Discovery of potent pteridine reductase inhibitors
to guide antiparasite drug development
Antonio Cavazzuti*, Giuseppe Paglietti†, William N. Hunter‡, Francisco Gamarro§, Sandra Piras†, Mario Loriga†,
Sergio Alleca†, Paola Corona†, Karen McLuskey‡, Lindsay Tulloch‡, Federica Gibellini*‡, Stefania Ferrari*,
and Maria Paola Costi*¶
*Dipartimento di Scienze Farmaceutiche, Università di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy; †Dipartimento Farmaco Chimico
Tossicologico, Università degli Studi di Sassari, via Muroni 23/a 07100 Sassari, Italy; ‡Division of Biological Chemistry and Drug Discovery, College of Life
Sciences, University of Dundee, Dundee, DD1 5EH, United Kingdom; and §Instituto de Parasitologia y Biomedicina ‘‘Lopez-Neyra,’’ Consejo Superior de
Investigaciones Cientificas, Parque Tecnológico de Ciencias de la Salud, Avenida del Conocimiento s.n., 18100 Armilla, Granada, Spain
Edited by Robert M. Stroud, University of California, San Francisco, CA, and approved November 26, 2007 (received for review May 10, 2007)
Pteridine reductase (PTR1) is essential for salvage of pterins by
parasitic trypanosomatids and is a target for the development of
improved therapies. To identify inhibitors of Leishmania major and
Trypanosoma cruzi PTR1, we combined a rapid-screening strategy
using a folate-based library with structure-based design. Assays
were carried out against folate-dependent enzymes including
PTR1, dihydrofolate reductase (DHFR), and thymidylate synthase.
Affinity profiling determined selectivity and specificity of a series
of quinoxaline and 2,4-diaminopteridine derivatives, and nine
compounds showed greater activity against parasite enzymes
compared with human enzymes. Compound 6a displayed a Ki
of 100 nM toward LmPTR1, and the crystal structure of the
LmPTR1:NADPH:6a ternary complex revealed a substrate-like binding mode distinct from that previously observed for similar compounds. A second round of design, synthesis, and assay produced
a compound (6b) with a significantly improved Ki (37 nM) against
LmPTR1, and the structure of this complex was also determined.
Biological evaluation of selected inhibitors was performed against
the extracellular forms of T. cruzi and L. major, both wild-type and
overexpressing PTR1 lines, as a model for PTR1-driven antifolate
drug resistance and the intracellular form of T. cruzi. An additive
profile was observed when PTR1 inhibitors were used in combination with known DHFR inhibitors, and a reduction in toxicity of
treatment was observed with respect to administration of a DHFR
inhibitor alone. The successful combination of antifolates targeting
two enzymes indicates high potential for such an approach in the
development of previously undescribed antiparasitic drugs.
antitrypanosomatid agents 兩 antifolates 兩 drug discovery
P
rotozoan parasites of the order Kinetoplastida are the causal
agents of serious human diseases, including African sleeping
sickness, Chagas’ disease, and leishmaniasis. There is an urgent
need for new, more effective drugs targeting these neglected
diseases, because those in current use are toxic, expensive, and
often difficult to administer. The problem is compounded by an
increase in drug resistance and lack of progress in drug development. Only a single new effective treatment has been developed in
the last 25 years, Miltefosine (hexadecylphosphocholine), recently
approved in India (1).
Enzymes involved in the provision and use of reduced folate
cofactors such as dihydrofolate reductase (DHFR) and thymidylate
synthase (TS) are valued drug targets for the treatment of bacterial
infections (2), cancer (3), and certain parasitic diseases, notably
malaria (4). DHFR catalyzes the two-step reduction of folate to
tetrahydrofolate, which is then transformed to N5,N10-methylene
tetrahydrofolate and is used by TS as a methyl donor and reducing
agent in the conversion of 2⬘-deoxyuridine-5⬘-monophosphate to
2⬘-deoxythymidine-5⬘-monophosphate. Inhibition of DHFR or TS
reduces the cellular pool of 2⬘-deoxythymidine-5⬘-monophosphate,
impairing DNA replication and resulting in cell death.
