Chemical reaction-induced multi-molecular polarization (CRIMP)

Hyperpolarized Magnetic Resonance and Artificial Intelligence: Frontiers of Imaging in Pancreatic Cancer

BACKGROUND

There is an unmet need for noninvasive imaging markers that can help identify the aggressive subtype(s) of pancreatic ductal adenocarcinoma (PDAC) at diagnosis and at an earlier time point, and evaluate the efficacy of therapy prior to tumor reduction. In the past few years, there have been two major developments with potential for a significant impact in establishing imaging biomarkers for PDAC and pancreatic cancer premalignancy: (1) hyperpolarized metabolic (HP)-magnetic resonance (MR), which increases the sensitivity of conventional MR by over 10,000-fold, enabling real-time metabolic measurements; and (2) applications of artificial intelligence (AI).

OBJECTIVE

Our objective of this review was to discuss these two exciting but independent developments (HP-MR and AI) in the realm of PDAC imaging and detection from the available literature to date.

METHODS

A systematic review following the PRISMA extension for Scoping Reviews (PRISMA-ScR) guidelines was performed. Studies addressing the utilization of HP-MR and/or AI for early detection, assessment of aggressiveness, and interrogating the early efficacy of therapy in patients with PDAC cited in recent clinical guidelines were extracted from the PubMed and Google Scholar databases. The studies were reviewed following predefined exclusion and inclusion criteria, and grouped based on the utilization of HP-MR and/or AI in PDAC diagnosis.

RESULTS

Part of the goal of this review was to highlight the knowledge gap of early detection in pancreatic cancer by any imaging modality, and to emphasize how AI and HP-MR can address this critical gap. We reviewed every paper published on HP-MR applications in PDAC, including six preclinical studies and one clinical trial. We also reviewed several HP-MR-related articles describing new probes with many functional applications in PDAC. On the AI side, we reviewed all existing papers that met our inclusion criteria on AI applications for evaluating computed tomography (CT) and MR images in PDAC. With the emergence of AI and its unique capability to learn across multimodal data, along with sensitive metabolic imaging using HP-MR, this knowledge gap in PDAC can be adequately addressed. CT is an accessible and widespread imaging modality worldwide as it is affordable; because of this reason alone, most of the data discussed are based on CT imaging datasets. Although there were relatively few MR-related papers included in this review, we believe that with rapid adoption of MR imaging and HP-MR, more clinical data on pancreatic cancer imaging will be available in the near future.

CONCLUSIONS

Integration of AI, HP-MR, and multimodal imaging information in pancreatic cancer may lead to the development of real-time biomarkers of early detection, assessing aggressiveness, and interrogating early efficacy of therapy in PDAC.

Indirect detection of intermediate in decarboxylation reaction of phenylglyoxylic acid by hyperpolarized 13C NMR

The decarboxylation reaction of phenylglyoxylic acid with hydrogen peroxide is studied by real-time hyperpolarized carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy at room temperature. A non-observable reaction intermediate is identified using blind selective saturation pulses in the expected chemical shift range, thereby revealing information on the reaction mechanism.

Assessing Metabolic Intervention with a Glutaminase Inhibitor in Real-Time by Hyperpolarized Magnetic Resonance in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive hematopoietic disease characterized by glutamine-dependent metabolism. A novel glutaminase (GLS) inhibitor, CB-839, is currently under evaluation for treatment of hematopoietic malignancies and solid tumors. Our purpose was to measure cellular changes in AML associated with CB-839 treatment and to test the ability of hyperpolarized pyruvate for interrogating these changes to OCI-AML3 cells. Our results show that treatment with CB-839 interfered with the citric acid cycle, reduced the NADH/NAD⁺ ratio and ATP levels, reduced cell proliferation and viability, and reduced the basal and maximal respiratory capacities [oxygen consumption rate (OCR)]. We observed a reduction of the conversion of hyperpolarized pyruvate to lactate in cell lines and in a mouse AML model after CB-839 treatment. Our in vitro and in vivo results support the hypothesis that, in AML, glutamine is utilized to generate reducing equivalents (NADH, FADH₂) through the citric acid cycle and that reduction in redox state by GLS inhibition decreases the rate of pyruvate to lactate conversion catalyzed by lactate dehydrogenase. We propose hyperpolarized pyruvate/lactate measurement as a method for direct monitoring of metabolic changes occurring in AML patients receiving CB-839. With further optimization, this method may provide a noninvasive imaging tool to assess the early efficacy of therapeutic intervention with GLS inhibitors.

