Salen-manganese complexes as catalytic scavengers of hydrogen peroxide and cytoprotective agents: Structure-activity relationship studies

Article abstract of DOI:10.1021/jm020207y

Synthetic catalytic scavengers of reactive oxygen species (ROS) may have broad clinical applicability. In previous papers, two salen-manganese complexes, EUK-8 and EUK-134, had superoxide dismutase (SOD) and catalase activities and prevented ROS-associate

Full text of DOI:10.1021/jm020207y

J . Med. Chem. 2002, 45, 4549-4558
4549
Sa len -Ma n ga n ese Com p lexes a s Ca ta lytic Sca ven ger s of Hyd r ogen P er oxid e
a n d Cytop r otective Agen ts: Str u ctu r e-Activity Rela tion sh ip Stu d ies
Susan R. Doctrow,*,† Karl Huffman,† Catherine Bucay Marcus,† Georges Tocco,† Evelyne Malfroy,†
Christy A. Adinolfi,† Henry Kruk,‡ Keith Baker,† Natalie Lazarowych,‡ J ulius Mascarenhas,‡ and
Bernard Malfroy†
Eukarion, Inc., 6F Alfred Circle, Bedford, Massachusetts 01730, and Dalton Chemical Laboratories, Inc., Toronto, Ontario
Received May 14, 2002
Synthetic catalytic scavengers of reactive oxygen species (ROS) may have broad clinical
applicability. In previous papers, two salen-manganese complexes, EUK-8 and EUK-134, had
superoxide dismutase (SOD) and catalase activities and prevented ROS-associated tissue injury.
This study describes two series of salen-manganese complexes, comparing catalytic ROS
scavenging properties and cytoprotective activities. The compounds vary widely in ability to
scavenge hydrogen peroxide, with this activity most influenced by salen ring alkoxy subsitution
and aromatic bridge modifications. In contrast, all compounds show comparable SOD activities.
The most active alkoxy-substituted catalase mimetics protected cultured cells from hydrogen
peroxide, and a subset of these were also neuroprotective in a rodent stroke model. Thus,
structural modification of the prototype EUK-8 yields compounds with enhanced catalase
activity and, in turn, biological effectiveness. This supports the concept that salen-manganese
complexes represent a class of SOD and, in particular, catalase mimetics potentially useful
against ROS-associated diseases.
In tr od u ction
cyclic ligand complexes,8,9 and the salen-manganese
complexes that are the focus of this research.10 Certain
of these manganese complexes have shown protective
activity in various experimental disease models.9,11-15
One class of nonmetal-containing compounds, carboxy-
fullerenes, have also been reported to have catalytic
ROS scavenging as well as neuroprotective activities.16
A low molecular weight selenium complex known as
Ebselen has been developed as a mimetic for glutathione
peroxidase and has also shown protection in various
models for oxidative stress.17
Oxygen, though essential for aerobic metabolism, can
be converted to toxic metabolites, such as the superoxide
anion and hydrogen peroxide, collectively known as
reactive oxygen species (ROS). Increased ROS formation
occurs under numerous pathological conditions includ-
ing, broadly, ischemic and inflammatory processes.1
When ROS generation exceeds the capacity of endog-
enous antioxidant defense systems to neutralize them,
a condition known as oxidative stress,2 tissues become
vulnerable to damage. Antioxidant therapies have long
been considered for a wide variety of disorders in which
oxidative stress is believed to play a significant role.1,3
Endogenous antioxidant enzymes include the super-
oxide dismutases (SODs) (Cu-, Zn-, and Mn-containing
forms) and catalase, which catalyze the neutralization
of superoxide and hydrogen peroxide, respectively.
Other enzymes acting against hydrogen peroxide in-
clude peroxidases and glutathione peroxidase. Exo-
genously administered SOD has been tested in various
clinical indications with limited success. This has led
to an interest in developing synthetic SOD mimetics
with more favorable pharmaceutical properties than a
protein such as SOD. The first low molecular weight
SOD mimetics to be described were copper complexes
that showed antitumor-promoting activity.4 Subse-
quently, several classes of manganese complexes pur-
ported to mimic manganese SOD were described. These
include complexes formed between the manganese and
the chelator desferrioxamine,5 a series of manganese-
porphyrin complexes,6,7 a class of manganese-macro-
We have previously reported10 that synthetic salen-
manganese complexes are SOD mimetics. The series of
compounds described in that study was either inactive
or exhibited a range of SOD activities over 2 orders of
magnitude. These compounds included EUK-8, one of
the more active and the most water soluble of the series.
EUK-8 was subsequently shown to exhibit catalase18,19
and peroxidase19 activities, showing that salen-man-
ganese complexes can scavenge hydrogen peroxide in
addition to superoxide. (Of the other classes of Mn-
containing SOD mimetics listed above, the metallopor-
phyrins have also been found to exhibit hydrogen
peroxide scavenging activities,7 while the macrocyclic
complexes reportedly show specificity for superoxide and
are being designed to optimize SOD activity.9) Because
hydrogen peroxide is produced in vivo during oxidative
stress, is cytotoxic, and is stable in solution as compared
to superoxide, the ability to consume hydrogen peroxide
is regarded as an advantageous property of the salen-
manganese complexes.19 Despite this, relatively little
effort has focused on the development and optimization
of catalase mimetics for possible therapeutic use.
In support of the potential clinical value of these
multifunctional catalytic ROS scavengers, EUK-8 is
protective in numerous pharmacological models for
* To whom correspondence should be addressed. Tel: 781-275-0424.
Fax: 781-275-0752. E-mail: s.doctrow@eukarion.com.
† Eukarion, Inc.
‡ Dalton Chemical Laboratories, Inc.
10.1021/jm020207y CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/28/2002

4550 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20
Doctrow et al.
ROS-associated tissue damage. For example, EUK-8
protected organotypic hippocampal cultures from toxic-
ity by the â-amyloid peptide.20 Administered in vivo, it
inhibited the degeneration of dopaminergic neurons in
two murine models for Parkinson’s Disease19 and sup-
pressed the development of paralysis in mouse experi-
mental autoimmune encephalomyelitis, a model for
multiple sclerosis.21 Intravenously infused EUK-8 pre-
served pulmonary function in a highly stringent porcine
endotoxin-induced model for the adult respiratory dis-
tress syndrome.18,22 Subsequently, EUK-134, a 3,3′-
methoxy salen ring-disubstituted EUK-8 analogue with
equivalent SOD activity but enhanced catalase activity,
was described and shown to be more neuroprotective
than EUK-8 in a rodent stroke model.23 EUK-134
protected hippocampal neurons and suppressed several
indicators of stress and tissue damage in a kainic acid-
induced seizure model in rats,24 and both EUK-8 and
EUK-134 extended the lifespan in the nematode worm
Caenorhabditis elegans.25 More recently, EUK-8 and
EUK-134 were shown to prolong survival and attenuate
indicators of spinal cord oxidative stress in a mouse
model for familial amyotropic lateral sclerosis,26 and
three salen-manganese complexes were found to pro-
long survival and rescue oxidative pathologies in mice
lacking manganese SOD.27
Thus, a substantial body of evidence exists for the
protective efficacy of salen-manganese complexes, as
exemplified by EUK-8 and EUK-134, in models for ROS-
associated pathologies. In this paper, we describe the
synthesis and evaluation of two series of salen-
manganese complexes, all structural analogues of EUK-
8. The evaluation includes comparing their hydrogen
peroxide and superoxide scavenging properties as well
as their cytoprotective activities in a cell culture model.
