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Biomedical subjects

R Weissleder

Publications and source records attributed to R Weissleder.

At least 73 records · Page 4Linked to original sources
PubMed1998

Continuous assessment of perfusion by tagging including volume and water extraction (CAPTIVE): a steady-state contrast agent technique for measuring blood flow, relative blood volume fraction, and the water extraction fraction.

A new technique, CAPTIVE, that is a synthesis of arterial spin labeling (ASL) blood flow and steady-state susceptibility contrast relative blood volume imaging is described. Using a single injection of a novel, long half-life intravascular magnetopharmaceutical with a high tissue:blood susceptibility difference (deltachi) to deltaR1 ratio, changes in tissue transverse relaxivity (deltaR2 or deltaR2*) that arise from changes in blood volume were measured, while preserving the ability to measure blood flow using traditional T1-based ASL techniques. This modification permits the continuous measurement of both blood flow and blood volume. Also, because the contrast agent can be used to remove the signal from intravascular spins, it is possible to measure the first-pass water extraction fraction. Contrast-to-noise is easily traded off with repetition rate, allowing the use of non-EPI scanners and more flexible imaging paradigms. The basic theory of these measurements, several experimental scenarios, and validating results are presented. Specifically, the PaCO2-reactivity of microvascular and total relative cerebral blood volume (rCBV), cerebral blood flow (CBF), and the water extraction-flow product (EF) in rats with the new contrast agent MPEG-PL-DyDTPA is measured, and the values are concordant with those of previous literature. As an example of one possible application, continuous flow and volume measurements during transient focal ischemia are presented. It is believed that CAPTIVE imaging will yield a more complete picture of the hemodynamic state of an organ, and has further application for understanding the origins of the BOLD effect.

Animals↗
PubMed1998

Novel gliosarcoma cell line expressing green fluorescent protein: A model for quantitative assessment of angiogenesis.

Angiogenesis is essential for tumor proliferation and metastasis. The extent of angiogenesis is measured by microvessel density (MVD) which has been identified as an independent prognostic factor for relapse-free survival in cancer patients. Existing methods of MVD assessment measure the microvessel count in the most active area of neovascularization ("hot spots") using antibodies against vascular endothelial antigens. This may produce unreliable results because of tissue volume loss and misshapening during the fixation and dehydration procedures. We report here a genetically engineered 9L cell line constitutively expressing green fluorescent protein that can be visualized using fluorescence microscopy without additional histological staining. The model developed in this study allows for the performance of simple and easy MVD counting, assuming that nonfluorescent "black spots" visible by fluorescence microscopy within the borders of the tumor tissue represent blood vessels. This assumption was confirmed by a comparative study utilizing conventional histological methods, anti-CD31 immunohistology, and Hoechst 33258 dye exclusion. This model is also useful for delineation of the true borders between tumor and normal brain tissue, including microscopic tumor extensions, without multiple histological staining. The suggested model allows quantification of tumor angiogenesis in tissue specimens, thus providing independent prognostic information about tumor growth and regression. It is expected to be most valuable in evaluating the efficacy of anti-angiogenic therapy.

Animals↗
PubMed1998

The development of in vivo imaging systems to study gene expression.

Imaging techniques are currently being developed to map the topography and level of gene expression following gene therapy. To date, two different imaging strategies have been investigated--using marker genes encoding either intracellular enzymes or cell-surface receptors. The first approach employs the ability of certain enzymes to modify imaging prodrugs, so that tissue accumulation of such drugs reflects the expression. The second approach utilises cell-surface expression of a ligand-binding receptor that can be detected using imaging tracers. In this review, we discuss nuclear-and magnetic-resonance-image techniques that have been developed to detect gene expression.

Binding Sites↗
PubMed1998

Non-invasive in vivo mapping of tumour vascular and interstitial volume fractions.

Non-invasive measurement of haemodynamic parameters and imaging of neovasculature architecture is of importance in determining tumour prognosis, in directing tissue sampling and in assessing treatment efficacy. In the current research we investigated a dual tracer nuclear magnetic resonance (NMR) technique to map the tumour vascular (VVF) and interstitial volume fraction (IVF) non-invasively in vivo. We hypothesised that a NMR signal emanating after intravenous administrations of a vascular paramagnetic probe (MPEG-PL-GdDTPA) can be maximised so that additional signal after administration of a second interstitial probe (GdDTPA) would only reflect the IVF but not the VVF. The method and its assumptions were verified and experimental conditions optimised both in phantoms and in C6 glioma bearing rats. Data derived from in vivo studies show tumoral VVF and IVF values that are consistent with histology data and literature values; the relative ranking order of values was tumour > muscle > brain. Image maps showed intratumoral and intertumoral heterogeneity of both parameters at submillimetre pixel resolution. The method is applicable to a wide variety of tumour models and can theoretically be performed repeatedly to study tumour growth or involution during therapy.

