Hyperpolarized [1,4-C]fumarate as an imaging agent of tumor cell death

Magnetic resonance spectroscopy (MRS) is a technique that allows us to non-invasively detect multiple small metabolites within cells or extracellular spaces (1-3). The clinical use of MRS as an adjunct to magnetic resonance imaging (MRI), particularly in cancer detection and treatment response evaluation, has expanded dramatically over the past several years. Although MRS is theoretically applicable to any nucleus possessing spin, the most important and more frequently investigated applications are in proton (H) and carbon-13 (C) (4-6). C MRS is superior to H MRS in many respects (3, 6-8). C MRS can provide specific information about the identity and structure of biologically important compounds. The chemical shift range for carbon (250 ppm) is much larger than that for proton (15 ppm), allowing for improved resolution of metabolites. In addition, the T relaxation time of C in small molecules is much longer than that of H (0.1–2.0 s in a magnetic field of 0.1–3.0 T), allowing the generation of hyperpolarized C-labeled tracers outside the subject and the MRI scanner. Some of these tracers are endogenous and therefore have lower toxicity than drugs and exogenous contrast media (6). However, C MRS is limited by the low natural abundance of C (1.1%) and its low magnetogyric ratio (γ of C is one quarter that of H) (2, 3). Several techniques have been used to overcome the C MRS limitation through enhancing the polarization of nuclear spins. One technique is proton decoupling, which eliminates the coupling of H with C by irradiating the entire H resonance absorption range and consequently collapsing C resonances to singlets (4, 5). The signal/noise ratio of C resonances has been shown to be significantly increased with proton decoupling. Another technique is known as dynamic nuclear polarization (DNP), which introduces one or more C molecules into a metabolic substrate (2, 3, 8). DNP transfers high electron spin polarization to nuclear spins microwave irradiation. Nearly 100% nuclear polarization for H and 50% for C can be achieved in various organic molecules when DNP is performed in a strong magnetic field and at cryogenic temperatures. Replacing the C (98.9% natural abundance) isotope with the C isotope at a specific carbon or carbons in a metabolic substrate does not affect the substrate's biochemistry. With C MRS, the body tissues are virtually invisible, and only regions where the hyperpolarized C-labeled substance is present will appear in the generated images. Thus, C-labeled substrates can provide >10,000-fold enhancement of the C MRS signals from the substrate and its subsequent metabolic products, allowing the assessment of changes in metabolic fluxes through glycolysis, citric acid cycle, and fatty acid synthesis (1, 9-13). Vascular and perfusion imaging can also be performed without background signal from surrounding tissues (3, 14). Gallagher et al. generated a hyperpolarized small molecule, [1,4-C]fumarate, as an imaging agent of C MRS for evaluation of early tumor response to treatment and other pathological states in which cell death plays a role, such as toxic insults or ischemia (1). Fumarate, a tricarboxylic acid cycle intermediate, is hydrated to produce malate in a reaction catalyzed by fumarase (or fumarate hydratase). The transport of fumarate across the plasma membrane is mediated by the sodium-dependent dicarboxylate acid transporter, which also transports succinate. Fumarate transport across the mitochondrial membrane occurs in exchange for malate, inorganic phosphate, or aspartate. The reaction enzyme fumarase exists as both cytosolic and mitochondrial isoforms, and the relative activity of the two isoforms varies markedly between tissues and between species. The investigators showed that [1,4-C]malate production from [1,4-C]fumarate is increased in treated lymphoma cells and tumors, and that this increase is caused by tumor cell necrosis. The formation of hyperpolarized [1,4-C]malate from [1,4-C]fumarate appears to be a sensitive marker of tumor cell death and could be used to detect the early response of tumors to treatment (1).