1448 –1453 兩 PNAS 兩 February 5, 2008 兩 vol. 105 兩 no. 5
Because trypanosomatids are auxotrophic for folates and pterins,
the inhibition of the enzymes depending on them should provide
suitable treatments. However, antifolates are not used in the
therapy of trypanosomatid infections mainly because of pteridine
reductase (PTR1). Although the bifunctional DHFR-TS used by
trypanosomatids can exclusively reduce folic acid, the short-chain
dehydrogenase/reductase PTR1 shows a much broader range of
activity catalyzing successive reductions of conjugated (folate) and
unconjugated (biopterin) pterins (5, 6). Under physiological conditions, PTR1 is responsible of reduction of ⬇10% of the folic acid
required by the cell (7), but when classical antifolate drugs inhibit
DHFR-TS, PTR1 can be overexpressed, allowing for significant
reduction of the necessary amounts of folates to ensure parasite
survival (8). This compensatory mechanism suggests that treatment
of trypanosomatid infections could be achieved through inhibition
of both DHFR and PTR1 by a single drug or a combination of
molecules that are specific inhibitors of the two targets (7). A series
of diaminopteridines and quinazolines have been tested against
these enzymes and parasites, leading to identification of inhibitors
with micromolar Ki (inhibition constant) values (9). Structures of
PTR1 complexes with substrates, products, and inhibitors have
illuminated aspects of ligand recognition and enzyme mechanism
(10–12). Based on these data and with the intention of identifying
potent inhibitors of Leishmania major and Trypanosoma cruzi
enzymes, we analyzed a library of 440 synthetic folate-like compounds. Approximately one-third of these were selected for rapid
screening against a panel of folate-dependent enzymes. The molecular interactions between two of the most potent inhibitors with
PTR1 were elucidated by x-ray crystallography. The compounds
that displayed the best inhibition profiles in vitro were subjected to
biological evaluation on the protozoan parasites L. major and T.
cruzi WT and overexpressing PTR1 lines, as well as on the
intracellular form of T. cruzi. When the molecules were tested in
combination with known DHFR inhibitors, additive profiles of
inhibition were observed.
Results and Discussion
Our study considered a collection of 440 folate-related compounds
originally designed as potential anticancer agents (13–15). Of these,
72 were recently synthesized to enrich the library. Compounds were
Author contributions: M.P.C. and S.F. designed research; and G.P., S.P., M.L., S.A., P.C.,
W.N.H., K.M., L.T., F. Gibellini, F. Gamarro, and A.C. performed research.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
Data deposition: The atomic coordinates and structure factors have been deposited in the
Protein Data Bank, www.pdb.org (PDB ID codes 2P8K and 2QHX).
¶To
whom correspondence should be addressed. E-mail: mariapaola.costi@unimore.it.
This article contains supporting information online at www.pnas.org/cgi/content/full/
0704384105/DC1.
© 2008 by The National Academy of Sciences of the USA
www.pnas.org兾cgi兾doi兾10.1073兾pnas.0704384105
evaluated on the basis of molecular diversity, lipophilicity, the
conformity to Lipinsky rules (16) and the presence of ionizable
functional groups [following integrated Ferguson’s principle (17)].
Previously reported toxicity data were considered and those compounds showing no cell-based anticancer activity preferred. The
131 molecules that fit the selection criteria were advanced to a rapid
screening assay evaluation against a panel of folate-dependent
enzymes, including TS from pathogenic and nonpathogenic species,
L. major DHFR-TS, and L. major and T. cruzi PTR1. Inhibition of
human TS and DHFR was used to estimate the toxicity/specificity
of the compounds, and three biological profiles were observed: (i)
no activity against both human enzymes at 190 M (48% of
compounds); (ii) inhibition of only one human enzyme (45%); and
(iii) inhibition of both human enzymes (7%) (data not shown).
Compounds that showed a specificity index (Ki human enzyme/Ki
target enzyme) ⱖ100 with respect to at least one human enzyme
(DHFR or TS) were considered suitable as antiparasitic leads. The
inhibition profiles identified groups of compounds with strong
specificity for LmPTR1, with a lack of activity against LmPTR1 and
hDHFR or relevant activity with lack of specificity (Fig. 1A, regions
1, 2, and 3, respectively). This trend reflects a significant level of
molecular diversity within the library and offers useful tools for
structure–activity relationship analysis. A first selection was performed based on solubility in water and calculated Ki ⱕ 30 M (at
least 30% of inhibition at 190 M). (Fig. 1B, red dashed line).
Compounds that inhibited the enzymes with Ki values ⬍3 M (Fig.
1B, blue dashed line), corresponding to 70% inhibition at 20 M
inhibitor concentration, were selected for testing against the protozoan parasites L. major and T. cruzi. Finally, we selected representatives of all main chemical classes present in the original library:
2,4 diaminopteridines and quinoxalines with substituents in positions 2 and 6 (Table 1). Ki-based selection led to a reduction from
Cavazzuti et al.
