F O C U S O N M O L E C U L A R I M A G I N G

Hyperpolarized 13C MR for Molecular Imaging of ProstateCancer

David M. Wilson and John Kurhanewicz

Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, California

Hyperpolarization using dissolution dynamic nuclear polarizationhas emerged as a versatile method to dramatically improve theMR signal of low-sensitivity nuclei. This technique facilitates thestudy of real-time metabolism in vitro and in vivo using 13C-enriched substrates and has been applied to numerous modelsof human disease. In particular, several mechanisms underlyingprostate cancer have been interrogated using hyperpolarized13C MR spectroscopy. This review highlights key metabolic shiftsseen in prostate cancer, their study by hyperpolarized 13C MRspectroscopy, and the development of new platforms for meta-bolic study.

Key Words: PET/MR imaging; PET/MRI; 13C-magnetic reso-nance; prostate; prostate cancer; hyperpolarized 13C; metabolism

J Nucl Med 2014; 55:1–6DOI: 10.2967/jnumed.114.141705

The recent first-in-man hyperpolarized 13C MR imagingstudy in prostate cancer patients has confirmed the clin-ical potential of this remarkable technology (1). Hyper-polarization using dynamic nuclear polarization (DNP) isnow a robust technique used to dramatically increase thesignal-to-noise ratio of nuclei that have a spin of one half.This increased sensitivity has facilitated the study of chem-ical phenomena in solution and the probing of a variety ofmetabolic pathways in vitro and in vivo (2). The techniqueis particularly sensitive to the metabolic reprogrammingthat occurs in malignancy and has been applied to severalcell and animal models of prostate cancer.Prostate cancer is now the most commonly diagnosed

cancer in men in the United States, with an estimated233,000 new cases of the disease in 2014 (3). The landscapeof the disease is changing rapidly, motivating new therapeu-tic and diagnostic approaches. There is considerable debateabout prostate cancer management, based on the efficacy ofcommon treatments (most commonly antiandrogen therapy,

radiotherapy, and radical prostatectomy), side effects, andrecent progress in the understanding of prostate cancer bi-ology. Radical prostatectomy remains the most frequentlyrecommended treatment for patients with a life expectancyof greater than 10 y but has severe side effects, includingerectile dysfunction and urinary incontinence. Expectantmanagement, or watchful waiting, may be the answer forpatients in whom disease is likely to be indolent. However,the diagnostic tools needed to predict disease aggressive-ness are lacking, making patient-specific therapy diffi-cult.

These difficulties are highlighted by therapies targetingthe androgen receptor. Androgen deprivation therapy iscurrently a mainstay of prostate cancer treatment, withandrogen antagonists (e.g., bicalutamide) prescribed asmonotherapy or in conjunction with other therapies. Re-cently, two highly potent antiandrogen agents have beenapproved, namely the androgen receptor antagonist enza-lutamide and the androgen biosynthesis inhibitor abirater-one acetate (4,5). A durable response to these agents is notuniversal, with relapse occurring in a significant subset ofpatients within 1 y. Furthermore, there is a small but pro-vocative body of evidence that potent, chronic androgenreceptor inhibition can drive transformation to androgen-independent neuroendocrine prostate cancer (6,7). Thistreatment-induced evolution of neuroendocrine prostatecancer (also known as small cell carcinoma) from adeno-carcinoma is thought to be an adaptive response with anincreasing incidence in the clinic. Metabolic markers of thistransition are urgently needed to treat patients receivingantiandrogen therapy appropriately.

The development of neuroendocrine prostate cancer inpatients receiving androgen receptor therapy is just oneexample of the numerous ways prostate cancer can evadetherapy, escape detection, and behave in unpredictableways. Increasingly, cancer is considered a metabolic disease,with the metabolic shifts enforced by oncogenes and tumorsuppressors (8). This concept differs from the traditionalview of cancer as a primarily genetic, proliferative phenom-enon. The focus of this mini review is identification of sev-eral of the metabolic shifts seen in cancer that may beexploited for diagnostic purposes, in particular by the agentsand platforms developed for hyperpolarized 13C MR spec-troscopy (MRS).

Received Jun. 9, 2014; revision accepted Jul. 30, 2014.For correspondence or reprints contact: John Kurhanewicz, Department of

Radiology and Biomedical Imaging, 1700 4th St., Byers Hall 203, SanFrancisco, CA 94158.E-mail: john.kurhanewicz@ucsf.eduPublished online ▪▪▪▪▪▪▪▪▪▪▪▪.COPYRIGHT © 2014 by the Society of Nuclear Medicine and Molecular

Imaging, Inc.

