mcres-oxedaravone (1)

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O R I G I N A L R E S E A R C H

The tautomerism influence on the antioxidant predictionof oxederavone

Sheise A. S. Brgida Ednilson Orestes Taina G. Barros

Carlos A. L. Barros Agnaldo S. Carneiro

Alberico B. F. da Silva Rosivaldo S. Borges

Received: 14 November 2012 / Accepted: 18 February 2013

Springer Science+Business Media New York 2013

Abstract A detailed theoretical prediction of oxedaravone

(a hydro-soluble derivative of edaravone) as antioxidant wasperformed. The DFT method with B3LYP/6-31G(d) basis

sets was used for the tautomerism study in gas phase and

solvent medium (water and methanol) and the antioxidant

capacity was predicted from HOMO, ionization energy (IE),

and bond dissociation energies (BDE). All calculations

showed that the ketoenol tautomerism of oxedaravone is

thermodynamically more favored than the imineenamine

tautomerism andthe solvent effect reduced theisomerization

energy barriers for the OH or NH tautomers. HOMO and

IE values showed that the NH tautomer is a better antioxi-

dant, while the BDE values showed that the OH tautomer is

the better antioxidant. Our results showed that the oxeda-

ravone may be a good strategy for designing polar deriva-

tives of edaravone.

Keywords Edaravone derivatives DFT Antioxidant Ionization Tautomerism Scavenging

Introduction

Edaravone 1, a recently developed free-radical scavenger

for clinical use, can quench radicalar reactions by trapping

a variety of free-radical oxygen and nitrogen species

(Tabrizchi, 2000). It was developed as a medical drug forbrain ischemia process (Kawai et al., 1997; Watanabe

et al., 1986) and has been reported to be effective for

myocardial ischemia, as well (Wu et al., 2002) as an effi-

cient antioxidant, which is considered to be the basis of its

protective effect against ischemia and has been reported to

prevent against hydroperoxy polyunsaturated fatty acids,

an oxidation product generated by lipoxygenase-induced

vascular endothelial cell injury and irradiation-induced in

endothelial nitric oxide synthase expression (Yamamoto

et al., 1997).

Its pharmacological effect arises from the scavenging

activity against free-radicals. In fact, it is efficient as

scavenging hydroxyl (HO) as DPPH radicals (DPPH)

(Tabrizchi, 2000). The HO is one of the most reactive

oxygen species in nature; it is not surprising that edaravone

possesses a high HO scavenging activity (Wang and

Zhang, 2003).

The study of its antioxidant mechanism through density

functional theory (DFT) calculations revealed that the

H-atom-abstraction rather than electron-transfer reaction is

involved in the radical-scavenging process of edaravone,

and the H-atom at position 4 is ready to be abstracted

(Wang and Zhang, 2003). The bond dissociation energy of

the methylene moiety (BDECH) of edaravone is higher than

the bond dissociation energy of the hydroxyl moiety

(BDEOH) of a-tocopherol, accounting for the activity dif-

ference between the two antioxidants (Wang and Zhang,

2003). As substituents have little influence on the BDECH,

2-pyrazolin-5-one is recognized as the active center for

edaravone (Wang and Zhang, 2003). However, other

authors have considered that edaravone anion is reported to

be an active form in scavenging free-radicals by a one-

electron-transferring mechanism (Watanabe et al., 1986),

S. A. S. Brgida T. G. Barros C. A. L. Barros A. S. Carneiro R. S. Borges (&)Nucleo de Estudos e Selecao de Biomoleculas da Amazonia,

Instituto de Ciencias da Saude, Universidade Federal do Para,

CP 11101, Belem, PA 66075-110, Brazil

e-mail: [email protected]

E. Orestes A. B. F. da Silva R. S. BorgesInstituto de Qumica de Sao Carlos, Universidade de Sao Paulo,

CP 780, Sao Carlos, SP 13560-970, Brazil

123

Med Chem Res

DOI 10.1007/s00044-013-0553-0

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that is a certain contradiction between the authors, since

some authors have related the antioxidant activity of eda-

ravone with the hydrogen abstraction of the ring pirazolone

(Wang and Zhang, 2003) and other have related the anti-

oxidant activity of edaravone with the electron donation of

the corresponding anionic form, an enolate ion (Watanabe

et al., 1986).