In addition, a subset of compounds are evaluated for
neuroprotective efficacy in a rodent stroke model.
F igu r e 1. Structures of salen-manganese complexes. Sub-
stituents are as indicated in the table. OMe, methoxy (CH3O);
OEt, ethoxy (CH3CH2O); OAc, acetoxy (CH3COO).
pound EUK-8, have SOD activity10 but, subsequently,
EUK-8 was also shown to have catalase activity, as
detected by the generation of a stoichiometric excess of
oxygen in the presence of hydrogen peroxide.18,19 The
catalase activity of EUK-8 is not saturable over a wide
range of hydrogen peroxide concentrations (0.1-100
mM). Similarly, kinetic analyses of mammalian cata-
lases indicate that the enzymes lack a Km for hydrogen
peroxide and therefore exhibit increased activity as the
intracellular hydrogen peroxide concentration increases.29
(This is in contrast to dimanganese-containing bacterial
catalases, which reportedly display saturation kinet-
ics.30) EUK-8 also exhibits peroxidase activity,19 which
is consistent with its function as a catalase. Catalases
are well-known to be capable of undergoing peroxidative
as well as catalatic reactions.29,31 In this manner, in the
presence of a suitable electron donor, catalase driven
decomposition of hydrogen peroxide proceeds via par-
titioning between catalatic and peroxidatic reactions.31
EUK-8 and other salen-manganese complexes have a
broader peroxidase substrate specificity than that re-
ported for catalase proteins, for example, enabling the
convenient assay of this activity with colorimetric
substrates such as 2,2′-azino-bis(3-ethylbenz-thiazoline-
6-sulfonic acid) (ABTS). As with its catalase activity,
the peroxidase activity of EUK-8 was not saturable with
respect to hydrogen peroxide. Both catalase and per-
oxidase activities of EUK-8 were found to be pH sensi-
tive, with activity increasing at increasing pH in the
range of pH 6 to pH 9.
Resu lts a n d Discu ssion
Sa len Rin g Su bstitu tion s Affect Hyd r ogen P er -
oxid e Sca ven gin g Activities of Sa len -Ma n ga n ese
Com p lexes. Figure 1 shows the structures of the ring-
substituted series of salen-manganese complexes evalu-
ated in this study. The Schiff base ligands used to
complex manganese(III) are derivatives of the tetraden-
tate ligand bis(salicylaldehyde)ethylenediamine (salen-
H2).28 Compounds were synthesized as described in the
Experimental Section, with either chloride or acetate
as axial ligand. In all cases, those compounds with an
axial acetate ligand were found to be more water soluble
than the corresponding chlorides. The prototype com-
pound, EUK-8, contains a chloride axial ligand and
unsubstituted salen ligand. It has previously been
reported to exhibit SOD activity of about 700 units/
mM.10 The other complexes contain ligands with sym-
metrical substitutions on the salen rings as shown in
Figure 1. This includes several chloride and acetate
pairs, which differ only in the axial ligand. The two
members of each pair showed similar, if not identical,
activity in the various assay systems, as discussed
further below. Two compounds, EUK-160 and EUK-189,
are represented only as the more water soluble acetate
complexes.
Catalase and peroxidase activities of the salen ring-
substituted analogues are compared in Table 1. Catalase
activity was assayed by monitoring the conversion of
hydrogen peroxide to oxygen as described in the Ex-
perimental Section. The table presents initial rates as
well as the maximal amount of oxygen produced,
We have previously reported that salen-manganese
compounds, including the unsubstituted prototype com-

Salen-Manganese Complexes as Catalytic Scavengers
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20 4551
Ta ble 1. Catalytic Hydrogen Peroxide Scavenging Activities of
Ring-Modified Salen-Manganese Complexesa
activities as compared to the unsubstituted EUK-8 and
EUK-108. The presence of methoxy groups at the
4-positions, in EUK-114 and EUK-115, markedly re-
duces the catalase and peroxidase activities relative to
the unsubstituted analogues. The activity is even fur-
ther weakened in EUK-118, with methoxy groups at the
4,6-positions. The 3,5-disubstituted analogue, EUK-160,
showed catalytic activities approximately equivalent to
those of EUK-113 and EUK-134, as did a 3-ethoxy
compound, EUK-189.
catalase rate
endpoint
peroxidase rate
compd
(µM O2/min) (maximal µM O2) (µM ABTS/min)
EUK-8
EUK-108
148 ( 27
149 ( 21
27 ( 6
25 ( 4
22.4 ( 4.1
23.6 ( 4.7
EUK-121
EUK-122
201 ( 39
197 ( 27
28 ( 2
28 ( 1
22.3 ( 0.7
23.6 ( 0.9
EUK-15
EUK-123
186 ( 2
112 ( 14
84 ( 1
55 ( 4
25.3 ( 5.5
19.9 ( 0.5
EUK-134
EUK-113
243 ( 18
260 ( 45
81 ( 2
79 ( 9
37.8 ( 9.6
36.0 ( 7.0
These studies yielded no evidence that the catalase
and peroxidase activities are dissociable from one
another since the rank order of the compounds was the
same for both activities. This was not unexpected since
the two reactions are hypothesized to arise, as alterna-
tive arms, from the same catalytic cycle. However, it is
possible that some analogues will show differences in
preference for one reaction pathway over the other. For
example, EUK-118 showed measurable catalase activity
but no detectable peroxidase activity, even though the
latter assay is the more sensitive. Furthermore, we have
found that analogues with longer chain 3-alkoxy sub-
stituents retain catalase activity but exhibit decreased
peroxidase activity (data not shown). While the likeli-
hood exists that certain structural alterations might
produce a catalase without peroxidase activity, or vice-
versa, in this ring-modified series of salen-manganese
complexes, we conclude that the catalase and peroxidase
activities are closely correlated with one another.
EUK-114
EUK-115
70 ( 8
81 ( 14
11 ( 3
12 ( 0.2
0.8 ( 0.2
0.8 ( 0.2
EUK-124
EUK-118
nd
35 ( 6
nd
9 ( 4
nd
-0.1 ( 0.1
EUK-160
235 ( 47
180 ( 7
57 ( 9
80 ( 3
44.8
30.0
EUK-189
a
Catalase and peroxidase activities were conducted as described
in the Experimental Section. Where indicated (nd), the assay was
not performed because of poor solubility of the compound EUK-
124.
calculated from time-dependent plots as described. The
latter provides an indication of total substrate turnovers
completed prior to inactivation of the compound.18 As
previously reported for EUK-8, the compounds are
inactivated under the catalase assay conditions. The
mechanism of inactivation may involve peroxidative
decomposition of the compound and/or formation of a
µ-oxo or similar higher oxidation species,32 but this has
not been studied and is currently unknown. At present,
we regard both parameters, initial rate and maximal
oxygen production, as being useful to compare the
hydrogen peroxide scavenging properties of various
analogues, and consequently, both parameters are
reported here. As shown in Table 1, salen ring substitu-
tion influences catalase activity in this series of com-
pounds. For example, analogues with methoxy substi-
tutions at the 3- (EUK-113, EUK-134) or 5-positions
(EUK-15, EUK-123) completed 2-3 times as many
reaction turnovers as EUK-8. The 3-substituted com-
pounds had, in addition, an initial rate about twice as
fast as EUK-8 and comparable analogues and were,
therefore, the most active catalase mimetics of this
series. The compound with the lowest catalase activity
was EUK-118, the 4,6-methoxy-substituted compound.
