Access Type

Open Access Dissertation

Date of Award

January 2024

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Eduard Y. Chekmenev

Abstract

Over the years, there has been research geared towards mitigating disease conditions such as cancers and tumors by molecular sensing of their aberrant metabolism in humans and animal models of diseases. Some of these biomolecules of interest are pyruvate, α-ketoglutarate, and ketoisocaproate, that play a central role in eukaryotic and human metabolic pathways. Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) have become predominant in investigating and imaging cancers and tumors for early detection and treatment. Although conventional NMR spectroscopy is limited by low sensitivity, we overcame this barrier through the use of hyperpolarization techniques. NMR hyperpolarization improves NMR sensitivity by 4-6 orders of magnitude by increasing the nuclear spin-order relative to the thermal equilibrium polarization level.

We used Parahydrogen-Induced Polarization (PHIP) and Signal Amplification by Reversible Exchange (SABRE) hyperpolarization techniques suitable for transferring parahydrogen-derived polarization to these biomolecules. SABRE is accomplished by simultaneous reversible exchange of parahydrogen (p-H2) and to-be-hyperpolarized contrast agent molecule on a hexacoordinate Iridium catalyst. Specially, we used a variant of SABRE called SABRE-SHEATH (Signal Amplification by Reversible Exchange in Shield Enables Alignment Transfer to Heteronuclei) where polarization is transferred from p-H2 to heteronucleus of the contrast media in the microtesla magnetic field. Chapters 1 and 2 are dedicated to creating an understanding of this study through careful explanation of the principles of NMR, MRI, hyperpolarization techniques and RASER effect.

Chapter 3 focuses on SABRE-SHEATH technique as it is explored to rapidly hyperpolarize 13C spins of [1-13C]pyruvate using parahydrogen as the source of spin order. The pyruvate interacts with an iridium-based polarization transfer catalyst with temperature and co-ligation of dimethyl sulfoxide and H2O playing key roles in controlling the exchange. By reducing the temperature, [1-13C]pyruvate exchange slows, achieving over 50% polarization in under 30 seconds. Using a 1.4 T NMR spectrometer, 39% polarization is measured and extrapolated to above 50% at saturation. The highest polarization of a 30 mM pyruvate sample, considering both free and bound forms, reaches 13%. Fast polarization buildup, outpacing spin-relaxation rates, and the ability to release hyperpolarized pyruvate through temperature cycling offer promise for future biomedical applications.

Chapter 4 explores the use of the SABRE-SHEATH technique to achieve fast hyperpolarization of 13C-labeled α-ketoglutarate. α-Ketoglutarate, a key metabolite in the tricarboxylic acid cycle, plays a vital role in metabolic pathways and is linked to conditions such as cancer. In this study, the SABRE-SHEATH technique generated 9.7% 13C polarization within just 1 minute. This method uses parahydrogen as a nuclear spin order source, and the polarization transfer was optimized at a field strength of 0.4 μT. The efficient hyperpolarization was driven by favorable relaxation dynamics, with a polarization buildup time of 11 seconds and a decay constant of 18.5 seconds. A higher ¹³C polarization value (17.3%) was achieved using natural-abundance α-ketoglutarate. Interestingly, deuteration had minimal effects on relaxation rates. This research highlights the potential of α-ketoglutarate as a hyperpolarized contrast agent, providing new opportunities for metabolic imaging, particularly in cancer diagnostics and brain function studies. The findings pave the way for developing hyperpolarized agents for real-time metabolic imaging applications.

Chapter 5 highlights the use of SABRE-SHEATH to rapidly hyperpolarize [1-13C]ketoisocaproate, a metabolic probe involved in branched-chain amino acid pathways. This technique achieved up to 18% polarization within one minute, a significant improvement compared to previous efforts. The optimization of temperature, pH, and p-H2 pressure played a key role in enhancing the hyperpolarization levels. We also explored the hyperpolarization of natural-abundance ketoisocaproate, which enabled the independent analysis of C-1 and C-2 carbons. Additionally, hyperpolarized [1-13C]ketoisocaproate was successfully detected in a post-mortem mouse using a 0.35 T clinical MRI scanner. These findings highlight the potential of ketoisocaproate as a hyperpolarized contrast agent for metabolic imaging, with applications in cancer and neurodegenerative disease research.

Chapter 6 focuses on advancements in NMR sensing using wireless maser detectors. We demonstrated how hyperpolarized nuclear spins can generate stimulated emission signals, similar to masers, improving the detection of hyperpolarized contrast agents like [1-13C]pyruvate, which is under investigation for cancer imaging. By enhancing the quality factor of the NMR detector through parametric pumping, a 22-fold increase in quality factor (up to 1,670) was reported, lowering the threshold for inducing nuclear spin masing. This approach allows for better signal detection under challenging conditions, such as low magnetic field homogeneity. This technology has potential for future applications in preclinical MRI scanners.

My dissertation will strengthen our understanding of the physics and chemistry of polarization transfer governing hyperpolarization of [1-13C]pyruvate, α-ketoglutarate, and ketoisocaproate in microtesla magnetic fields as well as RASER effects. HP contrast media utility for real time monitoring of diseases is already in clinical trials having gained attention and approval from relevant authorities because it a safe protocol that boosts MRI signals by several orders of magnitude. This work has the potential to augment or replace the 2.5h-long-PET exam by a non-ionizing ~10-minute-long HP MRI procedure.

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