Responsive fluorinated lanthanide probes for 19F magnetic resonance spectroscopy

Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.

Development of Ln(III) Derivatives as 19F Parashift Probes

19F parashift probes with paramagnetically shifted reporter nuclei provide attractive platforms to develop molecular imaging probes. These probes enable ratiometric detection of molecular disease markers using a direct detection technique. Here, we describe a series of trivalent lanthanide (Ln(III)) complexes that are structural analogues of the clinically approved MR contrast agent (CA) ProHance to obtain LnL 19F parashift probes. We evaluated trans-gadolinium paramagnetic lanthanides compared to diamagnetic YL for 19F chemical shift and relaxation rate enhancement. The paramagnetic contribution to chemical shift (δPCS) for paramagnetic LnL exhibited either shifts to lower frequency (δPCS < 0 for TbL, DyL, and HoL) or shifts to higher frequency (δPCS > 0 for ErL, TmL, and YbL) compared to YL 19F spectroscopic signal. Zero-echo time pulse sequences achieved 56-fold sensitivity enhancement for DyL over YL, while developing probe-specific pulse sequences with fast delay times and acquisition times achieved 0.6-fold enhancement in limit of detection for DyL. DyL provides an attractive platform to develop 19F parashift probes for ratiometric detection of enzymatic activity.

Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive 1 H and 19 F Relaxation Properties

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d6 /H2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (dMn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T1 and T2 relaxation times. Mn-TPP-p-CF3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.

19 F VT NMR: Novel Tm3+ and Ce3+ Complexes Provide New Insight into Temperature Measurement Using Molecular Sensors

In the past few decades, magnetic resonance spectroscopy (MRS) and MR imaging (MRI) have developed into a powerful non-invasive tool for medical diagnostic and therapy. Especially 19 F MR shows promising potential because of the properties of the fluorine atom and the negligible background signals in the MR spectra. The detection of temperature in a living organism is quite difficult, and usually external thermometers or fibers are used. Temperature determination via MRS needs temperature-sensitive contrast agents. This article reports first results of solvent and structural influences on the temperature sensitivity of 19 F NMR signals of chosen molecules. By using this chemical shift sensitivity, a local temperature can be determined with a high precision. Based on this preliminary study, we synthesized five metal complexes and compared the results of all variable temperature measurements. It is shown that the highest 19 F MR signal temperature dependence is detectable for a fluorine nucleus in a Tm3+ -complex.

Copper(II) complexes of cyclams with N-(2,2,2-trifluoroethyl)-aminoalkyl pendant arms as potential probes for 19F magnetic resonance imaging

A series of Cu(II) complexes with cyclam-based ligands containing two N-(2,2,2-trifluoroethyl)-aminoalkyl pendant arms in 1,8-positions (L1: 1,2-ethylene spacer, L2: 1,3-propylene spacer; L3: 1,4-butylene spacer) was studied in respect to potential use as contrast agents for ¹⁹F magnetic resonance imaging (MRI). A number of structures of the complexes as well as of several organic precursors were determined by single-crystal X-ray diffraction analysis. Geometric parameters (especially distances between fluorine atoms and the central metal ion) were determined for each complex and the identity of isomeric complex species present in solution was established. The NMR longitudinal relaxation times (T₁) of ¹⁹F nuclei in the ligands at clinically relevant fields and temperatures (1-2 s) were significantly shortened upon Cu(II) binding to 7-10 ms for [Cu(L1)]²⁺, 20-30 ms for [Cu(L2)]²⁺ and 20-50 ms for [Cu(L3)]²⁺. The trend of the relaxation time shortening is in accordance with the distance and number of chemical bonds between fluorine atoms and the Cu(II) ion. The signals show promising T₂*/T₁ ratios in the range 0.25-0.55, assuring their good applicability to ¹⁹F NMR/MRI. The results show that even the Cu(II) ion, with a small magnetic moment, causes significant relaxation enhancement with a long-range effect and can be considered as a highly suitable metal ion for efficient ¹⁹F MRI contrast agents.