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TARGETING METABOLISM: THE SEARCH FOR NEW INVIVO BIOMARKERS

A snapshot of prostate cancer metabolism is frequentlyobtained via 1H MRS, acquired in conjunction with ana-tomic data during a prostate MR imaging scan. MRS sig-nificantly improves the local evaluation of prostate cancerpresence and volume, particularly when performed athigher field-strengths (3T) in conjunction with other func-tional MR methods, namely dynamic contrast-enhancedimaging and diffusion-weighted imaging. Three-dimensionalMR spectroscopic imaging (MRSI) allows metabolic map-ping of the entire prostate gland, whereby resonances cor-responding to citrate, creatine, choline, and polyaminesmay be visualized. Changes in the concentrations of thesemetabolites can be used to identify cancer with high spec-ificity. The spectra taken from regions of prostate cancershow significantly reduced or absent citrate and poly-amines, whereas choline is elevated relative to spectra takenfrom surrounding healthy tissue. These changes in citrateare attributed to a shift from citrate-producing to citrate-oxidizing metabolism (9). The total choline resonance isincreased by numerous mechanisms, including overexpres-sion and activation of choline cycle enzymes (10). There-fore, the choline-to-citrate ratio ([choline 1 creatine]/citrate) is commonly used for spectral analysis since thecholine and creatine peaks are poorly resolved. The meta-bolic reprogramming seen in prostate cancer, as well asa characteristic 1H MRSI scan, are depicted in½Fig: 1� Figure 1.The development of hyperpolarized 13C MRS, and its

recent application to prostate cancer patients, have signifi-cantly expanded metabolic targets for imaging. Metabolicshifts in cancer are too numerous to describe in full here butinclude accelerated aerobic glycolysis, increased consump-tion of glutamine, and redox adaptation, whereby cancercells accumulate intracellular antioxidants. These metabolic

shifts are enforced by oncogenes (most notably PI3K/AKT,MYC, and HIF-1) and the loss of tumor suppressor genessuch as P53. The following summaries are offered to sug-gest how these transitions may be exploited by new meta-bolic imaging technologies.

The Warburg Hypothesis: The Glycolytic Phenotype

The pioneering studies of Otto Warburg were reported in1926, describing the altered metabolism of tumors. Thesestudies reported the remarkable preference of tumors foraerobic glycolysis, whereby the pyruvate produced byglycolysis is converted to lactate, rather than entering thetricarboxylic acid cycle. This preference for fermentationexists even when O2 is present—a surprising result indeed:why would a tumor cell prefer the 2 adenosine triphos-phates generated by glycolysis to the 36 adenosine triphos-phates afforded by complete oxidation of glucose? Thispreference may represent the selection of cells adapted tohypoxia, given the tendency of tumors to outgrow theirblood supply. Alternatively, suppression of reactive oxygenspecies formed during oxidative phosphorylation likely rep-resents another adaptive mechanism. The “why” remainscontroversial, but the Warburg phenotype has proved par-ticularly useful in the detection of cancers, which arelargely glucose-avid. PET scans using 18F-FDG are cur-rently the standard detection method for identifying meta-static disease in the body for a variety of human cancers.However, 18F-FDG PET offers poor tissue contrast in theprostate, and its use is complicated by proximity to theexcretory system. Furthermore, the fundamental switch inprostate cancer is altered carbon use, not just glucose up-take. Therefore, use of fast 13C MR after injection of hyper-polarized [1-13C]pyruvate to measure this metabolic switchand its relationship to cancer aggressiveness and therapeu-tic response makes biologic sense.

The shunting of pyruvate, an end product of glycolysis,to lactate in tumor is facilitated byincreased expression of lactate dehy-drogenase, as well as expression ofmonocarboxylate transporters that havea role in both monocarboxylate uptakeand, significantly, lactate export (11,12).In particular, monocarboxylate trans-porter 4 is a high-affinity lactate trans-porter that cotransports lactate anda proton out of the cell, contributingto acidification of the extracellularmatrix. Lactate dehydrogenase has beenexplored as an anticancer target, withdecreased expression in xenograftmodels resulting in lowered tumori-genicity (13).

A Glutamine Addiction?

