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