Xenon Host-Guest Interactions for Applications in NMR and MRI
- Author(s): Ramirez, Richard Matthew
- Advisor(s): Pines, Alexander
- et al.
The ability to detect the presence of specific analytes, whether in vivo or in heterogeneous solutions in vitro, is important for biomedical research as well as chemical sensing devices. These applications demand sensitive detection in order to report on small quantities of the material of interest. For many diseases such as cancer, early detection of the important biomarkers when they exist at very low concentrations improves patient prognosis.
NMR is noninvasive and does not require ionizing radiation, making it ideal for passive monitoring, and compatible with biological systems. It can be used to gain spectroscopic information, or alternatively to map the spatial distribution of a signal of interest, as is done with water protons in a 1H MRI experiment. The intrinsic sensitivity of NMR is low, however, due to low polarization which means that only ten protons out of a million are actually contributing to the observed signal. This is acceptable in an ordinary clinical MRI scan, where the in vivo concentration of protons is high ([H2O] $sim; 55 M), but severely limits its utility for low detection applications. Although contrast agents and methods have been successfully developed to address this problem they still lack the sensitivity to compete with nuclear medicine radioactive tracers which can be detected at sub-nanomolar concentrations—concentrations which better reflect the detection thresholds necessary for applications such as biomarker screening.
This work considers the use of dissolved 129Xe gas as the imaging medium, rather than water protons. 129Xe can be “hyperpolarized” to increase its NMR signal by greater than 10,000-fold, resulting in signals comparable to that of water from considerably smaller concentrations (e.g. millimolar). Since xenon, a noble gas, cannot easily be functionalized for targeted applications, agents that interact with both xenon and a target are required. These hosts provide a unique environment for xenon such that a unique signature arises in the 129Xe NMR spectrum. This text describes the development, characterization, and application of xenon hosts with a sharp focus on creating new agents with improved sensitivity over state-of-the-art.
Three different host platforms are presented. The first system builds upon a decade of previous work using the cage-like molecule cryptophane-A as the xenon host. Here, an agent composed of several hundred cryptophane-A molecules covalently attached to an M13 bacteriophage was synthesized and detected at sub-picomolar concentrations. The second system utilized emulsified, nanometer-sized perfluorocarbon-in-water droplets as the xenon host. This platform benefits from increased xenon payload and faster dynamics as opposed to cryptophane-based agents. Although a similar detection threshold was achieved for these nanoemulsion droplets, the time required for their detection was reduced by about 75%, and their production is much more straightforward and inexpensive. Finally, the third system was based upon gas vesicles (GVs), which are sub-micron sized “balloons” composed of proteins produced by buoyant photosynthetic bacteria. Detectable at picomolar concentrations, GVs hold the promise of a sensitive genetic reporter analogous to green fluorescent protein. What's more, it was found that GVs from different species give rise to unique chemical shifts in the 129Xe NMR spectrum, enabling multiplexed imaging.
Targeted cell labeling was performed with the phage-based agent and GVs to highlight their potential as exogenous molecular imaging agents. The bacteriophage-based agent utilized antibodies to target epidermal growth factor receptor (EGFR), a cell surface biomarker, while isolated GVs were functionalized against HER2 (also an EGFR) via intermediate biotin-avidin conjugation. Independent of these studies, the capability for imaging gene expression was demonstrated by transforming the GV-producing gene cluster into E. coli and placing it under control of an exogenous promoter.
Taken together, these results underscore the viability and potential of hyperpolarized 129Xe NMR/MRI contrast agents for low-detection applications.