New anti-MRSA cephalosporins with a basic aminopyridine at the C-7 position

Article abstract of DOI:10.1016/S0960-894X(00)00603-X

Incorporation of a basic aminopyridine into the C-7 position of 3-(amine-substituted arylthio)-3-norcephalosporins, as in 3, afforded high potency against MRSA and acceptable solubility for intravenous administration.

Full text of DOI:10.1016/S0960-894X(00)00603-X

Bioorganic & Medicinal Chemistry Letters 11 (2001) 137±140
New Anti-MRSA Cephalosporins with a Basic Aminopyridine at
the C-7 Position
Aesop Cho,Ã Tomasz W. Glinka, Maria Ludwikow, Andrew T. Fan, Michael Wang
and Scott J. Hecker
Microcide Pharmaceuticals, Inc., 850 Maude Avenue, Mountain View, CA 94043, USA
Received 18 September 2000; accepted 25 October 2000
AbstractÐIncorporation of a basic aminopyridine into the C-7 position of 3-(amine-substituted arylthio)-3-norcephalosporins, as
in 3, a€orded high potency against MRSA and acceptable solubility for intravenous administration. # 2001 Elsevier Science Ltd.
All rights reserved.
The usefulness of b-lactam drugs continues to be eroded
because of the emergence and spread of resistant
pathogens such as methicillin-resistant Staphylococcus
aureus (MRSA).1 Thus, there have been considerable
e€orts to restore e€ectiveness by building potency
against such pathogens into this class of antibiotics.2
neutral heteroaromatic ring systems. As a way of
attaining the desired level of solubility, we sought to
introduce an additional basic functionality at C-7. Since
high anti-MRSA activity also appears to correlate with
high lipophilicity of the C-7 side chain,6 8 a similar
strategy to that used successfully at C-3 was considered
attractive.
We recently reported the discovery of RWJ-54428 (MC-
02,479, 1), a new cephalosporin that displays potent
activity against MRSA.3 This compound has several
unique structural features, notably a 4-pyridinethiol
directly linked to the cephalosporin core (i.e., no methy-
lene spacer between them) and a 2-aminoethylthio-
methyl substituent. The pyridine ring, in particular,
proves to be crucial for both activity and solubility. We4
and others5 observed earlier that higher anti-MRSA
activity is achieved with more lipophilicity at C-3. The
pyridine of 1 is unprotonated at physiological pH and
thereby meets this lipophilicity criterion. At acidic pH,
on the other hand, the pyridine is positively-charged,
thus a€ording acceptable solubility for intravenous
administration.
Compound 2, with a nonbasic aromatic ring at C-3,
displayed excellent antibacterial activity (see Table 1).
As expected, however, this compound was sparingly
soluble in water due to its zwitterionic nature, which
precluded further pharmacological pro®ling. This lack
of solubility also discouraged us from exploring other
ÃCorresponding author. Tel.: +1-650-428-3513; fax: +1-650-428-
3550; e-mail: acho@microcide.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00603-X

