Serum concentrations of thioridazine, its major metabolites and serum neuroleptic‐like activities in schizophrenics with and without tardive dyskinesia

Two groups of elderly chronic schizophrenic patients, one with and one without tardive dyskinesia (TD), were studied. In addition to estimation of the neuroleptic‐like radioreceptor activity (RRA) of the serum, serum concentrations of thioridazine (THD) and its major metabolites, THD‐2‐sulfoxide, THD‐2‐sulfone and THD‐5‐oxide, were measured with high performance liquid chromatography. Blood samples were collected at two different occasions when the patients were considered to be in a pharmacological “steady state”. The concentrations of THD and its metabolites, as well as RRA values, were similar at both occasions. In general, THD‐2‐sulfoxide and THD‐5‐oxide were present in higher concentrations than the parent compounds, and the serum concentrations of THD, its metabolites and the RRA values were positively correlated with the administered daily dose of THD. However, no significant difference in serum levels of THD, its metabolites, or RRA values existed between the patients with and without TD. These data suggest that pharmacodynamic rather than pharmacokinetic differences may be more important in producing high risk for TD in patients on long‐term neuroleptic treatment.