Structures of LmPTR1–NADPH Inhibitor Complexes. Crystal structures
of the ternary complex of enzyme, reduced cofactor, and the
inhibitors 6a and 6b were determined at 2.4- and 2.6-Å resolution,
respectively. The PTR1 subunit is a seven-stranded parallel -sheet
with three ␣-helices on either side. The active site is an elongated
cleft ⬇22 by 15 Å, created by the C-terminal sections of several of
the -strands, two ␣-helices, and an extended loop region (10). The
cofactor binds in an extended conformation with the nicotinamide,
creating the floor of the catalytic center, with Phe-113 forming an
overhang under which the pterin-binding pocket is formed. Nearby
are three important residues (Fig. 2). Tyr-194 is the active-site base,
which acts in concert with Asp-181 to acquire and pass on one
reducing equivalent. Lys-198 positions the nicotinamide through
hydrogen-bonding interactions with the cofactor ribose and may
reduce the pKa of Tyr-194, thereby assisting catalysis (10, 12). The
LmPTR1-6a complex structure contains two tetramers, subunits
labeled A-H, in the asymmetric unit. In each active site, the
inhibitor adopts an orientation similar to that observed for substrate
with the C7-N8 bond near Asp-181 and Tyr-194. The inhibitor
forms hydrogen bonds with Ser-111, Tyr-194, the phosphate and
ribose components of the cofactor, and an ordered water molecule
(Fig. 2A). The asymmetric unit of the LmPTR1-6b complex structure contains a tetramer, subunits labeled A-D, and the inhibitor
displays two modes of binding (Fig. 2B). All four subunits of the
asymmetric unit position the inhibitor in a methotrexate (MTX)like binding mode, with the C7-N8 bond directed outward from the
active site (Fig. 2B). The occupancy of 6b in the MTX orientation
in subunits C and D is 100%; however, in subunits A and B, the
PNAS 兩 February 5, 2008 兩 vol. 105 兩 no. 5 兩 1449
BIOCHEMISTRY
Fig. 1. Inhibition activity profiles of the 131 selected compounds. (A) Distribution profile of compounds toward human DHFR (white dot) and LmPTR1
(solid black dots). It is possible to distinguish a first set of compounds with high
specificity for LmPTR1 (region 1), characterized by low-micromolar Ki on
LmPTR1 and lack of inhibition on human DHFR, followed by a series of
compounds with no activity on both enzymes (region 2) and finally compounds inhibiting both enzymes (region 3). (B) Enzyme inhibition profiles (Ki).
Logarithmic representation of Ki obtained in in vitro assay of enzyme inhibition for hDHFR (black), LmDHFR-TS (red), LmPTR1 (green), and TcPTR1 (yellow). Cutoff lines for inhibitors with Ki ⬍ 30 M (red) and Ki ⬍ 3 M inhibitors
(blue) are shown. Spots aligned at the top of the graph show no inhibition at
190 M.
131 to 9 compounds (Table 1, first round). These nine compounds
displayed a specificity index ⬎100 against at least one human
enzyme or showed an interesting enzyme profile and structure.
Notably, compound 6a, with a Ki of 7 M against TcPTR1 of 100
nM toward LmPTR1 and of 4 M against LmDHFR, was the best
inhibitor of LmPTR1 and one of the most selective compounds in
the series, with no detectable inhibition of hTS and a specificity
index of equal or ⬎100 toward hDHFR. Compounds 10n and 9m
inhibited LmPTR1 with Ki values of 7 and 6 M, respectively, and
8-fold lower ratio of affinity/no affinity toward hDHFR. Compounds 65, 66, and 55 were moderately good inhibitors of LmDHFR and showed no affinity toward hDHFR. The 2,4diaminopteridine derivatives are similar to the natural substrates/
cofactors for DHFR-TS and PTR1, and in general were not strong
inhibitors of these enzymes with the notable exception of 6a.
However, the quinoxaline scaffold displayed an interesting profile
of inhibition, suggesting an alternative approach to antifolate
candidates with high substrate selectivity.
