Stability of 1% rifampin suspensions prepared in five syrups

The stability and microbiological activity (potency) of rifampin in 1% w/v suspensions prepared extemporaneously using five syrups was studied. The contents of four 300-mg rifampin capsules (Rifadin, Merrell Dow Pharmaceuticals Inc.) were crushed and placed in amber glass bottles. The rifampin powder was combined with 120 mL of each of the following syrups: Syrup, NF; two commercially available simple syrups (Humco Laboratories and Whiteworth, Inc.), wild cherry syrup (Eli Lilly and Company), and a fruit-flavored syrup (Syrpalta). The bottles were then stored at room temperature or under refrigeration for six weeks. Portions of each suspension were removed and assayed using high-performance liquid chromatography (HPLC) and a microbiological assay technique initially and after two, four, and six weeks of storage. No significant changes in rifampin concentrations were detected by either assay technique over the study period. However, an increase in the concentration of rifampin degradation products was detected by HPLC assay, especially between the fourth and sixth weeks. A 1% w/v suspension of rifampin prepared using Rifadin capsules and any of the five syrups studied is stable for four weeks at room temperature or under refrigeration.