Hyperpolarized C-labeled bicarbonate (HCO) for pH measurement with C magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is a technique that allows the non-invasive detection of multiple small metabolites within cells or extracellular spaces (1-4). Although MRS is theoretically applicable to any nucleus possessing spin, the more frequently investigated applications are in proton (H) and carbon-13 (C) (5-7). C MRS is superior to H MRS in many respects (3, 7-9). 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. However, C MRS is limited by the low natural abundance of C (1.1%) and its very low nuclear spin polarization (2.5 × 10 polarization at 3 T and 37ºC) (2, 3). Several techniques have been used to overcome these limitations, including dynamic nuclear polarization (DNP), which introduces one or more C molecules into a metabolic substrate (2, 3, 9). Because 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), hyperpolarized C-labeled tracers can be generated outside the subject and the magnetic resonance scanner (7). 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 isotope (98.9% natural abundance) with the C isotope at a specific carbon or carbons in a metabolic substrate does not affect the substrate’s biochemistry. Hyperpolarized 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 and the imaging of blood vessels and tissue perfusion without background signal from surrounding tissues (1, 3, 4, 10-14). C MRS with DNP technique has also been investigated for measuring tissue pH (4, 15). HCO is the primary extracellular buffer, and it resists changes in pH through interconversion with CO in the reaction catalyzed by carbonic anhydrase. In principal, tissue pH can be determined from C MRS measurements of endogenous HCO and CO because their concentration ratio can be used to calculate pH from the Henderson-Hasselbalch equation with an acid dissociation constant (pa) of 6.17 . On the basis of this principal, Schroeder et al. measured the pH in diseased and healthy cardiac myocytes with simultaneous detection of hyperpolarized [1-C]pyruvate-derived HCO and CO (15). Their results suggest that hyperpolarized [1-C]pyruvate with MRS detection of its derived HCO and CO can be used to measure the intracellular pH (pH) of cardiomyocytes . Similarly, Gallagher et al. generated a non-toxic, pH-probe, hyperpolarized HCO and exploited the pH in tumors with measurement of the HCO and CO concentration ratio after administration of hyperpolarized HCO (4). The tumor microenvironment is characterized by low extracellular pH (pH) and neutral-to-alkaline pH (16, 17). The average pH could be as low as 6.0. A pH gradient (pH > pH) exists across the cell membrane in tumors. This gradient is contrary to that found in normal tissues, in which pH (7.2–7.4) is lower than pH. In addition, diffusion of the H ions along concentration gradients from tumors into adjacent normal tissues creates a peritumoral acid gradient. Accurate measurement of the pH in tissues is of diagnostic and therapeutic value. Imaging with small-molecule agents has been tested for measuring tumor pH. However, agents based on H, P, or F MRS are limited by the inherent low sensitivity of spectroscopy and small pH-dependent chemical shift of these agents (18, 19). The approach with gadolinium (Gd) chelate relaxation agents, which show a pH-dependent hydrogen exchange to the Gd-bound water, requires an accurate determination of the agent concentration, which in practice is difficult to achieve (20). Although positron emission tomography and optical imaging are sensitive, they appear have difficulty obtaining a pH map at high resolution (21-23). Furthermore, most of the published probes predominantly measure the pH within cells, which is more resistant to pH changes than the extracellular space. The data obtained by Gallagher et al. from hyperpolarized HCO indicated that hyperpolarized HCO provided a means to measure the pH rather than the pH (4). Given the range of pathological conditions in which the acid–base balance is altered, this technique may prove to be of diagnostic value not only in oncology but also in the imaging of ischemia and inflammation (4).