A Ln(III)-3-hydroxypyridine pH responsive probe optimized by DFT

Differences in tissue pH can be diagnostic of cancer and other conditions that shift cell metabolism. Paramagnetic probes are promising tools for pH mapping in vivo using magnetic resonance spectroscopy (MRS) as they provide uniquely shifted MR signals that change with pH. Here, we demonstrate a 3-hydroxy-6-methylpyridyl coordinating group as a new pH-responsive reporter group for Ln(III) MRS probes. The pH response of the complex was observed by UV-Vis, fluorescence, and NMR spectroscopies, and modeled using DFT. These results provide insight into the observed pH-dependent NMR spectrum of the complex. The protonation state of the hydroxypyridine changes the coordinating ability of the ligand, affecting the dipolar field of the lanthanide and the chemical shift of nearby reporter nuclei. The favorable pH response and coordination properties of the 3-hydroxypyridyl group indicates its potential for further development as a dual responsive-reporter group. Incorporation into optimized scaffolds for MRS detection may enable sensitive pH-mapping in vivo.

Design, Characterization and Molecular Modeling of New Fluorinated Paramagnetic Contrast Agents for Dual 1H/19F MRI

One major goal in medical imaging is the elaboration of more efficient contrast agents (CAs). Those agents need to be optimized for the detection of affected tissues such as cancers or tumors while decreasing the injected quantity of agents. The paramagnetic contrast agents containing fluorine atoms can be used for both proton and fluorine magnetic resonance imaging (MRI), and they open the possibility of simultaneously mapping the anatomy using 1H MRI and accurately locating the agents using 19F MRI. One of the challenges in this domain is to synthesize molecules containing several chemically equivalent fluorine atoms with relatively short relaxation times to allow the recording of 19F MR images in good conditions. With that aim, we propose to prepare a CA containing a paramagnetic center and nine chemically equivalent fluorine atoms using a cycloaddition reaction between two building blocks. These fluorinated contrast agents are characterized by 19F NMR, showing differences in the fluorine relaxation times T1 and T2 depending on the lanthanide ion. To complement the experimental results, molecular dynamics simulations are performed to shed light on the 3D-structure of the molecules in order to estimate the distance between the lanthanide ion and the fluorine atoms.

19F Magnetic Resonance Activity-Based Sensing Using Paramagnetic Metals

Fluorine magnetic resonance imaging (¹⁹F MRI) is a promising bioimaging technique due to the favorable magnetic resonance properties of the ¹⁹F nucleus and the lack of detectable biological background signal. A range of imaging agents have been developed for this imaging modality including small molecule perfluorocarbons, fluorine-rich macromolecules and nanoparticles, and paramagnetic metal-containing agents. Incorporation of paramagnetic metals into fluorinated agents provides a unique opportunity to manipulate relaxation and chemical shift properties of ¹⁹F nuclei. Paramagnetic centers will enhance relaxation rates of nearby ¹⁹F nuclei through paramagnetic relaxation enhancement (PRE). Further, metals with anisotropic unpaired electrons can induce changes in ¹⁹F chemical shift through pseudocontact shift (PCS) effects. PRE and PCS are dependent on the nature of the metal center itself, the molecular scaffold surrounding it, and the position of the ¹⁹F nucleus relative to the metal center. One intriguing prospect in ¹⁹F magnetic resonance molecular imaging is to design responsive agents that can serve to provide a read out biological activity, including the activity of enzymes, redox activity, the activity of ions, etc. Paramagnetic agents are well suited for this activity-based sensing as metal complexes can be designed to respond to specific biological activities and give a corresponding ¹⁹F response that results from changes in the metal complex structure and subsequently PRE/PCS. Broadly speaking, when designing paramagnetic ¹⁹F MR biosensors, one can envision that in response to changes in analyte activity, the number of unpaired electrons of the metal changes or the ligand conformation/chemical composition changes. This Account highlights activity-based probes from the Que lab that harness paramagnetic metals to modulate ¹⁹F signal. We discuss probes that use conversion from Cu²⁺ to Cu⁺ in response to reducing environments to dequench the ¹⁹F MR signal. Probes in which oxidants convert Co²⁺ to Co³⁺, resulting in chemical shift responses, are also described. Finally, we explore our foray into using Ni²⁺ coordination switching to furnish probes with different ¹⁹F signals when they are converted between 4-coordinate square planar and higher coordination numbers. A major barrier for ¹⁹F MR molecular imaging is in vivo application, as signal sensitivity is relatively low, requiring long imaging times to detect imaging agents. Nanoparticle and macromolecular agents show promise due to their higher fluorine density and longer circulation times; however, their analyte scope is limited to analytes that induce cleavage events. A grand challenge for researchers in this area is adapting lessons learned from small molecule paramagnetic probes with promising in vitro activities for the development of probes with enhanced in vivo utility for basic biological and clinical applications.