Previous results showed that the electron abstraction isrelated to electron-donating groups (EDG) at position 3,

decreasing the ionization energy (IE) when compared to the

substitution at position 4 (Perez-Gonzalez and Galano,

2012). However, the hydrogen abstraction is related to the

electron-withdrawing groups EDG at position 4, decreasing

the BDECH when compared to other substitutions, resulting

in a better antioxidant activity (Perez-Gonzalez and Galano,

2012; Borges et al., 2012). The unpaired electron after a

hydrogen abstraction from the CH group of the pyrazole

ring is localized at 2, 4, and 6 positions. The highest scav-

enging activity prediction is related to the lowest contribu-

tion at the carbon atom. The likely mechanism is related tohydrogen transfer. It was found that the antioxidant activity

depends on the presence of EDG at the C2 and C4 positions

and there is a correlation between IE and BDE. Our and other

results have identified three different classes of new deriv-

atives more potent than edaravone (Perez-Gonzalez and

Galano, 2012; Borges et al., 2012).

Nevertheless, in previous theoretical studies few com-

pounds are water soluble or polar (Watanabe et al., 1986;

Wang and Zhang, 2003; Borges et al., 2012). Furthermore,

the ischemia process is related to the free-radical species in

plasmatic phase (Yamamoto et al., 1997). Therefore, it is

interesting to clarify the scavenging mechanism and the

influence of the hydro-soluble derivative of edaravone 1,

named oxederavone 2 (Fig. 1), which will be helpful to

elucidate the structureactivity relationship in the oxidation

process.

Methodology

All calculations were performed with the Gaussian 03

molecular package (Frisch et al., 2003). Prior to any DFT

(Parr et al., 1999) calculation, all isomers were submitted

to a PM3 (Stewart, 1986) geometry conformational search.

All the lowest geometries obtained by semi-empirical PM3

were reoptimized by means of DFT/B3LYP (Becke, 1993;

Lee et al., 1988) with the 6-31G* basis sets (Hehre et al.,

1986).

The IE were calculated as the energy difference between

a neutral molecule (EMXH) and the respective cation free-radical (EMXH?) (Eq. 1), where M is the molecule and X

is the methylene, hydroxyl, or imine groups.

IE EMXH EMXH 1

The bond dissociation energies of methylene, hydroxyl,

and amine moieties (BDEXH) were calculated as the energy

difference between a neutral molecule (EMXH) and the

respective semiquinone (EMX) plus hydrogen radical or

hydrogen atom (EH) (Eq. 2).

BDEXH EMX EH EMXH 2

All calculations were performed in gas phase with thepurpose of obtaining the intrinsic properties of the tautomers

studied, free of any interaction. The solvent effect was

calculated in the presence of water or methanol simulated

using the polarizable continuum model (PCM) (Hehre et al.,

1986) implemented in the Gaussian 03W package. Then, the

molecular electrostatic potential surfaces were drawn using

the Gaussian 03 (Frisch et al., 2003). The molecular

visualization was performed with the program Molekel 4.2

(Portmann and Luthi, 2000).

In the present work, we aimed at exploring the hydro-

soluble derivative of edaravone 1, oxedaravone 2, for the

antioxidant properties and tautomerism taking into accountsolvent effects. We are interested in knowing the role

played for the different five tautomer of this molecule 2

(Fig. 2), to a better understanding of the oxedaravone

oxidation and its possible application as an antioxidant.

Results and discussion

Oxedaravone tautomerism

The dicarbonylic form (2a) is the most stable tautomer of

oxederavone and the values of the relative barriers of tau-

tomerization energies related to other tautomers are 15.62

(2b),66.73(2c), 62.95 (2d), and 71.56 (2e). These values are

shown in Table 1. Consequently, the imine and hydroxylated

forms are less favored thermodynamically. Therefore, as

show in the Fig. 3, there is no difference between the ox-

ederavone form 2a and 2b in water or methanol by PCM

methods. A low relative barrier was found between 2a and 2b

with values of 15.62, 16.67, and 16.34 kJ mol-1 in gas

phase, water, and methanol, respectively. Nevertheless, the

(1)

NN

H3C

O

(2)

NN

HO

O1

2

34

5

67

8

9

10

11

12

13

Fig. 1 Structure and numbering of edaravone (1) and oxederavone

(2)

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imine and hydroxylated forms have the lowest values in

water or methanol than in gas phase.

