Potent and Selective Amidopyrazole Inhibitors of IRAK4 That Are Efficacious in a Rodent Model of Inflammation

Article abstract of DOI:10.1021/acsmedchemlett.5b00106

IRAK4 is a critical upstream kinase in the IL-1R/TLR signaling pathway. Inhibition of IRAK4 is hypothesized to be beneficial in the treatment of autoimmune related disorders. A screening campaign identified a pyrazole class of IRAK4 inhibitors that were d

Full text of DOI:10.1021/acsmedchemlett.5b00106

Letter
pubs.acs.org/acsmedchemlett
Potent and Selective Amidopyrazole Inhibitors of IRAK4 That Are
Efficacious in a Rodent Model of Inflammation
William T. McElroy,*,† Zheng Tan,† Ginny Ho,† Sunil Paliwal,† Guoqing Li,† W. Michael Seganish,†
Deen Tulshian,† James Tata,† Thierry O. Fischmann,‡ Christopher Sondey,§ Hong Bian,∥ Loretta Bober,∥
James Jackson,∥ Charles G. Garlisi,§ Kristine Devito,§ James Fossetta,∥ Daniel Lundell,∥ and Xiaoda Niu§
‡
§
∥
†Discovery Chemistry, Structural Chemistry, In Vitro Pharmacology, and Respiratory and Immunology, Merck Research
Laboratories, 2015 Galloping Hill Road, Kenilworth, New Jersey 07033, United States
S
* Supporting Information
ABSTRACT: IRAK4 is a critical upstream kinase in the IL-1R/
TLR signaling pathway. Inhibition of IRAK4 is hypothesized to be
beneficial in the treatment of autoimmune related disorders. A
screening campaign identified a pyrazole class of IRAK4 inhibitors
that were determined by X-ray crystallography to exhibit an
unusual binding mode. SAR efforts focused on the identification
of a potent and selective inhibitor with good aqueous solubility
and rodent pharmacokinetics. Pyrazole C-3 piperidines were well
tolerated, with N-sulfonyl analogues generally having good rodent
oral exposure but poor solubility. N-Alkyl piperidines exhibited
excellent solubility and reduced exposure. Pyrazoles possessing N-
1 pyridine and fluorophenyl substituents were among the most active. Piperazine 32 was a potent enzyme inhibitor with good
cellular activity. Compound 32 reduced the in vivo production of proinflammatory cytokines and was orally efficacious in a mouse
antibody induced arthritis disease model of inflammation.
KEYWORDS: Interleukin-1 receptor-associated kinase 4, inflammation, drug discovery, SAR, structure-based drug design
nflammation, the response by an organism to nonself
molecules, pathogens, cellular damage, or cellular debris, is
initiates a signaling cascade terminating in activation of the
transcription factors activator protein-1 (AP-1) and nuclear
factor κ-light-chain-enhancer of activated B cells (NF-κB),
leading to the production of proinflammatory signaling agents
including tumor necrosis factor α (TNFα) and interleukin-1
(IL-1).9
I
highly controlled in mammalian biology. The dysregulation of
this process is the underlying cause of autoimmune disorders,
including rheumatoid arthritis and inflammatory bowel disease,
which afflict millions worldwide.1,2 Chronic inflammation
furthermore plays a role in numerous other disease states
including cancer3 and cardiovascular disease.4 While a number
of orally administered anti-inflammatory and analgesic agents
are available to address the symptoms of inflammation, most of
the current medicines that modify the autoimmune disease
state itself are biologics.5 These therapies are generally
efficacious, but are expensive and require IV administration.
The development of an orally available agent to treat
autoimmune disorders remains an area of intense investiga-
tion.6
The interleukin-1 receptor/Toll-like receptor (IL-1R/TLR)
inflammation signaling pathway is an attractive target for
therapeutic intervention in autoimmune disease.7 Members of
the IL-1R/TLR superfamily of proteins recognize foreign
pathogens and endogenous inflammation signaling agents, and
are key upstream drivers of inflammation. These proteins all
possess a conserved intracellular Toll/interleukin-1R (TIR)
domain. Upon ligand binding to the receptor, the TIR domain
recruits the scaffolding protein myeloid differentiation primary
response gene 88 (MYD88). The resulting complex activates
interleukin-1 receptor-associated kinase 4 (IRAK4).8 IRAK4
The role of IRAK4 in innate immunity is supported by
mouse knockout studies.10 Importantly, cells derived from a
small human population that lack the IRAK4 protein do not
respond to ligands known to activate the TIR pathway.11 The
genetic association of IRAK4 with inflammation and the fact
that it is the most proximal kinase to the IL-1R/TLR family
render it an attractive target for drug discovery. A number of
organizations have described IRAK4 inhibitor programs,
although the poor kinase selectivity associated with many of
these chemotypes limits a full understanding of their in vivo
efficacies.12−15 We report herein the discovery of a novel, highly
selective IRAK4 inhibitor chemotype and the identification of a
compound, which reduces the in vivo production of TLR2-
induced cytokines and is orally efficacious in a rodent model of
inflammation.
