Annexin A5: an imaging biomarker of cardiovascular risk

Apoptosis, a form of programmed cell death (PCD), plays an important role in the initiation and progression of a number of cardiovascular disease, such as heart failure, myocardial infarction, and atherosclerosis. One of the most prominent characteristics of apoptosis is the externalisation of phosphatidylserine (PS), a plasma cell membrane phospholipid, which in healthy cells only is present on the inner leaflet of the plasma cell membrane. Annexin A5, a 35 kD plasma protein, has strong affinity for PS in the nano-molar range. Through the coupling of Annexin A5 to contrast agents, visualization of apoptotic cell death in vivo in animal models and in patients has become feasible. These imaging studies have provided novel insight into the extent and kinetics of apoptosis in cardiovascular disease. Furthermore, Annexin A5 imaging has proven to be a suitable imaging biomarker for the evaluation of cell death modifying compounds and plaque stabilizing strategies. Recent insight in PS biology has shown that PS externalisation not only occurs in apoptosis, but is also observed in activated macrophages and stressed cells. In addition, it has been shown that Annexin A5 not only binds to exteriorized PS, but is also internalized through an Annexin A5 specific mechanism. These latter findings indicate that Annexin A5 imaging is not exclusively valuable for apoptosis detection, but can also be used to visualize inflammation and cell stress. This will open novel opportunities for imaging and drug delivery strategies. In this review we will discuss the introduction of Annexin A5 in preclinical and clinical imaging studies and provide an outlook on novel opportunities of Annexin A5 based targeting of PS.