2-Pyridylquinolone antimalarials with improved antimalarial activity and physicochemical properties

Article abstract of DOI:10.1039/c5md00062a

A series of 2-pyridylquinolones has been prepared in 5-7 steps and through lead optimisation, antimalarial activity as low as 12 nM against Plasmodium falciparum (Pf) has been achieved. Compared with previous analogues in this series, selected molecules h

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Page 1 of 13
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DOI: 10.1039/C5MD00062A
2-Pyridylquinolone Antimalarials with Improved Antimalarial
Activity and Physicochemical Properties
Sitthivut Charoensutthivarakul,a W. David Hong,a Suet C. Leung,a Peter D. Gibbons, a Paul T.P.
Bedingfield,b Gemma L. Nixon,a Alexandre S. Lawrenson,a Neil G. Berry,a Stephen A. Ward,b
Giancarlo A. Biaginib and Paul M. O’Neill*a
A series of 2ꢀpyridylquinolones has been prepared in 5ꢀ7 steps and through lead optimisation, antimalarial activity as low as
12 nM against Plasmodium falciparum (Pf) has been achieved. Compared with previous analogues in this series, selected
molecules have improved solubility, a reduced potential for offꢀtarget toxicity and improved metabolic stability profiles.
Docking studies performed with a homology model of the Pfbc1 complex target demonstrate a key role for the Tyr16 residues
in the recognition of highly active quinolone based inhibitors.

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Introduction
Malaria is caused by parasites of the genus Plasmodium which are widespread in tropical and subtropical regions of the world
and is responsible for one million deaths every year1. Plasmodium falciparum is the most deadly form of the parasite transmitted to
the human host by Anopheles mosquitoes2 and has developed resistance to many of the established classes of drugs3. Due to drug
resistance, the discovery of novel drug candidates remains a top priority. The development of compounds focused on hitting new
targets is seen as a viable way forward and is an established approach to avoid cross resistance with available antimalarials4.
There are a large number of recent studies based on the development of antimalarials with quinoloneꢀrelated structures5ꢀ7. One
of these programmes resulted in the discovery of ELQꢀ300, a selective cytochrome bc1 inhibitor now in preꢀclinical development8ꢀ
10. Our recent work has shown that inhibition of two mitochondrial enzymes in the electron transport chain – the cytochrome bc1
complex and the recently identified PfNDH2 (Type II NADH:ubiquinone oxidoreductase)11, 12 – results in the collapse of the
mitochondrial membrane potential and to the inhibition of de novo pyrimidine biosynthesis and ultimately parasite death13. Since
effective dual inhibition of these proteins can be observed with the 2ꢀaryl and 2ꢀpyridylquinolone pharmacophore14a a number of
inhibitors based on this structure have been identified. Two such lead compounds are CK-2-68 and SL-2-25
(
1
)
–
both of which
16
confer potent nanomolar activity against asexual and liver stages of P. falciparum15,
(Figure 1). Here, we describe further
modifications of the quinolone Aꢀring and 2ꢀpyridyl side chain with the aim of reducing lipophilicity in an attempt to provide
compounds with improved activity/solubility profiles.
In order to increase polarity of our quinolone hit series we incorporated a hydroxyl group into the Aꢀring at one of the 6, 7 or 8ꢀ
positions. The inclusion of the hydroxyl group in the quinolone Aꢀring offered the option of exploring proꢀdrugs for the series by
derivatisation to provide either phosphate or carbamate proꢀdrugs. Since the corresponding methoxy analogues were prepared en
route to 23 ꢀ 25, these compounds were also screened against Plasmodium falciparum.
Results and discussion
The synthesis of the methoxy and hydroxy series was accomplished in 5ꢀ7 steps from commercially available starting materials
(see Scheme 1). Aldehyde , synthesised from corresponding boronic acid and aromatic halide in excellent yield (see supporting
information), was utilised in a Grignard reaction to give alcohol in 32ꢀ83% yields. Alcohol was then oxidised using PCC to
yield ketone in good yields. Oxazole was prepared in yields of 31ꢀ98% from the respective isatoic anhydride which was
synthesised by adding diphosgene into methoxyꢀsubstituted anthranilic acid17 (see Supporting information). Reaction of oxazole
with ketone in the presence of catalytic trifluoromethane sulfonic acid gave the desired quinolones
22 in 4ꢀ62% yields18.
Some selected quinolones were further demethylated by adding BBr3 to obtain hydroxyl analogues 23-25 in 10ꢀ69 % yields.
2
3
3
4
6
5
6
4
7 ꢀ
From previous work, we had identified the
pꢀOCF3 substituent on the Dꢀring provided excellent antimalarial activity. More
recent work within our group on a related quinolone series revealed some advantages in 3,4ꢀdichloro substitution (Figure 2) and
for this reason we also briefly investigated this substitution pattern within the quinolone Dꢀring sideꢀchain in analogues 16-17
.
Antimalarial activities
Analysis of the in vitro data reveals that excellent activity can be achieved by the introduction of a methoxyl group in the 7ꢀ
position (see Table 2, entries 9, 11 and 13). A clear trend is seen in the pyridyl series where 7ꢀmethoxy analogues provide optimal
activity (see for example direct comparison of SL-2-25 and 9). In contrast, 5, 6 and 8ꢀmethoxy analogues of SL-2-25 resulted in a
complete loss in antimalarial activity.
Hydroxyl substitution is less favorable when compared with their methoxylated products (see Table 2, for example comparison
between and 24). Some regioisomers of (eg. 11 and 13) exhibit potent activity against the parasite 3,4ꢀDichlorophenyl
9
9
.
substitution also is tolerated as seen in 17 and 22 both of which show excellent antimalarial efficacy. Although 7ꢀmethoxy
analogues provide the best activity, we also noted that analogues 18, 20 and 22, which lack this substituent, are potent nanomolar
antimalarials.
Looking into their molecular structures, compound
side chain devoid of flexibility show low to moderate antimalarial activities. On the other hand, 11
meta substitution configured side chains exhibit good to outstanding efficacy against the 3D7 parasite strain.
9
,
15, 16, and 21 containing a 7ꢀOMe group and a similar para substitution
–
14 and 17 20 containing
ꢀ
Additionally, some selected compounds were tested against drug resistant strains of P. falciparum including chloroquine
resistant W2 and atovaquone resistant TM90C2B (Table 3). Compound 11 and 13, both contain flexible side chains, showed
excellent activities against both W2 and TM90C2B strains that are comparable to marketed drugs (chloroquine and atovaquone)
and SL-2-25. Notably, against the atovaquone resistant strain TM90C2B both 11 and 13 express excellent activity and show no
cross resistance with atovaquone. Both 11 and 13 were also tested against 3D7ꢀyDHODH·GFP, a transgenic derivative of P.
falciparum 3D7 containing yeast dihydroorotate dehydrogenase,14a,b and were shown to be inactive (IC50 >1 ꢁM), consistent with
these inhibitors targeting the electron transport chain of the parasite mitochondria.

