Organometallic Derivatization of the Nematocidal Drug Monepantel Leads to Promising Antiparasitic Drug Candidates

Article abstract of DOI:10.1002/chem.201602851

The discovery of novel drugs against animal parasites is in high demand due to drug-resistance problems encountered around the world. Herein, the synthesis and characterization of 27 organic and organometallic derivatives of the recently launched nematocidal drug monepantel (Zolvix) are described. The compounds were isolated as racemates and were characterized by¹H,¹³C, and¹⁹F NMR spectroscopy, mass spectrometry, and IR spectroscopy, and their purity was verified by microanalysis. The molecular structures of nine compounds were confirmed by X-ray crystallography. The anthelmintic activity of the newly designed analogues was evaluated in vitro against the economically important parasites Haemonchus contortus and Trichostrongylus colubriformis. Moderate nematocidal activity was observed for nine of the 27 compounds. Three compounds were confirmed as potentiators of a known monepantel target, the ACR-23 ion channel. Production of reactive oxygen species may confer secondary activity to the organometallic analogues. Two compounds, namely, an organic precursor (3 a) and a cymantrene analogue (9 a), showed activities against microfilariae of Dirofilaria immitis in the low microgram per milliliter range.

Full text of DOI:10.1002/chem.201602851

DOI: 10.1002/chem.201602851
Full Paper
&
Medicinal Chemistry
Organometallic Derivatization of the Nematocidal Drug
Monepantel Leads to Promising Antiparasitic Drug Candidates
Jeannine Hess,[a] Malay Patra,[a] Loganathan Rangasamy,[a] Sandro Konatschnig,[a]
Olivier Blacque,[a] Abdul Jabbar,[b] Patrick Mac,[c] Erik M. Jorgensen,[c] Robin B. Gasser,*[b] and
Gilles Gasser*[a]
Abstract: The discovery of novel drugs against animal para-
sites is in high demand due to drug-resistance problems en-
countered around the world. Herein, the synthesis and char-
acterization of 27 organic and organometallic derivatives of
the recently launched nematocidal drug monepantel
(Zolvixꢀ) are described. The compounds were isolated as
analogues was evaluated in vitro against the economically
important parasites Haemonchus contortus and Trichostrongy-
lus colubriformis. Moderate nematocidal activity was ob-
served for nine of the 27 compounds. Three compounds
were confirmed as potentiators of a known monepantel
target, the ACR-23 ion channel. Production of reactive
oxygen species may confer secondary activity to the organo-
metallic analogues. Two compounds, namely, an organic pre-
cursor (3a) and a cymantrene analogue (9a), showed activi-
ties against microfilariae of Dirofilaria immitis in the low mi-
crogram per milliliter range.
1
racemates and were characterized by H, 13C, and 19F NMR
spectroscopy, mass spectrometry, and IR spectroscopy, and
their purity was verified by microanalysis. The molecular
structures of nine compounds were confirmed by X-ray crys-
tallography. The anthelmintic activity of the newly designed
now quite widespread.[1a,3] Therefore, the development of new
drugs or chemical modification of existing drugs is crucial to
ensure sustainable chemical control of parasites in the future.
In this context, the discovery of the structurally new amino-
acetonitrile class of synthetic antiparasitic compounds (AADs,
see Figure 1) by Novartis Animal Health was a major break-
through in 2008.[4] An extensive structure–activity relationship
(SAR) study resulted in the development of monepantel (AAD
1566, Figure 1), which was released under the trade name
Zolvixꢀ in 2009 for the treatment of nematode infections in
sheep.[5]
Introduction
Multicellular parasites, including roundworms (nematodes),
flatworms (trematodes and cestodes), and arthropods (e.g.,
fleas, flies, and ticks), cause morbidity and mortality in animals
worldwide,[1] which result in a substantial loss to global food
production annually.[1b,g] The control of parasites relies largely
on the use of antiparasitic drugs.[2] However, drug resistance is
[a] J. Hess, Dr. M. Patra, Dr. L. Rangasamy, S. Konatschnig, Dr. O. Blacque,
Prof. Dr. G. Gasser
Department of Chemistry, University of Zurich
Winterthurerstrasse 190, 8057 Zurich (Switzerland)
E-mail: gilles.gasser@chem.uzh.ch
[b] Dr. A. Jabbar, Prof. Dr. R. B. Gasser
Faculty of Veterinary and Agricultural Sciences
The University of Melbourne
Parkville, Victoria 3010 (Australia)
E-mail: robinbg@unimelb.edu.au
[c] P. Mac, Dr. E. M. Jorgensen
Howard Hughes Medical Institute
Department of Biology, University of Utah
Salt Lake City, UT 84112-0840 (USA)
Figure 1. General structure of amino-acetonitrile derivatives (AADs) and
monepantel (AAD 1566).
Supporting information and ORCID number from the author for this article
are available on the WWW under http://dx.doi.org/10.1002/
chem.201602851. It contains general remarks (materials, instrumentation
and methods, X-ray crystallography, cell culture, cytotoxicity studies, ROS
generation, biological assays, electrophysiology methods), synthesis and
characterization of 3a–3n, 5b–5k, 7a, 7b, 8c, 9a, 9b, NMR spectra, mo-
lecular structures of 3a, 3b, 3g, 5a, 5c, 5h, and 5k, crystal data and
structure refinement for 3a, 3b, 3 f, 3g, 5a, 5c, 5d, 5h, and 5k, antipara-
sitic activity against C. felis, L. cuprina, and R. sanguineus of organometal-
lic derivatives 3a–3n, 5a–5k, 7a, 7b, 9a, and 9b), and1H NMR spectra of
3a and 9a at different time intervals.
Monepantel targets ligand-gated ion channels in the nema-
tode-specific DEG-3 family.[4] These channels are related to nic-
otinic acetylcholine receptors, but they are gated by betaine
and choline rather than acetylcholine.[6] Monepantel acts on
MPTL-1 receptors in Haemonchus contortus and the homo-
logues ACR-20 and ACR-23 in the free-living nematode Caeno-
rhabditis elegans. The drug acts as a positive allosteric modula-
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tor, hyperactivating these channels.[6c,7] An in vitro selection
procedure by Rufener et al. revealed that monepantel resist-
ance in H. contortus can develop relatively rapidly by a single
loss-of-function mutation. Indeed, monepantel-resistant worms
in livestock were reported within a few years.[3d,e,8]
vivo activity of monepantel.[18] Therefore, with the intention to
keep the benzamide part in place, we designed our first organ-
ometallic monepantel analogues by replacing the aryloxy part
of monepantel with a ferrocenyl unit (Scheme 1). The ferro-
cene/ferrocenium system has redox properties favorable for
production of ROS, which can improve toxicity, as shown for
ferroquine and ferrocifen.[11b,19] Subsequently, we employed
two different strategies to study the SAR of monepantel ana-
logues. In Strategy I, a series of organic and organometallic de-
rivatives were synthesized in which the functional group at the
4-position of the benzamide moiety was varied (Scheme 1). In
Strategy II (Scheme 1), we designed organic and organometal-
lic derivatives of 5a in order to evaluate the importance of the
organometallic moiety. For this purpose, the ferrocenyl subunit
was swapped with two different organometallic moieties,
namely, ruthenocene and cymantrene. Replacing the FeII core
with a comparatively inert RuII center may allow us to probe
the relevance of FeII/FeIII-mediated redox activity of the com-
pound. By contrast to ferrocene and ruthenocene, cymantrene
conjugates of organic drugs are rarely studied for their antipar-
asitic potency.[20] The metal center in cymantrene is isoelec-
tronic to that of ferrocene and the two metallocenes are nearly
isosteric. However, they have different redox behavior, and the
metal centers are in +1 and +2 oxidation states in cyman-
trene and ferrocene, respectively. Therefore, comparison of
these metallocenyl analogues should illuminate any metal-
mediated modes of action of our compounds.