These results are in accordance with the study of the

edaravone tautomerism (Queiroz et al., 2010). In this

study, the more stable carbonyl tautomer is responsible for

radical-scavenging reaction of edaravone and was mea-

sured in gas phase and solvents such as water and methanol

(Ohara et al., 2006).

Our result showed that keto tautomerism of oxedaravone

is more favored than the enol form. However, it is possible

that the keto form has the larger radical-scavenging activity

than the enol form. In fact, previous works have demon-

strated that the BDE of C7H is higher than C4H for

edaravone. Nevertheless, the BDENH and BDEOH corre-

sponding to positions 3 and 6, respectively, are much lower

than the BDECH corresponding to position 4. Moreover, the

relative barrier between CH and NH or OH tautomers

of edaravone is 32.76 and 43.22 kJ mol-1, respectively

(Wang and Zhang, 2003; Queiroz et al., 2010).

In accordance with Wang and Zhang, CH tautomer of

edaravone is unstable. Nevertheless, the solvent effect by

PCM methods decreased the energy barriers in NH or OH

tautomerization of edaravone for 11.59 or 25.14 kJ mol-1 in

water and 12.71 or 26.15 kJ mol-1 in methanol, respec-

tively. Furthermore, the solvent effects decreased the energy

barriers for the NH or OH of edaravone tautomerization(Wang and Zhang, 2003; Queiroz et al., 2010).

The edavarone tautomerization can be associated with

the scavenging activity of edavarone in ischemia and the

hydrogen at the 2, 4, 6, and 13 positions are acid. The

ionized form of edaravone (anion) is reported to be an

active form in scavenging free-radicals by a one-electron-

transfer mechanism (Watanabe et al., 1986) and its effi-

ciency as radical scavenger was assumed to be due to the

increasing of its anion form as active form (Nakagawa

(2a)

(2b)(2e)

(2d) (2c)

HNN

O

O

NN

HO

O

NN

HO

OHHN

N

HO

O

HNN

O

OH

Fig. 2 Five tautomeric forms

of oxedaravone

Table 1 Relative energy barriers of oxedaravone tautomers

Position DEgas phase DEin water DEin methanol

2a 0 0 0

2b 15.62 16.67 16.34

2c 66.73 51.99 53.01

2d 62.95 37.14 39.09

2e 71.56 46.16 47.05

Fig. 3 Relative energy barriers of the five tautomeric forms of the

oxedaravone

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et al., 2006). Nevertheless, the ischemia is a result of the

marked pH reduction of the tissue by generation of lactic

acid as well as protons (Ohara et al., 2006).

This way, the changes of EDG in 3-position of pyraz-

olone ring through substitution of methyl to carbonyl or its

tautomer hydroxyl group may facilitate the increase of

polarity and acidity of the pyrazolone ring.

The difference between the electronic behaviors of oxe-daravone tautomers can be observed by molecular electro-

static potentials (MEPs). These MEPs for the five

oxedaravone tautomers are presented in Fig. 4. All car-

bonyl, hydroxyl, imine, and enamine moieties present

negative charges (red and yellow), but the intermediate

charges are located on the carbon atoms (green) of the

aromatic or aliphatic moieties. Otherwise, the positive

charges (blue) are located on the hydrogen atoms. Actually,

the heteroatoms directly attached to the pyrazolone ring are

the ones responsible for the electronic characteristics pre-

sented by the molecules studied here. In fact, the electro-

static profiles for keto groups are significantly differentfrom the hydroxyl compounds.

