TRIAZOLE DERIVATIVES WITH ANTIFUNGAL ACTIVITY: A ...

Acta Poloniae Pharmaceutica ñ Drug Research, Vol. 65 No. 6 pp. 795ñ798, 2008 ISSN 0001-6837
Polish Pharmaceutical Society
TRIAZOLE DERIVATIVES WITH ANTIFUNGAL ACTIVITY:
A PHARMACOPHORE MODEL STUDY
ALICJA NOWACZYK* and BOØENA MODZELEWSKA-BANACHIEWICZ
Department of Organic Chemistry, Faculty of Pharmacy, Collegium Medicum in Bydgoszcz, Nicolaus
Copernicus University, Sk≥odowskiej-Curie 9, 85-094 Bydgoszcz, Poland
Keywords: antifungal activity, pharmacophore model of triazole alcohols, UR-9825
The incidence of fungal infections has
increased significantly in the past two decades (1,
2). The first generation of azoles antifungal
inhibitors of CYP51, have revolutionized treatment
of some serious fungal infections. Triazoles has
been the leading agents for the control of fungal
diseases of humans and animals for over 20 years
(3-5). According to this, azole derivatives are currently
the most widely studied class of antifungal
agents. The triazole alcohols such as 2-(2,4-difluorophenyl)-1,3-di(1H-1,2,4-triazol-1-yl)propan-2-ol
i.e. fluconazole and (2R,3R)-2-(2,4-difluorophenyl)-3-(5-fluoropyrimidin-4-yl)-1-(1H-1,2,4triazol-1-yl)butan-2-ol
i.e. voriconazole are the
representatives of the second generation of triazole
antifungal drug. The newest compound 7-chloro-3-
((2R,3R)-3-(2,4-difluorophenyl)-3-hydroxy-4-
(1H-1,2,4-triazol-1-yl)butan-2-yl)quinazolin-
4(3H)-one, labeled as UR-9825, exhibits two
important types of activity against certain fungal
pathogens i.e. activity against yeasts and filamentous
fungi (6). Up to date the name of compound
UR-9825 is albaconazole and it is undergoing the
Phase II clinical trials, but its future is uncertain (7,
8). Our recent docking experiments for fluconazole,
voriconazole and UR-9825 into the catalytic
site of MT-CYP51 (pdb code: 1ea1) as template (9)
showed that the molecules bind to the catalytic site
adopting the similar bioactive conformation as
observed in the crystallized complex of fluconazole
with the enzyme (10). The aim of this study is
a comparison of pharmacophore features of active
conformation obtained via docking of the drug in
Mycobacterium tuberculosis CYP51, with the pharmacophore
model proposed for the antifungal
drugs (11). In order to define more precisely the
* Corresponding author: e-mail: alicja@cm.umk.pl
795
structure-activity relationship within the investigated
compounds a molecular modeling study was
undertaken.
EXPERIMENTAL
Molecular modeling and docking were carried
out with the commercially available CAChe
WorkSystem Pro (version 7.5.0.85) software package.
All computations were performed on a HP-6200
wx workstation. The crystallographic structure of the
enzyme CYP51 were obtained from the Brookhaven
Protein Databank, accession number 1EA1. Missing
atoms in the crystal structure were added and the
structure was optimized. Structures of all drugs were
constructed using 3D-sketcher module available in
the software. All the molecule geometries were optimized
using the molecular mechanics methods MM3
until the root mean square (RMS) gradient value
becomes smaller than 0.1 kcal/mol Å. Force field calculations
were used to ascertain whether the resulting
structures corresponded to energy minima. Docking
experiments were preformed using a knowledgebased
strategy (11, 12). The optimized drug structure
was docked into the active site of MT-CYP51 automatically
by placing the nitrogen atom in position 4
of the triazole ring at the 2.37Å from the heme iron
atom on the center orthogonal of the heme plane in
MT-CYP51. The above value is the average distance
of Fe-N coordination bonds observed in the crystallized
complexes of MT-CYP51 with fluconazole and
4-phenylimidazole. First, the validation of the docking
method was carried out by redocking fluconazole
into its crystal structure in 1EA1.pdb. After successful
validation, voriconazole and albaconazole were
docked into MT-CYP51. The active structures

796 ALICJA NOWACZYK and BOØENA MODZELEWSKA-BANACHIEWICZ
obtained due to this optimization of the drugs were
then compared by superimposition procedure.
RESULTS AND DISCUSSION
Antifungal activity of triazole agents is mainly
attributed to the coordination abilities of the nitrogen
atom N4(3) of the triazole ring to the iron atom
of heme. The halogenated phenyl group of azole
inhibitor is deep in the hydrophobic binding cleft
within the heme environment of the enzymes.
Additionally, the chirality at C2 and C3 atoms is
important to antifungal activity. The hydroxyl group
attached to C2 atom and the methyl group attached
to the C3 position have been favorable for antifungal
activity.
All the fungal CYP51 proteins that had been
characterized were membrane-bound and it is difficult
to solve their crystal structures. Up to date the
Table 1. The active conformations of the investigated drugs.
only available solved X-ray crystal structure of
CYP51 was obtained from Mycobacterium tuberculosis,
but the explicit information about the enzyme
binding site of pathogenic fungi is not available yet.
Even when the structure is available, it is still very
difficult to deduce the relative contribution of each
individual residue to the total binding energy of each
inhibitor. According to this, the study of the interaction
between an azole and fungal CYP51 can only
be done by methods of molecular modeling and a
number of docking experiments have been reported
in the literature (13-15). However, until now any
interaction involving the oxygen atom of hydroxyl
group attached to C2 was not found. It is because the
activity site is so large that there is a significant distance
between the oxygen atom and the groups
around it (16). Hydrophobic interaction was predicted
to be stronger in the case of voriconazole than
that of fluconazole. It is explained by the presence of
Table 2. The distances (Å) between pharmacophore features for the investigated drugs and references modeling data.
Fluconazole Albaconazole Voriconazole ref. (11)
a1 (Å) 4.528 4.302 4.253 (4.104 ñ 4.872)
b1 (Å) 3.545 3.591 3.642 (3.165 ñ 3.678)
a2 (Å) 6.233 6.574 6.596 (6.545 ñ 6.934)
b2 (Å) 3.638 3.896 4.059 (3.810 ñ 3.887)
c (Å) 5.193 5.079 4.888 (4.016 ñ 4.575)
Table 3. The superimposition of the investigated compounds.