It is of increasing importance to search for structurally or
functionally unique classes of compounds to control para-
sites.[3a] To date, this quest has been focused mainly on organic
compounds, but metal-based compounds have had an impres-
sive success in other fields of medicinal chemistry.[9] Specifically,
recent efforts have identified anticancer,[10] antimalarial,[11] anti-
trypanosomal,[12] antibacterial,[13] and antischistosomal[14] com-
pounds. Organometallic complexes (i.e., compounds with at
least one direct metalꢀcarbon bond) are a class of metal com-
pounds that have shown promise as anti-infectives.[13,15] Ferro-
quine, the ferrocenyl analogue of the antimalarial drug chloro-
quine, is the most prominent example of an organometallic
drug candidate used against parasites. The introduction of
a ferrocenyl moiety alters the mode of action of the parent
drug and renders it active against chloroquine-resistant Plas-
modium falciparum strains.[11a] Similarly, an altered mode of
action was evident in the ferrocenyl derivative of the anticanc-
er drug tamoxifen, named ferrocifen. Whereas tamoxifen is
only active against estrogen-positive breast cancers, ferrocifen
is active against both estrogen-negative and estrogen-positive
cancers.[16] To the best of our knowledge, organometallic com-
plexes have rarely been tested for nematocidal activity.[17]
With the aim of developing new classes of nematocidal
drugs, we recently initiated a program to design and test or-
ganometallic analogues of the organic anthelmintic monepan-
tel. The aryloxy unit of the original molecule was substituted
with sandwich (ferrocene, ruthenocene) and half-sandwich
(cymantrene) organometallic moieties in order to introduce
metal-specific modes of action, as well as with various organic
moieties. The functional groups at the benzamide unit of the
organometallic and organic analogue were further varied
(SCF3, OCF3, CF3, F, Cl, Br, I, etc.). The presence of different func-
tional groups is expected to modulate the lipophilicity, biodis-
tribution, and pharmacokinetic properties, thereby influencing
the biological properties of the newly designed monepantel
analogues. These modifications resulted in a library of 27 or-
ganic/organometallic derivatives of monepantel for SAR stud-
ies. The initial biological screenings reported herein demon-
strate that some of the compounds retain activity against
monepantel targets, while gaining novel properties such as
the production of reactive oxidative species (ROS). Additionally,
we demonstrate for the first time that AADs, including our or-
ganic and organometallic analogues, show activity against Dir-
ofilaria immitis microfilariae.
Synthesis and characterization
Organometallic analogues 5a–5h were prepared in a two-step
procedure as outlined in Scheme 2. The central core, 2-amino-
2-hydroxymethylproprionitrile (1), was synthesized by follow-
ing the procedure described by Gauvry et al.[21] In a subsequent
reaction, the amide bond between 1 and the commercially
available compounds 2a–2h bearing a chlorocarbonyl moiety
was formed. Initially, we attempted to synthesize 3a by follow-
ing the procedure of Gauvry et al.[21] However, instead of the
desired compound, two different products, namely 2-cyano-2-
{4-[(trifluoromethyl)thio]benzamido}propyl acetate (3l) and 2-
cyano-2-{4-[(trifluoromethyl)thio]benzamido}propyl 4-[(trifluoro-
methyl)thio]benzoate (3m) were isolated in yields of 21% and
3%, respectively (Scheme 2). Therefore, we improved the pro-
tocol by changing the base from sodium hydroxide to triethyl-
amine, which resulted in the formation of the desired 3a in
74% yield. By following the same procedure, compounds 3b–
3h were isolated in moderate to good yields. Compound 3a
was further treated with benzoyl chloride under basic condi-
tions, and compound 3n was obtained in 86% yield. Finally,
esterification of 3a–3h with ferrocenecarboxylic acid (4) yield-
ed the desired compounds 5a–5h as yellow or orange solids.
Although it was shown that the efficacy of AADs against nem-
atodes is enantioselective, we decided to perform the synthe-
sis and biological evaluation with the organometallic race-
mates (5a–5h). Lower potency is expected for racemic mix-
tures rather than enantiopure compounds, but this approach
provided an initial estimation of potency.
Results and Discussion
Design of organometallic analogues
Monepantel consists of an aryloxy and a benzamide unit con-
nected by a chiral C2 spacer. Preliminary metabolism studies in-
dicated that the benzamide unit plays a crucial role in the in
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Scheme 1. Schematic representation of the design of organometallic derivatives based on the lead structure of AAD 1566 (monepantel).
Scheme 2. Reagents and conditions: i) NEt3, dry CH2Cl2, 1.5 h–24 h, r.t., 26% ꢀ74%. ii) a) Ferrocenecarboxylic acid (4), oxalyl chloride, dry CH2Cl2, r.t.; b) NEt3,
dry CH2Cl2, overnight (o.n.), r.t., 25%– 94% after two steps. iii) 4-(Trifluoromethylthio)benzoyl chloride, dry ethyl acetate, 1m NaOH, 3 h, r.t., 21% (3l) and 3%
(3m). iv) Benzoyl chloride, NEt3, dry CH2Cl2, 2 h, r.t. 86% (3n).
Monepantel is rapidly metabolized in vivo to the sulfone de-
rivative, with the corresponding sulfoxide as an intermedi-
ate.[18,22] Because these are active metabolites,[22] we synthe-
sized the sulfone and sulfoxide derivatives of 5a (5j and 5k,
Scheme 3). For this purpose, the SCF3 functionality of 3a was
selectively oxidized with different equivalents of meta-chloro-
peroxybenzoic acid (m-CPBA) at ꢀ788C. The resulting organic
precursors 3j and 3k were isolated as colorless solids in mod-
erate yields. Esterification of 3j or 3k with 4 yielded the final
ferrocenyl-based sulfoxide and sulfone derivatives, 2-cyano-2-
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Scheme 3. Reagents and conditions: (i) m-CPBA, dry CH2Cl2, ꢀ788C!r.t.,
45% (3j) and 43% (3k). ii) a) Oxalyl chloride, dry CH2Cl2, r.t.; b) NEt3, dry
CH2Cl2, o.n., r.t., 70% (5j) and 87% (5k) over two steps.