Fluorinated MRI contrast agents and their versatile applications in the biomedical field

MRI has been recognized as one of the most applied medical imaging techniques in clinical practice. However, the presence of background signal coming from water protons in surrounding tissues makes sometimes the visualization of local contrast agents difficult. To remedy this, fluorine has been introduced as a reliable perspective, thanks to its magnetic properties being relatively close to those of protons. In this review, we aim to give an overall description of fluorine incorporation in contrast agents for MRI. The different kinds of fluorinated probes such as perfluorocarbons, fluorinated dendrimers, polymers and paramagnetic probes will be described, as will their imaging applications such as chemical exchange saturation transfer (CEST) imaging, physico-chemical changes detection, drug delivery, cell tracking and inflammation or tumors detection.

Low-molecular-weight paramagnetic 19F contrast agents for fluorine magnetic resonance imaging

Objective

¹⁹F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable ¹⁹F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested.

Methods

Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. ¹⁹F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo ¹⁹F MRI was performed at 4.7 T.

Results

¹⁹F relaxation times strongly depended on the chelated lanthanide(III) ion. T₁ ranged from 6.5 to 287 ms, T₂ from 3.9 to 124.4 ms, and T₂* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; ¹⁹F MRI in vivo confirmed the in vivo applicability of the probe.

Conclusion

DOTP-based ¹⁹F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd³⁺ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

Fluorinated Paramagnetic Complexes: Sensitive and Responsive Probes for Magnetic Resonance Spectroscopy and Imaging

Fluorine magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) of chemical and physiological processes is becoming more widespread. The strength of this technique comes from the negligible background signal in in vivo¹⁹F MRI and the large chemical shift window of ¹⁹F that enables it to image concomitantly more than one marker. These same advantages have also been successfully exploited in the design of responsive ¹⁹F probes. Part of the recent growth of this technique can be attributed to novel designs of ¹⁹F probes with improved imaging parameters due to the incorporation of paramagnetic metal ions. In this review, we provide a description of the theories and strategies that have been employed successfully to improve the sensitivity of ¹⁹F probes with paramagnetic metal ions. The Bloch-Wangsness-Redfield theory accurately predicts how molecular parameters such as internuclear distance, geometry, rotational correlation times, as well as the nature, oxidation state, and spin state of the metal ion affect the sensitivity of the fluorine-based probes. The principles governing the design of responsive ¹⁹F probes are subsequently described in a "how to" guide format. Examples of such probes and their advantages and disadvantages are highlighted through a synopsis of the literature.

