Functionalized 129Xe contrast agents for magnetic resonance imaging

The concept of ‘xenon biosensor’ for magnetic resonance imaging (MRI) was first proposed by a Berkeley team in 2001, with evidence that hyperpolarized 129Xe bound to a biotin-labeled cryptophane can detect streptavidin at much lower concentrations (nM–μM) than is typical for contrast-enhanced MRI experiments. 129Xe biosensors have undergone many recent developments to address challenges in molecular imaging. For example, cryptophanes that exhibit 10-fold higher xenon affinity with distinct 129Xe magnetic resonance spectra have been synthesized. Also relevant are dendrimeric cryptophane assemblies and inorganic zeolites that localize many 129Xe atoms to rare targets. Finally, this article considers biosensors that produce measurable changes in 129Xe chemical shift based upon the activity of oligonucleotides, proteins, or enzymes, and includes the first cell studies.