Lead optimization of a pyrazole sulfonamide series of trypanosoma brucei N -myristoyltransferase inhibitors: Identification and evaluation of CNS penetrant compounds as potential treatments for stage 2 human african trypanosomiasis

Article abstract of DOI:10.1021/jm500809c

Trypanosoma brucei N-myristoyltransferase (TbNMT) is an attractive therapeutic target for the treatment of human African trypanosomiasis (HAT). From previous studies, we identified pyrazole sulfonamide, DDD85646 (1), a potent inhibitor of TbNMT. Although this compound represents an excellent lead, poor central nervous system (CNS) exposure restricts its use to the hemolymphatic form (stage 1) of the disease. With a clear clinical need for new drug treatments for HAT that address both the hemolymphatic and CNS stages of the disease, a chemistry campaign was initiated to address the shortfalls of this series. This paper describes modifications to the pyrazole sulfonamides which markedly improved blood-brain barrier permeability, achieved by reducing polar surface area and capping the sulfonamide. Moreover, replacing the core aromatic with a flexible linker significantly improved selectivity. This led to the discovery of DDD100097 (40) which demonstrated partial efficacy in a stage 2 (CNS) mouse model of HAT.

Full text of DOI:10.1021/jm500809c

Article
pubs.acs.org/jmc
Lead Optimization of a Pyrazole Sulfonamide Series of Trypanosoma
brucei N‑Myristoyltransferase Inhibitors: Identification and
Evaluation of CNS Penetrant Compounds as Potential Treatments for
Stage 2 Human African Trypanosomiasis
Stephen Brand, Neil R. Norcross, Stephen Thompson, Justin R. Harrison, Victoria C. Smith,
David A. Robinson, Leah S. Torrie, Stuart P. McElroy, Irene Hallyburton, Suzanne Norval, Paul Scullion,
Laste Stojanovski, Frederick R. C. Simeons, Daan van Aalten, Julie A. Frearson, Ruth Brenk,
Alan H. Fairlamb, Michael A. J. Ferguson, Paul G. Wyatt, Ian H. Gilbert,* and Kevin D. Read*
Drug Discovery Unit, Division of Biological Chemistry and Drug Discovery, College of Life Sciences, University of Dundee, Sir James
Black Centre, Dundee DD1 5EH, U.K.
S
* Supporting Information
ABSTRACT: Trypanosoma brucei N-myristoyltransferase (TbNMT) is an attractive therapeutic target for the treatment of
human African trypanosomiasis (HAT). From previous studies, we identified pyrazole sulfonamide, DDD85646 (1), a potent
inhibitor of TbNMT. Although this compound represents an excellent lead, poor central nervous system (CNS) exposure
restricts its use to the hemolymphatic form (stage 1) of the disease. With a clear clinical need for new drug treatments for HAT
that address both the hemolymphatic and CNS stages of the disease, a chemistry campaign was initiated to address the shortfalls
of this series. This paper describes modifications to the pyrazole sulfonamides which markedly improved blood−brain barrier
permeability, achieved by reducing polar surface area and capping the sulfonamide. Moreover, replacing the core aromatic with a
flexible linker significantly improved selectivity. This led to the discovery of DDD100097 (40) which demonstrated partial
efficacy in a stage 2 (CNS) mouse model of HAT.
glycine of a large number of proteins, a modification which is
implicated in localization and/or activation of the substrate.4,5
The enzyme operates via a Bi−Bi mechanism in which it first
binds myristoyl-CoA, causing a conformational rearrangement,
which subsequently reveals the peptide binding site.6 In T.
brucei, RNAi knockdown of NMT has been shown to be lethal
in cell culture7 and to abrogate infectivity in animal models of
HAT.8 Bioinformatics analysis suggests that about 60 proteins
are myristoylated in the parasite,9 although there is incomplete
knowledge of the downstream targets.10 NMT has also been
investigated as a potential target for the treatment of other
parasitic diseases including malaria,11 leishmaniasis,12 and
Chagas disease.13
INTRODUCTION
■
Human African trypanosomiasis (HAT) is caused by two
subspecies of the protozoan parasite Trypanosoma brucei,
Trypanosoma brucei gambiense and Trypanosoma brucei
rhodesiense, transmitted by the bite of an infected tetse fly.1,2
The disease is fatal unless treated. It has two stages: an initial
(hemolymphatic) peripheral infection during which the para-
sites are found in the bloodstream and gives rise to nonspecific
symptoms, and a second stage during which the parasites enter
the central nervous system (CNS), giving rise to the classic
symptoms of HAT, eventually leading to coma and death.
Currently, there are five treatments available, although none of
them are satisfactory, due to toxicity, treatment failures, and the
requirement for parenteral administration that is inappropriate
in a rural African setting.3
N-Myristoyltransferase (NMT) catalyzes the cotranslational
Received: May 27, 2014
transfer of myristate from myristoyl-CoA to the N-terminal
© XXXX American Chemical Society
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Recently, we have reported the discovery of DDD85646 (1,
Figure 1), a very potent inhibitor of T. brucei NMT (TbNMT)
RESULTS AND DISCUSSION
■
In this paper, we report the systematic optimization of both the
blood−brain barrier penetration and selectivity of 1, which has
led to the discovery of CNS penetrant TbNMT inhibitors
suitable for progression into in vivo proof-of-concept studies in
mice for the second stage of HAT. A summary of our
structure−activity findings is shown in Figure 2.
Figure 1. Biological and physicochemical profile of the prototypical
NMT inhibitor 1.
(IC50 = 0.002 μM) and of the growth of T. brucei (EC50 = 0.002
μM), which delivers potent activity in mouse models of stage 1
HAT (fully curative at 12.5 mg/kg b.i.d. po for 4 days, T. b.
brucei S427; fully curative at 50 mg/kg b.i.d. po for 4 days, T. b.
rhodesiense STIB900).14 Combined with further biological
mode of action studies, this has provided excellent validation
of NMT as a therapeutic target for the peripheral stage of
HAT.15
Figure 2. Summary of structure−activity relationships of (1).
Unfortunately, we were unable to obtain crystal structures of
TbNMT. However, we were able to obtain structural
information for the Leishmania major NMT (LmNMT) and
Aspergillus fumigatus NMT (Af NMT). The TbNMT, LmNMT,
Af NMT, and HsNMT have very high sequence identity in the
binding site for these class of inhibitors (Supporting
Information, Table S7). Therefore, we decided to use the
LmNMT and Af NMT as surrogates. These should be of use in
determining the binding mode of the inhibitors and suggesting
vectors for chemistry optimization. However, given the
similarities of the active sites, using the crystallography to
derive selectivity may be very challenging, especially where
factors such as the conformational flexibility of the protein and
interactions with water may play a key role.
Optimization of the Pyrazole Head Group. We had
previously shown that we were unable to vary the pyrazole
headgroup for other heterocycles without significant loss in
potency and that removal of the N-methyl or replacements with
other alkyl groups gave a significant loss in activity.14 From the
Leishmania major cocrystal structure with 1, the N1-methyl
group binds in a hydrophobic pocket and is important in fixing
N2 as hydrogen bond acceptor in a key hydrogen bonding
interaction with the hydroxyl side chain of a highly conserved
serine residue. However, substituents bigger than methyl on N1
would be expected to suffer a clash in the active site (see Figure
3).
