A dual-labeled Annexin A5 is not suited for SPECT imaging of brain cell death in experimental murine stroke

Cell death is one of the pathophysiological hallmarks after stroke. Markers to image cell death pathways in vivo are highly desirable. We previously showed that fluorescently labeled Annexin A5 (AnxA5), which binds specifically to phosphatidylserine (PS) on dead/dying cells, can be used in experimental stroke for monitoring cell death with optical imaging. Here we investigated whether dual-labeled AnxA5 (technetium and fluorescence label) can be used for single-photon emission computed tomography (SPECT) of cell death in the same model. C57Bl6/N mice were subjected to 60-minute middle cerebral artery occlusion (MCAO) and underwent SPECT imaging at 24, 48, and 72 hours afterwards. They were injected intravenously with either PS-binding AnxA5 or the nonfunctional AnxA5 (negative control), labeled with 99mTc and Alexa Fluor 568, respectively. After SPECT imaging, brain sections were cut for autoradiography and fluorescence microscopy. Ethanol-induced cell death in the femur muscle was used as positive control. We detected dual-labeled AnxA5 in the model of ethanol-induced cell death in the femur muscle, but not after MCAO at any time point, either with SPECT or with ex vivo autoradiography or fluorescence microscopy. Dual-labeled AnxA5 appears to be unsuited for visualizing death of brain cells in this MCAO model.