Discovery of Indoline-2-carboxamide Derivatives as a New Class of Brain-Penetrant Inhibitors of Trypanosoma brucei

Article abstract of DOI:10.1021/acs.jmedchem.5b00596

There is an urgent need for new, brain penetrant small molecules that target the central nervous system second stage of human African trypanosomiasis (HAT). We report that a series of novel indoline-2-carboxamides have been identified as inhibitors of Trypanosoma brucei from screening of a focused protease library against Trypanosoma brucei brucei in culture. We describe the optimization and characterization of this series. Potent antiproliferative activity was observed. The series demonstrated excellent pharmacokinetic properties, full cures in a stage 1 mouse model of HAT, and a partial cure in a stage 2 mouse model of HAT. Lack of tolerability prevented delivery of a fully curative regimen in the stage 2 mouse model and thus further progress of this series.

Full text of DOI:10.1021/acs.jmedchem.5b00596

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Article
pubs.acs.org/jmc
Discovery of Indoline-2-carboxamide Derivatives as a New Class of
Brain-Penetrant Inhibitors of Trypanosoma brucei
Laura A. T. Cleghorn, Sebastien Albrecht, Laste Stojanovski, Frederick R. J. Simeons, Suzanne Norval,
́
Robert Kime, Iain T. Collie, Manu De Rycker, Lorna Campbell, Irene Hallyburton, Julie A. Frearson,
Paul G. Wyatt, Kevin D. Read, and Ian H. Gilbert*
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: There is an urgent need for new, brain
penetrant small molecules that target the central nervous
system second stage of human African trypanosomiasis
(HAT). We report that a series of novel indoline-2-
carboxamides have been identified as inhibitors of Trypanoso-
ma brucei from screening of a focused protease library against
Trypanosoma brucei brucei in culture. We describe the
optimization and characterization of this series. Potent
antiproliferative activity was observed. The series demon-
strated excellent pharmacokinetic properties, full cures in a stage 1 mouse model of HAT, and a partial cure in a stage 2 mouse
model of HAT. Lack of tolerability prevented delivery of a fully curative regimen in the stage 2 mouse model and thus further
progress of this series.
(iii) in 2009, a combination treatment of nifurtimox and
eflornithine (NECT) was introduced, although this is also not
effective against T. b. rhodesiense.8−10
There is undeniably a need to identify new, safer, and easier to
use antitrypanosomal agents that can cross the blood−brain
barrier to be suitable for use as a stage 2 therapy. The ideal
candidate would be one that is suitable for treating both the stage
1 (peripheral) and stage 2 (CNS) infection.11
One strategy used in the Drug Discovery Unit in Dundee to
find new starting points is phenotypic (in vitro whole cell)
screening of compounds against Trypanosoma brucei brucei. A
mammalian cell counter screen, for example MRC-5 cells, is then
used as a counter screen to exclude nonselective com-
pounds.12−14 Using this approach, our in-house designed
protease set (∼3400 compounds) was screened against T. b.
brucei. The protease library was selected by analysis of known
protease inhibitors from the literature to identify motifs that are
found to bind to proteases. A selection of compounds was then
made that contained these motifs and retained lead-like
properties (generally compliant with the rule of 4).15 From the
primary screening, carried out at single concentration, 93
compounds showed a greater than 50% inhibition of parasite
growth at 30 μM. The hits from the single-point screen were
cherry-picked and rescreened to determine potency (EC50
value). This led to identification of a series of indoline-2-
carboxamides, typified by 1 (Figure 1).
INTRODUCTION
■
Human African trypanosomiasis (HAT) or sleeping sickness
occurs in sub-Saharan Africa and is caused by two subspecies of
the protozoan parasite Trypanosoma brucei; Trypanosoma brucei
gambiense in West and Central Africa and Trypanosoma brucei
rhodesiense in East Africa. This neglected disease is transmitted by
the bite of a tsetse fly and can be fatal if not treated. In 2009, the
number of reported cases dropped to 10000 for the first time in
50 years; the trend continued in 2010 with around 7000 cases
reported, although the estimated number of actual cases is
30000.1−3
Many people in the affected populations live in remote areas
with limited access to adequate health care, impeding diagnosis
and treatment. There are two stages of the disease; in stage 1, the
trypanosomes multiply in subcutaneous tissues, blood, and the
lymphatic system, resulting in bouts of fever, headaches, joint
pains, and itching. In the second stage, the parasites cross the
blood−brain barrier to infect the central nervous system, leading
to confusion, sensory disturbance, poor coordination and
disruption of the sleep cycle, coma, and death. Unfortunately,
diagnosis often does not occur until the patient has stage 2
disease. There is no effective vaccine.4 Pentamidine (T. b.
gambiense) and suramin (T. b. rhodesiense) are currently used to
treat infection at stage 1, while there are three treatments for
stage 2: (i) melarsoprol, an arsenical, can be used to treat both
strains but has severe side effects5 and is given by intravenous
injection over an extended time period; (ii) eflornithine is less
toxic than melarsoprol but only effective against T. b. gambiense
strain and is not suitable for a rural African setting given the
requirement for an extended period of intravenous dosage;6,7 and
Received: April 17, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jmedchem.5b00596
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Journal of Medicinal Chemistry
Article
aromatic ring (Scheme 2). In the most general procedure, a range
of substituted indolines was prepared starting with methyl
azidoacetate 4, which was formed from sodium azide and methyl
bromoacetate. Upon reaction with an aldehyde, this formed
vinylazide 5.20 A rhodium-catalyzed reaction was utilized to
cyclize vinylazide 5 to indole 6.21 Reduction using magnesium
turnings afforded the desired methyl indolinecarboxylate 722 that
was subsequently reacted with methylamine and then the
appropriate acid chloride to yield aromatically substituted
diamide indolines 8. In the case of 5-fluoroindoline, higher
yields were achieved using an alternative synthesis (9−13).23−25
5-Fluoro-2-iodobenzoic acid was reduced with borane, and the
resulting alcohol reacted to form mesylate 9, which was reacted
with N-(diphenylmethylene)glycine ethyl ester to give ethyl 2-
((diphenylmethylene)amino)-3-(5-fluoro-2-iodophenyl)-
propanoate 10. The amine was deprotected with citric acid and
reacted with the desired acid in a standard TBTU coupling to
give 11. The indoline ring was cyclized by copper catalysis and
the ethyl ester hydrolyzed with lithium hydroxide to give 12.