The importance of both the pyrazole 3- and 5-methyl groups
can be seen by comparing 2, 3, and 4 with 1 (see Table 1),
which were prepared according to Scheme 1. It was possible to
remove each of the methyls independently without affecting the
enzyme potency, but removal of both methyls caused a
significant loss in enzyme potency. Interestingly, removal of the
3-methyl group appeared to cause a reduction in parasite
potency in contrast to removal of the 5-methyl group (i.e., 2 vs
3). The 3- and 5-methyl groups both make hydrophobic
contacts with the protein. It was possible to increase the size of
the substituent at either the 3- or 5-positions (5 and 6),
Unfortunately, 1 is not active in the stage 2 HAT mouse
model (T. b. brucei GVR35; 100 mg/kg b.i.d. po for 5
days).14,15 However, this was unsurprising, as 1 poorly
penetrates the blood−brain barrier and is a substrate for P-
glycoprotein, thus preventing delivery of efficacious free brain
concentration at a tolerated dose (blood:brain ratio is <0.05 in
mouse and <0.09 in rat, rising to 0.27 in rat in the presence of
the Pgp inhibitor GF120918;16 experimental in Supporting
Information). Furthermore, 1 has relatively poor/no selectivity
at the enzyme level compared to the two human orthologues
HsNMT1 and HsNMT2 (IC50 = 0.003 μM), although this did
not translate to low selectivity at the cellular level (cellular
selectivity ratio, MRC-5 EC50/T. brucei EC50 = 150). While
there was no observable toxicity in rodents at therapeutic doses,
there was a relatively low therapeutic index (stage 1 HAT
minimal curative dose = 12.5 mg/kg b.i.d. po [T. b. brucei S427]
or 50 mg/kg b.i.d. po [T. b. rhodesiense STIB900]; MTD = 100
mg/kg b.i.d. po). It is unknown if the low safety margin in vivo
is caused by inhibition of mouse NMT or another off-target
effect. However, to try and eliminate the former, it was decided
to improve the enzymatic selectivity. Typically, selectivity
between two enzymes of interest is expressed as a ratio of their
biochemical IC50 values (which in this case would be HsNMT/
TbNMT). However, during the course of this study, we
discovered that many compounds were very potent inhibitors
of TbNMT, which due to the enzyme concentration used in the
assay (10 nM) have IC50 values approaching the observable
tight-binding limit (as evidenced by Hill slopes ≫1) and there
is thus very little differentiation between the most potent
compounds. Because we have already shown for this series that
TbNMT IC50 is proportional to T. brucei EC50, our preferred
means of defining selectivity (S) was therefore to use activity
against the parasite as a surrogate for TbNMT IC50 when
comparing activity against inhibition of HsNMT, to give a good
rank order:
S = IC50HsNMT/EC50T. brucei
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a
Scheme 1. Synthesis of Compounds in Table 1
a
(i) RNH2, pyridine/DCM; (ii) Pd(PPh3)4, 2-(1-piperazinyl)pyridine-
4-boronic acid pinacol ester, K3PO4, THF/H2O.
Figure 3. Crystal structure of 1 bound to LmNMT (PDB 2WSA),
highlighting the pyrazole sulfonamide pocket. The pyrazole N-methyl
packs tightly into the hydrophobic pocket, however, there is scope to
project from the 3- and 5- substituents and the solvent exposed
sulfonamide nitrogen.
than 1, compound 6 was nonselective, with no overall
improvement in stage 1 HAT therapeutic window (2−3-fold;
MTD in mice 20 mg/kg b.i.d. po). Indeed, the very high
potency of 6 for HsNMT could be the driving force for a high
although there was no apparent increase in enzyme potency.
However, the iso-butyl substituent on the 3-position (6) gave a
significant increase in activity against the parasite.
As well as being one of the most active compounds from this
series, compound 6 also benefited from having nearly a 10-fold
antiproliferative effect against the MRC-5 cell line (EC50 =
0.090 μM), thus eliminating it from further progression for
HAT. Moreover, compound 6 also had very low brain
penetration in mouse (brain:blood ratio <0.1). So, although
potency was increased, there was no overall gain in terms of
selectivity or brain penetration.
higher oral exposure in mouse than 1 (10 mg/kg po AUC0−8
,
3400 μg min/mL cf. 392 μg min/mL; bioavailability 93% cf. 20
%) and correspondingly demonstrated superior efficacy to 1 in
the stage 1 model in mice infected with T. b. rhodesiense
STIB900 (minimal curative dose was 6 mg/kg cf. 50 mg/kg
when dosed for 4 days b.i.d. po). Although it had higher efficacy
Investigation of SAR at the Central Aromatic Ring.
One or both of the chloro substituents on the central aromatic
ring of 1 were replaced with fluorine, methyl, or hydrogen,
intended to modulate localized van der Waals interactions, but
this did not have a significant effect on activity or selectivity for
Table 1. Modifications to the Head Group Pyrazole
a
b
IC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2-fold from the IC50. Compounds with
Hill slopes >1.5. S = IC50 HsNMT/EC50T. brucei. B:B is the brain:blood ratio in mouse. nd = not determined.
C
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TbNMT (see Scheme S1 and Table S1, Supporting
Information).
activity at either enzyme or cellular level. Removal of the
piperazine nitrogen atom in addition to the pyridine (i.e., 9) did
not significantly affect activity against TbNMT, although there
was a reduction in activity (∼10-fold) against the parasite.
Although 8 and 9 have reduced PSA compared to 1 (PSA = 79
and 76 Å2 vs 92 Å2, respectively), the desired improvement in
the brain:blood ratio was not forthcoming with 8. Capping
(alkylating) of the piperazine nitrogen (10) caused a noticeable
reduction in potency against the parasite.
The Effect of Modifications to the Linker. Previous
work14 had indicated that compounds with an alkyl chain
replacing the lower biaryl aromatic linker of 1 had a better
degree of selectivity (up to 60-fold) at the enzyme level (see
compounds in Figure 4). Therefore, it was decided to
Optimization of the Piperazine Ring. It was decided to
investigate the influence of substituents α- to the terminal
nitrogen of the piperazine on the selectivity of 1. Reference to
the structure of 1 in LmNMT showed a pocket which was not
accessed but could accommodate extra functionality (see Figure
S1, Supporting Information). This investigation found that
small substituents were tolerated but did not give a significant
rise in selectivity, S. Larger substituents caused a loss in activity
(see Scheme S2 and Table S2, Supporting Information).
The effect of removing one or both of the pyridyl and
internal piperazine nitrogens (Scheme 2) was investigated with
a
Scheme 2. Synthesis of Compounds in Table 2
a
(i) Pd(PPh3)4, K3PO4, THF/H2O and a boronate ester, (2-(1-
piperazinyl)pyridine-4-boronic acid pinacol ester for compound 1, 2-
(1-piperazinyl)phenyl-4-boronic acid pinacol ester for compound 8, 2-
(4-piperidinyl)phenyl-4-boronic acid pinacol ester for compound 9,
and 2-(4-methyl-1-piperazinyl)pyridine-4-boronic acid pinacol ester
for compound 10 (see Table 2 for precise structures).
Figure 4. Key compounds as reported previously in ref 14.
the aim of reducing the PSA to increase CNS penetration
because high PSA is well-known to negatively correlate with
passive membrane permeability (see Table 2).17,18 Removal of
the pyridyl nitrogen atom (i.e., 8) had virtually no effect on
investigate modification of the linker. Scheme 3 summarizes
the synthetic routes employed to prepare linker-modified
compounds 12−23. Compounds 12 and 13 were prepared
using the Sonogashira reaction19 of sulfonamide 7 with 5-
Table 2. Modifications to the Piperazine
a
b
IC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2-fold from the IC50. Compounds with
Hill slopes >1.5. S = IC50 HsNMT/EC50 T. brucei. B:B is the brain:blood ratio in mouse. nd = not determined.
D
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a
Scheme 3. Synthesis of Compounds in Table 3
a
(i) Alkene, 9-BBN, THF, 90 °C, then aryl bromide 7 or 11, Pd(PPh3)4, K3PO4, THF/H2O; (ii) alkene, 9-BBN, THF, 90 °C, then aryl bromide 7 or
11, Pd(PPh3)4, K3PO4, THF/H2O followed by TFA, DCM; (iii) 7, alkyne, CuI, Pd(PPh3)4, HNiPr2.
ethynyl-1-methyl-1H-imidazole and 1-methyl-4-(prop-2-yn-1-
yl)piperazine, respectively. In general, sulfonamides 14−23,
which bear a pendant amine attached via a saturated alkyl chain,
were prepared using a 9-BBN mediated boron-alkyl Suzuki-type
coupling reaction20 between aryl bromide 7 or 11 and the
appropriate alkene, i.e. compounds 14−17. Compounds 20 and
21 were prepared by direct addition of the appropriate alkene
while secondary amines 18, 19, 22, and 23 were prepared in
two steps by coupling with the corresponding BOC-protected
alkenes, followed by deprotection with TFA/DCM.