After the identification of compound 6a as a potent and selective
inhibitor of the parasite enzymes and having determined the crystal
structure in complex with cofactor and LmPTR1 (see below), the
compound library was reevaluated to identify similar compounds
for further testing (Table 1, second round). The selected compounds present a classical folate core structure with pteridinic or
quinoxalinic head groups linked through the carboxylic group to
different terminal groups (pyrrolidinic, piperidinic or glutamic
groups). Compounds 6b, 6c, 59, and 60 were tested against
LmPTR1 and the best inhibitors also against hDHFR. 6b displayed
Ki values of 37 and 820 nM against LmPTR1 and hDHFR,
respectively (Table 1). The other compounds showed little or no
inhibition of LmPTR1. Notably, compound 6c is at least 3 orders
of magnitude less active than compound 6a, even though the only
differences between them reside in the terminal portion of the
glutamate tail (a piperidine moiety in 6a vs. a pyrrolidine moiety in
6c). Thus, even if the portion of the enzyme active site accommodating the terminus of the inhibitor is wide and disordered (due to
the presence of flexible loops, as indicated in the x-ray structure),
this part of the inhibitor can still influence the binding affinity of the
molecule, possibly by steric hindrance and/or solvation effects.
Table 1. Inhibition constants (Ki, M) of compounds selected for further biological evaluation on parasites
TS
Name
Structure
DHFR
PTR1
hTS
LmTS
hDHFR
LmDHFR
LmPTR1
NI
NI
10
4
0.1
7
10n
41
NI
56
40
7
75
10l
13
NI
54
17
—
20
9t
15
NI
24
75
NI
38
9j
13
NI
NI
88
NI
40
9m
12
86
NI
27
6
106
65
NI
NI
NI
3
—
—
66
41
NI
NI
4
—
—
55
3
NI
NI
10
NI
NI
—
NI
0.8
—
0.037
—
6c
—
—
—
—
30%*
—
59
—
—
—
—
NI*
—
60
—
—
—
—
NI*
—
MTX
—
0.6
3.4†
130‡
0.18
0.11
a-First-round screening
6a
b-Second-round screening
6b
TcPTR1
NI, no inhibition at 190 M; —, not tested.
*Tested at maximum concentration of 75 M.
†K expressed in picomolar (39).
i
‡K expressed in picomolar (8).
i
occupancy is 50:50 between the MTX orientation and a substratelike orientation. Our observation is consistent with a previous
report that an MTX-type antifolate can adopt a nonclassical
orientation in the DHFR active site (18).
In the substrate-like conformation (Fig. 2B), the head group
of 6b is sandwiched between the nicotinamide and Phe-113 with
1450 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0704384105
N1, N2, N3, and N4 atoms interacting with the cofactor. N1 and
N2 form hydrogen bonds with the ribose 2⬘ OH, whereas N3
interacts with a cofactor phosphate. N2 interacts with the side
and main chains of Ser-111, whereas N4 is separated from the
Arg-17-NH2 by a distance of 3.3 Å. In subunit B, the terminal
portion ranging from the pterin head to the piperidine ring can
Cavazzuti et al.
Fig. 2. PTR1 inhibition. (A) Compound 6a adopts the substrate-like orientation. (B) Stereoview of the two orientations adopted by compound 6b in subunit
B of the ternary complex. C atoms of LmPTR1 and NADPH are gray, all O atoms are red, N atoms are blue, and the S atom of Met-233 is yellow. Two water molecules
are shown as red spheres. Green dashed lines identify potential hydrogen bond interactions; Phe-113 and Tyr-191 are ghosted for clarity. The C atoms of the
inhibitor that binds in the substrate-like orientation are purple and yellow for the inhibitor when it adopts the MTX-type orientation.
On-Parasite Assays. Growth inhibition assays were conducted on
WT lines of epimastigote T. cruzi and promastigote L. major to test
compound cytotoxicity. Dose–effect curves were obtained for all
compounds and different profiles observed. Based on the EC50
values, only a few of the candidates were able to completely inhibit
growth of parasites, with effects ranging from 40% of T. cruzi
growth inhibition for 9j to 100% of L. major growth inhibition for
9m (data not shown). These results indicate that processes critical
for parasite viability were targeted by the inhibitors. The implications of ptr1 gene knockout on parasite viability are published (20);
here, we evaluated the contribution of PTR1 in the mechanism of
action of inhibitors by testing against parasite lines overexpressing
this enzyme. A relative drug resistance (RDR) value, calculated by
dividing EC50 obtained from the PTR1-overexpressing lines by the
EC50 value obtained from WT lines (Fig. 3, EC50 in columns and
RDR in line), was assessed by using as controls both WT parasites
Cavazzuti et al.
and parasites transfected with the empty vector (pTEX for T. cruzi
and pX for L. major). The difference in biological activity (induced
resistance) between the controls and overexpressing lines was
assumed to be due to the different expression levels of PTR1.
Classical DHFR inhibitors, such as MTX, pyrimethamine (PYR),
aminopterin (AMP), and trimethoprim (TMP), were assayed for
comparison, and two trends of biological activity were evident;
compounds with RDR equal to 1 and those with RDR ⬎1 (Fig. 3).
