Improving HyperCEST performance by favorable xenon exchange conditions in liposomal nanocarriers.

MRI reporters that combine signal enhancement from saturation transfer with hyperpolarized 129 Xe show nanomolar detection sensitivity for in vitro studies. However, they need further improvement for accelerated CEST build-up that sufficiently dominates the intrinsic loss of hyperpolarization under in vivo conditions. This study introduces liposomes with a HyperCEST-active lipopeptide to enhance the efficiency of a well known Xe host, CrA-ma, with medium Xe exchange kinetics in aqueous environment, by two orders of magnitude. The depolarization time for constant saturation power but increasing saturation time is used as a comparative measure to rank different nanocarrier formulations. A variable cage load illustrates that the available CEST sites should be well distributed throughout the nanocarriers to avoid inefficiency from back exchange. For a liposome loading with only 2 mol% CrA-lipopeptide, the higher exchange kinetics allowed us to work even with 17-fold lower saturation power than for CrA-ma itself to achieve significant image contrast with 129 Xe. Overall, this study illustrates the wide parameter space that is now available when incorporating CrA-labelled lipopeptides into liposomal carriers.

Microscopic multifrequency MR elastography for mapping viscoelasticity in zebrafish.

PURPOSE: The zebrafish (Danio rerio) has become an important animal model in a wide range of biomedical research disciplines. Growing awareness of the role of biomechanical properties in tumor progression and neuronal development has led to an increasing interest in the noninvasive mapping of the viscoelastic properties of zebrafish by elastography methods applicable to bulky and nontranslucent tissues. METHODS: Microscopic multifrequency MR elastography is introduced for mapping shear wave speed (SWS) and loss angle (φ) as markers of stiffness and viscosity of muscle, brain, and neuroblastoma tumors in postmortem zebrafish with 60 µm in-plane resolution. Experiments were performed in a 7 Tesla MR scanner at 1, 1.2, and 1.4 kHz driving frequencies. RESULTS: Detailed zebrafish viscoelasticity maps revealed that the midbrain region (SWS = 3.1 ± 0.7 m/s, φ = 1.2 ± 0.3 radian [rad]) was stiffer and less viscous than telencephalon (SWS = 2.6 ± 0. 5 m/s, φ = 1.4 ± 0.2 rad) and optic tectum (SWS = 2.6 ± 0.5 m/s, φ = 1.3 ± 0.4 rad), whereas the cerebellum (SWS = 2.9 ± 0.6 m/s, φ = 0.9 ± 0.4 rad) was stiffer but less viscous than both (all p < .05). Overall, brain tissue (SWS = 2.9 ± 0.4 m/s, φ = 1.2 ± 0.2 rad) had similar stiffness but lower viscosity values than muscle tissue (SWS = 2.9 ± 0.5 m/s, φ = 1.4 ± 0.2 rad), whereas neuroblastoma (SWS = 2.4 ± 0.3 m/s, φ = 0.7 ± 0.1 rad, all p < .05) was the softest and least viscous tissue. CONCLUSION: Microscopic multifrequency MR elastography-generated maps of zebrafish show many details of viscoelasticity and resolve tissue regions, of great interest in neuromechanical and oncological research and for which our study provides first reference values.

Rapid analytical CEST spectroscopy of competitive host-guest interactions using spatial parallelization with a combined approach of variable flip angle, keyhole and averaging (CAVKA).