Animals↗
PubMed1998

Efficacy of thrombolytic therapy in pulmonary embolism determined by MION-enhanced MRA: an experimental study in rabbits.

RATIONALE AND OBJECTIVES: This study determined whether contrast-enhanced magnetic resonance angiography could be used as a noninvasive imaging technique to determine the therapeutic effect and endpoint in thrombolysis of acute pulmonary embolism in an animal model. METHODS: New Zealand white rabbits (n = 18) were anesthetized and mechanically ventilated. Single (n = 12 emboli) or dual (n = 12 emboli in 6 animals) pulmonary emboli were created by injecting autologous thrombi through a right internal jugular venous approach. Three-dimensional time of flight (TOF) magnetic resonance angiograms were obtained after intravenous administration of 2 mg Fe of a long circulating monocrystalline iron oxide. Animals then received 5000 IU heparin and 1.3 mg recombinant tissue plasminogen activator/kg intravenously, and magnetic resonance angiography was repeated 30 minutes and 60 minutes after initiation of thrombolytic therapy. RESULTS: MION-enhanced magnetic resonance angiography accurately detected pulmonary emboli in all rabbits. Thrombolysis during the observation period was successful in 8 of the 18 animals. In animals with a single embolus, the revascularization rate was 50% (6 of 12 emboli). The rate was 33% (4 of 12 emboli) in animals with multiple emboli. Magnetic resonance angiography allowed determination of thrombus resolution or thrombus persistence. CONCLUSIONS: It was feasible to diagnose pulmonary embolism accurately in this experimental study and to monitor thrombolysis of pulmonary emboli by MION-enhanced magnetic resonance angiography.

Acute Disease↗
PubMed1998

MR imaging of gene delivery to the central nervous system with an artificial vector.

PURPOSE: To determine whether gene delivery by means of a synthetic no viral DNA delivery system that is capable of gene transfer can be mapped with magnetic resonance (MR) imaging. MATERIALS AND METHODS: The DNA delivery system consisted of aminated (poly-L-lysine-conjugated) dextran chains anchored together with a central superparamagnetic core. Three different types of constructs were synthesized that differed in their amino content and, thus, DNA-loading capacity. The model plasmid consisted of complementary DNA encoding for humanized green fluorescent protein. Constructs were tested in cell culture and in vivo in a rat model. RESULTS: All three constructs were capable of transfecting human cells in culture with transfection efficiencies ranging from 0.3% to 4.1%, which is similar to that of diethylaminoethyl-dextran. MR imaging experiments showed that DNA constructs induced signal intensity changes that co-localized with phosphorus-33-labeled plasmid distribution at autoradiography. After injection of the constructs into the corpus callosum of rats, weak green fluorescent protein expression of neuronal and glial cells could be detected at immunohistologic examination. CONCLUSION: Dextran-based nonviral DNA delivery systems are capable of transfecting cells and can be visualized with MR imaging.

Amino Acids↗
PubMed1998

Intracellular magnetic labeling of lymphocytes for in vivo trafficking studies.

Lymphocyte adhesion and trafficking is difficult to observe in vivo over time. We used magnetic resonance imaging (MRI) to identify magnetically labeled lymphocytes in phantom experiments and in tissue. A method of lymphocyte labeling was developed that is based on fluid-phase endocytosis of nanometer-sized biocompatible superparamagnetic particles. The maximum cell uptake in culture was 0.11 ng Fe/cell corresponding to 5 x 10(6) particles/lymphocyte. Cells stably retained the label and were fully viable for at least 3 days. Labeled lymphocytes showed adhesion to human endothelial cells similar to unlabeled cells, indicating no effect of labeling on cell surface expression of adhesion proteins. No particle-mediated cytotoxicity could be observed. The detection threshold of MRI for detecting labeled lymphocytes in the current study was 2.5 x 10(6) cells/30 microL sampling volume. Following intravenous injection of labeled lymphocytes into rats, cells accumulated in spleen, lymph nodes and liver with a similar bio-distribution as unlabeled cells. Lymphocyte accumulation in the spleen resulted in MRI signal intensity changes readily detectable by MRI. These findings suggest that intracellular lymphocyte labeling with superparamagnetic particles is feasible, does not alter the viability or tissue distribution of labeled cells and allows the detection of labeled lymphocytes by MRI.

