Synthesis and in vitro stability of I-123-labelled annexin V: A potential agent for spect imaging of apoptotic cells

Human recombinant annexin V was radiolabelIed with iodine-123 by electrophi lic substitution of the containing tyrosine residues using carrier-free sod ium iodide-123 as the isotope source. The synthesis of I-123-labelled annex in V was performed using IodoBeads as oxidative solid phase iodinating agen t and reaction parameters were optimized and evaluated. Purification by str aight-phase HPLC gave [I-123]-annexin V in st maximum yield of approximatel y 70 % with a radiochemical purity of more than 95 % and a specific activit y of about 10 MBq / mu g of protein. Furthermore, in vitro stability experi ments indicated that [I-123]-annexin V was stable for at least 8 hours in a n aqueous phosphate buffer solution. In plasma however it decreased more ra pidly. The biological activity of [I-123]-annexin V remained intact in comp arison with FITC labelled annexin V as confirmed by in vitro binding experi ments with apoptotic cells.