Researchers in the Pines group, in collaboration with Berkeley professors David Wemmer and Matt Francis, are investigating the interactions of xenon with supramolecular hosts to develop biosensors capable of detecting biomolecules and chemical analytes[1]. By combining the remarkable sensitivity enhancements afforded by laser-polarized xenon nuclear magnetic resonance, detection schemes optimized for high-sensitivity, and unique synthetic constructs, these xenon-based molecular sensors are detectable at concentrations below the threshold of conventional NMR and can reveal important information regarding their local chemical environment. Furthermore, these molecular sensors are compatible with MRI techniques, making them ideal candidates for high sensitivity clinical imaging.

The basic ideas of the xenon-based molecular sensor project are described in an article of the Berkeley Lab Magazine; a first demonstration of molecular imaging can be found in this LBL press release.

Current research focuses on the following aspects:

The typical xenon-based molecular sensor consists of a xenon host molecule, usually a cryptophane cage, attached to a targeting moiety via a linker. When this sensor is titrated with its molecular target, its resonances broaden and change frequency. The sensors are synthesized in-house, primarily in collaboration with the Francis group, and are optimized for solubility, sensitivity, and targeting specificity, as well as for biocompatibilty[2]. Current work focuses on the synthesis of biochemical targets in certain cancers or physiological parameters like pH and temperature. Furthermore, the incorporation of naturally occurring host scaffolds, e.g. viral capsids, affords dramatic improvements in the sensitivity of the sensors. In addition to the biologically compatible constructs, sensors are also being developed for chemical analysis of various complex mixtures.

Magnetic resonance imaging (MRI) features the great advantage of being totally insensitive to the opacity of the object of interest. It is therefore a powerful tool for non-invasive, tomographic imaging of biological samples. Application of MRI methods to functionalized biosensors will provide a new technique of molecular imaging with high potential for biomedical applications, e.g. tumor identification and localization. We demonstrated development of a perfusion setup that enables delivery of xenon-saturated liquids to a phantom cell inside the NMR magnet. This cell contains the medium/object that is labeled with the biosensor and can be used to test MRI techniques for accurate mapping of the spatial sensor distribution[3]. The transition to setups allowing xenon imaging of living cells that can be targeted by the sensor is an ongoing project. Pulsed magnetic field gradients are applied to achieve spatial encoding of the biosensor resonance. This method images the composition of inhomogeneous samples and emphasizes the potential of the xenon-based molecular sensor as a selective imaging contrast agent. The inherent spectral information of these chemical shift imaging techniques makes this approach suitable for multiplexing, in which a number of different sensors with unique xenon chemical shifts bind to different targets.

A detection method developed in the Pines lab, Hyper-CEST (Hyperpolarized xenon Chemical Exchange Saturation Transfer), gives a huge sensitivity enhancement and reduces the acquisition time for molecular sensor imaging by several orders of magnitude[4]. By exploiting the dynamic process of chemical exchange over a long period of time (seconds), Hyper-CEST facilitates the acquisition of chemical and spatial information with high specificity and contrast can now be obtained within minutes. Using Hyper-CEST, xenon MRI could become an important technique for molecular imaging and overcome the sensitivity constraints of NMR when compared to other techniques such as positron emission tomography (PET) or single photon emission tomography (SPECT) [5]. The Hyper-CEST technique is currently used to enable temperature-sensitive imaging[6], as described below, and to push the detection limit into new dimensions, surpassing performance of optical spectroscopy for the first time[7]. Improving the performance of the Hyper-CEST method is an area of active research, with recent improvements yielding enhanced frequency resolution of the sensor. This enhanced frequency resolution facilitates the detection of many sensors in chemically similar environments that would previously have been indistinguishable.

Noninvasive, accurate, and spatially resolved temperature measurement in the human body is a promising technology for many biomedical applications, including hyperthermic treatment of cancer and detection of vulnerable atherosclerotic plaques. Researchers in the Pines group have developed a new MRI thermometry approach using hyperpolarized xenon and molecular sensors. Because the NMR frequency of xenon is exquisitely sensitive to temperature, we can detect changes in temperature as small as 0.1°C. This technology is currently being incorporated into a system compatible with in vivo imaging.

[1] Spence, M. M. et al. Functionalized xenon as a biosensor. Proc. Natl. Acad. Sci. USA 98, 10654-10657 (2001)
[2] Lowery, T. J. et al. Optimization of xenon biosensors for detection of protein interactions. ChemBioChem 7, 65-73 (2006)
[3] Hilty, C., Lowery, T. J., Wemmer, D. E. & Pines, A. Spectrally resolved magnetic resonance imaging of a xenon biosensor. Angew. Chem. Int. 45, 70-73 (2006)
[4] Schröder, L., Lowery, T.J., Hilty, C., Wemmer, D.E., and Pines, A., Science 314, 446-449 (2006)
[5] Driehuys, B., Science 314, 432-433 (2006).
[6] Schröder, L., Chavez, L., Meldrum, T., Smith, M., Lowery, T.J., Wemmer, D.E., Pines, A., Angew. Chem. Int. Ed. 47, 4316-4320 (2008).
[7] Schröder, L., Meldrum, T., Smith, M., Lowery, T.J., Wemmer, D.E., Pines, A. Phys. Rev. Lett. 100 (25), 257603 (2008)