Synthesis, Characterization, and Application of a Highly Hydrophilic Triarylmethyl Radical for Biomedical EPR

Line-Shifting Triarylmethyl Radicals for Imaging of Enzyme Activity Using Overhauser-Enhanced Magnetic Resonance Imaging: Application to Alkaline Phosphatase

Enzyme catalytic activities are critical biomarkers of tissue states under physiological and pathophysiological conditions. However, the direct measurement and imaging of enzyme activity in vivo remains extremely challenging. We report the synthesis and characterization of the first stable triarylmethyl (TAM) radical substrate of alkaline phosphatase (TAM-ALPs). The enzymatic dephosphorylation of TAM-ALPs results in a drastic change in its electron paramagnetic resonance (EPR) spectrum that can be used to image enzyme activity using EPR-based technologies. TAM-ALPs and their enzyme products were fully characterized using EPR and HPLC-MS techniques. A proof of concept of imaging enzyme activity using Overhauser-enhanced magnetic resonance imaging was demonstrated in vitro. This study clearly demonstrates the potential of EPR-based imaging technologies associated with TAM spin probes to map enzyme activity in vivo in future studies.

Nitroxide radical contrast agents for safe magnetic resonance imaging: progress, challenges, and perspectives

Magnetic resonance imaging (MRI) is considered one of the most valuable diagnostic technologies in the 21st century. To enhance the image contrast of anatomical features, MRI contrast agents have been widely used in clinical MRI diagnosis, especially those based on gadolinium, manganese, and iron oxide. However, these metal-based MRI contrast agents show potential toxicity to patients, which urges researchers to develop novel MRI contrast agents that can replace metal-based MRI contrast agents. Metal-free nitroxide radical contrast agents (NRCAs) effectively overcome the shortcomings of metal-based contrast agents and also have many advantages, including good biocompatibility, prolonged systemic circulation time, and easily functionalized structures. Importantly, since NRCAs acquire MRI signals with standard tissue water 1H relaxation mechanisms, they have great potential to realize clinical translation among many metal-free MRI contrast agents. At present, NRCAs have been proposed as an effective substitute for metal-based MRI contrast agents. Herein, this review first briefly introduces NRCAs, including their composition, classification, mechanism of action, application performances and advantages. Then, this review highlights the progress of NRCAs, including small molecule-based NRCAs and polymer-based NRCAs. Finally, this review also discusses the challenges and future perspectives of NRCAs.

In Situ Generation and High Bioresistance of Trityl-based Semiquinone Methide Radicals Under Anaerobic Conditions in Cellular Systems

Bioreduction of spin labels and polarizing agents (generally stable radicals) has been an obstacle limiting the in-cell applications of pulsed electron paramagnetic resonance (EPR) spectroscopy and dynamic nuclear polarization (DNP). In this work, we have demonstrated that two semiquinone methide radicals (OXQM⋅ and CTQM⋅) can be easily produced from the trityl-based quinone methides (OXQM and CTQM) via reduction by various reducing agents including biothiols and ascorbate under anaerobic conditions. Both radicals have relatively low pKa's and exhibit EPR single line signals at physiological pH. Moreover, the bioreduction of OXQM in three cell lysates enables quantitative generation of OXQM⋅ which was most likely mediated by flavoenzymes. Importantly, the resulting OXQM⋅ exhibited extremely high stability in the E.coli lysate under anaerobic conditions with 76- and 14.3-fold slower decay kinetics as compared to the trityl OX063 and a gem-diethyl pyrrolidine nitroxide, respectively. Intracellular delivery of OXQM into HeLa cells was also achieved by covalent conjugation with a cell-permeable peptide as evidenced by the stable intracellular EPR signal from the OXQM⋅ moiety. Owing to extremely high resistance of OXQM⋅ towards bioreduction, OXQM and its derivatives show great application potential in in-cell EPR and in-cell DNP studies for various cells which can endure short-term anoxic treatments.

SOX71, A Biocompatible Succinyl Derivative of the Triarylmethyl Radical OX071 for In Vivo Quantitative Oxygen Mapping Using Electron Paramagnetic Resonance

PURPOSE

This study aimed to develop a biocompatible oximetric electron paramagnetic resonance (EPR) spin probe with reduced self-relaxation, and sensitivity to oxygen for a higher signal-to-noise ratio and longer relaxation times at high oxygen concentration, compared to the reference spin probe OX071.

PROCEDURES

SOX71 was synthesized by succinylation of the twelve alcohol groups of OX071 spin probe and characterized by EPR at X-Band (9.5 GHz) and at low field (720 MHz). The biocompatibility of SOX71 was tested in vitro and in vivo in mice. A pharmacokinetic study was performed to determine the best time frame for EPR imaging. Finally, a proof-of-concept EPR oxygen imaging was performed on a mouse model of a fibrosarcoma tumor.

RESULTS

SOX71 was synthesized in one step from OX071. SOX71 exhibits a narrow line EPR spectrum with a peak-to-peak linewidth of 66 mG, similar to OX071. SOX71 does not bind to albumin nor show cell toxicity for the concentrations tested up to 5 mM. No toxicity was observed after systemic delivery via intraperitoneal injection in mice at twice the dose required for EPR imaging. After the injection, the probe is readily absorbed into the bloodstream, with a peak blood concentration half an hour, post-injection. Then, the probe is quickly cleared by the kidney with a half-life of ~ 45 min. SOX71 shows long relaxation times under anoxic condition (T1e = 9.5 µs and T2e = 5.1 µs; [SOX71] = 1 mM in PBS at 37 °C, pO2 = 0 mmHg, 720 MHz). Both the relaxation rates R1e and R2e show a decreased sensitivity to pO2, leading to twice longer relaxation times under room air conditions (pO2 = 159 mmHg) compared to OX071. This is ideal for oxygen imaging in samples with a wide range of pO2. Both the relaxation rates R1e and R2e show a decreased sensitivity to self-relaxation compared to OX071, with a negligible effect of the probe concentration on R1e. SOX71 was successfully applied to image oxygen in a tumor.

