).Key words Ibuprofen-nitroxide; reduction kinetics; ascorbic acid; methyl radical; Fenton reaction; electron paramagnetic resonance intensityThe representative nitroxide radicals, a six-memberedring nitroxide, tetramethylpiperidine-1-oxyl (TEMPO), and a five-membered-ring nitroxide, tetramethylpyrrolidine-1-oxyl (PROXYL), have been used as in-vivo probes. Recently, various nitroxides in which the tetramethyl groups around the nitroxyl group were replaced by bulkier groups have been synthesized. They had different reduction rates for reductants such as ascorbic acid (AsA), and these were applied as an invivo probe. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] The rate constant of reduction of a nitroxide probe in vitro is expected to be a guideline of its half-life in vivo.9-11) The rate constants of them are greatly different and could not measure under the same conditions. 9,10,17) For the nitroxides resistant to AsA, we planned to examine the measurement of their rate constants using methyl radicals formed by Fenton reaction. The balance between their sensitivity for reductants and half-life is important for use of nitroxide as an in-vivo probe. TEMPO radical is highly sensitive for reductants and a good detection reagent for AsA, 5,11) but, its half-life as an in-vivo probe was too short and unusable. 14) It has been shown that ibuprofen-PROXYL passed through the blood-brain barrier (BBB) in mice and was indicated the anti-inflammatory action in inflamed mice brains. Real-time changes in the redox status of mice brains were successfully visualized by magnetic resonance imaging (MRI) and electron paramagnetic resonance (EPR) imaging. Both shows a functionality as a theranostic compound.18) However, for exploring the redox status of brain, nitroxides with different reduction rate for reductants such as reactive oxygen species (ROS) or vitamin C or E are required. Five nitroxides with different reduction activities and their condensates with ibuprofen were synthesized (Fig. 1). The reduction reactivity of the five ibuprofen-nitroxides were measured by addition of AsA, and for the reduction-resistant nitroxides by the addition of the methyl radicals produced by the Fenton reaction. The reduction rate constants of the five ibuprofen-nitroxides for AsA or methyl radicals were determined and compared with the suitable time-dependent reduction-decay curve. Results and DiscussionSynthesis Six nitroxides (6-11) were synthesized by the synthetic methods shown in Chart S1. Sterically shielded nitroxides 7, 10 and 11 were synthesized according to the method established by Yamada's group 4-7) and Rajca's group. [9][10][11] Since there were some low-yield reaction steps in the synthesis of the sterically shielded nitroxides (7, 10, 11), the reaction conditions were partially improved and synthesis was achieved.Nitroxides with tetramethyl groups 6, 8 and 9 were synthesized by conventional methods. [19][20][21]

The drug-nitroxide radical hybrid-compound 7-N-((2,2,5,5-tetramethylpyrrolidine-1-yloxy(PROXYL))-3-yl-methyl)theophylline (3) was synthesized by coupling 7-N-tosyltheophylline with 3-hydroxymethyl-PROXYL, HMP). The stability of 3 relative to that of HMP was examined in the presence of the anti-oxidant, ascorbic acid (AsA). The initial reduction rate constants of 3 and HMP were 11.9±5.3 and 6.1±5.2 M min, respectively. In the presence of glutathione (GSH), these constants increased slightly to 22.3±6.8 and 9.1±2.4 M min, respectively. Two-dimensional cranial electron paramagnetic resonance imaging of mice intravenously injected with 3 via the tail vein revealed that probe 3 enters the mouse brain by passing through the blood-brain barrier (BBB).