Determination of 8-methoxypsoralen in suction-blister fluid and serum by liquid chromatography

A method is described for determination of 8-methoxypsoralen to 0.2 mL of suction-blister fluid or 1 mL of serum from psoriatic patients being treated with this drug. The drug is extracted from the biological matrix at pH 9.0 with a mixture of dichloromethane and light petroleum ether. 5-Methoxypsoralen is used as internal standard. Separation and quantitation are performed on a "high-performance" liquid chromatograph with use of an RP 18 reversed-phase column and detection at 245 nm. Accuracy and precision are good. Some benzodiazepines and their metabolites interfere. The lowest detectable concentration is 10 microgram/L, which means that the method is sufficiently sensitive to measure the drug concentrations in serum and suction-blister fluid for pharmacokinetic studies in patients being treated with a therapeutic dosage.