Effect of lipophilicity modulation on inhibition of human rhinovirus capsid binders

Article abstract of DOI:10.1016/j.bmcl.2011.08.083

To try and generate broad spectrum human rhinovirus VP1 inhibitors with more attractive physicochemical, DMPK and safety profiles, we explored the current SAR of known VP1 compounds. This lead to the identification of specific structural regions where reduction in polarity can be achieved, so guiding chemistry to analogues with significantly superior profiles to previously reported inhibitors.

Full text of DOI:10.1016/j.bmcl.2011.08.083

Bioorganic & Medicinal Chemistry Letters 21 (2011) 6031–6035
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Effect of lipophilicity modulation on inhibition of human rhinovirus
capsid binders
Andrew Morley a, , Nicholas Tomkinson a, Andrew Cook a, Catherine MacDonald b, Richard Weaver b,
⇑
Sarah King a, Lesley Jenkinson c, John Unitt c, Christopher McCrae c, Tim Phillips c
a Department of Chemistry, AstraZeneca R&D Charnwood, Bakewell Road, Loughborough LE11 5RH, UK
b Department of DMPK, AstraZeneca R&D Charnwood, Bakewell Road, Loughborough LE11 5RH, UK
c Department of Bioscience, AstraZeneca R&D Charnwood, Bakewell Road, Loughborough LE11 5RH, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
To try and generate broad spectrum human rhinovirus VP1 inhibitors with more attractive physicochem-
ical, DMPK and safety profiles, we explored the current SAR of known VP1 compounds. This lead to the
identification of specific structural regions where reduction in polarity can be achieved, so guiding chem-
istry to analogues with significantly superior profiles to previously reported inhibitors.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 18 July 2011
Revised 15 August 2011
Accepted 17 August 2011
Available online 24 August 2011
Keywords:
VP1
Human rhinovirus
Lipophilicity
Lipophilic ligand efficiency (LLE)
Human rhinovirus (HRV) is a sub-class of the picornaviridae
family and is associated with several respiratory diseases such
as the common cold, chronic obstructive pulmonary disease
(COPD) and asthma.1 Although HRV infections are mostly mild
and self-limiting they represent a significant economic burden,
especially in loss of working hours to society and commerce.2
This global financial burden is increased further by recent find-
ings that HRV is a common pathogen associated with acute exac-
erbations in asthma and chronic obstructive pulmonary disease.3–
ing major group is represented in both A and B homology groups.
All minor group serotypes fall into the HRV-A group.9 The third
group, HRV-C, has only been recently identified.10 The receptor
specificity and inhibitor sensitivity of serotypes within this group
is as yet unknown.
At the base of the capsid canyon, VP1 (along with components
of VP3) forms a conserved hydrophobic pocket. Virus binding to
ICAM-1 or LDLr destabilises the capsid unit, resulting in viral
uncoating, an essential early step in the viral replication cycle.1,8
In the late 70’s, a series of compounds was discovered from
screening of compound libraries for their ability to inhibit
rhinovirus-induced cytopathic effect (CPE) in cell culture.11 The
mechanism of action of these compounds was subsequently found
to be via binding to and stabilisation of the hydrophobic pocket of
VP1. Subsequently, inhibitors such as Pleconaril (1) were identified
and demonstrated a significant reduction in viral load12,13 in clini-
cal trials against the common cold. Pleconaril induced CYP3A4
expression in human studies and was subsequently not approved
by the FDA.14,15 Subsequent in vitro studies showed 1 induced
CYP3A4 in human hepatocytes with an pEC50 of 4.8.16 This effect
can potentially be attributed to the lipophilic nature of 1, which
is a common feature of the VP1 inhibitors that have been
published.1,8 Related second generation inhibitors include the
pyridazine analogues 2 and 3.17,18
7
The capsid shell of human rhinovirus consists of 60 units, each
comprised of four proteins (VP1–4). VP1, 2 and 3 form a can-
yon-like cleft, which the virus uses to attach to host cells as the
first stage towards entry.8 The HRV family has been sub-divided
into two groups, depending on which protein is utilised for viral
attachment to the host cell. The major group (ꢀ90% of serotypes)
binds intercellular adhesion molecule 1 (ICAM-1), whereas the
minor group utilise the low-density lipoprotein receptor (LDLr).
An alternative classification of HRV serotypes also exists, based
on sequence homology (HRV-A, -B and -C).9,10 The ICAM-1-utilis-
⇑
Corresponding author. Tel.: +44 1509 644772.
E-mail address: andy.morley@astrazeneca.com (A. Morley).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.08.083

