Mark J. Albers

Hyperpolarized 13C Lactate, Pyruvate, and Alanine: Noninvasive Biomarkers for Prostate Cancer Detection and Grading

An extraordinary new technique using hyperpolarized (13)C-labeled pyruvate and taking advantage of increased glycolysis in cancer has the potential to improve the way magnetic resonance imaging is used for detection and characterization of prostate cancer. The aim of this study was to quantify, for the first time, differences in hyperpolarized [1-(13)C] pyruvate and its metabolic products between the various histologic grades of prostate cancer using the transgenic adenocarcinoma of mouse prostate (TRAMP) model. Fast spectroscopic imaging techniques were used to image lactate, alanine, and total hyperpolarized carbon (THC = lactate + pyruvate + alanine) from the entire abdomen of normal mice and TRAMP mice with low- and high-grade prostate tumors in 14 s. Within 1 week, the mice were dissected and the tumors were histologically analyzed. Hyperpolarized lactate SNR levels significantly increased (P < 0.05) with cancer development and progression (41 +/- 11, 74 +/- 17, and 154 +/- 24 in normal prostates, low-grade primary tumors, and high-grade primary tumors, respectively) and had a correlation coefficient of 0.95 with the histologic grade. In addition, there was minimal overlap in the lactate levels between the three groups with only one of the seven normal prostates overlapping with the low-grade primary tumors. The amount of THC, a possible measure of substrate uptake, and hyperpolarized alanine also increased with tumor grade but showed more overlap between the groups. In summary, elevated hyperpolarized lactate and potentially THC and alanine are noninvasive biomarkers of prostate cancer presence and histologic grade that could be used in future three-dimensional (13)C spectroscopic imaging studies of prostate cancer patients.

Hyperpolarized C-13 spectroscopic imaging of the TRAMP mouse at 3T—Initial experience

The transgenic adenocarcinoma of mouse prostate (TRAMP) mouse is a well‐studied murine model of prostate cancer with histopathology and disease progression that mimic the human disease. To investigate differences in cellular bioenergetics between normal prostate epithelial cells and prostate tumor cells, in vivo MR spectroscopic (MRS) studies with non‐proton nuclei, such as 13C, in the TRAMP model would be extremely useful. The recent development of a method for retaining dynamic nuclear polarization (DNP) in solution permits high signal‐to‐noise ratio (SNR) 13C MRI or MRSI data to be obtained following injection of a hyperpolarized 13C agent. In this transgenic mouse study, this method was applied using a double spin‐echo (DSE) pulse sequence with a small‐tip‐angle excitation RF pulse, hyperbolic‐secant refocusing pulses, and a flyback echo‐planar readout trajectory for fast (10–14 s) MRSI of 13C pyruvate (pyr) and its metabolic products at 0.135 cm3 nominal spatial resolution. Elevated 13C lactate (lac) was observed in both primary and metastatic tumors, demonstrating the feasibility of studying cellular bioenergetics in vivo with DNP hyperpolarized 13C MRSI. Magn Reson Med, 2007.

In vivo 13carbon metabolic imaging at 3T with hyperpolarized 13C-1-pyruvate

We present for the first time dynamic spectra and spectroscopic images acquired in normal rats at 3T following the injection of 13C‐1‐pyruvate that was hyperpolarized by the dynamic nuclear polarization (DNP) method. Spectroscopic sampling was optimized for signal‐to‐noise ratio (SNR) and for spectral resolution of 13C‐1‐pyruvate and its metabolic products 13C‐1‐alanine, 13C‐1‐lactate, and 13C‐bicarbonate. Dynamic spectra in rats were collected with a temporal resolution of 3 s from a 90‐mm axial slab using a dual 1H‐13C quadrature birdcage coil to observe the combined effects of metabolism, flow, and T1 relaxation. In separate experiments, spectroscopic imaging data were obtained during a 17‐s acquisition of a 20‐mm axial slice centered on the rat kidney region to provide information on the spatial distribution of the metabolites. Conversion of pyruvate to lactate, alanine, and bicarbonate occurred within a minute of injection. Alanine was observed primarily in skeletal muscle and liver, while pyruvate, lactate, and bicarbonate concentrations were relatively high in the vasculature and kidneys. In contrast to earlier work at 1.5T, bicarbonate was routinely observed in skeletal muscle as well as the kidney and vasculature. Magn Reson Med 58:65–69, 2007.

Evaluation of lactate and alanine as metabolic biomarkers of prostate cancer using 1H HR-MAS spectroscopy of biopsy tissues

The goal of this study was to investigate the use of lactate and alanine as metabolic biomarkers of prostate cancer using 1H high‐resolution magic angle spinning (HR‐MAS) spectroscopy of snap‐frozen transrectal ultrasound (TRUS)‐guided prostate biopsy tissues. A long‐echo‐time rotor‐synchronized Carr‐Purcell‐Meiboom‐Gill (CPMG) sequence including an electronic reference to access in vivo concentrations (ERETIC) standard was used to determine the concentrations of lactate and alanine in 82 benign and 16 malignant biopsies (mean 26.5% ± 17.2% of core). Low concentrations of lactate (0.61 ± 0.28 mmol/kg) and alanine (0.14 ± 0.06 mmol/kg) were observed in benign prostate biopsies, and there was no significant difference between benign predominantly glandular (N = 54) and stromal (N = 28) biopsies between patients with (N = 38) and without (N = 44) a positive clinical biopsy. In biopsies containing prostate cancer there was a highly significant (P < 0.0001) increase in lactate (1.59 ± 0.61 mmol/kg) and alanine (0.26 ± 0.07 mmol/kg), and minimal overlap with lactate concentrations in benign biopsies. This study demonstrates for the first time very low concentrations of lactate and alanine in benign prostate biopsy tissues. The significant increase in the concentration of both lactate and alanine in biopsy tissue containing as little as 5% cancer could be exploited in hyperpolarized 13C spectroscopic imaging (SI) studies of prostate cancer patients. Magn Reson Med 60:510–516, 2008.

