Detecting tumor responses to treatment with magnetic resonance imaging
Authors: Kevin Brindle
Journal: Clinical Cancer Research (2008)

ED04-03   Patients with similar tumor types frequently have markedly different responses to the same therapy. The development of novel targeted cancer therapies could benefit significantly, therefore, from the introduction of imaging methods that allow an early assessment of treatment response in individual patients. These would allow an oncologist to rapidly assess the effectiveness of a new therapy. Ineffective treatments could be abandoned at an early stage and more effective treatments selected, with attendant welfare benefits for the patient and cost benefits for the health care system (1, 2).  We have been developing non-invasive and clinically applicable magnetic resonance-based methods for detecting the early responses of tumors to therapy. A primary focus has been on the development of methods for detecting tumor cell death, since the level of tumor cell death following drug treatment has been shown, in preclinical and clinical studies, to be a good prognostic indicator for treatment outcome. Thus, by monitoring tumor cell death we may get an indication of whether a particular drug is working very early during treatment, possibly within 24-48 hours, and long before there is any evidence of tumor shrinkage. I will briefly describe the different approaches that we, and others, have taken in detecting and imaging cell death in tumors using magnetic resonance [reviewed in (3)].  There are several metabolic markers of cell death that can be detected using  31 P MRS; the problem with these markers, however, is that they are relatively insensitive and therefore lack both temporal and spatial resolution. Cell death can also be detected from  1 H MRS measurements of cytoplasmic neutral lipid accumulation. Although more sensitive, this technique is unlikely to be able to detect apoptosis in a tumor when the percentage of apoptotic cells is relatively low. A potentially much more sensitive approach is to use a targeted contrast agent that allows detection of apoptotic cells by MRI. We showed that the C2A domain of the protein synaptotagmin would bind to the phosphatidylserine that appears on the surface of apoptotic cells. When conjugated to small paramagnetic iron oxide (SPIO) nanoparticles this protein allowed MRI detection of apoptotic cells, both  in vitro  and  in vivo , in a tumor treated with chemotherapeutic drugs (4). More recently we have developed Gd 3+ -based versions of this agent, which give positive contrast and are therefore easier to detect, and which are smaller and thus more readily enter the tumor interstitium. The unbound material also clears more easily, thus generating better tissue contrast (5-7). However, the increased accumulation of the simplest of these Gd 3+ -based agents in treated tumors, which was based on analysis of the whole tumor volume, was relatively modest (1.19Â±0.04). We knew from the histology, however, that cell death was localized within the tumor, which suggested that we might be able to better detect contrast agent accumulation in a treated tumor by analyzing its distribution as well as its concentration in the tissue. We have shown that 2D Minkowski functionals, which have been used previously in cosmology as precise morphological and structural descriptors of the evolution and morphology of galaxies (8-10), can be used to parameterize the heterogeneity of contrast agent accumulation, and that these parameters allow us to better discriminate between treated and untreated tumors. This image analysis approach substantially increases the sensitivity and specificity of detection of the contrast agent in drug-treated tumors and could be applied in the analysis of uptake of other targeted agents.  A fundamental limitation of magnetic resonance is its relatively low sensitivity due to the very low levels of nuclear spin polarization that can be achieved, even in high-field magnets. Recently a practicable method has been developed to enhance the nuclear spin polarization of nuclei such that gains in sensitivity greater than 10,000-fold can be achieved (11). This has enabled the imaging of  13 C-labelled cellular metabolites  in vivo  and, more importantly, their enzymatic transformation into other species. We showed recently that exchange of hyperpolarized  13 C label between the carboxyl groups of lactate and pyruvate, in the reaction catalyzed by the enzyme lactate dehydrogenase, could be imaged in tumors and that this flux was decreased in treated tumors undergoing drug-induced cell death (12). We have suggested that this technique could be used in the future for response monitoring in the clinic, in much the same way as  18 FDG has been used with PET. We have also shown that we can monitor the conversion of hyperpolarized [5- 13 C]glutamine to glutamate in human hepatoma cells  in vitro  (13). Since glutaminase activity has been correlated with the rate of cellular proliferation, measurements of its activity  in vivo  could be used to detect the early responses of tumors to cytotoxic and cytostatic drugs, as has been done with 39- deoxy-39-[ 18 F]fluorothymidine (FLT) and PET.  Since alterations in tissue pH underlie many pathological processes, the capability to image tissue pH in the clinic could offer new ways of detecting disease and response to treatment. We have shown that tissue pH can be imaged  in vivo  from the ratio of the signal intensities of hyperpolarized H 13 CO 3 - and  13 CO 2  following intravenous injection of hyperpolarized H 13 CO 3 - [14]. The technique was demonstrated with a study on a mouse tumor model, which showed that the average tumor pH was significantly lower than the surrounding tissue. Since bicarbonate is already used intravenously in humans, we propose that this technique could be used clinically to image tumors and other pathological processes that are associated with alterations in tissue pH such as ischemia and inflammation.   References  1. Neves AA, Brindle KM. Assessing responses to cancer therapy using molecular imaging. Biochim Biophys Acta 2006;1766:242-61.  2. Brindle KM. New approaches for imaging tumor responses to treatment. Nature Rev Cancer 2008;8:1-14.  3. Kettunen MI, Brindle KM. Apoptosis detection using magnetic resonance imaging and spectroscopy. Prog Nucl Mag. Reson Spectrosc. 2005;47:175-85.  4. Zhao M, Beauregard DA, Loizou L, Davletov B, Brindle KM. Non-invasive detection of apoptosis using magnetic resonance imaging and a targeted contrast agent. Nat Med 2001;7:1241-4.  5. Jung HI, Kettunen MI, Davletov B, Brindle KM. Detection of apoptosis using the C2A domain of synaptotagmin I. Bioconjug Chem 2004;15:983-7.  6. 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