Pharmacology: Pharmacodynamics: Mechanism of action: Moxifloxacin has in vitro activity against a wide range of Gram-positive and Gram-negative pathogens.
The bactericidal action of moxifloxacin results from the inhibition of both type II topoisomerases (DNA gyrase and topoisomerase IV) required for bacterial DNA replication, transcription and repair. It appears that the C8-methoxy moiety contributes to enhanced activity and lower selection of resistant mutants of Gram-positive bacteria compared to the C8-H moiety. The presence of the bulky bicycloamine substituent at the C-7 position prevents active efflux, associated with the norA or pmrA genes seen in certain Gram-positive bacteria.
Pharmacodynamic investigations have demonstrated that moxifloxacin exhibits a concentration dependent killing rate. Minimum bactericidal concentrations (MBC) were found to be in the range of the minimum inhibitory concentrations (MIC).
Effect on the intestinal flora in humans: The following changes in the intestinal flora were seen in volunteers following oral administration of moxifloxacin:
Escherichia coli,
Bacillus spp.,
Enterococcus spp., and
Klebsiella spp. were reduced, as were the anaerobes
Bacteroides vulgatus,
Bifidobacterium spp.,
Eubacterium spp., and
Peptostreptococcus spp. For
Bacteroides fragilis there was an increase. These changes returned to normal within two weeks.
Mechanism of resistance: Resistance mechanisms that inactivate penicillins, cephalosporins, aminoglycosides, macrolides and tetracyclines do not interfere with the antibacterial activity of moxifloxacin. Other resistance mechanisms such as permeation barriers (common in
Pseudomonas aeruginosa) and efflux mechanisms may also effect susceptibility to moxifloxacin.
In vitro resistance to moxifloxacin is acquired through a stepwise process by target site mutations in both type II topoisomerases, DNA gyrase and topoisomerase IV. Moxifloxacin is a poor substrate for active efflux mechanisms in Gram-positive organisms.
Cross-resistance is observed with other fluoroquinolones. However, as moxifloxacin inhibits both topoisomerase II and IV with similar activity in some Gram-positive bacteria, such bacteria may be resistant to other quinolones, but susceptible to moxifloxacin.
Breakpoints: EUCAST clinical MIC and disk diffusion breakpoints for moxifloxacin (01.01.2011): See Table 1.
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Microbiological Susceptibility: The prevalence of acquired resistance may vary geographically and with time for selected species and local information of resistance is desirable, particularly when treating severe infections. As necessary, expert advice should be sought where the local prevalence of resistance is such that utility of the agent in at least some types of infections is questionable. (See table 2.)
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Pharmacokinetics: Moxifloxacin is readily absorbed from gastrointestinal track with an absolute bioavailability of about 90%. It is widely distributed throughout the body tissues and is about 50% bound to plasma proteins. Moxifloxacin has an elimination half-life of about 12 hours, allowing once-daily dosing. It is metabolised principally via sulfate and glucuronide conjugation and is excreted in the urine and the faeces as unchanged drug and as metabolites, the sulfate conjugate primarily in the faeces and the glucuronide exclusively in the urine.