{4-[(trifluoromethyl)sulfinyl]benzamido}propyl ferrocenoate (5j)
and 2-cyano-2-{4-[(trifluoromethyl)sulfonyl]benzamido}propyl
ferrocenoate (5k, Scheme 3).
Scheme 5. Reagents and conditions: i) 4-(Trifluoromethylthio)benzoyl chlo-
ride, NEt3, dry THF, 16 h, r.t., 32%. ii) 3-Fluoro-4-(trifluoromethyl)benzonitrile,
NaH, dry THF, 08C!r.t., 36 h, 16%.
The ferrocenyl moiety of 5a was replaced with two different
organometallic moieties to assess their effect on the anthel-
mintic activity. In contrast to the ferrocenyl derivative 5a, the
cymantrene derivative 9a and ruthenocene derivative 9b
(Scheme 4) are not expected to produce ROS. Compounds 9a
and 9b were synthesized by following the synthetic pathway
outlined in Scheme 4. Initially, cymantrene carboxylic acid (8a)
and ruthenocenyl carboxylic acid (8b) were synthesized ac-
cording to published procedures.[23] Whereas 8a was linked to
3a by a one-step Steglich esterification reaction, 8b required
activation with oxalyl chloride and then reaction with 3a.
tallic precursors ferrocenylmethylamine (6a) and hydroxyme-
thylferrocene (6b) were synthesized following published
procedures.[24] Compound 7a was then readily prepared by
amide bond formation of 6a with commercially available 4-(tri-
fluoromethylthio)benzoyl chloride under alkaline conditions
and N-ferrocenyl-4-(trifluoromethylthio)benzamide (7a) was
isolated as a bright yellow solid. 3-(Ferrocenyloxy)-4-(trifluoro-
methyl)benzonitrile (7b) was isolated after an aromatic nucleo-
philic substitution reaction with the ferrocenyl alcohol
(Scheme 5).
All new compounds were unambiguously characterized by
1H, 13C, and 19F NMR spectroscopy, mass spectrometry, and IR
spectroscopy, and their purities were verified by elemental
analysis (see Experimental Section and Supporting Information
for further details). The 13C NMR spectra of the compounds
containing a fluorine atom (3a, 3b, 3g, 3h, 3j, 3k, 3l, 3m,
3n, 5a, 5b, 5g, 5h, 5j, 5k, 7a, 7b, 9a, and 9b) showed a char-
acteristic coupling to the adjacent carbon atom resulting in
a quadruplet splitting pattern. Most of the derivatives were de-
tected mainly as their [M+H]+ species by ESI mass spectrome-
try (positive detection mode) with traces of [M+Na]+ and
[M+K]+ species present.
X-ray crystallography
The structures of compounds 3a, 3b, 3 f, 3g, 5a, 5c, 5d, 5h,
and 5k were further corroborated by single-crystal X-ray dif-
fraction studies. Example structures of 3 f and 5d are shown in
Figure 2 (see Figure S1–S3 in the Supporting Information for
the other structures). The X-ray diffraction studies confirmed
the formation of the amide bond between the 2-amino-2-hy-
droxymethylproprionitrile species 1 and the substituted com-
pounds 2a–2h through the carbonyl group to form the mone-
pantel analogues. The reported crystal structure of monepantel
contains four crystallographically independent molecules of
the S enantiomer, which only differ from each other in the ori-
Scheme 4. Reagents and conditions: i) 3a, dicyclohexylcarbodiimide, 4-dime-
thylaminopyridine, dry Et2O/CH2Cl2, 24 h, 08C!r.t., 74%. ii) a) Oxalyl chloride,
dry CH2Cl2; b) 3a, NEt3, dry CH2Cl2, r.t., o.n., 41% after two steps.
To assess the importance of the chiral C2 spacer between
the aryloxy and benzamide units, two additional organometal-
lic derivatives were synthesized. Compounds 7a and 7b con-
tain benzamide and aryloxy units, respectively, which are con-
nected to a ferrocenyl moiety via an amide or an ether func-
tionality, rather than the C2 spacer (Scheme 5). The organome-
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Figure 2. Molecular structure of compounds 3 f (left) and 5d (right) with atomic numbering schemes.
entation of the terminal SCF3 group relative to the rest of the
molecule.[5] The central part of the monepantel enantiomer ex-
hibits a Z-type conformation between the chiral carbon atom
C* and the carbonyl oxygen atom (on the same side of the
central CꢀN bond), together with an E-type conformation be-
tween the carbonyl carbon atom and the methyl group (on
opposite sides of the NꢀC* bond) along the (O=)CꢀN(H)ꢀC*ꢀ
CH3 moiety. Indeed, the O-C-N-C* torsion angles in the inde-
pendent molecules are close to 08 (Z-type), between 1.9(5) and
5.7(5)8. This very narrow range was expected because of the
pronounced double-bond character of the central bond, due
to electron delocalization with the carbonyl group (as revealed
by relatively short CꢀN bond lengths in the range 1.355(4)–
66(5) ꢂ). Despite the single-bond character of the NꢀC* bond
(1.452(6)–62(5) ꢂ), the C-N-C*-C torsion angles also lie in
a narrow range close to 1808 (E-type; 165.4(4)–169.0(4)8).
metric space group Pna21, with two crystallographically inde-
pendent S and R enantiomer molecules in the asymmetric unit
in a 1:1 ratio.
Activity against H. contortus and Trichostrongylus colubrifor-
mis
H. contortus and T. colubriformis are common parasitic nemato-
des of ruminants and are responsible for major economic
losses on farms worldwide.[1e] H. contortus is located in the
abomasum (stomach) of the host, where it feeds on blood,
whereas T. colubriformis affects the anterior small intestine.[1e]
Since these species can coexist in similar climatic regions and
co-infect animals, it is desirable to have anthelmintics that effi-
ciently kill both parasites.[1e] Here, all organic precursors and or-
ganometallic analogues of monepantel were screened for their
activities against these two parasites in a larval development
assay (LDA, see Supporting Information for details), and the re-
sults are summarized in Tables 1 and 2.
At the highest concentration tested (10 mgmLꢀ1), nine of the
27 compounds showed moderate activities against H. contortus
and T. colubriformis, with EC60 values ranging from 1.80 to
9.50 mgmLꢀ1 (3.30–31.20 mm). The potencies of our compounds
were lower than that of the parent compound monepantel
against H. contortus and T. colubriformis.[5] Most of the organic
intermediates 3a–3k were not active. However, the insertion
of the ferrocenyl moiety generally increased activity, as ob-
served for compounds 5a, 5e, 5h, and 5k. Specifically, 3a, the
organic precursor of 5a, had EC60 values of 9.50 mgmLꢀ1
(31.20 mm) and >10.00 mgmLꢀ1 (>32.90 mm) against H. contor-
tus and T. colubriformis, respectively; by contrast, 5a showed
EC60 values of 2.10 mgmLꢀ1 (4.07 mm) and 6.30 mgmLꢀ1
(12.20 mm), respectively. Within the series of halide-contain-
ing organometallic compounds (5b–5e), 5e, with an
iodo substituent, showed an EC60 of 4.5 mgmLꢀ1 (8.30 mm)
against H. contortus. However, this compound was inactive
(>10 mgmLꢀ1/>18.45 mm) against T. colubriformis; 5e was the
only analogue shown to be active against H. contortus but not
against T. colubriformis.
