Monitoring enzyme activity with a hyperpolarized MRI biosensor

The pioneering work of Paul Lauterbur (New York) and Sir Peter Mansfield (Nottingham) in the early 1970s lead to the development of magnetic resonance imaging (MRI) that culminated into the award of the 2003 Noble Price in Medicine. Over time, the technique has transformed biomedical research and clinical diagnostics with broad availability of MRI scanners in our hospitals. The technological development of MRI is however far from completed as the technique holds many potential keys to unlock further insights in biological systems.
Molecular imaging is at the forefront of current MRI developments. Most MRI scans in hospitals generally provide anatomical images with millimeter resolution, while standard research systems can reach micrometer resolution. Molecular imaging, however, goes further and allows the visualization of the activity of specific target molecules hidden deep within an organism, providing novel insights into biological function and processes. Unfortunately, the inherently low signal intensity due to dilute molecular concentrations typically prevents the direct observation of target molecules with MRI. A solution is the combination of the high molecular specificity in molecular imaging with the strong signals typically available through hyperpolarized noble gas MRI.
Combining molecular imaging with hyperpolarized MRI is possible through biosensor molecules. The sensor molecules interact with the noble gas xenon but also with specific target molecules of interest in the body. The biosensor molecules will, depending on the presence of the target molecule, change the characteristic hyperpolarized MRI signal and hence the obtained MRI contrast. A new type of biosensor molecule will be synthesized, designed to effectively switch the local xenon MRI signal off. Normally, strong signals are expected from the target organ exposed to the xenon, leading to bright images. However, the xenon images will remain dark if the sensor molecules are administered as a contrast agent switching the signal off. Only once the biosensors detect the presence of the target enzyme, will the image return to normal brightness. A small quantity of target molecules can activate a large number of xenon molecules. This amplification effect will lead to easily visible, bright areas in the MRI images, and allows the visualization of the enzyme activity in the living organism.

We are proposing to develop a proof of concept for a novel tool for molecular imaging. We will design a new molecular sensor that can detect target enzymes with high sensitivity and specificity in vivo using magnetic resonance imaging (MRI). The novel tool, based on a radically new physical concept we described recently, uses a combination of hyperpolarized (hp) imaging, which dramatically increases the sensitivity to nM concentrations of the molecular target, and a molecular switch derived from standard MRI contrast agents, which allows the sensor to detect a wide range of enzymes with high specificity.

With this sensor, users will be able to study enzyme activity in opaque media, non-invasively, with high spatial resolution and in three dimensions in intact tissue samples, isolated organs and in vertebrate models in vivo using MRI. As a first demonstration of the tool, we will show that we can monitor the release of the enzyme matrix metalloproteinase MMP-9 in a culture of macrophages. The technique uses a widely available standard preclinical MRI setup, with the addition of a hyperpolarizer, which is increasingly affordable and user-friendly. The production of hyperpolarized 129Xe has significantly improved over the last decade, resulting in better image quality at potentially much reduced cost. Dedicated analysis software will be made publicly available as part of this project.

We are developing new MRI methodology that is part of a larger current research effort to radically improve non-invasive molecular MRI. The beneficiaries of this work will not only be academic but also those working in the pharmaceutical industry. If successful, the new class of xenon biosensors will allow those researchers to monitor molecular activity in vivo in a spatially resolved fashion. It will also allow for more efficient use of the individual animals used in the studies and therefore satisfy the desire for an overall reduction in animal number and animal suffering.
MMP-9 is chosen for the proof of concept work as we already have related 19F MRI data for the sensor cleaving of this member of the matrix metalloproteinase (MMP) family. However, the concept can easily be extended to other MMPs and hydrolytic enzymes. The medium-term goal is to developed pre-clinical (cells, tissue, small animal) MRI protocols to enable researchers to apply these tools without detailed technical knowledge in hyperpolarized MRI.
After successful proof of concept, we will seek follow up grants from BBSRC to advance the exploration of MRI measurable parameter as biomarkers. The sensor concept will be further developed as a contrast agent in the lungs and the brain as target organs. The brain is a difficult target for the contrast agent enhanced MRI due to the blood brain barrier, however the concept can be extended to other organs.
Commercialization. The advances made in this project will be used to commercialize the biosensor, potentially together with the TM's hyperpolarizer technology. In particular, as detailed in the Pathway to Impact, the focus will be on commercialization for pre-clinical applications and research.