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Drug Solubility Profiles
The poor aqueous solubility associated with the 2ꢀ and 3ꢀaryl/pyridyl quinolone series of antimalarial provides development
challenges for this class of drug. Previous research indicates that quinolones generally have poor aqueous solubility due to planar
aggregation via πꢀstacking of their ring system16. This interaction also leads to higher observed melting points19. Several strategies
can be used to improve the solubility; here, we explored the incorporation of a polar head group with modification of the pyridyl
sideꢀchain geometry. For those derivatives expressing higher potency, a 96ꢀwell plate aqueous solubility assessment was
performed19. The results are shown in Table 4. A clear relationship between melting point and solubility was observed. Compound
18 had the best aqueous solubility amongst the selected quinolones that correlated with its low melting point. Although 3,4ꢀ
dichlorophenyl analogues (16, 17 and 22) gave excellent activity, it is worth noting that a high melting point is observed which
possibly reflects a decrease in aqueous solubility. Inspection of the side chain suggests that the pyridyl function exclusively affects
both antimalarial activity and solubility. The presence of the pyridyl substitution (3,5ꢀdisubstituted pyridyl) as in 11, 17 and 18
gives excellent potency and a range of melting points that were significantly lower than SL-2-25’s. This relationship can be used
within this compound class to compare solubility.
Metabolic Stability Profiles
Selected potent quinolones were also profiled in microsomal stability assessment in human liver microsomes and rat
hepatocytes. Human microsomal stability assessment shows how stable the drug is when enters CYP450 oxidation stage, while rat
hepatocytic clearance shows the compound stability in both phase I and phase II metabolism20. Comparing 7ꢀOMe (11 and 17) to
7ꢀH (18
,
20
,
22) analogues, 7ꢀOMe compounds are more sensitive to CYP450 turnover, possibly as a result of 7ꢀ
Oꢀdemethylation.
3,4ꢀDichlorophenyl compounds (17
,
22) are more stable in human microsomal media than their ꢀOCF3 counterparts; the reverse
p
is seen in rat hepatocytes with greater stability observed for the latter pꢀsubstituent.
Potential for Off-target bc1 Inhibition
Previous antimalarial projects that have focused on the development of bc1 inhibitors have had to be terminated because of
safety concerns regarding cardiotoxicity21. Therefore, a bovine bc1 counterscreen was established to investigate potential
mammalian mitochondrial toxicity22. Some highly potent compounds were assessed in vitro, using bovine heart bc1 inhibition as
displayed in Table 6
.
The singleꢀpoint inhibition experiments were run at two different concentrations of select compounds – 100 nM and 1 ꢁM. A
low % inhibition represents that the compound is less active in the bovine heart bc1 screen. There is no obvious trend when
comparing 7ꢀH compounds to their 7ꢀOMe counterparts Generally, the substitutions on pyridyl ring as in 11, 17 and 18 provide
molecules with lower affinity for the mammalian bc1 target. Further studies are required to examine how this relationships extends
to human heart tissue derived isolated bc1 complexes.
Molecular Modeling
23
We have recently shown that 4ꢀ(1H)ꢀpyridones bind to the Qi site in cytochrome bc1
.
Consequently molecular modeling
studies were undertaken to provide insight as to how the compounds bind to this site. Shown in Figure 3 is the docking pose
between and a Plasmodium homology model of the yeast bc1 complex.
9
A homology model of cytochrome bc1 based on the primary sequence Q02768 and the 1NTK B. taurus bc1 complex template
was created with the PHYRE online homology modelling tool24. The model was used to predict the binding of
active site.
9 within the Qi
Using GOLD and the methods developed in our group25 compounds were docked into the binding site. We observed that our
potent inhibitors,
9 and 17, were predicted to bind tightly to the Qi site with the average PLPscores of 64 and 61, respectively.
In line with previous bc1 crystal structures with molecules bound in the Qi site (GW844520, GSK93212123, antimycin or
NQNO26), it was found that the interaction of quinolones with the bc1 target also involved several interactions – polar and
hydrophobic. These include a hydrogen bond between Asp218 and the quinolone’s NH group whilst there is another weak
hydrogen bond between Ser196 and the carbonyl group, very similar to the interactions of GW844520, GSK93212123 and
antimycin.26 Of particular importance is the hydrophobic interaction of 7ꢀmethoxy within a hydrophobic pocket created by
Leu214, Ile22, Phe210 and Trp26. The pyridyl group of
9 forms π stacking interactions with Phe30 and His12 whilst the OCF3
moiety appears to form hydrophobic contacts with Ile34 and Phe37, which is similar to the interaction of the pentyl moiety in
antimycin and alkyl chain in NQNO with the same two residues. This latter interaction is likely to be an important factor in
providing extra levels of antimalarial activity as seen for 9, 11, 13 and 17.
Conclusion
A 5ꢀ7 step synthesis of an array of 2ꢀpyridylquinolones with potent antimalarial activity has been described. Subtle changes to
the geometry of the side chain have a profound effect on the inherent antimalarial activity, physicochemical, metabolic and bc1

