Ion-Responsive 19F MRI Contrast Agents for the Detection of Cancer Cells

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.

Fluorinated Ionic Liquid Based Multicolor 19 F MRI Nanoprobes for In Vivo Sensing of Multiple Biological Targets

Multicolor imaging, which maps the distribution of different targets, is important for in vivo molecular imaging and clinical diagnosis. Fluorine 19 magnetic resonance imaging (¹⁹ F MRI) is a promising technique because of unique insights without endogenous background or tissue penetration limit. Thus multicolor ¹⁹ F MRI probes, which can sense a wide variety of molecular species, are expected to help elucidate the biomolecular networks in complex biological systems. Here, a versatile model of activatable probes based on fluorinated ionic liquids (ILs) for multicolor ¹⁹ F MRI is reported. Three types of ILs at different chemical shifts are loaded in nanocarriers and sealed by three stimuli-sensitive copolymers, leading to "off" ¹⁹ F signals. The coating polymers specifically respond to their environmental stimuli, then degrade to release the loaded ILs, causing ¹⁹ F signals recovery. The nanoprobes are utilized for non-invasive detection of tumor hallmarks, which are distinguished by their individual colors in one living mouse, without interference between each other. This multicolor imaging strategy, which adopts modular construction of various ILs and stimuli-responsive polymers, will allow more comprehensive sensing of multiple biological targets, thus, opening a new realm in mechanistic understanding of complex pathophysiologic processes in vivo.

Biological Utility of Fluorinated Compounds: from Materials Design to Molecular Imaging, Therapeutics and Environmental Remediation

The applications of fluorinated molecules in bioengineering and nanotechnology are expanding rapidly with the controlled introduction of fluorine being broadly studied due to the unique properties of C-F bonds. This review will focus on the design and utility of C-F containing materials in imaging, therapeutics, and environmental applications with a central theme being the importance of controlling fluorine-fluorine interactions and understanding how such interactions impact biological behavior. Low natural abundance of fluorine is shown to provide sensitivity and background advantages for imaging and detection of a variety of diseases with ¹⁹F magnetic resonance imaging, ¹⁸F positron emission tomography and ultrasound discussed as illustrative examples. The presence of C-F bonds can also be used to tailor membrane permeability and pharmacokinetic properties of drugs and delivery agents for enhanced cell uptake and therapeutics. A key message of this review is that while the promise of C-F containing materials is significant, a subset of highly fluorinated compounds such as per- and polyfluoroalkyl substances (PFAS), have been identified as posing a potential risk to human health. The unique properties of the C-F bond and the significant potential for fluorine-fluorine interactions in PFAS structures necessitate the development of new strategies for facile and efficient environmental removal and remediation. Recent progress in the development of fluorine-containing compounds as molecular imaging and therapeutic agents will be reviewed and their design features contrasted with environmental and health risks for PFAS systems. Finally, present challenges and future directions in the exploitation of the biological aspects of fluorinated systems will be described.

Machine-Learning-Guided Discovery of 19F MRI Agents Enabled by Automated Copolymer Synthesis

Modern polymer science suffers from the curse of multidimensionality. The large chemical space imposed by including combinations of monomers into a statistical copolymer overwhelms polymer synthesis and characterization technology and limits the ability to systematically study structure-property relationships. To tackle this challenge in the context of 19F magnetic resonance imaging (MRI) agents, we pursued a computer-guided materials discovery approach that combines synergistic innovations in automated flow synthesis and machine learning (ML) method development. A software-controlled, continuous polymer synthesis platform was developed to enable iterative experimental-computational cycles that resulted in the synthesis of 397 unique copolymer compositions within a six-variable compositional space. The nonintuitive design criteria identified by ML, which were accomplished by exploring <0.9% of the overall compositional space, lead to the identification of >10 copolymer compositions that outperformed state-of-the-art materials.