Reaction with methylamine and TBTU afforded the desired 5-
fluoroindoline diamide 13.
An enantiomerically pure synthesis of the substituted indoline
core was also carried out using a literature procedure (Scheme
3).26 A racemic mixture of methyl indolinecarboxylate 7 was
reacted with commercially available lipase A, Candida antartica
CAL-A (CLEA) and allyl 3-methoxyphenyl carbonate (14) to
afford a 50:50 mixture of the (R)-methylindoline-2-carboxylate
15 and (S)-1-allyl-2-methylindoline-1,2-dicarboxylate 16, which
were separable by column chromatography. Details of other
synthetic routes used to synthesize individual compounds are
described in the Supporting Information.
Figure 1. Hit (1) identified from the screen.
Compound 1 was the most potent example in the series of
indoline-2-carboxamides identified from screening, showing
good antiproliferative activity (EC50 = 27 nM) with selectivity
over mammalian cells (>1600-fold). As a starting point,
compound 1 contained several desirable features for a potential
therapeutic agent for stage 2 HAT: low molecular weight (344),
clogP (2.4) compatible with oral and CNS bioavailability, and
low PSA (59 Å2). These properties are all consistent with CNS
penetration, and 1 was predicted to cross the blood−brain barrier
using StarDrop (www.optibrium.com). Early in vitro DMPK
profiling indicated a good plasma protein free fraction (fraction
unbound, mouse (Fu) = 0.26) and solubility (>100 μg mL−1),
although the hepatic microsomal turnover was higher than
desired. The measured intrinsic clearance (Clint) for the
compound was mouse/rat/human = 4.4/4.4/2.6 mL min−1
g−1. Phenoxylacyl diamide indolines have not previously been
described in the literature as potential drug candidates in other
disease areas. A chemistry program was initiated to optimize the
original hit. To satisfy the target product profile for stage 2 HAT,
the molecule should be blood−brain barrier penetrant. Ideally,
the compound should also be orally bioavailable. Therefore, the
initial aim was to increase potency and improve metabolic
stability while retaining a low PSA (ideally <70 Å2),16 low MW,
low clogP, and a high fraction unbound.17−19
RESULTS AND DISCUSSION
■
All compounds were screened using a T. b. brucei whole cell
parasite assay27 and counter screened against a mammalian
MRC-5 cell line to exclude any compounds that showed general
cytotoxicity. None of the compounds reported here showed any
toxicity in the counter screen.
CHEMISTRY
■
Two routes were employed to synthesize the indoline-2-
carboxamides with an unsubstituted aromatic core. In the first
route, 2-indolinecarboxylic acid (enantiomerically pure R and S
are available commercially) was reacted in a two-step one-pot
synthesis (Scheme 1). First, it was coupled to an appropriate
Initial Hits. An initial hit expansion was carried out with 10
indoline-2-carboxamides contained within our protease set and a
further small number of analogues that were commercially
available. Table 1 illustrates selected results. The potency data
from the screening hits and commercially available hit expansion
compounds showed the general trend that NHMe > NHEt >
NH2 at the R2 position. Both ether and benzyl pendants at R1
showed potential for further development. Early investigations
established that there were defined structure−activity relation-
ships, indicating a precise molecular target(s). Thus, changing
the R2 amine and varying R1 could dramatically change the
potency.
Enantiomers. Examination of the individual enantiomers of
1 showed that (R)-isomer (27) was ∼1400-fold more potent
(EC50 value) than (S)-isomer (26), although the single
enantiomers displayed a similar level of hepatic microsomal
turnover to the racemic 1 (Table 2). Initial work focused on using
the racemic 2-indolinecarboxylic acid, as it was found to be
equipotent within experimental error to the (R)-enantiomer;
subsequently, analogues of interest were synthesized as the (R)-
enantiomer.
a
Scheme 1
a
Conditions: (a) R1CO2H (1 equiv), TBTU, (1 equiv), DIPEA, DCM,
room temperature, 2 h or R1COCl (1 equiv), TEA, DCM, 0 °C to
room temperature, 2 h; (b) R2NH2 (2 equiv), TBTU, (1 equiv),
DIPEA, room temperature, 2 h, 2−85% yield for steps (a) and (b)
combined. R1 and R2 are the substituents, as exemplified in Tables 1
and 3−6.
carboxylic acid using O-(benzotriazol-1-yl)-N,N,N′,N′-tetrame-
thyluronium tetrafluoroborate (TBTU) as the peptide coupling
agent, and second to an appropriate amine, again in the presence
of TBTU. Alternatively, R1 could be introduced using an acid
chloride.
There are only a limited number of substituted 2-
indolinecarboxylic acids available commercially. Several routes
were developed to achieve different substitution patterns on the
Initial Evaluation. To follow up the initial screening, we
initiated a more extensive hit-to-lead evaluation. There were
essentially three points on the indoline-2-carboxamide scaffold
that could be explored to look for improvements in potency
B
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Article
a
Scheme 2
a
Conditions (4−8): (a) NaN3, DMF, 0 °C to room temperature, 16 h, under Ar; (b) methyl azidoacetate (3 mol equiv), aldehyde (1 mol equiv),
THF, NaOMe, MeOH, −10 °C, 3 h; (c) Rh2(O2CC3F7)4, toluene, 65 °C, 48 h; (d) Mg, MeOH, 0 °C to room temperature, 16 h, under Ar; (e)
MeNH2 in THF, room temperature, 16 h, then RCOCl, DIPEA, DCM, room temperature under argon. Conditions (9−13): (a) BH3 1 M in THF 0
°C to room temperature, 16 h; (b) MsCl, DIPEA, THF, 0 °C, 2 h; (c) TBAI, 5 mol % in toluene, 50% (w/v) NaOH, 0 °C to room temperature, 16
h; (d) citric acid, 0 °C to room temperature, 3 h; (e) RCO2H, TBTU, DIPEA, room temperature, 2 h; (f) CsOAc, CuI, DMSO, room temperature,
1.5 h; (g) LiOH, THF, room temperature; (h) TBTU, MeNH2 (2 M in MeOH), DCM, room temperature, 2 h. Overall 46% yield from
commercially available 5-fluoro-2-iodobenzoic acid.
a
Scheme 3
a
Conditions: (a) lipase Cal-A, TBME, room temperature, 5 h, under argon, 35% of 15 (expect 50%, 70% of desired product overall).
