Novobiocin Enhances Polymyxin Activity by Stimulating Lipopolysaccharide Transport

Gram-negative bacteria are challenging
to kill with antibiotics
due to their impenetrable outer membrane containing lipopolysaccharide
(LPS). The polymyxins, including colistin, are the drugs of last resort
for treating Gram-negative infections. These drugs bind LPS and disrupt
the outer membrane; however, their toxicity limits their usefulness.
Polymyxin has been shown to synergize with many antibiotics including
novobiocin, which inhibits DNA gyrase, by facilitating transport of
these antibiotics across the outer membrane. Recently, we have shown
that novobiocin not only inhibits DNA gyrase but also binds and stimulates
LptB, the ATPase that powers LPS transport. Here, we report the synthesis
of novobiocin derivatives that separate these two activities. One
analog retains LptB-stimulatory activity but is unable to inhibit
DNA gyrase. This analog, which is not toxic on its own, nevertheless
enhances the lethality of polymyxin by binding LptB and stimulating
LPS transport. Therefore, LPS transport agonism contributes substantially
to novobiocin–polymyxin synergy. We also report other novobiocin
analogs that inhibit DNA gyrase better than or equal to novobiocin,
but bind better to LptB and therefore have even greater LptB stimulatory
activity. These compounds are more potent than novobiocin when used
in combination with polymyxin. Novobiocin analogs optimized for both
gyrase inhibition and LPS transport agonism may allow the use of lower
doses of polymyxin, increasing its efficacy and safety.