The relative peroxidase activities of the salen ring-
substituted analogues showed a good correlation to their
relative catalase activities (Table 1), as might be
expected based upon the proposed relationship between
catalatic and peroxidatic reactions. EUK-113 and EUK-
134 were the fastest peroxidases while EUK-118 was
the poorest, in this case having undetectable activity.
For comparison, under these catalase assay conditions,
bovine liver catalase (29 units/mL) produced oxygen at
the rate of ∼0.33 mM/min with no apparent inactiva-
tion. Under the peroxidase assay conditions, the same
concentration of bovine liver catalase showed no per-
oxidase activity toward ABTS.
It has been reported that some peroxidases use
organic peroxides as alternative substrates to hydrogen
peroxide. In an analogous spectrophotometric peroxi-
dase assay, EUK-8 exhibited weak peroxidase activity
with 1 mM tert-butylhydroperoxide, oxidizing ABTS at
a rate about 0.5% of that observed with the same
concentration of hydrogen peroxide. (In comparison,
horseradish peroxidase utilized tert-butylhydroperoxide
with an ABTS oxidation rate that was about 5.4% of
the rate with hydrogen peroxide.) EUK-113 was about
a 3-fold faster peroxidase with tert-butylhydroperoxide
than EUK-8. Interestingly, EUK-121 and EUK-122
were both even faster peroxidases with tert-butylhydro-
peroxide, about twice as fast as EUK-113. This observa-
tion implies that the various salen-manganese com-
plexes may, as peroxidases, differ in their relative
hydroperoxide specificities. EUK-118 had less than 2%
of the peroxidase activity toward tert-butylhydroperox-
ide as did EUK-8. The ability to consume organic
peroxides may have some relevance to an ability of
compounds to scavenge lipid peroxides in tissues.
Br id ge Mod ifica tion s F u r th er Affect Ca ta la se
Activities of Sa len -Ma n ga n ese Com p lexes. A sec-
ond series of salen-manganese complexes was next
synthesized, containing bridge structures different from
the ethano bridge of the previous series. On the basis
of the structure-activity data obtained with the first
series, some of these bridge-modified analogues were
also prepared with 3-methoxy symmetrical salen ring
substitution. The structures of the compounds in the
bridge-modified series are shown in Figure 2.
Overall, it is apparent that catalase and peroxidase
activities differ markedly among the various compounds
in this series. Most notably, the presence of methoxy
substituents at the 3-positions, as exemplified by EUK-
113 and EUK-134, increases the catalase and peroxidase
The catalase activities of these compounds are com-
pared in Table 2. Because the axial ligand was found
not to influence the catalytic activities, these compounds
are analyzed only in the more water soluble form with

4552 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20
Doctrow et al.
apparent peroxidase activity, as explained in the Ex-
perimental Section. This indicates that unlike the
ethano-bridged compounds, these analogues show a
preference of the catalatic vs the peroxidatic reaction
pathway in the catabolism of hydrogen peroxide, at least
under these experimental conditions.)
As in any research with metal complexes, the pos-
sibility exists that activities are attributable to the
metal and/or the ligand liberated by dissociation of the
complex. These possibilities were regarded as unlikely,
primarily because the salen-manganese complexes are
highly stable in aqueous solution, while the dissociated
ligands are unstable. For example, EUK-8, EUK-134,
and EUK-189 show no detectable decomposition, based
on high-performance liquid chromatography (HPLC)
analysis, over storage for many months at room tem-
perature in aqueous solution. The aromatic bridge
compounds have somewhat lower stability; for example,
EUK-178 decomposes over several days in aqueous
solution. In all cases, however, stability of the metal
complexes is significantly greater than that of the metal
free ligands. For example, the ligands for EUK-8 and
EUK-134 have half-lives of less than 15 min in aqueous
solution and the other ligands are similarly unstable.
The hydrogen peroxide scavenging activities of these
components were examined by assaying control com-
pounds, comprising the metal, the ligands, or the
breakdown products of the ligand, namely, the diamines
and salicylaldehydes, for catalase and peroxidase activi-
ties as described in the Experimental Section. The
following control compounds were tested for catalase
activity: five different ligands (EUK-8/108, EUK-134/
113, EUK-189, EUK-178, and EUK-172), 3-O-methoxy-
salicylaldehyde, 3-O-ethoxysalicylaldehyde, ethylene-
diamine, Mn(II) acetate, and Mn(III) acetate. None of
these compounds showed oxygen generation in excess
of background levels for the oxygen electrode assay with
respect to the initial rates (calculated values for controls
ranged from -7 to 8 µM/min) or total oxygen generation
(calculated values for controls did not exceed 1 µM). In
addition, none of these compounds showed detectable
peroxidase activity. Therefore, it is concluded that the
catalytic hydrogen peroxide activities summarized in
Tables 1 and 2 are attributable to the intact salen-
manganese complexes.
F igu r e 2. Structures of bridge-modified salen-manganese
complexes. Abbreviations as for Figure 1.
Ta ble 2. Catalase Activities of Bridge-Modified
Salen-Manganese Complexesa
catalase rate
endpoint
compd
(µM O2/min)
(maximal µM O2)
EUK-108
EUK-161
EUK-177
149 ( 21
890 ( 144
552 ( 131
25 ( 4
206 ( 19
119 ( 13
EUK-113
EUK-172
EUK-178
EUK-159
EUK-163
260 ( 45
1073 ( 174
814 ( 155
251 ( 29
36 ( 6
79 ( 9
300 ( 14
230 ( 7
91 ( 5
5 ( 1
a
Compounds were assayed for catalase activity as described
for Table 1. Values for the appropriate reference compounds (EUK-
108 and EUK-113) without bridge modification are included again
for comparison.
acetate as the axial ligand. Reference compounds in-
cluded in Table 2 from the previous series for compari-
son are EUK-108 and EUK-113. This series of com-
pounds showed an even wider range of catalase activities
than the ring-modified series. For example, compounds
containing six-membered aromatic bridge structures
(EUK-161 and EUK-177) exhibited dramatically in-
creased catalase activity as compared to the reference
compound EUK-108 as well as to the best compounds
identified in the salen ring-substituted series. Addition
of 3-methoxy substituents (EUK-172 and EUK-178,
respectively) resulted in even greater catalase activity.