Fluorinated Mesoporous Silica Nanoparticles for Binuclear Probes in 1H and 19F Magnetic Resonance Imaging

The development of molecular and cellular magnetic resonance imaging (MRI) procedures has always represented a challenge because of the fact that conventional MRI contrast agents are not directly detected in vivo; in proton MRI (e.g., with the nucleus ¹H), their local concentration is measured through the effect they exert on the signal of hydrogen protons present in their immediate vicinity. Because the contrast effects generated by conventional MRI probes superpose to and can often impede the anatomical information contained in ¹H MRI images, new probes based on a nucleus other than ¹H, are being developed. In this study, we report on the development of fluorinated mesoporous silica nanoparticles (MSNs), which could represent an interesting dual probe that allows two MRI modes: ¹H for high-resolution anatomical information and ¹⁹F for the detection of MSNs used as drug delivery agents. MSNs were synthesized and covalently functionalized either with fluorosilane (FMSNs) or polyfluorosiloxane (polyFMSNs) to enable their detection in ¹⁹F MRI. Then, gadolinium chelates were grafted on the particles to enhance their detectability in ¹H MRI. The physicochemical, textural, and relaxometric properties (¹H and ¹⁹F relaxation times) of the nanoparticles were measured and compared. The ¹⁹F relaxation properties were found to be dependent on the concentration of fluorine; they were also highly sensitive to the presence of gadolinium. The shortest relaxation times were obtained with polyFMSNs. At clinical magnetic field strengths, high ¹H relaxivities and low relaxometric ratios (r₂/r₁ = 1.45; 2.2 for nanoparticles entrapped in hydrogel) were found for both nanoparticle systems. Finally, the visibility of both systems was confirmed in ¹H, and the detectability of polyFMSNs was confirmed in ¹⁹F MRI. This physicochemical and relaxometric study opens the door to the applications of fluorinated silica nanoparticles as theranostic materials allowing dual MRI (¹H and ¹⁹F).

Spin-crossover and high-spin iron(ii) complexes as chemical shift 19F magnetic resonance thermometers

The potential utility of paramagnetic transition metal complexes as chemical shift 19F magnetic resonance (MR) thermometers is demonstrated. Further, spin-crossover FeII complexes are shown to provide much higher temperature sensitivity than do the high-spin analogues, owing to the variation of spin state with temperature in the former complexes. This approach is illustrated through a series of FeII complexes supported by symmetrically and asymmetrically substituted 1,4,7-triazacyclononane ligand scaffolds bearing 3-fluoro-2-picolyl derivatives as pendent groups (L x ). Variable-temperature magnetic susceptibility measurements, in conjunction with UV-vis and NMR data, show thermally-induced spin-crossover for [Fe(L1)]2+ in H2O, with T1/2 = 52(1) °C. Conversely, [Fe(L2)]2+ remains high-spin in the temperature range 4-61 °C. Variable-temperature 19F NMR spectra reveal the chemical shifts of the complexes to exhibit a linear temperature dependence, with the two peaks of the spin-crossover complex providing temperature sensitivities of +0.52(1) and +0.45(1) ppm per °C in H2O. These values represent more than two-fold higher sensitivity than that afforded by the high-spin analogue, and ca. 40-fold higher sensitivity than diamagnetic perfluorocarbon-based thermometers. Finally, these complexes exhibit excellent stability in a physiological environment, as evidenced by 19F NMR spectra collected in fetal bovine serum.

Fe- and Ln-DOTAm-F12 Are Effective Paramagnetic Fluorine Contrast Agents for MRI in Water and Blood