Fluorine-Containing Block and Gradient Copoly(2-oxazoline)s Based on 2-(3,3,3-Trifluoropropyl)-2-oxazoline: A Quest for the Optimal Self-Assembled Structure for 19F Imaging

The use of fluorinated contrast agents in magnetic resonance imaging (MRI) facilitates improved image quality due to the negligible amount of endogenous fluorine atoms in the body. In this work, we present a comprehensive study of the influence of the amphiphilic polymer structure and composition on its applicability as contrast agents in ¹⁹F MRI. Three series of novel fluorine-containing poly(2-oxazoline) copolymers and terpolymers, hydrophilic-fluorophilic, hydrophilic-lipophilic-fluorophilic, and hydrophilic-thermoresponsive-fluorophilic, with block and gradient distributions of the fluorinated units, were synthesized. It was discovered that the CF₃ in the 2-(3,3,3-trifluoropropyl)-2-oxazoline (CF₃EtOx) group activated the cationic chain end, leading to faster copolymerization kinetics, whereby spontaneous monomer gradients were formed with accelerated incorporation of 2-methyl-2-oxazoline or 2-n-propyl-2-oxazoline with a gradual change to the less-nucleophilic CF₃EtOx monomer. The obtained amphiphilic copolymers and terpolymers form spherical or wormlike micelles in water, which was confirmed using transmission electron microscopy (TEM), while small-angle X-ray scattering (SAXS) revealed the core-shell or core-double-shell morphologies of these nanoparticles. The core and shell sizes obey the scaling laws for starlike micelles predicted by the scaling theory. Biocompatibility studies confirm that all copolymers obtained are noncytotoxic and, at the same time, exhibit high sensitivity during in vitro ¹⁹F MRI studies. The gradient copolymers provide the best ¹⁹F MRI signal-to-noise ratio in comparison with the analogue block copolymer structures, making them most promising as ¹⁹F MRI contrast agents.

19F Magnetic Resonance Imaging and Spectroscopy in Neuroscience

¹H magnetic resonance imaging (MRI) has established itself as a key diagnostic technique, affording the visualization of brain anatomy, blood flow, activity and connectivity. The detection of other atoms (e.g. ¹⁹F, ²³Na, ³¹P), so called hetero-nuclear MRI and spectroscopy (MRS), provides investigative avenues that complement and extend the richness of information that can be gained from ¹H MRI. Especially ¹⁹F MRI is increasingly emerging as a multi-nuclear (¹H/¹⁹F) technique that can be exploited to visualize cell migration and trafficking. The lack of a ¹⁹F background signal in the brain affords an unequivocal detection suitable for quantification. Fluorine-based contrast material can be engineered as nanoemulsions, nanocapsules, or nanoparticles to label cells in vitro or in vivo. Fluorinated blood substitutes, typically nanoemulsions, can also carry oxygen and serve as a theranostic in poorly perfused brain regions. Brain tissue concentrations of fluorinated pharmaceuticals, including inhalation anesthetics (e.g. isoflurane) and anti-depressants (e.g. fluoxetine), can also be measured using MRS. However, the low signal from these compounds provides a challenge for imaging. Further methodological advances that accelerate signal acquisition (e.g. compressed sensing, cryogenic coils) are required to expand the applications of ¹⁹F MR imaging to, for instance, determine the regional pharmacokinetics of novel fluorine-based drugs. Improvements in ¹⁹F signal detection and localization, combined with the development of novel sensitive probes, will increase the utility of these multi-nuclear studies. These advances will provide new insights into cellular and molecular processes involved in neurodegenerative disease, as well as the mode of action of pharmaceutical compounds.

Tuning the thermoresponsive properties of PEG-based fluorinated polymers and stimuli responsive drug release for switchable 19F magnetic resonance imaging

Switching on of the 19F MRI signal via stimuli-responsive release of hydrophobic drug from PEG-based partly-fluorinated polymers due to change in thermoresponsive properties.

Fluoropolymers in biomedical applications: state-of-the-art and future perspectives

Biomedical applications of fluoropolymers in gene delivery, protein delivery, drug delivery, ¹⁹F MRI, PDT, anti-fouling, anti-bacterial, cell culture, and tissue engineering.