The Z-type conformation is observed in all our new amino-
acetonitrile derivatives. The largest O-C-N-C* torsion angle was
found in the crystal structure of 3a with a value as small as
8.6(2)8. More interestingly, the E-type conformation is also gen-
erally observed in our compounds: all C-N-C*-C torsion angles
lie in the range 160.4(3)–179.0(1)8, except for 3a with 72.9(2),
62.0(2)8 and 5k with 59(2)/68.0(6)8 (two-component disorder).
The crystal structures of 5c, 5d, 5 f, 5g, and 5k confirmed co-
ordination of the organic species on a ferrocenyl moiety to
form a carboxyl group. The CO2 fragment is essentially copla-
nar with the five-membered ring, and the CꢀC bond between
the ring and the carboxyl group lies in the normal range for
a
C(sp2)ꢀC(sp2) single bond (from 1.459(2) ꢂ for 5k to
1.484(6) ꢂ for 5 f). The relative orientation of the rest of the
molecule with the ferrocenyl moiety mainly depends on the in-
termolecular interactions that link the molecules in the solid
state, mainly CꢀH···O, CꢀH···N, and NꢀH···O hydrogen bonds,
as observed in the crystal structures of the organic molecules
3a, 3b, 3 f, and 3g. All compounds except 3g crystallized in
triclinic, monoclinic, or orthorhombic centrosymmetric space
groups, which indicates the presence of both R and S enantio-
mers (racemates) in the crystals. Compound 3g crystallized in
a racemic crystal structure as well, but in the noncentrosym-
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Table 1. Activity of organic intermediates 3a–3n against Haemonchus contortus and Trichostrongylus colubriformis in an LDA and against Dirofilaria immitis
microfilariae in vitro. The experiments were performed in two series of triplicate dose responses leading to six individual measurements. EC values were
calculated from the means of these individual measurements. The experimental errors are not reported, as they are considered too low to influence the
overall EC values.
Compound
EC60 value
T. colubriformis
[mgmLꢀ1
EC50 value
D. immitis, 48 h
[mgmLꢀ1
H. contortus
D. immitis, 24 h
[mgmLꢀ1
]
[mm]
]
[mm]
[mgmLꢀ1
]
[mm]
]
[mm]
9.50
31.20
>10.00
>32.90
>10.00
>32.90
0.49
1.61
3a
>10.00
>10.00
>10.00
>10.00
>10.00
>10.00
>10.00
>45.00
>41.90
>35.32
>30.29
>39.94
>36.73
>34.70
>10.00
>10.00
>10.00
>10.00
>10.00
>10.00
6.60
>45.00
>41.90
>35.32
>30.29
>39.94
>36.73
22.89
>10.00
>10.00
>10.00
>10.00
>10.00
>10.00
>10.00
>45.00
>41.90
>35.32
>30.29
>39.94
>36.73
>34.70
>10.00
6.60
>45.00
27.65
3b
3c
3d
3e
3 f
>10.00
>10.00
>10.00
>10.00
>10.00
>35.32
>30.29
>39.94
>36.73
>34.70
3g
3h
>10.00
>31.22
>10.00
>31.22
>10.00
>31.22
6.60
20.61
3j
>10.00
3.00
>29.74
8.66
>10.00
4.00
>29.74
11.55
>10.00
>10.00
>10.00
>29.74
>28.88
>19.67
>10.00
6.60
>29.74
19.06
3k
3l
>10.00
>19.67
>10.00
>19.67
6.60
12.98
3m
4.20
10.28
5.80
14.20
0.07[a]
>10.00
>24.49
>10.00
>24.49
3n
0.01[a]
0.022[a]
0.032[a]
2.40
5.25
2.20
4.81
AAD 85
0.01[a]
0.021[a]
0.001[a]
0.032[a]
0.01[a]
0.07[a]
n.d.i.[b]
n.d.i.
n.d.i.
n.d.i.
AAD 96
ivermectin
0.001[a]
0.001[a]
1.00–3.00
1.14-3.43
1.00–3.00
1.14-3.43
[a] EC100 value.[5] [b] n.d.i.=non-disclosable information.[25]
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Table 2. Activity of organometallic analogues of monepantel 5a–5h, 7a, 7b, 9a and 9b against Haemonchus contortus and Trichostrongylus colubriformis
in an LDA and against Dirofilaria immitis microfilariae in vitro. The experiments were performed in two series of triplicate dose responses leading to six in-
dividual measurements. EC values were calculated from the means of these individual measurements. The experimental errors are not reported, as they
are considered too low to influence the overall EC values.
Compound
EC60 value
T. colubriformis
[mgmLꢀ1
EC50 value
D. immitis, 48 h
[mgmLꢀ1
H. contortus
D. immitis, 24 h
[mgmLꢀ1
]
[mm]
]
[mm]
[mgmLꢀ1
]
[mm]
]
[mm]
2.10
4.07
6.30
12.20
>10.00
>19.37
>10.00
>19.37
5a
>10.00
>10.00
>10.00
4.50
>23.03
>22.19
>20.20
8.30
>10.00
>10.00
>10.00
>10.00
>23.03
>22.19
>20.20
>18.45
>10.00
>10.00
>10.00
>10.00
>23.03
>22.19
>20.20
>18.45
6.60
6.60
4.20
6.60
15.20
14.64
8.48
5b
5c
5d
12.17
5e
5 f
>10.00
>10.00
3.00
>21.63
>20.65
6.00
>10.00
>10.00
7.00
>21.63
>20.65
14.00
>10.00
>10.00
>10.00
>21.63
>20.65
>20.00
>10.00
6.60
>21.63
13.63
5g
5h
>10.00
>20.00
>10.00
>18.79
>10.00
>18.79
>10.00
>18.79
>10.00
>18.79
5j
1.80
3.30
4.60
8.40
>10.00
2.00
>18.24
5.25
>10.00
2.10
>18.24
5.10
5k
7a
6.00
14.31
>26.00
8.30
19.79
>26.00
>10.00
>10.00
>10.00
>26.00
>10.00
>26.00
7b
>10.00
>18.71
>10.00
>18.71
>10.00
>18.71
1.80
3.37
9a
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Table 2. (Continued)
Compound
EC60 value
T. colubriformis
EC50 value
D. immitis, 48 h
H. contortus
D. immitis, 24 h
[mgmLꢀ1
]
[mm]
[mgmLꢀ1
]
[mm]
[mgmLꢀ1
]
[mm]
[mgmLꢀ1
]
[mm]
>10.00
>17.80
>10.00
>17.80
>10.00
>17.80
>10.00
>17.8
9b
0.01[a]
0.022[a]
0.032[a]
0.07[a]
2.40
5.25
2.20
4.81
AAD 85
0.01[a]
0.021[a]
0.001[a]
0.032[a]
0.01[a]
0.07[a]
0.01[a]
n.d.i.[b]
n.d.i.
n.d.i.
n.d.i.
