A survey of cyclic replacements for the central diamide moiety of inhibitors of inosine monophosphate dehydrogenase

Article abstract of DOI:10.1016/S0960-894X(02)00641-8

A series of heterocyclic replacements for the central diamide moiety of 1, a potent small molecule inhibitor of inosine monophosphate dehydrogenase (IMPDH) were explored. The synthesis and the structure-activity relationships (SARs), derived from in vitro

Full text of DOI:10.1016/S0960-894X(02)00641-8

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3125–3128
A Survey of Cyclic Replacements for the Central Diamide Moiety
of Inhibitors of Inosine Monophosphate Dehydrogenase
T. G. Murali Dhar,* Chunjian Liu, William J. Pitts, Junquing Guo, Scott H. Watterson,
Henry Gu, Catherine A. Fleener, Katherine Rouleau, N. Z. Sherbina, Joel C. Barrish,
Diane Hollenbaugh and Edwin J. Iwanowicz
Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, NJ 08543-4000, USA
Received 30 April 2002; accepted 30 July 2002
Abstract—Aseries of heterocyclic replacements for the central diamide moiety of 1, a potent small molecule inhibitor of inosine
monophosphate dehydrogenase (IMPDH) were explored The synthesis and the structure–activity relationships (SARs), derived
from in vitro studies, for these new series of inhibitors is given.
# 2002 Elsevier Science Ltd. All rights reserved.
Inosine monophosphate dehydrogenase (IMPDH), an
enzyme in the de novo synthesis of guanosine nucleo-
tides, catalyzes the irreversible NAD-dependent oxida-
tion of inosine-50-monophosphate (IMP) to xanthosine-
50-monophosphate (XMP).1 Two distinct cDNA’s
encoding IMPDH have been identified and isolated.
These transcripts labeled type I and type II are of iden-
tical size (514 amino acids).2ꢀ4 IMPDH II activity is
markedly upregulated in actively proliferating cell types
including cancers and activated peripheral blood lym-
phocytes.5
Recently we have disclosed a new class of inhibitors of
IMPDH, exemplified by diamide 1 (Fig. 1).11 This report
describes a series of structural modifications aimed at
replacing the diamide linkage. An alternative approach to
IMPDH inhibition has been described by Pankiewicz.12
Ligand-based modeling suggested that the diamide moiety
might be replaced by either five– and six–membered ring
systems. This strategy would maintain a hydrogen bond
between the NH of the inhibitor and the carboxylate of
Asp 274 in the IMPDH enzyme active site. This hydrogen
CellCept1 (mycophenolate mofetil, MMF), a prodrug
of mycophenolic acid (MPA), has clinical use due to its
inhibition of IMPDH, for the treatment of transplant
rejection. Dose-limiting gastrointestinal (GI) toxicity
exhibited from administration of either MMF or MPA
limits this drugs potential for treatment of other auto-
immune disorders, such as psoriasis and rheumatoid
arthritis.6 The formation of conjugates at either the
carboxylic acid or phenolic residues has been postulated
as a contributing factor in the poor therapeutic index
observed for MMF.7,8 Structurally related analogues,
devoid of acid and phenolic functionalities, are reputed
to have an improved therapeutic window with regard to
dose limiting GI toxicity.9,10
Figure 1. The chemical structures of MPAand diamide 1. In addition,
the schematic showing the key hydrogen bond between the NH and
the carboxylate of Asp 274 and the site of heterocyclic insertion (Het.).
*Corresponding author. Tel.:+1-609-252-4158; fax:+1-609-252-6804;
e-mail: dharm@bms.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00641-8

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T. G. M. Dhar et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3125–3128
bond appears to be a critical interaction supported by
protein/inhibitor crystallographic studies and SAR (Fig.
1).13 Additionally, replacement of the diamide with five-
and six- membered ring systems, permit the terminal
phenyl group (A) to project into a similar region of
space.
The synthesis of the various heterocycles is described in
Schemes 1 and 2. 5-(4-amino-2-methoxyphenyl)-oxazole
2
these heterocycles. The 2-(N-aryl)-1,3-oxazole 16 was
synthesized employing the tandem iminophosphorane/
heterocumulene mediated annulation.15 The 1,10-
14 is used as a common intermediate in the synthesis of
Scheme 1. Reagents and conditions: (a) 1,10thiocarbonyldi-2(1H)-pyridone, CH2Cl2, rt (94%); (b) NaNHCN, EtOH, rt (67%); (c) phenylhydrazine,
EDC, DMF, 50 ꢁC (36%); (d) PhCOMe, NaH, DMF, MeI, rt (70%); (e) NH2OH, EtOH, reflux (15%); (f) PhC(O)NCS, CH2Cl, rt (94%); (g)
hydrazine, THF/EtOH, reflux, (32%); (h) PhC(O)NHNH2, EtOH, (94%); (i) CH3SO3H, toluene, reflux, (14%); (j) CDI, THF, PhC(O)NHNH2,
DMAP, EtOH, (27%); (k) C2Cl4Br2, PPh3 CH3CN, Et3N, rt (64%); (l) NH2CH2CH(OH)Ph, dioxane, reflux (75%); (m) 2-chloro-3-ethylbenzoxa-
zolium tetrafluoroborate CH3CN, Et3N, rt, (70%); (n) PhC(O)CH2N3, PPh3, CH2Cl2, rt, (80%); (o) PhCONCS, CH2Cl2, rt then 0.1 NaOH, MeI
(98%); (p) NH2OH, EtOH, reflux (98%); (q) NH3, dioxane, rt, (90%); (r) PhC(O)CH2Br, EtOH, reflux, (50%); (s) BocNHC(NBoc)SMe, DMF, rt
(100%); (t) TFA, rt, then, AG1-X8 hydroxide form resin (81%); (u) PhC(O)CH2Br, DMF (20%); (v) PhC(O)NHCH2CO2H, CDI, THF, rt (80%);
(w) Lawesson’s reagent, pyridine, 100 ꢁC (60%).