Kinetic modeling of hyperpolarized 13C1-pyruvate metabolism in normal rats and TRAMP mice

PURPOSE To investigate metabolic exchange between (13)C(1)-pyruvate, (13)C(1)-lactate, and (13)C(1)-alanine in pre-clinical model systems using kinetic modeling of dynamic hyperpolarized (13)C spectroscopic data and to examine the relationship between fitted parameters and dose-response. MATERIALS AND METHODS Dynamic (13)C spectroscopy data were acquired in normal rats, wild type mice, and mice with transgenic prostate tumors (TRAMP) either within a single slice or using a one-dimensional echo-planar spectroscopic imaging (1D-EPSI) encoding technique. Rate constants were estimated by fitting a set of exponential equations to the dynamic data. Variations in fitted parameters were used to determine model robustness in 15 mm slices centered on normal rat kidneys. Parameter values were used to investigate differences in metabolism between and within TRAMP and wild type mice. RESULTS The kinetic model was shown here to be robust when fitting data from a rat given similar doses. In normal rats, Michaelis-Menten kinetics were able to describe the dose-response of the fitted exchange rate constants with a 13.65% and 16.75% scaled fitting error (SFE) for k(pyr-->lac) and k(pyr-->ala), respectively. In TRAMP mice, k(pyr-->lac) increased an average of 94% after up to 23 days of disease progression, whether the mice were untreated or treated with casodex. Parameters estimated from dynamic (13)C 1D-EPSI data were able to differentiate anatomical structures within both wild type and TRAMP mice. CONCLUSIONS The metabolic parameters estimated using this approach may be useful for in vivo monitoring of tumor progression and treatment efficacy, as well as to distinguish between various tissues based on metabolic activity.

Imaging Considerations for In Vivo 13C Metabolic Mapping Using Hyperpolarized 13C-Pyruvate

One of the challenges of optimizing signal‐to‐noise ratio (SNR) and image quality in 13C metabolic imaging using hyperpolarized 13C‐pyruvate is associated with the different MR signal time‐courses for pyruvate and its metabolic products, lactate and alanine. The impact of the acquisition time window, variation of flip angles, and order of phase encoding on SNR and image quality were evaluated in mathematical simulations and rat experiments, based on multishot fast chemical shift imaging (CSI) and three‐dimensional echo‐planar spectroscopic imaging (3DEPSI) sequences. The image timing was set to coincide with the peak production of lactate. The strategy of combining variable flip angles and centric phase encoding (cPE) improved image quality while retaining good SNR. In addition, two aspects of EPSI sampling strategies were explored: waveform design (flyback vs. symmetric EPSI) and spectral bandwidth (BW = 500 Hz vs. 267 Hz). Both symmetric EPSI and reduced BW trended toward increased SNR. The imaging strategies reported here can serve as guidance to other multishot spectroscopic imaging protocols for 13C metabolic imaging applications. Magn Reson Med, 2009.

Evaluation of the ERETIC method as an improved quantitative reference for 1H HR-MAS spectroscopy of prostate tissue.

The Electronic REference To access In vivo Concentrations (ERETIC) method was applied to 1H HR‐MAS spectroscopy. The accuracy, precision, and stability of ERETIC as a quantitative reference were evaluated in solution and human prostate tissue samples. For comparison, the reliability of 3‐(trimethylsilyl)propionic‐2,2,3,3‐d4 acid (TSP) as a quantitation reference was also evaluated. The ERETIC and TSP peak areas were found to be stable in solution over the short‐term and long‐term, with long‐term relative standard deviations (RSDs) of 4.10% and 2.60%, respectively. Quantification of TSP in solution using the ERETIC peak as a reference and a calibrated, rotor‐dependent conversion factor yielded results with a precision ≤2.9% and an accuracy error ≤4.2% when compared with the expected values. The ERETIC peak area reproducibility was superior to TSPs reproducibility, corrected for mass, in both prostate surgical and biopsy samples (4.53% vs. 21.2% and 3.34% vs. 31.8%, respectively). Furthermore, the tissue TSP peaks exhibited only 27.5% of the expected area, which would cause an overestimation of metabolite concentrations if used as a reference. The improved quantification accuracy and precision provided by ERETIC may enable the detection of smaller metabolic differences that may exist between individual tissue samples and disease states. Magn Reson Med 61:525–532, 2009.

Semi-parametric time-domain quantification of HR-MAS data from prostate tissue

High Resolution – Magic Angle Spinning (HR‐MAS) spectroscopy provides rich biochemical profiles that require accurate quantification to permit biomarker identification and to understand the underlying pathological mechanisms. Meanwhile, quantification of HR‐MAS data from prostate tissue samples is challenging due to significant overlap between the resonant peaks, the presence of short

Methods for metabolic evaluation of prostate cancer cells using proton and 13C HR‐MAS spectroscopy and [3‐13C] pyruvate as a metabolic substrate

T_{2}^{*}

Double spin-echo sequence for rapid spectroscopic imaging of hyperpolarized 13C.

metabolites such as citrate or polyamines (T2 from 25 to 100 msec) and macromolecules, and variations in chemical shifts and