Bug drug. Mutations in two topoisomerase genes in S. aureus have led to strains that are resistant to premafloxacin, an 8-methoxyfluoroquinolone.

Resistance generally arises from mutations in the so-called quinolone-binding domain in the primary topoisomerase target. Mutations in the secondary topoisomerase target also contribute, but only when the primary target is affected. A quinolone that targets both of these topoisomerases and results in a low frequency of resistant mutant selection in vitro is a good candidate, because the probability of a resistant mutant in clinical settings is low.

Premafloxacin, an 8-methoxyfluoroquinolone, has enhanced activity against S. aureus, although its interactions with target enzymes and the mechanisms of S. aureus resistance are unknown. Dilek Ince and David C. Hooper investigated the reaction and resistance mechanisms of premafloxacin (Antimicrob. Agents Chemother. 2000, 44, 3344–3350). The effect of mutations in the type-2 topoisomerases was studied using mutants selected on prema floxacin-containing medium and those previously identified.

The primary target of premafloxacin is topoisomerase IV. Premafloxacin is very potent against wild-type S. aureus and strains with topoisomerase IV mutations. It is promising that the drug is a poor substrate for the NorA multidrug resistance efflux protein, as NorA overexpression has no effect on premafloxacin activity.

Surprisingly, some mutations in topoisomerase IV arising from selection on premafloxacin occurred outside the quinolone-binding domain. One mutation was identified in an area involved in dimer formation, and experiments indicated that this mutation is responsible for resistance. Furthermore, the same mutation was identified in a resistant clinical isolate of Mycoplasma hominis. Another S. aureus strain was identified with a mutation in both type-2 topoisomerases, although the mutation in DNA gyrase does not appear to contribute to resistance. In this case, a leucine replaced a proline residue in topoisomerase IV that lies below the active site tyrosine. Another mutant S. aureus strain carried a mutation at alanine-176, which is implicated in protein–DNA contacts. A mutation was also found at the highly conserved arginine-43, which may be involved in catalysis.

These mutations suggest that premafloxacin interacts with topoisomerase IV/DNA complexes in a different manner from that of other quinolones. The discovery of other novel mutations will expand the understanding of drug–target interactions and the mechanisms of quinolone resistance due to target alteration.