138
A. Cho et al. / Bioorg. Med. Chem. Lett. 11 (2001) 137±140
Goto et al. reported that aminopyridines are viable iso-
steres of the aminothiazole at the C-7 position found in
other types of cephalosporins.9 We envisioned that
replacement of the aminothiazole moiety of 2 with the
more basic aminopyridine (e.g., as in 3) would a€ord
enhanced solubility at low pH. Furthermore, the
aminopyridine group is expected to be uncharged at
physiological pH (predicted pKa=ꢀ 6), thus providing
the required lipophilicity for anti-MRSA activity. We
were pleased to discover that 3 displayed excellent anti-
MRSA activity (see Table 1) as well as dramatically
increased solubility (>20 mg/mL at pH 4.5). Utilizing
this new C-7 side chain, we proceeded to synthesize new
dibasic analogues with various aromatic systems at C-3
and evaluate their antibacterial activity.10
C-7 side chain 4, the cephem core 5,11 and the thiadia-
zole thiol 6 (Scheme 1). Reaction of the enol mesylate 5
with the thiol 6 in a biphasic medium (EtOAc/aq
sodium bicarbonate) provided 7. Importantly, no
unwanted C-2 double bond isomerization12 was
observed under these conditions. Subsequent acylation
of 7 by mixing with the acid 4 and phosphorus oxy-
chloride followed by simultaneous removal of all the
protecting groups a€orded the desired 3.13
The aminopyridine 4 was synthesized as shown in
Scheme 2. Commercially available 2,6-pyridinedicar-
boxylic acid 9 was converted to the monoamino acid 10
via Curtius rearrangement, and then to the methyl ester
11. The protected ketoester 14 was obtained by a mod-
i®ed literature method:9 Pummerer rearrangement of
the sulfoxide 12 with tri¯uoroacetic anhydride followed
by addition of methanol gave the dithioorthoformate
13, which was oxidized with sodium perborate to form
the methyl ester 14. Treatment of 14 with hydroxyl-
amine a€orded a mixture of syn- and anti-oximes (ratio
Chemistry
The synthesis of 3 consisted of sequential couplings of
three fully protected intermediates: the aminopyridine
Table 1. In vitro anti-MRSA activitiesa of dibasic cephalosporins
Compounds MRSAb 76 MRSA ATCC 335893 MRSAb Spain #356 Compounds MRSA 76 MRSA ATCC 33593 MRSA Spain #356
1
2
3
22a
22b
22c
22d
22e
22f
22g
1
1
1
4
2
2
2
2
2
2
1
1
1
2
1
2
2
1
1
2
1
1
1
4
2
2
2
2
1
2
22h
22i
22j
22k
22l
22m
22n
22o
8
4
1
1
1
2
4
1
1
2
2
8
8
32
1
4
4
1
1
2
2
2
16
16
32
16
16
32
1
Imipenem
Vancomycin
0.5
aMICs (minimum inhibitory concentrations, mg/mL) were determined in Mueller±Hinton broth supplemented with 2% NaCl. MICs were read after
24 h incubation at 35 oC.
bClinical isolates.

A. Cho et al. / Bioorg. Med. Chem. Lett. 11 (2001) 137±140
139
of ca. 5:1), which were separated after tritylation.
Hydrolysis of the syn-isomer 16 a€orded the protected
C-7 side-chain acid 4.
In a similar way, other C-3 aryl analogues 22a±22o were
obtained from couplings of
corresponding aryl thiols.
4 and 5 and the
The synthesis of the thiadiazole intermediate 6 is sum-
marized in Scheme 3. Starting with 1,3-dichloroacetone,
the appropriately functionalized 1,2,3-thiadiazole 20
was constructed by Hurd±Mori cyclization.14 Both
displacement with 2-aminoethanethiol and subsequent
N-t-BOC protection, giving 21, were achieved in one
pot. Finally, base-promoted b-elimination, with vacuum
assisted removal of the volatile acrylic ester by-product,
a€orded 6 as the sodium salt.
Biological Results
The in vitro antibacterial activity of the newly-prepared
cephalosporins was determined against MRSA strains
by broth microdilution assay as recommended by the
NCCLS.15 The results are shown in Table 1, with both
imipenem and vancomycin included as reference
agents.
Scheme 1. Reagents and conditions: (a) EtOAc±aq NaHCO3, 99%; (b) 4, POCl3, Hunig's base, THF, 45%; (c) TFA, Et3SiH, CH2Cl2, 84%.
Scheme 2. Reagents and conditions: (a) (PhO)2PON3, TEA, t-BuOH, 45%; (b) POCl3, DMF, EtOAc; then MeOH, 98%; (c) NaH,
CH3SCH2S(O)CH3, DMF, 65%; (d) TFAA, Py, CH2Cl2; then MeONa, 85%; (e) NaBO3, AcOH, 40%; (f) NH2OH, Py, EtOH, 100%; (g) TrCl,
TEA; then chromatography, 84%; (h) NaOH, i-PrOH±H2O, 92%.
Scheme 3. Reagents and conditions: (a) EtO2CCH2CH2SH, TEA, THF; (b) NH2NHCO2Et, cat. TsOH, mol sieve, CH3CN; (c) SOCl2, CH2Cl2,
45% from 17; (d) NaI, HSCH2CH2NH2, t-BOC2O, aq NaHCO3±dioxane, 85%; (e) MeONa, i-PrOH, 100%.

