New carbon-linked azole oxazolidinones with improved potency and pharmacokinetics

Article abstract of DOI:10.1016/j.bmcl.2006.10.063

Substitution of phenyl oxazolidinones with carbon-linked azoles resulted in the discovery of a new class of potent oxazolidinones that have excellent Gram-positive activity. In addition, replacement of the C-5 acetamide side chains with a 4-methyl triazol

Full text of DOI:10.1016/j.bmcl.2006.10.063

Bioorganic & Medicinal Chemistry Letters 17 (2007) 337–340
New carbon-linked azole oxazolidinones with improved
potency and pharmacokinetics
Sheila I. Hauck,* Christer Cederberg, Amanda Doucette, Lena Grosser, Neil J. Hales,
Grace Poon and Michael B. Gravestock
AstraZeneca R&D Boston, Infection Discovery, 35 Gatehouse Park, Waltham, MA 02451, USA
Received 31 July 2006; revised 17 October 2006; accepted 23 October 2006
Available online 26 October 2006
Abstract—Substitution of phenyl oxazolidinones with carbon-linked azoles resulted in the discovery of a new class of potent
oxazolidinones that have excellent Gram-positive activity. In addition, replacement of the C-5 acetamide side chains with a
4-methyl triazole diminished monoamine oxidase activity. The synthesis and biological evaluation of these compounds are
reported.
ꢀ 2006 Elsevier Ltd. All rights reserved.
O
N
F
As a result of emerging multi-drug resistance to antibac-
terials, the search for new anti-infective agents is relent-
less. The oxazolidinones are one of the few new classes
of antibacterials developed in the past thirty years. Orig-
inally discovered by Dupont scientists in the 1980s,1 they
have potent activity against Gram-positive organisms,
including multi-drug resistant strains. Their mode of ac-
tion involves the inhibition of protein synthesis at a un-
ique binding site on the 50S ribosomal subunit.2
However, oxazolidinone antibacterials, as a class, often
exhibit undesirable side effects including bone marrow
suppression and monoamine oxidase A (MAO-A) inhi-
bition.3 The MAO-A inhibition could potentially elevate
blood pressure in conjunction with certain foods, or lead
to drug–drug interactions;4 therefore, it is important to
discover oxazolidinones that do not exhibit this side-ef-
fect. Pfizer’s linezolid (Zyvoxꢁ), launched in 2000, is use-
ful against Gram-positive pathogens but is inactive
against most Gram-negative organisms. Pfizer later
reported that certain N-linked azole analogs 1 and 2
have moderate Gram-negative activity (Haemophilus
influenzae minimum inhibitory concentrations {MICs}
of 4–8 lg/ml).5
O
N
H
O
N
N
O
linezolid
O
N
O
N
F
F
O
O
N
N
H
N
H
N
N
N
O
O
1
2
Johnson & Johnson reported the synthesis and activity
of their 3-isoxazolinyl (3) and 3-isoxazolyl (4) oxazolid-
inone analogs shown below. Both series include com-
pounds with activity against Gram-positive strains that
is comparable to linezolid in vitro, but were found to
be less efficacious in vivo; the authors attributed this
to either insolubility in the vehicle or poor pharmacoki-
netics.6
O
N
O
N
F
F
R
R
O
O
H
H
N
N
O
O
N
N
O
O
3
4
In addition, Dong-A Pharmaceuticals described a series of
5-isoxazolyl derivatives such as 5 and 6 that have microbi-
ological activity equal or superior to vancomycin.7
Keywords: Triazoles; Isoxazoles; Oxazolidionones.
*
Corresponding author. Tel.: +1 781 839 4678; fax: +1 781 839
4630; e-mail: Sheila.Hauck@astrazeneca.com
0960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.10.063