No enhancement of activity was obtained with a six-
membered aliphatic bridge (EUK-159), which showed
catalase activity equivalent to that of the reference
compound EUK-113. An analogue containing a three
carbon aliphatic bridge (EUK-163) exhibited very low
catalase activity, comparable to that of the weakest
catalase in the salen-substituted series. These findings
suggest that a more planar structure, as might be
afforded by the aromatic bridges and lost in EUK-163,
might facilitate optimal catalase activity. In addition,
the electronic effects of the aromatic bridge and ex-
tended aromatic conjugation are likely to play a role in
enhancing catalase activity. (Peroxidase activity values
are not presented for the bridge-modified series of
compounds because the high catalase activity of ana-
logues with aromatic bridge structures quenched any
Rin g Su bstitu tion s a n d Br id ge Mod ifica tion s Do
Not Affect SOD Act ivit y of Sa len -Ma n ga n ese
Com p lexes. The SOD activities of both sets of com-
pounds were determined as described in the Experi-
mental Section. The IC50 values given for each com-
pound in the SOD assay are equivalent to 1 international
unit of SOD.6,10 For all compounds except two, the SOD
activities did not vary, with a mean IC50 value of 0.7 (
0.2 (SD) for the entire set. Two analogues, EUK-118 and
EUK-124, showed slightly lower SOD activities, with
IC50 values of 2 and 5 µM, respectively (with EUK-124’s
poor solubility perhaps contributing to its lower appar-
ent activity). Primarily, in comparison to other salen-
manganese complexes tested previously, which had IC50
values ranging from about 0.3 to 30 µM,10 all compounds
in the present series might be regarded as having
similar to identical SOD activity to one another and
comparable SOD activities to the best molecules in the
series reported by Baudry et al.10 Their SOD activities

Salen-Manganese Complexes as Catalytic Scavengers
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20 4553
are also comparable to that of MnTMPyP, a metallopor-
phyrin SOD mimetic with a reported rate constant for
reaction with superoxide of 3.9 × 107.6 This compound,
tested in parallel with the salen-manganese complexes
in our assay system, had an IC50 value of 0.4 ( 0.1 µM,
as compared to the published value of 0.7 µM.6
Thus, in contrast to the SOD activities of these and
similar compounds, their catalytic hydrogen peroxide
scavenging activities are profoundly influenced by the
ligand structure. A potential mechanism for the catalase
activity we’ve observed, taking into account the ability
to catalyze a peroxidative pathway, can be described by
the following scheme, where AH2 is an oxidizable
substrate such as ABTS:
F igu r e 3. Cytoprotection by salen ring-substituted salen-
manganese complexes. Cytoprotection studies were performed
as described in the Experimental Section. Absorbance values
given are the means ( SD of triplicate samples. Control cells
(open circle) received no glucose oxidase. Catalase-treated
(solid circle) cells received glucose oxidase (0.02 units/mL) as
well as bovine liver catalase (290 units/mL). Other samples
received the same dose of glucose oxidase and the indicated
concentrations of salen-manganese complex: EUK-134 (open
triangle), EUK-113 (solid triangle), EUK-15 (open square),
EUK-123 (solid square), EUK-8 (open inverted triangle), and
EUK-118 (solid inverted triangle). EUK-108, EUK-121, EUK-
122, EUK-114, and EUK-115 showed poor cytoprotective
activity, comparable to that of EUK-8, and have been omitted
from the figure for clarity. In separate experiments, EUK-160
(data not shown) and EUK-189 (Figure 5) showed equivalent
cytoprotection activity to that of EUK-134. Inactivity in this
assay was not found to be associated with cytotoxicity of the
compounds for the cells, which was tested in parallel as
described in the Experimental Section. For example, EUK-8
and EUK-15 showed no cytotoxicity at up to 400 µM.
(ligand)Mn(III) + H2O2 f
(ligand)Mn(V)dO + H2O (1)
(ligand)Mn(V)dO + H2O2 f
(ligand)Mn(III) + O2 + H2O (2)
(ligand)Mn(V)dO + AH2 f
(ligand)Mn(III) + A + H2O (3)
Catalase reaction:
salen-ligandMn(III) + 2H2O2 f
salen-ligandMn(III) + O2 + 2H2O (1 + 2)
Peroxidase reaction:
tion of hydroxyl radicals by the Fenton reaction.1 While
SOD and SOD mimetics are protective in some biologi-
cal settings, they are also potentially deleterious due
to their production of hydrogen peroxide. Overall, this
implies that catalase activity would be a key attribute
for synthetic ROS scavenging compounds. We next
conducted studies to investigate the potential value of
these catalase mimetics as cytoprotective agents. One
goal was to assess the predictive value of catalase
structure-activity relationships observed with these
two series of salen-manganese complexes in such
biological efficacy studies. We employed two biological
models, an in vitro model for hydrogen peroxide-de-
pendent cellular toxicity and an in vivo model for
cerebral ischemia.
Cytop r otective Activity. Salen-manganese com-
plexes were tested for protection of human der-
mal fibroblasts (HF cells) against glucose and glucose
oxidase, a hydrogen peroxide-generating system. As
shown in Figure 3, addition of glucose oxidase to the
HF cell culture system resulted in complete lethality
and bovine liver catalase afforded full protection. Most
of the salen ring-modified salen-manganese complexes
were essentially ineffective at protecting HF cells in this
paradigm, with the exception of the 3- and 5-alkoxy-
substituted compounds. Thus, cytotoxicity by glucose
and glucose oxidase was highly discriminating between
the various salen-manganese complexes. The most
effective cytoprotective compounds, EUK-113 and EUK-
134, were also the most active in catalase/peroxidase
assays. EUK-160 (data not shown) and EUK-189 (Fig-
ure 4) showed equivalent cytoprotective activities. Only
slightly less effective were EUK-15 and EUK-123.
Altogether, these six compounds were highly efficacious,
salen-ligandMn(III) + H2O2 + AH2 f
salen-ligandMn(III) + A + 2H2O (1 + 3)
In this proposed scheme, (ligand)Mn(V)dO is func-
tionally analogous to “compound I”, the intermediate
first described for the haem catalases.29 While the exact
structure of compound I is not known for this or for
other metal complexes and metalloenzymes conducting
such reactions, the intermediate is believed to result
from a two electron oxidation of the catalytic complex
by hydrogen peroxide.1 When salen-manganese com-
plexes are incubated with hydrogen peroxide, they
undergo a spectral change consistent with formation of
an oxidized intermediate (data not shown) and an
analogous oxomanganese species has been proposed as
the active intermediate in oxidative reactions by por-
phyrin-manganese33 and salen-manganese34 com-
plexes. This scheme is also consistent with the obser-
vation that electron-donating alkoxy substituents en-
hance activity when located ortho or para to the metal
complexation site of the salen ring (Table 1). Further
enhancement of activity by aromatic bridge structures
(Table 2) might also be predicted, through electronic
contributions, most likely via resonance in the vicinity
of the metal-binding nitrogen sites. The relative inde-
pendence of these two effects is consistent with their
apparent additivity (Table 2).
The relative involvement of different ROS in a given
pathological setting is controversial. However, super-
oxide can dismutate spontaneously in solution to form
hydrogen peroxide, which is long-lived, believed to freely
diffuse across cell membranes, and is well-documented
to be cytotoxic, either directly or by secondary produc-

4554 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20
Doctrow et al.
this model. Commensurate with their enhanced catalase
activity, they were cytoprotective at lower doses than
analogous compounds with the ethylenediamine bridge
(e.g., EUK-113). However, interestingly, the aromatic
bridged compounds lacking 3-methoxy substituents
(EUK-161 and EUK-177) showed poor cytoprotective
activity, despite their very high catalase activity. Taken
together with data obtained with the ring-substituted
series, this implies that both catalase activity and
alkoxy substitution at the salen rings are required for
cytoprotection in this HF model. Neither alone is
sufficient, as evidenced, for example, by the poor cyto-
protectiveness of methoxy-substituted compounds with
poor catalase activity, such as EUK-163 and EUK-118.
