Purification and identification of an impurity in bulk hydrochlorothiazide

Hydrochlorothiazide (6-chloro-3,4-dihydro-2H-1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide) (HCTZ) 1 is a widely used diuretic and anti-hypertensive. Recently, the Pharmeuropa recognized a new impurity initially thought to be an HCTZ dimer 6, which consists of the active drug (HCTZ) linked via the former beta-ring methylene to a known degradate, 5-chloro-2,4-disulfamylaniline 2. In an effort to meet a new requirement, an analytical high-pressure liquid chromatography method was developed that was selective and sensitive to the subject impurity. The impurity was concentrated and purified using a combination of solid phase extraction and reverse-phase high-pressure liquid chromatography. Subsequently, the impurity has been identified as a specific HCTZ-CH2-HCTZ isomer utilizing a variety of analytical techniques, including hydrolysis, ultraviolet spectroscopy, liquid chromatography/mass spectrometry, and 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. The data resulting from the application of these analytical techniques confirm the identity of the impurity as a methylene bridged pair of HCTZ molecules; however, a total of six possible isomers 7a-f exist because of the presence of three reactive amines/sulfonamides on each HCTZ molecule. One unique molecular structure (4-[[6-chloro-3,4,-dihydro-2H-1,2,4-benzothiadiazine-7-sulfonamide-1,1-dioxide]-methyl]-chloro-3-hydro-H-1,2,4-benzothiadiazine-7-sulfonamide-1,1-dioxide) 7f was identified using two-dimensional COSY, NOESY, and TOCSY 1H NMR experiments.