Received: March 13, 2015
Accepted: May 12, 2015
© XXXX American Chemical Society
A
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a
A screening campaign identified pyrazoles 1 and 2 as modest
IRAK4 kinase inhibitors (IC50 values = 2.2 and 0.69 μM,
respectively, Figure 1). Interest in this series was initially based
Scheme 1
a
Reagents and conditions: (a) p-tolylhydrazine, montmorillonite K-10,
i-PrOH, Δ, 75%; (b) pyrazolo[1,5-a]pyrimidine-3-carbonyl chloride
(5), DIPEA, CH2Cl2, 28%.
Letter.17 α-Ketonitrile 3 was condensed with 4-methylphenyl-
hydrazine to form 4.18 Aminopyrazole 4 was allowed to react
with acid chloride 5 to give 6. The analogous reaction between
4 and a carboxylic acid using peptide coupling agents was
unsuccessful as only starting materials were recovered even
under forcing conditions. In instances where the starting
ketonitriles were not commercially available, they were
prepared from their corresponding esters and the anion derived
from acetonitrile.
Initial SAR studies focused on the pyrazole C-3 position
(Table 1). In addition to IRAK4 inhibition, kinetic aqueous
solubility at pH 7 was measured for most analogues. Rat oral
exposure was determined in cassette mode for select inhibitors.
In these studies,19 Sprague−Dawley rats were dosed p.o. at 10
mg/kg, in 20% HPbCD, with drug plasma concentrations
measured at multiple time points and AUC values from 0 to 6 h
calculated. The pyrazole C-3 phenyl analogue 2 was chosen as a
starting point for SAR development. Since the C-3 substituent
points toward solvent in the X-ray structure, it was
hypothesized that the incorporation of polar groups on the
phenyl ring may improve enzyme activity through desolvation.
This was generally found to be true. The addition of a para-
methoxyphenyl substituent (6) resulted in an approximately 4-
fold increase in potency relative to 2. Inhibitor 6 was also
assessed for rat pharmacokinetics (PK) and kinase selectivity.
Although 6 exhibited no oral exposure, it was found to be very
selective: against a panel of 108 kinases, >80% inhibition was
observed for only 2 kinases (IRAK4 and MINK1) at 10 μM.
This confirmed the expectation that analogues in this series are
highly selective.
The dimethoxyphenyl analogue 7 exhibited excellent IRAK4
inhibition (IC50 = 44 nM). As with 6, compound 7 had no
measurable plasma exposure in rat. The poor PK was
investigated using metabolite identification studies, whereby
analogue 7 was incubated with rat hepatocytes. Monooxidation
was found to be the dominant route of metabolism, with the
oxidation occurring on the tolyl, pyrazole, or dimethoxyphenyl
ring. Although this study did not conclusively identify the site
of metabolism, it was suspected that the oxidation may be
occurring at the electron rich dimethoxyphenyl ring.
Figure 1. Pyrazoles 1 and 2 and X-ray cocrystal structure of 1 with
IRAK4. Not shown on this picture is the interaction with the peptide
bond between Met192 and Gly193, which lies above the top clipping
plane.
on the unusual binding mode of 1 as determined by X-ray
crystallography. Three H-bonds between 1 and the kinase hinge
were observed, with one ring proton from each pyrazole ring
functioning as hydrogen bond donors and the amide carbonyl
of 1 acting as a hydrogen bond acceptor. An intramolecular
hydrogen bond between the inhibitor amide N−H and
pyrimidine nitrogen stabilizes the pyrazolopyrimidine moiety
into a planar conformation that allows for a favorable slanted
face-to-edge π−π interaction with the gatekeeper Tyr262
phenol. The pyrazole phenyl substituent is positioned against
the peptide bond between Met192 and Gly193, at the N-
terminal end of a glycine rich loop of the protein, and is
oriented at about 55° relative to the pyrazole ring. Finally, the
pyrazole C-3 methyl group points toward the solvent exposed
region.
