Structure-activity relationship of ortho- and meta-phenol based LFA-1 ICAM inhibitors

Article abstract of DOI:10.1016/j.bmcl.2008.08.062

LFA-1 ICAM inhibitors based on ortho- and meta-phenol templates were designed and synthesized by Mitsunobu chemistry. The selection of targets was guided by X-ray co-crystal data, and led to compounds which showed an up to 30-fold increase in potency over reference compound 1 in the LFA-1/ICAM1-Ig assay. The most active compound exploited a new hydrogen bond to the I-domain and exhibited subnanomolar potency.

Full text of DOI:10.1016/j.bmcl.2008.08.062

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5245–5248
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Structure–activity relationship of ortho- and meta-phenol based LFA-1
ICAM inhibitors
*
Edward Yin-Shiang Lin , Kevin M. Guckian, Laura Silvian, Donovan Chin, P. Ann Boriack-Sjodin,
Herman van Vlijmen, Jessica E. Friedman, Daniel M. Scott
Research and Development, Biogen Idec Inc., 14 Cambridge Center, Cambridge, MA 02142, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
LFA-1 ICAM inhibitors based on ortho- and meta-phenol templates were designed and synthesized by
Mitsunobu chemistry. The selection of targets was guided by X-ray co-crystal data, and led to compounds
which showed an up to 30-fold increase in potency over reference compound 1 in the LFA-1/ICAM1-Ig
assay. The most active compound exploited a new hydrogen bond to the I-domain and exhibited subn-
anomolar potency.
Received 28 July 2008
Revised 14 August 2008
Accepted 18 August 2008
Available online 22 August 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
LFA-1
LFA-1/ICAM inhibitors
ICAM
LFA-1 (leukocyte function-associated antigen-1) is a cell surface
adhesion protein expressed on all leukocytes, and plays a key role
in the inflammatory process. Interaction of LFA-1 with its counter-
receptors (intracellular adhesion molecules ICAM-1, -2, -3) pro-
motes the migration of leukocytes to the site of inflammation.
Blocking the LFA-1/ICAM interaction is of great interest and has
potential in the treatment of autoimmune diseases.1–4
Figure 1. General structure for LFA-1 ICAM inhibitors.
Inhibition of the LFA-1/ICAM-1 interaction is possible at two
sites, including the metal-ion dependent adhesion site (MIDAS)
that is the location of the direct interaction between LFA-1 and
the ICAMs. Alternatively, inhibitors may target the I-domain allo-
steric site (IDAS). Binding at this remote site prevents conforma-
tional changes in LFA-1 necessary for interaction with the ICAMs.5,6
Previous studies at Abbott Laboratories7–13 had identified p-
arylthio cinnamide compounds, as shown in Figure 1, that inhibit
the interaction of LFA-1 and ICAM-1 at the allosteric site with mod-
erate potency. This series of compounds is characterized by A and B
phenyl rings, connected by a sulfide linker. Appended to the B-ring
is a third ring (C-ring) linked via a trans-cinnamide moiety. These
studies indicate that 2,3-bis(trifluoromethyl)phenyl and 4-carbo-
xypiperidine were the preferred B- and C-rings, respectively. Com-
pound 1 (Fig. 2) was among the most active12 in inhibiting the
interaction of LFA-1 and ICAM-1, and showed 12 nM potency in
the LFA-1/ICAM1-Ig assay.9
Figure 2. Abbott LFA-1 ICAM inhibitor compound 1.
the A-ring, while the B- and C-rings are kept constant. Modeling
studies suggested potential hydrophobic interactions and hydro-
gen bonding interactions when the A-ring is properly substituted
at the ortho- or meta-position. This study intends to examine these
interactions through the synthesis of appropriate A-ring ethers.
Note that morpholine was chosen as a C-ring replacement for 4-
carboxypiperidine, as this substitution simplifies the synthesis
and had earlier been shown to result in no loss of potency.12
Previously reported chemistry14,15 (Scheme 1) was applied to
prepare the common phenol intermediates, compounds 6a and
6b. Amide coupling of carboxylic acid 2 to morpholine gave amide
3. Dienophile hexafluorobutyne was condensed with 3, which was
The scope of this study is to improve on the potency of 1 by
exploring the structure–activity relationship of substitution on
* Corresponding author.
E-mail address: ted.lin@biogenidec.com (E.Y.-S. Lin).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.062