AAD 96
ivermectin
0.001[a]
1.00–3.00
1.14-3.43
1.00–3.00
1.14-3.43
[a] EC100 value.[5] [b] n.d.i.=non-disclosable information.[25]
Exchange of the SCF3 group in 5a with an OCF3 group in 5h
led to a slight decrease in activity against both parasites (EC60
increased by 43% for H. contortus and by 11% for T. colubrifor-
mis). Oxidizing the SCF3 group of 5a to its sulfone in 5k in-
creased the activity slightly (EC60 decrease of 14% for H. con-
tortus and 27% for T. colubriformis). However, the sulfoxide 5j
had no activity at the highest concentration tested (>
10 mgmLꢀ1, >18.79 mm). Overall, the order of potencies for
substituents at the benzamide unit of the active analogues of
5a is S(O)2CF3 ꢁSCF3 ꢁOCF3 >I.
gans expressed in oocytes of Xenopus laevis. Monepantel acts
as a positive allosteric modulator, increasing ion channel cur-
rent when it is co-applied with an agonist.[6a,c,7] To quantify the
potentiation of this channel by our compounds, we co-applied
them with the agonist betaine (300 mm, ꢂEC10) and compared
the current with control recordings for betaine alone.
All three compounds showed decent efficacies in this assay.
The least potent and efficacious compound was 3l, with an
EC50 of 9 mm and maximal potentiation of ninefold compared
with the current induced by betaine alone. The potency and
efficacy of compounds 3a and 5a were similar; 3a had an EC50
of 2.1 mm, with maximal potentiation of 30-fold, and 5a had an
EC50 of 1.2 mm, with maximal potentiation of 32-fold (Figure 3).
The less than twofold difference in EC50 values between these
two compounds is negligible, and seems to result mostly from
a considerable difference in the slope of the response curves
(Hill coefficient: 2.1 for 3a and 11.5 for 5a). The steep slope of
the curve for 5a could be caused by limited aqueous solubility
affecting the measurements at higher concentrations.
Replacement of the ferrocenyl moiety of 5a with a cyman-
trenyl and ruthenocenyl unit in 9a and 9b, respectively, ren-
dered the compound inactive against H. contortus and T. colu-
briformis. The inactivity of 9b, in particular, suggests that 5a
may have a partial metal-coupled mode of action. Based on
previous biological studies on ferrocenyl and ruthenocenyl de-
rivatives,[11b,16b,19a,26] it seems reasonable to speculate that the
ferrocene moiety of 5a, but not the ruthenocene moiety in
9b, can cause ROS generation inside the parasite, and that this
ROS is at least partly responsible for the activity of 5a. Indeed,
we found that ROS generation by 5a is significantly higher
than that of 9b, as described below.
The efficacy of these three compounds potentiating ACR-23
channels follows the order 5aꢂ3a>3l. This efficacy differs
from that against H. contortus and T. colubriformis, which fol-
lows the order 5aꢂ3l>3a. The marked efficacy of 3a when
assayed on the ion channel demonstrates that the ferrocenyl
group of 5a and the corresponding aryloxy group of mone-
pantel are not essential for receptor potentiation. Therefore,
the aryloxy group of monepantel is likely to contribute to fa-
vorable pharmacokinetic properties or pharmacological poten-
cy, rather than efficacy. The ferrocenyl group of 5a either par-
tially recapitulates these properties, or confers novel toxic
properties. These results are consistent with the differences in
antiparasitic activity of compounds 5a and 9b, and indicate
that redox-mediated toxic properties of ferrocene may contrib-
ute to the toxicity of 5a.
Potentiation of ion-channel current
The ferrocenyl compound 5a was more efficacious against H.
contortus and T. colubriformis than its organic counterpart 3a.
This difference may arise from the ability of the ferrocenyl
compound to potentiate the ion channels better than the or-
ganic counterpart. Alternatively, the presence of the hydropho-
bic ferrocenyl entity in 5a might enhance the accumulation of
the compound in the worm’s tissues, or impart a secondary
function, such as ROS production. To test whether better po-
tentiation of ion channel currents contributed to the increased
efficacy of 5a, we compared the ability of compounds 3a, 3l,
and 5a to potentiate the monepantel target ACR-23 of C. ele-
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Figure 4. Level of ROS in HeLa cells treated with 3a, 5a, 9a and 9b. TBH:
tert-butyl hydroperoxide.
which results from the attachment of a ferrocenyl moiety to
3a, produced a 2.4-fold higher level of ROS than organic pre-
cursor 3a alone. ROS induced by the ruthenocene analogue
9b was significantly (P<0.001) lower than for 5a. This differ-
ence in ROS production might be one of the factors responsi-
ble for the difference in potency between 5a and 9b against
H. contortus and T. colubriformis (Figure 4). Additionally, we
evaluated ROS production following exposure to the cyman-
trene analogue 9a. The ROS level induced by 9a was compara-
ble to its organic precursor (3a).
Figure 3. Potentiation of ACR-23 channels expressed in Xenopus laevis
oocytes. A) Compound 3l. B) Compound 3a. C) Compound 5a. Left: concen-
tration-response curves. Insets indicate the chemical groups that are differ-
ent between the three compounds. x-Fold potentiation represents the cur-
rent after 1 min of sustained co-application with betaine (300 mm), normal-
ized to the current from applying betaine alone. n=5–8 recordings from in-
dependent oocytes. Error bars are standard errors of the mean. Right: repre-
sentative current traces from co-application of AAD compounds with the
agonist betaine.
