Bio

Honors & Awards


  • Junior Fellow, International Society of Magnetic Resonance in Medicine (2014)
  • Summa Cum Laude, International Society of Magnetic Resonance in Medicine (2014)
  • Magna Cum Laude, International Society of Magnetic Resonance in Medicine (2013)

Professional Education


  • Doctor of Philosophy, Stanford University, EE-PHD (2012)
  • Master of Science, Stanford University, EE-MS (2009)

Stanford Advisors


Research & Scholarship

Lab Affiliations


Publications

Journal Articles


  • The feasibility of assessing branched-chain amino acid metabolism in cellular models of prostate cancer with hyperpolarized [1-(13)C]-ketoisocaproate. Magnetic resonance imaging Billingsley, K. L., Park, J. M., Josan, S., Hurd, R., Mayer, D., Spielman-Sun, E., Nishimura, D. G., Brooks, J. D., Spielman, D. 2014; 32 (7): 791-795

    Abstract

    Recent advancements in the field of hyperpolarized (13)C magnetic resonance spectroscopy (MRS) have yielded powerful techniques capable of real-time analysis of metabolic pathways. These non-invasive methods have increasingly shown application in impacting disease diagnosis and have further been employed in mechanistic studies of disease onset and progression. Our goals were to investigate branched-chain aminotransferase (BCAT) activity in prostate cancer with a novel molecular probe, hyperpolarized [1-(13)C]-2-ketoisocaproate ([1-(13)C]-KIC), and explore the potential of branched-chain amino acid (BCAA) metabolism to serve as a biomarker. Using traditional spectrophotometric assays, BCAT enzymatic activities were determined in vitro for various sources of prostate cancer (human, transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse and human cell lines). These preliminary studies indicated that low levels of BCAT activity were present in all models of prostate cancer but enzymatic levels are altered significantly in prostate cancer relative to healthy tissue. The MR spectroscopic studies were conducted with two cellular models (PC-3 and DU-145) that exhibited levels of BCAA metabolism comparable to the human disease state. Hyperpolarized [1-(13)C]-KIC was administered to prostate cancer cell lines, and the conversion of [1-(13)C]-KIC to the metabolic product, [1-(13)C]-leucine ([1-(13)C]-Leu), could be monitored via hyperpolarized (13)C MRS.

    View details for DOI 10.1016/j.mri.2014.04.015

    View details for PubMedID 24907854

  • Dynamic Metabolic Imaging of Hyperpolarized [2-C-13]Pyruvate Using Spiral Chemical Shift Imaging with Alternating Spectral Band Excitation MAGNETIC RESONANCE IN MEDICINE Josan, S., Hurd, R., Park, J. M., Yen, Y., Watkins, R., Pfefferbaum, A., Spielman, D., Mayer, D. 2014; 71 (6): 2051-2058

    Abstract

    In contrast to [1-(13) C]pyruvate, hyperpolarized [2-(13) C]pyruvate permits the ability to follow the (13) C label beyond flux through pyruvate dehydrogenase complex and investigate the incorporation of acetyl-coenzyme A into different metabolic pathways. However, chemical shift imaging (CSI) with [2-(13) C]pyruvate is challenging owing to the large spectral dispersion of the resonances, which also leads to severe chemical shift displacement artifacts for slice-selective acquisitions.This study introduces a sequence for three-dimensional CSI of [2-(13) C]pyruvate using spectrally selective excitation of limited frequency bands containing a subset of metabolites. Dynamic CSI data were acquired alternately from multiple frequency bands in phantoms for sequence testing and in vivo in rat heart.Phantom experiments verified the radiofrequency pulse design and demonstrated that the signal behavior of each group of resonances was unaffected by excitation of the other frequency bands. Dynamic three-dimensional (13) C CSI data demonstrated the sequence capability to image pyruvate, lactate, acetylcarnitine, glutamate, and acetoacetate, enabling the analysis of organ-specific spectra and metabolite time courses.The presented method allows CSI of widely separated resonances without chemical shift displacement artifact, acquiring multiple frequency bands alternately to obtain dynamic time-course information. This approach enables robust imaging of downstream metabolic products of acetyl-coenzyme A with hyperpolarized [2-(13) C]pyruvate. Magn Reson Med, 2013. 2013 Wiley Periodicals, Inc.

