ACS Medicinal Chemistry Letters
Letter
some bacteria and does not reach an intracellular concentration
that creates a bacteriostatic effect.24,25 This effect is clear in our
MIC measurements of novobiocin against E. coli cells with and
without the essential drug efflux pump component, TolC. The
MIC of novobiocin against wildtype E. coli MG1655 cells was
320 μM, while the MIC against E. coli BW25113 ΔtolC was 1.2
μM (Table S1). To test this hypothesis, we codosed E. coli
MG1655 cells with varying concentrations of novobiocin and
10. A checkerboard plot demonstrates no significant change in
the antibacterial activity of novobiocin when codosed with
different concentrations of 10 (Figure 3B), suggesting that 10
does not alter membrane properties in a manner that influences
the transport or passive diffusion of novobiocin into cells.
In conclusion, our retrosynthetic analysis led us to
disconnect 1 into three structural components: (1) hetero-
aromatic ring, (2) linker region containing the secondary
hydroxyl group, and (3) amine-linked tail region (Figure 1).
We found that the stereochemistry of the linker had no
influence on the biological activity of 1, signifying it does not
target a specific protein but directly interacts with the cellular
membrane. Through these studies we established that the
carbazole moiety with chlorine substitutions is an important
element for biological activity suggesting that its planar,
hydrophobic, and dipolar characteristics are important for
orienting the molecule in the membrane. Manipulation of the
tail group affects the potency of the compound, and increasing
its hydrophobicity improves the activity of the molecule,
presumably through interacting with membranes through the
lipophilic tail of the phospholipids. We discovered that the
antibacterial activity of 10 can be enhanced by codosing with
other antibiotics. Mouse infection model studies of 10 and 11
against F. tularensis and B. anthracis are currently underway with
USAMRIID and will provide insight into the clinical capabilities
of this class of antibiotics.
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ASSOCIATED CONTENT
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S
* Supporting Information
All synthetic procedures, compound characterization data, and
biological methods. This material is available free of charge via
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AUTHOR INFORMATION
■
Corresponding Author
*Phone: +1 608/890-1342. Fax: +1 608/265-0764. E-mail:
Funding
This research was supported by grants from the National
Institutes of Health (1DP2OD008735), the Human Frontiers
Science Program (RGY0076/2013), and Defense Threat
Reduction Agency under USAMRIID (Proj. #922141).
K.A.H. acknowledges a fellowship from the American
Foundation for Pharmaceutical Education (AFPE).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Stephanie Halasohoris and Lynda Miller at the
USAMRIID facility for their assistance. We thank V.H.S. Head
and C. Truise for inspiration. Opinions, interpretations,
conclusions and recommendations are those of the authors
and are not necessarily endorsed by the U.S. Army.
E
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