Rigid propargylic systems bearing protonatable tail groups
did not have appreciable activity (i.e., 12 and 13) (see Table 3).
Attachment of the terminal piperazine to the aryl core via a
flexible propyl linker (i.e., 14) not only retained reasonable
activity but resulted in a marked increase in isoform selectivity
(>100-fold) relative to 1. We cannot rationalize the origin of
this selectivity by crystallography, as we do not have structures
of the TbNMT and HsNMT complexes. Homology models
predict very similar structures in the active sites; therefore, the
selectivity may be due to differences in the protein structures
remote from the active site, differential protein dynamics21 or
the differential effects of interactions with water in the active
site.
was fully curative in a stage 1 mouse model (T. b. brucei S427)
at a dose of 4 × 50 mg/kg b.i.d. po for 4 days. Work
subsequently focused on increasing CNS penetration (brain:-
blood ratio of 14 < 0.1) and selectivity of this new framework.
Removal of the internal piperazine nitrogen of 14 resulted in a
10-fold improvement in potency (16) (EC50 = 0.002 μM) and
also a small reduction in PSA. Although deletion of this
nitrogen appeared to cause a reduction of selectivity at the
biochemical level compared to 14 (from >100-fold to
approximately 16-fold), comparison of selectivity (S) shows
that this modification is selectivity neutral.
Extending the linker by an additional methylene unit (i.e., 20
cf. 16 and 22 cf. 18) increased activity substantially, resulting in
subnanomolar potency against the parasite, although notable
gains in selectivity were not achieved with the increased
molecular flexibility. Comparison of the structures of 19 and 23
in LmNMT shows that the extra methylene in the linker does
not significantly impact the binding mode of the inhibitor (data
not shown). Removal of the piperidine N-methyl groups of 16
and 20 appeared to reduce potency marginally. Removal of the
chlorine atoms from 14, 16, 18, 20, and 22 consistently
reduced potency by at least 10-fold (i.e., 14 vs 15; 16 vs 17 and
20 vs 21 etc.). Indeed, this appeared to be a general
phenomenon within this subseries. The impact of removing
the piperidine methyl group on selectivity was greater for the
three-carbon linker than the four-carbon linker. Unfortunately,
none of the compounds in Table 3 have a brain:blood ratio
>0.1 (14, 16, and 17 were tested).
The structure of 14 bound to Af NMT (Figure 5) confirmed
that substitution of the rigid biaryl system with a flexible linker
does not alter the binding mode with the piperazine NH
interacting with the C-terminal carboxylate via a conserved
water molecule, as described for 1.
Compound 14 was selected as a new lead because, in
addition to having significantly improved selectivity (S = 20), it
Improving CNS Penetration by “Capping” the
Secondary Sulfonamide. It is notable that there are very
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Table 3. Modifications to the Linker
a
b
IC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2-fold from the IC50. Compounds with
Hill slopes >1.5. S = IC50 HsNMT/EC50 T. brucei.
7.0 0.5, calculated using ACDlabs software, version 11.02),
suggesting the sulfonamide is significantly ionized at physio-
logical pH. Moreover, the secondary sulfonamide exerts a
considerable contribution to the overall PSA of compounds in
this series (approximately 54 Å2). Therefore, the sulfonamide
was substituted with a variety of capping groups designed to
simultaneously reduce PSA and preclude deprotonation of the
sulfonamide.
Capping the sulfonamide nitrogen of 7 with a methyl group
(i.e., 24) was found to significantly enhance brain penetration
in mice (brain:blood ratio of 7 = <0.1 cf. brain:blood ratio of 24
= 3.7) without unduly affecting potency. Indeed, this
modification caused a significant enhancement in CNS
Figure 5. Binding mode of 14 (C atoms gold) bound to Af NMT
showing the interaction between the piperazine NH and a water
molecule coordinated with the C-terminal carboxylate. The binding
mode of 1 bound to LmNMT (C atoms slate) is shown for
comparison of the piperazine moieties.
penetration across a variety of inhibitors bearing different
terminal amines, implicating the sulfonamide moiety as the
principal cause of the poor CNS penetration of the parent
compounds (see Table 4).
In general, the compounds bearing a tethered amine in Table
4 were prepared via a Suzuki-type reaction20 with sulfonamides
24−28, which had been modified by N-alkylation of
sulfonamide 7 (Scheme 4). For instance, compound 40 was
prepared by the 9-BBN mediated addition of 4-allyl-1-
methylpiperidine with 28. Notably, we believe this is the first
few examples of CNS penetrant secondary sulfonamides in the
literature.22−24 The secondary sulfonamide has an appreciable
acidity, particularly in compounds where the sulfonyl aromatic
bears two halogen atoms (i.e., pKa of 7 = 6.9 0.5, pKa of 1 =
F
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Table 4. Effect on the Brain:Blood Ratio of “Capping” the Sulfonamide with an Alkyl Group
a
b
IC50 values are shown as mean values of two or more determinations. Y = Cl for all compounds except compounds 30, 31, 32, and 38, where Y =
c
H. The blood:brain ratio was unchanged when measured in wild-type and mdr1a-deficient mice, indicating that neither are Pgp substrates.
d
e
Compounds with Hill slopes >1.5. Cli measured in mL min−1 g−1. B:B is the brain:blood ratio in mouse. nd = not determined.
reported application of the difluoromethylation of a secondary
sulfonamide, achieved by reaction of 7 with sodium
chlorodifluoroacetate.
supports this theory (see the metabolite identification study,
Table S4 in Supporting Information).
Optimization of the Sulfonamide Capping Group. A
variety of compact fluoroalkyl groups were surveyed as
sulfonamide caps with the aim of improving metabolic stability
and therefore oral exposure (compounds 40 to 42 in Table 4).
The 2-fluoroethyl analogue was not included in this study due
to its reported potential for conversion to fluoroacetic acid, a
metabolic poison.25 On the basis of higher potency and lower
PSA, piperidine 37 instead of 33 was adopted as the test-bed
for this investigation.
Replacement of the methyl group of 37 with a 2,2-
difluoroethyl group (i.e., 41) and 2,2,2-trifluoroethyl group
(i.e., 42) successfully resulted in a reduction in microsomal
turnover as desired (Cli = 7.3, 2.2, and 1.1 mL/min/g,
respectively), albeit at the expense of potency, which was
reduced approximately 5-fold in both cases. Encouragingly,
however, the novel difluoromethylated sulfonamide 40
demonstrated a potency profile similar to the parent compound
37 and had significantly improved stability (i.e., Cli 2.5 mL/
min/g compared to 7.3 mL/min/g for 37). In addition to
benefiting from greater mouse microsomal stability compared
to 33, 34, 37, and 39, compound 40 did not undergo
sulfonamide N-dealkylation (see metabolite identification
study, Table S4 in Supporting Information). Consistent with
greater sulfonamide stability relative to 33, compound 40
demonstrated good oral exposure in mouse and crucially
Methylation of the sulfonamide of lead compound 14 (i.e.,
33) demonstrated that a compound within this series could
deliver the desired profile for potency (TbNMT IC50 = 0.003
μM, EC50 = 0.010 μM), selectivity (S = 19), and good brain
penetration (brain:blood ratio = 0.6). However, 33 exhibited
very poor metabolic stability when incubated with mouse
microsomes relative to its parent secondary sulfonamide 14 (Cli
= 7.4 cf. 1.7 mL/min/g, respectively). While both microsomal
stability and CNS exposure could be improved by resorting to
the ethyl sulfonamide analogue 34 (brain:blood ratio = 1.3 cf.
