Sequential electrochemical reduction, solvent partition, and automated thiol colorimetry for urinary captopril and...

Analysis of urinary captopril was necessary for dosage form bioavailability and dose titration studies. The necessity for long-term storage of samples prior to analysis and the presence of an oxidation-prone thiol of captopril required development of an acid-chelate stabilization method for urinary captopril. An electrochemical reduction released disulfide-conjugated captopril for thiol colorimetry. Of several rugged reduction cells evaluated, one with a porous glass disk separating the anode and the mercury pool cathode was preferred. Methylene chloride partitions from acidified salt-saturated urines, before and after reduction, allowed the measurement of free and disulfide-conjugated captopril. The drug partitioned into the solvent, whereas the aqueous phase retained acid protonated, amino group-bearing thiols like cystine. Subsequent solvent evaporation volatilized other potential colorimetric interferences. An automated thiol colorimetry of 25 samples/hr was developed for analysis of the aqueous reconstitutes. Results were confirmed by a subsequently developed HPLC method with electrochemical detection.