Development and Characterization of Annexin V Mutants with Endogenous Chelation Sites for 99mTc

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Abstract
[99mTc]Annexin V can be used to image organs undergoing cell death during cancer chemotherapy and organ transplant rejection. To simplify the preparation and labeling of annexin V for nuclear-medicine studies, we have investigated the addition of peptide sequences that will directly form endogenous chelation sites for 99mTc. Three mutant molecules of annexin V, called annexin V-116, -117, and -118, were constructed with N-terminal extensions of seven amino acids containing either one or two cysteine residues. These molecules were expressed cytoplasmically in Escherichiacoli and purified to homogeneity with a final yield of 10 mg of protein/L of culture. Analysis in a competitive binding assay showed that all three proteins retained full binding affinity for erythrocyte membranes with exposed phosphatidylserine. Using SnCl2 as reducing agent and glucoheptonate as exchange agent, all three proteins could be labeled with 99mTc to specific activities of at least 50−100 μCi/μg. The proteins retained membrane binding activity after the radiolabeling procedure, and quantitative analysis indicated a dissociation constant (Kd) of 7 nmol/L for the annexin V-117 mutant. The labeling reaction was rapid, reaching a maximum after 40 min at room temperature. The radiolabeled proteins were stable when incubated with phosphate-buffered saline or serum in vitro. Proteins labeled to a specific activity of 25−100 μCi/μg were injected intravenously in mice at a dose of 100 μg/kg, and biodistribution of radioactivity was determined at 60 min after injection. Uptake of radioactivity was highest in kidney and liver, consistent with previous results obtained with wild-type annexin V. Cyclophosphamide-induced apoptosis in vivo could be imaged with [99mTc]annexin V-117. In conclusion, annexin V can be modified near its N-terminus to incorporate sequences that form specific chelation sites for 99mTc without altering its high affinity for cell membranes. These annexin V derivatives may be useful for in vivo imaging of cell death.