0.6; Cli = 3.1 cf. 7.4 mL/min/g for 34 and 33, respectively),
microsomal instability was still higher than compound 1, and
this compound had poor oral exposure in mice. Similarly, the
more potent piperidine congener of 33, i.e., 37, also exhibited a
higher microsomal turnover relative to its parent secondary
sulfonamide (i.e., compare 37 vs 16, Cli = 7.3 cf. 4.3 mL/min/g,
respectively). An hepatic portal vein study in male Sprague−
Dawley rat dosed orally with 33 (3 mg/kg) indicated almost
total hepatic extraction of the parent (see Figure S2 in
Supporting Information), and sulfonamide dealkylation was
suspected as the principal mode of elimination. Incubation of
compounds 33, 34, 37, and 39 with mouse microsomes, which
showed rapid disappearance of parent and the N-dealkylated
sulfonamide as the predominant metabolite for all compounds,
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a
Scheme 4. Synthesis of Compounds in Table 4
a
−
(i) NaH, DMF, MeI or EtBr; (ii) CF3CH2OMs, K2CO3, CH3CN; (iii) CHF2CH2I, K2CO3, CH3CN; (iv) Na+ CF2ClCO2 , K2CO3, CH3CN; (v)
alkene, THF, 9-BBN, then aryl bromide 24−28, Pd(PPh3)4, K3PO4, THF/H2O; (vi) 3-formylphenyl boronic acid, Pd(PPh3)4, K3PO4, THF/H2O;
(vii) piperazine, NaBH(OAc)3, CHCl3; (viii) Pd(PPh3)4, 2-(1-piperazinyl)pyridine-4-boronic acid pinacol ester, K3PO4, THF/H2O; (ix) pent-4-en-
1-ol for compound 35 or hex-5-en-1-ol for compound 36, THF, 9-BBN, then aryl bromide 24, Pd(PPh3)4, K3PO4, THF/H2O; (x) MsCl, NEt3,
pyridine, then imidazole.
Figure 6. Stereo diagram showing comparison of binding modes of 43 (C atoms gold) and 1 (C atoms cyan) showing retention of sulfonamide
configuration after capping with a CHF2 group.
retained appreciable CNS penetration (brain:blood ratio = 1.6).
Unfortunately, some of the compounds described above
showed significant inhibition of hERG, which would need
further investigation if these compounds were to be progressed
(i.e., 40 hERG IC50 = 0.6 μM cf. 1 hERG IC50 = 28 μM).
Further Optimization of the Linker and Amine on a
Tertiary Sulfonamide. Due to its superior potency and
improved metabolic stability the difluoromethyl group was
retained as the preferred sulfonamide cap in a final study sought
to further optimize potency and selectivity of 40 by making
modifications in both the tether and the pendant amine
portions. The crystal structure of 43 bound to AfNMT
confirms that the binding mode of the sulfonamide is largely
unaffected by capping with the CHF2 group (see Figure 6).
Compounds were prepared using analogous procedures to
those already described according to the conditions shown in
Scheme 5. This study resulted in the identification of a large
number of highly trypanocidal compounds, some with good
CNS penetration (see Table 5 and Table S3, Supporting
Information). Unfortunately, however, it proved challenging to
obtain substantial selectivity at the enzyme level and there was
not an observable trend. Although primary potencies against
TbNMT remain substantially unvaried in the low nanomolar
range, there is wider variation in activity against the parasite;
this is probably a consequence of the inability of the enzyme
H
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a
Scheme 5. Synthesis of Compounds in Table 5
a
(i) Alkene, THF, 9-BBN, 90 °C for 1 h, then sulfonamide 28, Pd(PPh3)4 or Pd(dppf)2·DCM, K3PO4, THF/H2O; (ii) TFA, DCM; (iii) MsCl,
NEt3, DCM , then HNMe2, DCM; (iv) Paraformaldehyde, NaBH(OAc)3, CHCl3.
assay to discriminate the most potent compounds due to the
tight binding nature of enzyme inhibition (many compounds
had high Hill slopes, i.e. ≫ 1).
Information) and compounds with high selectivity at the
biochemical level (i.e., HsNMT/TbNMT is >100-fold for 50).
However, there was no substantial improvement in selectivity
(S), relative to 40, with these compounds with the exception of
compound 50 (S = 47), nor a substantially greater window at
the wholly cellular level (i.e., T. brucei EC50 vs MRC-5)
achieved compared with 40, which already has an approx-
imately 1000-fold differential. Additionally, these compounds
generally exhibited higher microsomal intrinsic clearances than
40 and were therefore ruled out of selection for progression
into animal models. Comparison of compounds 49−52 (i.e., 49
and 50, cf. 51 and 52) suggests that the S-enantiomers show a
marginally greater effect on selectivity (S) compared to the R-
enantiomers.
In addition to those compounds described in Tables 4 and 5,
a variety of other amine-bearing cores appended via flexible
linkers of varied length to the benzenesulfonamide were
evaluated, although none were able to provide the desired
profile in terms of potency and selectivity (see Table S3,
Supporting Information).
Efficacy in a Stage 2 Model of HAT for “Capped”
Sulfonamide Lead Compounds. On the basis of their
trypanocidal activity, oral pharmacokinetics, brain:blood ratio,
and confirmed fully curative activity in the stage 1 mouse model
(T. b. brucei S427, 50 mg/kg b.i.d. po for 4 days), compounds
40, 46, and 42 were selected for evaluation in the stage 2 mouse
model of HAT (T. b. brucei GVR35). With the assumption that
the brain interstitial fluid is the biophase for effective cure of
stage 2 HAT and that EC99 drug levels are required,15 it was
calculated (using T. b. brucei S427 EC50, mouse oral
pharmacokinetics, measured brain total concentration, and
brain free fraction) that an oral dose of 20 mg/kg b.i.d., 15 mg/
kg b.i.d., or 250 mg/kg b.i.d. for 40, 46, and 42, respectively
would be needed to deliver stage 2 cures based on maintaining
the brain free concentration (as a surrogate of brain interstitial
fluid levels26) above the T. brucei EC99 (see Table S5,
Supporting Information for dose calculations). All the
compounds were dosed in the stage 2 mouse model of HAT
(T. b. brucei GVR35) at the MTD (100, 50, or 150 mg/kg b.i.d.
po for 40, 46, and 42, respectively) for 5 days. However, some
Key points noted from these studies:
• In the one example for which there is data, converting
the terminal tertiary amine to a secondary amine (40 cf.
43) caused a large drop in the brain:blood ratio (falls
from 1.6 to 0.03).
• Replacement of the piperidine of 40 with a piperazine
(i.e., 44) caused an approximately 10-fold reduction in
cellular potency, consistent with previous observations.
• Compound 53, a bicyclic analogue of 44, exhibited the
best CNS penetration observed within this series
(brain:blood ratio = 2.6). However, this compound
suffered from high microsomal clearance (Cli = 15 mL/
min/g) and no improvement to oral exposure in mouse
compared to the lead compound 40.
• An increase in the length of the propyl tether of 40 by an
additional methylene (i.e., 46), or an oxygen atom (i.e.,
47) resulted in similar or slightly better trypanocidal
activity while retaining brain:blood ratio.
• Removal of the piperidine ring and replacement by an
acyclic linker of comparable length to 46 and 47 was also
tolerated (i.e., 48, T. brucei EC50 = 0.002 μM). However,
these acyclic linkers were very sensitive to the
substituents on the terminal amine, with potency
decreasing rapidly as the size of substituents on the
basic nitrogen is increased (see Table S3 in Supporting
Information). While this makes sense intuitively, because
the N-substituent could potentially impede contact
between the basic and acidic centers, it was hoped to
strike a potential balance between potency and selectivity
by branching into unaccessed space.