The observation that inhibitor 1 bound to the IRAK4 hinge
without a typical kinase binding motif16 such as 2-amino-
pyrimidine or benzimidazole led us to suspect that analogues in
this series may exhibit high selectivity for IRAK4, although a
more potent inhibitor would be required in order to accurately
interpret selectivity data. With this assumption, an SAR
program was undertaken with the goal of identifying an
analogue that could be used in a rodent model of inflammation
proof-of-concept study. Such a compound should be a potent
(<10 nM) enzyme inhibitor with excellent kinase selectivity.
Additionally, the analogue sought should possess both oral
rodent exposure and good aqueous solubility, which would
allow for dosing using various routes of administration.
The general synthesis depicted in Scheme 1, using analogue
6 as an example, was employed for most of the pyrazoles in this
Replacement of the methoxyphenyl ring with more electron
deficient aromatic systems was explored next. The 4-pyridyl
analogue 8 maintained good IRAK4 activity and selectivity (6/
108 kinases, >80% inhibition at 10 μM). However, no
improvement in plasma exposure was observed.
Analogue 9 was designed with the expectation that the
addition of the methoxy group would both improve IRAK4
activity, in a similar manner as seen with the phenyl analogues,
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Collectively the above findings demonstrated that modifica-
tion of the pyrazole C-3 substituent was a viable strategy to
improve both IRAK4 potency and rodent PK in this series. It
was hypothesized that the poor solubilities of 2 and 6−11 were
due to the expected planar nature of the compounds.20 In order
to address this, alicyclic C-3 substituents were explored next.
Replacement of the methyl group in the original hit 1 (IRAK4
IC50 = 2.2 μM) with a cyclopropyl ring (12) gave an
approximately 6-fold improvement in IRAK4 potency. It was
encouraging that 12 also exhibited better solubility and oral
exposure than analogues in the (hetero)aryl series. However,
the more lipophilic cyclohexyl analogue 13 was only equipotent
to 1 and was poorly soluble.
Table 1. SAR Studies at Pyrazole C-3 Substituent
In a similar manner as described above, heteroatom
containing substituents were prepared with the goals of
improving IRAK4 potency and decreasing lipophilicity and
thus improving solubility relative to 13. The THP species 14
was a modest inhibitor of IRAK4, but showed improved
solubility and measurable exposure. Encouraged by this result,
piperidine analogues were explored next. The unsubstituted 4-
piperidine 15 was a relatively potent enzyme inhibitor with
excellent solubility, likely due to the charged nature of this
species at pH 7. N-Alkylated analogues 16 and 17 displayed
similar activities and maintained excellent solubilities relative to
N-H piperidine 15. Disappointingly, exposure was below the
limit of detection for 15 and minimal for N-iso-propyl analogue
17.
Replacement of the piperidine N-alkyl substituents with
other functional groups was explored with the hope that this
may improve rat PK. N-Acetamide 18 had a similar profile to
the parent piperidine 15, although improved pharmacokinetics.
This result led to the design of piperidines with other nitrogen
functional groups. The urea 19 was somewhat less potent than
15. Importantly, methanesulfonamide 20 was identified as
having reasonable IRAK4 potency and high plasma exposure in
rat. Furthermore, the excellent kinase selectivity observed with
earlier analogues was preserved, as 20 inhibits only 2/108
kinases with >80% inhibition at 10 μM (IRAK4, TRKB).
Although the enzyme potency of 20 was below that of our
desired criteria, the excellent selectivity and rat oral exposure
made this compound suitable for the development of an in vivo
biomarker assay. In this experiment, female Lewis rats (n = 9)
were dosed by oral gavage at 100 mg/kg with 20. One hour
later, the rats were injected intraperitoneally with the TLR2
agonist PAM2CSK4.21 Three hours postinjection, plasma was
collected and levels of TLR2-derived cytokines were measured
relative to the control group that was dosed with only the
vehicle. Modest but not statistically significant reductions of
TNFα (21%) and IL-1β (28%) were observed, with no
decrease seen of either IFNγ or IL-6. The average exposure of
20 in this study was 2.66 μM at 3 h postinjection.