6032
A. Morley et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6031–6035
CF3
N
N
O
O
O
N
1
Pleconaril
clog P 4.0 logD >4.5
N
N
N
N
N
N
OEt
O
N
O
O
CO2Et
3
2
Pirodavir
clog P 4.4
clog P 4.2 logD 4.4
Like 1, this series of compounds also have undesirably high
calculated lipophilicity values (cLogP19) > 4. Following the seminal
work of Lipinski, there is a growing body of evidence that mole-
cules which are hydrophobic tend to show greater levels of promis-
cuity and subsequently have higher rates of attrition due to safety
in oral drug development.20–22
enantiomeric preferences weren’t carried out as part of the initial
evaluation.
Attention then focused on the right hand aromatic ring and
substituents (O–Ph–R2) primarily looking to increase polarity
Table 1
We envisaged that if a broad serotype inhibitor of VP1 could be
identified with an acceptable lipophilicity profile (ideally
cLogP < 3) then it could be a more useful therapy for several
respiratory indications. Most VP1 inhibitors consist of the general
pharmacophore, shown below.
Linker SAR
CO2Et
N
N
R
Compd
R
HRV16 pIC50
cLogP
LEb
LLEc
3
a
R2
Ph
X
N
Het
O
2
7.2
4.2
0.27
R1
O
N
N
Our strategy was to probe the SAR of the linker (X–O) and the
right hand aromatic (Ph–R2) to see where reductions in lipophilic-
ity could be achieved without impacting on VP1 inhibition. The
pyridazine motif, present in 2 and 3 was considered a useful phar-
macophore that brought a good balance of VP1 potency and cross
4
5
7.0
7.6
3.2
2.6
0.26
0.29
3.8
5
O
O
serotype activity whilst being adequately polar (cLogP = À0.08319
)
O
O
and was thus kept constant during our early assessment. HRV sero-
type 16 was selected for initial VP1 inhibitory evaluation, as it is a
member of the largest HRV cluster (major class A serotype). Com-
pounds were tested for their ability to inhibit HRV16 induced CPE
in HeLa OHIO cells.23 Pirodavir (2) was chosen as a reference and
initial SAR exploration around the linker unit (X–O) was under-
taken. Several cyclic amines possessing different polarities, substi-
tution vectors and linker lengths were evaluated as part of this
process, together with varying the position of the ether motif.
These changes generated quite significant differences in lipophilic-
ity values and data for key analogues is highlighted in Table 1.
Several modifications are tolerated for inhibiting HRV16 in-
duced CPE, especially where lipophilicity is significantly (>1 unit)
reduced, compared to 2. Some of the key variations include moving
the ether away from conjugation with the aromatic ring, which
produces a 1 unit reduction in lipophilicity. Various heterocyclic
ring systems also provide good inhibition, in some cases showing
profound changes in cLogP, LE24 and LLE.21 Attempts to add polar
substituents at the bridgehead of the piperidine (e.g., 7) did reduce
cLogP, but were less effective at inhibiting HRV16 effects. Addi-
tionally it’s well known that increasing structural complexity
through introduction of chiral centres can have a beneficial effect
on parameters like solubility.25 Compounds such as 6 and 10
showed good inhibition of HRV16 induced CPE. Evaluation of the
N
N
6
7
7.2
6.8
3.2
2.9
0.27
0.23
4
O
N
CN
3.9
N
N
8
9
7.1
7.7
3.4
3.1
0.27
0.31
3.7
4.6
O
O
N
10
11
12
7.5
7.2
7.2
3.1
2.6
2.8
0.3
0.3
0.3
4.4
4.2
4.4
N
N
O
O
a
b
c
Values mean of n = 2 experiments.
LE (Ligand efficiency)⁄ Values are calculated as pIC50/heavy atom count.
LLE (Lipophilic ligand efficiency)⁄ Values are calculated as pIC50ÀcLogP.