Electron abstraction

The antioxidant capacity of keto, hydroxyl, imine, or enam-

ine tautomers of oxedaravone were theoretically predicted

using HOMO and IE values. The HOMO energy is an

important parameter for molecular electronic structure. The

molecule which has the lower HOMO energy has a weak

electron donating ability. On the contrary, the higher

HOMO energy implies that the molecule is a good electron

donor (Queiroz et al., 2009). The HOMO disposition

around the hydroxylpyrazolone rings can indicate the site

of reaction with free-radicals qualitatively because the

H-abstraction reaction undergoes electron transfer. These

tautomers have every position rich in electrons, as shown in

the HOMO maps (see Fig. 5). In fact, all phenyl, methyl,and hydroxyl or carbonyl groups at the 1 and 2-positions

have great contribution for the electronic formation of

HOMO, increasing the chemistry reactivity of these

molecules.

The HOMO values are showed in Tables 1, 2. The

tautomers showed the following HOMO values in gas

phase: -6.06 to -5.61 eV. Therefore, all hydroxyl and

imine tautomers are more nucleophilic than the carbonyl

tautomers.

Also, the IE values in gas phase for the tautomers are

760.75700.14 kJ mol-1. These results are in accordance

with the HOMO values. In fact, previous work hasdemonstrated a good tendency between HOMO and IE

(Queiroz et al., 2009). Therefore, the result indicates that

the OH tautomer increased the HOMO and decreased the

IE values.

All calculations showed that the 2b tautomer is more

nucleophilic than all other tautomers, decreasing the IE

values. Therefore, the electron donation is more favored. In

this case, the electron transfer is stabilized by the electron

conjugation between hydroxyl and imine groups. In accor-

dance with the electron abstraction, the hydroxyl group is

Fig. 4 MEPs of the five tautomeric forms of oxedaravone Fig. 5 HOMO of the five tautomeric forms of oxedaravone

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responsible for the more antioxidant property of oxede-

ravone than edaravone.

In fact, the spin density distribution showed a character-

istic contribution of the tautomer 2b on the 3C, 4C, 5C, and

6C positions (see Table 3). Consequently, the tautomer that

has the lowest global contribution for the phenyl and pyr-

azolone rings is more nucleophilic. These results showed

that the hydroxyl-imine tautomer can be responsible for the

antioxidant capacity by electron abstraction due to its higherHOMO and lower IE values.

Hydrogen abstraction

The bond dissociation energies (BDEXH) for oxedaravone

tautomers for different CH, OH, or NH substitutions

were calculated. These values represent the easiness of the

hydrogen donation of oxedaravone tautomers to produce

semiquinone derivatives. The hydrogen abstraction was

another antioxidant mechanism studied (Queiroz et al.,

2009; Cao et al., 2003). Therefore, molecules with the

lowest BDEXH are more active.As can be seen from Fig. 6, the BDEXH values to the

tautomer of lower energy (2a) in gas phase are

398.47 kJ mol-1 for CH and 335.56 kJ mol-1 for NH.

The next tautomer with lower energy (2b) has BDEXH values

of 348.58 kJ mol-1 for CH and 319.93 kJ mol-1 for OH.

The BDEXH values for other tautomers are of higher energies

(2c, 2d, and 2e).

The hydrogen donation at CH position is more difficult.

Nonetheless, OH and NH bond dissociation energies are

favored due to electronic interactions mainly for the tau-

tomer 2d. This result suggests that hydroxyl or carbonyl

tautomers of oxedaravone have more electronic stabiliza-tion than the methyl moiety.

In accordance with the electron abstraction, the hydro-

gen abstraction showed that the NH and OH tautomers

can be responsible for the antioxidant capacity by hydrogen

abstraction due to its higher HOMO and lower IE and

BDEXH values. Nevertheless, edaravone has the lowest

BDEXH values, i.e., 343.67, 310.91, and 300.45 kJ mol-1

for CH, NH, or OH, respectively.

Contrarily to the electron abstraction, the spin density

distribution after hydrogen abstraction with semiquinone

compounds showed a lower spincontribution on the oxygen or

nitrogen atoms, 1,3-positions of pyrazolone ring and globalspin contribution for the phenyl and pyrazolone rings (see

Table 4). Consequently, these results showed that the anti-

oxidantcapacity of oxedaravone can be determined mainly by

the stability of the semiquinone radical, generated after the

hydrogen donation. Therefore, a symmetric contribution

between the phenyl and pyrazolone rings was needed for the

more stable tautomer. The result of this work canbe important

for the design of new edaravone derivatives.