Several cancers increase glutamineconsumption through induction of glu-tamine transporters, as well as enhanced

FIGURE 1. Metabolic reprogramming in prostate cancer, observed by 1H MRS. (A)Change in glucose metabolism when comparing normal prostate glandular epithelialcells and malignant prostate cells. (B) Representative T2-weighted imaging andcorresponding 3-dimensional MRSI array in patient with Gleason 3 1 4 cancer inright base of prostate gland. Inlaid spectra corresponding to normal and malignantvoxel demonstrate differences in metabolism for these regions, with large citrateresonance observed in normal prostate tissue and abnormal choline peak in cancer.GLUTs5 glucose transporters; ZIP15 zinc transporter; AcCoA5 acetyl-CoA; ACCH5acetylcholine; αKG 5 α-ketoglutarate; Lac 5 lactate; LDH 5 lactate dehydrogenase;MCT45monocarboxylate transporter 4; OAA5 oxaloacetate; Pyr5 pyruvate; TCA5tricarboxylic acid.

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expression of enzymes that metabolize glutamine, namelyglutaminases and glutamate oxaloacetate transaminases. Insome cancer cells, withdrawal of glutamine results in celldeath. Enhanced glutamine metabolism is believed to bea compensatory mechanism related to the flow of citrate outof the tricarboxylic acid cycle, needed to provide acetyl-CoA for fatty acid biosynthesis. Mitochondrial citrate caneither undergo oxidation in the mitochondria (via aconitase)to isocitrate or be exported into the cytosol for lipogenesis.Glutamine, converted to a-ketoglutarate, thus provides anessential carbon source for the tricarboxylic acid cycle inthe tumor metabolic phenotype. This altered metabolismmay be driven by c-MYC, a transcription factor whose de-regulation is common in cancer. MYC has been shown toinduce both the expression of glutamine transporters andupregulation of glutaminases (14,15).

Redox Adaptation

Radiation therapy, with or without concomitant hor-monal therapy, is a mainstay of prostate cancer therapy. Ofnewly diagnosed prostate cancer patients, approximately30% have high-risk disease (stage T3–T4 or prostate-specificantigen level. 20 mg/L or Gleason score of 8–10) and are atrisk for biochemical and clinical failure after therapy (16).Even with modern conformal radiation therapy, treatmentfailure occurs in approximately 45% of patients with lo-cally defined disease (17). Increasingly, redox (reductionand oxidation) mechanisms are considered critical in ra-diation resistance (18). The cytotoxic effect of radiationtherapy is mediated by generation of reactive oxygen spe-cies in tissues, which cause oxidative damage in DNA,proteins, and lipids, leading to DNA strand breaks, lossof protein function, and membrane damage. Recently, ithas been demonstrated that reduced nicotinamide adeninedinucleotide phosphate oxidase enzymes are upregulatedin response to androgens, stimulating the production ofstress molecules and antioxidative enzymes; inhibition ofthese enzymes sensitizes human prostatic carcinoma (PC)cells in vitro to radiation (19). Additional studies havefound a critical role for glutathione in mediating radiationresistance in prostate cancer (20,21). Intracellular redoxstatus is often characterized by the ratio of reduced tooxidized glutathione, and decreasing this ratio in vitro(by addition of selenite) was found to radiosensitize pros-tate cancer cells. Elevated glutathione and other antioxi-dants seen in cancer are increasingly viewed as redoxadaptation, which several new therapies aim to exploit,including the thiol-depleting agent TDZD-8 (4-benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione) (22).

MOTIVATION FOR HYPERPOLARIZED 13C MR IMAGING

The extraordinary technique hyperpolarized MR usingDNP has the potential to revolutionize the way we use MRimaging in the risk assessment of prostate cancer patients.In particular, hyperpolarization addresses some of theintrinsic limitations of 1H MRS, including low sensitivity,

overlap of key resonances, and lack of information aboutmetabolic fluxes. Hyperpolarized 13C methodology is basedon polarizing nuclear spins in an amorphous solid state atapproximately 1.2 K through coupling of the nuclear spinswith unpaired electrons that are added to the sample via anorganic free radical. In the solid state, the high-electron-spinpolarization is in part transferred to the nuclear spins bymicrowave irradiation, and then the sample is rapidly dis-solved for injection into a system of interest (23). 13C-labeledsubstrates have been recently polarized to obtain dramaticenhancements of the 13C NMR signals (.50,000-fold at3-T) of the substrate as well as subsequent metabolic products.The enhancement that is achieved is lost in time as a function ofthe spin-lattice relaxation time of the nucleus (T1).

