Apoptosis (programmed cell death) plays an important role in the pathophysiology of many diseases, such as cancer, neurodegenerative disorders, vascular disorders, and chronic hepatitis, as well as in the biology of normal cells, such as epithelial cells and immune cells (1). Apoptosis is gene-regulated (2) and is the result of proteolysis of intracellular components by activation of a series of proteolytic enzymes called caspases and changes of plasma membrane structure by translocase, floppase, and scramblase (3-5). As a result, there is rapid redistribution of phosphatidylserine (PS) from the inner membrane leaflet to the outer membrane leaflet, exposing the anionic head group of PS. On the other hand, PS is also accessible for annexin V binding in necrosis because of disruption of the plasma membrane.
Annexin V is a 36-kDa endogenous human protein produced in particular by epithelial cells from many tissues, such as the placenta, umbilical vessels, liver, spleen, kidney, heart, uterus, and skeletal muscle, as well as by erythrocytes, leukocytes, endothelial cells, and platelets (6). Annexin V binds to PS with high affinity (Kd = 7 nM) (3, 7, 8). Apoptosis can be induced by chemicals, radiation, cytokines, hormones, and various pathological conditions (5); therefore, the ability to monitor apoptosis in association with disease progression or regression should provide important information for clinical applications. Annexin V has been radiolabeled with 18F, 123I, 125I, and 99mTc for imaging (9-12). However, annexin V is not able to distinguish between apoptosis and necrosis. However, PS is expressed in both cell necrosis and apoptosis and therefore cannot distinguish between the two, whereas caspase 3/7 is only activated in apoptosis. Caspase substrate may be an attractive alternative for imaging cells undergoing apoptosis because caspases are key enzymes that mediate apoptosis (13). Chu et al. (14) reported that isatin sulfonamide analogs exhibited selective inhibition of caspase-3 and caspase-7 (executioner caspases) over caspase-1, -6, and -8 (initiator caspases). 1-[4-(2-[18F]Fluoroethoxy)-benzyl]-5-(2-phenoxymethyl-pyrrolidine-1-sulfonyl)-1H-indole-2,3-dione ([18F]WC-II-89) showed higher uptake in the liver and spleen in cycloheximide-treated rats when compared with untreated control rats (15). (S)-1-(4–2-[11C]Methoxybenzyl)-5-(2-phenoxymethyl-pyrrolidine-1-sulfonyl)-1H-indole-2,3-dione ([11C]WC-98), an analog of [18F]WC-II-89, has been synthesized for imaging caspase-3 activation in apoptosis (16).