Measurement of duloxetine in blood using high-performance liquid chromatography with spectrophotometric detection and column switching.

A method using high-performance liquid chromatography (HPLC) with column switching and ultraviolet (UV) spectroscopy was developed for the determination of duloxetine in human plasma. After centrifugation and addition of venlafaxine as internal standard, plasma samples were injected into the HPLC system and precleaned on a column (10 x 4.0 mm) filled with cyanopropyl (CN)-modified silica of 20 microm particle size, with use of 8% (vol/vol) acetonitrile in deionized water as eluent. Duloxetine was eluted and separated on a LiChrospher 100 CN (5-microm particle size; column size, 250 x 4.6 mm I.D.) using acetonitrile-water-potassium dihydrogenphosphate trihydrate buffer (pH, 6.4; 50:50 vol/vol) and detected at 218 nm. Duloxetine could be analyzed within 30 minutes. The limit of quantification was 5 ng/mL. At duloxetine concentrations up to 138 ng/mL that resulted from therapeutic doses of 30 to 120 mg per day, the interassay reproducibility of quality control samples was better than 12%. The method was found to be robust and stable. With the exception of chlorprothixene and desmethylclomipramine, other drugs that may be used as comedication were not found to exhibit retention times similar to duloxetine. In serum samples from 37 patients treated with 30 to 120 mg per day for at least 7 days, the mean steady state serum concentration of duloxetine was 40 ng/mL, the median was 37 ng/mL, and the 25th and 75th percentiles were 22 and 55 ng/mL, respectively. At 60, 90, and 120 mg/day, the mean +/- SD serum concentrations were 33+/-22.0, 43+/-22.2, and 48+/-17.0 ng/mL, respectively. There was a statistically significant correlation (P<0.05, r2=0.26) between prescribed daily doses and serum concentrations of duloxetine. In patients without or with comedication with other drugs, such as inhibitors of cytochrome P450 2D6 (eg, metoprolol or propranolol), serum concentrations of duloxetine were not significantly different. HPLC with column switching and ultraviolet detection as described here is suitable for pharmacokinetic studies and therapeutic drug monitoring of duloxetine.