18F-Labeled wild-type annexin V: comparison of random and site-selective radiolabeling methods.

Early stage apoptosis is characterized by the externalization of phosphatidylserine (PS) from the inner leaflet of the plasma membrane to the outer periphery. Consequently, PS represents an excellent target for non-invasive imaging of apoptosis by positron emission tomography. Annexin V is a 36 kDa protein which binds with high affinity to PS. Radiolabeling of wild-type annexin V with fluorine-18 ((18)F) can be accomplished via random acylation of 23 amine groups (22 lysine residues and one N-terminal amine) with [(18)F]SFB or site-specific alkylation reaction on cysteine residue at position 315 with maleimide-containing prosthetic groups like [(18)F]FBEM. The effect upon random and site-directed (18)F labeling of annexin V was studied with EL4 mouse lymphoma cells. Both, randomly and site-selectively radiolabeled annexin V demonstrated comparable binding to apoptotic EL4 cells. This finding suggests that the (18)F radiolabeling method has no significant effect on the ability of (18)F-labeled wild-type annexin V to bind PS in apoptotic cells.