A serious limitation of high resolution 129Xe chemical exchange saturation transfer (CEST) NMR spectroscopy for comparing competitive host-guest interactions from different samples is the long acquisition time due to step-wise encoding of the chemical shift dimension. A method of optimized use of 129Xe spin magnetization to enable the accelerated and simultaneous acquisition of CEST spectra from multiple samples or regions in a setup is described. The method is applied to investigate the host-guest system of commercially available cucurbit[7]uril (CB7) and xenon with competing guests: cis-1,4-bis(aminomethyl)cyclohexane, cadaverine, and putrescine. Interactions with the different guests prove that the observed CEST signal is from a CB6 impurity and that CB7 itself does not produce a CEST signal. Instead, rapid interactions between xenon and CB7 manifest in the spectrum as a broad saturation response that could be suppressed by cis-1,4-bis(aminomethyl)cyclohexane. This guest prevents interactions at the CB7 portals. The suggested method represents a type of spectroscopic imaging that is capable of capturing the exchange kinetics information of systems that otherwise suffer from shortened T2 times and yields multiple spectra for comparing exchange conditions with a reduction of >95% in acquisition time. The spectral quality is sufficient to perform quantitative analysis and quantifications relative to a CB6 standard as well as relative to a known blocker concentration (putrescine) that both reveal an unexpectedly high CB6 impurity of ca. 8%.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe.

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Probing Reversible Guest Binding with Hyperpolarized 129Xe-NMR: Characteristics and Applications for Cucurbit[n]urils.

Cucurbit[n]urils (CB[n]s) are a family of macrocyclic host molecules that find various applications in drug delivery, molecular switching, and dye displacement assays. The CB[n]s with n = 5-7 have also been studied with 129Xe-NMR. They bind the noble gas with a large range of exchange rates. Starting with insights from conventional direct detection of bound Xe, this review summarizes recent achievements with chemical exchange saturation transfer (CEST) detection of efficiently exchanging Xe in various CB[n]-based supramolecular systems. Unprecedented sensitivity has been reached by combining the CEST method with hyperpolarized Xe, the production of which is also briefly described. Applications such as displacement assays for enzyme activity detection and rotaxanes as emerging types of Xe biosensors are likewise discussed in the context of biomedical applications and pinpoint future directions for translating this field to preclinical studies.

High Exchange Rate Complexes of 129 Xe with Water-Soluble Pillar[5]arenes for Adjustable Magnetization Transfer MRI.

Macrocyclic host structures for generating transiently bound 129 Xe have been used in various ultra-sensitive NMR and MRI applications for molecular sensing of biochemical analytes. They are based on hyperpolarized nuclei chemical exchange saturation transfer (Hyper-CEST). Here, we tested a set of water-soluble pillar[5]arenes with different counterions in order to compare their potential contrast agent abilities with that of cryptophane-A (CrA), the most widely used host for such purposes. The exchange of Xe with such compounds was found to be sensitive to the type of ions present in solution and can be used for switchable magnetization transfer (MT) contrast that arises from off-resonant pre-saturation. We demonstrate that the adjustable MT magnitude depends on the interplay of saturation parameters and found that the optimum MT contrast surpasses the CrA CEST performance at moderate saturation power. Since modification of such water-soluble pillar[5]arenes is straightforward, these compounds can be considered a promising platform for designing various sensors that may complement the field of Xe HyperCEST-based biosensing MRI.

Complete Generation of a 129Xe Biosensor on the Solid Support by Systematic Backbone Assembly.

Xenon biosensors are an emerging tool for different molecular imaging approaches. For many applications, their development requires peptide synthesis steps, followed by the selective installation of a xenon host onto the peptide backbone in solution. In this study, three different strategies were attempted for generating entire Xe biosensors on the solid support. Notably, one strategy involving CryA-da was beneficial by directly integrating this host into the growing construct on a low loaded resin via modification of the administered subcomponent equivalents and by prolonging the coupling procedure. Subsequently, installation of additional amino acids or of additional labels onto the growing construct was achieved by a procedure in which an excess amine was administered to the activated CryA-da (acid) anchored onto the resin. Further, the as-generated Xe biosensor was tested for its NMR and MRI capabilities in H2O and compared to the performance of CryA-ma. Xe NMR of the biosensor indicated a clear CEST response and the Xe MR images revealed similar contrast compared to the reference host. These observations suggest that functionalizing CryA-da on both sides with multiple labels did not alter significantly its NMR capabilities. Hereby, we could show the successful and complete synthesis of a CryA-da-based xenon biosensor on the solid support without any notable side reactions and without the necessity of multiple purification steps.

