Influence of pH on hydrolytic decomposition of diethylpropion hydrochloride: stability studies on drug substance and tablets using high-performance liquid chromatography.

The influence of pH on the hydrolysis of diethylpropion hydrochloride was investigated. Hydrolytic decomposition of the drug in solution at 45 degrees occurred at a very slow and constant rate at pH 3.5 and below but increased rapidly as the pH was raised above 3.5. Results of stability studies performed on six commercial tablet products from five manufacturers showed a similar relationship between the decomposition rate and the formulation pH. A reaction pathway based on the formation of an enamine structure is proposed for the pH-dependent hydrolysis. A high-performance liquid chromatographic assay for the drug is presented, which also separates and detects the hydrolysis product 1-phenyl-1,2-propanedione and the compound assumed to be the enol tautomer of this product.