Enzymatic Triggered Release of Imaging Probe from Paramagnetic Liposomes
Sara Figueiredo¹, Enzo Terreno², Joao Nuno Moreira³, Carlos F.G.C. Geraldes¹, Silvio Aime^2
1Dep. of Biochemistry and Technology and Center for Neurosciences and Cell Biology, University of Coimbra, Coimbra, Portugal; ²Department of Chemistry and Molecular Imaging Center, University of Torino, Torino, Italy; ³Lab. of Pharmaceutical Technology and Center for Neurosciences and Cell Biology, University of Coimbra, Coimbra, Portugal

The design of imaging probes reporting about a given enzymatic activity is an important task in Molecular Imaging investigations. The aim of this work was to prepare paramagnetic liposomes encapsulating the clinically approved Gd-HPDO3A complex and able to release the imaging probe in the presence of a specific enzyme upregulated in a given disease. To do this, an amphiphilic lipopeptide acting as substrate for MMP (Matrix Metallo Proteinases) was prepared and incorporated in liposomes. It has been reported that in the presence of MMP like collagenase, the liposomes release its content, thus determining the detection of a T1 contrast enhancement.

A Novel Dual MRI-PARACEST/Fluorescent Probe for the Detection of Cathepsin-D Activity in Alzheimer's Disease
Robert Ta^1,2, Alex Li¹, Mojmir Suchy, ^1,3, Robert Hudson³, Stephen Pasternak^4,5, Robert Bartha^1,2
1Imaging Research Group, Robarts Research Institute, London, Ontario, Canada; ²Medical Biophysics, University of Western Ontario, London, Ontario, Canada; ³Chemistry, University of Western Ontario, London, Ontario, Canada; ⁴Molecular Brain Research Group, Robarts Research Institute, London, Ontario, Canada; ⁵Clinical Neurological Sciences, University of Western Ontario, London, Ontario, Canada

A novel dual magnetic resonance/fluorescent probe has been designed for molecular targeting of Cathepsin D in Alzheimer's disease.  The MRI contrast of this probe has been detected using the on-resonance paramagnetic agent chemical exchange effect  (OPARACHEE) method.  Greater than a 1% OPARACHEE contrast was observed in 1.5 mM Tm³⁺-DOTA-Glycine in a 5% BSA phantom.  The dual probe demonstrated uptake into neuronal cells by confocal microscopy and had no toxic effects on these cells at the concentrations tested.

Self-Degrading, MRI-Detectable Hydrogels with Picomolar Target Sensitivity
Jason Colomb¹, Katherine Louie¹, Stephen P. Massia¹, Kevin M. Bennett^1
1School of Biological and Health Systems Engineering , Arizona State University, Tempe, AZ, United States

Nanostructured hydrogels have been developed as synthetic tissues, tissue scaffolds for cell and drug delivery, and as guides for tissue regeneration. A fundamental problem with hydrogels is that implanted gel structure is difficult to monitor noninvasively. Here we demonstrate that the aggregation of cationic magnetic nanoparticles, attached to specific macromolecules in biological and synthetic hydrogels, can be controlled to detect changes in gel macromolecular structure with MRI.  Sensitivity of the gels to target molecules is finely controlled using an embedded zymogen cascade amplifier and we show that these gels self-degrade when they come into contact with pM concentrations of enterokinase.

Direct Detection of Cytosine Deaminase Enzymatic Activity Using CEST MRI
Guanshu Liu^1,2, Segun M. Bernard^2,3, Terence Tse², Piotr Walczak^2,3, Michael T. McMahon^1,2, Jeff W.M. Bulte^2,3, Peter C.M. van Zijl^1,2, Assaf A. Gilad^2,3
1F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States; ²Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, MD, United States; ³Institute for Cell Engineering, Johns Hopkins University School of Medicine, Baltimore, MD, United States

A new MRI method for assessing cytosine deaminase (CD) enzymatic activity was developed. This method allows the direct detection and quantification of CD by observing the changes in Chemical Exchange Saturation Transfer (CEST) signal when the substrates cytosine and 5-Fluorocytosine (5-FC) are converted to products uracil and 5-Fluorouracil (5-FU) by CD respectively. In addition, this method is capable of continuously monitoring the CD activity using the natural compounds in the cytosine/uracil system. Possible applications for this method include monitoring of in vivo CD activity and CD gene therapy for cancer.

A Novel Class of S-Gal^TM Analogs as ¹H MRI  LacZ Gene Reporter Molecules
Praveen Kumar Gulaka¹, Vikram D. Kodibagkar^1,2, Jian-Xin Yu², Ralph P. Mason, ^12
1Biomedical Engineering, UT Arlington and UT Southwestern Medical Center at Dallas, Dallas, Texas, United States; ²Radiology, UT Southwestern Medical Center at Dallas, Dallas, Texas, United States

Extensive implementation of gene therapy as a therapeutic strategy for cancers has been hampered by difficulties in quantitatively assessing the success of gene transfection and longevity of gene expression. Therefore development of non-invasive reporter techniques based on appropriate molecules and imaging modalities may help to assay gene expression. We have evaluated a range of S-Gal™ analogs as novel 1H MR lacZ gene-reporter molecules in vitro and have identified C3-GD as an optimal agent for in vivo studies.

