid (DTPA) (anxA5-DTPA) for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging with a single purification step. The use of a recombinant annexin A5 (cys-anxA5) with a single thiol group allowed regionally specific coupling, without affecting the binding domain of cys-anxA5. RESULTS The metal complexing capacity of anxA5-DTPA was investigated by labelling with 111In3+ and Eu3+. Binding of modified anxA5-DTPA to apoptotic cells was tested in competition experiments with a fluorescent anxA5 derivative (anxA5-FITC) using flow cytometry and compared with that of wildtype anxA5 or non-binding anxA5-DTPA (M1234-anxA5-DTPA). The binding affinity to apoptotic cells of the anxA5-DTPA conjugate does not differ from that of wildtype anxA5. CONCLUSIONS This two-step synthesis of annexin A5-DTPA resulted in biologically active anxA5-DTPA, which can be labelled with radionuclides for use in SPECT and PET imaging.