Enabling Stroke Theranostics with Hyperpolarized Magnetic Resonance

The severe social and economic burden of stroke is a strong motivation for the exploration of innovative medical approaches. In this context, magnetic resonance (MR) of hyperpolarized (HP) molecular agents via dissolution dynamic nuclear polarization (dDNP) allows tracking their metabolic fate in vivo. This may not only provide novel insights into the underlying metabolic mechanisms, but also pave the way to a theranostic approach for stroke when applied to neuroprotective agents.

This thesis focuses on the development of novel theranostic approaches to acute ischemic stroke based on HP neuroprotective agents, specifically [1-13C] lactate and [1-13C] pyruvate, in the preclinical setting. To achieve this goal, it entails two parallel aspects: exploring their aptitude as molecular biomarkers for stroke, and reinforcing the exploitation of this potential by optimizing technical and methodological aspects of HP MR experiments.

The dynamics of HP lactate and pyruvate metabolism were quantified via global MR spectroscopic measurements and kinetic modeling. Both substrates underscored their potential as biomarkers for acute ischemic stroke, showing distinct biochemical transport and metabolism in a mouse model of stroke compared to healthy animals. Although lactate is more challenging to acquire as a result of lower polarization and metabolism than pyruvate, it is of greater interest in theranostics with better demonstrated neuroprotective effects.

The effects of the nitroxyl radical TEMPOL, a dDNP polarizing agent and known neuroprotective agent, on cerebral lactate metabolism were studied to understand the dissimilar metabolic trends observed compared to previous studies. The simultaneous injection of this radical with a bolus of lactate, prepolarized with the trityl radical OX063, resulted in a higher pyruvate labeling. This emphasizes that free radicals used as polarizing agents in dDNP, despite their low dose, are not biologically inert and must be carefully considered to avoid experimental bias.

A successful HP MR experiment requires repeatable production of HP substrates. To this end, the custom fluid path (CFP) and the corresponding cryogenic probe were implemented on a dDNP polarizer to reduce user variability. This not only improved the consistency of both the polarization and concentration, but also enhanced the overall cryogenic and DNP performance, and facilitated the optimization of sample formulations via longitudinally detected electron spin resonance (LOD-ESR) measurements.

A multi-sample dDNP probe was built to extend the application range of HP MR. Up to three samples can be polarized simultaneously while being individually monitored, even with distinct nuclei and radicals, and then sequentially dissolved. This increases the throughput of dDNP several folds and could enable the investigation of HP multi-agent theranostic approaches to gain a more comprehensive view and target multiple facets of stroke.

To characterize the dynami