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inhibitory profiles of related quinolone analogues. In terms of balance of properties, analogues 17 and 18 have improved
properties over the initial lead compound with a reduced propensity for offꢀtarget mammalian bc1 inhibition coupled with
improved solubility/ metabolic stability. These observations encourage selection of molecules in this class for additional in vivo
PK and mouse activity assessment in the near future.
Experimental
Airꢀ and moistureꢀsensitive reactions were carried out in ovenꢀdried glassware, sealed with rubber septa, under nitrogen from a
balloon. Airꢀ and moistureꢀsensitive liquids and reagents were transferred via syringe. Reactions were stirred using Teflonꢀcoated
magnetic stir bars. All commercial reagents were used without further purifications. NMR spectra were recorded in CDCl3 or
DMSOꢀd6 on a Brucker AMX400 spectrometer (1H 400 MHz, 13C 100 MHz). Chemical shifts (δ) were expressed in ppm relative
to tetramethylsilane (TMS) as an internal standard. J values are in hertz (Hz) and the splitting patterns were designed as follows: s,
singlet; d, doublet; t, triplet; q, quartet; dd, double doublet; m, multiplet. Mass spectra were recorded on either a Micromass LCT
Mass Spectrometer using electrospray ionisation (ESI) or Trioꢀ1000 Mass Spectrometer using chemical ionisation (CI). Elemental
analysis (%C, %H, %N) was performed in the University of Liverpool microanalysis laboratory. All melting points were
determined with Gallenkamp melting point apparatus and were uncorrected. See supporting information for experimental and data
on all intermediates.
General procedure for the synthesis of quinolones
To a solution of oꢀoxazolineꢀsubstituted anilines (1.1 eq, 1.1 mmol) and ketones (1.0 eq, 1.0 mmol) in dry nꢀbutanol (15 mL)
o
was added trifluoromethane sulfonic acid (20 mol %) and the mixture was stirred under reflux (135 C) for 24 hours. The reaction
was cooled to room temperature and the solvent was evaporated under vacuum. Saturated sodium carbonate solution (20 mL) was
added. The aqueous solution was extracted with EtOAc (3 x 20 mL), washed with brine, dried over MgSO4, and concentrated
under vacuum. The crude mixture was purified by column chromatography to give quinolones.
o
1
9
J
J
: white solid (yield 51%); mp 324 ꢀ325 C; H NMR (400 MHz, DMSO) δ 11.60 (s, 1H, NH), 8.88 (d,
= 8.8 Hz, 2H), 8.24 (d, = 8.2 Hz, 1H), 8.15 (dd, = 8.2, 1.9 Hz, 1H), 8.04 (d, = 9.0 Hz, 1H), 7.55 (d,
= 2.3 Hz, 1H), 6.93 (dd, J
= 9.0, 2.3 Hz, 1H), 3.84 (s, 3H, 7ꢀOMe), 1.93 (s, 3H, 3ꢀMe). 13C NMR (101 MHz, DMSO) δ 162.1,
J
= 1.9 Hz, 1H), 8.34
(d,
(d,
J
J
J
J = 8.8 Hz, 2H), 6.99
155.4, 149.8, 138.5, 130.3, 129.2, 129.1, 121.7, 121.6, 120.4, 118.0, 115.2, 113.6, 99.0, 55.7, 12.3. ES HRMS: m/z calculated for
C23H18N2O3F3 ([M+H]+) 427.127, found 427.129.
11: white solid (yield 34%); mp 255 ꢀ258 oC; 1H NMR (400 MHz, DMSO) δ 11.57 (s, 1H, NH), 9.10 (d,
J
