Stability of allopurinol suspension compounded from tablets

The stability of allopurinol as an extemporaneous suspension compounded form tablets was studied. The allopurinol suspension (20 mg/mL) was prepared by incorporating pulverized 300-mg allopurinol tablets into the suspending vehicle containing sodium carboxymethylcellulose and magnesium aluminum silicate (Veegum). Lycasin (a syrup vehicle that is 75% maltilol was added to this mixture. Appropriate antioxidants, preservatives, sweeteners and flavoring agents also were used. Samples of the suspension were stored in amber-colored glass bottles at 50 deg, 60 deg, 70 deg, and 80 deg C. At various times during the 97-day study period, the concentration of allopurinol in each sample was determined by a stability-indicating high performance liquid chromatography assay procedure. At the same time, samples were inspected visually for signs of caking or settling and evaluated for redispersibility and pourability. The aqueous solubility of allopurinol as a function of temperature also was studied. It was found that the aqueous solubility of allopurinol increased with an increase in the temperature. A zero-order reaction was assumed for the suspension. The kinetics of degradation were determined and the energy of activation and shelf-life were calculated using the Arrhenius plot. During the study period all samples remained homogenous and showed no signs of caking or settling. The allopurinol suspension compounded from tablets was found to be pharmaceutically acceptable and easily pourable and redispersible. The energy of activation for the suspension was found to be 21.92 kcal/mol. The shelf-life (t90) of the suspension was found to be 8.3 years at room temperature.