The lack of a robust decrease in cytokine levels observed with
20 was attributed to its modest IRAK4 enzyme activity.
Although permeability22 for 20 was good (Papp = 31 × 10−6
cm/s), we were concerned that the poor solubility of 20 (<2
μM) conflated an understanding of its cellular activity (IC50
=
and reduce any oxidative metabolism of the pyridine nitrogen
and increase rat PK. Although some loss in activity was
observed, it was significant that 9 became the first compound in
this chemotype to exhibit any rat exposure. The regioisomeric
pyridines 10 and 11 displayed similar IRAK4 IC50 values
relative to their 4-pyridyl counterparts. It is noteworthy that the
unsubstituted 3-pyridine 10 also exhibited measurable but poor
rodent exposure.
2.4 μM). We thus undertook SAR investigations into the
pyrazole N-1 position. For these studies, the C-3 substituents
were restricted to either the piperidine sulfonamide (due to the
high exposure of 20) or N-iso-propyl piperidine (given the
excellent potency and solubility of 17). The findings are
summarized in Table 2. Rat AUC values were measured over 6
or 8 h as indicated.
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involving the addition of an ortho substituent was designed.
This was expected to improve solubility by forcing the pyrazole
and phenyl rings to adopt a noncoplanar arrangement in
solution. It was also known from the X-ray structure of 1 that
these groups are oriented at a 55° angle relative to one another
in the enzyme active site. It was thus believed that the ortho
substituent would have the additional benefit of preorganizing
the inhibitor into its biologically active conformation, as the
predicted dihedral angle is 5° larger for an analogue with an
ortho-fluoro substituent.
Table 2. SAR Studies at Pyrazole C-3 and N-1 Substituents
ortho-Fluorophenyl analogue 25 had IRAK4 IC50 = 40 nM.
Although solubility remained poor, 25 was the first compound
in the sulfonamide series for which a measurable value (5 μM)
could be obtained. Replacement of the para-methyl substituent
with a cyclopropyl group (26) resulted in a slight decrease in
potency and a loss of whatever solubility had been gained with
25.
It was believed that replacing the fluorophenyl rings of 25
and 26 with their bioisosteric pyridines may lead to an
improvement in solubility. Although this was not observed with
pyridines 27 and 28, these compounds were slightly more
potent than their phenyl or fluorophenyl counterparts. Further,
it was found that 27 was highly selective: in a panel of 108
kinases, the only kinase with >80% inhibition at 10 μM was
IRAK4. We attempted to further capitalize on this finding by
using pyrimidine 29, but this led to a loss in IRAK4 activity
relative to 27.
Having been unsuccessful in the quest to use N-1
substituents to improve solubility, C-3 N-alkyl piperidines
were revisited. Analogue 30 was designed with a fully basic
nitrogen for solubility and an ortho-fluorophenyl pyrazole for
improved potency. Compound 30 was a potent enzyme
inhibitor (IC50 = 13 nM) with excellent solubility and modest
exposure in rat. An X-ray cocrystal structure shows that 30
retained the same interactions with the protein as compound 1
(Figure 2). The pyrazolopyrimidine and Tyr262 are oriented at
approximately 40° to one another (slanted face-to-edge
interaction). The pyrazole and fluorophenyl rings are as
expected perpendicular to one another. The fluorine atom
engages in a hydrogen bond with the N−H of Ser269.
Replacement of the fluorophenyl with a pyridine gave analogue
31, which was the first inhibitor in this series to possess IRAK4
IC50 < 10 nM. We suspect that the additional interactions
between the fluorine atom and, by extended reasoning, the
pyridine nitrogen are responsible for the improved potencies of
30 and 31.
A key finding was the discovery that replacement of the
pyrazole C-3 piperidine ring with a piperazine provided
analogues displaying small but significant improvements in
both enzyme inhibition and exposure. Thus, N-methyl
piperazine 32 had IRAK4 IC50 = 5 nM, excellent aqueous
solubility, and rat AUC = 1.7 μM·h (0−8 h). As expected,
replacement of the N-methyl group with a methanesulfonamide
(33) improved plasma exposure but decreased solubility.