It was decided to see if selectivity (S) could be improved by
the introduction of chiral amines, with substituents which could
potentially access different subpockets in the region bound by
the terminal amine (see compounds 49−53 in Table 5). These
modifications resulted in some compounds with subnanomolar
potency against T. brucei (see Table S3, Supporting
I
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Table 5. Optimization of the Linker and the Terminal Amine of a CHF2-Capped Sulfonamide
a
b
c
IC50 values are shown as mean values of two or more determinations. Compounds with Hill slopes >1.5. For analogues where the size of the N-
d
substituent was varied, see Table S3 in Supporting Information. Cli measured in mL min−1 g−1. B:B is the brain:blood ratio in mouse. S = IC50
HsNMT/EC50 T. brucei. nd = not determined.
toxicity was observed at this dose regimen in infected mice for
both 40 (1/5 mice) and 46 (2/5 mice). Although 42 was well
tolerated, none of the mice were cured, which was somewhat
expected based on the above calculation. For 40 and 46, further
dosing regimens were tested in order to try and achieve in vivo
proof-of-concept in a stage 2 HAT model (40: 50 mg/kg b.i.d.
po for 10 days, 100 mg/kg b.i.d. po for 8 days, and three rounds
of 3 day dosing at 100 mg/kg with a two day “drug holiday”
between rounds. 46: 25 mg/kg b.i.d. for 10 days, 50 mg/kg
b.i.d. po for 8 days). Encouragingly, the two 100 mg/kg
extended dosing schedules for 40 were partially efficacious (1/5
mice). However, no cures were achieved at 50 mg/kg b.i.d. po
for 10 days with either dose regimen of 46. Indeed, significant
toxicity was observed for the extended 50 mg/kg dose regimen
of 46. Kaplan−Meier graphs for efficacy of compound 40 in
continuous and pulsed dosing regimens are shown in Figure 7.
Although this outcome is encouraging for 40, it was
disappointing that full cures were not achieved considering a
20 mg/kg b.i.d. oral dose was predicted to be efficacious. A
possible explanation for this is the difference observed in
minimal curative efficacy to different parasite strains, as has
already been seen for key NMT compounds. For example, an
efficacy study of compound 46 showed approximately a 4-fold
reduction in efficacy in an acute model of T. b. brucei GVR35
(minimum curative dose 25 mg/kg b.i.d. po for 4 days)
compared to T. b. brucei S427 (minimum curative dose 6 mg/
kg b.i.d., po for 4 days). The predicted efficacy of 40 was based
on T. b. brucei S427 data, and if efficacy to T. b. brucei GVR35 is
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Figure 7. Kaplan−Meier survival graph for compound 40 in the T. b. brucei GVR35 stage 2 HAT efficacy model ((A) continuous; (B) pulsed
dosing).
4−5-fold less sensitive (not tested), as was observed for 46, a
100 mg/kg b.i.d. po dose regimen of 40 would be on the cusp
of efficacy. It was therefore unfortunate that a lack of tolerability
prevented higher dosing to deliver full cures. While compounds
49 and 50 have better selectivity (S) than 40 and may therefore
be better tolerated at a higher dose required for efficacy, their
high metabolic instability ruled them out from further
evaluation.
brain at a tolerated dose, further confounded by the lower
efficacy of these compounds toward the T. b. brucei GVR35
strain compared to the T. b. brucei S427 strain. Compounds
with better selectivity would hopefully allow dosing at either
higher levels or for longer duration and therefore result in a
fully curative stage 2 TbNMT lead. It is, however, encouraging
that some cures and prolongation of survival in the stage 2
model is observed via blockade of TbNMT because this
increases confidence in the validity of this target for the
development of therapeutics for stage 2 HAT. The compounds
described above, including compound 40, and the increased
understanding of how to optimize compounds for both BBB
penetration and selectivity represent a very useful platform for
further studies.
CONCLUSIONS
■
In this article, we describe optimization of the lead pyrazole
sulfonamide 1 which resulted in compound 40, which has
markedly increased blood−brain barrier penetration and
improved selectivity. We have a more complete understanding
of the SAR of this series and how to achieve blood−brain
barrier penetration by capping the sulfonamide group which
reduces the PSA and acidity. While a greater degree of
selectivity has also been obtained, achieved through introduc-
tion of a flexible linker between the pyrazole and amine, the
reasons for this are not fully understood. It is important to note
that the downside of these modifications to 1 is a significant
increase in lipophilicity (cLogP of 1 and 40 is 3.1 and 4.7
respectively), which, when combined with a reduction in PSA,
potentially increases the risk of off-target activity.27 This may be
the cause of the poor tolerability of compound 40.
EXPERIMENTAL SECTION
■
General Experimental Information. Chemicals and solvents
were purchased from the Aldrich Chemical Co., Fluka, ABCR, VWR,
Acros, Fisher Chemicals, and Alfa Aesar and were used as received
unless otherwise stated. Air- and moisture-sensitive reactions were
carried out under an inert atmosphere of argon in oven-dried
glassware. Analytical thin-layer chromatography (TLC) was performed
on precoated TLC plates (layer 0.20 mm silica gel 60 with fluorescent
indicator UV254, from Merck). Developed plates were air-dried and
analyzed under a UV lamp (UV254/365 nm). Flash column
chromatography was performed using prepacked silica gel cartridges
(230−400 mesh, 40−63 μm, from SiliCycle) using a Teledyne ISCO
Combiflash Companion or Combiflash Retrieve. 1H NMR, 13C NMR,
19F NMR, and 2D-NMR spectra were recorded on a Bruker Avance
DPX 500 spectrometer (1H at 500.1 MHz, 13C at 125.8 MHz, 19F at
470.5 MHz). Chemical shifts (δ) are expressed in ppm recorded using
Evaluation of key tool compounds from this series in a CNS
model of HAT resulted in partial efficacy. Lack of full cure is
likely to be a consequence of insufficient therapeutic window to
be able to deliver fully efficacious free drug concentration to the
K
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the residual solvent as the internal reference in all cases. Signal splitting
patterns are described as singlet (s), doublet (d), triplet (t), quartet
(q), multiplet (m), broad (b), or a combination thereof. Coupling
constants (J) are quoted to the nearest 0.1 Hz. LC-MS analyses were
performed with either an Agilent HPLC 1100 series connected to a
Bruker Daltonics MicrOTOF or an Agilent Technologies 1200 series
HPLC connected to an Agilent Technologies 6130 quadrupole LC/
MS, where both instruments were connected to an Agilent diode array
detector. LC−MS chromatographic separations were conducted with a
Waters Xbridge C18 column, 50 mm × 2.1 mm, 3.5 μm particle size;
mobile phase, water/acetonitrile + 0.1% HCOOH, or water/
acetonitrile +0.1% NH3; linear gradient from 80:20 to 5:95 over 3.5
min and then held for 1.5 min; flow rate of 0.5 mL min−1. All assay
compounds had a measured purity of ≥95% (by TIC and UV) as
determined using this analytical LC−MS system. High resolution
electrospray measurements were performed on a Bruker Daltonics
MicrOTOF mass spectrometer. Microwave-assisted chemistry was
performed using a Biotage Initiator Microwave Synthesizer.
(dd, J = 4.9 Hz 13.9 Hz, 1H) 2.98 (dd, J = 9.3 Hz 13.9 Hz, 1H), 1.06
(s, 3H), 0.99 (s, 3H). m/z (ES+, 70 V) 404.1 (MH+).
4-Bromo-2,6-dichloro-N-(difluoromethyl)-N-(1,3,5-trimethyl-1H-
pyrazol-4-yl)benzenesulfonamide (28). A well-stirred slurry of 4-
bromo-2,6-dichloro-N-(1,3,5-trimethyl-1H-pyrazol-4-yl)-
benzenesulfonamide (3.0 g, 7.2 mmol), potassium carbonate (3.0 g,
21.0 mmol), and sodium chlorodifluoroacetate (3.3 g, 21.0 mmol) in
acetonitrile (100.0 mL) was heated to 60 °C for 48 h. The resulting
slurry was then concentrated in vacuo, diluted with DCM (100 mL)
and water (100 mL), the organic phase separated, dried (MgSO4), and
concentrated in vacuo. Trituration of the residue with diethyl ether
gave a precipitate, which was collected by vacuum filtration and dried
to give the title compound 28 as a fine white powder (2.05 g, 4.43
mmol, 62%). 1H NMR (500 MHz, CDCl3): δ 7.61 (2H, s), 7.34 (1H,
dd, J = 59.4 Hz 61.2 Hz), 3.67 (3H, s), 2.01 (3H, s), 1.71 (3H, s). m/z
(ES+, 70 V) 464.1 (MH+).
