In vivo detection of apoptotic cell death: a necessary measurement for evaluating therapy for myocarditis, ischemia, and heart failure.

If life is to continue, cells that have completed their useful function(s) must die in a timely manner. Apoptosis, programmed cell death, is a natural, orderly, energy-dependent process that causes cells to die without inducing an inflammatory response. In the heart, apoptosis plays pivotal roles in the development of myocarditis, cardiomyopathies, transplant rejection, the periinfarct zone in myocardial infarction, and reperfusion injury. Apoptosis is triggered either by a decrease in factors required to maintain the cell in good health or by an increase in factors which cause damage to the cell. When these factors tilt in the direction of death and the cell has sufficient time to respond, a common proteolytic cascade involving cysteine aspartic acid-specific proteases (caspases) is activated to initiate apoptosis. Cells that die by apoptosis autodigest their DNA and nuclear proteins, change the phospholipid composition on the outer surface of their cell membrane, and form lipid enclosed vesicles, which contain noxious intracellular contents, organelles, autodigested cytoplasm, and DNA. The compositional cell membrane phospholipid change that occurs with the onset of apoptosis is marked by the sudden expression of phosphatidylserine (PS), a phospholipid that ordinarily appears on the inner leaflet of the membrane, on the external leaflet of the membrane. The constant exposure of PS during apoptosis makes it an attractive target for radiopharmaceutical imaging. An endogenous human protein, annexin V, has a high affinity (kd = 7 nmol/L) for PS bound to the cell membrane. Fluorescence-labeled annexin V is used for histologic and cell-sorting studies to identify apoptotic cells. Annexin has been radiolabeled and binds to cells undergoing apoptosis in vivo. This review outlines some of the key features of apoptosis as contrasted to necrosis (unregulated cell death) and describes how these processes can be imaged with radionuclide techniques.