A. Morley et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6031–6035
6033
through either the addition or repositioning of heteroatoms. This is
summarised in Table 2.
Following this preliminary SAR assessment, more detailed
analysis was undertaken across the series to evaluate other VP1
serotype inhibition. In addition to screening HRV16, active com-
pounds were also tested against HRV14 (major class B serotype)
and HRV2 (minor class A serotype). Although this only represents
3% of the HRV serotype family, sequence analysis suggests these
are the best combination to efficiently ascertain the spectrum of
activity.26 Selectivity across these three serotypes is shown in Fig-
ure 1. Each point is coloured according to its cLogP. There are
examples of compounds with pIC50 values >7 against all three ser-
otypes (located in the top right hand corner ofFig. 1) and in most of
these cases the LLE21 are >4, demonstrating broad serotype cover-
age can be achieved with compounds that are not highly lipophilic.
Templates from Table 1 with broad spectrum CPE inhibition in-
clude 2, 4, 8, 10, and 11. The other templates showed at least a
log unit reduction in potency against either or both HRV 14 or 2
CPE.
In order to evaluate physicochemical and DMPK parameters of
the series, the ester motifs of analogues in Table 1 with CPE
pIC50 > 6 against HRV serotypes 16, 14 and 2 were replaced with
more stable H bond acceptor analogues, based on literature
SAR.1,8,11,17,18 Compounds were evaluated in a range of physico-
chemical and in vitro DMPK assays to understand the effect of
reducing lipophilicity on these parameters. Figure 2 highlights
the effect of cLogP on two key parameters, solubility (Fig. 2a, weak
linear trend, R2 value of 0.3, p value?) and human plasma protein
binding (Figure 2b, good correlation, R2 value of 0.95, p value?).
Protein binding is not a key parameter in its own right for com-
pound progression, but does have strong contribution to defining
whole blood potencies and in vivo pharmacokinetic profiles. The
ability to accurately predict this value would be beneficial for fu-
ture compound prioritisation.
Changes in this region were not as successful as in the linker
unit and indeed conversion of the phenyl ring to pyridine was
the only modification that maintained potency whilst reducing
lipophilicity. However the more basic ortho pyridyl ester (13)
was ꢀ1.5 log units less active, though LLE, compared to 2, is the
same. Further attempts to modify the bi-cycles by introduction of
heteroatoms at the bridgehead were unsuccessful as was attempts
to replace the ester with more polar isosteres.
Table 2
Capping motif SAR
N
N
N
OR
cLogP
Compd
R
HRV16 pIC50
7.2
LE
LLE
3.7
CO2Et
CO2Et
N
N
13
3.5
2.7
3.4
0.27
N
14
15
5.6
5.8
0.21
0.21
2.9
2.4
OEt
N
N
OEt
N
16
5.8
3.2
0.21
2.6
N
N
SO2Me
Metabolic stability, measured in human microsomes and rat
hepatocytes did not correlate with lipophilicity, though there are
weak trends associated with CYP inhibition and clogP.
17
18
5.7
2.8
2.8
0.21
—
2.9
—
SOMe
<4.5
Figure 1. A plot of pIC50 values for HRV serotypes 16, 14 and 2. Each point is
coloured according to its cLogP value. Compounds 1 and 2 are highlighted as
references.
Figure 2. Correlation plots.