Production of highly polarized [1-13 C]acetate by rapid decarboxylation of [2-13 C]pyruvate - application to hyperpolarized cardiac spectroscopy and imaging

PURPOSE

The objective of the present work was to develop and implement an efficient approach to hyperpolarize [1-¹³ C]acetate and apply it to in vivo cardiac spectroscopy and imaging.

METHODS

Rapid hydrogen peroxide induced decarboxylation was used to convert hyperpolarized [2-¹³ C]pyruvate into highly polarized [1-¹³ C]acetate employing an additional step following rapid dissolution of [2-¹³ C]pyruvate in a home-built multi-sample dissolution dynamic nuclear polarization system. Phantom dissolution experiments were conducted to determine optimal parameters of the decarboxylation reaction, retaining polarization and T₁ of [1-¹³ C]acetate. In vivo feasibility of detecting [1-¹³ C]acetate metabolism is demonstrated using slice-selective spectroscopy and multi-echo imaging of [1-¹³ C]acetate and [1-¹³ C]acetylcarnitine in the healthy rat heart.

RESULTS

The first in vivo signal was observed ~23 s after dissolution. At the corresponding time point in the phantom experiments, 97.9 ± 0.4% of [2-¹³ C]pyruvate were converted into [1-¹³ C]acetate by the decarboxylation reaction. T₁ and polarization of [1-¹³ C]acetate was determined to be 29.7 ± 1.9% and a 47.7 ± 0.5 s. Polarization levels of [2-¹³ C]pyruvate and [1-¹³ C]acetate were not significantly different after transfer to the scanner. In vivo, [1-¹³ C]acetate and [1-¹³ C]acetylcarnitine could be detected using spectroscopy and imaging.

CONCLUSION

Decarboxylation of hyperpolarized [2-¹³ C]pyruvate enables the efficient production of highly polarized [1-¹³ C]acetate that is applicable to study short-chain fatty acid metabolism in the in vivo heart.

Assessing Therapeutic Efficacy in Real-time by Hyperpolarized Magnetic Resonance Metabolic Imaging

Precisely measuring tumor-associated alterations in metabolism clinically will enable the efficient assessment of therapeutic responses. Advances in imaging technologies can exploit the differences in cancer-associated cell metabolism as compared to normal tissue metabolism, linking changes in target metabolism to therapeutic efficacy. Metabolic imaging by Positron Emission Tomography (PET) employing 2-fluoro-deoxy-glucose ([18F]FDG) has been used as a routine diagnostic tool in the clinic. Recently developed hyperpolarized Magnetic Resonance (HP-MR), which radically increases the sensitivity of conventional MRI, has created a renewed interest in functional and metabolic imaging. The successful translation of this technique to the clinic was achieved recently with measurements of 13C-pyruvate metabolism. Here, we review the potential clinical roles for metabolic imaging with hyperpolarized MRI as applied in assessing therapeutic intervention in different cancer systems.

Real-Time Interrogation of Aspirin Reactivity, Biochemistry, and Biodistribution by Hyperpolarized Magnetic Resonance Spectroscopy

Hyperpolarized magnetic resonance spectroscopy enables quantitative, non-radioactive, real-time measurement of imaging probe biodistribution and metabolism in vivo. Here, we investigate and report on the development and characterization of hyperpolarized acetylsalicylic acid (aspirin) and its use as a nuclear magnetic resonance (NMR) probe. Aspirin derivatives were synthesized with single- and double-¹³ C labels and hyperpolarized by dynamic nuclear polarization with 4.7 % and 3 % polarization, respectively. The longitudinal relaxation constants (T₁ ) for the labeled acetyl and carboxyl carbonyls were approximately 30 seconds, supporting in vivo imaging and spectroscopy applications. In vitro hydrolysis, transacetylation, and albumin binding of hyperpolarized aspirin were readily monitored in real time by ¹³ C-NMR spectroscopy. Hyperpolarized, double-labeled aspirin was well tolerated in mice and could be observed by both ¹³ C-MR imaging and ¹³ C-NMR spectroscopy in vivo.