A series of fluorinated macrocyclic complexes, M-DOTAm-F12, where M is La^III, Eu^III, Gd^III, Tb^III, Dy^III, Ho^III, Er^III, Tm^III, Yb^III, and Fe^II, was synthesized, and their potential as fluorine magnetic resonance imaging (MRI) contrast agents was evaluated. The high water solubility of these complexes and the presence of a single fluorine NMR signal, two necessary parameters for in vivo MRI, are substantial advantages over currently used organic polyfluorocarbons and other reported paramagnetic ¹⁹F probes. Importantly, the sensitivity of the paramagnetic probes on a per fluorine basis is at least 1 order of magnitude higher than that of diamagnetic organic probes. This increased sensitivity is due to a substantial-up to 100-fold-decrease in the longitudinal relaxation time (T₁) of the fluorine nuclei. The shorter T₁ allows for a greater number of scans to be obtained in an equivalent time frame. The sensitivity of the fluorine probes is proportional to the T₂/T₁ ratio. In water, the optimal metal complexes for imaging applications are those containing Ho^III and Fe^II, and to a lesser extent Tm^III and Yb^III. Whereas T₁ of the lanthanide complexes are little affected by blood, the T₂ are notably shorter in blood than in water. The sensitivity of Ln-DOTAm-F12 complexes is lower in blood than in water, such that the most sensitive complex in water, Ho^III-DOTAm-F12, could not be detected in blood. Tm^III yielded the most sensitive lanthanide fluorine probe in blood. Notably, the relaxation times of the fluorine nuclei of Fe^II-DOTAm-F12 are similar in water and in blood. That complex has the highest T₂/T₁ ratio (0.57) and the lowest limit of detection (300 μM) in blood. The combination of high water solubility, single fluorine signal, and high T₂/T₁ of M-DOTAm-F12 facilitates the acquisition of three-dimensional magnetic resonance images.

Coevolution and ratiometric behaviour in metal cation-driven dynamic covalent systems

Coevolution can be defined as the correlated changes of structurally and/or functionally connected entities. Dynamic Covalent Libraries (DCLs) have been used to demonstrate coevolution and ratiometric behaviour on a molecular level using dynamic covalent molecules such as imines and hydrazones.

Dynamic Covalent Libraries (DCLs) have been used to demonstrate coevolution behaviour on a molecular level using dynamic covalent molecules such as imines and hydrazones. Two systems are presented: the first system is based on a dialdehyde and two diamines in combination with Zn(ii) and Hg(ii) to form a 2 × 2 Constitutional Dynamic Network (CDN) of four complexes of macrocyclic bis-imines. Whereas the two metal ions, when reacted separately form a complex with each macrocycle with low selectivity, when applied together, each cation yields selectively a complex with one of the two macrocycles. Thus, the simultaneous application of both cations, where one might have expected the formation of four different complexes, results in the synergistic evolution (co-evolution) towards a simpler, more selective outcome under agonist amplification. The second system of 4 components, 2 amines and 2 aldehydes displays metalloselection together with a correlated evolution in distribution on complexation of Zn(ii) and Cu(i) with the dynamic ligand constituents and exhibits a dynamic ratiometry process related to the antagonistic behaviour of a pair of ligand constituents.

Nickel(ii) complexes of N-CH2CF3 cyclam derivatives as contrast agents for 19F magnetic resonance imaging

Nickel(ii) complexes of N,N′′-2,2,2-trifluoroethyl cyclam derivatives show significant ¹⁹F NMR relaxation rate enhancement useful for ¹⁹F MRI imaging.

A ratiometric NMR pH sensing strategy based on a slow-proton-exchange (SPE) mechanism

Real time and non-invasive detection of pH in live biological systems is crucial for understanding the physiological role of acid-base homeostasis and for detecting pathological conditions associated with pH imbalance. One method to achieve in vivo pH monitoring is NMR. Conventional NMR methods, however, mainly utilize molecular sensors displaying pH-dependent chemical shift changes, which are vulnerable to multiple pH-independent factors. Here, we present a novel ratiometric strategy for sensitive and accurate pH sensing based on a small synthetic molecule, SPE1, which exhibits exceptionally slow proton exchange on the NMR time scale. Each protonation state of the sensor displays distinct NMR signals and the ratio of these signals affords precise pH values. In contrast to standard NMR methods, this ratiometric mechanism is not based on a chemical shift change, and SPE1 binds protons with high selectivity, resulting in accurate measurements. SPE1 was used to measure the pH in a single oocyte as well as in bacterial cultures, demonstrating the versatility of this method and establishing the foundation for broad biological applications.

