ideally into a narrow pocket in the active site of the enzyme.11
Therefore, to the best of our knowledge, emmacin represents
the first member of a new structural sub-class of bacterial-
selective DHFR inhibitors.31
D. A. Smith and B. C. Jones, J. Comput. Aided Mol. Des., 2001, 15,
273–286.
16 Details of the synthetic routes employed for analogue generation
are available in the supplementary information and in our original
report on the DOS library synthesis (see ref. 14); structures shown
in supporting information appendix.
In conclusion, we have exploited a DOS small molecule
collection in the discovery of a structurally novel antibacterial
agent called emmacin. Emmacin inhibited the growth of two
epidemic strains of MRSA in vitro, and crucially, was found to be
inactive in a variety of mammalian cytotoxicity assays. It was
shown to act as a prokaryote-selective, uncompetitive and rever-
sible inhibitor of EMRSA-16 DHFR (DfrBEMRSA16). Due to its
dihydropyrimidine core, emmacin represents a new structural sub-
class of DHFR inhibitors, which could potentially be exploited in
the development of critically-needed, new antibacterial agents.
We gratefully acknowledge financial support from the
EPSRC, AstraZeneca and Pfizer. We also acknowledge the
EPSRC National Mass Spectrometry Service Centre,
Swansea, for providing mass spectrometric data and Syngenta
for carrying out preliminary mode of action studies.
17 Analysis of this data provided some insight into possible SAR
exhibited by these nitrogen-based scaffolds; bicyclic species 3 were
typically more active with halogen atoms (Br or Cl) at the R4
position, bicyclic compounds 4 generally displayed higher levels of
activity when R1 was an aryl halide rather than an alkyl substituent
and the replacement of an iso-butyl group with an aryl halide at the
R3 position of 2 also appeared to confer a higher level of activity.
For selected biological screening data consult the supplementary
information.
18 R. Milcent, J. C. Malanda, G. Barbier and J. Vaissermann,
J. Heterocycl. Chem., 1997, 34, 329–336.
19 Emmacin is a chiral molecule; the data presented in this report
refer to the use of a racemic sample of emmacin synthesised by the
route outlined in Scheme 1. Studies towards the isolation and
independent biological screening of the two enantiomers of
emmacin are ongoing.
20 N. V. Kovalevskaya, E. D. Smurnyi, B. Birdsall, J. Feeney and V.
I. Polshakov, Pharm. Chem. J., 2007, 41, 350–353.
21 J. M. Blaney, C. Hansch, C. Silipo and A. Vittoria, Chem. Rev.,
1984, 84, 333–407.
22 S. Hawser, S. Lociuro and K. Islam, Biochem. Pharmacol., 2006,
71, 941–948.
23 The IC50 values for inhibition of S. aureus DHFR enzyme
by trimethoprim and iclaprim are both 7 nM; P. Schneider,
S. Hawser and K. Islam, Bioorg. Med. Chem. Lett., 2003, 13,
4217–4221.
24 W. J. Peppard and C. D. Schuenke, Curr. Opin. Invest. Drugs,
2008, 9, 210–225.
25 The effect of emmacin on DfrBEMRSA16 activity was determined
using a commercially available DHFR assay kit (Sigma Aldrich,
Product Code CS0340). Human DHFR was not inhibited by
emmacin. See supplementary information for more detail.
26 D. Voet, J. G. Voet and C. W. Pratt, Principles of Biochemistry,
John Wiley & Sons, Inc., New Jersey, 2008.
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31 It is conceivable that emmacin’s dihydropyrimidine is oxidized to
give a heteroaromatic core under assay conditions. However, this is
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This journal is The Royal Society of Chemistry 2008
4964 | Chem. Commun., 2008, 4962–4964