Access Type
Open Access Embargo
Date of Award
January 2023
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Chemistry
First Advisor
Eduard Chekmenev
Abstract
The detection of hypoxia in hypoxic-related diseases can provide crucial information for treatment planning and assessment of therapy response. Although Positron Emission Tomography (PET) was employed as a research tool for imaging of hypoxia detection, it has several limitations. Our motivation is to develop a hyperpolarized (HP) molecular probe for hypoxia sensing. My dissertation presents the utility of Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) to hyperpolarize a wide range of N-heterocyclic compounds, which can be employed as MRI contrast agents for hypoxia sensing and other molecular imaging applications.SABRE-SHEATH is a rapid, simple, cost-effective parahydrogen (pH2)-based hyperpolarization method. This hyperpolarization technique transfers the non-equilibrium spin order from parahydrogen to the nuclear spins of exchangeable substrates using an Ir-based catalyst. The SABRE-SHEATH technique hyperpolarizes 15N nuclei and increases NMR sensitivity. Using 15N spins is advantageous because it leads to longer lifetimes of the hyperpolarized state than other hyperpolarized nuclei (e.g., 13C or 1H), allowing more time to acquire NMR signals. The Ir-based SABRE catalyst, [IrCl(COD)(IMes)], was utilized in all our SABRE-SHEATH experiments. This highly efficient catalyst transfers polarization from pH2 to the exchangeable substrate molecules, enhancing their NMR polarization by 4-6 orders of magnitude. Some previous studies reported on the SABRE-SHEATH hyperpolarization of [15N3]metronidazole. However, the relaxation dynamics study was not investigated in detail in sub-microtesla fields, where polarization is generated. Therefore, I have investigated the 15N spin-lattice relaxation dynamics of the [15N3]metronidazole and [15N2]metronidazole isotopologues discussed in Chapter 3. The study reveals that avoiding 14N spins in 15N-15N spin networks can prevent polarization loss from quadrupolar relaxation. For example, the presence of a 14N nucleus in the scalar coupling network reduces 15N T1 relaxation values by approximately 3-fold for all 15N sites in [15N2]isotopologue compared to [15N3]isotopologue across different [IrCl(COD)(IMes)] catalyst concentrations. Moreover, our near approach of avoiding spin-spin coupled 14N nucleus allows us to improve %P15N by a factor of 2-4. Nimorazole, an imidazole-based antibiotic, is now in Phase 3 clinical trial in Europe as a hypoxia radiosensitizer. We hypothesize the use of [15N3]nimorazole as a theragnostic hypoxia contrast agent that can be employed on the next-generation MRI-LINAC systems. Chapter 4 explored the hyperpolarization profile of [15N3]nimorazole. The study shows that creating a hyperpolarized state that lasts for tens of minutes is possible. The resulting level of 15N nuclear spin polarization was a 67,000-fold enhancement at 1.4 T, corresponding to %P15N of 3.2%. Chapter 5 demonstrates the successful use of [15N]dalfampridine in 15N SABRE-SHEATH hyperpolarization experiments, resulting in %P15N up to 2.0%. HP [15N]dalfampridine can be potentially employed for future in vivo applications. It was also revealed that pyridine co-ligand reduces catalyst activation time for the active SABRE complex with [15N]dalfampridine. The 15N T1 relaxation time of [15N]dalfampridine at 1.4 T is sufficiently long for potential MRI applications. Chapter 6 focuses on the studies of the SABRE-SHEATH hyperpolarization process of 15N-sites and 19F-site of fluoro-[15N3]metronidazole. Fluoro-[15N3]metronidazole exhibited high %P15N values (4.19% to 6.22%) at all three 15N sites, consistent with our prior spin-relayed polarization transfer study via the 2J15N-15N network in HP [15N3]metronidazole. Furthermore, polarization level achieved for 19F in fluoro-[15N3]metronidazole was %P19F of 0.157%. All compounds studied via the SABRE-SHEATH process in this dissertation, except [15N]fampridine, contain a nitroimidazole moiety in their structure. It is anticipated that nitroimidazole moiety would undergo an electronic reduction in hypoxic in vivo conditions. This process leads to the -NO2 (nitro) group stepwise reduction to a -NH2 (amino) group. We have performed ab initio computational studies of 15N chemical shifts of [15N3]nimorazole (Chapter 4), fluoro-[15N3]metronidazole (Chapter 6), and their putative metabolites. These potential HP contrast agents can be used for chemical shift hypoxia sense because they have been shown to be reduced during metabolism by hypoxic tissues, leading to changes in their 15N chemical shifts. All 15N chemical shifts in [15N3]nimorazole and fluoro-[15N3]metronidazole studied exhibited a clear sensitivity trend to the reduction process for all three 15N sites, i.e., their 15N chemical shifts can provide a sensitive gauge for 15N hypoxia sensing in vivo. HP fluoro-[15N3]metronidazole was deemed the optimum choice in hypoxia sensing based on the dynamic range of 15N chemical shift changes during the stepwise reduction. We anticipate that [15N3]metronidazole likely acts similarly well in the context of hypoxia sensing due to the similarity of the chemical structure between [15N3]metronidazole and fluoro-[15N3]metronidazole. Based on the results of our 15N SABRE-SHEATH experimental hyperpolarization studies and theoretical ab initio calculations, it has been concluded that fluoro-[15N3]metronidazole would be the optimal choice as a potential MRI contrast agent for hypoxia sensing applications. Furthermore, in vivo analysis of the chemical shifts of fluoro-[15N3]metronidazole can provide insights into the metabolic kinetics of its in vivo metabolism in hypoxic conditions, which is not available from PET studies employing radioactive nitroimidazole tracers. This information could be of potential utility to MRI and PET imaging of hypoxia. This dissertation focuses on exploring the use of SABRE-SHEATH hyperpolarization in enhancing the sensitivity of MRI and NMR. Combining in vitro SABRE-SHEATH experiments with ab initio calculations was found to help identify the best lead for next-generation MRI contrast agents. These new contrast agents can potentially sense hypoxia by detecting metabolic processes in vivo. The results demonstrate the potential of this technique in advancing the field of MRI and facilitating the development of new contrast agents for detecting hypoxia. This work has potential applications for basic research and future clinical diagnosis. Overall, this dissertation significantly contributes to the development of nuclear spin hyperpolarization, and the results provide a foundation for further research in this exciting study area.
Recommended Citation
Kabir, Mohammad Shah Hafez, "Hyperpolarized Nitrogen-15 Mri Contrast Agents For Hypoxia Sensing Via Reversible Exchange With Parahydrogen" (2023). Wayne State University Dissertations. 3857.
https://digitalcommons.wayne.edu/oa_dissertations/3857