Enhanced Cell Death Imaging Using Multivalent Zinc(II)-bis(dipicolylamine) Fluorescent Probes

There is a clinical need for imaging technologies that can accurately detect cell death in a multitude of pathological conditions. Zinc(II)-bis(dipicolylamine) (Zn2BDPA) coordination complexes are known to associate with the anionic phosphatidylserine that is exposed on the surface of dead and dying cells, and fluorescent monovalent Zn2BDPA probes are successful cell death imaging agents. This present study compared the membrane targeting ability of two structurally related deep-red fluorescent probes, bis-Zn2BDPA-SR and tetra-Zn2BDPA-SR, with two and four appended Zn2BDPA units, respectively. Vesicle and cell microscopy studies indicated that a higher number of Zn2BDPA targeting units improved probe selectivity for phosphatidylserine-rich vesicles, and increased probe localization at the plasma membrane of dead and dying cells. The fluorescent probes were also tested in three separate animal models, (1) necrotic prostate tumor rat model, (2) thymus atrophy mouse model, and (3) traumatic brain injury mouse model. In each case, there was more tetra-Zn2BDPA-SR accumulation at the site of cell death than bis-Zn2BDPA-SR. The results indicate that multivalent Zn2BDPA probes are promising molecules for effective imaging of cell death processes in cell culture and in living subjects.