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-Zn ₂BDPA-SR accumulation at the site of cell death than bis-Zn ₂BDPA-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.