A low molecular weight zinc2+-dipicolylamine-based probe detects apoptosis during tumour treatment better than an annexin V-based probe

Molecular Imaging of Apoptosis in Atherosclerosis by Targeting Cell Membrane Phospholipid Asymmetry

BACKGROUND

Apoptosis in atherosclerotic lesions contributes to plaque vulnerability by lipid core enlargement and fibrous cap attenuation. Apoptosis is associated with exteriorization of phosphatidylserine (PS) and phosphatidylethanolamine (PE) on the cell membrane. Although PS-avid radiolabeled annexin-V has been employed for molecular imaging of high-risk plaques, PE-targeted imaging in atherosclerosis has not been studied.

OBJECTIVES

This study sought to evaluate the feasibility of molecular imaging with PE-avid radiolabeled duramycin in experimental atherosclerotic lesions in a rabbit model and compare duramycin targeting with radiolabeled annexin-V.

METHODS

Of the 27 rabbits, 21 were fed high-cholesterol, high-fat diet for 16 weeks. Nine of the 21 rabbits received ^99mTc-duramycin (test group), 6 received ^99mTc-linear duramycin (duramycin without PE-binding capability, negative radiotracer control group), and 6 received ^99mTc-annexin-V for radionuclide imaging. The remaining normal chow-fed 6 animals (disease control group) received ^99mTc-duramycin. In vivo microSPECT/microCT imaging was performed, and the aortas were explanted for ex vivo imaging and for histological characterization of atherosclerosis.

RESULTS

A significantly higher duramycin uptake was observed in the test group compared with that of disease control and negative radiotracer control animals; duramycin uptake was also significantly higher than the annexin-V uptake. Quantitative duramycin uptake, represented as the square root of percent injected dose per cm (√ID/cm) of abdominal aorta was >2-fold higher in atherosclerotic lesions in test group (0.08 ± 0.01%) than in comparable regions of disease control animals (0.039 ± 0.0061%, p = 3.70·10^-8). Mean annexin uptake (0.060 ± 0.010%) was significantly lower than duramycin (p = 0.001). Duramycin uptake corresponded to the lesion severity and macrophage burden. The radiation burden to the kidneys was substantially lower with duramycin (0.49% ID/g) than annexin (5.48% ID/g; p = 4.00·10^-4).

CONCLUSIONS

Radiolabeled duramycin localizes in lipid-rich areas with high concentration of apoptotic macrophages in the experimental atherosclerosis model. Duramycin uptake in atherosclerotic lesions was significantly greater than annexin-V uptake and produced significantly lower radiation burden to nontarget organs.

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.

Targeting phosphatidylethanolamine and phosphatidylserine for imaging apoptosis in cancer

INTRODUCTION

Both phosphatidylethanolamine (PE) and phosphatidylserine (PS) can be externalized to the outer cell membrane in apoptosis. Thus the objective was to determine whether PE-targeting ¹⁸F-duramycin and PS-targeting ¹⁸F-Zn-DPA could be used for imaging apoptosis.

METHODS

Duramycin and Zn-DPA were labeled with either ¹⁸F-Al or ¹⁸F-SFB. U937 cells were incubated with four different concentrations of camptothecin (CPT). For assessing the effect of incubation time on uptake, 37 MBq of radiotracer was added to cells incubated for 15, 30, 60, and 120 min at 37 °C. For blocking experiments, 150 μg duramycin and 40 μg Zn-DPA were added to cells for 15 min prior to the addition of either duramycin or Zn-DPA labeled with ¹⁸F. Apoptosis was measured by flow cytometry using an annexin-V/PI kit. Cells were co-stained with Hoechst, Cy5-duramycin, and PSVue480 (FITC-Zn-DPA) to localize fluorescent dye uptake in cells.

RESULTS

Apoptosis in cells increased proportionally with CTP as confirmed by both flow cytometry and fluorescent staining. Both FITC-Zn-DPA and FITC-duramycin localized mainly on the cell membrane during early apoptosis and then translocated to the inside during late apoptosis. Uptake of FITC-duramycin, however, was higher than that of FITC-Zn-DPA. Cellular uptake of four different radiotracers was also proportional to the degree of apoptosis, increasing slightly over time and reaching a plateau at about 1 h. The blocking experiments demonstrated that uptake in all the control groups was predominantly non-specific, whereas the specific uptake in all the treated groups was at least 50% for both ¹⁸F labeled duramycin and Zn-DPA.

CONCLUSION

Both PE-targeting ¹⁸F-duramycin and PS-targeting ¹⁸F-Zn-DPA could be considered as potential radiotracers for imaging cellular apoptosis. Advances in knowledge and implications for patient care: Cellular data support the further development of radiotracers targeting either PE or PS for imaging apoptosis, which can associate with clinical outcome for cancer patients.

