HP MR Probes for Translation

Many probes have been recently developed to study cancer metabolism and response to therapy, including HP pyruvate, fructose, bicarbonate, and fumarate. These probes take advantage of long T1 relaxation times, which govern the lifetime of the probe, making it useful for imaging. In order to develop new HP probes, we utilize a combination of biochemical and biophysical approaches, with a specific metabolic aberration of interest.

Recently, we have been very interested in oxidative stress and the effects of reactive oxygen species (ROS) on the cancer cell. Reducing and oxidizing (Redox) state is a balance that is tightly regulated in the cell, maintained by a wide range of enzymatic and signaling pathways. At the center of this interplay is our predominant antioxidant glutathione, vitamin C, and other redox species such as NADPH.

With the increased generation of ROS, the redox state of the cell is stressed and this can affect a huge number of pathways in cancer. With this in mind we have developed an endogenous HP redox sensor, [1-13C] dehydroascorbate (DHA), which is the oxidized form of vitamin C. HP DHA can rapidly enter the cell and be reduced to vitamin C (1) (Figure 1). This rate of reduction is indicative of the cells capacity to reduce substrates and allows DHA to inform on the cellular redox network.

We’ve found that while some cells overall become oxidized, the internal concentration of reductant is still dramatically increased, providing an environment where cells can continue to rapidly reduce substrates (2). Broadly, this suggests a prognostic role for this new redox sensor in determining the vulnerability of both normal and abnormal tissues to ROS. In the case of cancer, this could impact many of our chemotherapeutics and radiation regiments, where the cancer cell can use this mechanism to survive therapy.

Figure 1 -- <em>Biochemical mechanism, hyperpolarisation, and reduction of [1-13C] DHA. </em> (A) Relationship between redox pairs NADPH/NADP, GSSG/GSH and vitamin C/DHA with associated enzymes. (B) Reversible reduction of labeled DHA to vitamin C demonstrating the position of the hyperpolarized carbon. (C) Sequential coronal T2-weighted images and corresponding 13C 3D MRSI demonstrating distribution of hyperpolarized DHA and vitamin C (VitC) in a transgenic adenocarcinoma model of prostate (TRAMP) mouse post–intravenous injection of 350uL of 15mM hyperpolarized [1-13C] DHA. This data set was collected using a variable tip-angle scheme initiated 25s following this injection, with individual voxels describing the relative reduction of [1-13C] DHA to [1-13C] VitC at the same time point. The liver and kidneys are best seen in (top image) and prostate tumor in (bottom image), but both imaging slices contain significant amounts of liver, kidney, and tumor tissue. Regions of liver, kidney and prostate tumor are segmented and superimposed on the spectral grid (color-coded dashed lines).  Differences in metabolite ratios are seen between normal organs and between prostate tumor and normal surrounding tissues. Representative 13C spectra from liver, kidney, and prostate tumor in a TRAMP mouse are shown to the right. (D) Axial T2-weighted images and corresponding color overlays of hyperpolarized DHA and VitC signal in a normal rat brain.