338
S. I. Hauck et al. / Bioorg. Med. Chem. Lett. 17 (2007) 337–340
A palladium-catalyzed Sonagashira coupling of trimeth-
O
O
N
O
N
F
F
ylsilyl acetylene with the iodophenyloxazolidinone 7 fol-
lowed by removal of the silyl group gave the alkyne
intermediate 8.11 Cycloaddition with p-methoxybenzyl
azide gave a mixture of regioisomers, which, upon
deprotection with neat trifluoroacetic acid, gave the car-
bon-linked triazoles 9–11. The microbiological activity
of a few examples is shown in Table 1.
EtO
O
O
H
H
RO
N
N
N
N
O
O
O
O
5
6
All of these compounds utilize the acetamide side chain
because early structure–activity relationships had indi-
cated it was required for potency.8 We have previously
demonstrated that the acetamide side chain in oxazolid-
inones can be replaced with five-membered heterocycles
such as aminoisoxazoles, hydroxyisoxazoles, and tria-
zoles, and still retain comparable activity.9 More recent-
ly, we have disclosed that 4-substituted triazoles can
diminish MAO-A inhibition.10 In the course of our
investigations to find a broader spectrum antibacterial,
these heterocycles were studied in combination with var-
ious carbon-linked azole substituents on the phenyl ring,
resulting in new antibacterials with exemplary microbio-
logical potency and DMPK properties described here.
The carbon-linked triazoles 9–11 had disappointing
antimicrobial potency against both Gram-positive and
Gram-negative strains, less active than linezolid. Alkyl-
ation of the phenyl-triazole (compounds 12 and 13) did
not significantly improve activity. Subsequently, other
carbon-linked azoles, such as isoxazoles, were investi-
gated. The synthesis of the carbon-linked isoxazoles is
shown in Scheme 2.
The isoxazolyl stannane 14 was prepared using a litera-
ture procedure for cycloaddition of nitrile oxides with
tributylethynyl stannane.12 Palladium-catalyzed Stille
couplings gave the isoxazole oxazolidinones 15–18 in
moderate to high yields. Removal of the Boc-protecting
group from 16 gave compound 19. Although the car-
bon-linked isoxazoles have only moderate Gram-nega-
tive activity compared to nitrogen-linked analogs, they
We initially examined substitution of the phenyl ring
with carbon-linked triazoles in combination with our
heterocyclic side chains using the route in Scheme 1.
SiMe3
1.
O
O
F
Pd2dba3, P-(2-furyl)3, CuI
THF, TEA
F
O
O
I
N
N
R
R
2. KOH, MeOH
7
8
BOC
N
H
N
H
N
N
N
O
N
N
N
R =
O
O
MeO
1.
2.
O
F
N3
N
N
O
1, 4-dioxane, 100 ˚C
N
HN
R
Trifluoroacetic acid
H
H
N
N
N
N
N
N
O
R =
O
9
10
11
Scheme 1. Synthesis of carbon-linked triazole oxazolidinones.
Table 1. In vitro activity (lg/ml) of oxazolidinones 9–13
H
N
H
O
F
N
N
N
O
N
O
N
R =
N
N
O
N
N
R
R'
9
10
11
Compound
R
R0
Hin
Sau
Spn
9
10
Acetamide
H
8
16
4
8
2
16
32
16
2
Aminoisoxazole
Triazole
Triazole
H
11
12
H
Me
>64
>64
32
64
32
4
13
Linezolid
Triazole
Cyano-methyl
8
2
1
Hin, Haemophilus influenzae; Sau, Staphylococcus aureus; Spn, Streptococcus pneumoniae.