Ligation driven (19)F relaxation enhancement in self-assembled Ln(III) complexes

Strong bidentate ligation between a fluorinated isophthalate and binuclear lanthanide-DO3A species yields a new class of (19)F NMR agent with very high nuclear relaxation rates at physiologically-relevant pH.

Dual-frequency calcium-responsive MRI agents

Responsive or smart magnetic resonance imaging (MRI) contrast agents are molecular sensors that alter the MRI signal upon changes in a particular parameter in their microenvironment. Consequently, they could be exploited for visualization of various biochemical events that take place at molecular and cellular levels. In this study, a set of dual-frequency calcium-responsive MRI agents are reported. These are paramagnetic, fluorine-containing complexes that produce remarkably high MRI signal changes at the (1)H and (19)F frequencies at varying Ca(2+) concentrations. The nature of the processes triggered by Ca(2+) was revealed, allowing a better understanding of these complex systems and their further improvement. The findings indicate that these double-frequency tracers hold great promise for development of novel functional MRI methods.

Injectable hyaluronic acid hydrogel for 19F magnetic resonance imaging

We report on a 19F labeled injectable hyaluronic acid (HA) hydrogel that can be monitored by both 1H and 19F MR imaging. The HA based hydrogel formed via carbazone reaction can be obtained within a minute by simple mixing of HA-carbazate and HA-aldehyde derivatized polymers. 19F contrast agent was linked to with carbazate and thiol dually functionalized HA via orthogonal Michael addition reaction which afforded cross-linkable and 19F labeled HA. The 19F labeling of HA polymer did not affect the mechanical properties of the formed hydrogel. As a result, the shape of a hydrogel sample could be imaged very well by both 1H MRI and high resolution 19F MRI. This hydrogel has high potential in clinical applications since it is injectable, biocompatible, and can be tracked in a minimally invasive manner. The present approach can be applied in preparation of injectable 19F labeled hydrogel biomaterials from other natural biomacromolecules.

Detection of (19)F-labeled biopharmaceuticals in cell cultures with magnetic resonance

Magnetic resonance (MR) studies of the therapeutic efficacy of fluorinated drugs have recently become possible due to improvements in detection including the application of very strong magnetic fields up to 9.4Tesla (T). These advances allow tracking, identification, and quantification of (19)F-labeled biopharmaceuticals using (19)F MR imaging ((19)F MRI) and spectroscopy ((19)F MRS). Both techniques are noninvasive, are nondestructive, and enable serial measurements. They also allow for controlled and systematic studies of cellular metabolism in cancerous tissue in vivo (small animals and humans) and in vitro (body fluids, cells culture, tissue extracts and isolated tissues). Here we provide an overview of the (19)F MRI and (19)F MRS techniques used for tracking (19)F labeled anticancer chemotherapeutics and antibodies which allow quantification of drug uptake in cancer cells in vitro.

Paramagnetic 19F chemical shift probes that respond selectively to calcium or citrate levels and signal ester hydrolysis

Paramagnetic magnetic resonance chemical shift probes containing a proximal CF(3) group have been characterised. Different systems have been created that report reversible changes in calcium ion concentrations in the millimolar regime, signal the presence of citrate selectively in competitive aqueous media and allow the monitoring of remote ester/amide hydrolysis in relayed, irreversible transformations. Chemical shift non-equivalence is amplified by the presence of the proximate lanthanide ion, with a mean separation between the CF(3) group and the metal ion of 6.4 Å found for a thulium complex, in an X-ray structure of the metal complex aqua adduct. The enhanced rate of longitudinal relaxation of the (19)F nucleus allows faster data acquisition.