Several of the previously discussed metabolic shifts havebeen explored using hyperpolarized MR methods. Spe-cifically, the glycolytic phenotype has been interrogatedby hyperpolarized [U-13C, U-2H]glucose, hyperpolarized[2-13C]fructose, hyperpolarized [1-13C]pyruvate, hyperpo-larized [1-13C]lactate, and hyperpolarized [1-13C]alanine(2). Hyperpolarized [1-13C]pyruvate has been the mostwidely used probe for in vivo studies since it polarizes well,has a long T1, is rapidly taken up by the cell, and is metab-olized at the juncture of glycolysis, the tricarboxylic acidcycle, amino acid biosynthesis, and other critical pathways.Numerous hyperpolarized [1-13C]pyruvate studies have beenperformed in transgenic and implanted tumors, perfusedorgans, and engineered cell systems, or bioreactors. Increasedconversion of hyperpolarized [1-13C]pyruvate to hyperpolar-ized [1-13C]lactate has been detected in a variety of preclinicalcancer models and found to correlate with tumor grade ina transgenic adenocarcinoma of the mouse prostate (TRAMP)model (24).

Probes to measure extracellular pH have also exploitedthe Warburg phenotype. As stated previously, acidificationof the extracellular matrix is related, in part, to the exportof excess lactic acid generated by aerobic glycolysis. Theacidic tumor microenvironment may enhance both localinvasion and distant spread of disease (metastases). Numer-ous other molecular imaging methods (including 31P MRSand PET-pHLIP [pH (low) insertion peptide]) have been de-veloped to evaluate tumor acidity in vivo, but a recent prom-ising method has used hyperpolarized 13C-bicarbonate. Theinterconversion of bicarbonate and CO2 is facilitated by thefamily of carbonic anhydrases because the noncatalyzedrate of interconversion is slow relative to the metabolicfluxes. 13C-cesium bicarbonate was polarized by Gallagheret al., rapidly converted to 13C-sodium bicarbonate usingan ion-exchange resin, and used to demonstrate cancer-catalyzed interconversion both in vitro and in tumor-bearingmice, demonstrating lower pH within tumor (25). The ratioof 13C-HCO3 to 13CO2 is governed by the Henderson–Hasselbalch equation, and pH can readily be determinedfrom integrating the corresponding resonances on a voxel-by-voxel basis. A subsequent study extended this ap-proach to hyperpolarizing 13C-sodium bicarbonate itself

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and showing acidification of TRAMP tumors in a murinecohort (26).Enriched glutamine is also readily polarized using the

DNP technique, with hyperpolarized [5-13C]glutamineand a partially deuterated version, hyperpolarized [5-13C,4-2H2]glutamine, both reported (27,28). The latter probewas designed to increase the apparent T1 of the 13C nucleus,since this parameter determines a probe’s useful hyperpo-larized lifetime. Hyperpolarized glutamine has not yet beenreported in conjunction with preclinical models of prostatecancer, but rapid glutaminolysis of the probe has been describedin both human hepatocellular carcinoma and brain tumor cells.The redox adaptation of tumors may be explored using

approaches that target pools of intracellular antioxidants or,directly, reactive oxygen species. The former approach ishighlighted by hyperpolarized [1-13C]dehydroascorbate,the oxidized form of vitamin C, which is rapidly taken intocells by glucose transporters and reduced (29,30). Thisconversion has been attributed to reduced glutathione, theprincipal small-molecule thiol antioxidant in cells. Reductionof hyperpolarized [1-13C]dehydroascorbate takes place rapidlyin the brain, liver, kidneys, and prostate tumor in the TRAMPmodel. Because the intracellular redox network is vast andcomplex, it is unlikely that this conversion is dependent solelyon glutathione concentration or on the redox state of cells,often defined as the ratio of glutathione to oxidized glutathi-one. The exact mechanism of hyperpolarized [1-13C]dehy-droascorbate to hyperpolarized [1-13C]vitamin C reductionis currently under investigation, but it appears likely that thisconversion reflects the redox capacity of tissues in responseto the oxidant stressor dehydroascorbate. Other approachesto imaging endogenous oxidizing species include hyper-polarized 13C-benzoylformic acid, which decomposes toa carboxylic acid (13C-benzoic acid), a reaction readilyobservable by chemical shift (31). This method is far fromclinical application but highlights the way hyperpolarizedMR imaging may be used to target the chemical microen-vironment of tumors.