Activity against filarial nematodes
As organometallic derivatization was shown previously to
modulate the activity profiles of organic drugs,[11b,16b,26] we as-
sayed the activity of our new class of anthelmintics against
other parasites. We were eager to assess the activity of our
compounds on taxonomically and biologically disparate
groups of parasites, including D. immitis (canine heartworm),
Ctenocephalides felis (cat flea), Lucilia cuprina (blow fly), and
Rhipicephalus sanguineus (brown dog tick). Neither AAD 85 nor
the compounds synthesized in this study had activity (at the
highest concentration tested) on C. felis (100 mgmLꢀ1), L. cupri-
na (32 mgmLꢀ1), or R. sanguineus (100 mgmLꢀ1 and
640 mgmLꢀ1) (Supporting Information, Table S4). Compound
5a and four other compounds effective against H. contortus
and T. colubriformis in an LDA (3h, 3n, 5h, and 5k) exhibited
no activity against D. immitis in a motility assay. However, 12
out of 27 compounds showed considerable activity against D.
immitis microfilariae, in a similar range to that of the standard
anti-heartworm agent ivermectin.[28] The organic precursor 3a
was extremely active against this parasite, with an EC50 value
of 0.49 mgmLꢀ1 (1.61 mm) after 48 h. Other organic precursors
(3c, 3j, 3l, and 3m) had lower activities, with EC50 values of
6.60 mgmLꢀ1 (12.98–27.65 mm). The activities of organometallic
derivatives were moderate; 5b, 5c, 5d, 5e, 5g, and 7a
showed EC50 values of 2.00–6.60 mgmLꢀ1 (5.10–15.20 mm). Inter-
estingly, the organometallic analogue 7a, bearing only the
benzamide unit and lacking the C2 spacer, was the only com-
Production of ROS
The rationale behind the design of our organometallic-contain-
ing monepantel derivatives was the introduction of metal-spe-
cific modes of action into the organic drug. For example, we
envisaged that the redox properties of the ferrocene/ferroceni-
um system in 5a might lead to the production of toxic ROS
and thus improve antiparasitic activity of the organic drug.[27]
As discussed earlier, 5a exhibited moderate activity against H.
contortus and T. colubriformis, but ruthenocenyl analogue 9b is
inactive. To confirm that our compounds have the predicted
redox properties, we assessed ROS induced by selected com-
pounds (3a, 5a, 9a, and 9b) in living cells. Although it would
have been ideal to investigate ROS production in parasites, for
practical and technical reasons, we used a mammalian cervical
cancer cell line (HeLa). To this end, cells were treated with dif-
ferent compounds (25 mm for 20 h), and the intracellular ROS
was quantified with the oxidation-sensitive fluorescent indica-
tor 2’,7’-dichlorofluorescein diacetate (H2DCFDA); tert-butyl hy-
droperoxide (TBH) was employed as a positive control (100 mm
for 6 h). The ROS level in cells treated with 3a was similar to
that of the untreated control cells. By contrast, compound 5a,
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pound that was active against all three parasitic nematodes
tested (i.e., H. contortus, T. colubriformis, and D. immitis); 7a is
also the only compound that displayed activity after only 24 h.
The replacement of ferrocene in 5a with a cymantrene unit
makes the resultant analogue 9a active against D. immitis,
with an EC50 value of 1.80 mgmLꢀ1 (3.37 mm).
Because AAD activity against filarial nematodes has not
been reported previously, we decided to also test a close ana-
logue of monepantel, namely, AAD 85. This compound has ac-
tivity similar to that of 7a; it is less potent than 3a, but acts
within 24 h. The D. immitis genome does not appear to
encode a close homologue of the receptors targeted by mone-
pantel, which suggests that this species has a novel AAD
target. Moreover, the lack of a correlation between ROS pro-
duction and activity against D. immitis indicates that the activi-
ty against D. immitis is not specifically caused by ROS.
pounds (3a, 7a and 9a). Among these three most active com-
pounds, 7a is a unique candidate showing activity against all
three parasitic nematodes. The SAR of compounds active
against H. contortus and T. colubriformis was distinct from the
SAR of compounds active against D. immitis. This suggests
a unique mechanism of action for the compounds in the
canine heartworm.
Experimental Section
Synthesis and characterization of organic precursors (3a–3g, 3j–
3n) and organometallic analogues (5a–5k, 7a, 7b, 9a, 9b) can be
found in the Supporting Information.
Acknowledgements
This work was financially supported by the Swiss National Sci-
ence Foundation (Professorships N8 PP00P2_133568 and
PP00P2_157545 to G.G.), the University of Zurich (G.G.), the
Stiftung fꢃr wissenschaftliche Forschung of the University of
Zurich (G.G.), the Novartis Jubilee Foundation (G.G), and the
Swiss Government Excellence Scholarship for Postdoctoral Re-
searcher (R. L.). R.B.G.’s research program is supported by the
Australian Research Council (ARC), the National Health and
Medical Research Council (NHMRC), Melbourne Water Corpora-
tion, Yourgene Bioscience, the Alexander von Humboldt Foun-
dation, and The University of Melbourne. E.M.J. is supported
by the US National Institutes of Health (NINDS R01-NS034307)
and is an Investigator of the Howard Hughes Medical Institute
(HHMI). R.B.G. and E.M.J. are grateful recipients of Professorial
Humboldt Research Awards. The authors would like to thank
Dr. Jacques Bouvier (Novartis Animal Health, St-Aubin, Switzer-
land) and Dr. Noꢄlle Gauvry (Novartis Animal Health, Basel,
Switzerland) for their help with the biological assays.
Cytotoxicity and stability of 3a and 9a
An ideal antiparasitic compound would be selective in killing
worms while being nontoxic to the mammalian host. We eval-
uated the selectivity of the two most active compounds
against D. immitis, 3a and 9a. We determined their cytotoxici-
ty using noncancerous human embryonic kidney (HEK293)
cells. The metal-based anticancer drug cisplatin was used as
positive control. Cisplatin inhibits the growth of HEK293 cells
with an IC50 value of 39.62ꢃ2.61 mm. Encouragingly, both of
the compounds 3a and 9a were nontoxic up to 100 mm (the
highest concentration assayed), and this demonstrates selectiv-
ity of the compounds towards parasites.
Another important parameter is the stability of the drug
candidates. The stabilities of 3a and 9a were evaluated by
1H NMR spectroscopy. The compounds were dissolved in
[D6]DMSO/D2O [1/4 (3a), 2/3 (9a), v/v] and the solutions kept
1
in the dark at room temperature. H NMR spectra were record-
ed at different time intervals. No decomposition of the com-
pounds was observed after a 48 h incubation time, and this
confirms the stability of 3a and 9a in aqueous media (see Fig-
ure S4 and S5 of the Supporting Information).
Keywords: anthelmintics · bioorganometallic chemistry · drug
design · medicinal chemistry · metallocenes
[1] a) D. Otranto, R. Wall, Med. Vet. Entomol. 2008, 22, 291–302; b) N. D.
Sargison, Vet. Parasitol. 2012, 189, 79–84; c) R. K. Grencis, Annu. Rev. Im-
munol. 2015, 33, 201–225; d) D. Traversa, Parasit. Vectors 2012, 5, 91–
91; e) F. Roeber, A. Jex, R. Gasser, Parasit Vectors 2013, 6, 153; f) R. Ka-
minsky, L. Rufener, J. Bouvier, R. Lizundia, S. Schorderet Weber, H. Sager,
Vet. Parasitol. 2013, 195, 286–291; g) A. R. Sykes, Anim. Sci. 1994, 59,
155–172.
[2] a) L. Holden-Dye, R. J. Walker, Anthelmintic drugs, The C.elegans Re-
search Community ed., WormBook, 2007; b) C. Epe, R. Kaminsky, Trends
Parasitol. 2013, 29, 129–134; c) B. E. Campbell, M. Tarleton, C. P.
Gordon, J. A. Sakoff, J. Gilbert, A. McCluskey, R. B. Gasser, Bioorg. Med.