= 2.2 Hz, 1H), 8.80
= 7.9 Hz, 2H), 6.97 (d,
= 9.0, 2.3 Hz, 1H), 3.84 (s, 3H, 7ꢀOMe), 1.94 (s, 3H, 3ꢀMe). 13C NMR (101 MHz, DMSO) δ 176.6,
(d,
J
= 2.2 Hz, 1H), 8.35 (t,
J
= 2.2 Hz, 1H), 8.05 (d,
J
= 9.0 Hz, 1H), 8.01 (d,
J
= 7.9 Hz, 2H), 7.55 (d,
J
J
= 2.3 Hz, 1H), 6.93 (dd,
J
162.1, 148.9, 148.7, 144.2, 141.7, 136.1, 135.1, 134.3, 131.4, 129.6, 127.3, 122.1, 118.1, 115.3, 113.6, 99.0, 55.7, 12.3.ESI
HRMS: m/z calculated for C23H18N2O3F3 ([M+H]+) 427.127, found 427.127.
13: white solid (yield 34%); mp 245 ꢀ247 oC; 1H NMR (400 MHz, DMSO) δ 11.56 (s, 1H, NH), 8.89 (d,
J
= 5.0 Hz, 1H), 8.34
= 8.0 Hz, 2H), 7.00 (d,
= 9.0, 2.4 Hz, 1H), 3.85 (s, 3H, 7ꢀOMe), 1.93 (s, 3H, 3ꢀMe). 13C NMR (101 MHz, DMSO) δ 176.6, 162.1,
(d,
J
= 8.0 Hz, 2H), 8.22 (br s, 1H), 8.06 (d,
J
= 9.0 Hz, 1H), 7.60 (dd,
J
= 5.0, 1.5 Hz, 1H), 7.53 (d,
J
J =
2.4 Hz, 1H), 6.94 (dd,
J
155.5, 150.4, 144.9, 144.2, 141.7, 137.7, 129.2, 127.3, 123.2, 121.6, 120.7, 118.1, 114.7, 113.7, 111.4, 99.1, 55.7, 12.2. ESI
HRMS: m/z calculated for C23H18N2O3F3 ([M+H]+) 427.127, found 427.127.
17: yellow solid (yield 17%); mp 290 ꢀ292 oC; 1H NMR (400 MHz, DMSO) δ 11.59 (s, 1H, NH), 9.15 (d,
J
= 2.0 Hz, 1H), 8.81
= 8.5, 2.2 Hz, 1H), 7.82
J = 8.9, 2.4 Hz, 1H), 3.85 (s, 3H, 7ꢀOMe), 1.93 (s, 3H, 3ꢀMe). ESI HRMS:
(d,
(d,
J
J
= 2.0 Hz, 1H), 8.43 (t,
= 8.5 Hz, 1H), 6.98 (d,
J
J
= 2.0 Hz, 1H), 8.22 (d,
= 2.4 Hz, 1H), 6.94 (dd,
J = 2.2 Hz, 1H), 8.05 (d, J = 8.9 Hz, 1H), 7.91 (dd, J
m/z calculated for C22H1162O235Cl2 ([M+H]+) 411.067, found 411.067.
o
1
18: pale yellow solid (yield 54%); mp 217 – 220 C; H NMR (400 MHz, DMSO) δ 11.77 (s, 1H, NH), 9.12 (d,
J
= 2.1 Hz,
= 8.8 Hz, 2H), 7.66 (ddd, = 8.3,
= 7.9, 6.9, 1.2 Hz, 1H), 1.96 (s, 3H, 3ꢀMe). 13C
1H), 8.82 (d,
J
= 2.1 Hz, 1H), 8.38 (t,
J
= 2.1 Hz, 1H), 8.16 (dd,
J
= 8.3, 1.2 Hz, 1H), 8.01 (d,
J
J
6.9, 1.5 Hz, 1H), 7.59 (d,
J
= 7.9 Hz, 1H), 7.56 (d,
J
= 8.8 Hz, 2H), 7.34 (ddd,
J
NMR (101 MHz, DMSO) δ 177.0, 148.9, 148.7, 144.7, 139.9, 136.0, 135.2, 134.2, 131.9, 131.4, 129.6, 125.4, 123.5, 123.3, 122.1,
118.5, 115.6, 12.4. ESI HRMS: m/z calculated for C22H16N2O3F3 ([M+H]+) 397.116, found 397.117.
20: offꢀwhite solid (yield 56%); mp 244 – 246 oC; 1H NMR (400 MHz, DMSO) δ 11.78 (s, 1H, NH), 8.90 (dd,
J
= 4.9, 0.7 Hz,
= 8.9 Hz, 2H), 8.26 (br s, 1H), 8.16 (dd, = 8.2, 6.8, 1.5 Hz, 1H), 7.63 (dd, = 4.9,
= 7.8 Hz, 1H), 7.53 (d, = 8.9 Hz, 2H), 7.34 (ddd, J
= 7.8, 6.8, 1.1 Hz, 1H), 1.95 (s, 3H, 3ꢀMe). 13C NMR
1H), 8.35 (d,
J
J
= 8.2, 1.1 Hz, 1H), 7.67 (ddd,
J
J
1.5 Hz, 1H), 7.60 (d,
J
J
(101 MHz, DMSO) δ 150.4, 144.1, 139.9, 131.9, 129.2, 125.4, 123.5, 123.3, 123.3, 123.2, 121.6, 120.7, 118.6, 115.0, 12.3. ESI
HRMS: m/z calculated for C22H16N2O3F3 ([M+H]+) 397.116, found 397.117.
22: a pale yellow solid (yield 25%); mp = 270 ꢀ 272 oC; 1H NMR (400 MHz, DMSO) δ 11.75 (s, 1H, NH), 9.15 (d,
J
= 2.1 Hz,
1H), 8.82 (d,
J
= 2.1 Hz, 1H), 8.45 (t,
J
= 2.1 Hz, 1H), 8.22 (d,
J
= 2.2 Hz, 1H), 8.15 (dd,
J
= 8.0, 1.1 Hz, 1H), 7.91 (dd, = 8.4,
J