Lack of cytoprotective activity cannot be explained by
toxicity of the compounds for HF cells. For example, of
the bridge-modified compounds tested in Figure 5, EUK-
161 was the least toxic, displaying no apparent cyto-
toxicity at 400 µM. This does not, however, eliminate
the potential role of a hydrogen peroxide-dependent
toxicity caused by compounds lacking alkoxy substitu-
ents. In support of this possibility, in the presence of
EUK-161 (200 µM), HF cells were about 2-4-fold more
sensitive to GO, as evidenced by cell death at lower GO
concentrations. By comparison, EUK-161 at 100 µM and
EUK-8 at both concentrations did not significantly affect
the dose-dependent sensitivity of HF cells to GO, while
EUK-134 at both concentrations was protective (data
not shown). While it is apparent that alkoxy substitu-
ents are advantageous for protective activity of salen-
manganese complexes in this cellular model, the reasons
for this element of the cytoprotection structure-activity
relationship are not yet understood, but in some cases,
a synergistic toxicity with hydrogen peroxide might play
a role.
F igu r e 4. Comparative cytoprotection activity of EUK-134
and control compounds. Cytoprotection studies were performed
as described, with control and catalase-treated cells as indi-
cated in the legend for Figure 3. The compounds tested were
EUK-134 (open triangle), EUK-189 (solid diamond), Mn(II)
acetate (open square), and 3-methoxysalicylaldehyde (solid
triangle).
F igu r e 5. Cytoprotection by bridge-modified salen-manga-
nese complexes. Cytoprotection studies were performed as
described, with control and catalase-treated cells as indicated
in the legend for Figure 3. The compounds tested were EUK-
113 (open triangle), EUK-172 (solid square), EUK-178 (open
square), EUK-161 (open diamond), and EUK-177 (solid tri-
angle). EUK-163 was tested separately at up to 400 µM and
showed no cytoprotective activity (data not shown). Of the
compounds shown, only EUK-178 showed some moderate
cytotoxicity in the absence of glucose oxidase at 200 µM while
EUK-177 and EUK 172, as well as EUK-178, showed cytotox-
icity only at 400 µM.
A relatively simple factor, such as lipophilicity, does
not appear to play a role in this cytoprotection model,
since EUK-134, the ethoxy analogue EUK-189 (Figure
4), and the cyclohexyl-bridged compound EUK-159 (data
not shown) show identical cytoprotective activities,
consistent with their equivalent catalase activities
(Tables 1 and 2), and without regard to the greater
lipophilicity of both EUK-189 and EUK-159 in octanol
partitioning studies (data not shown). In a previously
published study, EUK-134 and EUK-189 were highly
protective and similarly effective at protecting rat
primary neurons from hydrogen peroxide, in this case,
administered directly as a bolus, rather than enzymati-
cally.35 Interestingly, in contrast to these observations
with extracellular hydrogen peroxide, EUK-189 was
recently shown to be approximately 2 orders of magni-
tude more potent than EUK-134 at inhibiting apoptosis
in primary neurons36 as well as being more effective in
a mouse model for mitochondrial oxidative stress,27
presumably via intracellular mechanisms more likely
to be influenced by lipophilicity. It should also be noted
that EUK-8, EUK-134, and EUK-189 have comparable
activities and are all highly effective at protecting
primary neurons and PC12 cells from toxicity by the
peroxynitrite-generating agent sin-1.35 Protection against
sin-1 toxicity may be via a SOD activity and/or a direct
interaction with the reactive nitrogen species nitric
oxide or peroxynitrite. Manganese porphyrins, which,
as noted above, have catalytic properties similar to those
displaying virtually complete protection. In contrast, the
other complexes displayed little or no protection at any
concentration tested. EUK-118 showed a complete lack
of cytoprotection in the model, consistent with its virtual
inactivity in the catalase/peroxidase assays. Control
compounds lacking catalase activity were also inactive
in this model. For example (Figure 4), 3-methoxysali-
cylaldehyde and Mn(II)acetate were inactive, as com-
pared in parallel to the fully protective salen-manga-
nese complexes EUK-134 and EUK-189. Ethylenedi-
amine was cytotoxic for HF cells, as were various
ligands, due most likely to their rapid dissociation to
release the toxic diamine.
The bridge-substituted compounds were also evalu-
ated for protective activity in the HF cell cytotoxicity
model. The results (Figure 5) showed that compounds
with aromatic bridges as well as 3-methoxy substituents
(EUK-172 and EUK-178) were highly cytoprotective in

Salen-Manganese Complexes as Catalytic Scavengers
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20 4555
Ta ble 3. Neuroprotective Activity of Salen-Manganese
of the salen-manganese complexes, are known to
scavenge peroxynitrite,37 and there has been indirect
evidence that salen-manganese complexes can do so as
well.38 The details of this property, and structure-
activity relationships of various salen-manganese com-
plexes in potential interactions with reactive nitrogen
species, are not known, but mechanisms analogous to
the peroxidatic reactions described here are likely to
contribute. The ability of mammalian catalase itself to
scavenge nitric oxide in a hydrogen peroxide-dependent
fashion has, in fact, been reported.39 More directly, a
very recent study now shows that EUK-8 and EUK-134
can react with peroxynitrite and nitric oxide via oxidant-
dependent processes.40 Altogether, the experience with
these compounds or, indeed, with any multifunctional
catalytic ROS scavenger, in cytoprotection models sug-
gests that structure-activity relationships will be highly
dependent on the nature of the model, including factors
such as type(s) of ROS generated, intra- or extracellular
site of ROS insult, and specific cell type, including
available endogenous defense mechanisms. This is not
at all surprising and, in fact, quite interesting. It
demonstrates that the predictive value of any single in
vitro model is limited and must be interpreted carefully
in relationship to in vivo activities.
Complexes in a Rodent Stroke Modela
% infarct significant
treatment
dose
vehicle
n
(SD)
vs control
control
control (M)
control (M2)
saline
5% mannitol
5% mannitol, pH 7.6
43 7.4 (3.2)
48 7.8 (3.0)
4
7.7 (2.7)
EUK-134
0.05 saline
0.5 saline
10 6.3 (5.3)
12 3.6 (3.2)
ns
*
*
*
*
5
5
53
saline
5% mannitol
5% mannitol
8
6
4
1.3 (1.8)
1.4 (2.5)
2.1 (1.5)
EUK-8
5
39
saline
5% mannitol
8
8
3.7 (3.9)
4.1 (3.2)
ns
ns
EUK-113
EUK-172
5
saline
5
1.7 (1.3)
*
0.5 5% mannitol,pH 7.6
5% mannitol,pH 7.6
5
8
6.1 (3.4)
2.0 (1.6)
ns
*
5
EUK-178
0.5 5% mannitol
7
4
5
2.5 (2.6)
5.7 (1.8)
9.6 (3.8)
*
ns
ns
5
5% mannitol
5% mannitol
17
EUK-163
EUK-189
5
5
5
saline
6
5
7
6.6 (5.7)
0.9 (1.0)
5.2 (2.3)
ns
*
5% mannitol
5% mannitol
EUK-161
ns
a
The rat middle cerebral artery occlusion model was performed
as described in the Experimental Section. Compounds were
dissolved in the indicated vehicle for intravenous injection and
compared to the appropriate vehicle control group as described (*
) p < 0.01). Doses are in µmol/kg. In addition, two salicylalde-
hydes, each tested at 10 µmol/kg, had no significant effect. Groups
(n ) 3) treated with salicylaldehyde and 3-methoxysalicylaldehyde
had mean percent infarcts of 10.7 ( 3.4 and 6.8 ( 2.9, respectively.