6034
A. Morley et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6031–6035
As Pleconaril (1) has been shown to induce CYP3A414 it was
a trend with lipophilicity across the series ( Fig. 3, R2 value of
0.6). Extrapolating from this small data set reinforces the hypoth-
esis that targeting a lower lipophilicity range is likely to reduce
CYP induction risks.
We successfully generated compounds (e.g., 19) that met our
Lead Optimisation criteria by combining our own early SAR find-
ings with that in the literature (Table 3).
important to assess compounds in a suitable in vitro screen to
gauge the effects of lipophilicity on this parameter. The pregnane
X receptor (PXR) is activated by a range of compounds that induce
CYP3A4, including dexamethasone and rifampicin.27 Subsequently,
compounds were tested in a PXR reporter gene assay, as a surro-
gate marker for possible CYP3A4 induction liabilities.28 Com-
pounds
1
and
3
did show maximal effects in this assay,
The profile of 19 is encouraging when compared to 1, with
greatly increased solubility and free fraction, alongside signifi-
cantly lower activity in the PXR assay. Human microsomal Clints
are modest, with metabolite identification studies showing
hydroxylation of the methyloxadiazole as the major product. It
has previously been shown that replacement of the methyl motifs
appended to oxadiazole analogues which are closely related to 1,
results in significant increases in microsomal stability.12 Reduction
in lipophilicity has a benefit on CYP inhibition profiles, with 19
being clean against all 5 isoforms tested.
exhibiting free absence of protein binding? pEC50 of 6.6 and 7.8,
respectively, (absolute maximal effect?). Broader testing showed
These studies demonstrate that broad spectrum VP1 inhibition
can be achieved with compounds possessing significantly reduced
lipophilicity to those reported in the literature. The physicochem-
ical and most importantly predicted in vitro safety profiles of com-
pounds like 19 are superior to compounds that have failed to
progress through clinical development. This will increase confi-
dence that VP1 inhibitors can de discovered that possess cLogP
profiles <3 and exhibit a broad HRV serotype activity. Compounds
such as 19 can potentially provide a foundation to identify ana-
logues having a better chance of successfully progressing to market
for HRV related viral diseases.
Figure 3. Plot of cLogP versus PXR free pEC50
.
Table 3
Profiles of 19 and 1
N
O
N
CF3
N
N
O
O
O
O
N
N
N
1
N
19
Parameter
Lead criteria29
19
1
HRV16, 2 pIC50
cLogP/LogD
Mol wt
pIC50 > 7
<3.0
<450
7.1, 7.3
2/2.6
337
6.0, 7.0
4/>4.5
381
LE (HRV16)
LLE (HRV16)
Solubility30
Hu/rat
0.29
5.1
125
0.22
2
<2 lM
>100
l
M
lM
Prot binding
% free
6/5
26
5
0.13/<0.2
22
Hu Micsa
<30
<15
l
l
L/min/mg
Rat Hepsb
L/min/106cells
20
31
Chem stability
PAMPA
t
> 100 h
>200
NDc
½
(Papp 10À6 cm/s)
47
<5 (5/5)
<4.3(5/5)
NDc
<5 (3/5) 5.9e, 5.2f
<4.3(5/5)
d
Cyp Inhib pIC50
pIC50 < 5
pIC50 < 5
CypTDIg
PXR
(%Emax, pEC50
)
19%, 4.5
9/2.2/1.5/60
88%, 6.6
NT
Rat PK Cl/t1/2/Vdss/Fh
t1/2 > 1 h
pIC50 values are the means of at least three experiments.
a
Human microsome metabolism intrinsic clearance (l
L/min/mg).32
Rat Sprague–Dawley hepatocyte metabolism intrinsic clearance (
ND–not determined.
b
c
d
e
f
l
L/min/106 cells).33
Inhibition of cytochrome P450 isoforms: 1A2, 2C9, 2C19, 2D6 and 3A4.
CYP 3A4 pIC50
CYP 2D6 pIC50
.
.
g
h
TDI–time dependant inhibition
Compounds dosed at 2 mg/kg iv, n = 2 animals (Cl, mL/min/kg, t1/2, h, Vdss, L/kg, F,%)

A. Morley et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6031–6035
6035
15. Ma, J. D.; Nafziger, A. N.; Rhodes, G.; Liu, S.; Bertino, J. S., Jr. Drug Metab. Dispos.
2006, 34, 783.
Acknowledgment
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18. Brown, R. N.; Cameron, R.; Chalmers, D. K.; Hamilton, S.; Luttick, A.; Krippner,
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19. CLogP, Version 4.3, BioByte (www.biobyte.com).
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We would like to thank BioDuro for synthetic support towards
compounds highlighted in this publication.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.08.083.
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