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Molecular imaging in vivo: an introduction

GlaxoSmithKline, 891–995 Greenford Road, Middlesex UB6 0HE, UK

View larger version (86K): [in a new window]   Figure 1. Demonstrating gene transfection using MRI. In this technique tumour cells were engineered to overexpress the transferrin receptor (a cell membrane receptor involved in regulating cellular iron uptake). As a result the tumour overexpressing transferrin receptors accumulated iron in the form of MIONs (monocrystalline iron oxide nanoparticles). The MIONs induce a high susceptibility (the left-hand tumour in these animals – the control tumour is on the right flank) imaged as signal loss by MR. The images were acquired on a 1.5 T system. Reproduced with permission [26].

View larger version (78K): [in a new window]   Figure 2. In vivo use of a gadolinium based smart contrast agent to demonstrate gene transfection. When the enzyme -galactosidase is expressed in engineered cells, the -galactopyranose ring protecting the Gd3+ is cleaved allowing bulk water access to the paramagnetic gadolinium ion. The images labelled A and B are of Xenopus (African claw toed frog) embryos that express -galactosidase or not. The embryo labelled +mRNA is expressing -galactosidase and significantly more detail is seen in this embryo than the one labelled –mRNA. The bottom embryo is oriented upside down compared with the top one. Reproduced with permission [28].

View larger version (23K): [in a new window]   Figure 3. Example of a "smart" MRI probe. Ca2+ causes a conformational change in the molecule such that access of bulk water to the Gd3+ can be accessed or denied. Access of bulk water to the Gd3+ causes shortening in the longitudinal relaxation time of water protons and therefore signal enhancement on T1 imaging. Chemical structures courtesy of Prof. T Meade, NorthWestern University, USA.

View larger version (16K): [in a new window]   Figure 4. In vivo 19F-MRS (magnetic resonance spectroscopy) demonstration of gene transfection. In this example a tumour was grown on the flank of an animal the cells of which had been transfected with the cytosine deaminase (CD) gene (CD is a fungal gene and has no mammalian counterpart). CD catalyses the conversion of 5-fluorocytosine (5FC) into the active anticancer drug 5-fluorouracil (5FU). 5FU has a different resonant frequency to 5FC allowing its identification in vivo and confirming transfection with the CD gene. FNuc, Fluoronucleotides; FßAl, Fluoro-beta-alanine. Reproduced with permission [36].

View larger version (73K): [in a new window]   Figure 5. Non-invasive demonstration of transfection of the herpes simplex virus thymidine kinase (HSV-Tk) gene into a murine tumour using PET. The C6tk+ tumour cells were transfected with HSV-Tk and implanted into nude mice. Control tumours had C6 cells only. HSV-Tk causes phosphorylation and intracellular trapping of the radiotracer 9-[(1-[18F]Fluoro-3-hydroxy-2-propoxy)methyl]guanine ([18F]FHPG). Note persistence of activity in the C6tk+ tumour indicating gene transfection. Reproduced with permission [91].

View larger version (43K): [in a new window]   Figure 6. Non-invasive demonstration of protein–protein interactions using in vivo PET. The radiotracer [9-(4-[18F]Fluoro-3-hydroxymethylbutyl)guanine] is trapped inside tumour cells that bind p53 and TAg (transforming factor SV40 large T antigen) protein. Gal4 is a DNA binding domain and VP16 an enhancer domain. Binding of p53 and TAg initiates expression of herpes simplex thymidine kinase that phosphorylates the radiotracer and traps it intracellularly. Intracellular trapping is demonstrated via persistent activity over the transformed murine tumour on the right. The EGFP domain can be induced to express green fluorescent protein providing fluorescent confirmation of VP16 TAg binding. Reproduced with permission [53].

View larger version (111K): [in a new window]   Figure 7. Confirmation of therapeutic inhibition of the enzyme matrix metalloproteinase-2 (MMP-2) in a murine tumour model using near infrared (NIR) imaging and a "smart" fluorescent probe. In the absence of the MMP inhibitor the NIR probe binds to its MMP target and is converted to a fluorescent substrate. The smart NIR probe will not be activated without binding to the enzyme. Reproduced with permission [60].

View larger version (118K): [in a new window]   Figure 8. Examples of bioluminescence imaging. (a) Migration of neural progenitor stem cells across the midline towards an implanted glioma in a mouse. The neural progenitor cells were labelled with the luciferin (luc) gene ex vivo so that cells expressing luciferin exhibit luminescence under appropriate conditions. (b) luc-labelled ovarian cancer cells implanted at different densities in the peritoneal cavity of a nude mouse. Note that cellular densities as low as 5 x 103 cells can be imaged. Reproduced with permission [55].

View larger version (83K): [in a new window]   Figure 9. Cartoon depicting the molecular structure of green fluorescent protein (GFP). GFP consists of a 11-stranded -barrel in addition to a central helix that carries the chromophore. GFP therefore acts as an energy acceptor for the protein aequorin efficiently transforming blue light (470 nm) emitted by aequorin, into green light (508 nm). Structure downloaded from Protein Data Bank (1EMA) http://www.rcsb.org/pdb/index.html.

View larger version (90K): [in a new window]   Figure 10. An example of the jellyfish Aequorea victoria from which the bioluminescent protein green fluorescent protein was originally extracted. Although many examples of bioluminescence exist in nature the image does not represent bioluminescence. This jellyfish was illuminated by a light during photography. (Reproduced with permission of the author.)

View larger version (142K): [in a new window]   Figure 11. Representative images from a multifunctional study of a patient with a locally advanced rectal carcinoma (outlined by the yellow arrows). (a) Axial T2 section through the section of the tumour at mid-rectal level. The soft tissue mass superior to the tumour is a recently gravid uterus. (b) The parametric permeability (capillary leakiness) map of the same tumour. Each pixel in the tumour is colour coded according to the amount of permeability (white–high, blue–low). (c) The perfusion map. High signal intensity pixels correspond to a high relative perfusion. The data indicate a poorly perfused tumour. Note the high perfusion of the recently gravid uterus. (d) The apparent diffusion coefficient map. Areas of high signal intensity indicate restricted diffusion. These functional data were acquired during the same scanning session in under 1 h.