Figure 1. The pharmacophore model of triazole antifungal alcohols.
extra methyl group attached to the C3 atom of
voriconazole molecule (15). In the literature, the
pharmacophore model was proposed for azole antifungals
(11, 15).
According to the literature we have to define
pharmacophore points such as: A ñ coordination site
in heme, B ñ hydrogen bond acceptor atom, C ñ
hydrophobic interaction groups [i.e. the center of
diphenylphenyl ring and methyl group or prochiral
hydrogen atom (17)]. The intermolecular distances
and angles in the proposed pharmacophore model
are as follows: a 1 ñ the distance between the heterocyclic
N4(3) nitrogen atom of triazole moiety and
the hydrogen bond acceptor a 2 ñ the distance
between N4(3) atom and the center of hydrophobic
interaction; b 1 ñ the distance between OH group
attached to C2 and the center of diphenylphenyl
ring; b 2 ñ the distance between CH 3 group or prochiral
hydrogen atom attached to C3 atom and the center
of diphenylphenyl ring; c ñ the distance between
the N4(3) and the center of diphenylphenyl ring. The
investigated active structures were compared by
means of the distances listed above. In Figure 1
pharmacophore feature scheme is presented for the
investigated compounds. The obtained values for
the compounds and references modeling data form
the literature are shown in Table 2.
It was found that the distances between the
pharmacophore features, estimated for the selected
ligands, are within the following range: a 1 = 4.253 ñ
Triazole derivatives with antifungal activity... 797
4.528 Å; b 1 = 3.545 ñ 3.642 Å; a 2 = 6.233 ñ 6.596 Å;
b 2 = 3.638 ñ 4.059 Å; c = 4.888 ñ 5.193 Å. The
results obtained show that the distance values are in
the range of references data. Based on this we can
treat the geometries of drugs obtained in this study
as the active conformation for the investigated triazole
alcohols.
To obtain additional information concerning
the shape of the investigated drug, the active conformations
were chosen for superimposition. The
atoms (except hydrogen atoms) common to these
molecules were selected for the fitting procedure.
Their similarity was calculated as RMS fit. The
RMS routine provided estimates of how closely
molecules fit to each other. The lower the RMS
value, the better similarity.
The RMS deviations for each group are as follows:
0,399 Å (fluconazole ñ albaconazole); : 0,497
Å (fluconazole ñ voriconazole); 0,610 Å (voriconazole
ñ albaconazole). A comparison of the geometries
of compounds shown above indicates high similarity
in the orientation of the all pharmacophore
points. These results confirm that the chosen group
is an important structural unit for antifungal activity.
The pair voriconazole ñ albaconcazole the highest
RMS value in the set is due to the terminal aromatic
fragments (5-fluoropyrimidin-4-yl and 7-chloroquinazolin-4(3H)-one,
respectively) that have different
orientation, this does not influence the pharmacophore
features of the molecule though.

798 ALICJA NOWACZYK and BOØENA MODZELEWSKA-BANACHIEWICZ
CONCLUSION
According to obtained results concerning the
pharmacophore features of antifungal drugs, it is
seen that active conformations of analyzed drugs
preserve the conditions imposed by existing pharmacophore
model derived for antifungal drugs.
According to this one can state that application in
the docking studies Mycobacterium tuberculosis
CYP51 is reasonable approach in case of absence of
genuine fungal CYP51.
The analysis of similarity of obtained active
structures shows that all three compounds adopt
very similar conformation in the target site of the
enzyme. The only significant deviation of the structure
concern fragment apart of the pharmacophoreís
points. The presence of hydroxyl group in the second
position and the methyl group in the third position
of the propyl chain in the triazole alcohol is a
crucial structure feature determining the affinity of
compounds to target site. More extensive structureactivity
relationship studies are in progress and will
be reported in due course.
Acknowledgment
This study was supported by the research grant
from the UMK no. 13/2008.
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Erratum:
The names of authors of the paper ìGADOLINIUM Gd(III) COMPLEXES WITH DERIVATIVES
OF NITRILOACETIC ACID: SYNTHESIS AND BIOLOGICAL PROPERTIESî from Acta Pol.
Pharm. Drug Res. 65, issue 5, 535 (2008) are: Boles≥aw Karwowski, Ma≥gorzata Witczak, Eløbieta
Mikiciuk-Olasik, and Micha≥ Studniarek.

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