Dextran-conjugated tetrathiatriarylmethyl radicals as biocompatible spin probes for EPR spectroscopy and imaging

Abstract

Tetrathiatriarylmethyl (TAM) radicals represent soluble paramagnetic probes for biomedical electron paramagnetic resonance (EPR)-based spectroscopy and imaging. There is an increasing demand in the development of multifunctional, biocompatible and targeted trityl probes hampered by the difficulties in derivatization of the TAM structure. We proposed a new straightforward synthetic strategy using click chemistry for the covalent conjugation of the TAM radical with a water-soluble biocompatible carrier exemplified here by dextran. A set of dextran-grafted probes varied in the degrees of Finland trityl radical loading and dextran modification by polyethelene glycol has been synthesized. The EPR spectrum of the optimized macromolecular probe exhibits a single narrow line with high sensitivity to oxygen and has advantages over the unbound Finland trityl of being insensitive to interactions with albumin. In vivo EPR imaging of tissue oxygenation performed in breast tumor-bearing mouse using dextran-grafted probe demonstrates its utility for preclinical oximetric applications. Synthesis of the first organic free radical, triphenylmethyl, was reported by Gomberg in 1900 1. By the late 1990s, these compounds with sterically protected trivalent carbon regained attention as the core structural fragment for the synthesis of stable organic radicals. Nycomed Innovation designed the sterically crowded trityl radicals, TAMs (tetrathiatriarylmethyl), in order to avoid hydrogen hyperfine coupling and enhance radical stability and water solubility 2-3. Currently TAMs represent one of the major classes of soluble paramagnetic probes characterized by extraordinary stability toward tissue redox processes, long relaxation time and narrow line width making them particularly attractive for electron paramagnetic resonance (EPR)-based spectroscopy and imaging applications 3-7. Figure 1 shows the most popular structures of TAM oximetric probes, Finland trityl (FTr) 8-9 and OX063 4 , their deuterated derivatives 5, 10-12 , and the recently synthesized multifunctional monophosphonated probe, HOPE 13-14

Ox‐SLIM: Synthesis of and Site‐Specific Labelling with a Highly Hydrophilic Trityl Spin Label

“…[27] Therefore, efforts have been undertaken to increase the hydrophilicity of trityl radicals. For imaging and sensing purposes,c onjugation of 1C to dendritic PEGesters, [28,29] dextrans, [30] or oligopeptides [31] addressed this issue. However,t he resulting radicals become veryl arge and are therefore not suitable as spin labels.…”

Methanethiosulfonate Derivative of OX063 Trityl: A Promising and Efficient Reagent for Side‐Directed Spin Labeling of Proteins

“…One strategy to circumvent the lipophilicity of the FTAM core is to bury it in larger hydrophilic structures, such as polyesters, dendritic PEG esters, dendrimeric amides, oleogels or emulgels, and dextrans . However, the resulting structure is much too large in size for SDSL applications.…”

Development of multifunctional Overhauser-enhanced magnetic resonance imaging for concurrent in vivo mapping of tumor interstitial oxygenation, acidosis and inorganic phosphate concentration

“…The multifunctional monophosphonated trityl probe, dpTAM, used in this work, is applicable for intratissue delivery only due to its interaction with plasma albumin 41 . Further synthetic work to improve the probe biocompatibility and suitability for systemic delivery is in progress based on two alternative strategies, (i) incorporation of monophosphonate group in core structure of Ox071 trityl probe suitable for systemic delivery 15,25 ; and (ii) conjugation of dpTAM probe with a biocompatible carrier, such as dextran 42 . In summary, the multifunctional OMRI technique significantly broadens the area of pre-clinical EPR-based applications allowing for mapping and correlation of physiologically-relevant tissue parameters in various disease models of cancers and beyond.…”