140
A. Cho et al. / Bioorg. Med. Chem. Lett. 11 (2001) 137±140
Most of compounds tested displayed good to excellent
anti-MRSA activity. Activity trends in six-membered
ring analogues (22k>22l>22m) re¯ect the importance
of lipophilicity. On the other hand, the ®ve-membered
ring thiadiazole 3 was somewhat more potent than the
thiazole 22e, suggesting that other factors such as elec-
tronic e€ects might also be operative. While the amino-
ethylthiomethyl ring appendage could be positioned
either ortho (1,2) or meta (1,3) to the cephem core in the
thiazole series (i.e., 22b and 22e), the meta analogue 22a
was inferior in activity to the ortho analogue 3 in the
thiadiazole series. The meta analogues 22n and 22o in
the six-membered ring diazine series were also less active
than the ortho analogue 22m. With the exception of 22j
(possessing an extra basic amine), the activities of the
series of 1,3-thiazol-5-yl analogues (22e±22i) appear to
correlate with lipophilicity.
3. (a) Hecker, S. J.; Glinka, T. W.; Cho, A.; Zhang, Z. J.;
Chamberland, S.; Grith, D.; Lee, V. J. J. Antibiot. in press.
(b) Glinka, T. W.; Cho, I.-S. (Cho, A.); Zhang, Z. J.; Price,
M.; Case, L.; Crase, J.; Frith, R.; Liu, N.; Ludwikow, M.;
Rea, D.; Chamberland, S.; Lee, V. J.; Hecker, S. J. Program
and Abstracts of the 37th Intersci. Conf. on Antimicrob. Agents
Chemother. 1997, F176.
4. Hecker, S. J.; Cho, I.-S.; Glinka, T. W.; Zhang, Z. J.; Price,
M. E.; Lee, V. J.; Christensen, B. G.; Boggs, A.; Chamberland,
S.; Malouin, F.; Parr, T. R.; Annamalai, T.; Blais, J.; Bond,
E. L.; Case, L.; Chan, C.; Crase, J.; Frith, R.; Grith, D.;
Harford, L.; Liu, N.; Ludwikow, M.; Mathias, K.; Rea, D.;
Williams, R. J. Antibiot. 1998, 51, 722.
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Bioorg. Med. Chem. 1998, 6, 1009. (b) Ternansky, R. J.; Dra-
heim, S. E.; Pike, A. J.; Bell, F. W.; West, S. J.; Jordan, C. L.;
Wu, C. Y. E.; Preston, D. A.; Alborn, W., Jr.; Kasher, J. S.;
Hawkins, B. L. J. Med. Chem. 1993, 36, 1971. For carbape-
nem series, see: (c) Laub, J. B.; Greenlee, M. L.; DiNinno, F.;
Huber, J. L.; Sundelof, J. G. Bioorg. Med. Chem. Lett. 1999,
9, 2973. (d) Imamura, H.; Ohtake, N.; Shimizu, A.; Jona, H.;
Sato, H.; Nagano, R.; Ushijima, R.; Yamada, K.; Hashi-
zume, T.; Morishima, H. Bioorg. Med. Chem. Lett. 2000, 10,
109.
Unlike that of zwitterionic compound 2, solubility of
this series of compounds was sucient to conduct vari-
ous pharmacological studies. Results of further pro®ling
of selected compounds, including in vivo ecacy,
serum e€ects and pharmacokinetic properties, have
been presented.10
6. Kim, O. K.; Hudyma, J. D.; Matiskella, J. D.; Ueda, Y.;
Bronson, J. J.; Mansuri, M. M. Bioorg. Med. Chem. Lett.
1997, 7, 2753.
