Simultaneous administration of antacids containing magnesium or aluminium and ciprofloxacin or other quinolones decreases the gastrointestinal absorption of those antibacterial agents. Current speculation about the mechanism of this interaction has focused on drug-cation chelation. The present study was designed to detect the protonation in solutions and the formation of the complex species at the pH levels typical of the gastrointestinal tract. It involves the study of ciprofloxacin in aqueous solutions containing Al3+ and (or) Mg2+ by combining the results of potentiometric and spectroscopic (1H nuclear magnetic resonance) techniques. Calculations were only performed for data in the range 4.5 < pH < 5.5 (pH levels typical of gastrointestinal tract) and the results of both methods are made self-consistent, assuming an equilibrium model including complex species MHL, MLOH (where H2L denotes ciprofloxacin and M is Al3+ or Mg2+); their formation constants are given.

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Gibbs energy, enthalpy and entropy for the reactions of ciprofloxacin and lomefloxacin with aluminium ion were determined. No significant differences were observed for Gibbs energy values for these fluorquinolones while significant differences were found for enthalpy and entropy. The highly positive values observed for the entropy give evidence for a strong cation-ligand interaction and point out the role of hydration in the complex formation. As the value of the entropy for the reaction with ciprofloxacin is higher than that observed for lomefloxacin one concludes that the former ligand interacts more strongly with the metal ion than the latter.