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

This feature article describes the development of synthetic zinc(ii)-dipicolylamine (ZnDPA) receptors as selective targeting agents for anionic membranes in cell culture and living subjects. There is a strong connection between anionic cell surface charge and disease, and ZnDPA probes have been employed extensively for molecular imaging and targeted therapeutics. Fluorescence and nuclear imaging applications include detection of diseases such as cancer, neurodegeneration, arthritis, and microbial infection, and also quantification of cell death caused by therapy. Therapeutic applications include selective targeting of cytotoxic agents and drug delivery systems, photodynamic inactivation, and modulation of the immune system. The article concludes with a summary of expected future directions.

Non-invasive in vivo imaging of arthritis in a collagen-induced murine model with phosphatidylserine-binding near-infrared (NIR) dye

Introduction

Development of non-invasive molecular imaging techniques that are based on cellular changes in inflammation has been of active interest for arthritis diagnosis. This technology will allow real-time detection of tissue damage and facilitate earlier treatment of the disease, thus representing an improvement over X-rays, which detect bone damage at the advanced stage. Tracing apoptosis, an event occurring in inflammation, has been a strategy used. PSVue 794 is a low-molecular-weight, near-infrared (NIR)-emitting complex of bis(zinc²⁺-dipicolylamine) (Zn-DPA) that binds to phosphatidylserine (PS), a plasma membrane anionic phospholipid that becomes flipped externally upon cell death by apoptosis. In this study, we evaluated the capacity of PSVue 794 to act as an in vivo probe for non-invasive molecular imaging assessment of rheumatoid arthritis (RA) via metabolic function in murine collagen-induced arthritis, a widely adopted animal model for RA.

Methods

Male DBA/1 strain mice were treated twice with chicken collagen type II in Freund’s adjuvant. Their arthritis development was determined by measuring footpad thickness and confirmed with X-ray analysis and histology. In vivo imaging was performed with the NIR dye and the LI-COR Odyssey Image System. The level of emission was compared among mice with different disease severity, non-arthritic mice and arthritic mice injected with a control dye without the Zn-DPA targeting moiety.

Results

Fluorescent emission correlated reliably with the degree of footpad swelling and the manifestation of arthritis. Ex vivo examination showed emission was from the joint. Specificity of binding was confirmed by the lack of emission when arthritic mice were given the control dye. Furthermore, the PS-binding protein annexin V displaced the NIR dye from binding, and the difference in emission was numerically measurable on a scale.

Conclusions

This report introduces an economical alternative method for assessing arthritis non-invasively in murine models. Inflammation in feet and ankles can be measured longitudinally using the PSVue 794 probe for cell death and with a commonly available multipurpose imager. This technique provides metabolic and functional information that anatomical measurement of footpad swelling or visual determination of arthritic index cannot. It also may decrease the number of animals required per experiment because tissue damage will not necessarily require evaluation by harvesting joints for histology.

Bis(zinc-dipicolylamine), Zn-DPA, a new marker for apoptosis

PURPOSE

To characterize the labeling of apoptotic cells with a molecular probe of bis(zinc(II)-dipicolylamine) (Zn-DPA) conjugated with a fluorescent reporter in a rat model of retinal ganglion cell (RGC) degeneration induced by N-methyl-D-aspartate (NMDA).

METHODS

Adult Wistar rats were given unilateral intravitreal injections of 3 μL 40 mM neutralized NMDA and euthanized at 1, 2, 4, 24, and 48 hours. One hour before euthanasia, 3 μL Zn-DPA conjugated with fluorescein (Zn-DPA 480) was intravitreally injected. Prelabeling of RGC with retrograde fluorogold (FG), TUNEL, and immunohistochemistry with III β-tubulin and vimentin were performed.

RESULTS

Fluorescence labeling of Zn-DPA 480 was observed in the retinas from 1 hour up to 24 hours after NMDA injection, whereas the labeling was reduced at 48 hours postinjection. At both 4 and 24 hours postinjection, most Zn-DPA 480-positive cells in the RGC layer were labeled by FG and III β-tubulin. The number of TUNEL-positive cells increased from 4 to 24 hours. At 24 hours, 95.7% of Zn-DPA 480-positive cells were TUNEL positive, whereas 95.1% of TUNEL-positive cells were Zn-DPA 480 positive. The numbers of Zn-DPA 480-positive cells at 1 and 2 hours after NMDA injection were significantly higher than TUNEL.

CONCLUSIONS

Our findings demonstrate that intravitreal injection of fluorescent Zn-DPA 480 labels retinal neurons undergoing apoptosis and that recognition of exposed phosphatidylserine appears earlier than detection of DNA fragmentation, indicating the potential of Zn-DPA as an imaging probe for tracking degenerating retinal neurons.

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.