Effect of β-Cyclodextrin on the Degradation Rate of Amoxicillin in Acidic Solution

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The formation of an inclusion complex of amoxicillin (AMX) with β-cyclodextrin (β-CD) in aqueous solution was confirmed by a solubility method and proton nuclear magnetic resonance (1H-NMR) spectroscopy. The apparent stability constant for the inclusion complex was 10.72 M-1 in water at 25°C. The effect of α-CD, β-CD, and γ-CD on the degradation of AMX in a pH 1.2 solution at 37°C was investigated. β-CD and γ-CD reduced the rate of degradation. α-CD had no effect. These results were consistent with those of 1H-NMR spectroscopy. The effect of β-CD on the degradation rate was studied in more detail. The apparent first order rate constant for the degradation of AMX in the pH 1.2 solution at 37°C was 0.1121 h-1 (t1/2=6.18 h), which decreased with the addition of β-CD. The rate constants and t1/2 values for the concentrations of β-CD added, corresponding to molar ratios of AMX to β-CD of 1:0.5, 1:1, 1:2, 1:5, and 1:10, were 0.1051 h-1 and 6.59 h, 0.0992 h-1 and 6.98 h, 0.0893 h-1 and 7.76 h, 0.0697 h-1 and 9.95 h, and 0.0509 h-1 and 13.61 h, respectively. The activation energy for the degradation of AMX in the pH 1.2 solution was increased from 6.9×104 J/mol (AMX alone) to 8.0×104 J/mol (AMX:β-CD=1:10).