Stability of metronidazole benzoate in suspensions

A stability-indicating HPLC assay method has been developed to quantify metronidazole benzoate in suspensions. A study of the stabilities of two suspensions (16.0 mg/ml) of metronidazole benzoate in commercially available vehicles, Ora-Plus and a mixture of Ora-Plus and Ora-Sweet, showed that both suspensions were stable for at least 90 days at room temperature. The mobile phase required to elute metronidazole benzoate contained 40% acetonitrile versus approximately 10% for the free base. The solubility of the ester in water at 25 degrees C was found to be approximately 0.1 mg/ml versus 10 mg/ml for the free base. The ester did not hydrolyse significantly to the free base after storage for 8 h at 37 degrees C in simulated gastric fluid, and 5 h at 37 degrees C in simulated intestinal fluid. The bitter tasting metronidazole may not be satisfactorily substituted with its tasteless ester in the treatment of local gastrointestinal infections.