Programmed cell death (PCD) is essential for physiological homeostasis and contributes to various pathologies. Numerous drugs, including cytostatics, modulate PCD. Consequently, in vivo detection and quantification of PCD can provide valuable information on disease progression and/or treatment efficacy.
One of the earliest events during cell death is the externalization of phosphatidylserine (PS) on the cell membrane, which persists until cell clearance. Annexin A5 binds PS with high affinity on dying cells, a property largely retained after fluorescent or radiolabeling. This enabled the development of fluorescent and radiolabeled annexin A5 for molecular imaging, which has been evaluated in animal models.
Annexin A5’s role as a cell death detection agent was discovered serendipitously. Initially characterized as an anticoagulant due to its affinity for PS on platelets, annexin A5 was later found to bind PS on apoptotic cells—identified by Fadok et al.—signaling phagocytosis. Subsequent studies confirmed annexin A5’s high-affinity, specific binding to externalized PS on apoptotic cells. The first patient imaging using technetium-labeled annexin A5 was conducted at Maastricht University Hospital, enabling visualization of myocardial ischemia and reperfusion injury. However, existing technetium-labeled probes proved unsuitable for imaging the abdomen, restricting the assessment of tumors in organs such as the colon, liver, and kidney. Additionally, the quality of annexin preparations could not be reliably assessed.
This thesis aims to better understand and improve annexin A5-based nuclear imaging protocols. The advent of new labeling technologies necessitated the use of three different annexin A5-99mTc conjugation methods. The first part of this thesis focuses on quality control and the biological functionality of annexin A5 preparations. The second part addresses imaging challenges and biodistribution limitations of early technetium-labeled annexin probes. The final research chapter demonstrates the use of 99mTc-annexin A5 as a tool for treatment response assessment.