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High-pressure liquid chromatographic determination of chlorothiazide and hydrochlorothiazide in plasma and urine: preliminary results of clinical studies.

High-pressure liquid chromatographic procedures were developed for the determination of chlorothiazide and hydrochlorothiazide in plasma and urine. The plasma assay incorporates a preextraction procedure that eliminates interference by endogenous substances. Chromatography is carried out on an octadecyl reversed-phase column. Mobile phases are 15% methanol in 0.01 M acetic acid for plasma and 4% acetonitrile in 0.01 M sodium perchlorate, adjusted to pH 4.6, for urine. At a flow rate of 2.5 ml/min, the retention times for chlorothiazide and hydrochlorothiazide are 3.5 and 4.6 min for plasma and 10.5 and 13.5 min for urine, respectively. Preliminary results of a clinical study in fasting male volunteers showed that the plasma levels and urinary excretion rate of chlorothiazide peaked at 1-2 hr following a 500-mg oral dose and subsequently declined irregularly. On the other hand, the plasma levels and urinary excretion rate of hydrochlorothiazide peaked at 2-3 hr following a 50-mg oral dose and subsequently declined in biphasic fashion. Urinary excretion rates of both chlorothiazide and hydrochlorothiazide closely resemble their concentration profiles in plasma.