レボフロキサシン

Levofloxacin is bactericidal and exerts its antimicrobial effects via inhibition of bacterial DNA replication. It has a relatively long duration of action in comparison with other antibiotics that allows for once or twice daily dosing. Levofloxacin is associated with QTc-interval prolongation and should be used with caution in patients with other risk factors for prolongation (e.g. hypokalemia, concomitant medications). Levofloxacin has demonstrated _in vitro_ activity against a number of aerobic gram-positive and gram-negative bacteria and may carry some activity against certain species of anaerobic bacteria and other pathogens such as _Chlamydia_ and _Legionella_. Resistance to levofloxacin may develop, and is generally due to mutations in DNA gyrase or topoisomerase IV, or via alterations to drug efflux. Cross-resistance may occur between levofloxacin and other fluoroquinolones, but is unlikely to develop between levofloxacin and other antibiotic classes (e.g. macrolides) due to significant differences in chemical structure and mechanism of action. As antimicrobial susceptibility patterns are geographically distinct, local antibiograms should be consulted to ensure adequate coverage of relevant pathogens prior to use.

Levofloxacin, like other fluoroquinolone antibiotics, exerts its antimicrobial activity via the inhibition of two key bacterial enzymes: DNA gyrase and topoisomerase IV. Both targets are type II topoisomerases, but have unique functions within the bacterial cell. DNA gyrase is an enzyme found only in bacteria that introduces negative supercoils into DNA during replication - this helps to relieve torsional strain caused by the introduction of positive supercoils during replication, and these negative supercoils are essential for chromosome condensation and the promotion of transcription initiation. It is comprised of four subunits (two A subunits and two B subunits) of which the A subunits appear to be the target of fluoroquinolone antibiotics. Bacterial topoisomerase IV, in addition to contributing to the relaxation of positive supercoils, is essential at the terminal stages of DNA replication and functions to “unlink” newly replicated chromosomes to allow for the completion of cell division. Inhibition of these enzymes by levofloxacin likely occurs via complexation with the topoisomerase enzymes. The end result is a blockade of DNA replication, thus inhibiting cell division and resulting in cell death.