Piperazine 32 was selected for further profiling (Figure 3). In
order to determine its cellular activity, THP1-XBlue cells were
preincubated with inhibitor 32. Inflammatory signaling was
then stimulated with either the TLR4 agonist LPS, which
signals through IRAK4, or TNFα, which acts through IRAK4
independent pathways. Activation of the downstream tran-
scription factor NF-κB was then determined. Analogue 32
exhibits an IC50 = 83 nM in cells stimulated by LPS, and no
measurable inhibition at concentrations up to 31.6 μM in cells
Movement of the para-methyl group to the meta position
(21) led to marked decrease in IRAK4 inhibition compared to
20, while the removal of this substituent altogether (22) gave a
4-fold loss in activity. Subsequent studies thus focused on
analogues with para-substituted aromatic rings at the N-1
position. Replacement of the methyl group with a methoxy
functionality had no effect on potency but did result in a
dramatic loss in rat exposure, possibly due to metabolism of 23
via demethylation of the ether. Replacement of the methyl
group with a cyclopropyl substituent gave analogue 24 that had
similar enzyme potency to 20 and approximately 4-fold less
plasma exposure. This was curious since it was expected that
the cyclopropyl group would confer improved metabolic
stability relative to its methyl counterpart.
None of Table 2 analogues thus far (21−24) exhibited
improved solubility or potency relative to 20. A strategy
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Figure 2. X-ray cocrystal of 30 with IRAK4.
stimulated by TNFα. Compound 32 was highly selective, as the
only kinase with >80% inhibition at 1 μM in a panel of 108 is
IRAK4. Bioavailability when dosed at 10 mg/kg was 28%, with
21% unbound in rat plasma. This analogue was assessed for its
ability to reduce proinflammatory cytokine production in vivo,
using a similar experiment as performed with 20 (vide supra).
Good reduction in levels of TNFα (32%), IL-1β (28%, p <
0.05), IL-6 (85%, p < 0.005), IL-8 (34%), and IFNγ (30%, p <
0.005) were observed upon dosing at 100 mg/kg.
Figure 3. Profile of analogue 32 and paw size reduction in mouse
ABIA study.
Analogue 32 was assessed in the antibody induced arthritis
model of inflammation in mice.23 In this study, BALB/c female
mice were immunized with an arthritogenic monoclonal
antibody (CIA-MAB-50, 2.5 mg) on day 0. On day 3, the
mice were given LPS (25 μg i.p.) as a booster. On days 3−10,
mice were dosed q.d. by oral gavage with 100 mg/kg of
compound 32 or dexamethasone (2 mg/kg) as a positive
control. Paw size was measured on days 0, 3, 6, 8, and 10. On
day 10 the study was terminated. The results of the paw
measure on day 10 are shown in Figure 3. Compound 32 gave a
robust decrease in paw size relative to the control vehicle group
(dosed with methylcellulose, MC) and displayed similar
efficacy to dexamethasone.
In summary, a series of potent, selective, and orally
bioavailable pyrazole IRAK4 inhibitors have been described.
A screening campaign identified pyrazoles 1 and 2 as promising
hits, and an X-ray structure of 1 indicated that the pyrazole C-3
substituent, pointing toward the solvent exposed portion of the
protein, and N-1 substituent offered opportunities to improve
IRAK4 activity. SAR studies at the C-3 position indicated that
piperidine and piperazine substituents were well tolerated, and
the identity of the nitrogen substituent could be used to
modulate potency, solubility, and rat oral exposure. Although
sulfonamides conferred excellent rat PK, they were universally
poorly soluble. In contrast, alkyl substituted piperidines and a
piperazine were highly soluble and very potent. Limited SAR
studies at the pyrazole N-1 position indicated that ortho-fluoro
and pyridine substituents provided the best levels of potency.
These studies ultimately led to the discovery of analogue 32,
which reduced the production of TLR4-mediated cytokines and
was efficacious in a mouse antibody induced arthritis model.
ASSOCIATED CONTENT
■
S
* Supporting Information
Compound synthesis and characterization, and assay protocols.
The Supporting Information is available free of charge on the
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H.; Al-Jumaah, S.; Al-Hajjar, S.; Al-Mohsen, I. Z.; Frayha, H. H.;
Rucker, R.; Hawn, T. R.; Aderem, A.; Tufenkeji, H.; Haraguchi, S.;
Day, N. K.; Good, R. A.; Gougerot-Pocidalo, M. A.; Ozinsky, A.;
Casanova, J. L. Pyogenic bacterial infections in humans with IRAK-4
deficiency. Science 2003, 299, 2076−2079.
