We implemented a new set of instruments to prepare HP molecules by dynamic nuclear polarization (DNP) for metabolic imaging, in particular a stand-alone hyperpolarizer containing a cryogen-free magnet that can be operated at any field up to 7 T and does not require the use of liquid helium, an expensive consumable. This novel system is by far the most performant preclinical hyperpolarizer that has been developed to date, providing up to three doses of HP molecules with more than 50% carbon-13 (13C) polarization. It will be a model for the future commercial systems. It was used for both in vitro and in vivo HP MR experiments.
We demonstrated that most keto acids, in particular pyruvic acid and phenylglyoxylic acid, which are endogenous molecules, have the propensity to yield radicals under photo-excitation in the ultraviolet-visible spectrum. These non-persistent radicals play the role of polarizing agent and they disappear when the HP molecules are brought to room-temperature, forming a small amount of endogenous byproduct. While investigating these keto acids, we also unravelled the key photochemical mechanism enabling the creation of non-persistent radicals in pyruvic acid. This had remained elusive despite more than 85 years of research.
We investigated the feasibility of reducing the pyruvate dose used for in vivo HP MR experiments. In particular, we demonstrated that that liver metabolism can be measured in vivo with HP pyruvate administered at near basal plasma concentration. We showed that hepatic gluconeogenesis can be directly probed in vivo with HP pyruvate. We also investigated the potential of glucose and lactate as HP substrates for in vivo metabolic studies at physiological level. More specifically, we demonstrated that it is possible to measure cerebral glucose metabolism in vivo with sub-second time resolution. This unique method allows direct detection of glycolysis in vivo in the healthy brain in a noninvasive manner. We also studied lactate uptake and intracellular metabolism in the brain. We demonstrated that HP lactate can be used to detect a variation in cerebral lactate uptake of <40 nmol in a healthy brain during an in vivo experiment lasting only 75 s.
We proposed a new ex vivo application of HP MR to detect the metabolic changes induced by the activation of immune cells. We demonstrated that the metabolic adaptation triggered by human T lymphocytes stimulation can be rapidly and noninvasively detected by HP MR. We imaged, with a sub-cellular resolution, the 13C enrichment in macromolecules following the injection of 13C-glucose in tumors, highlighting the higher 13C enrichment in the tumor region as compared with healthy tissue.
In terms of dissemination, these results led to the publication of 13 articles in scientific journals, 4 book chapters, 21 conference abstracts, and I gave 13 invited talks at international conferences. We also submitted two patent applications and are aiming at exploiting the technology developed in this project to make HP MR a clinically viable modality.