Responsive polymers for medical diagnostics

Stimulus-responsive polymers have been used in improving the efficacy of medical diagnostics through different approaches including enhancing the contrast in imaging techniques and promoting the molecular recognition in diagnostic assays. This review overviews the mechanisms of stimulus-responsive polymers in response to external stimuli including temperature, pH, ion, light, etc. The applications of responsive polymers in magnetic resonance imaging, capture and purification of biomolecules through protein-ligand recognition and lab-on-a-chip technology are discussed.

Tuning of the Aggregation Behavior of Fluorinated Polymeric Nanoparticles for Improved Therapeutic Efficacy

Incorporation of fluorinated moieties in polymeric nanoparticles has been shown in many instances to increase their uptake by living cells and, hence, has proven to be a useful approach to enhancing delivery to cells. However, it remains unclear how incorporation of fluorine affects critical transport processes, such as interactions with membranes, intracellular transport, and tumor penetration. In this study, we investigate the influence of fluorine on transport properties using a series of rationally designed poly(oligo(ethylene glycol) methyl ether acrylate)-block-perfluoropolyether (poly(OEGA)_m-PFPE) copolymers. Copolymers with different fluorine contents were prepared and exhibit aggregate in solution in a manner dependent on the fluorine content. Doxorubicin-conjugated poly(OEGA)₂₀-PFPE nanoparticles with lower fluorine content exist in solution as unimers, leading to greater exposure of hydrophobic PFPE segments to the cell surface. This, in turn, results in greater cellular uptake, deeper tumor penetration, as well as enhanced therapeutic efficacy compared to that with the micelle-state nanoaggregates (poly(OEGA)₁₀-PFPE and poly(OEGA)₅-PFPE) with higher fluorine content but with less PFPE exposed to the cell membranes. Our results demonstrate that the aggregation behavior of these fluorinated polymers plays a critical role in internalization and transport in living cells and 3D spheroids, providing important design criteria for the preparation of highly effective delivery agents.

Fluorinated ZnFeIII Hollow Metal-Organic Framework as a 19F NMR Probe for Highly Sensitive and Selective Detection of Hydrogen Sulfide

Hydrogen sulfide (H2S) is considered as a highly toxic environmental pollutant and an important signal transmitter in physiological processes, and the selective and reliable detection of H2S is of great concern and remains challenging. Herein, we report a smart sensitive "off-on" 19F NMR sensor for H2S by partially introducing a fluorinated ligand to construct a hollow dual metal-organic framework (MOF) nanosystem, F-ZnFeIII hMOF, in which the fluorinated ligand acts as the 19F signal source but is initially quenched due to the strong paramagnetic relaxation enhancement (PRE) effect from neighboring Fe3+ nodes. Upon exposure to sulfide ions, reduction of Fe3+ to Fe2+ is specifically triggered, which attenuates PRE efficiency, thus turning on the 19F NMR signal. The unique hollow MOF architecture benefits the mobility of 19F atoms, thereby improving the response sensitivity. Meanwhile, the desirable H2S-sorption feature and appropriate redox potential of Fe3+/Fe2+ account for the favorable selectivity. The increase in the 19F signal is linear with the concentration of sulfide in the range of 20 to 150 μM with a detection limit of 2.8 μM. The probe is well demonstrated by analyzing H2S in complex matrixes such as biological and foodstuff samples.

General criteria for evaluating suitable polymer ligands for the synthesis of aluminum nanocrystals

This work proposes general criteria for evaluating suitable polymer ligands for shape- and size-controlled synthesis of Al nanocrystals.