(Figure 2) and ideally to reduce the hepatic microsomal
turnover. First, looking at the R1 pendant substituent both as
two-carbon ether and one-carbon linkers (A); second, varying
the R2 amine (B); and last, investigating alternative cores such as
aza indoline, tetrahydroquinoline, and substituted indolines (C).
Analysis of compound 1 using a cytochrome P450 (P450)
metabolism predictor (StarDrop) suggested that metabolism of
the 5-position of the indoline may be contributing to metabolic
instability.
Variation of the Pendant Ether. Initially, the substituent
on the ether was varied to block potentially metabolically labile
positions and attempt to improve potency by finding new
interactions (Table 3). The activity was generally found to be
very sensitive to the nature of the substituent. Increasing chain
length and flexibility by incorporating a benzyl ether 28 led to
∼70-fold loss in potency (as meansured by EC50 value). The 3-
fluorophenyl derivative 29 was found to be equipotent to 1 but
showed no improvement in intrinsic clearance. A chloro in the 3-
position of 30 led to a 15-fold loss in potency and a significant
increase in intrinsic clearance. An unsubstituted phenyl 31 was
found to be equipotent, indicating that halogens are not
necessarily required for binding in the active site. Interestingly,
this compound showed an increased intrinsic clearance for
mouse and rat but a similar level for human. Replacing the
aromatic group with an aliphatic such as 3-tetrahydropyran 32
showed a 37-fold loss in potency; however, a much lower
intrinsic clearance was observed. A heteroaromatic ring
(pyridine) 33 was tolerated, although a 10-fold loss in potency
was observed. 4-Methyl 34 and 4-fluoro 35 are equipotent with 1
within experimental error. The 4-fluoro showed improved
intrinsic clearance (at least in mice). The single (R)-enantiomer
of this was synthesized 36 and had a similar profile. 3,4-
C
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Journal of Medicinal Chemistry
Article
Table 1. Potency of Selected Initial Screening Hits and
Commercially Available Hit Expansion
Figure 2. Planned optimization studies of the hit compound.
Difluorophenyl 37 was synthesized as a single enantiomer and
was equipotent to 1.
Variation of the Benzyl Moiety. The pendant ether was
replaced by a methylene linker to give a series of benzyl
derivatives. Switching to a benzyl slightly lowered the PSA; such
modifications are favorable when designing a molecule to cross
the BBB. The SAR observed for the benzyl derivatives (Table 4)
followed a similar trend to that observed in the pendant ether
series. For comparison purposes, 21, an unelaborated phenyl
with a potency of 0.22 μM, was used as the start point in this
study. Increasing the steric bulk of the pendant substituent from
phenyl to naphthyl 39 showed complete loss of potency,
suggesting there is a limited space in the adopted binding pose.
The 4-chlorophenyl analogue 40 showed approximately 3-fold
improvement in activity; however, it had the same intrinsic
clearance as 1. Bulky alkyl group substituents on the aromatic
ring such as isopropyl 41 were not well tolerated and led to a
significant increase in mouse intrinsic clearance (35 mL min−1
g−1). 4-Trifluorophenyl 42 and 4-fluorophenyl 43 were
equipotent at 0.1 μM. Compound 43 showed very good
metabolic stability, similar to that observed for 4-fluorophenyl
35 in the ether series. Addition of a fluoro in the 2-position was
detrimental to activity when combined with 4-fluoro 44. A fluoro
in the 3-position either with 4-chloro 45 or 4-fluoro 46 did not
significantly affect the potency and displayed a detrimental effect
on metabolic stability, particularly in the case of the 4-chloro
derivative. The (R)-enantiomer of 43, 47, retained good potency,
showed improved metabolic stability, lower plasma protein
binding compared to the “best” original hit 1 (Fu, mouse [fraction
unbound] 0.59 vs 0.26) combined with a good free fraction in
brain tissue (Fu, male SD rat = 0.43).
a
EC50 values are shown as mean values of two or more determinations.
b
Standard deviation is typically within 2−3-fold from the EC50. The
upper concentration was set to 15 μM not 50 μM in this assay.
Methylamide Replacements. The methylamide substitu-
ent was investigated to see if potency improvements could be
gained at this position and because the methyl group is a point of
potential metabolic vulnerability (Table 5). Investigation into the
methylamine suggested that there were space limitations around
this section of the molecule in the binding pocket. Capping the
free NH of 48 gave a 200-fold drop in potency, suggesting either
the NH is important for binding or substituents in two different
vectors were not tolerated because of steric restrictions.
Extending the chain length from methyl to ethyl 49 showed a
slight loss in potency yet extending to butyl amine 50 was not
tolerated, leading to ∼700 fold loss in potency together with a
dramatically increased metabolic instability. Incorporating a
slightly bulkier group such a cyclopropyl 51 resulted in ∼14-fold
drop in potency, again hinting at size limitations within the
pocket as observed with 48. Fluorination of the ethylamine
substituent at the methyl group 52 gave ∼17-fold loss in potency
compared to the ethylamine substituent. In an attempt to
improve the metabolic stability a methylamine-d3 analogue was
synthesized 53; deuteration has previously been reported as a
Table 2. Stereochemical Investigation around the Original Hit
T. b. brucei EC50 MRC-5 EC50
mouse microsomal intrinsic
a
a
(μM)
0.027
41
(μM)
clearance (mL min−1 g−1
)
1
>50
>50
4.4
4.0
(S)-
26
(R)-
27
0.029
>50
3.6
a
EC50 values are shown as mean values of two or more determinations.
Standard deviation is typically within 2−3-fold from the EC50.
D
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Journal of Medicinal Chemistry
Article
Table 3. Pendant Ether Modifications
b
microsomal intrinsic clearance
(mL min−1 g−1
r
)
a
a
compd no.
stereochem
R
T. b. brucei EC50 (μM)
MRC-5 EC50 (μM)
m
h
1
R/S
R/S
R/S
R/S
R/S
R/S
R
4-chlorophenyl
benzyl
0.03
1.9
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
4.4
ND
4.4
9.1
6.9
1.2
ND
6.5
1.2
1.2
3.2
ND
4.4
ND
13.5
ND
9.0
2.6
ND
3.3
28
29
30
31
32
33
34
35
36
37
38
4-chloro-3-fluorophenyl
3,4-dichlorophenyl
phenyl
0.03
0.40
0.04
1.1
ND
1.4
3-tetrahydropyran
4-methyl-3-pyridyl
4-methylphenyl
4-fluorophenyl
4-fluorophenyl
3,4-difluorophenyl
phenyl-d5
ND
ND
47
ND
ND
2.4
0.39
0.03
0.02
0.04
0.03
0.05
R/S
R/S
R
ND
4.9
ND
1.3
R
ND
ND
ND
ND
R
a
b
EC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2−3-fold from the EC50. m/r/h =
mouse/rat/human; ND = not determined.