Prototypical Procedure for Coupling of an Aryl Bromide with a 9-
BBN-Derived Trialkylborane under B-Alkyl Suzuki−Miyaura Con-
ditions: 2,6-Dichloro-N-(difluoromethyl)-4-(3-(1-methylpiperidin-4-
yl)propyl)-N-(1,3,5-trimethyl-1H-pyrazol-4-yl)benzenesulfonamide
(40). A solution of 4-allyl-1-methylpiperidine (776 mg, 5.54 mmol) in
THF (10.0 mL) under nitrogen at rt was treated dropwise with 9-BBN
(0.5 M in THF, 13.2 mL, 6.6 mmol). The reaction was then heated in
a microwave for 30 min at 90 °C. The resulting solution was then
reduced to approximately half its original volume by passage of
nitrogen then transferred via cannula into a stirred mixture of 28 (2.5
g, 5.4 mmol) and K3PO4 (1.27 g, 6.0 mmol) in DMF (10.0 mL) and
water (2.0 mL) under argon. After bubbling argon through the
reaction for 5 min at rt, Pd(PPh3)4 (160 mg, 0.14 mmol) was added
and the reaction vessel sealed and then heated in a microwave at 80 °C
for 30 min. The cooled reaction mixture was then concentrated in
vacuo, diluted with DCM (50 mL) and aqueous ammonia solution (50
mL), and the organic phase separated, washed with brine (2 × 25 mL),
dried (MgSO4), and concentrated in vacuo. Chromatography (SiO2,
EtOAc:MeOH:saturated aqueous ammonia solution 10:1:0.1) gave the
Prototypical Procedure for the Suzuki Reaction between an Aryl
Bromide and a Boronic Acid/Boronate Ester: 2,6-Dichloro-4-(2-
piperazin-1-yl-pyridin-4-yl)-N-(1,3,5-trimethyl-1H-pyrazol-4-yl)-ben-
zenesulfonamide (1). A deoxygenated solution of 4-bromo-2,6-
dichloro-N-(1,3,5-trimethyl-1H-pyrazol-4-yl)-benzenesulfonamide
(13.84 g, 33.3 mmol), 2-(1-piperazinyl)pyridine-4-boronic acid pinacol
ester (11.57 g, 40.0 mmol), K3PO4 (9.73 g, 44.0 mmol), and
Pd(PPh3)4 (1.50 g, 0.96 mmol) in DMF (200 mL) and water (40 mL)
in a round-bottomed flask under argon was heated at 120 °C for 1 h.
The reaction mixture was then concentrated in vacuo, diluted with
DCM (400 mL), washed with saturated aqueous ammonia solution (2
× 100 mL), dried (MgSO4), and concentrated in vacuo. The residual
solid was triturated from Et2O and collected by filtration to give a
solid, which was recrystallized from EtOAc to give the title compound
1
1 as an off-white powder (15.22 g, 30.7 mmol, 92%). H NMR (500
MHz, DMSO-d6): δ 9.79 (s, 1H), 8.25 (d, J = 5.9 Hz, 1H), 8.20 (s,
2H), 7.61 (s, 1H), 7.40 (d, J = 5.9 Hz, 1H), 4.08 (s br, 4H), 3.63 (s,
3H), 3.28 (s br, 4H), 2.00 (s, 3H), 1.77 (s, 3H). 13C NMR (125 MHz,
DMSO-d6): 147.5, 147.3, 143.8, 137.4, 136.3, 135.2, 129.8, 111.8,
111.7, 109.1, 108.9, 42.7, 42.0, 36.2, 10.4. HRMS (m/z): [M + H]+
calcd for C21H25N6SO2Cl2, 495.1131; found, 495.1124.
1
title compound 40 as a white powder (2.41 g, 4.6 mmol, 85%). H
NMR (500 MHz, DMSO-d6): δ 7.69 (t, J = 60.3 Hz, 1H), 7.62 (s,
2H), 3.63 (s, 3H), 3.35 (s br, 2H), 2.90−2.80 (m, 2H), 2.67 (s, 3H),
2.63 (t, J = 7.4 Hz, 2H), 1.86 (s, 3H), 1.84−1.77 (m, 2H), 1.62 (s,
3H), 1.60−1.55 (m, 2H), 1.49−1.32 (m, 3H), 1.20−1.13 (m, 2H).
HRMS (m/z): [M + H]+ calcd for C22H31N4SO2Cl2F2, 523.1507;
found, 523.1490. 13C NMR (125 MHz, DMSO-d6): δ 151.1, 145.9,
140.3, 135.2, 132.0, 129.5, 111.1, 105.9, 53.3, 42.5, 36.5, 34.6, 33.8,
32.3, 28.9, 26.8, 10.2, 8.6. 19F NMR (470 MHz, DMSO-d6): δ −92.2
(dd, J = 60.3 Hz 207.8 Hz, 1F), −94.4 (dd, J = 60.3 Hz 207.5 Hz, 1F),
Prototypical Procedure for Preparation of a Sulfonamide from
an Amine and a Sulfonyl Chloride: 4-Bromo-N-(1,3,5-trimethyl-1H-
pyrazol-4-yl)-benzenesulfonamide (11). 4-Bromobenzenesulfonyl
chloride (5.0 g, 19.6 mmol) was added portionwise to a stirred
solution of 4-amino-1,3,5-trimethyl-1H-pyrazole (2.45 g, 19.6 mmol)
in pyridine (50 mL) at rt. The reaction was stirred for 24 h then
concentrated to dryness in vacuo. The resulting residue was diluted
with DCM (100 mL), washed with aqueous sodium hydroxide
solution (0.5M, 100 mL), organic phase separated, dried (MgSO4),
filtered, and concentrated to dryness in vacuo. Trituration from Et2O
and collection by vacuum filtration gave the title compound 11 as a
1
spectrum was not H-decoupled.
Enzyme Inhibition Assay. N-Myristoyltransferase is an enzyme
that catalyzes the addition of myristic acid from myristoyl-CoA to the
N-terminal glycine residue of numerous substrate proteins and
peptides with the subsequent release of coenzyme A. 3H-labeled
myristoyl-CoA (GE Healthcare) can be used in the reaction to transfer
1
fine off-white solid (5.1 g, 14.8 mmol, 79%). H NMR (500 MHz,
3 H-myristic acid to
a biotinylated substrate peptide
DMSO-d6): δ 9.21 (1H, s), 7.79 (d, J = 8.5 Hz, 2H), 7.56 (d, J = 8.5
Hz, 2H), 3.56 (s, 3H), 1.82 (s, 3H), 1.61 (s, 3H). HRMS (m/z): [M +
H]+ calcd for C12H15N3SO2Br, 344.0063; found, 344.0059.
(GCGGSKVKPQPPQAK(biotin)-amide, Pepceuticals Inc.). The
reaction can be measured by the subsequent binding of the labeled
peptide to streptavidin-coated scintillation proximity assay (SPA)
beads (GE Healthcare) and monitoring of β-particle excitation of the
embedded scintillant. Measurement of the ability of compounds to
inhibit the N-myristoyltransferase enzyme(s) of human (HsNMT-1
and HsNMT-2) and kinetoplast (T. brucei, T. cruzi, and L. major)
species was performed using a modification of the scintillation
proximity assay platform described previously by Panethymitaki et al.28
as follows: Compounds were solubilized in DMSO at a top
concentration of 10 mM and serially diluted in half log steps to
achieve a range of final assay concentrations of 100 μM to 1.5 nM.