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E. Y.-S. Lin et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5245–5248
Table 1
Activity of ether compounds: saturated ringsa
LFA-1 ICAM1-Ig IC50 (nM)
R-
ortho-
meta-
7a
8.9
2
7b
530
8a
8b
3.9
Scheme 1. Synthetic scheme for preparation of 6a and 6b. Reagents and conditions:
(a) i—SOCl2, CH2Cl2, 0 °C to RT, o/n; ii—morpholine, CH2Cl2, RT, 2 h; (b) hexafluoro-
2-butyne, dichloroethane, À78 °C to 115 °C, o/n; (c) boron trifluoride diethyl
etherate, dichloroethane, reflux, 2 h; (d) triflic anhydride, pyridine, CH2Cl2, 0 °C to
RT, 2 h; (e) 2-mercaptophenol or 3-mercaptophenol, K2CO3, acetone, reflux, 1 h.
9a
2.5
9b
34
10a
11a
12a
13a
14a
55
10b
11b
12b
13b
14b
2.1
18
51
4
860
810
3
followed by Lewis-acid catalyzed rearrangement to give phenol 4.
Conversion of phenol 4 to the triflate 5 with triflic anhydride was
followed by reaction with 2- or 3-mercaptophenol to give the
key intermediates, 6a and 6b. Phenols 6a and 6b were found to
be active in the LFA-1/ICAM1-Ig assay, with IC50 of 9.7 and
17 nM, respectively.
From phenols 6a and 6b, elaboration to the A-ring ethers was
accomplished by Mitsunobu reaction with the appropriate alcohols
to give the ethers 7a–22a and 7b–22b (Scheme 2). Note that for
certain multifunctional alcohols, a subsequent deprotection was
required to complete the synthesis.
1.8
18
a
Assay performed as described in the Ref. 9.
A series of saturated-ring alcohols was used and the data for the
ether products is summarized in Table 1. Each ether shows moder-
ate to high potency in the LFA-1/ICAM1-Ig assay.
effect of aryl rings on the structure–activity relationship, and to
see the effect of inserting an additional methylene into the linker.
The data for the ether products is summarized in Table 2. As in the
previous series, the ortho-substituted ethers (15a–20a) were more
potent than the meta-substituted analogs (15b–20b). As a whole,
the series shows retention of activity, and in some cases improved
activity relative to phenols 6a or 6b. Benzyl (17a, 24 nM) was tol-
erated but was less active than each of the pyridinemethyl isomers,
18a–20a. This is consistent with the preference for polar
substitutions.
3- and 4-pyridinemethyl-substituted, 19a and 20a, were the
most active compounds in Table 2, and have IC50 of 2.5 and
3.7 nM, respectively. Crystal structure of 19a and LFA-1 complex
(Fig. 3) suggests that no new hydrogen bonds are formed between
the protein and the ether-modified A-ring of the small molecule;
most of the improved potency is likely due to increased hydropho-
bic interactions. Note that in the co-crystal structure, the pyridine
moiety is exposed to solvent, leading to the preference for pyridi-
nemethyl over the less hydrophilic benzyl. Interestingly, the crys-
tal structure showed that the pyridine ring of 19a is in close
proximity to the phenol OH of Tyr-257 (5.45 Å). Interaction of
the small molecule with this residue directly (via a longer linker)
or through a water molecule would be expected to increase po-
tency significantly.
In general, the ortho-substituted ethers (7a–14a) are either
equipotent or more active than the meta-substituted analogs
(7b–14b).16 The presence of a polar heteroatom in the ring in-
creased potency, as the piperidine (8a, 2 nM) and tetrahydropyran
compounds (9a, 2.5 nM) were more active than the corresponding
cyclohexyl compound (7a, 8.9 nM). This suggests that a polar
group in this position is preferred. This result is supported by the
substituted cyclohexane compounds, 11a–14a. Comparison of 4-
methylcyclohexyl (cis-11a, 18 nM; trans-12a, 51 nM) and 4-carbo-
xycyclohexyl (cis-13a, 4 nM; trans-14a, 1.8 nM) shows that the
more polar 4-carboxycyclohexyl is favored.
To complement the data on saturated ring ethers, a series of
arylmethanols was used. These were chosen to determine the
The saturated ring series led to the discovery of the polar 4-car-
boxy substitution. The unsaturated series confirmed the preference
for substitution by polar groups and also suggested that an addi-
tional methylene in the linker could bring the compound into con-
tact with Tyr-257. Close examination of the X-ray co-crystal
structure suggested that the non-planar cyclohexane would be a
better scaffold than the planar phenyl for reaching and interacting
Scheme 2. Synthetic scheme for preparation of 7a–22a and 7b–22b. Reagents and
conditions: (a) R-OH, PPh3, diisopropyl azodicarboxylate, THF, RT, o/n.

