Measurement of blood levels of neuroleptics and metabolites by combined high performance liquid chromatography-radioreceptor assay for D2 and sigma sites.

The dopamine (D2) receptor blocking property of antipsychotic medications has been proposed as the mechanism of the therapeutic activity of this class of drugs. This property has also been exploited as a method to quantify therapeutic levels of these drugs in patients. However, the lack of correlation among dosage, blood levels and clinical response has resulted in a contradictory literature on both mechanism and quantification of these drugs. Bioactivity and chemical identity of the commonly prescribed neuroleptic drug fluphenazine and its metabolites in human plasma were determined by a new method which combines the selectivity of chemical methods with the sensitivity and bioassay of the radioreceptor assay (RRA) method. Fluphenazine and its metabolites were separated and identified in human plasma by an ion-pairing reverse phase high performance liquid chromatographic method with electrochemical detection. A volatile buffer system was employed which was compatible with facile sample preparation for post-column analyses, and which provided sharp, symmetrical chromatographic peaks of parent compound and metabolites. Post chromatography, HPLC fractions were assayed by RRA for D2, alpha 1 and sigma receptors. More than one pattern of metabolism of the drug was seen, including biosynthesis of drug metabolites with biological activities at these receptor types. The individual differences with which this occurs may contribute to the variabilities seen in clinical response to neuroleptics, and to difficulties in neuroleptic blood level determinations.