Novel 7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones as IMPDH inhibitors

Article abstract of DOI:10.1016/j.bmcl.2005.06.108

The synthesis and biological activity of a novel series of 7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones are described. Some of these compounds were found to be potent inhibitors of inosine 5′-monophosphate dehydrogenase type II (IMPDH II).

Full text of DOI:10.1016/j.bmcl.2005.06.108

Bioorganic & Medicinal Chemistry Letters 15 (2005) 5335–5339
Novel 7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones
as IMPDH inhibitors
Helen L. Birch, George M. Buckley, Natasha Davies, Hazel J. Dyke, Elizabeth J. Frost,
Philip J. Gilbert, Duncan R. Hannah, Alan F. Haughan, Michael J. Madigan,
Trevor Morgan, William R. Pitt, Andrew J. Ratcliffe, Nicholas C. Ray,
Marianna D. Richard, Andrew Sharpe,* Alicia J. Taylor,
Justine M. Whitworth and Sophie C. Williams
UCB Celltech, Granta Park, Great Abington, Cambridge, CB1 6GS, UK
Received 13 September 2004; revised 2 June 2005; accepted 23 June 2005
Available online 3 October 2005
Abstract—The synthesis and biological activity of a novel series of 7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones are
described. Some of these compounds were found to be potent inhibitors of inosine 50-monophosphate dehydrogenase type II
(IMPDH II).
Ó 2005 Elsevier Ltd. All rights reserved.
Inosine 50-monophosphate dehydrogenase (IMPDH) is
an enzyme that catalyses the NAD dependent oxidation
of inosine 50-monophosphate (IMP) to xanthosine 50-
monophosphate (XMP). This is a key step in the de
novo synthesis pathway of guanine nucleotides, which
is heavily utilized by proliferating cells.1–4 Two isoforms
of the enzyme have been identified and designated type I
and type II.5 Of these it is IMPDH type II that is upreg-
ulated in actively proliferating cell type such as cancers
and activated peripheral blood lymphocytes.6,7 Thus,
inhibition of IMPDH II will result in inhibition of guan-
ine production and hence inhibition of cell proliferation
in the immune system. IMPDH II has therefore become
an interesting target for the treatment of autoimmune
diseases such as psoriasis, systemic lupus erythematosus
and rheumatoid arthritis as well as for transplant
rejection.
inhibitor of both IMPDH I and II8 showing an IC50 of
15 nM in our IMPDH II enzyme assay.9 The reported
gastrointestinal toxicity of this compound has resulted
in a search for alternative IMPDH inhibitors with im-
proved therapeutic window.10 The forerunner of these
was the urea VX-497 (in our assay, IC50 21 nM) that
reached Phase II clinical trials (Fig. 1).11
In an earlier publication, we reported the discovery of a
novel series of 7-methoxy-6-oxazol-5-yI-1H-quinazo-
line-2,4-diones exemplified by compound 1 as IMPDH
II inhibitors.12 Here, we describe how that template
was modified in order to improve its poor physical
An IMPDH inhibitor on the market for transplant rejec-
tion is CellceptÒ (mycophenolate mofetil, MMF), an es-
ter pro-drug of the active component mycophenolic acid
(MPA). MPA is a very potent, uncompetitive, reversible
*
Corresponding author. Fax: +44 1223 89640; e-mail: andrew.
sharpe@ucb-group.com
Present address: Argenta Discovery Ltd., 8/9 Spire Green Centre,
Flex Meadow, Harlow, CM 19 5TR, UK.
Figure 1. Structures of MPA, MMF and VX-497.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.108

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H. L. Birch et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5335–5339
properties resulting in a series of novel 7-methoxy-6-
oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones that were
also potent IMPDH II inhibitors.
IMPDH potency and attain favourable DMPK
properties.
Replacing the 3-methyl group in compound 2a with a
pyridyl-ethyl substituent (2b) gave no increase in activ-
ity against IMPDH II, much in the same way that vary-
ing this substituent in the quinazoline-2,4-dione series
had no dramatic effect on IMPDH II activity. A more
fruitful approach was to vary the 2-substituents of the
7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-
ones. Thus, replacing one of the 2-methyl groups with a
styrene moiety gave compound 2c and an improvement
in potency. However, it was found for this acyclic series
that both enzyme and cellular activities reached a pla-
teau not acceptable for our programme. It was our
Although only modest potency against IMPDH II was
achieved with the 7-methoxy-6-oxazol-5-yl-1H-quinazo-
line-2,4-diones, the major flaw with the series was its
poor solubility, which was postulated to result from
the overall flatness of the structure (compound 1, pH
6.5: 3 lg/ml). In order to improve this, the 2-oxo moiety
was replaced with a gem-dimethyl group resulting in
compound 2a. Pleasingly, this compound also showed
moderate IMPDH II potency in addition to the predict-
ed improvements in solubility (pH 6.5: 77 lg/ml). Our
objective with this series, therefore, was to improve the
Table 1. SAR of early 7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones
Compound
IMPDH II9 IC50 (nM)
PBMC15 (lM)
1
104
4.0
2a
2b
2c
2d
2e
2f
2g
192
300
49
21
37
3.0
8.5
11.6
1.9
0.97
328
526
96
104