BEYOND MODELS: NEW METABOLIC PLATFORMS INPROSTATE CANCER FOR HYPERPOLARIZED 13C MR

How well do commonly used prostate cancer modelsrecapitulate the human condition? Unfortunately, testing ofdrugs using currently available preclinical models has notresulted in effective therapies in the clinic, and the dearth ofrelevant preclinical models has been identified as a limita-tion in the development of new cancer drugs. Preclinicalprostate cancer models include monolayer primary andimmortalized cell cultures, cocultures of stromal and epithelialcells, 3-dimensional cultures in matrices, and murine modelsof disease. The hyperpolarized 13C MR literature is biasedtoward animal models because of the interest in developingrobust hardware, pulse sequences, and imaging protocolsto prove that this nascent technology is readily clinicallytranslatable. Transgenic models of prostate cancer include

the TRAMP mouse and phosphatase and tensin homologmodels, which recapitulate important features of humanprostate cancer but have significant limitations. Limitationsof these transgenic models include anatomic differences be-tween the rodent and human gland, rapid progression to poorlydifferentiated cancer, absence of bony metastases, and lackof genetic heterogeneity, with oncogenes continuously “on”throughout the gland. These differences make it difficult toextend results obtained in transgenic models to human disease.

Frequently used prostate cancer cell cultures may also beinadequate to recapitulate human disease, as it is difficult tomimic the complex cell–cell and cell–matrix interactionsfound in normal tissue. These differences were highlightedby a recent study using primary human tissue slice cultures(TSCs) in an MR-compatible bioreactor. The study com-pared the metabolism of TSCs to snap-frozen, patient-derived prostate biopsy samples, 2 common immortalizedprostate cancer cell-lines (PC-3 and VCaP), and primaryprostate cancer cells using 1H high-resolution magic anglespinning (HR-MAS) spectroscopy (11). Snap-frozen patient-derived prostate biopsy samples were considered the goldstandard, since these have been shown to best reflect the invivo human prostate. The TSCs used were precision-cutfrom fresh tissues obtained at the time of surgery and cul-tured in a manner to provide optimum nutrient and gasexchange. Steady-state concentrations of key metaboliteswere compared, in general demonstrating that TSCs showedsimilar 1H HR-MAS spectra to biopsy samples and thatimmortalized cancer cell lines and primary cancer cellsshowed significant differences from both TSCs and biopsies.These data are summarized in ½Fig: 2�Figure 2A. The first observa-tion is related to citrate and polyamine resonances, hall-marks of prostate metabolism seen by in vivo 1H MRS.These metabolites were not detected in the immortalizedcell lines or primary prostate cancer cells but were foundin the biopsy samples and TSCs. Of note, citrate levels werefound to be lower in the TSCs than in biopsy samples,possibly because of the different treatment of the tissuesbefore 1H HR-MAS (snap freezing vs. incubation in me-dium). Second, lactate found in TSCs was on the same orderas their biopsy counterparts, with much higher levels foundin PC-3, VCaP, and primary prostate cancer cells. Third, thesteady-state glutamate concentrations were similar in TSCsand biopsy samples, although dramatically different fromPC3 and primary cells (higher) and VCaP cells (lower).Finally, for PC3 cells choline metabolism differed signifi-cantly from the other tissues and cells studied. Theseexperiments highlighted the perils of translatingdata obtained in cell cultures to the clinic. In particular,the metabolic signatures of primary cell cultures did notrecapitulate those found in biopsy samples, which may re-flect selection of the most aggressive cell phenotypesin culture. Furthermore, these studies suggested that theTSC platform may better approximate the human con-dition than other available culture models. Bioreactor-loaded TSCs were further interrogated with hyperpolarized

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[1-13C]pyruvate, demonstrating increased pyruvate-to-lac-tate flux that correlated with lactate dehydrogenase andmonocarboxylate transporter expression (Fig. 2B). Thesemethods may be further extended to biopsy samples them-selves, with a potentially profound impact on patient care.

