Low neuroleptic serum levels in patients receiving fluphenazine decanoate

The authors monitored serum levels of fluphenazine in nine patients following injections of fluphenazine decanoate ranging from 10 to 75 mg. Levels were detected by a radioreceptor assay based on the ability of the drug to compete with ³H-spiroperidol for binding to dopamine receptors in rat caudate membranes. Serum levels of fluphenazine were quite stable over a 2- to 3-week period following single intramuscular injections of the decanoate and correlated with injected dose. Following decanoate treatment serum levels of fluphenazine are substantially lower than levels observed for most other neuroleptics administered orally. This raises questions as to how fluphenazine decanoate can exert therapeutic actions.