Plasma concentrations of quetiapine, N-desalkylquetiapine, o-desalkylquetiapine, 7-hydroxyquetiapine, and quetiapine sulfoxide in relation to quetiapine dose, formulation, and other factors.

N-Desalkylquetiapine may be a pharmacologically active quetiapine metabolite. However, information on plasma concentrations of N-desalkylquetiapine and other quetiapine metabolites attained during quetiapine therapy is scant. The aim of this study was to investigate plasma concentrations of quetiapine, N-desalkylquetiapine, O-desalkylquetiapine, 7-hydroxyquetiapine, and quetiapine sulfoxide attained during therapy and analyze the data with respect to prescribed dose and other variables. Quetiapine and its metabolites were measured in plasma samples submitted for quetiapine therapeutic drug monitoring (2009-2011). Concentration, metabolic ratio, and concentration corrected for dose (C/D) were investigated against quetiapine dose, age, sex, and formulation. Sample results were excluded if nonadherence with therapy was queried. There were 99 samples from 59 patients. N-Desalkylquetiapine plasma concentrations showed the strongest correlation with dose of all analytes, but O-desalkylquetiapine and quetiapine sulfoxide were strongly correlated to plasma quetiapine concentrations. There was no significant difference in C/D for any analyte between males and females and no correlation to age. Quetiapine and quetiapine sulfoxide C/D were significantly different (P < 0.01) between patients prescribed immediate- and extended-release formulations. Quetiapine, 7-hydroxyquetiapine and quetiapine sulfoxide C/D showed significant variation (P < 0.02) between those samples taken 10-14 hours postdose as compared with that of 16-24 hours postdose, but there was no significant effect as regards N-desalkylquetiapine. Plasma quetiapine, O-desalkylquetiapine, 7-hydroxyquetiapine, and quetiapine sulfoxide concentrations were significantly affected by formulation and/or time since last dose. Plasma N-desalkylquetiapine concentrations were not affected by either factor therefore may be a better marker for quetiapine exposure than plasma quetiapine concentrations.