Neu r op r otective Activity of Selected Sa len -
Ma n ga n ese Com p lexes in a Rod en t Str ok e Mod el.
We previously showed that EUK-134 is highly neuro-
protective in a rat stroke model involving permanent
regional cerebral ischemia, with EUK-8 showing sig-
nificantly less effectiveness.23 We have now extended
this study to test several other salen-manganese
complexes. For comparison among analogues, all were
administered at 5 µmol/kg, with other dose groups also
tested in some cases, as indicated. The results are
summarized in Table 3. Three compounds, EUK-113,
EUK-189, and EUK-172, exhibited neuroprotective
activity comparable to that of EUK-134. Two com-
pounds, EUK-161 and EUK-163, were not active. The
lack of activity of EUK-161 is of interest, as it was with
the HF cell model (Figure 4), considering its very high
catalase activity. Again, this implicates the importance
of alkoxy substituents for activity in this biological
model. The lack of activity of EUK-163, which has
3-methoxy substituents but poor catalase activity, should
also be noted. EUK-178 displayed a bell-shaped dose
response, most likely due to its cytotoxicity, which is
also evident in the dose-response behavior in the HF
model (Figure 5) and in separate cytotoxicity studies in
HF cells (Figure 5 legend). Overall, these findings
demonstrate some association between cytoprotective
activity in the HF cell model and neuroprotective
activity in the rat stroke model. In particular, no
compound lacking activity in the HF cell model showed
significant neuroprotective activity in vivo. Not surpris-
ingly, the structure-activity correlation between the
two models was not absolute. For example, EUK-178
and EUK-172 were not more effective than EUK-134
in the stroke model, despite their much higher catalase
activities and greater potencies in the HF cell model.
Factors such as pharmacokinetics, in vivo stability, and
blood-brain barrier permeability would influence the
compounds’ relative activities in the in vivo model while,
of course, would play no role in the cell culture system.
Also, while the HF cell model was designed specifically
to apply a hydrogen peroxide-dependent insult, it is
extremely unlikely that hydrogen peroxide is the sole
cause of neurological damage in the complex cascade
that follows permanent ischemia. For example, as
discussed earlier, reactive nitrogen species may con-
tribute and the structure-activity relationships of reac-
tive nitrogen scavenging properties of the salen-
manganese complexes are not yet known. Nonetheless,
it is well worth noting that the HF cell culture model
does appear to have some predictive value in selecting
compounds potentially active in the in vivo stroke
model.
Con clu sion s
The salen-manganese complexes described in this
study, all structural analogues of the prototype com-
pound EUK-8, exhibit catalase and SOD activities in
vitro. The hydrogen peroxide scavenging activities of
these compounds can be structurally modulated. For
example, catalase activity is increased by symmetrical
3- or 5-alkoxy substituents on the salen rings and by
aromatic bridge structures. Such changes do not, how-
ever, affect SOD activity. Many of the more active
catalases can effectively protect human fibroblasts in
culture against exogenous hydrogen peroxide. However,
at least within this series of compounds, this cytopro-
tection requires 3- and/or 5-alkoxy substituents, sug-
gesting that these substituents confer some advantage,
in addition to their enhancement of catalase activity,
toward the biologically protective characteristics of
salen-manganese complexes. Finally, four of the com-
pounds that are protective in cell culture are also
neuroprotective in vivo in a rat stroke model. Overall,
these data show that salen-manganese complexes

4556 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20
Doctrow et al.
6H, J ) 8.0 Hz), 3.89 (s, 6H), 3.34 (quartet, 2H, J ) 4.0 Hz),
1.51 to 1.99 (m’s, 8H); EUK-177 ligand (CDCl3): δ 13.05 (d,
2H, J ) 6.6 Hz), 8.65 (d, 2H, J ) 6.6 Hz), 6.81 to 7.58 (d’s and
t’s, 12H, J ) 7.5 Hz); EUK-178 ligand (DMSO-d6): δ 13.03 (s,
2H), 8.98 (s, 2H), 6.95 to 7.53 (d’s and t’s, 10H, J ) 8 Hz), 3.87
(s, 6H); EUK-161 ligand (CDCl3): δ 13.38 (s, 1H), 12.85 (s,
1H), 9.55 (s, 1H), 8.68 (s, 1H), 8.45 (t, 1H, J ) 2.0 Hz), 6.97 to
7.60 (d’s and t’s, 10H, J ) 8.0 Hz); EUK-172 ligand (CDCl3):
δ 13.46 (s, 1H), 12.91 (s, 1H), 9.50 (s, 1H), 8.67 (s, 1H), 8.43
(s, 1H), 6.90 to 7.53 (d’s and t’s, 9H, J ) 7.0 Hz), 3.97 (s, 3H),
3.92 (s, 3H); EUK-163 ligand (CDCl3): δ 8.44 (s, 2H), 6.86 to
6.98 (d’s and t’s, 6H, J ) 8.0 Hz), 3.93 (s, 6H), 3.75 to 3.89 (d
and m, 5H, J ) 4.7 Hz).
Elemental analyses of the final products were performed
by Canadian Microanalytical Services (Delta, BC, Canada),
and results were as follows: EUK-8, Anal. (C16H14ClMnN2O2,
0.25H2O) C, H, N; EUK-108, Anal. (C18H17MnN2O4, 0.5H2O)
C, H, N; EUK-113, Anal. (C20H21MnN2O6, 3.0H2O) C, H, N;
EUK-134, Anal. (C18H18ClMnN2O4, 3.2H2O) C, H, N; EUK-114,
Anal. (C20H21MnN2O6, 1.0H2O) C, H, N; EUK-115, Anal.
(C18H18ClMnN2O4, 2.5H2O) C, H, N; EUK-123, Anal. (C20H21N2-
MnO6, 2.5H2O) C, H, N; EUK-15, Anal. (C18H18ClMnN2O4,
1.2H2O) C, H, N; EUK-121, Anal. (C18H15F2MnN2O4, 2.5H2O)
C, H, N; EUK-122, Anal. (C16H12ClF2MnN2O2, 1.25H2O) C, H,
N; EUK-118, Anal. (C22H25MnN2O8, 0.75H2O) C, H, N; EUK-
124, Anal. (C20H22ClMnN2O6, 0.4H2O) C, H, N; EUK-189, Anal.
represent a class of catalase mimetics whose hydrogen
peroxide scavenging and cytoprotective activities can be
substantially manipulated. These findings support the
concept that compounds of this class, selected by
biochemical and biological criteria such as those de-
scribed here, have potential applications in the treat-
ment of ROS-associated diseases, including ischemic
tissue damage.
