Noninvasive molecular imaging of cell death in myocardial infarction using 111In-GSAO.

Acute insult to the myocardium is associated with substantial loss of cardiomyocytes during the process of myocardial infarction. In this setting, apoptosis (programmed cell death) and necrosis may operate on a continuum. Because the latter is characterized by the loss of sarcolemmal integrity, we propose that an appropriately labeled tracer directed at a ubiquitously present intracellular moiety would allow non-invasive definition of cardiomyocyte necrosis. A trivalent arsenic peptide, GSAO (4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid), is capable of binding to intracellular dithiol molecules such as HSP90 and filamin-A. Since GSAO is membrane impermeable and dithiol molecules abundantly present intracellularly, we propose that myocardial localization would represent sarcolemmal disruption or necrotic cell death. In rabbit and mouse models of myocardial infarction and post-infarct heart failure, we employed In-111-labelled GSAO for noninvasive radionuclide molecular imaging. (111)In-GSAO uptake was observed within the regions of apoptosis seeking agent- (99m)Tc-Annexin A5 uptake, suggesting the colocalization of apoptotic and necrotic cell death processes.

Noninvasive imaging of cell death using an Hsp90 ligand.

Cell death plays a central role in normal physiology and in disease. Common to apoptotic and necrotic cell death is the eventual loss of plasma membrane integrity. We have produced a small organoarsenical compound, 4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid, that rapidly accumulates in the cytosol of dying cells coincident with loss of plasma membrane integrity. The compound is retained in the cytosol predominantly by covalent reaction with the 90 kDa heat shock protein (Hsp90), the most abundant molecular chaperone of the eukaryotic cytoplasm. The organoarsenical was tagged with either optical or radioisotope reporting groups to image cell death in cultured cells and in murine tumors ex vivo and in situ. Tumor cell death in mice was noninvasively imaged by SPECT/CT using an (111)In-tagged compound. This versatile compound should enable the imaging of cell death in most experimental settings.

Fluorescence-enhanced europium-diethylenetriaminepentaacetic (DTPA)-monoamide complexes for the assessment of renal function.

Real-time, noninvasive assessment of glomerular filtration rate (GFR) is essential not only for monitoring critically ill patients at the bedside, but also for staging and monitoring patients with chronic kidney disease. In our pursuit to develop exogenous luminescent probes for dynamic optical monitoring of GFR, we have prepared and evaluated Eu(3+) complexes of several diethylenetriamine pentaacetate (DTPA)-monoamide ligands bearing molecular "antennae" to enhance metal fluorescence via intramolecular ligand-metal fluorescence resonance energy transfer process. The results show that Eu-DTPA-monoamide complex 18b, which contains a quinoxanlinyl antenna, exhibits large (ca. 2700-fold) Eu(3+) fluorescence enhancement. Indeed, complex 18b exhibits the highest fluorescent enhancement observed thus far in the DTPA-type metal complexes. The renal clearance property was assessed using the corresponding radioactive (111)In complex 18a, and the data suggest that this complex clears via a complex mechanism that includes glomerular filtration.

Characterization of novel histidine-tagged Tat-peptide complexes dual-labeled with (99m)Tc-tricarbonyl and fluorescein for scintigraphy and fluorescence microscopy.

To enable concurrent whole body scintigraphy and direct imaging of subcellular localization of permeation peptides, dual-labeled Tat-peptides useful for both radiometric analysis and fluorescence microscopy are desired for molecular imaging applications. Thus, novel dual-labeled D-Tat-peptides comprising Tat-basic domain (hgrkkrrqrrrgc), C-terminus conjugated with fluorescein-5-maleimide (FM) and N-terminus chelated with [(99m)Tc(CO)(3)] via histidine coordination, were synthesized and characterized. In human Jurkat cells, radiotracer uptake and washout studies revealed concentration-dependent accumulation of the dual-labeled Tat-peptide within cells. Subcellular localization of Tat-peptide was confirmed by fluorescence microscopy using an analogous [Re(CO)(3)] dual-labeled Tat-peptide. As seen with C-terminus single-labeled Tat-peptides, localization to the nucleoli was observed with the dual-labeled Tat-peptide, suggesting that the mechanism of Tat-peptide uptake and localization was not dependent on free peptide termini at either end. In Balb/c mice, biodistribution studies performed with the dual-labeled Tat-peptide showed fluorescence intensity by microscopic analysis that visually confirmed and correlated directly with scintigraphic and radiometric data. Of note, following intravenous administration, little brain penetration of these permeation sequences was observed in vivo. His[(99m)Tc(CO)(3)]-, DTPA[(99m)Tc(CO)(3)]-, and epsilon-lys-gly-cys[(99m)Tc(O)]-labeled Tat-peptides showed significant pharmacokinetic differences in liver and kidney depending on labeling strategy, indicating that Tat-peptide biodistribution can be impacted by the chelation moiety coordinated with (99m)Tc. Thus, we have shown that dual-labeled (99m)Tc-tricarbonyl Tat-peptide-FM conjugates can be conveniently synthesized and enable direct comparison of quantitative radiometric and qualitative fluorescence data both in vitro as well as in vivo.