Compound at each concentration was added to white 384-well plates
in a volume of 0.5 μL. N-Myristoyltransferase enzyme (HsNMT-1,
HsNMT-2, TcNMT, TbNMT, or LmNMT), dissolved to a working
concentration of 10 nM in assay buffer (30 mM Tris-HCl, pH 7.4, 0.5
mM EGTA, 0.5 mM EDTA, 1.25 mM DTT, 0.1% Triton X-100), was
then added to columns 1−11 and 13−23 of the plates in a volume of
Prototypical Procedure for N-Alkylation of a Sulfonamide with
an Alkyl Halide: 4-Bromo-N-methyl-N-(1,3,5-trimethyl-1H-pyrazol-
4-yl)-benzenesulfonamide (24). Sodium hydride (95% w/w, 88 mg,
3.48 mmol) was added portionwise to a solution of 4-bromo-N-(1,3,5-
trimethyl-1H-pyrazol-4-yl)-benzenesulfonamide (1.0 g, 2.91 mmol) in
DMF (10.0 mL) at 0 °C. When effervescence had ceased, methyl
iodide (217 μL, 3.48 mmol) was added dropwise and the reaction was
allowed to warm to rt over 4 h. The reaction was concentrated to
dryness in vacuo, diluted by addition of DCM (30 mL), washed with
water (2 × 15 mL), dried (MgSO4), and concentrated in vacuo. The
residue was triturated from Et2O and collected by vacuum filtration to
give the title compound 24 as a fine off-white solid (557 mg, 1.56
mmol, 54%). 1H NMR (500 MHz, DMSO-d6): δ 9.67 (s, 1H), 8.78 (d,
J = 5.7 Hz, 1H), 8.51 (d, J = 8.6 Hz, 1H), 8.09 (d, J = 5.8 Hz, 1H),
7.86 (d, J = 5.6 Hz, 1H), 7.50 (d, J = 5.7 Hz, 1H), 7.21 (d, J = 8.4 Hz,
2H), 4.17 (d, J = 8.4 Hz, 2H), 4.34 (s, 1H), 4.18−4.14 (m, 1H), 3.21
L
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20 μL. To columns 12 and 24, 20 μL of assay buffer was added to
provide a no-enzyme control. Following a 5 min incubation at room
temperature, the substrates (GCGGSKVKPQPPQAK(biotin)-amide
and myristoyl-CoA), dissolved in assay buffer, were added to all wells
in a volume of 20 μL to start the reaction. The final concentrations of
peptide and 3H-myristoyl-CoA were 0.5 μM and 125 nM, respectively,
and the specific activity of the radiolabel was 8 Ci/mmol. Plates were
then incubated at room temperature for up to 50 min (dependent
upon the period of linearity for the different enzyme species) before
SPA beads, suspended to 1 mg/mL in a stop solution (200 mM
phosphoric acid/NaOH, pH 4, 750 mM MgCl2), were added in a
volume of 40 μL. Plates were then read on a TopCount microplate
luminometer and data analyzed by calculating the percentage
inhibition compared to the maximum and minimum assay controls.
Concentration effect curves were fitted using nonlinear regression
using XLFit 4.2, and IC50 values were determined.
Mouse Pharmacokinetics. Test compound was dosed as a bolus
solution intravenously at 3 mg free base/kg (dose volume, 5 mL/kg;
dose vehicle, saline or 5% DMSO; 40% PEG400; 55% saline) to
female NMRI mice (n = 3) or dosed orally by gavage as a solution at
10 and/or 50 mg free base/kg (dose volume, 5 mL/kg; dose vehicle,
5% DMSO; 95% distilled water or 5% DMSO; 40% PEG400; 55%
distilled water) to female NMRI mice (n = 3/dose level). Female
NMRI mice were chosen as these represent the sex and strain used for
the stage 1 and stage 2 HAT efficacy models. Blood samples were
taken from each mouse at 2 (IV only), 5 (IV only), 15, and 30 min, 1,
2, 4, 6, 8, and 24 (po only) hours postdose and mixed with two
volumes of distilled water. After suitable sample preparation, the
concentration of test compound in blood was determined by UPLC-
MS/MS using a Quattro Premier XE (Waters, USA). Pharmacokinetic
parameters were derived from the mean blood concentration time
curve using PKsolutions software v 2.0 (Summit Research Services,
USA).
Cell Viability Assay. Measurement of the ability of the
compounds to inhibit human (MRC5, human lung fibroblast cells)
and trypanosome (T. b. brucei, BSF427, VSG118) cell growth was
performed using a modification of the cell viability assay previously
described by Raz et al.29 Compounds were dissolved in DMSO at a
top concentration of 10 mM and serially diluted in half log steps to
achieve a range of final assay concentrations of 50 μM to 0.5 nM.
Compound at each concentration (200-fold final) was added to clear
96-well tissue culture plates in a volume of 1 μL. Then 2000 cells per
well in relevant growth medium (HMI-9T for T. brucei, a modification
of HMI-9 as described by Hurumi et al.,30 where 0.2 mM 2-
mercaptoethanol was replaced with 0.056 mM thiolglycerol, and MEM
with 10% FBS for MRC-5) were then added to columns 1−11 of the
plates in a volume of 199 μL. To column 12, 200 μL of medium was
added to provide a no cells control. Plates were then incubated at 37
°C in an atmosphere of 5% CO2 for 69 h before the addition of 20 μL
of 500 μM rezasurin solution and a further incubation period of 4 h.
Plates were then read on a BioTek flx800 fluorescent plate reader, and
percentage inhibition was compared to the maximum and minimum
assay controls. Concentration effect curves were fitted using nonlinear
regression using XLFit 4.2 and EC50 values determined.
Protein Crystallization and Structure Determination. De-
tailed descriptions of protein expression, crystallization, and structure
determination for Af NMT complexes will be reported in a separate
manuscript (Fang et al., manuscript submitted). Ternary complexes of
Af NMT with myristoyl CoA (MCoA) and ligands of interest were
obtained by cocrystallization by incubating protein with 10 mM
MCoA plus 10 mM ligand diluted from a 100 mM stock in DMSO
prior to crystallization. Coordinates and associated diffraction data for
complexes of Af NMT + compounds 14 and 43 have been deposited in
the Protein Data Bank (PDB) with accession codes 4UWI and 4UWJ,
respectively. Data measurement and refinement statistics are shown in
Table S6 (Supporting Information).
Intrinsic Clearance (Cli) Experiments. Test compound (0.5 μM)
was incubated with female CD1 mouse liver microsomes (Xenotech
LLC ; 0.5 mg/mL 50 mM potassium phosphate buffer, pH7.4) and the
reaction started with addition of excess NADPH (8 mg/mL 50 mM
potassium phosphate buffer, pH 7.4). Immediately, at time zero, then
at 3, 6, 9, 15, and 30 min, an aliquot (50 uL) of the incubation mixture
was removed and mixed with acetonitrile (100 uL) to stop the
reaction. Internal standard was added to all samples, the samples
centrifuged to sediment precipitated protein, and the plates then
sealed prior to UPLC-MS/MS analysis using a Quattro Premier XE
(Waters corporation, USA).
XLfit (IDBS, UK) was used to calculate the exponential decay and
consequently the rate constant (k) from the ratio of peak area of test
compound to internal standard at each time point. The rate of intrinsic
clearance (Cli) of each test compound was then calculated using the
equation Cli (mL/min/g liver) = k × V × microsomal protein yield,
where V (mL/mg protein) is the incubation volume/mg protein added
and microsomal protein yield is taken as 52.5 mg protein/g liver.
Verapamil (0.5 μM) was used as a positive control to confirm
acceptable assay performance.
Mouse Brain Penetration. Each compound was dosed as a bolus
solution intravenously at 2 mg free base/kg (dose volume, 5 mL/kg;
dose vehicle, 5% DMSO; 95% saline) to female NMRI mice (n = 6).
At 5 and 30 min following intravenous bolus injection of test
compound, mice (n = 3/time point) were placed under terminal
anesthesia with isofluorane. A blood sample was taken by cardiac
puncture into two volumes of distilled water and the brain removed.
After suitable sample preparation, the concentration of test compound
in blood and brain was determined by UPLC-MS/MS using a Quattro
Premier XE (Waters, USA). For each mouse at each time point, the
concentration in brain (ng/g) was divided by the concentration in
blood (ng/mL) to give a brain:blood ratio. The mean value obtained
was quoted.