Animals↗
PubMed1998

Preclinical evaluation and phase I clinical trial of a 99mTc-labeled synthetic polymer used in blood pool imaging.

OBJECTIVE: To obtain initial data on the safety and efficacy of a novel polymeric, synthetic blood pool contrast agent [O-monomethoxypoly(ethylene glycol)-O'succinyl]poly(N-epsilon-L-lysyl [99mTc]diethylenetriamine pentaacetate monoamide, we performed a preclinical evaluation and phase 1 clinical trial under an investigator-sponsored investigational new drug application. MATERIALS AND METHODS: Methoxypoly(ethylene glycol)ethylenetriaminopentaacetic acid was formulated into a kit containing the polymer, stannous chloride, and a buffer. Kits were stored in frozen form for subsequent labeling with technetium-99m. Acute and subacute toxicity studies were carried out in rats and rabbits. Healthy human volunteers (n = 6) were then enrolled in a prospective, open-label phase 1 clinical study. RESULTS: Animal studies showed no signs of acute or subacute toxicity at doses 280 times the proposed dose for humans. In the clinical trial with humans, no significant abnormalities of laboratory values, ECG findings, or hemodynamic parameters were seen. One volunteer experienced facial flushing and palpitations. Four volunteers showed typical blood pool biodistribution, with a blood half-life of 20.6 +/- 2.3 hr. At 24 hr after administration, 22.1% +/- 2.5% of the injected dose had been excreted through the kidneys. Two other volunteers showed a different biodistribution (primarily to liver and spleen), presumably associated with labeling instability. CONCLUSION: Synthetic methoxypoly(ethylene glycol)-grafted polymers can have long circulation times in humans. Pharmaceuticals based on such polymers are expected to have clinical applications in cardiovascular imaging, gastrointestinal bleeding studies, and capillary leak imaging.

Adult↗
PubMed1998

Targeting gene therapy vectors to CNS malignancies.

Gene therapy offers significant advantages to the field of oncology with the addition of specifically and uniquely engineered mechanisms of halting malignant proliferation through cytotoxicity or reproductive arrest. To confer a true benefit to the therapeutic ratio (the relative toxicity to tumor compared to normal tissue) a vector or the transgene it carries must selectively affect or access tumor cells. Beyond the selective toxicities of many transgene products, which frequently parallel that of contemporary chemotherapeutic agents, lies the potential utility of targeting the vector. This review presents an overview of current and potential methods for designing vectors targeted to CNS malignancies through selective delivery, cell entry, transport or transcriptional regulation. The topic of delivery encompasses physical and pharmaceutic means of increasing the relative exposure of tumors to vector. Cell entry based methodologies are founded on increasing relative uptake of vector through the chemical or recombinant addition of ligand and antibody domains which selectively bind receptors expressed on target cells. Targeted transport involves the potential for using cells to selectively carry vectors or transgenes into tumors. Finally, promoter and enhancer systems are discussed which have potential for selectivity activating transcription to produce targeted transgene expression or vector propagation.

Central Nervous System Neoplasms↗
PubMed1997

Dynamic liver imaging with iron oxide agents: effects of size and biodistribution on contrast.

In vivo effective relaxation rates in normal rat liver were evaluated for four dextran coated iron oxide agents: monocrystalline iron oxide nanocolloid (MION) with a mean particle diameter of 3.9 nm, a polycrystalline agent (PION) with a larger mean diameter of 12 nm, and these two agents labeled with the asialofetuin (ASF) protein for high hepatocytic receptor binding affinity (MION-ASF and PION-ASF). Using echo planar imaging at 2 Tesla, dose response was measured with measured with high temporal resolution for 3 h after injection of agent, and by comparing with relaxivities in vitro and in brain, dominant in vivo contrast phenomena were elucidated. While transverse relaxivity for PION-ASF exceeded that for MION-ASF by almost a factor of 2 in solution, relaxation rates in vivo became equivalent. Liver relaxation using non-ASF agents was consistent with rapid water exchange between vascular and extravascular compartments, which dominated relaxation as a result of agent accumulation in Kupffer cells.

Animals↗
PubMed1997

Tumor cell endocytosis imaging facilitates delineation of the glioma-brain interface.