Evidence for a plasma membrane-mediated permeability barrier to Tat basic domain in well-differentiated epithelial cells: lack of correlation with heparan sulfate.

Membrane permeation peptides, such as Tat basic domain, have emerged as useful membrane transduction agents with potential utility in therapeutic delivery and diagnostic imaging. While generally thought to universally permeate all cells by a nonselective process, the mechanism of membrane transduction remains poorly characterized. To examine vectorial transport properties of Tat basic domain in well-differentiated epithelial cells possessing tight junctions, L and D stereoisomers of Tat(48-57) peptide conjugates labeled with (99m)Tc were quantitatively analyzed in confluent monolayers of MDCK renal epithelial and CaCo-2 colonic carcinoma cells grown in transwell configurations. In both cell lines, vectorial transepithelial apparent permeability coefficients (P(app)) for L- and D-[(99m)Tc]Tat-peptides ranged from 30 to 70 nm/s, comparable to values for the macromolecular impermeant marker inulin in both apical-to-basolateral and basolateral-to-apical directions, but 100-fold less than the P(app) values for propranolol, a highly permeable control compound. Upon direct instillation of [(99m)Tc]Tat-peptide into the urinary bladder of living rats in vivo, no transepithelial permeation into other tissues was identified. Furthermore, MDCK and CaCo-2 cells showed a complete lack of intracellular accumulation of fluorescein conjugated Tat-peptide. However, translocation into cells was induced by treatment with plasma membrane permeabilizing agents such as digitonin and acetone/methanol, while cholesterol depletion with beta-methyl-cyclodextrin and metabolic inhibition with CCCP or 4 degrees C showed no effect. By contrast, in Hela and KB 3-1 cells, epithelial lines that do not form tight junctions in monolayer culture, baseline cytoplasmic and nucleolar accumulation was readily observed. Because all four cell lines expressed heparan sulfate proteoglycans, putative receptors for Tat basic peptides, we found no correlation between heparan sulfate and the permeation barrier observed in MDCK and CaCo-2 cells. The unanticipated presence of a permeation barrier to Tat-peptides in well-differentiated epithelial cells suggests the existence of cell-specific mechanisms for mediated translocation of these permeation peptides.

Characterization of a novel 99mTc-carbonyl complex as a functional probe of MDR1 P-glycoprotein transport activity.

Multidrug resistance (MDR) mediated by overexpression of MDR1 P-glycoprotein (Pgp) is one of the best characterized barriers to chemotherapy in cancer patients. Furthermore, the protective function of Pgp-mediated efflux of xenobiotics in various organs has a profound effect on the bioavailability of drugs in general. Thus, there is an expanding requirement to noninvasively interrogate Pgp transport activity in vivo. We herein report the Pgp recognition properties of a novel 99mTc(I)-tricarbonyl complex, [99mTc(CO)3(MIBI)3]+ (Tc-CO-MIBI). Tc-CO-MIBI showed 60-fold higher accumulation in drug-sensitive KB 3-1 cells compared to colchicine-selected drug-resistant KB 8-5 cells. In KB 8-5 cells, tracer enhancement was observed with the potent MDR modulator LY335979 (EC50 = 62 nM). Similar behavior was observed using drug-sensitive MCF-7 breast adenocarcinoma cells and MCF-7/MDR1 stable transfectants, confirming that Tc-CO-MIBI is specifically excluded by overexpression of MDR1 Pgp. By comparison, net accumulation in control H69 lung tumor cells was 9-fold higher than in MDR-associated protein (MRP1)-expressing H69AR cells, indicating only modest transport by MRP1. Biodistribution analysis following tail vein injection of Tc-CO-MIBI showed delayed liver clearance as well as enhanced brain uptake and retention in mdr1a/1b(-/-) gene deleted mice versus wild-type mice, directly demonstrating that Tc-CO-MIBI is a functional probe of Pgp transport activity in vivo.