    View details for DOI 10.1002/mrm.24871

    View details for Web of Science ID 000336260900013

  • Hyperpolarized [1,4-C-13]-diethylsuccinate: a potential DNP substrate for in vivo metabolic imaging NMR IN BIOMEDICINE Billingsley, K. L., Josan, S., Park, J. M., Tee, S. S., Spielman-Sun, E., Hurd, R., Mayer, D., Spielman, D. 2014; 27 (3): 356-362

    Abstract

    The tricarboxylic acid (TCA) cycle performs an essential role in the regulation of energy and metabolism, and deficiencies in this pathway are commonly correlated with various diseases. However, the development of non-invasive techniques for the assessment of the cycle in vivo has remained challenging. In this work, the applicability of a novel imaging agent, [1,4-(13) C]-diethylsuccinate, for hyperpolarized (13) C metabolic imaging of the TCA cycle was explored. In vivo spectroscopic studies were conducted in conjunction with in vitro analyses to determine the metabolic fate of the imaging agent. Contrary to previous reports (Zacharias NM et al. J. Am. Chem. Soc. 2012; 134: 934-943), [(13) C]-labeled diethylsuccinate was primarily metabolized to succinate-derived products not originating from TCA cycle metabolism. These results illustrate potential issues of utilizing dialkyl ester analogs of TCA cycle intermediates as molecular probes for hyperpolarized (13) C metabolic imaging. Copyright 2014 John Wiley & Sons, Ltd.

    View details for DOI 10.1002/nbm.3071

    View details for Web of Science ID 000330798100013

  • In vivo investigation of cardiac metabolism in the rat using MRS of hyperpolarized [1-C-13] and [2-C-13]pyruvate NMR IN BIOMEDICINE Josan, S., Park, J. M., Hurd, R., Yen, Y., Pfefferbaum, A., Spielman, D., Mayer, D. 2013; 26 (12): 1680-1687

    Abstract

    Hyperpolarized (13) C MRS allows the in vivo assessment of pyruvate dehydrogenase complex (PDC) flux, which converts pyruvate to acetyl-coenzyme A (acetyl-CoA). [1-(13) C]pyruvate has been used to measure changes in cardiac PDC flux, with demonstrated increase in (13) C-bicarbonate production after dichloroacetate (DCA) administration. With [1-(13) C]pyruvate, the (13) C label is released as (13) CO2 /(13) C-bicarbonate, and, hence, does not allow us to follow the fate of acetyl-CoA. Pyruvate labeled in the C2 position has been used to track the (13) C label into the TCA (tricarboxylic acid) cycle and measure [5-(13) C]glutamate as well as study changes in [1-(13) C]acetylcarnitine with DCA and dobutamine. This work investigates changes in the metabolic fate of acetyl-CoA in response to metabolic interventions of DCA-induced increased PDC flux in the fed and fasted state, and increased cardiac workload with dobutamine in vivo in rat heart at two different pyruvate doses. DCA led to a modest increase in the (13) C labeling of [5-(13) C]glutamate, and a considerable increase in [1-(13) C]acetylcarnitine and [1,3-(13) C]acetoacetate peaks. Dobutamine resulted in an increased labeling of [2-(13) C]lactate, [2-(13) C]alanine and [5-(13) C]glutamate. The change in glutamate with dobutamine was observed using a high pyruvate dose but not with a low dose. The relative changes in the different metabolic products provide information about the relationship between PDC-mediated oxidation of pyruvate and its subsequent incorporation into the TCA cycle compared with other metabolic pathways. Using a high dose of pyruvate may provide an improved ability to observe changes in glutamate. Copyright 2013 John Wiley & Sons, Ltd.

