Stability of aqueous solutions of amoxicillin sodium in the frozen and liquid states.

The stability of aqueous admixtures of amoxicillin sodium in both the liquid and frozen (solid) states was studied. Admixtures of amoxicillin sodium were prepared in sterile water for injection to a theoretical concentration of 10 mg/mL. For each experimental run, 2-mL aliquots of the admixture were placed in stoppered glass volumetric flasks and stored at temperatures ranging from 19.5 degrees C to -30 degrees C; 16 flasks were stored at each temperature. After equilibration for approximately 20 minutes, duplicate flasks at each temperature were removed from storage conditions for time-zero assay. Subsequently, duplicate flasks were assayed at various times, depending on the storage temperature, for up to 13 days or until more than 80% of the drug had degraded. All samples were assayed at least in duplicate using high-performance liquid chromatography. When amoxicillin solutions were in the liquid state (at temperatures between 19.5 and 0 degrees C), the time required for the amoxicillin concentration to decrease to 90% of its initial value (t90) increased as temperature decreased. However, between 0 degree C and -7 degrees C, the t90 of frozen solutions decreased from two days to 1.08 hours. As temperature declined further, the rate of degradation decreased until the solution was completely frozen; at -30 degrees C, the t90 had increased to 13 days. Amoxicillin sodium is unstable in aqueous solutions stored between 0 degrees C and -20 degrees C. If admixtures of this drug are to be frozen for later use, the storage temperature should be below -30 degrees C.