Exp er im en ta l Section
Ma ter ia ls. All chemicals, except for solvents, used in the
synthesis of salen-manganese complexes were purchased from
Aldrich Chemical Co. (Milwaukee, WI). All solvents used in
synthesis of the compounds were reagent grade and were used
without further purification and were obtained from either
Caledon Laboratories (Georgetown, Ontario, Canada) or Com-
mercial Alcohols (Toronto, Ontario, Canada). MnTMPyP was
purchased from Alexis Corporation (San Diego, CA). The XTT
reagent was obtained from Boehringer Mannheim, Inc. (In-
dianapolis, IN). All components of tissue culture media were
purchased from BioWhittaker (Walkersville, MD), and tissue
culture plasticware was from Corning (Corning, NY). Materials
and chemicals used for the rodent stroke experiments were
as described previously.23 All other chemicals were obtained
from Sigma Chemicals (St. Louis, MO).
(C22H25MnN2O6, 1.8H2O) C, H, N; EUK-160, Anal. (C22H25
-
Syn th esis a n d Ch a r a cter iza tion of Sa len -Ma n ga n ese
Com p lexes. EUK-8 and EUK-108 were prepared using a
published procedure,28 which was modified to produce the
other complexes. The bis(salicylaldehyde)ethylenediamine
(salen-H2)-substituted ligands were prepared by the addition
of 1 equiv of ethylenediamine in absolute ethanol to a solution
of 2 equiv of the substituted aldehyde in absolute ethanol
(0.05-0.2 M solution). For the bridge-modified salen-man-
ganese complexes, the corresponding diamine was substituted
for ethylenediamine and reacted with either o-vanillin or
salicylaldehyde in a similar manner. For EUK-159, the trans
(() cyclohexyldiamine was used. For all ligands, the precipitate
formed in the reaction was filtered, washed with ethanol, and
air-dried to give the desired ligand in 79-96% yield. One
equivalent of solid manganese(II) acetate tetrahydrate was
added to a stirred suspension of 1 equiv of the ligand in 95%
ethanol (0.025-0.03 M), either at ambient temperature or at
reflux, and the reaction was then stirred for 1-2 h. The dark
brown solutions were then evaporated to dryness under a
stream of air. The crude product, generally a brown or dark
green solid, was washed with acetone, filtered, and air-dried.
The products were obtained at hydrates in 62-92% yield. The
acetate complexes were converted to the corresponding chlo-
rides by treating an aqueous solution (0.03-0.06 M) of the
acetate, warmed to 50 °C, with 5 equiv of KCl dissolved in
distilled water. A brown or dark green precipitate immediately
formed. The suspension was cooled in an ice/water bath and
then filtered, and the solid was washed with water and
acetone. The products were obtained as hydrates in 66-78%
yield.
1H NMR spectra of the ligands were obtained on a Bruker
ARX 400 MHz instrument. The 1H NMR data obtained for each
ligand were as follows: EUK-113/134 ligand (CDCl3): δ 13.59
(bs, 2H), 8.35 (s, 2H), 6.8 to 6.92 (d’s and t, 6H, J ) 8.0 Hz),
3.98 (s, 4H), 3.90 (s, 6H); EUK-114/115 ligand (CDCl3): δ 13.70
(bs, 2H), 8.22 (s, 2H), 7.10 (d, 2H, J ) 8.0 Hz), 6.44 (s, 2H),
6.39 (d, 2H, J ) 8.0 Hz), 3.87 (s, 4H), 3.81 (s, 6H); EUK-15/
123 ligand (CDCl3): δ 12.71 (bs, 2H), 8.32 (s, 2H), 6.91 (m,
4H), 6.75 (s, 2H), 3.96 (singlet, 4H), 3.77 (s, 6H); EUK-121/
122 ligand (DMSO-d6): δ 13.94 (bs, 2H), 8.68 (s, 2H), 7.28 (m,
4H), 6.75 (m, 2H), 3.95 (s, 4H); EUK-118/124 ligand (CDCl3):
δ 14.20 (bs, 2H), 8.46 (s, 2H), 5.97 (s, 2H), 5.71 (s, 2H), 3.79
(s, 10H), 3.75 (s, 6H); EUK-160 ligand (CDCl3): δ 8.25 (s, 2H),
6.54 (s, 2H), 6.28 (s, 2H), 3.95 (s, 4H), 3.86 (s, 6H), 3.74 (s,
6H); EUK-189 ligand (CDCl3): δ 13.61 (s, 2H), 8.34 (s, 2H),
6.76 to 6.94 (dd and t, 6H, J ) 8.0 Hz), 4.13 (quartet, 4H, J )
7.0 Hz), 3.97 (s, 4H), 1.49 (t, 6H, J ) 7.0 Hz); EUK-159 ligand
(CDCl3): δ 13.8 (s, 2H), 8.26 (s, 2H), 6.73 to 6.89 (d’s and t,
MnN2O8, 2.8H2O) C, H, N; EUK-159, Anal. (C24H27MnN2O6,
2.4H2O) C, H, N; EUK-177, Anal. (C22H17MnN2O4, 0.5H2O) C,
H, N; EUK-178, Anal. (C24H21MnN2O6, 1.6H2O) C, H, N; EUK-
161, Anal. (C21H16MnN3O4) C, H, N; EUK-172, Anal. (C23H20
-
MnN3O6, 1.9H2O) C, H, N; EUK-163, Anal. (C21H23MnN2O7,
3.33H2O) C, H, N.
Ca ta la se Activity. Catalase activity was assayed by moni-
toring the conversion of hydrogen peroxide to oxygen using a
Clark type polarographic oxygen electrode as described previ-
ously.23 Salen-manganese complexes were tested at 10 µM
in reaction mixtures containing hydrogen peroxide at 10 mM,
and initial rates and total amount of oxygen produced were
determined as described.23 Control compounds were tested at
the appropriate equivalent concentrations, that is, 20 µM for
salicylaldehydes and 10 µM for Mn salts, ligand, and ethyl-
enediamine. Stock solutions of complexes and control com-
pounds for this and other catalytic assays were prepared in
water or methanol, depending on solubility. Where present,
at a final concentration that did not exceed 5%, methanol did
not affect assay activities. Blank reactions, containing all
components except salen-manganese complex, yielded rate
and end point values of 1 ( 34 and 3 ( 2, respectively, and
were not subtracted from the values obtained with compounds.
For convenience and sensitivity, pH 8.1 was selected for
catalase and peroxidase activity (below) assays. The use of
millimolar hydrogen peroxide concentrations in the catalase
assays was dictated by the sensitivity of the oxygen measure-
ment system. However, under different assay conditions
monitoring hydrogen peroxide disappearance, salen-manga-
nese complexes also effectively neutralize lower concentrations
of hydrogen peroxide at physiological pH, for example, under
conditions of the cytoprotection assay described below.
P er oxid a se Activity. Peroxidase activity was assayed by
monitoring the hydrogen peroxide-dependent oxidation of
ABTS spectrophotometrically. Assay mixtures consisted of 50
mM sodium phosphate, pH 8.1, 0.9% sodium chloride, 0.5 mM
ABTS, 0.2 mM hydrogen peroxide, and 10 µM salen-
manganese complex. Assays were conducted at 27 ( 0.2 °C.
