Effects of order of magnesium exposure on the postantibiotic effect and bactericidal activity of ciprofloxacin.

Quinolone antibiotics are known to form chelates with various metal cations. It has also been recognized that the physicochemical properties of the chelated antibiotic differ significantly from its unchelated form, thereby causing a reduction in antimicrobial activity. In addition, the formation of metal chelates is also believed to be the reason for the significant reduction in oral bioavailability for the quinolones when concomitantly dosed with oral cation containing agents. This has prompted the adoption of an alternate dosing regimen by introducing an adequate interval between the two. As a result of this dosing strategy, pathogens are exposed to the quinolones and metal cations in alternate orders. Using magnesium, ciprofloxacin, and Escherichia coli as the test organism, investigations were conducted to study the changes in bactericidal activity and postantibiotic effect (PAE) in relation to the orders of cation/antibiotic exposure. Results showed a parallel decrease in both bactericidal activity and PAE when the test organism was exposed to the two agents simultaneously; however, no apparent influences on these two antimicrobial effects were observed when Mg2+ was presented before or after ciprofloxacin exposure. In line with the current dosing recommendations, the interval spaced between ciprofloxacin and Mg2+ should preserve both the bactericidal activity and PAE exhibited by the antibiotic. How the present data are to be extrapolated to other quinolones and cations should be the subject of future studies.