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Chem. Lett. 1994, 4, 1147.
8. Cho, A.; Ludwikow, M.; Glinka, T. W.; Zhang, Z. J.; Lee,
V. J.; Hecker, S. J. 7th International Conference on Chemistry
of Antibiotics and Related Microbial Products, Mierki,
Poland, 2±6 September, 2000 (Abstracts).
9. Goto, J.; Sakane, K.; Nakai, Y.; Teraji, T.; Kamiya, T. J.
Antibiot. 1984, 37, 532.
10. A portion of this work was presented. (a) Cho, A.; Lud-
wikow, M.; Liu, N.; Fan, A.; Glink, T.; Zhang, Z. J.; Price,
M.; Dudley, M. N.; Chamberland, S.; Lee, V. J.; Hecker, S. J.
Program and Abstracts of the 39th Intersci. Conf. on Anti-
microb. Agents Chemother. 1999, F-392. (b) Glinka, T. W.;
Huie, K.; Halas, S.; Cho, A.; Ludwikow, M.; Price, M.; Chen,
S.; Grith, D.; Chamberland, S.; Blais, J.; Hecker, S. J.; Lee,
V. J. Program and Abstracts of the 40th Intersci. Conf. on
Antimicrob. Agents Chemother. 2000, #1071.
Conclusions
A new C-7 side chain in which the aminothiazole is
replaced with a basic aminopyridine was incorporated
into 3-(amine-substituted arylthio)-3-norcephaolspor-
ins. Unlike the zwitterionic compound 2, the dibasic
analogue 3 possessing this C-7 side chain had solubility
sucient for parenteral (iv) administration, while retain-
ing excellent anti-MRSA activity. Various C-3 aromatic
ring analogues with this side chain were synthesized and
evaluated for their anti-MRSA activity. Some of them
were as potent as RWJ-54428, a clinical candidate.
Acknowledgements
11. Glinka, T. W.; Cho, A.; Chamberland, S.; Dudley, M. N.;
Grith, D.; Huie, K.; Ludwikow, M.; Zhang, Z. J.; Hecker, S.
J.; Lee, V. J. J. Antibiot. in press.
12. Farina, V.; Baker, S. R.; Hauck, S. I. J. Org. Chem. 1989,
54, 4962.
13. Spectral data for 3; 1H NMR (D2O, 400 MHz) 2.87 (t,
2H), 3.27 (t, 2H), 3.46 (d, 1H, J=18 Hz), 3.65 (d, 1H, J=18
Hz), 4.23 (ABq, 2H, CH2SCH2CH2N), 5.58 (d, 1H, J=5 Hz),
5.87 (d, 1H, J=5 Hz), 6.95 (d, 1H, J=8 Hz), 7.05 (d, 1H, J=8
Hz), 7.90 (t, 1H, J=8 Hz). MS (ES): 569 (M+H)+.
14. (a) Fujita, M.; Kobori, T.; Hiyama, T.; Kondo, K. Het-
erocycles 1993, 36, 33. (b) Hurd, C. D.; Mori, R. I. J. Am.
Chem. Soc. 1955, 77, 5359.
15. NCCLS (National Committee for Clinical Laboratory
Standards) Methods for Dilution of Antimicrobial Susceptibility
Tests for Bacteria that Grow Aerobically. NCCLS Document
M7-A4, Vol. 17, No. 2, 1997.
The work described herein was conducted as part of a
research collaboration with the R.W. Johnson Pharma-
ceutical Institute. The authors would like to thank
Monica Hoang, Cynthia Dinh and Christine Chan for
their MIC measurements, and also thank Dr. Ving J.
Lee and Dr. Michael N. Dudley for their helpful
guidance of this work.
References and Notes
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