Purpose

Improved metabolic imaging of liver tumors is important clinically, particularly for colon cancer metastases to the liver. Imaging liver tumor metabolism could assist in studying tumor viability, heterogeneity, aggressiveness, treatment response, tumor prognosis and to study metabolic biomarkers for development of novel therapies. Molecular imaging strategies such as FDG-PET have limited sensitivity for small intrahepatic lesions due to high background metabolism of normal hepatocytes. The metabolic conversion of pyruvate to lactate is upregulated in many tumors, a phenomenon known as the Warburg effect¹. Conversely, hyperpolarized (HP) ¹³C MRI is a powerful emerging molecular imaging technique for evaluating metabolism in vivo¹ but it is not selective for different cell types. Gadoxetate (Eovist™, Bayer Pharmaceuticals) is an FDA-approved gadolinium-based contrast agent that is used routinely for visualizing liver tumors. Gadoxetate is selectively taken into hepatocytes but excluded from tumors due to lack of organic anion transporting polypeptide 1 (OATP1²; Fig 1). Gadoxetate has been previously shown to selectively suppress the metabolic signal arising from the liver as compared to kidneys³. In this study, we separate the metabolic signals arising from tumor cells by using gadoxetate to selectively quench the background signals arising from normal hepatocytes.

Methods

CC531 rat-derived colon cancer cells (Cell Lines Services, Heidelberg Germany, 0.5-1 x 10⁶ cells) were implanted into the livers of WAG-Rij rats (n = 2, Charles River Laboratories). Hyperpolarization of 1-¹³C Pyruvate (24 ml) was performed using a 3.35T dynamic nuclear polarizer (Hypersense, Oxford Instruments) at 1.35 K for one hour. The hyperpolarized pyruvate was rapidly dissolved with superheated 80 mM NaOH dissolution buffer and 2-2.5 mL was injected into each rat over 12 secs. Imaging was performed on a clinical 3T MRI (MR750, General Electric) scanner with multinuclear imaging capability and a custom-built dual tuned 1H/13C volume coil. Dynamic ¹³C images of pyruvate and lactate were acquired using a spectrally-selective echo planar imaging strategy⁴, beginning 10 sec after the start of the HP 1-¹³Cpyruvate injection. Images were acquired with a FOV of 64 x 64 mm, matrix size of 16 x 16, 8 mm slice thickness and with TR = 200 ms/metabolite and TE = 18.5 ms. Pyruvate and lactate dynamic images were acquired using an independent flip angle of 10 and 30 degrees respectively. A total of 20 time-frames were acquired for each metabolite, with a temporal resolution of 3 s and a total scan time of 60s. ¹³C images were acquired prior to and 15 minutes following injection of gadoxetate (0.1 mmol/kg) (Fig 2). Metabolite maps (pyruvate and lactate) were analyzed from two different ROIs: one through normal liver and one through liver tumor (Fig 2).

Results

Proton images show expected hepatocyte enhancement following gadoxetate administration, with tumors appearing as defects in gadoxetate uptake (Fig 2B). Tumors demonstrated high levels of lactate at baseline, as expected (Fig 3). Under the influence of gadoxetate, the lactate/pyruvate ratio in normal livers reduced from 0.74 ± 0.22 to 0.55 ± 27 (26%), while lactate/pyruvate ratio in the tumors changed less (from 1.39 ± 0.26 to 1.14 ± 0.24, or 19%, Fig 4A). If only the later time points were used (30-45 s after injection), a much larger difference in the lactate/pyruvate ratio pre and post-gadoxetate between liver (0.96 ± 0.19 to 0.65 ± 0.17 (~32%)) and tumor (1.98 ± 0.39 to 1.97 ± 25 (~1%), Fig 4B) were observed, improving the contrast between the two.

Discussion

The use of targeted relaxation agents with selective compartmentalization is a novel and promising tool for increasing the specificity of hyperpolarized ¹³C MRI. High quality hyperpolarized ¹³C images can be acquired post administration of gadoxetate despite the presence of residual gadolinium circulating in the bloodstream. Gadoxetate was able to successfully suppress the hyperpolarized ¹³C signal arising from hepatocytes as compared to tumors. Greater selectivity was observed at later time points, likely because earlier time points were dominated by metabolites still present in the blood pool rather than cells. Further experiments in a larger sample size are required to confirm these observations. Further optimization of gadoxetate dose (increasing or decreasing the dosage to achieve better contrast between liver and tumor) and timing (>10secs interval between HP ¹³C pyruvate injection and acquisition of ¹³C images) will potentially further improve metabolite contrast in liver and tumor.

Acknowledgements

No acknowledgement found.