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Parahydrogen induced hyperpolarization of [1-13C] pyruvate for metabolic imaging in cancer research
Shingo Matsumoto1, Neil J Stewart1, Yuka Fukue1, and Hiroshi Hirata1

1Information Science and Technology, Hokkaido University, Sapporo, Japan

Synopsis

13C-labeled pyruvate is a clinically available key tracer for hyperpolarized 13C metabolic MRI study. An automated hyperpolarizing system of [1-13C]pyruvate using a low-cost parahydrogen induced polarization (PHIP) with side-arm hydrogenation (SAH), in which hyperpolarization is initially induced to the parahydrogen derived two 1Hs on unsaturated pyruvate precursor followed by the polarization transfer to a 13C of pyruvate, was developed. Magnetic field cycling process for 1H-to-13C polarization transfer process was optimized by the experimentally determined J-coupling network of pyruvate precursor and quantum chemical simulation, and its possible applications for cancer research are investigated in tumor-bearing mouse model.

Introduction

Pyruvate is a hub metabolite of glucose, which metabolic flux well reflects tumor microenvironment and so can be a useful molecular probe for predicting cancer treatments outcome (Fig.1)1,2. Success in clinical trials of hyperpolarized13C MRI of pyruvate metabolism using a dissolution dynamic nuclear polarization (dDNP) have reignited interest in parahydrogen induced polarization (PHIP) with a side arm hydrogenation (SAH) as a low-cost alternative means to prepare hyperpolarized 13C-labled pyruvate1. In PHIP-SAH, hyperpolarization of 1H is generated by parahydrogenation reaction on unsaturated precursor, then the polarization can be transferred to surrounding 13C nuclei by magnetic field cycling (MFC)1. The timing and field strength of MFC to obtain the maximum polarization can be theoretically simulated based on the spin-spin coupling (J-coupling) network of13C labeled tracer. Precise control of the MFC process is necessary to achieve high enough13C polarization and guided delivery of tracers for metabolic MRI applications. In this study, we developed a system to automate parahydrogenation reaction step and the MFC process in PHIP experiments, in which timing and magnetic field for MFC can be easily controlled by a LabVIEW/PXI system.

Method

All J-coupling values of pyruvate precursor were measured by a Bruker Avance III HD 800 MHz NMR spectrometer equipped with a 5 mm zgradient PATXI probe. Optimization of the timing of magnetic field changes for MFC was simulated as reported by 3using density matrix simulation (Fig.3). Mapping of the magnetic field distribution within a zero field chamber constructed from three layers of μ-metal was conducted in every 5 mm step using a three dimensional fluxgate type gauss meter. Based on the MFC timing and magnetic field mapping of the zero field chamber, parahydogenation reactor with electromagetic valves systems was placed around the center of zero-field area <0.1 μT. Optimized MFC was applied using a sweep coil placed inside the zero-field chamber controlled by LabVIEW/PXI system. Hyperpolarized 13C NMR spectra and chemical shift imaging (CSI) were obtained using a home-built preclinical 1.5T MRI scanner.

Results and Discussion

Spin order transfer of parahydrogenated [1-13C]ally pyruvate precursor using the developed PHIP hyperpolarizer system resulted in substantial enhancement of 13C NMR/MRI signal more than 20,000 times compared to that under thermal equilibrium at 1.5T. Side-arm PHIP technique was applied to generate hyperpolarized 13C pyruvate from the precursor by a rapid hydrolysis <10 sec. Although about 30% loss of 13C signal was observed during the hydrolysis process of ester side-arm, hyperpolarized [1-13C]pyruvate with >10,000 time signal enhancement was generated using the developed PHIP system. Chemical shift imaging (CSI) of a phantom containing the hyperpolarized [1-13C]pyruvate was conducted and the CSI image was overlaid on the T2-weighted 1H image (Fig.4).

Conclusion

Unsaturated precursors of 13C labeled metabolic probes were successfully polarized by more than 20,000 times compared to thermal equilibrium level using a PHIP based hyperpolarizer system. This automated PHIP system may prove useful as a low-cost alternative to dDNP type hyperpolarizers for 13C-pyruvate and other metabolic MRI study, especially in cancer applications.

Acknowledgements

This study was supported by JST PREST, Saitama, Japan.

References

  1. Reineri F, Boi T, Aime S. ParaHydrogen Induced Polarization of 13C carboxylate resonance in acetate and pyruvate. Nat Commun. 2015;6:5858.2.
  2. Matsumoto S, Kishimoto S, Saito K, et al. Metabolic and Physiologic Imaging Biomarkers of the Tumor Microenvironment Predict Treatment Outcome with Radiation or a Hypoxia-Activated Prodrug in Mice. Cancer Res. 2018;78(14):3783-3792.3.
  3. Stewart NJ, Kumeta H, Tomohiro M, et al. Long-range heteronuclear J-coupling constants in esters: Implications for 13C metabolic MRI by side-arm parahydrogen-induced polarization. J Magn Reson. 2018;296:85-92.

Figures

Fig. 1 Metabolic MRI of hyperpolarized [1-13C]-pyruvate using dDNP and cancer treatment response. (A) Pyruvate to lactate metabolic flux imaging in three pancreatic ductal adenocarcinoma (PDAC) xenografts. (B) Response of three PDAC tumors to three different cancer treatments; fractuatedx-irradiation, gemcitabine, and hypoxia activated prodrugTH302 (Cancer Res 2018)

Fig. 2 Scheme of preparing hyperpolarized [1-13C] pyruvate using a PHIP-SAH system.

Fig.3: Contour plots of simulated achievable 13C polarization as a function of Jcoupling values in a 3-spin system (para-H2, 13C; HH1C) for three cases: (A) magnetic fieldcycling (at a fixed adiabatic remagnetizationtime of 5 sec), (B) PH-INEPT+, and (C)ESOTHERIC spin order transfer methods (J Magn Reson 2018).

Fig. 4 Theoretically achievable 13C polarization when considering only direct (through 4/5-bonds) polarization transfer (3-spin system) compared with when considering the complete neighboring heteronuclear and homonuclearJ coupling network (6-spin system; “all spins”) (A), dynamic 13C NMR spectrum (B), and chemical shift imaging (C) of 13C-labled allyl ester precursor polarized by PHIP-SAH system with magnetic field cycling for spin order transfer.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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