Supramolecular Fluorine Magnetic Resonance Spectroscopy Probe Polymer Based on Passerini Bifunctional Monomer

A water-soluble fluorine magnetic resonance spectroscopy host-guest probe, P(HPA-co-AdamCF₃A), was successfully constructed from the facile synthesis of a bifunctional monomer via a quantitative Passerini reaction. Supramolecular complexation with (2-hydroxypropyl)-β-cyclodextrin promoted a change in the chemical environment, leading to modulation of both the relaxation properties as well as chemical shift of the fluorine moieties. This change was used to probe the supramolecular interaction by ¹⁹F MRI spectroscopy and give insight into fluorine probe formulation. This work provides a fundamental basis for an ¹⁹F MR imaging tracer capable of assessing host-guest inclusion and a potential model to follow the fate of a drug delivery system in vivo.

19F MRI of Polymer Nanogels Aided by Improved Segmental Mobility of Embedded Fluorine Moieties

Using fluorinated probes for 19F MRI imaging is an emerging field with potential utility in cellular imaging and cell tracking in vivo, which complements conventional 1H MRI. An attractive feature of 19F-based imaging is that this is a bio-orthogonal nucleus and the naturally abundant isotope is NMR active. A significant hurdle however in the 19F MRI arises from the tendency of organic macromolecules, with multiple fluorocarbon substitutions, to aggregate in the aqueous phase. This aggregation results in significant loss of sensitivity, because the T2 relaxation times of these aggregated 19F species tend to be significantly lower. In this report, we have developed a strategy to covalently trap nanoscopic states with an optimal degree of 19F substitutions, followed by significant enhancement in T2 relaxation times through increased segmental mobility of the side chain substituents facilitated by the stimulus-responsive elements in the polymeric nanogel. In addition to NMR relaxation time based evaluations, the ability to obtain such signals are also evaluated in mouse models. The propensity of these nanoscale assemblies to encapsulate hydrophobic drug molecules and the availability of surfaces for convenient introduction of fluorescent labels suggest the potential of these nanoscale architectures for use in multimodal imaging and therapeutic applications.

Design strategies for chemical-stimuli-responsive programmable nanotherapeutics

Chemical-stimuli-responsive nanotherapeutics have gained great interest in drug delivery and diagnosis applications. These nanotherapeutics are designed to respond to specific internal stimuli including pH, ionic strength, redox, reactive oxygen species, glucose, enzymes, ATP and hypoxia for site-specific and responsive or triggered release of payloads and/or biomarker detections. This review systematically and comprehensively addresses up-to-date technological and design strategies, and challenges nanomaterials to be used for triggered release and sensing in response to chemical stimuli.

High F-Content Perfluoropolyether-Based Nanoparticles for Targeted Detection of Breast Cancer by 19F Magnetic Resonance and Optical Imaging

Two important challenges in the field of ¹⁹F magnetic resonance imaging (MRI) are the maintenance of high fluorine content without compromising imaging performance, and effective targeting of small particles to diseased tissue. To address these challenges, we have developed a series of perfluoropolyether (PFPE)-based hyperbranched (HBPFPE) nanoparticles with attached peptide aptamer as targeting ligands for specific in vivo detection of breast cancer with high ¹⁹F MRI sensitivity. A detailed comparison of the HBPFPE nanoparticles (NPs) with the previously reported trifluoroethyl acrylate (TFEA)-based polymers demonstrates that the mobility of fluorinated segments of the HBPFPE nanoparticles is significantly enhanced (¹⁹F T₂ > 80 ms vs 31 ms), resulting in superior MR imaging sensitivity. Selective targeting was confirmed by auto- and pair correlation analysis of fluorescence microscopy data, in vitro immunofluorescence, in vivo ¹⁹F MRI, ex vivo fluorescence and ¹⁹F NMR. The results highlight the high efficiency of aptamers for targeting and the excellent sensitivity of the PFPE moieties for ¹⁹F MRI. Of relevance to in vivo applications, the PFPE-based polymers exhibit much faster clearance from the body than the previously introduced perfluorocarbon emulsions ( t_1/2 ∼ 20 h vs up to months). Moreover, the aptamer-conjugated NPs show significantly higher tumor-penetration, demonstrating the potential of these imaging agents for therapeutic applications. This report of the synthesis of polymeric aptamer-conjugated PFPE-based ¹⁹F MRI CAs with high fluorine content (∼10 wt %) demonstrates that these NPs are exciting candidates for detecting diseases with high imaging sensitivity.