Table 4. Benzyl Analogues
a
a
compd no. stereochem
R
T. b. brucei EC50 (μM)
MRC-5 EC50 (μM)
mouse microsomal intrinsic clearance (mL min−1 g−1
)
21
39
40
41
42
43
44
45
46
47
R/S
R/S
R/S
R/S
R/S
R/S
R/S
R
phenyl
0.22
50
>15
>50
>50
>50
>50
>50
>50
>50
>50
>50
ND
28
1-naphthyl
4-chlorophenyl
0.08
0.77
0.11
0.11
2.3
4.4
35
4-isopropylphenyl
4-trifluoromethylphenyl
4-fluorophenyl
3.0
0.8
0.5
8.4
1.9
1.9
2,4-difluorophenyl
4-chloro-3-fluorophenyl
3,4-difluorophenyl
4-fluorophenyl
0.08
0.11
0.05
R/S
R
a
EC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2−3-fold from the EC50. ND = not
determined.
method to reduce metabolism.28 Deuterated compound 53 was
equipotent with improved metabolic stability compared to 1 and
was similar to the undeuterated analogue 35.
moiety 55 showed improved metabolic stability with a good
plasma protein free fraction (Fu = 0.33 mouse). The potency and
metabolic stability were similar to the nonfluorinated indoline
43. Compound 55 also showed a good free fraction in brain
tissue (Fu = 0.29 rat). The (R)-isomer equivalent of 55, 56, was
also well tolerated. The addition of a fluoro substituent to the 6-
position 57, 7-position 58, or 4 and 6-positions 59 on the
aromatic portion of the indoline gave equipotent results but
offered no advantage over earlier compounds. The 5-bromoindo-
line analogue 60 was found to be equipotent, suggesting there
may be a large amount of space surrounding the aromatic portion
of the indoline in the binding mode. To test this further, the 5-
phenyl (61) and 5-(4-pyridyl) (62) derivatives were synthesized;
both led to approximately a 10-fold loss in potency, and
substituents in the position were not pursued further.
Core Modifications. Examination of 1 in a P450 metabolism
predictor (StarDrop) suggested that metabolism of the 5-
position of the indoline may be responsible for the observed
metabolic instability. In an attempt to block this potential
metabolic hot spot, synthesis was undertaken to replace the
hydrogen in this position with a fluorine, a strategy frequently
employed in medicinal chemistry that would hopefully not
impair binding or reduce the observed antiproliferative effect.29
Substituted indolines were prepared using the procedure
described in the Chemistry section (Scheme 2; compounds
made are listed in Table 6). 5-Fluoro indoline 54 was well
tolerated with a marginal loss in potency compared to 1 but only
marginal improvement in metabolic stability observed, if any. 5-
Fluoroindoline with a 4-fluorophenyl as the pendant ether
Alternative cores to the indoline were also examined: replacing
the indoline with pyrrolidine 64, removing the C3 carbon of the
E
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Journal of Medicinal Chemistry
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Table 5. Amide Modifications
a
a
compd no. stereochem
X
R
T. b. brucei EC50 (μM)
MRC-5 EC50 (μM)
mouse microsomal intrinsic clearance (mL min−1 g−1
)
1
R/S
R/S
R/S
R/S
R/S
R/S
R
Cl
Cl
Cl
Cl
Cl
Cl
F
MeNH−
Me2N−
EtNH−
n-BuNH−
cyclopropyl-NH−
CF3CH2NH−
d3-MeNH−
0.03
5.4
>50
>50
>50
>50
>50
>50
>50
4.4
ND
6.2
27
48
49
50
51
52
53
0.09
19
0.41
1.5
5.5
9.0
1.9
0.04
a
EC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2−3-fold from the EC50. ND = not
determined.
Table 6. Core Modifications
microsomal intrinsic
b
clearance (mL min−1 g−1
)
a
a
compd no.
stereochem
X
R4
R5
R6
R7
T. b. brucei EC50 (μM)
MRC-5 EC50 (μM)
m
r
h
1
R/S
R/S
R/S
R
Cl
Cl
F
H
H
F
H
H
0.03
0.06
0.08
0.04
0.09
0.08
0.07
0.08
0.24
0.44
0.92
36
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
>50
4.4
2.8
4.4
5.3
2.6
1.6
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
H
H
H
H
F
H
H
1.3
2.1
0.5
F
H
F
H
H
ND
2.0
ND
2.8
ND
1.9
R/S
R/S
R/S
R/S
R/S
R/S
R/S
F
H
F
F
H
F
H
F
H
F
2.9
ND
6.7
ND
4.3
Cl
F
F
F
F
H
3.7
H
Br
Ph
H
H
4.6
ND
25
ND
ND
ND
ND
ND
ND
ND
ND
ND
F
H
H
H
ND
ND
14
F
H
4-pyridyl
NA
H
H
ND
ND
ND
ND
ND
ND
ND
c
Cl
Cl
Cl
F
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
ND
4.7
NA
19
R
NA
37
ND
ND
ND
R/S
R/S
Cl
NA
0.46
>50
NA
a
b
EC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2−3 fold from the EC50. m/r/h =
mouse/rat/human. Synthetic scheme outlined in Supporting Information,30. ND = not determined. NA = not applicable.
c
indoline to give an open chain phenyl acetamide derivative 65, or
using 1,2,3,4-tetrahydroisoquinoline 66. None were tolerated.