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2.2 Hz, 1H), 7.81 (d,
J
= 8.4 Hz, 1H), 7.66 (ddd,
J
= 8.0, 6.9, 1.5 Hz, 1H), 7.59 (d, J = 8.0 Hz, 1H), 7.33 (ddd, J = 8.0, 6.9, 1.1 Hz,
1H), 1.95 (s, 3H, 3ꢀMe). HRMS (CI): m/z calculated for C21H14N2O35Cl2 ([M+H]+) 381.056, found 381.055.
Parasite Culture
Plasmodium blood stage cultures27 and drug sensitivity28 were determined by established methods. IC50s (50% inhibitory
concentrations) were calculated by using the fourꢀparameter logistic method (Grafit program; Erithacus Software, United
Kingdom).
Molecular Modeling
A homology model of P. falciparum cytochrome bc1 complex was constructed using the PHYRE online homology modelling
program24. The P. falciparum cytochrome
b
primary sequence Q02768 was obtained from UNIPROT29 used as the query
sequence. A one to one threading model was built using PHYRE and the model produced had a confidence score of 100%. The
docking was performed using default settings in Gold with ten docking poses calculated for each quinolone.
Acknowledgements
S.C. thanks the University of Liverpool and Mahidol University, Thailand for the financial support under the Mahidolꢀ
Liverpool Scholarship scheme. We thank Dr Denis E. Kyle (Department of Global Health, College of Public Health, University of
South Florida, USA) for the antimalarial testing contained in Table 3 and AstraZeneca for metabolic stability measurements.
Notes and references
a Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, United Kingdom.
b Liverpool School of Tropical Medicine, Pembroke Place, Liverpool. L3 5QA, United Kingdom.
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Page 7 of 13
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Figure Legends
Figure 1. The quinolone core structure and initial lead compounds CK-2-68 and SL-2-25 (1) – and their
antimalarial activity
Figure 2. Previous work on 4-trifluoromethoxybenzyl and 3,4-dichlorobenzyl analogues.
Figure 3. In silico docking of
cartoon in cyan with residues rendered as sticks.
9
into the Plasmodium falciparum bc1 homology model. Protein rendered as a
rendered as sticks (carbon – grey, hydrogen – white, oxygen –
9
red, nitrogen – blue, fluorine - cyan). Hydrogen bond indicated by black dashed line with distance given in
Ångstrom.
Scheme
O
O
H
O
O
O
OH
a
b
(b)
( )
(a)
( )
R
R
R
R
R
R
4
4
2
2
3
3
O
O
X
X
d
(d)
( )
+
+
N
R
R
N
O
O
N
N
H
H
X
X
X
X
-
7
22
7 - 22
c
O
O
(c)
( )
O
O
N
N
O
O
NH
NH22
H
H
5
5
6
6
O
O
O
O
e
M
H
O
HO
O
MeO
X
X
e
(e)
=
R
R =
N
N
N
R
R
N
R
R
N
N
H
H
H
H
-
-
23 25
8 10
23- 25
8-10
o
Conditions: (a) EtMgBr, THF,0 C, under N2, 1 h (b) PCC, DCM, 2 h (c) 2-amino-2-methyl-propanol, ZnCl2,
PhCl,135 oC (d) CF3SO3H, n-BuOH, under N2 ,130 oC ,24 h (e) BBr3, DCM, rt., overnight.
Scheme 1. Synthesis of Quinolones.