An RDR ⬎1 (6a, AMP, MTX in T. cruzi and 6a, and 6b in L. major)
indicates that PTR1 inhibition is implicated in antiparasite activity.
For compounds displaying an RDR value near 1 (9m, 9t for T.
cruzi), the growth inhibition of parasites is unrelated to PTR1
expression level, and the inhibitor likely has an alternative molecular target in both WT and overexpressing parasites. These results
with 9m and 9t for T. cruzi epimastigote forms suggest that
alternative, potentially valuable targets besides PTR1 and
DHFR-TS exist.
Additionally, we have assayed the activity of selected compounds
6a and 6b on amastigote intracellular forms of T. cruzi. Both
compounds were able to inhibit proliferation of amastigote T. cruzi
demonstrating their ability to cross the plasma membrane of
mammalian host Vero cells displaying their activity intracellularly.
The effect of compounds 6a and 6b on amastigote growth inhibition
was, respectively, 22% and 27% at 50 M (SI Appendix, SI Fig. 12).
Synergy Evaluation. Selected compounds were combined with
known DHFR inhibitors to test effects of coadministration on T.
cruzi and L. major WT parasite lines. For each compound tested,
at least one known inhibitor produced an additive inhibition, which
Fig. 3. EC50 and RDR values for T. cruzi (Left) and L. major (Right). Growth
inhibition effect (M EC50, left y axis) induced by main compounds singularly
administrated on WT (black bars) and PTR1 overexpressing lines (gray bars).
RDR values obtained dividing EC50 from PTR1-overexpressing line by EC50
obtained from WT lines for each pair of data are plotted with black pointed
line (values in right y axis).
PNAS 兩 February 5, 2008 兩 vol. 105 兩 no. 5 兩 1451
BIOCHEMISTRY
be more precisely assigned to one conformation, with the N10
methyl oriented toward a hydrophobic site formed by Leu-226
and -229.
An overlay of the two binding modes displayed by 6b is shown in
Fig. 2B. A significant feature is that the catalytic Tyr-194 donates
a hydrogen bond to the inhibitor when it adopts the substrate-like
orientation but accepts a hydrogen bond donated from 6b in the
MTX-like orientation. That we observe two orientations for similar
(6a, MTX) inhibitors or indeed the same molecule (6b) suggests
little difference in the energy of binding. For 6a, the steric freedom
that arises because of lack of an N10 methyl substituent may tilt the
balance toward the substrate-like orientation. For 6b, the lack of
interaction between the enzyme and the tail of the compound
suggests that hydrogen-bonding capacity, which principally resides
in the pteridine head group, is an important determinant of
orientation. The hydrogen-bonding capacity offered by the activesite residues can be satisfied by two orientations, and that is what
is observed. There may also be a solvent effect, because the N10
methyl group in one orientation, the MTX mode, is placed to
disrupt a well ordered water network observed in that region of the
active site (11, 12).
The differences between MTX or similar ligands binding to
hDHFR and LmPTR1 are striking [supporting information (SI)
Appendix, SI Figs. 7 and 8] (10, 11). MTX, 6a, and 6b bind the
PTR1:NADP⫹ complex with the pteridine head group forming
extensive interactions with the cofactor and the tail placed at the
periphery of the active site with few interactions observed between
ligand and enzyme. When MTX binds to hDHFR (19), there are
few interactions (van der Waals) with the cofactor, all confined to
the nicotinamide. The pteridine group and in particular the tail of
MTX, however, form extensive electrostatic and van der Waals
interactions with protein residues (SI Appendix, SI Fig. 8).
Fig. 4. Images obtained by fluorescence microscopy of samples stained with
DAPI (2-(4-amidinophenyl)-6-indolecarbamidine) for evaluation of mean
number of parasites per infected not treated (B) and treated (C) cells, compared with noninfected Vero cells (A). The nuclei of 9 –10 Vero cells are visible
in each camp. In B is shown as intracellular parasites (visualized as a small spot
given by fluorescence of amastigote kinetoplast and nucleus) distributed in a
monolayer when the adherent cell is not confluent with space to expand,
facilitating their counting. When parasitism is high, the shape of the infected
area corresponds to an area covered by the adherent cell. It is evident that in
C, parasites that entered during the infection phase, before drug administration were not able to proliferate as in B.