Elucidating the Impact of Molecular Structure on the 19F NMR Dynamics and MRI Performance of Fluorinated Oligomers

To understand molecular factors that impact the performance of polymeric ¹⁹F magnetic resonance imaging (MRI) agents, a series of discrete fluorinated oligoacrylates with precisely defined structure were prepared through the combination of controlled polymerization and chromatographic separation techniques. These discrete oligomers enabled thorough elucidation of the dependence of ¹⁹F NMR and MRI properties on molecular structure, for example, the chain length. Importantly, the oligomer size and dispersity strongly influence NMR dynamics (T₁ and T₂ relaxation times) and MR imaging properties with higher signal-to-noise ratio (SNR) observed for oligomers with longer chain length and shorter T₁. Our approach enables an effective pathway and thus opportunities to rationally design effective polymeric ¹⁹F MR imaging agents with optimized molecular structure and NMR relaxivity.

Self-Assembled Thermoresponsive Polymeric Nanogels for 19F MR Imaging

Magnetic resonance imaging using fluorinated contrast agents (¹⁹F MRI) enables to achive highcontrast in images due to the negligible fluorine background in living tissues. In this pilot study, we developed new biocompatible, temperature-responsive, and easily synthesized polymeric nanogels containing a sufficient concentration of magnetically equivalent fluorine atoms for ¹⁹F MRI purposes. The structure of the nanogels is based on amphiphilic copolymers containing two blocks, a hydrophilic poly[ N-(2-hydroxypropyl)methacrylamide] (PHPMA) or poly(2-methyl-2-oxazoline) (PMeOx) block, and a thermoresponsive poly[ N(2,2difluoroethyl)acrylamide] (PDFEA) block. The thermoresponsive properties of the PDFEA block allow us to control the process of nanogel self-assembly upon its heating in an aqueous solution. Particle size depends on the copolymer composition, and the most promising copolymers with longer thermoresponsive blocks form nanogels of suitable size for angiogenesis imaging or the labeling of cells (approximately 120 nm). The in vitro ¹⁹F MRI experiments reveal good sensitivity of the copolymer contrast agents, while the nanogels were proven to be noncytotoxic for several cell lines.

Low-cost LED-based Photo-CIDNP Enables Biocompatible Hyperpolarization of 19 F for NMR and MRI at 7 T and 4.7 T

Substrates containing ¹⁹ F can serve as background-free reporter molecules for NMR and MRI. However, in vivo applications are still limited due to the lower signal-to-noise ratio (SNR) when compared with ¹ H NMR. Although hyperpolarization can increase the SNR, to date, only photo-chemically induced dynamic nuclear polarization (photo-CIDNP) allows for hyperpolarization without harmful metal catalysts. Photo-CIDNP was shown to significantly enhance ¹⁹ F NMR signals of 3-fluoro-DL-tyrosine in aqueous solution using flavins as photosensitizers. However, lasers were used for photoexcitation, which is expensive and requires appropriate protection procedures in a medical or lab environment. Herein, we report ¹⁹ F MR hyperpolarization at 4.7 T and 7 T with a biocompatible system using a low-cost and easy-to-handle LED-based set-up. First hyperpolarized ¹⁹ F MR images could be acquired, because photo-CIDNP enabled repetitive hyperpolarization without adding new substrates.