    View details for DOI 10.1002/nbm.3003

    View details for Web of Science ID 000327157400007

  • Measuring mitochondrial metabolism in rat brain in vivo using MR Spectroscopy of hyperpolarized [2-C-13]pyruvate NMR IN BIOMEDICINE Park, J. M., Josan, S., Grafendorfer, T., Yen, Y., Hurd, R. E., Spielman, D. M., Mayer, D. 2013; 26 (10): 1197-1203

    Abstract

    Hyperpolarized [1-(13) C]pyruvate ([1-(13) C]Pyr) has been used to assess metabolism in healthy and diseased states, focusing on the downstream labeling of lactate (Lac), bicarbonate and alanine. Although hyperpolarized [2-(13) C]Pyr, which retains the labeled carbon when Pyr is converted to acetyl-coenzyme A, has been used successfully to assess mitochondrial metabolism in the heart, the application of [2-(13) C]Pyr in the study of brain metabolism has been limited to date, with Lac being the only downstream metabolic product reported previously. In this study, single-time-point chemical shift imaging data were acquired from rat brain in vivo. [5-(13) C]Glutamate, [1-(13) C]acetylcarnitine and [1-(13) C]citrate were detected in addition to resonances from [2-(13) C]Pyr and [2-(13) C]Lac. Brain metabolism was further investigated by infusing dichloroacetate, which upregulates Pyr flux to acetyl-coenzyme A. After dichloroacetate administration, a 40% increase in [5-(13) C]glutamate from 0.014??0.004 to 0.020??0.006 (p?=?0.02), primarily from brain, and a trend to higher citrate (0.002??0.001 to 0.004??0.002) were detected, whereas [1-(13) C]acetylcarnitine was increased in peripheral tissues. This study demonstrates, for the first time, that hyperpolarized [2-(13) C]Pyr can be used for the in vivo investigation of mitochondrial function and tricarboxylic acid cycle metabolism in brain. Copyright 2013 John Wiley & Sons, Ltd.

    View details for DOI 10.1002/nbm.2935

    View details for Web of Science ID 000324462100001

  • Effects of Isoflurane Anesthesia on Hyperpolarized C-13 Metabolic Measurements in Rat Brain MAGNETIC RESONANCE IN MEDICINE Josan, S., Hurd, R., Billingsley, K., Senadheera, L., Park, J. M., Yen, Y., Pfefferbaum, A., Spielman, D., Mayer, D. 2013; 70 (4): 1117-1124

    View details for DOI 10.1002/mrm.24532

    View details for Web of Science ID 000325136300023

  • Metabolic response of glioma to dichloroacetate measured in vivo by hyperpolarized C-13 magnetic resonance spectroscopic imaging NEURO-ONCOLOGY Park, J. M., Recht, L. D., Josan, S., Merchant, M., Jang, T., Yen, Y., Hurd, R. E., Spielman, D. M., Mayer, D. 2013; 15 (4): 433-441

    Abstract

    The metabolic phenotype that derives disproportionate energy via glycolysis in solid tumors, including glioma, leads to elevated lactate labeling in metabolic imaging using hyperpolarized [1-(13)C]pyruvate. Although the pyruvate dehydrogenase (PDH)-mediated flux from pyruvate to acetyl coenzyme A can be indirectly measured through the detection of carbon-13 ((13)C)-labeled bicarbonate, it has proven difficult to visualize (13)C-bicarbonate at high enough levels from injected [1-(13)C]pyruvate for quantitative analysis in brain. The aim of this study is to improve the detection of (13)C-labeled metabolites, in particular bicarbonate, in glioma and normal brain in vivo and to measure the metabolic response to dichloroacetate, which upregulates PDH activity.An optimized protocol for chemical shift imaging and high concentration of hyperpolarized [1-(13)C]pyruvate were used to improve measurements of lactate and bicarbonate in C6 glioma-transplanted rat brains. Hyperpolarized [1-(13)C]pyruvate was injected before and 45 min after dichloroacetate infusion. Metabolite ratios of lactate to bicarbonate were calculated to provide improved metrics for characterizing tumor metabolism.Glioma and normal brain were well differentiated by lactate-to-bicarbonate ratio (P = .002, n = 5) as well as bicarbonate (P = .0002) and lactate (P = .001), and a stronger response to dichloroacetate was observed in glioma than in normal brain.Our results clearly demonstrate for the first time the feasibility of quantitatively detecting (13)C-bicarbonate in tumor-bearing rat brain in vivo, permitting the measurement of dichloroacetate-modulated changes in PDH flux. The simultaneous detection of lactate and bicarbonate provides a tool for a more comprehensive analysis of glioma metabolism and the assessment of metabolic agents as anti-brain cancer drugs.