CONCLUSION

SOX71, a succinylated derivative of OX071 was synthesized, characterized, and applied for in vivo EPR tumor oxygen imaging. SOX71 is highly biocompatible, and shows decreased sensitivity to oxygen and self-relaxation. This first report suggests that SOX71 is superior to OX071 for absolute oxygen mapping under a broad range of pO2 values.

Biocompatible Monophosphonated Trityl Spin Probe, HOPE71, for In Vivo Measurement of pO2, pH, and [Pi] by Electron Paramagnetic Resonance Spectroscopy

Hypoxia, acidosis, and elevated inorganic phosphate concentration are characteristics of the tumor microenvironment in solid tumors. There are a number of methods for measuring each parameter individually in vivo, but the only method to date for noninvasive measurement of all three variables simultaneously in vivo is electron paramagnetic spectroscopy paired with a monophosphonated trityl radical, pTAM/HOPE. While HOPE has been successfully used for in vivo studies upon intratissue injection, it cannot be delivered intravenously due to systemic toxicity and albumin binding, which causes significant signal loss. Therefore, we present HOPE71, a monophosphonated trityl radical derived from the very biocompatible trityl probe, Ox071. Here, we describe a straightforward synthesis of HOPE71 starting with Ox071 and report its EPR sensitivities to pO2, pH, and [Pi] with X-band and L-band EPR spectroscopy. We also confirm that HOPE71 lacks albumin binding, shows low cytotoxicity, and has systemic tolerance. Finally, we demonstrate its ability to profile the tumor microenvironment in vivo in a mouse model of breast cancer.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

Rapid Scan EPR Oxygen Imaging in Photoactivated Resin Used for Stereolithographic 3D Printing

Oxygen plays a critical role in the photopolymerization process resulting in the formation of solid structures from liquid resins during three-dimensional (3D) printing: it acts as a polymerization inhibitor. Upon exposure to light, oxygen is depleted. As a result, the polymerization process becomes activated. Electron paramagnetic resonance (EPR) imaging is described as a tool to visualize changes in oxygen distribution caused by light exposure. This nondestructive method uses radio waves and, therefore, is not constrained by optical opacity offering greater penetrating depth. Three proof-of-principle imaging experiments were demonstrated: (1) spatial propagation of the photopolymerization process; (2) oxygen depletion as a result of postcuring; and (3) oxygen visualization in a 3D printed spiral model. Commercial stereolithography (SLA) resin was used in these experiments. Lithium octa-n-butoxynaphthalocyanine (LiNc-BuO) probe was mixed with the resin to permit oxygen imaging. Li-naphthalocyanine probes are routinely used in various EPR applications because of their long-term stability and high functional sensitivity to oxygen. In this study, we demonstrate that EPR imaging has the potential to become a powerful visualization tool in the development of 3D printing technology, including bioprinting and tissue engineering.

Large-scale synthesis of a monophosphonated tetrathiatriarylmethyl spin probe for concurrent in vivo measurement of pO2, pH and inorganic phosphate by EPR

Low-field electron paramagnetic resonance spectroscopy paired with pTAM, a mono-phosphonated triarylmethyl radical, is an unmatched technique for concurrent and non-invasive measurement of oxygen concentration, pH, and inorganic phosphate concentration for in vivo investigations. However, the prior reported synthesis is limited by its low yield and poor scalability, making wide-spread application of pTAM unfeasible. Here, we report a new strategy for the synthesis of pTAM with significantly greater yields demonstrated on a large scale. We also present a standalone application with user-friendly interface for automatic spectrum fitting and extraction of pO₂, pH, and [P_i] values. Finally, we confirm that pTAM remains in the extracellular space and has low cytotoxicity appropriate for local injection.

Synthesis and Redox Properties of Water-Soluble Asymmetric Trityl Radicals

Tetrathiatriarylmethyl (trityl) radicals have been recently shown to react with biological oxidoreductants including glutathione (GSH), ascorbic acid (Asc), and superoxide anion radical (O2•-). However, how the substituents affect the reactivity of trityl radicals is still unknown. In this work, five asymmetric trityl radicals were synthesized and their reactivities with GSH, Asc, and O2•- investigated. Under aerobic conditions, GSH induces fast decays for the thioether- (TSA) and N-methyleneglycine-substituted (TGA) derivatives and slow decay for the 4-carboxyphenyl-containing one (TPA). Under anaerobic conditions, the direct reduction of these radicals by GSH also occurs with rate constants (kGSH) from 1.8 × 10-4 M-1 s-1 for TPA to 1.0 × 10-2 M-1 s-1 for TGA. Moreover, these radicals can also react with O2•- with rate constants (kSO) from 1.2 × 103 M-1 s-1 for ET-01 to 1.6 × 104 M-1 s-1 for TGA. Surprisingly, these radicals are completely inert to Asc in both aerobic and anaerobic conditions. Additionally, the substituents exert an important effect on redox potentials of these trityl radicals. This work demonstrates that the redox properties of the trityl radicals strongly depend on their substituents, and TPA with high stability toward GSH shows great potential for intracellular applications.