Convenient Synthesis of Multivalent Zinc(II)-Dipicolylamine Complexes for Molecular Recognition

An overview on the development of different optical sensing platforms for adenosine triphosphate (ATP) recognition

Adenosine triphosphate (ATP), one of the biological anions, plays a crucial role in several biological processes including energy transduction, cellular respiration, enzyme catalysis and signaling. ATP is a bioactive phosphate molecule, recognized as an important extracellular signaling agent. Apart from serving as a universal energy currency for various cellular events, ATP is also considered a factor responsible for numerous physiological activities. It regulates cellular metabolism by breaking phosphoanhydride bonds. Several diseases have been reported widely based on the levels and behavior of ATP. The variation of ATP concentration usually causes a foreseeable impact on mitochondrial physiological function. Mitochondrial dysfunction is responsible for the occurrence of many severe diseases such as angiocardiopathy, malignant tumors and Parkinson's disease. Therefore, there is high demand for developing a sensitive, fast-responsive, nontoxic and versatile detection platform for the detection of ATP. To this end, considerable efforts have been employed by several research groups throughout the world to develop specific and sensitive detection platforms to recognize ATP. Although a repertoire of optical chemosensors (both colorimetric and fluorescent) for ATP has been developed, many of them are not arrayed appropriately. Therefore, in this present review, we focused on the design and sensing strategy of some chemosensors including metal-free, metal-based, sequential sensors, aptamer-based sensors, nanoparticle-based sensors etc. for ATP recognition via diverse binding mechanisms.

Imaging Cell Death: Focus on Early Evaluation of Tumor Response to Therapy

Cell death plays a prominent role in the treatment of cancer, because most anticancer therapies act by the induction of cell death including apoptosis, necrosis, and other pathways of cell death. Imaging cell death helps to identify treatment responders from nonresponders and thus enables patient-tailored therapy, which will increase the likelihood of treatment response and ultimately lead to improved patient survival. By taking advantage of molecular probes that specifically target the biomarkers/biochemical processes of cell death, cell death imaging can be successfully achieved. In recent years, with the increased understanding of the molecular mechanism of cell death, a variety of well-defined biomarkers/biochemical processes of cell death have been identified. By targeting these established cell death biomarkers/biochemical processes, a set of molecular imaging probes have been developed and evaluated for early monitoring treatment response in tumors. In this review, we mainly present the recent advances in identifying useful biomarkers/biochemical processes for both apoptosis and necrosis imaging and in developing molecular imaging probes targeting these biomarkers/biochemical processes, with a focus on their application in early evaluation of tumor response to therapy.

Fluorescence Imaging of Interscapular Brown Adipose Tissue in Living Mice

Brown adipose tissue (BAT) plays a key role in energy expenditure and heat generation and is a promising target for diagnosing and treating obesity, diabetes and related metabolism disorders. While several nuclear and magnetic resonance imaging methods are established for detecting human BAT, there are no convenient protocols for high throughput imaging of BAT in small animal models. Here we disclose a simple but effective method for non-invasive optical imaging of interscapular BAT in mice using a micellar formulation of the commercially available deep-red fluorescent probe, SRFluor680. Whole-body fluorescence imaging of living mice shows extensive accumulation of the fluorescent probe in the interscapular BAT and ex vivo analysis shows 3.5-fold selectivity for interscapular BAT over interscapular WAT. Additional imaging studies indicate that SRFluor680 uptake is independent of mouse species and BAT metabolic state. The results are consistent with an unusual pharmacokinetic process that involves irreversible translocation of the lipophilic SRFluor680 from the micelle nanocarrier into the adipocytes within the BAT. Multimodal PET/CT and planar fluorescence/X-ray imaging of the same living animal shows co-localization of BAT mass signal reported by the fluorescent probe and BAT metabolism signal reported by the PET agent, ¹⁸F-FDG. The results indicate a path towards a new, dual probe molecular imaging paradigm that allows separate and independent non-invasive visualization of BAT mass and BAT metabolism in a living subject.

Selective recognition of anionic cell membranes using targeted liposomes coated with zinc(ii)-bis(dipicolylamine) affinity units

Zinc(ii)-bis(dipicolylamine) (Zn2BDPA) coated liposomes are shown to have high recognition selectivity towards vesicle and cell membranes with anionic surfaces. Robust synthetic methods were developed to produce Zn2BDPA-PEG-lipid conjugates with varying PEG linker chain length. One conjugate (Zn2BDPA-PEG2000-DSPE) was used in liposome formulations doped with the lipophilic near-infrared fluorophore DiR. Fluorescence cell microscopy studies demonstrated that the multivalent liposomes selectively and efficiently target bacteria in the presence of healthy mammalian cells and cause bacterial cell agglutination. The liposomes also exhibited selective staining of the surfaces of dead or dying human cancer cells that had been treated with a chemotherapeutic agent.

Library synthesis, screening, and discovery of modified Zinc(II)-Bis(dipicolylamine) probe for enhanced molecular imaging of cell death

Zinc(II)-bis(dipicolylamine) (Zn-BDPA) coordination complexes selectively target the surfaces of dead and dying mammalian cells, and they have promise as molecular probes for imaging cell death. A necessary step toward eventual clinical imaging applications is the development of next-generation Zn-BDPA complexes with enhanced affinity for the cell death membrane biomarker, phosphatidylserine (PS). This study employed an iterative cycle of library synthesis and screening, using a novel rapid equilibrium dialysis assay, to discover a modified Zn-BDPA structure with high and selective affinity for vesicles containing PS. The lead structure was converted into a deep-red fluorescent probe and its targeting and imaging performance was compared with an unmodified control Zn-BDPA probe. The evaluation process included a series of FRET-based vesicle titration studies, cell microscopy experiments, and rat tumor biodistribution measurements. In all cases, the modified probe exhibited comparatively higher affinity and selectivity for the target membranes of dead and dying cells. The results show that this next-generation deep-red fluorescent Zn-BDPA probe is well suited for preclinical molecular imaging of cell death in cell cultures and animal models. Furthermore, it should be possible to substitute the deep-red fluorophore with alternative reporter groups that enable clinically useful, deep-tissue imaging modalities, such as MRI and nuclear imaging.

Multivalent dendritic molecules as broad spectrum bacteria agglutination agents

This study reports the first set of synthetic molecules that act as broad spectrum agglutination agents and thus are complementary to the specific targeting of antibodies. The molecules have dendritic architecture and contain multiple copies of zinc(II)-dipicolylamine (ZnDPA) units that have selective affinity for the bacterial cell envelope. A series of molecular structures were evaluated, with the number of appended ZnDPA units ranging from four to thirty-two. Agglutination assays showed that the multivalent probes rapidly cross-linked ten different strains of bacteria, regardless of Gram-type and cell morphology. Fluorescence microscopy studies using probes with four ZnDPA units indicated a high selectivity for bacteria agglutination in the presence of mammalian cells and no measurable effect on the health of the cells. The high bacterial selectivity was confirmed by conducting in vivo optical imaging studies of a mouse leg infection model. The results suggest that multivalent ZnDPA molecular probes with dendritic structures have great promise as selective, broad spectrum bacterial agglutination agents for infection imaging and theranostic applications.

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.