Degradation kinetics of the new antibacterial fluoroquinolone derivative, orbifloxacin, in aqueous solution.

The degradation kinetics of orbifloxacin [1-cyclopropyl-5,6,8-trifluoro-1,4-dihydro-7-(cis-3,5-dimethyl-1-pipe raz inyl)-4-oxoquinoline-3-carboxylic acid] was investigated as a function of pH (1.5-10.5), temperature (100-120 degrees C) and buffer concentration (0.05-0.2 M) by means of high-performance liquid chromatography. The degradation of orbifloxacin in aqueous solution followed apparent first-order kinetics under all experimental conditions. No appreciable effect of buffer on the degradation of orbifloxacin was observed for any of the buffer species used in this study. The log k-pH profiles indicated specific-acid and specific-base catalyses and there were inflection points near pH 6 and 9 corresponding to the pKa1 and pKa2 values. From the Arrhenius plots, the activation energies for k'H, k'H2O, kH2O, k"H2O and k"OH were found to be 31.9, 36.9, 23.5, 26.5 and 19.0 kcal/mol, respectively. Arrhenius data obtained from this study showed that the degradation of orbifloxacin at room temperature was negligible at all pH values studied conditions (pH 1.5-10.5).