However, the 1,2,3,4-tetrahydroquinoline derivative 67 was
moderately well tolerated with submicromolar potency. When
the amide was moved to the 3-position (68), no activity was
observed, indicating that the shape and proximity of the amide is
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Table 7. In Vitro Data for Key Compounds
1
27
35
47
55
a
T. brucei EC50 (μM)
0.03
3.5
0.03
2.0
0.02
0.06
2.8
0.08
4.4
Hill slope
selectivity vs MRC-5
MWT
>1800
345
2.4
>1800
>2700
328
1.9
>860
312
2.0
>720
346
2.1
344
clogP
2.4
PSA
59
59
59
49
58
solubility
>100 μg/mL
Fu = 0.26 (mouse)
Fu = 0.15
ND
ND
64 μg/mL
>100 μg/mL
Fu = 0.33 (mouse)
Fu = 0.29
plasma protein binding
Fu = 0.16 (rat)
ND
Fu = 0.42 (mouse)
ND
Fu = 0.59 (mouse)
Fu = 0.43
brain tissue binding (rat)
microsomal intrinsic clearance (m/h) (mL min−1 g−1
b
)
4.4/2.6
3.6/ND
1.2/ND
1.9/ND
1.3/<0.5
a
b
EC50 values are shown as mean values of two or more determinations. Standard deviation is typically within 2−3-fold from the EC50. m/h =
mouse/human. ND = not determined. cLogP and PSA calculated using StarDrop. MWT = molecular weight; PSA = polar surface area; Fu = fraction
unbound.
important in the active binding pose. In an attempt to further
improve the metabolic stability, 3,3-dimethyl indoline 63 was
synthesized; unfortunately, this gave ∼34-fold loss in potency.
The molecular target (or targets) of these compounds is
unknown. However, there is a very tight structure−activity
relationship. It could be that the indoline-2-carboxamides may be
working on more than one target, the overlapping needs for the
SAR of each target resulting in a very tight SAR picture. Both the
pendant ethers or benzyl groups give a very similar SAR, so it is
possible with the flexibility of the linker chain that the
substituents are free to rotate and occupy very similar space.
There is a substantial difference between the enantiomers of the
indoline-2-carboxamide, with the R-isomer being >1350-fold
more potent than the S. The space around the methyl amide is
very constrained as exceptionally few modifications are tolerated
in this position, and removing the methyl to give an
unsubstituted amide results in a loss of potency as does
extending it. Substituted indolines are well tolerated, although
no potency gains were observed. Substituting with large aromatic
groups is tolerated in the 5-position, suggesting that there is a
large space surrounding this part of the molecule that plays no
role in the antiproliferative effect. Improved metabolic stability is
observed with some of the substituted cores (e.g., 5-fluoroindo-
line). Alternative cores are moderately well tolerated but offer no
improvement in physiochemical or antiproliferative properties.
consistent with blood−brain barrier penetration and relatively
low plasma protein binding.
Achieving Proof of Concept for the Series. Compound 1
had a higher than ideal microsomal metabolic instability in
mouse (Clint 4.4 mL min−1 g−1) and when dosed intra-
peritoneally to female NMRI mice (n = 3) at 10 mg/kg, half-
life was short (0.7 h), with blood free concentration below EC50
within 2 h of dosing (Tables 7 and 8). Therefore, to try and
achieve in vivo proof of concept for the series, the
pharmacokinetics of 1 was assessed in HRN hepatic CYP
reductase null mice31 (n = 3) following a single intraperitoneal
(IP) dose at 10 mg/kg. With the absence of hepatic CYP450
activity, a significant improvement in exposure was observed
(∼9-fold higher AUC and half-life increasing to 4.8 h). Notably,
blood free concentration remained significantly higher than EC99
for 8 h, such that a b.i.d. dose regimen at greater than 2 mg/kg IP
should deliver cures in a mouse model of infection, assuming
linearity of exposure. From previous work (unpublished), T. b.
brucei infection develops in female HRN mice consistently as for
female NMRI mice, making it a useful model for rapid in vivo
proof of concept for HAT with compounds poorly optimized for
stability to CYP450s, thus allowing commitment to medicinal
chemistry.
Compound 1 was therefore progressed to a stage 1 mouse
model of HAT using HRN mice infected with T. b. brucei S427.
Compound 1 was dosed (n = 3 infected mice) at 3 mg/kg IP
b.i.d.for 4 d, starting 3 d post infection. Parasitaemia was knocked
down and no relapse was observed up to 30 d post infection,
confirming full cures at this dose regimen. Interestingly, in a
subsequent study using female NMRI mice infected with T. b.
brucei S427, 1 delivered a partial curative regimen (2/3 mice
DRUG METABOLISM AND PHARMACOKINETICS
■
Analysis of the measured in vitro DMPK data indicated that, in
general, molecules synthesized with a higher predicted Log P
(StarDrop) showed a higher metabolic instability when
incubated with pooled mouse liver microsomes; compounds
with intrinsic clearance in the desired range of <5 mL min−1 g−1
had a cLogP < 2. In general, the compounds showed a good
plasma free fraction; with the nonether benzyl series having a
slightly higher free fraction.
1
surviving to 30 d post infection; /3 mice with parasitaemia
relapsed) when dosed at 10 mg/kg b.i.d. for 4 d. At this dose level,
blood free concentration was only higher than EC99 for 2 h,
rapidly falling below EC50 by 4 h. This indicates that whatever the
mechanism of action of the compound, blood free EC99 does not
need to be maintained for activity as seen for T. brucei N-
myristoyltransferase inhibitors, as observed previously.32 This
could be due to rapid cidality, commitment to parasite death, or
compound concentration selectively within the parasite.33
Submitting compound 1 to a static-cidal assay (Figure 3)
indicated the compound was rapidly cidal,34 with a clear
reduction of parasite counts by the first time point of the assay
(24 h) at concentrations as low as 0.6 μM.
Five key molecules were selected for more detailed
investigation. These were selected on the basis of potency,
microsomal stability, and some diversity in structure. Thus, 1 is
the start point, 27 the most active enantiomer, 35 is the fluoro
analogue, 47 is where the ether oxygen has been removed, and 55
is where there is a fluorine in the indoline ring to increase the
metabolic stability. A summary of the in vitro profile and mouse
pharmacokinetic data are shown in Tables 7 and 8, respectively.
The compounds selected all had drug-like properties: molecular
weight around 350, clogP around 2, and PSA < 60 Å2, which is
When 47 and 55 were dosed IP to female NMRI mice, a
significant increase in exposure was observed compared to 1
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Figure 3. Static-cidal profile for 1. T. brucei bloodstream form parasites
were exposed to 10 concentrations of compound 1, and the effect on
parasite numbers was measured at 24 and 48 h post compound
exposure. A negative slope indicates cidal activity at that concentration.
Concentrations are as follows (μM): (black circles) 50, (red triangles)
17, (green squares) 5.6, (yellow diamonds) 1.9, (blue triangles) 0.6,
(magenta hexagons) 0.2, (cyan circles) 0.07, (violet squares) 0.01, (olive
green triangles) 0. RFU = relative fluorescence units.
