Influence of metal cations on the solubility of fluoroquinolones.

Although a clinically relevant interaction between a fluoroquinolone and a metal cation was first described more than 20 years ago the biopharmaceutical mechanism of this interaction is still not understood. One of the obvious disagreements in the literature is about the effect of metal cations on the solubility of fluoroquinolones. Namely, metal cations are reported to increase the solubility of fluoroquinolones as well as to decrease it and thus cause the lowered bioavailability. Thus in this work the solubility of ciprofloxacin, norfloxacin and ofloxacin and the effect of metal cations on the solubility of these fluoroquinolones in aqueous media of different pH values were reevaluated. The results clearly show that the metal cations either do not affect or even increase the solubility of fluoroquinolones. Thus they surely do not influence the bioavailability of these drugs by decreasing their solubility. Additionally, possible explanations for the contradictory results reported in the literature are given.