permits a decrease in concentration of known DHFR-TS inhibitors,
characterized by a low therapeutic index, necessary to reach maximum effect. The importance of this observation resides in the low
toxicity of the selected compounds (SI Appendix, SI Table 5). PYR
was generally the best partner for inhibition of T. cruzi, where
almost all associations produced additive inhibition, and L. major
(SI Fig. 20). In the latter case, PYR was the only commercial
inhibitor showing an additive effect with all selected hits. MTX,
TMP, and AMP showed antagonist profiles against most of the
tested compounds. Compounds 65 and 66, in contrast, showed
additive inhibition effects. Some combinations gave inhibition
profiles that exceeded the expected value of 50%, as explained in
Materials and Methods. Compound 66 with PYR gave an inhibition
value of 63% against L. major, whereas 101 with PYR exhibited
greater than expected growth inhibition (60%) against T. cruzi. T.
cruzi displayed smaller differences in growth inhibition values than
did L. major when the mentioned compounds were combined with
known inhibitors (SI Appendix, SI Fig. 20). This differential effect
may be related to the presence of additional targets (5, 21) specific
to T. cruzi that could increase the growth inhibition caused by
nonspecific DHFR inhibition.
The result of the combination of each selected compound 6a and
6b at 50 M with PYR at 10 M (the latter producing 66%
amastigote growth inhibition when administered alone at that
concentration) on intracellular amastigote forms of T. cruzi was
additive, showing an amastigote growth inhibition of 83–88% when
both compounds were mixed at the above concentrations (SI
Appendix, SI Fig. 12). Compounds 6a and 6b showed a lower toxicity
on mammalian cells (EC50 values of 580 and 331 M, respectively)
with respect to PYR (EC50 22 M). Consequently, the coadministration of compound 6a (50 M) with PYR (10 M) efficiently
reduced the toxicity of the treatment necessary to obtain 85%
amastigote growth inhibition with a significantly reduced effect on
Vero cells (25% of Vero cell death vs. 39% of Vero cell death by
PYR required concentration to reach the same 85% amastigote
growth inhibition).
Concluding Remarks. The screening of a folate-analogue library
identified a potent selective inhibitor of LmPTR1, named compound 6a. Crystallographic analysis revealed 6a interacting with the
enzyme with a binding mode in which the pteridine is rotated by
180° with respect to the pteridine of MTX. Based on this finding,
1452 兩 www.pnas.org兾cgi兾doi兾10.1073兾pnas.0704384105
additional compounds were retrieved from the library, seeking to
investigate the structural features responsible for this altered binding mode and to improve binding affinity. Compound 6b, with an
N10 methyl substituent, is well suited to the purpose of understanding the involvement of this group in the establishment of a particular
binding mode. The crystal structure of LmPTR1-NADPH-6b
suggested that 6b is able to satisfy the hydrogen binding capacity
offered by the active site, and the cost of disruption of a water
network might explain the different orientations observed. The
database of folate analogues was reexamined to identify compounds showing similarity with 6a and inhibition constants determined. The best PTR1 inhibitors were tested against WT and
engineered L. major and T. cruzi parasites overexpressing PTR1,
but these compounds showed poor efficacy when tested singly.
However, when used in combination with known DHFR inhibitors,
especially PYR, an additive profile was observed to be deleterious
to parasite cell growth. The same behavior was observed when using
the intracellular amastigote forms of T. cruzi, with the advantage of
a reduced toxicity of treatment because of lower concentrations of
DHFR-TS targeting drug. This work describes a successful combination of two antifolates designed to target DHFR-TS and PTR1.
The challenge now is to exploit our understanding of this system to
explore and develop a more potent combination that provides
synergistic inhibition at a level that ultimately might provide an
alternative therapeutic approach against this important class of
pathogens.
Materials and Methods
Chemistry. The synthesis of 368 compounds in the library is published (e.g., refs.
22–24).
Synthesis of compound 6a. To a solution of methyl 1-(4-aminobenzoyl)piperidine4-carboxylate (compound 4a, SI Appendix, SI Text) (100 mg, 0.36 mmol) in 3 ml of
DMA was added an equimolar amount of 6-bromomethyl-2,4-diaminopteridine
hydrobromide (compound 7, SI Text), synthesized as referenced (23–25). The
solution was stirred and kept at room temperature for 90 h. The solvent was
removed by evaporation in vacuo (⬍1 mm Hg, bath to 45°C). The residue was
purified by flash chromatography using CHCl3/MeOH (9:1, vol/vol) as eluent to
afford the 6a as a solid (65 mg, 50% yield).