(1.5−6.5-fold higher AUC), consistent with the higher micro-
somal stability. Compound 27 had similar exposure compared to
1 (1.4-fold lower AUC), which was not surprising considering
similar microsomal metabolic stability (Clint for 1 and 27 = 4.4
and 3.6 mL min−1 g−1, respectively). Compound 35 also had
similar exposure to 1 (same AUC), yet microsomal stability was
greatly improved (Clint 1.2 mL min−1 g−1). All compounds
showed a significant loss in AUC when dosed orally.
With in vivo PoC achieved for the series in a stage 1 mouse
model of HAT, it was decided to progress one of the lead
molecules to a stage 2 mouse model.35 All the lead compounds
were brain penetrant and importantly had high brain fraction
unbound (Fu), similar to plasma Fu, such that significant dose
adjustment above stage 1 efficacious dose should not be
necessary to deliver efficacious free brain concentration.
Unfortunately, however, any dose adjustment that may be
required above the initial pharmacokinetic assessment would not
be possible from data obtained from maximum tolerated dose
(MTD) studies on 27 and 47 in female NMRI mice. Compound
47 was well tolerated at 15 mg/kg IP b.i.d. for 3 days but with
clinical signs of toxicity developing at 25 mg/kg after a single
dose. Compound 27 was poorly tolerated even at 15 mg/kg b.i.d.
IP with clinical signs of toxicity developing during the second day
of treatment.
Compound 47 was therefore chosen for the stage 2 efficacy
model at a maximum dose of 12.5 mg/kg b.i.d. IP. Stage 1 efficacy
data on 1 suggested time above a certain threshold was not
necessary for cures. Both 47 and 55 offered the greatest
improvement in Cmax following IP dosing, but 47 was chosen for
this initial study for ease of synthesis, not being a racemate and
MTD already defined. For stage 2 of infection, the standard
model is T. b. brucei GVR35 in which CNS infection is allowed to
develop in female NMRI mice before treatment starts, requiring
about 21 d. Mice (n = 5) were treated with 47 at 12.5 mg/kg IP
b.i.d. for 5 d from 21 d post infection and, although the majority
of mice eventually relapsed with parasitaemia (6−13 days later
than vehicle dosed infected mice), one mouse was cured, defined
as survival with no parasitaemia at 180 d post infection.
H
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Trypanosome Growth Assay. This was conducted as previously
described.27 Cell culture plates were stamped with 1 μL of an
appropriate stock concentration of test compound, followed by addition
of 200 μL of trypanosome culture (HMI9-T, 10% FCS) to each well at a
density of 2000 cells per well (except column 12 receiving media only).
Primary screening was carried out at 30 μM; 93 compounds showed a
greater than 50% inhibition of parasite growth. Potency was carried out
using 10-point dose−response curves.
Screening in Receptor Panel. The compound series is very
exciting as it shows partial cure in a stage 2 model of HAT.
However, the observed toxicity prevents further progression of
the compounds. To try and identify a possible driver for toxicity,
that may then offer an opportunity to design out this unwanted
effect, the key compounds were screened against a panel of
receptors at the NIMH Psychoactive Drug Screening Program
(PDSP).36,37 No significant activity against a large panel of
receptors, including dopamine, serotonin, histamine, and
muscarinic receptors (see Supporting Information) was
observed.
MRC-5 Growth Assay. This was conducted as previously described.27
MRC-5 cells (2000 cells per well) were seeded in 200 μL of DMEM,
10% FCS, and allowed to adhere overnight. One microliter of test
compound was added to each well on the day of assay initiation.
Trypanosome and MRC-5 plates were incubated at 37 °C in an
atmosphere of 5% CO2 for 69 h prior to the addition of 20 μL of
rezasurin (final concentration 50 μm). After a further 4 h, fluorescence
was measured (excitation 528 nm; emission 590 nm) using a BioTek
flx800 plate reader. For cell assays, ten-point screening was conducted
between 15 μM and 0.5 nM or between 50 μM and 2 nM, depending on
stock concentrations. The final DMSO level was 0.5% in all cases.
Static-Cidal Assay. The static-cidal assay was conducted as
previously described.34 In short, bloodstream-form T. brucei cells were
plated into 384-well assay plates (4 × 105 cell mL−1, 50 μL/well) and
incubated for 0, 20 or 44 h at 37 °C, 5% CO2. At each time-point, plates
were developed with resazurin as described above.
SUMMARY
■
A series of phenoxyacyl and benzyl acyl indoline-2-carboxamides
have been investigated as a new potential stage 2 therapeutic for
African sleeping sickness. The series was found to be highly
potent against T. b. brucei in culture displaying a strong
antiproliferative effect while retaining excellent selectivity over
the mammalian cell counter screen. The compounds showed
limited possibilities for optimization; most modifications led to
reduced potency or increased metabolic instability. However, a
carefully designed program led to derivatives 47 and 55 with
improved metabolic stability and enhanced in vivo exposure.
This demonstrates that it is possible to optimize phenotypic hits
in the absence of knowledge of the molecular target.38
Physicochemical Properties and in Vitro DMPK Parameters.
The software package StarDrop by Optibrium was used to calculate
physicochemical parameters including log P, molecular weight, and
polar surface area (PSA). The aqueous kinetic solubility of the
compounds was measured using laser nephelometry as previously
described.40 The plasma protein binding of the compounds was
determined by the equilibrium dialysis method.12
An exemplar in the series, 1 was able to cure a stage 1 model of
HAT. Initially, this was carried out in HRN mice, which gave
improved blood exposure. Interestingly, 1 was also curative in
wild-type NMRI mice at exposures at which coverage with
compound at the blood free concentration above EC99 was not
maintained. This indicates that whatever the mechanism of
action of the compound, blood free EC99 does not need to be
maintained for a set duration of time, in contrast to what was
required for N-myristolyltransferase inhibitors.32 This could be
due to rapid cidality, commitment to parasite death, or
compound concentration selectively within the parasite.33
This study demonstrates successful use of HRN mice to
rapidly deliver in vivo proof of concept at a relatively early stage
in the project. This gave confidence in the series and to commit
additional medicinal chemistry resources to further optimization,
knowing that the series was efficacious. Clearly one issue
addressed in the subsequent optimization was that of microsomal
stability.