Time-resolved monitoring of enzyme activity with ultrafast Hyper-CEST spectroscopy.

We propose a method to dynamically monitor the progress of an enzymatic reaction using NMR of hyperpolarized 129 Xe in a host-guest system. It is based on a displacement assay originally designed for fluorescence experiments that exploits the competitive binding of the enzymatic product on the one hand and a reporter dye on the other hand to a supramolecular host. Recently, this assay has been successfully transferred to NMR, using xenon as a reporter, cucurbit[6]uril as supramolecular host, and chemical exchange saturation transfer with hyperpolarized Xe (Hyper-CEST) as detection technique. Its advantage is that the enzyme acts on the unmodified substrate and that only the product is detected through immediate inclusion into the host. We here apply a method that drastically accelerates the acquisition of Hyper-CEST spectra in vitro using magnetic field gradients. This allows monitoring the dynamic progress of the conversion of lysine to cadaverine with a temporal resolution of ~30 s. Moreover, the method only requires to sample the very early onset of the reaction (<0.5% of substrate conversion where the host itself is required only at µM concentrations) at comparatively low reaction rates, thus saving enzyme material and reducing NMR acquisition time. The obtained value for the specific activity agrees well with previously published results from fluorescence assays. We furthermore outline how the Hyper-CEST results correlate with xenon T2 measurements performed during the enzymatic reaction. This suggests that ultrafast Hyper-CEST spectroscopy can be used for dynamically monitoring enzymatic activity with NMR.

NMR Hyperpolarization Techniques of Gases.

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Development of an antibody-based, modular biosensor for 129Xe NMR molecular imaging of cells at nanomolar concentrations.

Magnetic resonance imaging (MRI) is seriously limited when aiming for visualization of targeted contrast agents. Images are reconstructed from the weak diamagnetic properties of the sample and require an abundant molecule like water as the reporter. Micromolar to millimolar concentrations of conventional contrast agents are needed to generate image contrast, thus excluding many molecular markers as potential targets. To address this limitation, we developed and characterized a functional xenon NMR biosensor that can identify a specific cell surface marker by targeted (129)Xe MRI. Cells expressing the cell surface protein CD14 can be spatially distinguished from control cells with incorporation of as little as 20 nM of the xenon MRI readout unit, cryptophane-A. Cryptophane-A serves as a chemical host for hyperpolarized nuclei and facilitates the sensitivity enhancement achieved by xenon MRI. Although this paper describes the application of a CD14-specific biosensor, the construct has been designed in a versatile, modular fashion. This allows for quick and easy adaptation of the biosensor to any cell surface target for which there is a specific antibody. In addition, the modular design facilitates the creation of a multifunctional probe that incorporates readout modules for different detection methods, such as fluorescence, to complement the primary MRI readout. This modular antibody-based approach not only offers a practical technique with which to screen targets, but one which can be readily applied as the xenon MRI field moves closer to molecular imaging applications in vivo.

Continuous-wave saturation considerations for efficient xenon depolarization.

The combination of hyperpolarized Xe with chemical exchange saturation transfer (Hyper-CEST) is a powerful NMR technique to detect highly dilute concentrations of Xe binding sites using RF saturation pulses. Crucially, that combination of saturation pulse strength and duration that generates the maximal Hyper-CEST effect is a priori unknown. In contrast to CEST in proton MRI, where the system reaches a steady-state for long saturation times, Hyper-CEST has an optimal saturation time, i.e. saturating for shorter or longer reduces the Hyper-CEST effect. Here, we derive expressions for this optimal saturation pulse length. We also found that a pulse strength, B1, corresponding to five times the Xe exchange rate, k(BA) (i.e. B1 = 5 k(BA)/γ with the gyromagnetic ratio of (129)Xe, γ), generates directly and without further optimization 96% of the maximal Hyper-CEST contrast while preserving spectral selectivity. As a measure that optimizes the amplitude and the width of the Hyper-CEST response simultaneously, we found an optimal saturation pulse strength corresponding to √2 times the Xe exchange rate, i.e. B1=√2k(BA)/γ. When extremely low host concentration is detected, then the expression for the optimum saturation time simplifies as it approaches the longitudinal relaxation time of free Xe.