Chem. Lett. 2011, 21, 3277–3281; d) B. L. Blagburn, M. W. Dryden, Vet.
Clin. North Am. Small Anim. Pract. 2009, 39, 1173–1200.
[3] a) R. M. Kaplan, A. N. Vidyashankar, Vet. Parasitol. 2012, 186, 70–78;
b) I. A. Sutherland, D. M. Leathwick, Trends Parasitol. 2011, 27, 176–181;
c) A. J. Wolstenholme, I. Fairweather, R. K. Prichard, G. von Samson-Him-
melstjerna, N. C. Sangster, Trends Parasitol. 2004, 20, 469–476; d) R. Van
den Brom, L. Moll, C. Kappert, P. Vellema, Vet. Parasitol. 2015, 209, 278–
280; e) A. Mederos, Z. Ramos, G. Banchero, Parasit Vectors 2014, 7, 598;
f) A. S. Cezar, G. Toscan, G. Camillo, L. A. Sangioni, H. O. Ribas, F. S. F.
Vogel, Vet. Parasitol. 2010, 173, 157–160; g) T. Coles, M. Dryden, Parasit
Vectors 2014, 7, 8.
Conclusion
Given the emergence of drug resistance in some socioeconom-
ically important parasitic nematodes, new strategies are
needed to discover drugs with novel modes of action. In this
study, we explored metal-based variants to expand the chemi-
cal space for antiparasitic drug candidates. Starting from the
organic drug monepantel as a lead structure, we designed and
synthesized a series of organic and organometallic analogues
by employing two different strategies. The efficacies of the
structurally diverse organometallic analogues as well as various
organic intermediates were evaluated against H. contortus and
T. colubriformis. Compounds 3a, 3h, 3l, 3n, 5a, 5e, 5h, 5k,
and 7a showed moderate activity against these two nemato-
des. Further biological assessments on other parasites revealed
that 12 of our new compounds had activity against D. immitis
microfilariae in the low mgmLꢀ1 range of activity for three com-
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
10
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Full Paper
[4] R. Kaminsky, P. Ducray, M. Jung, R. Clover, L. Rufener, J. Bouvier, S. S.
Weber, A. Wenger, S. Wieland-Berghausen, T. Goebel, N. Gauvry, F. Pau-
trat, T. Skripsky, O. Froelich, C. Komoin-Oka, B. Westlund, A. Sluder, P.
Maser, Nature 2008, 452, 176–180.
[5] P. Ducray, N. Gauvry, F. Pautrat, T. Goebel, J. Fruechtel, Y. Desaules, S. S.
Weber, J. Bouvier, T. Wagner, O. Froelich, R. Kaminsky, Bioorg. Med.
Chem. Lett. 2008, 18, 2935–2938.
[14] a) J. Hess, J. Keiser, G. Gasser, Future Med. Chem. 2015, 7, 821–830;
b) M. Patra, K. Ingram, A. Leonidova, V. Pierroz, S. Ferrari, M. N. Robert-
son, M. H. Todd, J. Keiser, G. Gasser, J. Med. Chem. 2013, 56, 9192–9198;
c) M. Patra, K. Ingram, V. Pierroz, S. Ferrari, B. Spingler, R. B. Gasser, J.
Keiser, G. Gasser, Chem. Eur. J. 2013, 19, 2232–2235; d) M. Patra, K.
Ingram, V. Pierroz, S. Ferrari, B. Spingler, J. Keiser, G. Gasser, J. Med.
Chem. 2012, 55, 8790–8798.
[6] a) L. Rufener, J. Keiser, R. Kaminsky, P. Mꢅser, D. Nilsson, PLoS Pathog.
2010, 6, e1001091; b) L. Rufener, N. Bedoni, R. Baur, S. Rey, D. A. Glauser,
J. Bouvier, R. Beech, E. Sigel, A. Puoti, PLoS Pathog. 2013, 9, e1003524;
c) A. S. Peden, P. Mac, Y.-J. Fei, C. Castro, G. Jiang, K. J. Murfitt, E. A.
Miska, J. L. Griffin, V. Ganapathy, E. M. Jorgensen, Nat. Neurosci. 2013,
16, 1794–1801.
[7] R. Baur, R. Beech, E. Sigel, L. Rufener, Mol. Pharmacol. 2014, 87, 96–102.
[8] I. Scott, W. E. Pomroy, P. R. Kenyon, G. Smith, B. Adlington, A. Moss, Vet.
Parasitol. 2013, 198, 166–171.
[9] a) E. Alessio, Wiley-VCH Verlag, Weinheim, 2011, Bioinorganic Medicinal
Chemistry; b) J. C. Dabrowiak, Metals in Medicine, John Wiley & Sons
Ltd, Chichester, 2009; c) M. Patra, G. Gasser, ChemBioChem 2012, 13,
1232–1252.
[15] a) G. Gasser, N. Metzler-Nolte, Curr. Opin. Chem. Biol. 2012, 16, 84–91;
b) G. Gasser, I. Ott, N. Metzler-Nolte, J. Med. Chem. 2011, 54, 3–25; c) G.
Jaouen, N. Metzler-Nolte, in Topics in Organometallic Chemistry, Vol. 32;
and references therein, 1st ed., Springer, Heidelberg, Germany, 2010;
d) C. Hartinger, P. J. Dyson, Chem. Soc. Rev. 2009, 38, 391–401; e) ref.
[11]; f) C. Biot, D. Dive, in Medicinal Organometallic Chemistry, Vol. 32
(Eds.: G. Jaouen, N. Metzler-Nolte), Springer, Heidelberg, 2010, pp. 155–
193; g) C. Biot, W. Castro, C. Y. Botte, M. Navarro, Dalton Trans. 2012, 41,
6335–6349; h) D. Dive, C. Biot, ChemMedChem 2008, 3, 383–391.
[16] a) T. Dallagi, M. Saidi, A. Vessiꢈres, M. Huchꢉ, G. Jaouen, S. Top, J. Org.
Chem. 2013, 734, 69–77; b) G. Jaouen, S. Top, A. Vessiꢈres, G. Leclercq,
M. J. McGlinchey, Curr. Med. Chem. 2004, 11, 2505–2517; c) S. Top, A.
Vessiꢈres, G. Leclercq, J. Quivy, J. Tang, J. Vaissermann, M. Huchꢉ, G.
Jaouen, Chem. Eur. J. 2003, 9, 5223–5236.
[10] a) N. J. Farrer, P. J. Sadler, in Bioinorganic Medicinal Chemistry, Wiley-
VCH Verlag GmbH & Co. KGaA, 2011, pp. 1–47; b) P. C. A. Bruijnincx,
P. J. Sadler, Curr. Opin. Chem. Biol. 2008, 12, 197–206; c) C. G. Hartinger,
N. Metzler-Nolte, P. J. Dyson, Organometallics 2012, 31, 5677–5685.
[11] a) C. Biot, G. Glorian, L. A. Maciejewski, J. S. Brocard, O. Domarle, G.