(12) Dou, H.; Song, Y.; Liu, X.; Yang, L.; Jiang, N.; Chen, D.; Li, E.;
Tan, R.; Hou, Y. A novel benzenediamine derivate rescued mice from
experimental sepsis by attenuating proinflammatory mediators via
IRAK4. Am. J. Respir. Cell Mol. 2014, 51, 191−200.
(13) Tumey, L. N.; Boschelli, D. H.; Bhagirath, N.; Shim, J.; Murphy,
E. A.; Goodwin, D.; Bennett, E. M.; Wang, M. M.; Lin, L. L.; Press, B.;
Shen, M.; Frisbie, R. K.; Morgan, P.; Mohan, S.; Shin, J.; Rao, V. R.
Identification and optimization of indolo[2,3-c]quinoline inhibitors of
IRAK4. Bioorg. Med. Chem. Lett. 2014, 24, 2066−2072.
(14) For a recent review, see Chaudhary, D.; Robinson, S.; Romero,
D. L. Recent advances in the discovery of small molecule inhibitors of
interleukin-1 receptor-associated kinase 4 (IRAK4) as a therapeutic
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ACS Publications website at DOI: 10.1021/acsmedchem-
lett.5b00106.
Accession Codes
X-ray coordinates for compounds 1 and 30 in the Protein Data
Bank are 4YO6 and 4YP8, respectively.
AUTHOR INFORMATION
Corresponding Author
*Tel: 908-740-3852. E-mail: william.mcelroy@merck.com.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Merck departments of preclinical
development and pharmacokinetics, pharmacodynamics, and
drug metabolism for providing solubility and rat PK analyses,
respectively.
(15) For a recent review, see Hynes, J.; Nair, S. K. Advances in the
discovery of small-molecule IRAK4 inhibitors. Annu. Rep. Med. Chem.
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ABBREVIATIONS
■
IRAK4, interleukin-1 receptor-associated kinase 4; SAR,
structure−activity relationship; IL-1R/TLR, interleukin-1 re-
ceptor/Toll-like receptor; TIR, Toll/interleukin-1R; MYD88,
myeloid differentiation primary response gene 88; AP-1,
activator protein-1; NF-κB, nuclear factor κ-light-chain-
enhancer of activated B cells; TNFα, tumor necrosis factor α;
IL-1, interleukin-1; TLR2, Toll-like receptor 2; i-PrOH, iso-
propanol; DIPEA, diisopropylethylamine; N/A, not available;
HPbCD, (2-hydroxypropyl)-β-cyclodextrin; AUC, area under
curve; PK, pharmacokinetic; MINK1, misshapen-like kinase 1;
THP, tetrahydropyran; TRKB, tyrosine receptor kinase B;
PAM2CSK4, palmitoyl-2-cysteine-serine-lysine; IL-1β, interleu-
kin-1 β; IL-6, interleukin-6; IFNγ, Interferon γ; TLR4, Toll-like
receptor 4; LPS, lipopolysaccharide; IL-8, interleukin-8; ABIA,
antibody induced arthritis; MC, methylcellulose
(16) Toledo, L. M.; Lydon, N. B.; Elbaum, D. The structure-based
design of ATP-site directed protein kinase inhibitors. Curr. Med. Chem.
1999, 6, 775−805.
(17) See Supporting Information for complete description of
compound synthesis.
(18) Reddy, G. J.; Latha, D.; Rao, K. S. A clean and rapid synthesis of
5-amino and 5-alkoxycarbonylpyrazoles using montmorillonite under
acid free conditions. Org. Prep. Proced. Int. 2004, 36, 494−498.
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Wainhaus, S.; Broske, L.; Prelusky, D.; Nomeir, A.; White, R. E.
Cassette-accelerated rapid rat screen: a systematic procedure for the
dosing and liquid chromatography/atmospheric pressure ionization
tandem mass spectrometric analysis of new chemical entities as part of
new drug discovery. Rapid Commun. Mass Spectrom. 2001, 15, 335−
340.
(20) Ritchie, T. J.; Macdonald, S. J. F. The impact of aromatic ring
count on compound developability - are too many aromatic rings a
liability in drug design? Drug Discovery Today 2009, 14, 1011−1020.
(21) Palsson-McDermott, E. M.; O’Neill, L. A. J. The potential of
targeting Toll-like receptor 2 in autoimmune and inflammatory
diseases. Irish J. Med. Sci. 2007, 176, 253−260.
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