H. L. Birch et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5335–5339
5337
other strategy of cyclising the 2-gem-dimethyl substitu-
ents to give a spiro compound that proved to be of
greater long-term value. Towards this end, a number
of analogues were prepared starting with the cyclopen-
tyl derivative (2d), progressing through the tetrahydro-
pyranyl derivative (2e) and the thiophene-fused
cyclopentyl (2f), and culminating in the tert-butoxycar-
bonyl-pyrrolidine compound (2g). As can be seen, this
compound has enzyme potency similar to dione 1 but
with improved activity in our peripheral blood mono-
nuclear cells (PMBCs) proliferation assay (Table 1).15
In addition, this compound shows the favourable solu-
bility (pH 6.5: 101 lg/ml) seen by early examples in this
series.
Figure 2. Docking studies on tert-butyl 70-methoxy-60-(1,3-oxazol-5-
yl)-40-oxo-30,40-dihydro-1H,10H-spiro[pyrrolidine-3,20-quinazolinc]-1-
carboxylate.
Synthesis of these compounds was readily achieved from
known amino-acid 3. An EDC coupling with the appro-
priate amine (generally methylamine) gave amino-amide
4, which on condensation with a ketone using para-tol-
uenesulfonic acid in refluxing dichloroethane afforded
the desired quinazolinone 2 (Scheme 1).13,14
showed enzyme and cellular activities similar to those
of its tert-butyl analogue (2g), as did the tert-butyl
amide (5b). The ethyl urea (5c), despite maintaining
enzyme activity, was disappointing in the cellular as-
say. Similarly, a reduction in enzyme and cellular
activities was observed for the phenyl urea (5d). How-
ever, formation of the tetra-substituted urea 5e not
only improved potency against IMPDH II but also
more importantly restored cellular activity to that seen
for 2g. Unfortunately, this compound showed a higher
human microsomal turnover compared to 2g. To ad-
dress this issue, potentially metabolisable sites on the
phenyl ring were blocked with fluoro substituents
and this, as shown by compound 5f, was partially suc-
cessful without impairing potency. Aliphatic cyclic and
acyclic ureas, as exemplified by 5g, 5h and 5i, exhibit-
ed similar profiles to 5e. Formation of the oxy-urea
(5j) reduced microsomal clearance to a level similar
to that of 2g without seriously compromising enzyme
and cellular activities. All members of this series
showed an improvement in solubility over dione 1
(e.g., 5g, pH 6.5: 813 l/ml). Several of these com-
pounds were made chirally pure and, as for example
2g, most of the activity was associated with one enan-
tiomer (Table 2).
Clearly, in forming the spiro-centre, in many cases a
chiral centre is also generated. Compound 2g was of
sufficient interest for the enantiomers to be separated
using chiral HPLC. Encouragingly, most of the activity
was found to reside in one of the enantiomers (Enan-
tiomer 1 56 nM, Enantiomer 2 4255 nM) Unfortunately
all attempts to determine the absolute stereochemistry
of this centre using X-ray crystallography failed. How-
ever using the docking programme, GOLD,16 the cen-
tre was tentatively assigned an S configuration: for
this enantiomer the tert-butyloxy group fits snugly into
a lipophilic pocket in the enzyme active site. X-ray
crystal studies of VX-4973 predict that the oxazole
and methoxy groups make key interactions in the ac-
tive site orientating inhibitors in the manner shown
(Fig. 2).
Spiro-pyrrolidine 2g was also a useful compound from
which to start further explorations of the SAR of this
series. Removal of the tert-butyloxy group proceeds
smoothly using hydrogen chloride in diethyl ether to
afford the free amine, which was further reacted with
a variety of electrophiles. The ethyl carbamate (5a)
As has been seen repeatedly for this series, high micro-
somal turnover is a major issue. In an attempt to under-
stand this, in vivo PK experiments were performed on
compound 2g in which the metabolites were identified.
It was found that a major site of metabolism was the
pyrrolidine core of the molecule. One way of preventing
this oxidation was to block the labile site with a
substituent and towards this end a series of proline-de-
rived compounds was synthesized. The required enan-
tiomerically pure ketones were readily prepared in a
three-step synthesis from commercially available start-
ing materials.
Condensation with amino-amide 4 in the manner de-
scribed above produced the desired quinazolinones as
pairs of diastereomers (6a–d) that could be separated
by IIPLC. Some chiral induction was observed with an
approximately 4:1 ratio of diastereomers.
Scheme 1. Preparation of 7-methoxy-6-oxazol-5-yl-2,3-dihydro-1H-
quinazolin-4-ones.