Efficacy Experiments. Stage 1 efficacy experiments using T. b.
brucei S427 were performed according to methods described by
Frearson et al.15
T. b. brucei GVR35 Stage 2 Efficacy Protocol. NMRI mice used in
the experiment were purchased from Harlan, UK. Mice were
maintained under standard temperature/lighting conditions and
given food and water ad libitum. The trypanosomes used in this
study were of the cloned stabilate T. b. brucei GVR35, originally
derived from trypanosomes isolated from a wildebeest in the Serengeti
in 1966.31 By 13 days postinfection, parasites entered the CNS.32 If
treatment is delayed until 21 days after infection, successful cure is
therefore dependent on the ability of the compound to penetrate
through the blood−brain barrier.33 This model is widely used to
evaluate the efficacy of new compounds for treatment against stage 2
HAT.34
Cryopreserved T. b. brucei GVR35 trypomastigotes were diluted to
∼5 × 104 cells/mL in 20 mM Hanks Balanced Salt Solution with
glucose, and 0.2 mL of that dilution was used for infection (∼1 × 104
cells/mL per animal). All animals in the groups were intraperitoneally
infected (ip) on day 0. After infection, parasitaemia was monitored by
examination of tail blood smears. On day 21, post infection animals
were randomly allocated to five mice per group. Infected animals were
dosed with one of the following: diaminazene diaceturate (ip), 40 mg/
kg once only as a HAT stage 1 control (early stage tyrypanocide to
clear the parasites in the systemic circulation and in tissues other than
CNS); Melasoprol (ip), 10 mg/kg once daily for 5 days as a positive
control for HAT stage 2; or test compound (gavage) twice daily for 10
days (i.e., 40, 42, or 46) or twice daily for 3 × 3 days with 2 days rest
between dose intervals (i.e., 40). One group of five mice was an
untreated control group. The dose solutions were prepared daily, using
5% DMSO:40% PEG400:55% Milli-Q water for compound (i.e., 40)
and 10% DMSO:90% peanut oil for control drugs. To measure the
response to treatment, parasitaemia was monitored twice per week
until day 50. After day 50, the mice were checked only once per week
until day 180 post treatment. Mice surviving to the end of experiment
and blood smear aparasitaemic were considered cured.
All regulated procedures on living animals were carried out under
the authority of licenses issued by the Home Office under the Animals
(Scientific Procedures) Act 1986, as amended in 2012 (and in
compliance with EU Directive EU/2010/63). License applications
M
dx.doi.org/10.1021/jm500809c | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Article
were approved by the University’s Ethical Review Committee (ERC)
before submission to the Home Office.
(7) Price, H. P.; Menon, M. R.; Panethymitaki, C.; Goulding, D.;
McKean, P. G.; Smith, D. F. Myristoyl-CoA:protein N-myristoyl-
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(8) Price, H. P.; Guther, M. L. S.; Ferguson, M. A. J.; Smith, D. F.
Myristoyl-CoA:protein N-myristoyltransferase depletion in trypano-
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2010, 169, 55−58.
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Smith, D. F.; Brown, K. A. N-Myristoyltransferase: a prospective drug
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic details for all compounds, supplementary data tables,
additional information on ADMET and pharmacology, and
crystallographic data tables. This material is available free of
charge via the Internet at http://pubs.acs.org.
Accession Codes
Coordinates and associated diffraction data for complexes of
AfNMT with compounds 14 and 43 have been deposited in
the Protein Data Bank (PDB) with accession codes 4UWI and
4UWJ, respectively.
(11) Wright, M. H.; Clough, B.; Rackham, M. D.; Rangachari, K.;
Brannigan, J. A.; Grainger, M.; Moss, D. K.; Bottrill, A. R.; Heal, W. P.;
Broncel, M.; Serwa, R. A.; Brady, D.; Mann, D. J.; Leatherbarrow, R. J.;
Tewari, R.; Wilkinson, A. J.; Holder, A. A.; Tate, E. W. Validation of N-
myristoyltransferase as an antimalarial drug target using an integrated
chemical biology approach. Nature Chem. 2014, 6, 112−121.
(12) Tate, E. W.; Bell, A. S.; Rackham, M. D.; Wright, M. H. N-
Myristoyltransferase as a potential drug target in malaria and
leishmaniasis. Parasitology 2014, 141, 37−49.
AUTHOR INFORMATION
Corresponding Authors
*For I.H.G.: phone, +44 1382 386 240; E-mail, i.h.gilbert@
dundee.ac.uk.
■
*For K.D.R.: phone, +44 1382 388 688; E-mail, k.read@
dundee.ac.uk.
Notes
(13) Roberts, A. J.; Torrie, L. S.; Wyllie, S.; Fairlamb, A. H.
Biochemical and genetic characterisation of Trypanosoma cruzi N-
myristoyltransferase. Biochem. J. 2014, 459, 323−332.
The authors declare no competing financial interest.
(14) Brand, S.; Cleghorn, L. A.; McElroy, S. P.; Robinson, D. A.;
Smith, V. C.; Hallyburton, I.; Harrison, J. R.; Norcross, N. R.; Spinks,
D.; Bayliss, T.; Norval, S.; Stojanovski, L.; Torrie, L. S.; Frearson, J. A.;
Brenk, R.; Fairlamb, A. H.; Ferguson, M. A.; Read, K. D.; Wyatt, P. G.;
Gilbert, I. H. Discovery of a novel class of orally active trypanocidal N-
myristoyltransferase inhibitors. J. Med. Chem. 2012, 55, 140−152.
(15) Frearson, J. A.; Brand, S.; McElroy, S. P.; Cleghorn, L. A. T.;
Smid, O.; Stojanovski, L.; Price, H. P.; Guther, M. L. S.; Torrie, L. S.;
Robinson, D. A.; Hallyburton, I.; Mpamhanga, C. P.; Brannigan, J. A.;
Wilkinson, A. J.; Hodgkinson, M.; Hui, R.; Qiu, W.; Raimi, O. G.; Van
Aalten, D. M. F.; Brenk, R.; Gilbert, I. H.; Read, K. D.; Fairlamb, A. H.;
Ferguson, M. A. J.; Smith, D. F.; Wyatt, P. G. N-Myristoyltransferase
inhibitors as new leads to treat sleeping sickness. Nature 2010, 464,
728−732.
(16) Polli, J. W.; Jarrett, J. L.; Studenberg, S. D.; Humphreys, J. E.;
Dennis, S. W.; Brouwer, K. R.; Woolley, J. L. Role of P-glycoprotein on
the CNS disposition of amprenavir (141W94), an HIV protease
inhibitor. Pharm. Res. 1999, 16, 1206−1212.
(17) Di, L.; Rong, H.; Feng, B. Demystifying brain penetration in
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ACKNOWLEDGMENTS
■
Funding for this work was provided by the Wellcome Trust
(grants WT077705, WT083481, WT085622). We thank Gina
MacKay for performing HRMS analyses and for assistance with
performing other NMR and MS analyses, Daniel James for data
management, Bhavya Rao for performing parasite assays,
Dhananjay Joshi and Mary Gardiner for performing enzyme
assays, and Dr. Stephen Patterson for critical evaluation of the
manuscript.
ABBREVIATIONS USED
■
B:B, brain to blood ratio; 9-BBN, 9-borabicyclo(3.3.1)nonane;
Cli, intrinsic clearance; EC99, 99% of maximal effective
concentration; GVR35, Glasgow Veterinary Research-35 strain;
HAT, human African trypanosomiasis; MRC-5, Medical
Research Council-5; NMT, N-myristoyltranferase; NMRI,
Naval Medical Research Institute; STIB900, Swiss Tropical
Institute, Basel-900 strain
(18) Zakeri-Milani, P.; Tajerzadeh, H.; Islambolchilar, Z.; Barzegar,
S.; Valizadeh, H. The relation between molecular properties of drugs
and their transport across the intestinal membrane. DARU J. Pharm.
Sci. 2006, 14, 164−171.
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