Analysis of cancer metabolism by imaging hyperpolarized nuclei: prospects for translation to clinical research

Abstract

A major challenge in cancer biology is to monitor and understand cancer metabolism in vivo with the goal of improved diagnosis and perhaps therapy. Because of the complexity of biochemical pathways, tracer methods are required for detecting specific enzyme-catalyzed reactions. Stable isotopes such as (13)C or (15)N with detection by nuclear magnetic resonance provide the necessary information about tissue biochemistry, but the crucial metabolites are present in low concentration and therefore are beyond the detection threshold of traditional magnetic resonance methods. A solution is to improve sensitivity by a factor of 10,000 or more by temporarily redistributing the populations of nuclear spins in a magnetic field, a process termed hyperpolarization. Although this effect is short-lived, hyperpolarized molecules can be generated in an aqueous solution and infused in vivo where metabolism generates products that can be imaged. This discovery lifts the primary constraint on magnetic resonance imaging for monitoring metabolism-poor sensitivity-while preserving the advantage of biochemical information. The purpose of this report was to briefly summarize the known abnormalities in cancer metabolism, the value and limitations of current imaging methods for metabolism, and the principles of hyperpolarization. Recent preclinical applications are described. Hyperpolarization technology is still in its infancy, and current polarizer equipment and methods are suboptimal. Nevertheless, there are no fundamental barriers to rapid translation of this exciting technology to clinical research and perhaps clinical care.

Figure 1

(A) Diagram demonstrating the metabolism of [1-¹³C]pyruvate, which can be converted to [1-¹³C]lactate in a reaction catalyzed by the enzyme LDH, [1-¹³C]alanine in a reaction catalyzed by ALT, and ¹³C-carbon dioxide in a reaction catalyzed by PDH). The carbon dioxide released is subsequently interconverted with bicarbonate in a reaction catalyzed by carbonic anhydrase (CA). (B) The great strength of hyperpolarized ¹³C NMR is the ability to measure not only the uptake of the labeled substrate but also its metabolic products. For example, in the hyperpolarized ¹³C NMR spectrum of an isolated rat heart perfused with hyperpolarized [1-¹³C]pyruvate, metabolite resonances due to ¹³CO₂, H¹³CO₃ [1-¹³C]pyruvate, [1-¹³C]lactate, and [1-¹³C]alanine can be readily observed in a single acquisition. (C) Because of the dramatic increase in sensitivity of hyperpolarized MR, hyperpolarized products of pyruvate metabolism can also be measured with a 1-second temporal resolution and metabolic fluxes calculated from the kinetic information. Figure adapted from Merritt et al. [128].

Figure 2

(A) Representative hematoxylin and eosin-stained pathologic sections (magnification, x40) and hyperpolarized ¹³C spectra from a normal mouse prostate, an early stage and late transgenic mouse prostate tumor (TRAMP), and a lymph node metastases. Below the histologic sections are representative hyperpolarized ¹³C spectra acquired after injection of hyperpolarized [1-¹³C]pyruvate and normalized to correct for differences in polarization. The normalized spectra exhibited a visually clear increase of hyperpolarized lactate and hyperpolarized lactate-to-pyruvate ratio with progression from the normal to early and late-stage tumors and metastases. (B) Axial T₂-weighted ¹H image depicting the primary tumor and lymph node metastasis from a TRAMP mouse with a late-stage primary tumor and the overlay of hyperpolarized [1-¹³C]lactate image after the injection of 350 µl of hyperpolarized [1-¹³C]pyruvate. Hyperpolarized [1-¹³C] lactate increased in going from normal to prostate cancer and with disease progression. (C) A box plot quantitatively summarizing the peak area-to-noise ratios of the [1-¹³C]lactate-to-noise ratio for the four histologically defined groups. The lactate peak area SNR values were statistically different (P < .05) for all four groups, except that early stage tumors were not significantly different from lymph node metastases. In addition, there was minimal overlap between individual [1-¹³C]lactate-to-noise ratios between normal prostates and early and late-stage tumors. Figure adapted from Albers et al. [123].

Figure 3

(A) Transverse proton MR image of amouse with a subcutaneously implanted EL4 tumor (outlined in red). (B) pH map of the same animal calculated from the ratio of the H¹³CO₃ acquired 10 seconds after intravenous injection of 100 mM hyperpolarized H¹³CO₃^- and assuming a pK a of 6.17 (pH = pK _a + log ([HCO₃] / [CO₂]). Figure adapted from Gallagher et al. [137].

Figure 4

Prototype polarizer (top left) has been installed in a clean room adjacent to a 3-T clinical MR scanner in preparation for a phase 1 clinical trial of [1-¹³C]pyruvate in prostate cancer patients. The ¹³C/¹H coil designs shown (bottom left) are those that will be used in the patient studies. These coils have been tested in canine studies and provided the ¹³C MRSI data shown on the right. On the top right is a representative canine prostate ¹³C MRSI array obtained after injection of hyperpolarized [1-¹³C]pyruvate dose volume of 1.4 ml/kg of body weight, a dose that was determined to be safe in healthy human volunteer studies. The corresponding [1-¹³C]pyruvate and [1-¹³C]lactate images overlaid on a axial T₂-weighted image of the canine prostate is shown on the bottom right. The ¹³C MRSI data were collected in 15 seconds at a spatial resolution of 0.125 cm³ demonstrating high levels of [1-¹³C]pyruvate (SNR, ≥200) and lower [1-¹³C]lactate levels consistent with normal canine prostate tissue metabolism (right). These preliminary MR metabolic imaging studies demonstrated that the T ₁ of hyperpolarized [1-¹³C]pyruvate was sufficient to allow delivery to the prostate and metabolism in a large animal. In addition, there was sufficient sensitivity to detect metabolism throughout the prostate at a much greater spatial and temporal resolution than previously possible with other MR metabolic imaging techniques. Figure adapted from Nelson et al. [153].