Synthesis, Characterization, and Nanoencapsulation of Tetrathiatriarylmethyl and Tetrachlorotriarylmethyl (Trityl) Radical Derivatives—A Study To Advance Their Applicability as in Vivo EPR Oxygen Sensors

Abstract

Tissue oxygenation plays an important role in the pathophysiology of various diseases and is often a marker of prognosis and therapeutic response. EPR (ESR) is a suitable noninvasive oximetry technique. However, to reliably deploy soluble EPR probes as oxygen sensors in complex biological systems, there is still a need to investigate and improve their specificity, sensitivity, and stability. We reproducibly synthesized various derivatives of tetrathiatriarylmethyl and tetrachlorotriarylmethyl (trityl) radicals. Hydrophilic radicals were investigated in aqueous solution mimicking physiological conditions by, e.g., variation of viscosity and ionic strength. Their specificity was satisfactory, but the oxygen sensitivity was low. To enhance the capability of trityl radicals as oxygen sensors, encapsulation into oily core nanocapsules was performed. Thus, different lipophilic triesters were prepared and characterized in oily solution employing oils typically used in drug formulations, i.e., middle-chain triglycerides and isopropyl myristate. Our screening identified the deuterated ethyl ester of D-TAM (radical 13) to be suitable. It had an extremely narrow single EPR line under anoxic conditions and excellent oxygen sensitivity. After encapsulation, it retained its oxygen responsiveness and was protected against reduction by ascorbic acid. These biocompatible and highly sensitive nanosensors offer great potential for future EPR oximetry applications in preclinical research.

Tris[2,2,6,6-tetramethyl-8-(trimethylsilyl)benzo[1,2-d;4,5-d′]bis(1,3-dithiol)-4-yl]methanol diethyl ether monosolvate

“…Trityl radicals feature a very narrow linewidth in EPR spectra, slow spin-spin relaxation at room temperature and show line-broadening depending on the oxygen concentration in their surroundings. The latter property also makes them suitable as oxygen probes (Frank et al, 2015). However, most of the trityl radicals reported in the literature feature carboxylic acid derivatives as substituents in the para-position.…”

EPR Imaging Spin Probe Trityl Radical OX063: A Method for Its Isolation from Animal Effluent, Redox Chemistry of Its Quinone Methide Oxidation Product, and in Vivo Application in a Mouse

“…The potential use of these persistent radicals as electron paramagnetic resonance (EPR) probes in biological applications was recognized early on, and many water-soluble triarylmethyl (TAM) radicals have been synthesized and studied. 2,3 Of these, carboxylated TAMs have emerged as powerful tools for cancer biology and medicine, as they show good water-solubility, narrow line widths, and long relaxation times. 4 These radicals were initially designed as polarizing agents for dynamic nuclear polarization (DNP) and contrast agents in Overhauser-enhanced magnetic resonance imaging (OMRI).…”

Synthesis and EPR studies of the first water-soluble N@C60 derivative

“…The vast majority of spin probes currently used are either nitroxide-or trityl-based radicals, with water-soluble derivatives of these used for an array of biological applications such as oximetry measurements [8][9][10][11] and distance measurements in biomolecules. 12,13 In order to investigate if N@C60 could be used in such a capacity it is necessary for the inherently hydrophobic fullerene cage to be functionalised with hydrophilic groups to render it compatible with aqueous environments.…”