S. I. Hauck et al. / Bioorg. Med. Chem. Lett. 17 (2007) 337–340
339
O
F
PhNCO
TEA
O
I
N
R
O
F
O
SnBu3
7
N
+
-
O
O
N
N
Bu3Sn
O
N
R
50 ˚C
98%
O
Pd(Ph3P)Cl2
dioxane, 100 ˚C
overnight
14
B OC
N
H
N
N
N
N
N
N
N
N
O
R =
O
15
16
17
18
Scheme 2. Synthesis of carbon-linked isoxazole oxazolidinones.
Table 2. In vitro activity (lg/ml) and monoamine oxidase inhibition (lM) of oxazolidinones 15–19
H
H
O
F
N
N
N
N
N
N
N
N
N
O
R =
R
O
O
N
N
O
15
19
17
18
Compound
R
Hin
Sau
Spn
MAO-A Ki
15
19
Acetamide
4
16
8
0.5
0.5
0.5
0.5
2
0.25
0.25
0.25
0.25
1
0.95
0.42
2.9
Aminoisoxazole
Triazole
4-Methyl triazole
17
18
4
22.6
20
Linezolid
8
Hin, Haemophilus influenzae; Sau, Staphylococcus aureus; Spn, Streptococcus pneumoniae.
do have very potent Gram-positive activity relative to
linezolid as shown in Table 2. In addition, the data in
109
Table 2 demonstrate the decrease in monoamine oxidase
potency as the right hand side chain is varied from ami-
noisoxazole to 4-methyltriazole.
108
107
The pharmacokinetic (PK) properties of two of these
compounds (17 and 18) were evaluated in rat and the
106
data are shown in Table 3. Both compounds have low
clearance (Cl), low volume of distribution (Vss), and a
105
moderate half-life (t1/2). The oral bioavailability (F) is
quite good for both analogs.
104
103
Due to the excellent in vitro potency and PK properties
seen in this series, selected compounds were further tested
in vivo in a mouse lung infection model against Strepto-
coccus pneumoniae. The data are shown in Figure 1 versus
the vehicle and linezolid for comparison. While the reduc-
tion of bacterial colony forming units (CFU) by com-
pounds 15 and 19 was unexpectedly low (22% and 33%,
respectively) relative to linezolid (75%), compound 17
was extremely efficacious, yielding a 92% CFU reduction.
102
101
vehicle
linezolid
15
17
19
Figure 1. Efficacy of linezolid, 15, 17, and 19 in mice after Strepto-
coccus pneumoniae lung infection and three doses (25 mg/kg po) of
each compound.
Table 3. Pharmacokinetic parameters of oxazolidinones 17 and 18 in rat
Compound
Cl (mL/min/kg)
Vss (L/kg)
t1/2 (h)
F (%) 40 mg/kg
17
18
5
5
0.6
0.5
1.7
1.2
82
87

340
S. I. Hauck et al. / Bioorg. Med. Chem. Lett. 17 (2007) 337–340
In conclusion, we report the synthesis13 and biological
evaluation of a new class of oxazolidinones. The
3-methylisoxazole-phenyl oxazolidinones have excel-
lent Gram-positive activity and DMPK properties. In
particular, compound 17 shows exceptional efficacy
against S. pneumoniae infections. Replacement of the
typical acetamide group by 4-methyl triazole showed
diminished MAO-A inhibition in our series. Addition-
al work exploring the substitution of the isoxazole
ring is ongoing in order to improve the Gram-negative
activity.
References and notes
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In vitro and in vivo assays. The bacteria used in these
studies were taken from the AstraZeneca culture collec-
tion. Minimum inhibitory concentrations were generat-
ed by broth microdilution according to the Clinical and
Laboratory Standards Institute guidelines14 for the
majority of bacteria. Human liver monoamine
oxidase-A activity was assayed by adapting the method
of Flaherty.15 The Ki is expressed as a mean of three
experiments. Specific conditions were: 100 mM sub-
strate 4-(1-methyl-2-pyrryl)-1-methyl-1,2,3,6-tetrahy-
dropyridine, 82 nM human liver monoamine oxidase
A, 100 mM potassium phosphate buffer, pH 7.4, and
25 ꢂC. The mouse efficacy model experimental design
utilized the following protocol: S. pneumoniae
ARC548 was administered at 1 · 106 CFU to anesthe-
tized female C57BL/6 mice. Eighteen hours after infec-
tion, experimental compounds were administered orally
at 25 mg/kg, each mouse received two additional
administrations at 4-h intervals. Mice were sacrificed
and total CFU/lung were determined 2 days after
infection.
Acknowledgments
14. Clinical and Laboratory Standards Institute, Wayne, PA,
2003. Document M07-A6.
15. Flaherty, P.; Castagnoli, K.; Wang, Y.-X.; Castagnoli, N.,
Jr. J. Med. Chem. 1996, 39, 4756.
The authors thank Jeanette Jones and Adam Mondello
for the microbiological data and Tory Nash for MAO Ki
values.