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Tables
O
X
N
H
R
Table 1 Yields for the synthesis of quinolone 7-22.
Compound
R
X
No. of steps
% Yield
67
3
% Yield
4
% Yield
98
6
% Yield 7-22
7
OC
F
3
5-OMe
6-OMe
7-OMe
8-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-H
6
6
6
6
6
6
6
7
6
6
6
5
5
5
5
5
69
69
69
69
92
47
99
66
97
69
33
92
47
99
97
33
4
N
N
N
OCF3
OCF3
OCF3
8
67
48
22
51
25
34
40
34
42
62
23
17
54
55
56
62
25
9
67
73
10
11
12
13
14
15
16
17
18
19
20
21
22
67
30
N
OCF3
32
73
N
OCF3
N
73
73
OCF3
OCF3
55
73
N
32
73
N
OCF3
37
73
N
Cl
83
73
Cl
N
Cl
Cl
65
73
N
OCF3
OCF3
OCF3
32
31
N
N
7-H
73
31
7-H
55
31
N
OCF3
7-H
37
31
N
Cl
Cl
7-H
65
31
N

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DOI: 10.1039/C5MD00062A
O
X
N
H
R
Table 2 In vitro antimalarial Activities versus 3D7 P. falciparum , aqueous solubilities and melting points of quinolones
Compound
R
X
IC50 (nM) 3D7 ± SD
54 ± 6
Melting point
277 - 278 oC
OCF3
SL-2-25
7-H
N
OCF3
OCF3
OCF3
7
8
5-OMe
6-OMe
7-OMe
8-OMe
> 1000
> 1000
14 ± 2
> 1000
ND
N
312 - 314 oC
324 - 325 oC
182 - 185 oC
N
N
9
10
OCF3
N
OCF3
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-H
12 ± 3
149 ± 5
12 ± 3
255-258 oC
200-204 oC
245-247 oC
188-190 oC
255-258 oC
268-270 oC
290-292 oC
217-220 oC
187-192 oC
244-246 oC
239-241 oC
270-272 oC
decomposed at 250oC
ND
OCF3
N
N
OCF3
OCF3
N
150 ± 10
> 1000
200 ± 10
18 ± 3
N
OCF3
N
Cl
Cl
N
Cl
Cl
N
OCF3
41 ± 2
N
OCF3
OCF3
N
7-H
390 ± 30
21 ± 5
7-H
N
OCF3
7-H
> 1000
40 ± 4
N
Cl
Cl
7-H
N
OCF3
OCF3
OCF3
6-OH
7-OH
8-OH
326 ± 50
202 ± 79
> 1000
N
N
154 - 156 oC
N