Conclusion

The theoretical prediction of the oxedaravone tautomerism

was carried out using DFT-B3LYP/6-31G(d). Gas phase

and solvent effects showed that the ketoenol tautomers of

oxedaravone are thermodynamically more favored than the

Table 2 Calculated properties of oxedaravone tautomers

Position HOMO (eV) IE (kJ mol-1)

1 -5.73 723.99

2a -6.06 760.75

2b -5.66 700.14

2c -5.61 717.27

2d-

5.69 726.03

2e -5.67 724.27

Table 3 Spin density contribution in cation radical of the oxedaravone tautomers calculated by electron abstraction

Position 2a 2b 2c 2d 2e

1N 0.305918 0.294384 0.264365 0.324966 0.299915

2N 0.143442 0.042923 0.019815 0.087726 0.070777

3C -0.027008 0.080433 0.133693 -0.022708 -0.093880

4C 0.004539 -0.005398 0.062706 0.013520 0.302434

5C -0.058345 -0.028949 -0.010669 -0.058069 -0.106994

6O 0.138687 0.095594 0.016676 0.185821 -0.016531

7C 0.082953 0.054494 0.077003 0.085143 0.049817

8C 0.101649 0.148802 0.109278 0.113520 0.079937

9C -0.042654 -0.077893 -0.055846 -0.049831 -0.036699

10C 0.258679 0.305536 0.241651 0.269930 0.173009

11C -0.074866 -0.091523 -0.060561 -0.079264 -0.048886

12C 0.129823 0.160784 0.112177 0.137585 0.085885

13O 0.056023 0.048396 0.113920 0.002962 0.261715

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imineenamine tautomers. The HOMO and ionization

energy values showed that carbonyl or hydroxyl tautomers

are better antioxidants than the imine or enamine tautom-

ers. The calculated bond dissociation energies showed that

the OH tautomer is a better antioxidant than the NH or

CH ones. Our results revealed that the OH tautomer is

more important for the antioxidant property of oxedarav-one due to its lower bond dissociation energies.

Acknowledgments The authors are grateful to the Brazilian

Agencies CNPq and UFPA by financial support.

References

Becke AD (1993) Density-functional thermochemistry. III. The role

of exact exchange. J Chem Phys 98:56485652

Borges RS, Queiroz AN, Mendes APS, Araujo SC, Franca LCS,

Franco ECS, Leal WG, da Silva ABF (2012) Density functional

theory (DFT) study of edaravone derivatives as antioxidants. Int

J Mol Sci 13:75947606

Cao H, Pan X, Li C, Zhou C, Deng F, Li T (2003) Density functional

theory calculations for resveratrol. Bioorg Med Chem Lett

13:18691871

Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M,Nakajima T, Honda Y, Kitao O, Nakai H, Klene M, Li X, Knox

JE, Hratchian HP, Cross JB, Bakken V, Adamo C, Jaramillo J,

Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R,

Pomelli C, Ochterski JW, Ayala PY, Morokuma K, Salvador P,

Dannenberg JJ, Zakrzewski VG, Dapprich S, Daniels AD, Strain

MC, Farkas O, Malick DK, Rabuck AD, Raghavachari K,

Foresman JB, Ortiz JV, Cui Q, Baboul AG, Clifford S,

Cioslowski J, Stefanov BB, Liu G, Liashenko A, Piskorz P,

Komaromi I, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng

CY, Nanayakkara A, Challacombe M, Gill PMW, Johnson B,

Johnson B, Chen W, Wong MW, Gonzalez C, Pople JA (2003)

Gaussian 03, revision C.02. Gaussian Inc, Wallingford

(2a)

(2b)(2e)

(2d) (2c)

HNN

O

O

N N

HO

O

NN

HO

OHHN

N

HO

O

HN N

O

OH

H

H

H

H

398.47

348.58

319.93

335.56

326.68345.70

311.68

335.51 350.53

297.37

Fig. 6 BDEs of the five

tautomeric forms of

oxedaravone

Table 4 Spin density contribution in semiquinone radical of the oxedaravone tautomers calculated by hydrogen abstraction

