Applications of magnetic resonance in model systems: cancer therapeutics

NMR spectroscopy in beta cell engineering and islet transplantation

Islet transplantation is a promising method for restoring normoglycemia and alleviating the long term complications of diabetes. Widespread application of islet transplantation is hindered by the limited supply of human islets and requires a large increase in the availability of suitable insulin secreting tissue as well as robust quality assessment methodologies that can ensure safety and in vivo efficacy. We explore the application of nuclear magnetic resonance (NMR) spectroscopy in two areas relevant to beta cell engineering and islet transplantation: (1) the effect of genetic alterations on glucose metabolism, and (2) quality assessment of islet preparations prior to transplantation. Results obtained utilizing a variety of NMR techniques demonstrate the following: (1) Transfection of Rat1 cells with the c-myc oncogene (which may be involved in cell proliferation and cell cycle regulation) and overexpression of Bcl-2 (which may protect cells from stresses such as hypoxia and exposure to cytokines) introduce a wide array of alterations in cellular biochemistry, including changes in anaerobic and oxidative glucose metabolism, as assessed by 13C and 31P NMR spectroscopy. (2) Overnight incubation of islets and beta cells in the bottom of centrifuge tubes filled with medium at room temperature, as is sometimes done in islet transportation, exposes them to severe oxygen limitations that may cause cell damage. Such exposure, leading to reversible or irreversible damage, can be observed with NMR-detectable markers using conventional 13C and 31P NMR spectroscopy of extracts. In addition, markers of irreversible damage (as well as markers of hypoxia) can be detected and quantified without cell extraction using high-resolution magic angle spinning 1H NMR spectroscopy. Finally, acute ischemia in a bed of perfused beta cells leads to completely reversible changes that can be followed in real time with 31P NMR spectroscopy.

Magnetic resonance microscopy in cardiac development

Magnetic resonance microscopy (MRM) is a fast and non-destructive imaging technique that can analyze the three-dimensional structure of the embryonic heart both qualitatively and quantitatively. Intravascular contrast agents have been developed to accentuate the anatomy of cardiac chambers, the cardiac outflow tract, and major arteries and veins throughout the embryonic body. MRM generates non-distorted three-dimensional data of vascular anatomy in a fraction of the time required by conventional optical image reconstruction techniques. The three-dimensional nature of these data allows the creation of visual models that can be manipulated for fast and easy interpretation of the complex relationships between heart chambers and aortic arches. This is particularly helpful because these relationships change in complex ways during development. The non-destructive nature of MRM makes it well suited for investigating rare or valuable specimens and live subjects. MRM techniques have been developed for imaging the embryo in utero and in vitro, although MRM studies of fixed embryo specimens are easier to perform and produce data with better contrast and higher resolution.

Imaging biochemistry: applications to breast cancer

The use of magnetic resonance spectroscopy (MRS) to investigate breast tumour biochemistry in vivo is reviewed. To this end, results obtained both from patients in vivo and from tumour extracts and model systems are discussed. An association has been observed between transformation and an increase in phosphomonoesters (PMEs) detected in the 31P MRS spectrum, as well as an increase in choline-containing metabolites detected in the 1H spectrum. A decrease in PME content after treatment is associated with response to treatment as assessed by tumour volume. Experiments in model systems aimed at understanding the underlying biochemical processes are presented, as well as data indicating the usefulness of MRS in monitoring the uptake and metabolism of some chemotherapeutic agents.

Clinical imaging of cancer metastasis

Tumour imaging is an essential part of the practice of oncology, with a crucial role in screening programmes and in diagnosis and staging of established disease. Furthermore, the assessment of tumour size by imaging, usually with computer tomography (CT) scanning, is a key component in determining the tumour response to therapy both in clinical trials and in daily oncology practice. Techniques such as CT, ultrasound (US) and magnetic resonance imaging (MRI) provide high resolution anatomical images with detailed structural information. However, these imaging modalities yield limited functional information on the tumour tissues and often cannot distinguish residual disease from non-viable or necrotic tumour masses, nor can they detect minimal residual disease. In contrast, radiopharmaceutical imaging and, in particular, positron emission tomography (PET) can give some functional information about the underlying tissues. The possibility of refining these techniques and also the emergence of newer imaging modalities that can detect changes in cancers at the physiological, cellular or molecular levels, gives rise to the notion that these methods will have implications for drug development strategies and also future clinical management. In this review, we briefly discuss the current role of imaging in clinical practice, describe some of the advances in imaging modalities currently undergoing evaluation, and speculate on the future role of these techniques in developmental therapeutics programmes.

Diffusion MRI detects early events in the response of a glioma model to the yeast cytosine deaminase gene therapy strategy

Detection of a therapeutic response early in the course of cancer treatment, before tumor growth delay or regression, is not currently possible in experimental models or clinical medicine. New interim measures of therapeutic response would be particularly useful in the development of cancer chemosensitization gene therapy by facilitating optimization of gene transfer protocols and prodrug dosing schedules. Diffusion MRI is a sensitive technique producing quantitative and noninvasive images of the apparent mobility of water within a tissue. We investigated the utility of diffusion MRI for detecting early changes associated with a refined cytosine deaminase (CD)/5-fluorocytosine (5FC) chemosensitization gene therapy paradigm in orthotopic 9L gliomas stably expressing the recently cloned S. cerevisiae CD gene. Mean tumor diffusion increased 31% within 8 days of initiating 5-FC treatment, preceding tumor growth arrest and regression. Complete regression of the intracranial tumor was observed in four of five treated animals, and recurrent tumor in the remaining animal exhibited water diffusion behavior similar to primary, untreated tumors. These results demonstrate the efficacy of the yCD/5FC strategy for glioma and suggest that increased tumor water diffusion is an indicator of active therapeutic intervention.