Sensitive gas-chromatographic determination of fluphenazine in human plasma

We describe a method for quantitative determination of fluphenazine in human plasma, based on gas chromatography of its trimethylsilyl derivative and the use of a structurally similar internal standard. We determined its concentrations in plasma of four patients who were receiving the drug intramuscularly as its decanoate, in doses ranging from 12.5 to 250 mg; the concentrations in the plasma of these patients ranged from 0.25 to 11.8 microgram/liter. The precision (CV) of the method over the range 2 to 17 microgram/liter was 6.8% when fluphenazine was extracted from 5 ml of plasma. Of 18 other drugs tested, only thioridazine and the cis and trans forms of flupenthixol interfere with the determination.