Sheffield Hyperpolarised Imaging of Metabolism and MR Engineering Research

SHIMMER

Sheffield Dissolution DNP

Pyruvate is a commonly used substrate in hyperpolarised MRS studies due to its key location in a number of anabolic and catabolic transformations associated with in energy metabolism, see figure 1. Hyperpolarised dDNP has found major application in studying heart function and cancer of a range of tissues.

Under aerobic conditions functional mitochondria metabolises pyruvate to acetyl-CoA by oxidative phosphorylation. During anaerobic conditions pyruvate is fermented to lactate, which is catalysed by lactate dehydrogenase (Figure 1). Cancer cells often favour the lactate pathway even in the presence of oxygen (Warburg Effect). Therefore, methods for blocking this pathway have therapeutic potential.

In a CRUK funded project grant (CRUK project grant. C1276/A10345) at Sheffield we have been using dissolution DNP to study cancer metabolism. In a P22 fibrosarcoma tumour model we estimate the rate constant for pyruvate to lactate conversion in tumours in response to a hypoxic challenge, using hyperpolarised 13C1-pyruvate and magnetic resonance spectroscopy. This showed that there was that the rate constant for pyruvate to lactate conversion, kpl, responds significantly to a rapid reduction in tumour oxygenation (doi: 10.1016/j.radonc.2015.03.011).

Schematic of glycolytic pathway

Figure 1: Schematic of glycolytic pathway. 13C shows the potential pathways observable by MRS using hyperpolarised pyruvate.

The rate constant for conversion of hyperpolarised 13C1-pyruvate to lactate

Figure 2: The rate constant for conversion of hyperpolarised 13C1-pyruvate to lactate, kpl, (s-1), in P22 fibroscaromas for a) hypoxia group (n=10 pairs) and b) normoxia group (n=8 pairs). Each symbol represents an individual animal. Box plots show the median line, with the box edges representing the 25% and 75% quartiles. Whiskers extend to the furthermost value within 1.5 times the interquartile range from the 25% and 75% quartiles. Outliers are plotted beyond the whiskers. *p<0.05; **p<0.01.

As part of the CRUK project we have constructed automated MRI compatible injections systems, (Applied Magnetic Resonance,July 2012, Volume 43, Issue 1-2, pp 263-273 ) and an improved version (doi:10.1016/j.jmr.2013.10.024). This permitted a reproducible injection regime and could be operated over an arbitrary volume range, see figure 3. the system also increased the MR signal by ensuring that only hyperpolarised material was administered.

We have developed a method for estimating the arterial input function for hyperpolarised substrate without interference from background signal of surrounding tissue see figure 4. Our AIF, rather than the commonly used box-car AIF, provides realistic estimates of the rate constant of conversion of pyruvate to lactate, kpl, the rate constant of conversion of lactate to pyruvate klp, the clearance rate constant of pyruvate from blood to tissue, Kip. (doi: 10.1002/mrm.24546).

Demand volume versus average delivered volume for at least 3 measurements by mass of water

Figure 3: Demand volume versus average delivered volume for at least 3 measurements by mass of water. Error bars represent the standard deviation in delivered volume. Inset shows zoomed region of main figure in the volume range 0.00-2.00 ml.

A measured AIF (in the chamber), the fit to the measured AIF signal and the derived direct AIF

Figure 4: A measured AIF (in the chamber), the fit to the measured AIF signal and the derived direct AIF.