Position 2a 2b 2c 2d 2e

1N 0.204224 0.204325 0.181855 0.175018 0.152398

2N 0.489441 0.489277 0.399726 0.402683 -0.017143

3C -0.069564 -0.069552 -0.052092 -0.036102 -0.089154

4C 0.000242 0.000240 0.139876 0.124768 0.647614

5C -0.001973 -0.001941 -0.005920 -0.001044 -0.085697

6O 0.096487 0.096389 0.264290 0.258486 0.030896

7C -0.055340 -0.055400 -0.044160 -0.043576 -0.028073

8C 0.069729 0.069809 0.059048 0.058223 0.062638

9C-

0.035752-

0.035792-

0.030508-

0.030122-

0.03351610C 0.076581 0.076672 0.065818 0.064922 0.083401

11C -0.040271 -0.040320 -0.033825 -0.033384 -0.038423

12C 0.072364 0.072448 0.060829 0.059932 0.067143

13O 0.211571 0.211594 0.010378 0.014831 0.285859

Med Chem Res

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7/7

Hehre WJ, Radom L, Schleyer PVR, Pople JA (1986) Ab initio

molecular orbital theory. Wiley, New York

Kawai H, Nakai H, Suga M, Yuki S, Watanabe T, Saito KI (1997)

Effects of a novel free radical scavenger, MCl-186, on ischemic

brain damage in the rat distal middle cerebral artery occlusion

model. J Pharmacol Exp Ther 281:921927

Lee C, Yang W, Parr RG (1988) Development of the Collesalvetti

correlationenergy formula into a functional of the electron

density. Phys Rev B 37:785789

Nakagawa H, Ohyama R, Kimata A, Suzuki T, Miyata N (2006)

Hydroxyl radical scavenging by edaravone derivatives: efficient

scavenging by 3-methyl-1-(pyridin-2-yl)-5-pyrazolone with an

intramolecular base. Bioorg Med Chem Lett 16:59395942

Ohara K, Fujii A, Ichimura Y, Sato K, Mukai K (2006) Kinetic study

of radical-scavenging and vitamin E-regenerating actions of

edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one). Bull Chem

Soc Jpn 79:421426

Parr RG, Szentpaly LV, Liu S (1999) Electrophilicity index. J Am

Chem Soc 121:19221924

Perez-Gonzalez A, Galano A (2012) OH radical scavenging activity

of edaravone: mechanism and kinetics. J Phys Chem B

116:11801188

Portmann S, Luthi HP (2000) Molekel: an interactive molecular

graphics. Chimia 54:766770

Queiroz AN, Gomes BAQ, Moraes WM Jr, Borges RS (2009) A

theoretical antioxidant pharmacophore for resveratrol. Eur J Med

Chem 44:16441649

Queiroz AN, Mendes APS, Leal MS, Chaves Neto AMJ, Borges RS

(2010) Tautomerism and radical-scavenging activity of edarav-

one by DFT methods. J Comput Theor Nanosci 7:153156

Stewart JJP (1986) Optimization of parameters for semiempirical

methods I method. J Comput Chem 10:209220

Tabrizchi R (2000) Edaravone Mitsubishi-Tokyo. Curr Opin Investig

Drugs 1:347354

Wang LF, Zhang HY (2003) A theoretical investigation on DPPH

radical-scavenging mechanism of edaravone. Bioorg Med Chem

Lett 13:37893792

Watanabe T, Yuki S, Egawa M, Nishi H (1986) Protective effects of

MCI-186 on cerebral ischemia: possible involvement of free

radical scavenging and antioxidant actions. J Pharmacol Exp

Ther 268:15971604

Wu TW, Zeng LH, Wu J, Fung KP (2002) Myocardial protection of

MCI-186 in rabbit ischemia- reperfusion. Life Sci 71:22492255

Yamamoto T, Yuki S, Watanabe T, Mitsuka M, Saito KI, Kogure K

(1997) Delayed neuronal death prevented by inhibition of

increased hydroxyl radical formation in a transient cerebral

ischemia. Brain Res 762:240242

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mcres-oxedaravone (1)

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