Synthesis of compound 6b. To a solution of methyl 1-(4-methylaminobenzoyl)piperidine-4-carboxylate (compound 5, SI Appendix, SI Text) (120 mg, 0.43
mmol) in 3 ml of DMA was added an equimolar amount of 6-bromomethyl2,4-diaminopteridine hydrobromide (compound 7, SI Appendix, SI Text),
synthesized as reported (22–24). The solution was stirred and kept at room
temperature for 84 h. The solvent was removed by evaporation in vacuo (⬍1
mm Hg, bath to 45°C). The residue was purified by flash chromatography using
CHCl3/MeOH (9:1, vol/vol) as eluent to afford 6b as a solid (110 mg, 50% yield).
Synthetic methods for the remaining 70 compounds together with selected
analytical data are described in the SI Appendix, SI Text.
Analytical Methods. Melting points were recorded on a Köfler hot stage or
Digital Electrothermal melting point apparatus and are uncorrected. Infrared
spectra were recorded as nujol mulls on NaCl plates with a Perkin–Elmer 781 IR
spectrophotometer and are expressed in (cm⫺1). UV spectra are qualitative and
were recorded in nanometers for solutions in EtOH with a Perkin—Elmer Lamba
5 spectrophotometer. NMR (1H-NMR and NOE difference) spectra were determined in CDCl3, DMSO-d6, CDCl3/DMSO-d6 (in the ratio 1:3) with a Varian XL-200
(200 MHz). Chemical shifts (␦ scale) are reported in parts per million downfield
from tetramethylsilane as internal standard. Splitting patterns are designated as
follows: s, singlet; d, doublet; t, triplet; q, quadruplet; m, multiplet; br s, broad
singlet; and dd, double doublet.
The assignment of exchangeable protons (OH and NH) was confirmed by the
addition of D2O. Mass spectroscopy was performed on combined HP 5790 –HP
5970 GC/MS apparatus or with a combined liquid chromatograph–Agilent 1100
series Mass Selective Detector (Agilent). Elemental analyses were performed on
a Perkin–Elmer 2400 instrument, and the results for C, H, and N were within ⫾
0.4% of theoretical values.
Materials. Analytical TLC was performed on Merck silica gel F-254. For flash
chromatography Merck silica gel 60 was used with a particle size 0.040 – 0.063
mm (230 – 400 mesh; Americal Society for Testing and Materials). Commercial
reagents were used without further purification.
Cavazzuti et al.
Crystallographic Analyses. Crystallographic analyses followed established methods (11, 27) and are detailed in SI Appendix together with statistics (SI Appendix,
SI Table 2) and examples of omit electron density maps (SI Appendix, SI Fig. 5).
Target Protein Expression and in Vitro Inhibition Assay of Purified Enzymes.
Proteins were purified as described (28 –36). For the determination of TS activity,
the oxidation of cofactor N5,N10-methylentetrahydrofolic acid was followed
spectrophotometrically at 340 nm (29, 30). For determination of reductase activity (hDHFR, LmDHFR, TcDHFR, LmPTR1, and TcPTR1), NADPH oxidation was
followed at 340 nm (8). All kinetic studies were performed in continuous assays
executed in a Beckman DU640 spectrophotometer (26). The activity assays for
PTR1 inhibition studies were performed at 303 K in the presence of NADPH
(usually 100 M) and folic acid (30 M) at pH 6.0 in sodium phosphate buffer. For
this reaction, PTR1 was incubated with folic acid and the reaction initiated with
NADPH. Km was determined by measuring the enzyme activity dependence on
substrate concentration using folic acid as a substrate. Average Km was 8 M for
PTR1. The inhibition assays were performed by adding increasing concentrations
of inhibitor solubilized in 100% DMSO. DMSO concentration was kept below the
concentration affecting enzymes activity (1% for PTR1, 8% for TS).
pTEX, and WT-pTEX-PTR1 (35) epimastigote forms of T. cruzi (CL-Brener II strain)
strains were grown in liver infusion tryptose medium supplemented with 10%
hiFBS at 28°C. The parasite cultures were initiated with 4 ⫻ 106 cells per ml⫺1 and
collected in the exponential phase of parasite growth. Trypomastigote forms of
T. cruzi were obtained according to the method described (37) by infection of
Vero cells (American Type Culture Collection CRL-1586), maintained in RPMI
medium 1640 (Gibco BRL) supplemented with 5% FBS at 37°C. Semiconfluent
Vero cell cultures were infected with trypomastigotes to obtain both amastigote
and trypomastigote forms of T. cruzi.