    View details for DOI 10.1093/neuonc/nos319

    View details for Web of Science ID 000316965600005

    View details for PubMedID 23328814

  • Metabolite kinetics in C6 rat glioma model using magnetic resonance spectroscopic imaging of hyperpolarized [1-13C]pyruvate MAGNETIC RESONANCE IN MEDICINE Park, J. M., Josan, S., Jang, T., Merchant, M., Yen, Y., Hurd, R. E., Recht, L., Spielman, D. M., Mayer, D. 2012; 68 (6): 1886-1893

    Abstract

    In addition to an increased lactate-to-pyruvate ratio, altered metabolism of a malignant glioma can be further characterized by its kinetics. Spatially resolved dynamic data of pyruvate and lactate from C6-implanted female Sprague-Dawley rat brain were acquired using a spiral chemical shift imaging sequence after a bolus injection of a hyperpolarized [1-(13)C]pyruvate. Apparent rate constants for the conversion of pyruvate to lactate in three different regions (glioma, normal appearing brain, and vasculature) were estimated based on a two-site exchange model. The apparent conversion rate constant was 0.018 0.004 s(-1) (mean standard deviation, n = 6) for glioma, 0.009 0.003 s(-1) for normal brain, and 0.005 0.001 s(-1) for vasculature, whereas the lactate-to-pyruvate ratio, the metabolic marker used to date to identify tumor regions, was 0.36 0.07 (mean SD), 0.24 0.07, and 0.12 0.02 for glioma, normal brain, and vasculature, respectively. The data suggest that the apparent conversion rate better differentiate glioma from normal brain (P = 0.001, n = 6) than the lactate-to-pyruvate ratio (P = 0.02).

    View details for DOI 10.1002/mrm.24181

    View details for Web of Science ID 000311398600022

    View details for PubMedID 22334279

  • Application of hyperpolarized [1-13C]lactate for the in vivo investigation of cardiac metabolism NMR IN BIOMEDICINE Mayer, D., Yen, Y., Josan, S., Park, J. M., Pfefferbaum, A., Hurd, R. E., Spielman, D. M. 2012; 25 (10): 1119-1124

    Abstract

    In addition to cancer imaging, (13) C-MRS of hyperpolarized pyruvate has also demonstrated utility for the investigation of cardiac metabolism and ischemic heart disease. Although no adverse effects have yet been reported for doses commonly used in vivo, high substrate concentrations have lead to supraphysiological pyruvate levels that can affect the underlying metabolism and should be considered when interpreting results. With lactate serving as an important energy source for the heart and physiological lactate levels one to two orders of magnitude higher than for pyruvate, hyperpolarized lactate could potentially be used as an alternative to pyruvate for probing cardiac metabolism. In this study, hyperpolarized [1-(13) C]lactate was used to acquire time-resolved spectra from the healthy rat heart in vivo and to measure dichloroacetate (DCA)-modulated changes in flux through pyruvate dehydrogenase (PDH). Both primary oxidation of lactate to pyruvate and subsequent conversion of pyruvate to alanine and bicarbonate could reliably be detected. Since DCA stimulates the activity of PDH through inhibition of PDH kinase, a more than 2.5-fold increase in bicarbonate-to-substrate ratio was found after administration of DCA, similar to the effect when using [1-(13) C]pyruvate as the substrate.

    View details for DOI 10.1002/nbm.2778

    View details for Web of Science ID 000308710400003

    View details for PubMedID 22278751

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