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Table 3. Drug resistant parasites inhibition profiles of selected quinolones
IC50 (nM)
W2
IC50 (nM)
TM90C2B
14.5
Compound
chloroquine
atovaquone
SL-2-25
11
12.3
0.3
9908
156
48
4.0
7.2
13
4.2
7.0
Table 4. Selected compounds solubility profiles
Kinetic Solubility
(µM)
Compound Melting point
pH 1
pH 7.4
SL-2-25
277 - 278 oC
3.2
2.6
7.8
120
15
<1
<1
<1
<1
14
9
324 -325 oC
255 -258 oC
245 -247 oC
290 -292 oC
217 -220 oC
244 -246 oC
11
13
17
18
20
140
79
3.6
2.1
Table 5. Selected compounds metabolic stability profiles
Human Mics CLint
Rat Heps CLint
(µL/min/106 cell)
Compound
(µL/min/mg)
SL-2-25
11
20.6
25.9
14
ND
1.2
4.9
1.8
2.5
9.9
17
18
12.5
7.7
20
22
5.6
Table 6. Enzyme and parasite inhibition profiles of selected quinolones.
Bovine heart bc1
(% inhibition at 100 nM) (% inhibition at 1 μM)
Bovine heart bc1
Compound IC50 (nM) 3D7
9
14
12
12
18
41
21
55
19
63
13
20
52
98
81
85
20
25
81
11
13
17
18
20

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CK-2-68
X = Cl
IC50(3D7) = 31 nM
OCF3
OCF3
O
D
C
C
5
A
8
6
R =
B
SL-2-25 (1)
X = H
X
N
H
R
D
IC50(3D7) = 54 nM
N

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O
O
N
N
N
H
N
H
Cl
OCF3
Cl
IC50 (3D7) = 70 nM
IC50 (3D7) = 200 nM

Page 13 of 13 MedChemComm