T1 nuclear magnetic relaxation dispersion of hyperpolarized sodium and cesium hydrogencarbonate-13 C

In vivo pH mapping in tissue using hyperpolarized hydrogencarbonate-¹³ C has been proposed as a method to study tumor growth and treatment and other pathological conditions related to pH changes. The finite spin-lattice relaxation times (T₁ ) of hyperpolarized media are a significant limiting factor for in vivo imaging. Relaxation times can be measured at standard magnetic fields (1.5 T, 3.0 T etc.), but no such data are available at low fields, where T₁ values can be significantly shorter. This information is required to determine the potential loss of polarization as the agent is dispensed and transported from the polarizer to the MRI scanner. The purpose of this study is to measure T₁ dispersion from low to clinical magnetic fields (0.4 mT to 3.0 T) of different hyperpolarized hydrogencarbonate formulations previously proposed in the literature for in vivo pH measurements. ¹³ C-enriched cesium and sodium hydrogencarbonate preparations were hyperpolarized using dynamic nuclear polarization, and the T₁ values of different samples were measured at different magnetic field strengths using a fast field-cycling relaxometer and a 3.0 T clinical MRI system. The effects of deuterium oxide as a dissolution medium for sodium hydrogencarbonate were also analyzed. This study finds that the cesium formulation has slightly shorter T₁ values compared with the sodium preparation. However, the higher solubility of cesium hydrogencarbonate-¹³ C means it can be polarized at greater concentration, using less trityl radical than sodium hydrogencarbonate-¹³ C. This study also establishes that the preparation and handling of sodium hydrogencarbonate formulations in relation to cesium hydrogencarbonate is more difficult, due to the higher viscosity and lower achievable concentrations, and that deuterium oxide significantly increases the T₁ of sodium hydrogencarbonate solutions. Finally, this work also investigates the influence of pH on the spin-lattice relaxation of cesium hydrogencarbonate-¹³ C measured over a pH range of 7 to 9 at 0.47 T.

Interrogating Metabolism in Brain Cancer

This article reviews existing and emerging techniques of interrogating metabolism in brain cancer from well-established proton magnetic resonance spectroscopy to the promising hyperpolarized metabolic imaging and chemical exchange saturation transfer and emerging techniques of imaging inflammation. Some of these techniques are at an early stage of development and clinical trials are in progress in patients to establish the clinical efficacy. It is likely that in vivo metabolomics and metabolic imaging is the next frontier in brain cancer diagnosis and assessing therapeutic efficacy; with the combined knowledge of genomics and proteomics a complete understanding of tumorigenesis in brain might be achieved.

In vivo pH mapping of injured lungs using hyperpolarized [1-13 C]pyruvate

PURPOSE

To optimize the production of hyperpolarized ¹³ C-bicarbonate from the decarboxylation of hyperpolarized [1-¹³ C]pyruvate and use it to image pH in the lungs and heart of rats with acute lung injury.

METHODS

Two forms of catalysis are compared calorimetrically to maximize the rate of decarboxylation and rapidly produce hyperpolarized bicarbonate from pyruvate while minimizing signal loss. Rats are injured using an acute lung injury model combining ventilator-induced lung injury and acid aspiration. Carbon images are obtained from both healthy (n = 4) and injured (n = 4) rats using a slice-selective chemical shift imaging sequence with low flip angle. pH is calculated from the relative HCO3- and CO₂ signals using the Henderson-Hasselbalch equation.

RESULTS

It is demonstrated that base catalysis is more effective than metal-ion catalysis for this decarboxylation reaction. Bicarbonate polarizations up to 17.2% are achieved using the base-catalyzed reaction. A mean pH difference between lung and heart of 0.14 pH units is measured in the acute lung injury model. A significant pH difference between injured and uninjured lungs is also observed.