Compounds in the series were brain penetrant, and partial cure
in a stage 2 mouse model was achieved. Unfortunately, toxicity
prevented any dose escalation to explore the possibility to deliver
a fully curative stage 2 regimen. Initial attempts to identify the
mode of toxicity have not been successful. Although the
compounds were derived from a focused protease set, their
mode of action is unknown. Elucidating the molecular target(s)
may allow for a target-based drug discovery program, which may
then also allow the associated toxicity with the related
compounds to be dialled out.
Brain Tissue Binding. The methodology employed was a
modification of that reported previously (Summerfield et al., 2007).41
In brief, a 96-well equilibrium dialysis apparatus was used to determine
the free fraction in the brain for the test compound (HT Dialysis LLC,
Gales Ferry, CT). Membranes (12−14 kDa A cutoff) were conditioned
in deionized water for 60 min, followed by conditioning in 80:20
deionized water:ethanol for 20 min and then rinsed in artificial
cerebrospinal fluid (CSF) before use. Mouse brain was removed from
the freezer and allowed to thaw on the day of experiment. Thawed brain
tissue was homogenized with artificial CSF to a final composition of 1:2
brain:artificial CSF using a Covaris S2 (KBiosciences, Hoddesdon, UK).
Diluted brain homogenate was spiked with the test compound (10 ug/
g), and 150 μL aliquots (n = 6 replicate determinations) loaded into the
96-well equilibrium dialysis plate. Dialysis vs artificial CSF (150 μL) was
carried out for 5 h in a temperature controlled incubator at ca. 37 °C
(Stuart scientific Ltd., UK) using an orbital microplate shaker at 125 rpm
(Stuart scientific Ltd., UK). At the end of the incubation period, aliquots
of brain homogenate or artificial CSF were transferred to micronic tubes
(Micronic BV, The Netherlands) and the composition in each tube
balanced with control fluid such that the volume of artificial CSF to brain
homogenate is the same. Sample extraction was performed by the
addition of 400 μL of acetonitrile containing an appropriate internal
standard. Samples were allowed to mix for 1 min and then centrifuged at
1825g in 96-well blocks for 10 min (Allegra X12-R, Beckman Coulter,
USA). All samples were analyzed by means of UPLC/MS/MS on a
Quattro Premier XE mass spectrometer (Waters Corporation, USA).
The unbound fraction was determined as the ratio of the peak area in
artificial CSF to that in brain, with correction for dilution factor
according to eq 1.42
In summary, this study has demonstrated the ability to
optimize phenotypic hits to furnish compounds that are brain
penetrant and trypanocidal. It has also demonstrated the utility of
the HRN mice in establishing early in vivo proof of concept.
1/D
undiluted F =
u
((1/Fu,apparent) − 1) + 1/D
(1)
where D = dilution factor in brain homogenate and Fu,apparent is the
measured free fraction of diluted brain homogenate.
Intrinsic Clearance (Clint) Determination. Each test compound
(0.5 μM) was incubated with pooled female CDI mouse liver
microsomes, pooled male Sprague−Dawley rat microsomes, or pooled
mixed gender human liver microsomes (Tebu-Bio, UK; 0.5 mg/mL 50
EXPERIMENTAL SECTION
■
Assays. Cell-Based Assays. Proliferation assays using bloodstream
form T. brucei brucei and human MRC-5 fibroblasts were conducted
using a modification of the cell viability assay as previously reported.27,39
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mM potassium phosphate buffer, pH 7.4) 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 μL) of the incubation mixture was removed and mixed
with acetonitrile (100 μL) to stop the reaction. Internal standard was
added to all samples, and the samples were centrifuged at 1825g for 10
min to sediment precipitated protein. The plates were 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 (Clint) of each compound was then calculated using the
following eq 2:
analyzed under a UV lamp (UV 254/365 nm) and/or KMnO4 was used
for visualization. Flash column chromatography was performed on an
automated purification system (Teledyne ISCO Combiflash Com-
panion or Combiflash Retrieve) using Grace Resolve prepacked silica gel
cartridges (230−400 mesh, 40−63 μm, varying sizes depending on
1
reaction scale). H and 13C NMR spectra were recorded on a Bruker
Advance II 500 spectrometer operating at 500.1 and 125.8 MHz (unless
otherwise stated) using CDCl3 or DMSO-d6 solutions. Chemical shifts
(δ) are expressed in ppm recorded using the residual solvent as the
internal reference in all cases. Signal splitting patterns are described as
singlet (s), doublet (d), triplet (t), multiplet (m), broadened (b), or a
combination thereof. Coupling constants (J) are quoted to the nearest
0.1 Hz (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 spectrometer, where both instruments
were connected to an agilent diode array detector. LC-MS chromato-
graphic separations were conducted with a Water Xbridge C18 column,
50 mm × 2.1 mm, 3.5 μm particle size, using either methanol, methanol/
water (95:5), or water/acetonitrile (1:1) + 0.1% formic acid as the
mobile phase; 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 synthesized compounds
were determined to be of >95% purity by the LC-MS method (TIC and
UV) as determined using this analytical LC-MS system. High resolution
electrospray measurements were performed on a Bruker MicroTof mass
spectrometer. Microwave-assisted chemistry was performed using a
Biotage Initiator microwave synthesizer.
General Procedure A: Synthesis of Diamides from Indoline-2-
carboxylic Acid (Scheme 1). To a solution of R1CO2H (1 mol equiv) in
DCM (anhydrous, 5 mL mmol−1) at room temperature, O-
(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate
(TBTU) (1 mol equiv) was added and the reaction mixture stirred
for 30 min, indoline-2-carboxylic acid (1 mol equiv) was then added and
the reaction mixture stirred for 2 h. The progress of the reaction was
monitored by LCMS; after successful coupling of the first amide, a
further 1 equiv of TBTU was added to the reaction mixture and the
mixture stirred for 30 min, methylamine (2 M in MeOH or 2 M in THF,
5 mol equiv) was added, and the reaction mixture stirred for a further 1 h
or until complete consumption of the starting material was observed.
The reaction was quenched with H2O, passed through a hydrophobic
frit, and the organic layer concentrated in vacuo. Column chromatog-
raphy eluting with ether afforded the desired diamides.