We describe a method for measuring tumor cell endocytosis in vivo and provide the anatomic correlate of this tumor cell function using a superparamagnetic and histologically detectable marker for cell uptake (MION). Rats (n = 22) were intrahemispherically implanted with a thymidine kinase (TK)-positive 9L gliosarcoma cell line, where TK served as the tumor marker. Twenty-four hours after intravenous injection of 10 mg Fe/kg of MION, rat brains were removed and underwent MR imaging ex vivo at near-microscopic resolution (isotropic voxel size of 86 microm, 9.4 T) prior to histologic processing. The imaging probe accumulated within tumor cells adjacent to the hyperpermeable tumor-brain interface including microscopic deposits and along finger-like invasions of the tumor into brain, facilitating the demarcation of the true histologic tumor border in three dimensions by MR microscopy. The method has potential research and clinical implications for delineating the tumor-brain interface prior to therapy and/or for providing a rational basis for imaging nanocolloid drug delivery to solid tumors.

Animals↗
PubMed1997

Application of a stable cell culture assay for the functional assessment of novel MR contrast agents.

The purpose of this study was to apply a new cell culture assay that preserves hepatocyte orientation and differentiation for screening of MR contrast agents with hepatocyte specificity. Cultured hepatocytes were sandwiched between two layers of collagen, preserving both hepatocyte function and morphology over a prolonged period of time. Plain and rhodaminated monocrystalline iron-oxide particles (MION and MION-rh) and asialoglycoprotein receptor-specific rhodaminated asialofetuin coupled to MION (MION-ASF-rh) were prepared. Dose-dependent competition experiments of these agents were performed with D(+)-galactose to determine the specificity of galactose-mediated cell uptake. To assess the impact of cell integrity on cell uptake dose-dependent functional experiments with two hepatotoxins (ethanol and CCl4) were performed. Normal cell cultures showed significantly higher fluorescent-light emission after incubation with hepatocyte-directed ASF-MION-rh than after incubation with MION-rh. Competition experiments of ASF-MION with galactose showed a dose-dependent decrease in calibrated fluorescent-light emission. Cell cultures treated with hepatotoxins demonstrated a dose-dependent reduction in calibrated fluorescent-light emission following incubation with ASF-MION-rh. The validated assay system allows assessment not only of hepatocyte specificity, but also of hepatocyte damage. Because the assay can be applied to cells from any species (rat, pig, human), it may represent an ideal test system prior to clinical trials of new hepatocyte-directed MR contrast agents.

Animals↗
PubMed1997

In vivo localization of diglycylcysteine-bearing synthetic peptides by nuclear imaging of oxotechnetate transchelation.

A phenomenon of in vivo transchelation of oxotechnetate from a complex with glucoheptonic acid to synthetic peptides bearing oxotechnetate-binding motifs and a technique for in vivo visualization of these peptides are described. Using two model peptides bearing two tandem diglycylcysteine (GGC) motifs (P1) or three GGC motifs (P2), we demonstrated that: (i) these peptides efficiently transchelated oxo-[99mTc]technetate from a complex with glucoheptonic acid in vitro (a complex with peptides was stable at least 24 h; radiochemical purity exceeded 95% by high performance liquid chromatography); (ii) injection of peptides into the rectus femoris muscle (at 0.5-1 micromol of SH groups) followed by an intravenous injection of 99mTc-glucoheptonate (0.25-0.5 mCi per animal) yielded visualization of the injected muscle by nuclear imaging within 1 h after injection; (iii) the experimental/control (contralateral) thigh muscle ratio was 1.80 +/- 0.05 for peptide P1 and 3.0 +/- 0.1 for P2; (iv) the injection of a control peptide P2 with SH groups covalently modified with N-ethylmaleimide resulted in a ratio of 1.4 +/- 0.2. These findings argue for specific association of oxo-[99mTc]technetate with free thiols within the binding motif of injected peptides in vivo. In vivo transchelation of oxo-[99mTc]technetate may be useful for the purpose of noninvasive imaging of gene expression, i.e., when the expression product bears GGC motifs.

Animals↗
PubMed1997

Relative blood volume measurements by magnetic resonance imaging facilitate detection of testicular torsion.

RATIONALE AND OBJECTIVES: The authors determine the utility of relative blood volume measurements (rBV) using a blood pool marker for magnetic resonance imaging (MRI) in detection of early testicular torsion. METHODS: Testicular torsion was induced in rats by counterclockwise 720 degrees rotation and fixation of the testis in the scrotum. MPEG-PL-DTPA-Gd enhanced MRI (30 mumol Gd/kg bolus injection) was performed 1 hour after torsion at 1.5 T using fat-suppressed three-dimensional fast spoiled gradient-recalled sequence for relative blood volume measurement and three-dimensional time-of-flight sequence for MR angiography (MRA). RESULTS: The rBV of the torqued testes was significantly lower (13.3% +/- 13.5%) than that of testes with sham operation (97.7% +/- 5.3%; P < 0.05). Rats with testicular torsion showed larger regions of ischemia than did animals with sham operation (63.4% +/- 13.0% versus 4.0% +/- 2.8% of all pixels in testis; P < 0.01). The MRA of testicular torsion showed engorgement of the distal testicular vein as a sign of venous compression or total disappearance of the testicular vein, indicating arterial insufficiency. CONCLUSIONS: The authors conclude that MPEG-PL-DTPA-Gd can be used to obtain functional (rBV), morphologic (tunica enhancement), and angiographic (venous engorgement, arterial compromise) findings that should improve the diagnosis of testicular torsion in the acute setting.