T. G. M. Dhar et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3125–3128
3127
biphenylaniline 26 was synthesized employing the
Buchwald protocol.16 These compounds were tested for
the in vitro inhibition of the IMPDH II enzyme17 and
the data is summarized in Tables 1 and 2.
oxazole, isoxazole and oxadiazole analogues were the
more potent members of this series. The isoxazole 7,
was the least potent of this group whereas 16, 13 and 18
displayed similar inhibitory activity relative to 2. The
non-aromatic heterocycle 15, shows that an aromatic
system is not a requirement for potency. The low
potency seen with imidazole 23, suggests that the
IMPDH II enzyme does not tolerate basic residues in
this region of the inhibitor. This might be explained by
charge repulsion, since the negative charge of the car-
boxylate group of ASP 274 is satisfied by the close
proximity of the guanidine group of ARG 322.
Table
1 reports the in vitro inhibitor potencies
against IMPDH II for a series of inhibitors featuring
a central five-membered heterocyclic residue. All het-
erocycles are commonly substituted with a phenyl
and aniline moieties. Replacement of the diamide
linkage of 1 with triazole 5 looked favorable by
molecular modeling (Fig. 2) and was only a 3-fold
less potent than 1. This result suggested an in-depth
examination of other heterocyclic cores was war-
ranted. Thiadiazole 11 and related thiazole 20 dis-
played inhibitory activities of 300 nM or less. The
The IMPDH activity of 6-membered ring systems is
shown in Table 2. The methoxytriazine 31 was of
Table 1. SAR of the five-membered heterocyclic inhibitors of
IMPDH
Compd
R1
IMPDH II IC50, mM
1
NA0.052
5
0.15
0.12
7
9
0.19
Scheme 2. Reagents and conditions: (a) 3-bromobiphenyl, Pd2(dba)3,
BINAP, NaOt-Bu, toluene, 80 ꢁC, (11%); (b) HCO2H, 150 ꢁC, (96%);
(c) NaH, DMF, 28,18 then 4N HCl, 70 ꢁC, (75%); (d) PhB(OH)2,
DME, K2CO3, Pd(PPh3)4, 90 ꢁC (81%); (e) THF, rt (99%); (f)
NaOMe, dioxane, 80 ꢁC (75%).
11
13
15
16
18
20
23
25
0.33
0.044
Table 2. SAR of the six-membered heterocyclic inhibitors of IMPDH
0.11
0.020
Compd
R1
IMPDH II IC50, mM
26
30% @ 500
0.059
0.30
29
31
0.11
7% @ 100
0.041
0.76

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6. Sievers, T. M.; Rossi, S. J.; Ghobrial, R. M.; Arriola, E.;
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Figure 2. Overlap of minimized structures: diamide 1 (red) and
triazole 5 (magenta).
10. Jain, J.; Almquist, S. J.; Shlyakhter, D.; Harding, M. W.
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comparable potency to the more potent analogues in the
five-membered ring series. In contrast, replacement of
the six-membered heterocyclic core with a phenyl resi-
due, 26, led to a significant loss in affinity. Biphenyl 26
can not assume the same dihedral angle of 29 and 31
because of unfavorable steric interactions. This may be
responsible for the loss of activity observed with 26,
although a direct comparison is not possible due to the
lack of a methoxy group in the appropriate position.
In summary a number of ring systems were examined as
replacements for the diamide linkage in compound 1.
Several heteroaromatic systems (13, 16, 18 and 31)
inhibited IMPDH as potently as diamide 1. Compound
16 is similar in potency to MPA, and this series is the
subject of further effort to enhance potency and in vivo
efficacy in a T-cell mediated pharmacodynamic model.
15. Froyen, P. Phosphorous, Sulfur Silcon 1991, 60, 81.
16. Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem.
Soc. 1996, 118, 7215.
17. The enzymatic activity of human IMPDH II was quanti-
tated as follows. The conversion of NAD+ to NADH was
followed spectrophotometrically at 340 nm. Areaction mix-
ture containing 0.1 M Tris, 0.1 M KCl, 3 mM EDTApH 8.0,
400 mM IMP, 2 mM DTT and 40 nM IMPDH II was added
to the wells of flat bottom UV-transparent 96-well plates
(Costar 3635). To test inhibitors, compounds resuspended in
DMSO were diluted in the reaction to give a final DMSO
concentration of 2.5%. IMPDH II used in these assays was
purified from Escherichia coli expressing the gene for the
human Type II enzyme. The reaction was initiated by addi-
tion of NAD to a final concentration of 400 mM. After a 2-h
incubation at 25 ꢁC, readings were taken at 340 nM. The
concentrations of compound required to inhibit NADH
accumulation by 50% (IC50) were calculated using a four-
parameter logistic plot.
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