Blampain, P. Millet, A. J. Georges, H. Abessolo, D. Dive, J. Lebibi, J. Med.
Chem. 1997, 40, 3715–3718; b) F. Dubar, T. J. Egan, B. Pradines, D. Kuter,
K. K. Ncokazi, D. Forge, J.-F. o. Paul, C. Pierrot, H. Kalamou, J. Khalife, E.
Buisine, C. Rogier, H. Vezin, I. Forfar, C. Slomianny, X. Trivelli, S. Kapishni-
kov, L. Leiserowitz, D. Dive, C. Biot, ACS Chem. Biol. 2011, 6, 275–287.
[12] a) R. A. Sanchez-Delgado, A. Anzellotti, Mini-Rev. Med. Chem. 2004, 4,
23–30; b) A. Martꢆnez, T. Carreon, E. Iniguez, A. Anzellotti, A. Sanchez,
M. Tyan, A. Sattler, L. Herrera, R. A. Maldonado, R. A. Sanchez-Delgado,
J. Med. Chem. 2012, 55, 3867–3877; c) L. Otero, G. Aguirre, L. Boiani, A.
Denicola, C. Rigol, C. Olea-Azar, J. D. Maya, A. Morello, M. Gonzꢇlez, D.
Gambino, H. Cerecetto, Eur. J. Med. Chem. 2006, 41, 1231–1239; d) C. R.
Maldonado, C. Marin, F. Olmo, O. Huertas, M. Quiros, M. Sanchez-
Moreno, M. J. Rosales, J. M. Salas, J. Med. Chem. 2010, 53, 6964–6972;
e) M. Navarro, E. J. Cisneros-Fajardo, T. Lehmann, R. A. Sanchez-Delgado,
R. Atencio, P. Silva, R. Lira, J. A. Urbina, Inorg. Chem. 2001, 40, 6879–
6884; f) R. A. Sꢇnchez-Delgado, M. Navarro, K. Lazardi, R. Atencio, M.
Capparelli, F. Vargas, J. A. Urbina, A. Bouillez, A. F. Noels, D. Masi, Inorg.
Chim. Acta 1998, 275–276, 528–540; g) R. A. Sꢇnchez-Delgado, K. Lazar-
di, L. Rincon, J. A. Urbina, A. J. Hubert, A. N. Noels, J. Med. Chem. 1993,
36, 2041–2043; h) M. Navarro, T. Lehmann, E. J. Cisneros-Fajardo, A.
Fuentes, R. A. Sanchez-Delgado, P. Silva, J. A. Urbina, Polyhedron 2000,
19, 2319–2325; i) J. J. N. Silva, P. M. M. Guedes, A. Zottis, T. L. Balliano,
F. O. Nascimento Silva, L. G. FranÅa Lopes, J. Ellena, G. Oliva, A. D. Andri-
copulo, D. W. Franco, J. S. Silva, Br. J. Pharmacol. 2010, 160, 260–269;
j) E. Iniguez, A. Sanchez, M. Vasquez, A. Martinez, J. Olivas, A. Sattler, R.
Sanchez-Delgado, R. Maldonado, J. Biol. Inorg. Chem. 2013, 18, 779–
790; k) M. Navarro, C. Gabbiani, L. Messori, D. Gambino, Drug Discovery
Today 2010, 15, 1070–1078.
[17] P. M. Loiseau, J. J. Jaffe, D. G. Craciunescu, Int. J. Parasitol. 1998, 28,
1279–1282.
[18] a) A. Lifschitz, M. Ballent, G. Virkel, J. Sallovitz, P. Viviani, C. Lanusse, Vet.
Parasitol. 2014, 203, 120–126; b) L. Stuchlꢆkovꢇ, R. Jirasko, I. Vokral, J.
Lamka, M. Spulak, M. Holcapek, B. Szotakova, H. Bartikova, M. Pour, L.
Skalova, Anal. Bioanal. Chem. 2013, 405, 1705–1712.
[19] a) P. Pigeon, S. Top, A. Vessiꢈres, M. Huchꢉ, E. A. Hillard, E. Salomon, G.
Jaouen, J. Med. Chem. 2005, 48, 2814–2821; b) N. Chavain, H. Vezin, D.
Dive, N. Touati, J.-F. Paul, E. Buisine, C. Biot, Mol. Pharm. 2008, 5, 710–
716.
[20] a) L. Glans, W. Hu, C. Jost, C. de Kock, P. J. Smith, M. Haukka, H. Bruhn,
U. Schatzschneider, E. Nordlander, Dalton Trans. 2012, 41, 6443–6450;
b) P. V. Simpson, C. Nagel, H. Bruhn, U. Schatzschneider, Organometallics
2015, 34, 3809–3815.
[21] P. Ducray, N. Gauvry, T. Goebel, M. Jung, R. Kaminsky, F. Pautrat, (Ed.:
WO/2005/044784), Google Patents, 2005.
[22] D. Karadzovska, W. Seewald, A. Browning, M. Smal, J. Bouvier, J. M. Gir-
audel, J. Vet. Pharmacol. Ther. 2009, 32, 359–367.
[23] a) E. R. Biehl, P. C. Reeves, Synthesis 1973, 1973, 360–361; b) D. P. Cor-
mode, A. J. Evans, J. J. Davis, P. D. Beer, Dalton Trans. 2010, 39, 6532–
6541.
[24] a) P. D. Beer, D. K. Smith, J. Chem. Soc. Dalton Trans. 1998, 417–424;
b) D. C. G. Hardy, L. Ren, T. C. Tamboue, C. Tang, J. Polym. Sci. Part A
2011, 49, 1409–1420.
[25] For legal issues the efficacy of AAD96 cannot be disclosed.
[26] C. Biot, W. Daher, N. Chavain, T. Fandeur, J. Khalife, D. Dive, E. De Clercq,
J. Med. Chem. 2006, 49, 2845–2849.
[27] R. Rubbiani, O. Blacque, G. Gasser, Dalton Trans. 2016, 45, 6619–6626.
[28] D. D. Bowman, C. Mannella, Top. Companion Anim. Med. 2011, 26, 160–
172.
[13] M. Patra, G. Gasser, N. Metzler-Nolte, Dalton Trans. 2012, 41, 6350–
6358.
Received: June 15, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
11
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
&
Medicinal Chemistry
J. Hess, M. Patra, L. Rangasamy,
S. Konatschnig, O. Blacque, A. Jabbar,
P. Mac, E. M. Jorgensen, R. B. Gasser,*
G. Gasser*
&& – &&
Organometallic anthelmintics: The syn-
thesis and antiparasitic evaluation of or-
ganometallic compounds based on the
lead structure of the anthelmintic mon-
epantel (see scheme) against some eco-
nomically important parasitic worms of
animals are reported.
Organometallic Derivatization of the
Nematocidal Drug Monepantel Leads
to Promising Antiparasitic Drug
Candidates
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
12
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!