Multichannel MRI labeling of mammalian cells by switchable nanocarriers for hyperpolarized xenon.

We demonstrate a concept for multichannel MRI cell-labeling using encapsulated laser-polarized xenon. Conceptually different Xe trapping properties of two nanocarriers, namely macrocyclic cages as individual hosts or compartmentalization into nanodroplets, ensure a large chemical shift separation for Xe bound in either of the carriers even after cellular internalization. Two differently labeled mammalian cell populations were imaged by frequency selective saturation transfer resulting in a switchable "two-color" xenon-MRI contrast at micro- to nanomolar Xe carrier concentrations.

Supramolecular Assays for Mapping Enzyme Activity by Displacement-Triggered Change in Hyperpolarized (129)Xe Magnetization Transfer NMR Spectroscopy.

Reversibly bound Xe is a sensitive NMR and MRI reporter with its resonance frequency being influenced by the chemical environment of the host. Molecular imaging of enzyme activity presents a promising approach for disease identification, but current Xe biosensing concepts are limited since substrate conversion typically has little impact on the chemical shift of Xe inside tailored cavities. Herein, we exploit the ability of the product of the enzymatic reaction to bind itself to the macrocyclic hosts CB6 and CB7 and thereby displace Xe. We demonstrate the suitability of this method to map areas of enzyme activity through changes in magnetization transfer with hyperpolarized Xe under different saturation scenarios.

Live-cell MRI with xenon hyper-CEST biosensors targeted to metabolically labeled cell-surface glycans.

The targeting of metabolically labeled glycans with conventional MRI contrast agents has proved elusive. In this work, which further expands the utility of xenon Hyper-CEST biosensors in cell experiments, we present the first successful molecular imaging of such glycans using MRI. Xenon Hyper-CEST biosensors are a novel class of MRI contrast agents with very high sensitivity. We designed a multimodal biosensor for both fluorescent and xenon MRI detection that is targeted to metabolically labeled sialic acid through bioorthogonal chemistry. Through the use of a state of the art live-cell bioreactor, it was demonstrated that xenon MRI biosensors can be used to image cell-surface glycans at nanomolar concentrations.

Depolarization Laplace transform analysis of exchangeable hyperpolarized ¹²9Xe for detecting ordering phases and cholesterol content of biomembrane models.

We present a highly sensitive nuclear-magnetic resonance technique to study membrane dynamics that combines the temporary encapsulation of spin-hyperpolarized xenon ((129)Xe) atoms in cryptophane-A-monoacid (CrAma) and their indirect detection through chemical exchange saturation transfer. Radiofrequency-labeled Xe@CrAma complexes exhibit characteristic differences in chemical exchange saturation transfer-driven depolarization when interacting with binary membrane models composed of different molecular ratios of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). The method is also applied to mixtures of cholesterol and POPC. The existence of domains that fluctuate in cluster size in DPPC/POPC models at a high (75-98%) DPPC content induces up to a fivefold increase in spin depolarization time τ at 297 K. In POPC/cholesterol model membranes, the parameter τ depends linearly on the cholesterol content at 310 K and allows us to determine the cholesterol content with an accuracy of at least 5%.

Fast gradient-encoded CEST spectroscopy of hyperpolarized xenon.

Breaking speed limits: The acquisition of xenon-129 Hyper-CEST spectra is drastically accelerated by utilizing gradients to encode the chemical shift dimension. The signal is increased by using repeated spin-echo refocussing. The additional application of a variable flip angle makes the experiment independent from a constant Xe redelivery.