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H. L. Birch et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5335–5339
Table 2. SAR of 70-methoxy-30-methyl-60-(1,3-oxazol-5-yl)-10H-spiro[pyrrolidine-3,20-quinazolin]-40(30H)-ones
Compound
R
IMPDH II9 IC50 (lM)
PBMC15 (nM)
Clint15 (lL/min/mg)
2g
5a
5b
5c
5d
5c
5f
Ot-Bu
OEt
t-Bu
104
80
0.97
1.2
42
24
94
0.54
7.2
46
11
NHEt
NHPh
NMePh
119
393
64
16
19
0.86
1.2
144
97
51
5g
5h
NEt2
45
68
0.54
1.8
92
152
5i
5j
NiPr2
N(Me)OMe
71
79
0.07
1.9
612
38
Note. All compounds shown are racemic.
major product from the condensation with the ketone
derived from D-proline) shows the desired levels of activ-
ity against IMPDH II and hence was the only one tested
in our cellular assay. Compound 6d was deduced to have
S stereochemistry at the spiro-centre and showed an
improvement in enzyme potency as well as the predicted
improvement in metabolic stability over its unsubstitut-
ed analogue (2g) (Table 3). In view of its increased
enzyme potency over compound 2g, the activity in the
cell proliferation assay was a little disappointing
although still acceptable. However, as can be seen in Ta-
ble 2, compounds with similar enzyme potencies can
give rise to quite different cellular activities (e.g., 5i
and 5j).
Figure 3. NOE experiments on proline-derived quinazolinones.
Using the known stereochemistry within the molecule,
persistence with elegant NMR experiments looking at
long-range NOE effects between the a-proton on the
pyrrolidine ring and the 30N-methyl protons resulted
in determination of the absolute stereochemistry of the
spiro-centre (Fig. 3). Testing these compounds in our
enzyme assay also produced some interesting results.
As can be seen only one of the four diastereomers (the
In summary, we have identified a series of novel 7-meth-
oxy-6-oxazol-5-yl-2,3-dihydro-1H-quinazolin-4-ones as
Table 3. SAR of proline-derived 70-methoxy-30-methyl-60-(1,3-oxazol-5-yl)-1H-spiro[pyrrolidine-3,20-quinazolin]-40 (30H)-ones
Compound
Stereochemistry (spiro-centre, a-centre)
IMPDH II15 IC50 (nM)
PBMC15 (lM)
Clint17 (lL/min/mg)
6a
6b
6c
6d
R,S
S,S
R,R
S,R
948
369
n.t.
n.t.
n.t.
1.5
n.t.
n.t.
n.t.
18
26% at 10 lM
35
n.t., not tested.

H. L. Birch et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5335–5339
5339
11. Sorbera, L. A.; Silvestre, J. S.; Castaner, J.; Martin, L.
Drugs Future 2000, 25, 809.
potent IMPDH II inhibitors that also have physical
properties superior to those of our earlier series. The
in vivo PK profiles and pharmacological results of some
of these compounds will be reported elsewhere.
12. Buckley, G. M.; Davies, N.; Dyke, H. J.; Gilbert, P. J.;
Hannah, D. R.; Haughan, A. F.; Hunt, C. A.; Pitt, W. R.;
Profit, R. H.; Ray, N. C.; Richard, M. D.; Sharpe, A.;
Taylor, A. J.; Whitworth, J. M.; Williams, S. C. Bioorg.
Med. Chem. Lett. 2005, 15, 751.
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