Parasite Growth Inhibition Studies. The inhibition profile of selected compounds
on the WT and transfected protozoan parasites was assessed by culturing the cells
in the presence of different concentrations of inhibitors. After 72 h of incubation
at 28°C, the viability of parasites was determined by the colorimetric assay (MTT,
[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide]) (38, 39). For
drug evaluation on the intracellular amastigote forms of T. cruzi, we used an
experimental model of intracellular forms obtained after the infection of Vero
cells, as host mammalian cells, with the infective trypomastigote metacyclic forms
of T. cruzi (37). Infection was obtained by inoculation and in vitro culture of Vero
cells with trypomastigote forms at a rate of 10 parasite per cell. After 4 h of
infection, infected cell cultures were washed with PBS, and fresh culture media
containing the selected inhibitors were added. After 72 h of culture, samples
were fixed with p-formaldehyde 4% and stained with the nucleus-selective
fluorescent marker DAPI (2-(4-amidinophenyl)-6-indolecarbamidine). Counting
of intracellular amastigotes was performed by ‘‘Analize Particles’’ function in
image processing software ImageJ 1.38x (http://rsb.info.nih.gov/ij) 1.38x on 10
random selected visual camps for sample analysis were evaluated and an average
of 250 mammalian cells counted.
Synergy Evaluation. For combinatory-effect determination of selected inhibitors,
extracellular parasites were exposed in separate wells to EC50 doses of commercial
inhibitors and selected compounds, defined in previous cytotoxicity assays.
Within the same 96-well plate, the viability of parasites was assessed concomitantly with isoeffective concentrations of selected compounds and commercial
inhibitors (SI Appendix, SI Fig. 20). Using intracellular amastigotes of the protozoan parasite T. cruzi, we assessed the mean number of parasites per infected cell
after treatment with inhibitors administered singularly. The sum of the independent percentage of amastigote growth inhibition was compared with the effect
obtained by coadministration of inhibitors at the same concentrations (40, 41).
Cell Culture. WT, WT- pX, and WT-pX-PTR1 (SI Appendix, SI Text and SI Fig. 9)
promastigote forms of L. major (strain 260) were grown at 28°C in M-199 medium
(Gibco) supplemented with 10% heat-inactivated FBS (hiFBS, Gibco). WT, WT-
ACKNOWLEDGMENTS. This work was funded by Ministero Istruzioni Università
Ricerca–Fondi di Investimento per la Ricerca di Base RBAU01S38Z (M.P.C.), the
Wellcome Trust and Biotechnology and Biological Sciences Research Council
(W.N.H.), Spanish Grant ISCIII-Red de Investigación Cooperativa en Enfermedades
Tropicales (RICET) RD06/0021/0002 (to F. Gamarro), and the European Union
Marie Curie Host Fellowships (QLK2-CT-2001-60091 (to F. Gibellini). We thank the
European Synchrotron Radiation Facility for beam time, Charles Bond and Alexander Schüttelkopf for discussions, Eprova for providing substrates, and S. Beverley, Washington University, St. Louis, MO, for reagents.
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Cavazzuti et al.
PNAS 兩 February 5, 2008 兩 vol. 105 兩 no. 5 兩 1453
BIOCHEMISTRY
Selection of Inhibitors. Compounds were selected on the basis of a lack of
antiproliferative activity toward human cell lines with reference to the National
Cancer Institute, Bethesda [National Cancer Institute (NCI)] studies for anticancer
agents. The data were available following an in vitro disease-oriented antitumor
screening program against a panel of 60 human tumor cells lines, in the NCI. The
compounds showed an inhibitory potency ⬍40% in a dose–response curve, in the
NCI test that produced 50% growth inhibition, total growth inhibition, and 50%
cytotoxicity data (12, 25).
The second criterion was molecular diversity. The database contained quinoxalines and 2,4-diaminopteridines. Simple molecular properties such as solubility, presence of a glutamate tail, salt, or free carboxylic acid functionality were
also considered in selection. Finally, from ⬎400 compounds, 131 were tested in
rapid screening assays.
The molecules were tested against the following enzymes. Human TS and
DHFR, Escherichia coli TS, Lactobacillus casei TS, Enterococcus faecalis TS, L. major
DHFR-TS, T. cruzi DHFR-TS, L. major PTR1, T. cruzi PTR1. Raw IC50 values against
the different enzymes, assuming competitive inhibition with respect to substrate,
were obtained and Ki values calculated (26). For TS, the competitor substrate was
N5,N10-methylenetetrahydrofolate; for DHFR, dihydrofolic acid, and for PTR1,
folic acid. Data analysis was conducted and the first considerations were performed on the grouping of human and non-human enzyme inhibitors. Assays
were carried out in triplicate, and no individual measurement differed by ⬎20%
from the mean.