The strength of hydralazine hydrochloride (I) 10 mg tablets, containing starch as an excipient, decreases significantly with time. An investigation to determine the basis for the drop in strength showed that starch exposed to the drug exhibited fluorescence at 414 nm. As model compounds IIIa and V, which contain a triazolophthalazinyl moiety, also fluoresce at about 414 nm, it is proposed that the emission is due to a triazolophthalazine derivative (IIIc) resulting from hydralazine hydrochloride and starch. Degradation of IIIc generates s-triazolo[3,4-a]phthalazine (IIIb), and a small amount of IIIb is detected in aged tablets. IIIc in aged tablets can be converted to IIIb by acid-catalyzed hydrolysis. The quantity of IIIb determined by HPLC amounts to most of the "missing" hydralazine hydrochloride. The reaction between hydralazine hydrochloride and starch is believed to cause the unexpected drop in the strength of hydralazine hydrochloride tablets.
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