Coupled liquid-liquid extraction and column switching LC for the determination of a new antihistaminic H1 drug in human urine

Summary Mizolastine (SL 85.0324) is a new antihistaminic H₁ benzimidazole derivative which is excreted into urine almost completely metabolized; about 2% of the unchanged drug is excreted as conjugated compound which requires enzymatic deconjugation before analysis. Since the existing methods for plasma samples do not work on deconjugated human urine due to interferences, a new method was developed. The method is based on a diethyl-ether extraction of mizolastine and an internal standard from alkalinized urine. The ether extract is back-extracted with an aqueous buffer (pH=2.6), this extract is neutralized (pH=6.5) and an aliquot injected into a C₁₈ pre-column where clean up and preconcentration take place. The analytes are then desorbed from the pre-column and transferred to the analytical column. The analytical column is a C₁₈ type specially seactivated for basic compounds with an eluent of acetonitrile/phosphate solution (pH=4.5), 40/60, v/v, at a flow rate of 1 ml min^–1. Detection is at 285 nm. The method is linear in the range 10–500 ng ml^–1 with a lower limit of detection of 10 ng ml^–1. The precision and accuracy, evaluated during intra-day and inter-day assays, are satisfactory for pharmacokinetic investigations.