A unique 19F MRI agent for the tracking of non phagocytic cells in vivo

There is currently intense interest in new methods for understanding the fate of therapeutically-relevant cells, such as mesenchymal stem cells (MSCs). The absence of a confounding background signal and consequent unequivocal assignment makes 19F MRI one of the most attractive modalities for the tracking of injected cells in vivo. We describe here the synthesis of novel partly-fluorinated polymeric nanoparticles with small size and high fluorine content as MRI agents. The polymers, constructed from perfluoropolyether methacrylate (PFPEMA) and oligo(ethylene glycol) methacrylate (OEGMA) have favourable cell uptake profiles and excellent MRI performance. To facilitate cell studies the polymer was further conjugated with a fluorescent dye creating a dual-modal imaging agent. The efficacy of labelling of MSCs was assessed using 19F NMR, flow cytometry and confocal microscopy. The labelling efficiency of 2.6 ± 0.1 × 1012 19F atoms per cell, and viability of >90% demonstrates high uptake and good tolerance by the cells. This loading translates to a minimum 19F MRI detection sensitivity of ∼7.4 × 103 cells per voxel. Importantly, preliminary in vivo data demonstrate that labelled cells can be readily detected within a short acquisition scan period (12 minutes). Hence, these copolymers show outstanding potential for 19F MRI cellular tracking and for quantification of non-phagocytic and therapeutically-relevant cells in vivo.

Overcoming Surfactant-Induced Morphology Instability of Noncrosslinked Diblock Copolymer Nano-Objects Obtained by RAFT Emulsion Polymerization

RAFT emulsion polymerization techniques including polymerization-induced self-assembly (PISA) and temperature-induced morphological transformation (TIMT) are widely used to produce noncrosslinked nano-objects with various morphologies. However, the worm, vesicle and lamellar morphologies produced by these techniques typically cannot tolerate the presence of added surfactants, thus limiting their potential applications. Herein we report the surfactant tolerance of noncrosslinked worms, vesicles, and lamellae prepared by RAFT emulsion polymerizations using poly(di(ethylene glycol) ethyl ether methacrylate-co-N-(2-hydroxypropyl) methacrylamide) (P(DEGMA-co-HPMA)) as a macromolecular chain transfer agent (macro-CTA). Significantly, these P(DEGMA-co-HPMA) nanoparticles are highly stable in concentrated solutions of surfactants (e.g., sodium dodecyl sulfate (SDS)). We also demonstrate that the surfactant tolerance is related to the limited binding of SDS to the main-chain of the P(DEGMA-co-HPMA) macro-CTA constituting the particle shell. This work provides new insight into the interactions between surfactants and thermoresponsive copolymers and expands the scope of RAFT emulsion polymerization techniques for the preparation of noncrosslinked and surfactant-tolerant nanomaterials.

19F CEST imaging probes for metal ion detection

For detecting metal ions with ¹⁹F chemical exchange saturation transfer magnetic resonance imaging (¹⁹F CEST MRI), a class of novel fluorinated chelators with diverse fluorine contents and chelation properties were conveniently synthesized on gram scales. Among them, a DTPA-derived chelator with high sensitivity and selectivity was identified as a novel ¹⁹F CEST imaging probe for simultaneously detecting multiple metal ions.

3D shape change of multi-responsive hydrogels based on a light-programmed gradient in volume phase transition

We present a new type of anisotropic oligo(ethylene glycol) methacrylate hydrogel with multi-responsive and programmable 3D deformation behaviour.

The role of hydrophobic hydration in the LCST behaviour of POEGMA300 by all-atom molecular dynamics simulations

The role of hydrophobic hydration in the LCST phase transition of POEGMA₃₀₀ by means of the breakage of cage-like water formations around the side chains.

A pH-responsive nanoprobe for turn-on 19F-magnetic resonance imaging

A pH-responsive MRI nanoprobe was developed by partially replacing organic linkers in ZIF-8, which displays pH-responsive in vivo¹⁹F MRI ability.

Activatable probes for diagnosis and biomarker detection by MRI

Magnetic resonance imaging (MRI) is a non-invasive imaging technique with widespread use in diagnosis. Frequently, contrast in MRI is enhanced with the aid of a contrast agent, among which smart, responsive, OFF/ON or activatable probes are of particular interest. These kinds of probes elicit a response to selective stimuli, evidencing the presence of enzymes or acidic pH, for instance. In this review, we will focus on smart probes that are detectable by both ¹H and ¹⁹F MRI, frequently based on nanomaterials. We will discuss the triggering factors and the strategies employed thus far to activate each probe.