(R)-1-(2-(4-Chlorophenoxy)acetyl)-N-methylindoline-2-carboxa-
mide (27). Prepared using general procedure A, colorless solid, 207 mg,
67%. δH (500 MHz, CDCl3) 8.21 (bs, 1H, NH), 7.30−7.26 (m, 5H,
ArH), 7.15 (t, J = 7.5 Hz, 1H, ArH), 6.91 (dd, J = 6.9 and 2.2 Hz, 2H,
ArH), 5.79 and 5.06 (2 × bs, rotomer, 1H, CH2), 5.28 and 4.75 (2 × bs,
rotomer, 2H, CH2), 3.68 (bs, 1H, CH2), 3.30 (bs, 1H, CH2), 2.76 (bd,
rotomer, 3H, CH3). δC (125 MHz, DMSO-d6) 171.0, 165.9 (CO),
156.9, 142.8, 129.6, 129.0, 127.1, 124.6, 124.5, 123.7, 116.2 (ArC), 66.0,
59.6, 34.5, 25.9. LCMS (ES+): m/z (%) 345 [M + H]+. tR: 4.17 (20−
95% MeCN, acidic). HRMS (ES+) calcd for [C18H18ClN2O3 + H],
345.1000; found, 345.0988.
(R)-1-(2-(4-Fluorophenyl)acetyl)-N-methylindoline-2-carboxa-
mide (47). Prepared using general procedure A, colorless solid, 339 mg,
54%. δH (500 MHz, DMSO-d6) 8.28 (d, J = 4.4 Hz, 1H, NH), 8.05 (d, J =
8.1 Hz, 1H, ArH), 7.28−7.26 (m, 2H, ArH), 7.21 (d, J = 7.3 Hz, 1H,
ArH), 7.17−7.13 (m, 2H, ArH), 7.01 (t, J = 7.3 Hz, 1H, ArH), 5.11 (d, J
= 8.7 Hz, 1H, CH), 3.78 (d, J = 16.0 Hz, 1H, CH2), 3.59 (dd, J = 16.5 and
11.3 Hz, 1H, CH2), 3.47 (d, J = 16.0 Hz, 1H, CH2), 3.06 (d, J = 16.5, 1H,
CH2), 2.65 (d, J = 4.4 Hz, 3H, CH3). δC (125 MHz, DMSO-d6) = 171.4
(CO), 169.3 (CO), 131.6, 131.85, 131.2, 130.0, 127.0, 124.4,
123.4, 116.3, 114.9, 114.7 (ArC), 60.9, 34.1, 25.8. LCMS (ES+): m/z
(%) 313 [M + H]+. tR: 3.90 (20−95% MeCN, acidic). HRMS (ES+)
calcd for [C18H18FN2O2 + H], 313.1347; found, 313.1335.
Clint (mL min−1 g−1 liver) = k × V × microsomal protein yield
(2)
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.43
Verapamil was used as a positive control to confirm acceptable assay
performance.
In Vivo Mouse Pharmacokinetics and Brain Penetration.
Mouse Pharmacokinetics. Each compound was dosed as a bolus
solution intraperitoneally at 10 mg free base/kg (dose volume, 10 mL/
kg; dose vehicles used, 10% DMSO; 40% PEG400; 50% Milli-Q water or
5% DMSO; 40% PEG400; 55% saline) to female HRN (1 only) or
NMRI mice (n = 3) or dosed orally by gavage as a solution at 10 mg free
base/kg (dose volume, 10 mL/kg; dose vehicle, 10% DMSO; 40%
PEG400; 50% Milli-Q water) to female NMRI mice (n = 3). 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 5 (IP only), 15, and 30 min, 1, 2, 4, 6, and 8 h post
dose, mixed with two volumes of distilled water, and stored frozen until
UPLC/MS/MS analysis. Pharmacokinetic parameters were derived
from noncompartmental analysis of the blood concentration time curve
using PK solutions software v 2.0 (Summit Research Services, USA).
Mouse Brain Penetration. Each compound was dosed as a bolus
solution intravenously at 0.5 or 1 mg free base/kg (dose volume, 5 mL/
kg; dose vehicle, 10% DMSO; 40% PEG400; 50% Milli-Q water) 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 and mixed with two volumes of distilled water, the mice
decapitated, and the brain removed. Diluted blood and brains were
stored frozen until preparation and analysis by UPLC/MS/MS. 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.
Mouse Stage-1 and Stage-2 Efficacy Mouse Models. These
were carried out as previously described27,32 at the dose level presented
and using the same dose formulation as the pharmacokinetic studies.
HRN mice were used under the same infection and monitoring protocol
as described for female NMRI mice.
Animal Ethics. All regulated procedures on living animals were
carried out under the authority of a project license 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 were approved by the University’s Ethical Review
Committee (ERC) before submission to the Home Office. The ERC
has a general remit to develop and oversee policy on all aspects of the use
of animals on University premises and is a subcommittee of the
University Court, its highest governing body.
Chemistry/Compound Characterization. General Methods.
Chemicals and solvents were purchased from Aldrich Chemical Co.,
Alfa Aesar, Fluorochem, Apollo, and Fisher Chemicals 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
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jmed-
chem.5b00596.
■
S
Synthetic details for 26, 28−55, I3, 57−68; details of
receptors screened at the NIMH Psychoactive Drug
Screening Program (PDF)
Molecular formula strings (CSV)
AUTHOR INFORMATION
Corresponding Author
*Phone: +44 1382 386 240. E-mail: i.h.gilbert@dundee.ac.uk.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding for this work was provided by the Wellcome Trust
(grant ref. WT077705 and Strategic Award WT083481). We
thank Dr. Stephen Thompson and Margaret Huggett for
chemical synthesis around the indoline-2-carboxamide not
detailed here, Gina McKay for performing HRMS analyses,
and Daniel James for data management. Receptor binding
profiles were generously provided by the National Institute of
Mental Health’s Psychoactive Drug Screening Program, contract
no. HHSN-271-2008-00025-C (NIMH PDSP). The NIMH
PDSP is directed by Bryan L. Roth MD, Ph.D., at the University
of North Carolina at Chapel Hill and Project Officer Jamie
Driscol at NIMH, Bethesda MD, USA.
ABBREVIATIONS USED
■
HAT, human African trypanosomiasis; T.b., Trypanosoma brucei;
HTS, high throughput screening; Fu, fraction unbound; PPB,
plasma protein binding; SAR, structure−activity relationship;
DMPK, drug metabolism and pharmacokinetics; TBTU, O-
(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoro-
borate; CLEA, Candida antartica CAL-A; Clint, intrinsic
clearance; B:B, blood to brain ratio; CNS, central nervous system
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