CONCLUSION

It is demonstrated that hyperpolarized ¹³ C-bicarbonate can be efficiently produced from the base-catalyzed decarboxylation of pyruvate. This method is used to obtain the first regional pH image of the lungs and heart of an animal. Magn Reson Med 78:1121-1130, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Application of Good's buffers to pH imaging using hyperpolarized (13)C MRI

N-(2-Acetamido)-2-aminoethanesulfonic acid (ACES), one of Good's buffers, was applied to pH imaging using hyperpolarized (13)C magnetic resonance spectroscopy. Rapid NMR- and MRI-based pH measurements were obtained by exploiting the sensitive pH-dependence of its (13)C chemical shift within the physiologic range.

[Cu(o-phthalate)(phenanthroline)] Exhibits Unique Superoxide-Mediated NCI-60 Chemotherapeutic Action through Genomic DNA Damage and Mitochondrial Dysfunction

The in cellulo catalytic production of reactive oxygen species (ROS) by copper(II) and iron(II) complexes is now recognized as a major mechanistic model in the design of effective cytotoxins of human cancer. The developmental complex, [Cu(o-phthalate)(1,10-phenanthroline)] (Cu-Ph), was recently reported as an intracellular ROS-active cytotoxic agent that induces double strand breaks in the genome of human cancer cells. In this work, we report the broad-spectrum action of Cu-Ph within the National Cancer Institute's (NCI) Developmental Therapeutics Program (DTP), 60 human cancer cell line screen. The activity profile is compared to established clinical agents-via the COMPARE algorithm-and reveals a novel mode of action to existing metal-based therapeutics. In this study, we identify the mechanistic activity of Cu-Ph through a series of molecular biological studies that are compared directly to the clinical DNA intercalator and topoisomerase II poison doxorubicin. The presence of ROS-specific scavengers was employed for in vitro and intracellular evaluation of prevailing radical species responsible for DNA oxidation with superoxide identified as playing a critical role in this mechanism. The ROS targeting properties of Cu-Ph on mitochondrial membrane potential were investigated, which showed that it had comparable activity to the uncoupling ionophore, carbonyl cyanide m-chlorophenyl hydrazine. The induction and origins of apoptotic activation were probed through detection of Annexin V and the activation of initiator (8,9) and executioner caspases (3/7) and were structurally visualized using confocal microscopy. Results here confirm a unique radical-induced mechanistic profile with intracellular hallmarks of damage to both genomic DNA and mitochondria.

Low-temperature dynamic nuclear polarization of gases in Frozen mixtures

PURPOSE

To present a new cryogenic technique for preparing gaseous compounds in solid mixtures for polarization using dynamic nuclear polarization (DNP).

METHODS

(129) Xe and (15) N2 O samples were prepared using the presented method. Samples were hyperpolarized at 1.42K at 5 Tesla. (129) Xe was polarized at 1.65K and 1.42K to compare enhancement. Polarization levels for both samples and T1 relaxation times for the (129) Xe sample were measured. Sample pulverization for the (129) Xe and controlled annealing for both samples were introduced as additional steps in sample preparation.

RESULTS

Enhancement increased by 15% due to a temperature drop from 1.65K to 1.42K for the (129) Xe sample. A polarization level of 20 ± 3% for the (129) Xe sample was achieved, a two-fold increase from 10 ± 1% after pulverization of the sample at 1.42K. T1 of the (129) Xe sample was increased by more than three-fold by means of annealing. In the case of (15) N2 O, annealing led to a ∼two-fold increase in the signal level after DNP.

CONCLUSION

The presented technique for producing and manipulating solid gas/glassing agent/radical mixtures for DNP led to high polarization levels in (129) Xe and (15) N2 O samples. These methods show potential for polarizing other gases using DNP technology. Magn Reson Med 76:1007-1014, 2016. © 2015 Wiley Periodicals, Inc.

Metabolic Imaging as a Biomarker of Early Radiation Response in Tumors

(13)C-pyruvate hyperpolarized magnetic resonance imaging (HP-MRI) is emerging as a viable quantitative biomarker for solid tumor response and normal tissue toxicity after radiotherapy. This technology effectively predicts response related to metabolic agents or alterations in the tumor microenvironment, but challenges remain to be addressed to ensure successful translational implementation. See related article by Saito et al., p. 5073.