Dual 19F/1H MR gene reporter molecules for in vivo detection of β-galactosidase

Increased emphasis on personalized medicine and novel therapies requires the development of noninvasive strategies for assessing biochemistry in vivo. The detection of enzyme activity and gene expression in vivo is potentially important for the characterization of diseases and gene therapy. Magnetic resonance imaging (MRI) is a particularly promising tool, since it is noninvasive and has no associated radioactivity, yet penetrates deep tissue. We now demonstrate a novel class of dual (1)H/(19)F nuclear magnetic resonance (NMR) lacZ gene reporter molecule to specifically reveal enzyme activity in human tumor xenografts growing in mice. We report the design, synthesis, and characterization of six novel molecules and evaluation of the most effective reporter in mice in vivo. Substrates show a single (19)F NMR signal and exposure to β-galactosidase induces a large (19)F NMR chemical shift response. In the presence of ferric ions, the liberated aglycone generates intense proton MRI T(2) contrast. The dual modality approach allows both the detection of substrate and the imaging of product enhancing the confidence in enzyme detection.

19F-lanthanide complexes with increased sensitivity for 19F-MRI: optimization of the MR acquisition

Fluorine-19 magnetic resonance methods offer advantages for molecular or cellular imaging in vivo due to the absence of radioactivity, lack of naturally occurring background signal, and the ability to easily combine measurements with anatomical MRI. Previous studies have shown that (19) F-MRI sensitivity is limited to millimolar concentrations by slow longitudinal relaxation. In this study, a new class of macrocyclic fluorinated lanthanide complexes is investigated where relaxation rates are significantly shortened by proximity of the fluorine group to a paramagnetic lanthanide ion located within the same molecule. Longitudinal and transverse relaxation rates are field dependent and in the range 50-150 s(-1) and 70-200 s(-1), respectively, at 7 T. Relaxation rates in these complexes are a function of the molecular structure and are independent of concentration at biologically relevant levels, so can be used as criteria to optimize imaging acquisition. Phantom experiments at 7 T indicate a lower limit for detection by imaging of 20 μM.

Reversible signal regulation system of 19F NMR by redox reactions using a metal complex as a switching module

We present here the reversible signal regulation of (19)F NMR using the transition metal complex as a switching module. Water-soluble fluorinated dendrimers containing ferrocene were synthesized as a probe, and the signal intensities of (19)F NMR of the probes were monitored by changing the valence state of the ferrocene moiety. By oxidation of the ferrocene moiety, the relaxation of the nearby fluorine atoms was accelerated via the paramagnetic relaxation enhancement, as a result the (19)F NMR signal from the probe was significantly decreased. The reduction of the ferrocenium cation of the probe recovered the signal intensity. Finally, in combination with the difference of the fluorescence quenching ability between ferrocene and ferrocenium cation, we constructed the multimodal (19)F NMR/fluorescence probe based on the redox switching system.

Gadolinium-modulated 19F signals from perfluorocarbon nanoparticles as a new strategy for molecular imaging

Recent advances in the design of fluorinated nanoparticles for molecular magnetic resonance imaging (MRI) have enabled specific detection of (19)F nuclei, providing unique and quantifiable spectral signatures. However, a pressing need for signal enhancement exists because the total (19)F in imaging voxels is often limited. By directly incorporating a relaxation agent, gadolinium (Gd), into the lipid monolayer that surrounds the perfluorocarbon (PFC), a marked augmentation of the (19)F signal from 200-nm nanoparticles was achieved. This design increases the magnetic relaxation rate of the (19)F nuclei fourfold at 1.5 T and effects a 125% increase in signal--an effect that is maintained when they are targeted to human plasma clots. By varying the surface concentration of Gd, the relaxation effect can be quantitatively modulated to tailor particle properties. This novel strategy dramatically improves the sensitivity and range of (19)F MRI/MRS and forms the basis for designing contrast agents capable of sensing their surface chemistry.