Cell tracking with caged xenon: using cryptophanes as MRI reporters upon cellular internalization.

Caged xenon has great potential in overcoming sensitivity limitations for solution-state NMR detection of dilute molecules. However, no application of such a system as a magnetic resonance imaging (MRI) contrast agent has yet been performed with live cells. We demonstrate MRI localization of cells labeled with caged xenon in a packed-bed bioreactor working under perfusion with hyperpolarized-xenon-saturated medium. Xenon hosts enable NMR/MRI experiments with switchable contrast and selectivity for cell-associated versus unbound cages. We present MR images with 10(3) -fold sensitivity enhancement for cell-internalized, dual-mode (fluorescence/MRI) xenon hosts at low micromolar concentrations. Our results illustrate the capability of functionalized xenon to act as a highly sensitive cell tracer for MRI detection even without signal averaging. The method will bridge the challenging gap for translation to in vivo studies for the optimization of targeted biosensors and their multiplexing applications.

NMR of hyperpolarised probes.

Increasing the sensitivity of NMR experiments is an ongoing field of research to help realise the exquisite molecular specificity of this technique. Hyperpolarisation of various nuclei is a powerful approach that enables the use of NMR for molecular and cellular imaging. Substantial progress has been achieved over recent years in terms of both tracer preparation and detection schemes. This review summarises recent developments in probe design and optimised signal encoding, and promising results in sensitive disease detection and efficient therapeutic monitoring. The different methods have great potential to provide molecular specificity not available by other diagnostic modalities.

Biomembrane interactions of functionalized cryptophane-A: combined fluorescence and 129Xe NMR studies of a bimodal contrast agent.

Fluorescent derivatives of the (129)Xe NMR contrast agent cryptophane-A were obtained by functionalization with near infrared fluorescent dyes DY680 and DY682. The resulting conjugates were spectrally characterized, and their interaction with giant and large unilamellar vesicles of varying phospholipid composition was analyzed by fluorescence and NMR spectroscopy. In the latter, a chemical exchange saturation transfer with hyperpolarized (129)Xe (Hyper-CEST) was used to obtain sufficient sensitivity. To determine the partitioning coefficients, we developed a method based on fluorescence resonance energy transfer from Nile Red to the membrane-bound conjugates. This indicated that not only the hydrophobicity of the conjugates, but also the phospholipid composition, largely determines the membrane incorporation. Thereby, partitioning into the liquid-crystalline phase of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was most efficient. Fluorescence depth quenching and flip-flop assays suggest a perpendicular orientation of the conjugates to the membrane surface with negligible transversal diffusion, and that the fluorescent dyes reside in the interfacial area. The results serve as a basis to differentiate biomembranes by analyzing the Hyper-CEST signatures that are related to membrane fluidity, and pave the way for dissecting different contributions to the Hyper-CEST signal.

Functionalized 129Xe as a potential biosensor for membrane fluidity.

Using spin hyperpolarized xenon ((129)Xe) we investigate the impact of the local molecular environment on reversible host-guest interactions. We label Xe guest atoms that are temporarily bound to cryptophane-A hosts using the Hyper-CEST technique. By varying the length of the saturation pulse and utilizing an inverse Laplace transform we can determine depolarization times for the noble gas in different local environments, in this case biomembranes possessing different fluidity. We extend this technique to magnetic resonance imaging, mapping the spatial distribution of the different biomembranes. Such decays measured in biomembranes of 200 µM 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were characterized by mono-exponential decays with time constants of τPOPC = 3.00(-0.61)(+0.77) s and τDPPC(-4.16)(+5.19) = 22.15 s. Analyzing both environments simultaneously yielded a bi-exponential decay. This approach may give further insights into saturation transfer dynamics of reversibly bound Xe with applications extending into biomedical diagnostics.