Phenoxide-Bridged Zinc(II)-Bis(dipicolylamine) Probes for Molecular Imaging of Cell Death

Abstract

Cell death is involved in many pathological conditions, and there is a need for clinical and preclinical imaging agents that can target and report cell death. One of the best known biomarkers of cell death is exposure of the anionic phospholipid phosphatidylserine (PS) on the surface of dead and dying cells. Synthetic zinc(II)-bis(dipicolylamine) (Zn2BDPA) coordination complexes are known to selectively recognize PS-rich membranes and act as cell death molecular imaging agents. However, there is a need to improve in vivo imaging performance by selectively increasing target affinity and decreasing off-target accumulation. This present study compared the cell death targeting ability of two new deep-red fluorescent probes containing phenoxide-bridged Zn2BDPA complexes. One probe was a bivalent version of the other, and associated more strongly with PS-rich liposome membranes. But, the bivalent probe exhibited self-quenching on the membrane surface, so the monovalent version produced brighter micrographs of dead and dying cells in cell culture and also better fluorescence imaging contrast in two living animal models of cell death (rat implanted tumor with necrotic core and mouse thymus atrophy). An 111In-labelled radiotracer version of the monovalent probe also exhibited selective cell death targeting ability in the mouse thymus atrophy model, with relatively high amounts in dead and dying tissue and low off-target accumulation in non-clearance organs. The in vivo biodistribution profile is the most favorable yet reported for a Zn2BDPA complex and thus the monovalent phenoxide-bridged Zn2BDPA scaffold is a promising candidate for further development as a cell death imaging agent in living subjects.

Preassembled Fluorescent Multivalent Probes for the Imaging of Anionic Membranes

“…We have observed this surface enhanced probe self-quenching phenomenon before with other multivalent near-infrared fluorescent ZnDPA probes. 16 Probe self-aggregation on the membrane surface is triggered by electrostatic neutralization of the cationic ZnDPA units when they associate with the anionic POPS. The amount of self-aggregated probe on the membrane surface increases until the liposome surface is saturated with probe.…”

“…However, the effect is countered by probe self-aggregation on the surface which promotes quenching and lowers the fluorescence signal intensity. 16 …”

Small molecule additive enhances cell uptake of 5-aminolevulinic acid and conversion to protoporphyrin IX

“…The ZnBDPA compounds 1 and 2 were synthesized as described previously. 36 All other reagents and chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification.…”

Imaging and therapeutic applications of zinc(ii)-dipicolylamine molecular probes for anionic biomembranes

“…Several of our studies have noted self-aggregation of fluorescent ZnDPA probes on the membrane surface and decreased emission intensity due to an aggregation-induced quenching effect. 105, 138 Recently, a new fluorescent ZnDPA probe with ‘aggregation-induced emission’ was reported by Hu and coworkers for apoptosis detection. 139 The deep-red probe is weakly emissive in aqueous media but turns on when it associates with the surfaces of early apoptotic mammalian cells.…”