Animals↗
PubMed1997

Differentiation of liver hemangiomas from metastases and hepatocellular carcinoma at MR imaging enhanced with blood-pool contrast agent Code-7227.

PURPOSE: To evaluate differentiation of liver lesions at magnetic resonance (MR) imaging enhanced with Code-7227. MATERIALS AND METHODS: Thirty-five patients with 38 proved liver lesions (15 hemangiomas, 17 metastases, six hepatocellular carcinomas [HCCs]) underwent T1-weighted gradient-echo and T2-weighted fast-spin-echo MR imaging at 1.5 T before and after intravenous administration of Code-7227 (1.1 mg iron per kilogram of body weight). RESULTS: In hemangiomas, the mean contrast-to-noise ratio on precontrast and postcontrast images, respectively, increased from -4.51 +/- 4.7 (standard deviation) to 5.19 +/- 6.3 on T1-weighted images and decreased from 14.73 +/- 7.4 to 0.64 +/- 5.1 on T2-weighted images. In comparison, metastases remained hypointense to liver on T1-weighted images (from -5.77 +/- 5.9 to -7.8 +/- 6.8) and hyperintense on T2-weighted images (from 8.73 +/- 5.4 to 12.61 +/- 6.1). Although HCC enhanced more than metastases, they also remained hypointense to liver on T1-weighted images (from -4.87 +/- 6.1 to -1.79 +/- 5.7) and hyperintense on T2-weighted images (from 10.12 +/- 7.9 to 8.7 +/- 6.4). The degree of enhancement on T1-weighted images and of signal intensity drop on T2-weighted images were significantly lower in malignant liver masses than in hemangiomas (P < .001). CONCLUSION: Distinctly different enhancement patterns with Code-7227 helped accurate differentiation of liver lesions.

Adult↗
PubMed1997

Paramagnetic metal scavenging by melanin: MR imaging.

PURPOSE: To quantitate the binding of metals to synthetic melanin in vitro, which is believed to be the reason why melanotic melanomas are hyperintense on T1-weighted magnetic resonance (MR) images, and to test whether such binding by natural melanin can be detected in cultured melanoma cells in vivo with MR imaging. MATERIALS AND METHODS: Seven synthetic metallomelanins were prepared and their metal contents and relaxivities determined. Melanotic PC1A and amelanotic B16 melanoma cells were incubated with increasing concentrations of iron. MR images of synthetic melanin and cell phantoms were obtained. RESULTS: The iron-binding capacities and relaxivities of the different synthetic metallomelanins varied considerably, which reflects the heterogeneous structure of melanin and the complexity of its binding of metals. Nevertheless, the MR signal intensities of the synthetic melanin and cell phantoms show marked increases that scale, respectively, with increasing iron content and iron concentration in the incubation medium. CONCLUSION: Melanotic melanomas are hyperintense on T1-weighted images because of paramagnetic metal scavenging. This observation has implications for the interpretation of MR images, the improved detection of melanomas, and the development of imaging marker genes.

Animals↗
PubMed1997

MR imaging and scintigraphy of gene expression through melanin induction.

PURPOSE: To determine whether an expression vector that encodes for human tyrosinase, the key enzyme in the melanin production pathway, can be used to image gene expression with magnetic resonance (MR) imaging and scintigraphy. MATERIALS AND METHODS: Mouse fibroblasts and human embryonal kidney cells were transfected with an expression vector that contained a complete complementary DNA sequence that encodes the human tyrosinase gene (pcDNA3tyr). Transfected cells were assayed for messenger RNA presence, melanin staining, and indium-111 binding; scintigraphy and MR imaging were performed. RESULTS: Transfected cells contained tyrosinase messenger RNA and stained positively for melanin. Transfected cells had a higher In-111 binding capacity than nontransfected cells, a difference readily detectable with scintigraphy. MR imaging showed transfected cells to have markedly higher signal intensity after gene transfer than nontransfected cells. CONCLUSION: Gene transfer and expression in cell culture can be detected with MR imaging and scintigraphy. The proposed strategy of using an imaging marker gene may have a substantial effect on the noninvasive imaging of gene therapy.

Animals↗