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5247
Table 2
Table 3
Activity of ether compounds: unsaturated ringsa
Activity of ether compounds: 4-carboxycyclohexanemethanol derived ethersa
LFA-1 ICAM1-lg IC50 (nM)
LFA-1 I CAM1-lg lC50 (nM)
ortho-
R-
ortho-
meta-
R-
meta-
15a
16a
17a
18a
19a
20a
9.5
15b
—
68
21a
22a
0.4
0.6
21b
22b
3.4
3.4
11
—
24
17b
18b
19b
20b
220
70
aAssay performed as described in the Ref. 9.
9.7
2.5
3.7
69
52
a
Assay performed as described in the Ref. 9.
Figure 4. X-ray co-crystal structure of 21a in the I-domain. RCSB file name (3E2M).
The ether 21a exhibits poor pharmacokinetic properties when
dosed orally in male rats: 9% bioavailability and a 3.6 h iv half-life.
This trend was observed for each of the representative ortho-ana-
logs studied. However, the less potent meta-analog, 13b, showed
improved PK (31% OBA, 2.6 h iv half-life). Further investigation is
needed to determine if other meta-analogs show similar or better
PK, and whether the PK of the ortho-series can be improved.
In summary, a new series of potent LFA-1 antagonists featuring
A-ring ethers has been discovered. A new hydrogen bond interac-
tion between the carboxylate of the inhibitor and Tyr-257 of the
protein was discovered. The best compound in the series is two or-
ders of magnitude more potent than the reference compound 1 and
is subnanomolar in the LFA-1/ICAM-Ig assay. These compounds are
under further study.
Figure 3. X-ray co-crystal structure of 19a in the LFA-1 I-domain. RCSB file name
(3BQN).
with Tyr-257. Thus, 4-carboxycyclohexanemethyl substituted
ethers were synthesized and the data summarized in Table 3.
A significant increase in potency was seen in the best com-
pounds in this series over the compounds in Tables 1 and 2. The
cis-4-carboxycyclohexanemethyl-substituted 21a showed 10-fold
improvement in potency over the one carbon shorter 13a and
was the most active compound in the series: 0.4 nM in the LFA-
1/ICAM1-Ig assay. It is also 6 times more potent than the best aro-
matic compound shown in Table 2 and 100 times more active than
the reference compound, 1.
An X-ray co-crystal structure of 21a and LFA-1 was obtained
(Fig. 4), which showed a new interaction between the inhibitor
and the allosteric site. As hypothesized, the carboxyl group of
21a interacts with Tyr-257, making a new direct hydrogen bond
(3.06 Å) to the side-chain phenol OH. Close analysis of the crystal
structure also suggests an additional water-mediated interaction
between the carboxylate of 21a and the side-chain carboxylate of
Glu-284. These two interactions are the likely contributors to the
observed increase in potency.
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