HYPERPOLARIZED 13C MRIMAGING IN PROSTATECANCER PATIENTS

A decade after the first demonstrationof dissolution-DNP by Ardenkjaer-Larsen et al. (23), metabolic imagingusing hyperpolarized [1-13C]pyruvatewas accomplished in prostate cancerpatients at the University of CaliforniaSan Francisco (1). Importantly, thisstudy showed that injection of hyper-polarized [1-13C]pyruvate was safe in31 patients with biopsy-proven local-ized prostate cancer with a medianage of 63 y. Safety was important toestablish, given the difference in phar-macologic dose between 13C hyper-polarized MR and other metabolicimaging technologies such as PET(the difference between hyperpolar-ized [1-13C]pyruvate and 18F-FDGdoses is on the order of 107). Both2-dimensional dynamic MRSI andsingle-time-point MRSI were demon-strated, showing that in many casesthe observed hyperpolarized [1-13C]lactate metabolite accurately reflectedthe presence, location, and size of cancerrelative to surrounding benign tissues.A study performed using the highesthyperpolarized [1-13C]pyruvate dose isdepicted in ½Fig: 3�Figure 3. Significantly, inseveral cases hyperpolarized 13C MRSrevealed regions of the prostate that were

previously missed by state-of-the-art 1H multiparametric im-aging that was performed as part of the clinical trial. Thisstudy was highly promising and demonstrated the potentialto use hyperpolarized MR in a variety of clinical contextswith altered glycolysis.

THE FUTURE

The development of rapid-dissolutionDNP methods has, in a short time,afforded profound insights into pros-tate cancer metabolism. These insightshave been obtained largely in pre-clinical disease models, which reca-pitulate several of the key metabolicshifts found in cancer, namely accel-erated glycolysis, glutamine consump-tion, and redox adaptation. Metabolicstudies using both 1H and hyperpolar-ized 13C spectroscopy have reinforcedthe view that these models need tobetter recapitulate human in vivo pros-tate cancer, to provide an appropriate

FIGURE 2. Spectroscopic studies demonstrate challenges of studying metabolismusing cells in culture and validate tissue slices (TSCs) as platform for hyperpolarized13C research. (A) Quantified metabolite ratios for benign and malignant prostatetissues, obtained using 1H HR-MAS. Differences in metabolite ratios between TSCsand snap-frozen biopsy samples are compared, as well as those obtained fromprimary prostate cancer cells and immortalized cell lines PC3 and VCap. (B)Metabolic study of TSCs on bioreactor platform using hyperpolarized [1-13C]pyruvate. Both single spectrum acquired at 90 s and normalized dynamic datashow significantly increased conversion to hyperpolarized [1-13C]lactate inmalignant TSCs. Ala 5 alanine; BY 5 biopsy samples; Cho 5 choline; Cit 5 citrate;Glu 5 glutamic acid; GPC 5 glycerophosphocholine; Lac 5 lactate; Pol 5 polyamines;Pyr 5 pyruvate.

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FIGURE 3. Hyperpolarized 13C MRS study in human patient with biopsy-provenGleason grade 3 1 3 prostate cancer using 3-dimensional MRSI at a single timepoint. This patient received highest dose of hyperpolarized [1-13C]pyruvate (0.43 mL/kg).Axial T2-weighted image through malignant region is juxtaposed with corresponding13C spectral array, with area of putative tumor highlighted by pink shading. Increasedconversion to hyperpolarized [1-13C]lactate was seen in this region, consistent withabnormalities found on multiparametric 1H staging examination. Lac 5 lactate; Pyr 5pyruvate; LDH 5 lactate dehydrogenase; NAD 5 nicotinamide adenine dinucleotide;NADH 5 reduced nicotinamide adenine dinucleotide.

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platform for biomarker identification and validation. Weanticipate that miniaturization of perfused cellular systemswill allow rapid metabolic characterization of human bi-opsy samples, with the goal of tailoring therapy to individ-ual patient phenotypes.Although miniaturization is a significant goal in the

metabolic analysis of ex vivo samples, adaptation of preclinicalhyperpolarized MRS methods to humans requires significantinnovation. As the recent clinical trial using hyperpolarized[1-13C]pyruvate suggests, several of these challenges havealready been met. Future efforts in humans will be drivenby new hyperpolarized probes, improved methods for po-larization and delivery of 13C substrates, MR pulse sequen-ces using sparse sampling techniques, parallel imagingmethodologies, and improved coil design. Importantly, con-tinued attention to biochemical detail will be needed toestablish the clinical value of hyperpolarized 13C MRS along-side existing 1H methods in the multiparametric prostateMR examination.

DISCLOSURE

This work was supported by the National Institutes ofHealth (R01 CA166766, P41 EB013598). No other poten-tial conflict of interest relevant to this article was reported.

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