Multispectral MRI Contrast Through Cylindrical Nanoshell Agents
Gary Zabow^1,2, Stephen Dodd¹, John Moreland², Alan Koretsky^1
1NINDS, NIH, Bethesda, MD, United States; ²NIST, Boulder, CO, United States

Thanks to the processing control afforded by top-down microfabrication techniques, geometrically tailored magnetic microparticles have recently been shown able to produce tunable, multispectral MRI contrast.  Here we demonstrate a new form of such agent based on new cylindrical nanoshell structure designs.   These hollow magnetic cylinders can produce large NMR frequency shifts through the control of the cylinder materials, aspect ratios and wall thicknesses.  Apart from yielding distinct frequency shifted NMR peaks, it is also shown that these cylindrical nanoshell structures exhibit good mechanical robustness and automatically self-align (as is required) to the applied MRI B₀ field.

Eu³⁺-Based PARACEST Agents with Intermediate Water Exchange Rates Also Act as T₂ Exchange (T_2exch) Contrast Agents
Todd C. Soesbe¹, Federico A. Rojas-Quijano¹, Matthew E. Merritt¹, A. Dean Sherry^1,2
1Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States; ²Department of Chemistry, The University of Texas at Dallas, Dallas, TX, United States

In our initial in vivo murine studies of CEST agents, we observed a significant loss of MR signal in certain tissue types, most notably the kidneys (intravenous injection) and human cancer cell xenografts (intratumoral injection). This loss in signal was present even when the CEST saturation pulse was omitted from the imaging sequence, and appeared to be caused by a local decrease in T₂ due to the presence of the CEST agent. We hypothesized that the proton exchange that enables the CEST effect can also cause a decrease in T₂ for compounds with intermediate proton exchange rates.

MR Contrast from Ascorbic Acid (Vitamin C) in Phantoms and in Vivo
Christopher D. Lascola¹, Talaignair Venkatraman¹, Bjorn Engstrom¹, Haichen Wang^1
1Department of Radiology and Brain Imaging and Analysis Center, Duke University Medical Center, Durham, NC, United States

: L-ascorbic acid (vitamin C) is the most abundant intracellular antioxidant and an essential co-factor. Intracellular levels of ascorbic acid (AA) are remarkably high, where concentrations may exceed 10-30 mM. In this study, we show that AA in solution produces significant changes in T2 and T2* relaxivity at physiologically relevant concentrations. These results raise two important possibilities: first, that endogenous AA may be an important contributor to native T2 and T2* contrast in CNS and other tissues; and second, that both oxidized and reduced forms of ascorbic acid may have utility as novel MR contrast probes.

Hyperpolarized 89Y Complexes as PH Sensitive NMR Probes
Ashish Kumar Jindal¹, Matthew E. Merritt¹, Eul Hyun Suh¹, Craig R. Malloy^1,2, Alan Dean Sherry^1,3, Zoltan Kovacs^1
1Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States; ²Veterans Affairs, North Texas Health Care System, Dallas, TX, United States; ³Department of Chemistry, University of Texas at Dallas, Richardson, TX, United States

Hyperpolarization followed by fast dissolution provides tremendous gains in SNR in both NMR and MRI experiments, but a primary bottleneck in its application is the T₁ decay of the magnetization in the liquid state.  Due to its long T₁, hyperpolarized ⁸⁹Y makes an excellent candidate as an in vivo imaging agent.  Here we report the chemical shift dependence upon pH for two hyperpolarized ⁸⁹Y complexes and clearly demonstrate how such complexes can be used as sensitive spectroscopy/imaging probes to measure pH.

Remote MRI Sensing of PH and Cell Viability Using Immunoprotective Microcapsules Crosslinked with Polycationic DIACEST Peptides
Dian Respati Arifin^1,2, Kannie W.Y. Chan^1,2, Guanshu Liu^1,3, Amanda Cardona¹, Muksit Jamil¹, Jeff W.M. Bulte^1,2, Michael T. McMahon^1,3
1Radiology, Johns Hopkins University School of Medicine, Baltimore, MD, United States; ²Cell Imaging Section, Institute for Cell Engineering, Johns Hopkins University School of Medicine, Baltimore, MD, United States; ³F.M. Kirby Center for Functional Brain Imaging, Kennedy Krieger Institute, Johns Hopkins University School of Medicine, Baltimore, MD, United States

Cell transplantation is a potential treatment for various diseases such as type I diabetes, liver failure and cardiovascular disorders. Encapsulation of cells inside semi-permeable microcapsules offers immunoprotection for the cells and recipient. We have developed new biodegradable microcapsules using polycationic peptides from our library of CEST agents that are detectable by MRI. These DIACEST capsules are pH-responsive and can be used to monitor biological events, which are accompanied by pH changes. Human pancreatic cells encapsulated inside these microcapsules were alive and functional for at least 27 days in vitro. We also demonstrate that these microcapsules can detect cell apoptosis in vitro.