ABTS oxidation was monitored at 740 nm to eliminate
interference by the salen-manganese complexes. The amount
of oxidized ABTS was estimated using an ∆ꢀ740 of 20 300 M-1
cm-1 calculated based upon the published molar extinction
41
coefficient at 414 nm.
There was no detectable ABTS
oxidation in the absence of salen-manganese complex or
hydrogen peroxide. The most active catalases, namely, EUK-
161, EUK-172, EUK-177, and EUK-178, showed little or no
activity in the peroxidase assay. This was attributed to rapid

Salen-Manganese Complexes as Catalytic Scavengers
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 20 4557
destruction of hydrogen peroxide via their high catalase
activity, as evidenced by the ability of these compounds to also
quench apparent peroxidase activity of another analogue such
as EUK-134, when added to the reaction mixture. Because of
this artifact, peroxidase activities of the bridge-modified series
of compounds were not included in Table 2.
this to be more reproducibly lethal in the HF cell system, in
serum-containing medium, than bolus administrations of
hydrogen peroxide. However, in other cellular systems, for
example, rat primary neurons cultured in defined medium,
salen-manganese complexes effectively protect against bolus-
administered hydrogen peroxide.35 To ensure that there were
no direct effects of salen-manganese complexes on glucose
oxidase, the enzyme activity was assayed in the presence or
absence of EUK-134 or EUK-8. Glucose oxidase was assayed
by monitoring hydrogen peroxide generation, using a modifica-
tion of a horseradish peroxidase-coupled spectrophotometric
assay described previously,45 with horseradish peroxidase in
excess to ensure that the peroxidase and/or catalase activities
of the salen-manganese complexes did not interfere with the
assay. Assay mixtures contained glucose oxidase (0.016 units/
mL), glucose (1 g/L), ABTS (0.5 mM), horseradish peroxidase
(77.5 units/mL), and sodium phosphate, pH 7.4, with and
without added EUK-8 or EUK-134 (200 µM). ABTS oxidation
was monitored at 740 nm, as described above for the peroxi-
dase assay. Reaction rates were readily detectable and did not
differ significantly in the presence or absence of salen-
manganese complexes.
SOD Activity. SOD activity was assayed by following the
inhibition of the reduction of an electron acceptor molecule in
the presence of the free radical generating system xanthine/
42
xanthine oxidase as described by McCord and Fridovich,
using assay conditions essentially as described by Faulkner
et al.,6 except for the omission of ethylenediaminetetraacetic
acid (EDTA), due to artifacts described previously.10 Assays
were conducted at 27 ( 0.2 °C using a water-jacketed cell
holder in a Beckman DU7400 spectrophotometer. Oxidized
cytochrome c (0.13 mg/mL) was employed as acceptor, and its
reduction was monitored spectrophotometrically at 550 nm.6
To compare the SOD activities of salen-manganese complexes,
their IC50 values were determined from concentration-depend-
ent plots generated from 5 to 7 different concentrations.
Control reactions to ensure that the compounds did not directly
inhibit xanthine oxidase were performed by monitoring con-
version of xanthine to urate at 292 nm 43 in reaction mixtures
lacking cytochrome c. Selected salen-manganese complexes
(EUK-8, EUK-134, EUK-15, EUK-115, EUK-122, EUK-172,
EUK-159, EUK-177, EUK-178, EUK-163, EUK-161, and EUK-
189) were tested for xanthine oxidase inhibition in this
manner. While three compounds (EUK-8, EUK-15, and EUK-
122) showed interference with the assay at 10 µM, the others
did not, showing no effect on the rate of urate production. It
was concluded that the apparent SOD activity exhibited by
these compounds was not attributable to an inhibition of
xanthine oxidase. To eliminate the possibility of artifactual
peroxide-dependent oxidation of cytochrome c interfering in
this assay,44 selected compounds (EUK-8, EUK-108, EUK-113,
EUK-15, EUK-114, EUK-115, EUK-121, EUK-122, EUK-123,
EUK-124, EUK-134, EUK-160, EUK-189, EUK-159, EUK-161,
EUK-163, EUK-172, EUK-177, and EUK-178) were also as-
sayed for SOD activity in the presence of bovine liver catalase
(767 units/mL). In all cases, this was found not to affect the
IC50 value or to significantly affect the rate of cytochrome c
reduction in the xanthine/xanthine oxidase assay.
Rod en t Str ok e Mod el. Middle cerebral artery occlusion
was performed in Sprague-Dawley rats, and brain infarct
volumes were quantitated 21 h after occlusion, as described
previously.23 Salen-manganese complexes were administered
by intravenous bolus injection 3 h after occlusion. The choice
of injection vehicle was determined by the solubility properties
of the compound. For most of the compounds, either normal
saline or 5% mannitol was used. For EUK-172, which has a
pH-dependent solubility profile in the physiological range, the
5% mannitol vehicle was buffered with 20 mM sodium
phosphate (pH 7.6) to prevent precipitation of the compound
upon injection. Infarct volumes were not different for vehicle
control animals receiving either of these vehicles (Table 3).
The data are expressed as percent infarct volume, calculated
by comparing the volume of infarcted brain tissue to the total
brain volume of each animal. (As reported previously,23 this
corresponded to a mean infarct volume of about 100 mm3 in
the control groups.) Because of animal-to-animal variability
in collateral circulation, success of the ligature, and other
factors, data from brains showing no detectable infarct were
omitted from analysis. Statistical significance was assessed
by Dunnett’s t test (p < 0.01), comparing to the appropriate
vehicle control group.
Cytop r otection Assa y. This assay was conducted as
described previously.23 Essentially, human dermal fibroblasts
(HF cells) were incubated in multiwell plates with the hydro-
gen peroxide-generating system glucose and glucose oxidase,
in the presence of various doses of salen-manganese com-
plexes or various control substances, as indicated in the
figures. Stock solutions of salen-manganese complexes or
control substances were prepared in water or nondenatured
ethanol. Ethanol, where present, did not exceed 5% and was
found to have no effect on cell viability. After 18 h, cells were
washed and cell viability was assessed spectrophotometrically
using the XTT reagent, a tetrazolium dye converted to a
soluble colorimetrically detectable product by respiring cells,
per manufacturer’s instructions. Cells were examined under
the microscope to confirm cytotoxicity readings as well as to
eliminate any samples containing precipitated test agents. The
amounts of glucose oxidase used in the cytoprotection assays
corresponded to approximately 0.02 units/mL (0.8-1.0 µg/mL).
Because of normal variations associated with cell culture
conditions, the concentration of salen-manganese complex
required to overcome cytotoxicity also varied among individual
experiments in the course of this study. Thus, to compare
potencies of compounds tested on different days, an internal
reference compound, EUK-134 or EUK-113, was included in
each assay. The concentration of these reference compounds
showing half-maximal cytoprotection (ED50) varied from 25 to
124 µM. To assess cytotoxicity of the compounds alone, cells
were incubated with salen-manganese complexes without
glucose oxidase and cell viability was assessed as described
above. The glucose and glucose oxidase system was utilized
for these studies in order to expose cells for a period of hours
to continuously generated hydrogen peroxide. We have found
Ack n ow led gm en t. This research was supported in
part by a grant to Eukarion from the National Institutes
of General Medical Sciences (R43 GM 57770).
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