Quantification of midodrine and its active metabolite in plasma using a high performance liquid chromatography column switching technique

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Abstract
An automated column‐switching HPLC system is described for the simultaneous determination of midodrine, an alpha‐adrenergic stimulating drug, and its active metabolite, ST‐1059. Serum or plasma (850 μ L) is directly injected onto a RP 18 (30 μm particle size) pre‐column (9 × 4 mm ID) which acts as an on‐line liquid‐solid extractor and analyte enrichment system. The injection is followed by washing steps. The fraction containing the analytes is transferred onto an analytical RP18 column via step gradient elution where the final analysis is performed. Fluorescence detection is used (λex 290 nm and λem 322 nm), and method detection limits of 0.8 ng/mL plasma were reached. These were sufficiently low to determine the plasma concentration‐time profiles for both compounds following oral administration of 2.5 mg and 5 mg midodrine hydrochloride. The assay in serum or plasma was linear in the range of 1 to 15 ng analyte/mL, the recovery was >95%, and the reproducibility was sufficient. The assay was rugged and was maintained by routinely changing the home‐made, dry packed pre‐column every 20th serum injection.

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