Introduction:Inter-individual variability in clozapine levels poses a challenge for treatment optimization. Norclozapine, active metabolite, is a valuable marker for assessing exposure, adherence, efficacy, and side effects. Therapeutic drug monitoring (TDM) of both clozapine and norclozapine is recommended for dose adjustments and optimal treatment. Limited attention has been given to the interaction between total duration (TD) and norclozapine levels.

Research Question or Hypothesis:Are there any association between daily dose (DD), age, gender,TD, clozapine and norclozapine serum levels in patients with schizophrenia spectrum disorders (SSD)?

Study Design:Single-center, 7-years retrospective cohort study.

Methods:Clozapine and norclozapine level assessments served as a basis for subsequent multivariate modeling, including variables of age, gender, and TD. The data were analyzed using R(v4.2.2), and linear mixed-effects models were constructed using lme4 (v 1.1.31) and lmerTest (v 3.1.3) packages. Fixed effects of age, gender and TD on clozapine and norclozapine levels were hierarchically modeled. Nested approach within patients (random-effect) was used.

Results:Total of 220 patients, 87(48.3%female) had accurate TD information available and were included in analysis. Total of 316 observations of serum concentrations of clozapine and norclozapine were analyzed. Significant positive association was found between DD and clozapine serum levels (β=0.8645, SE=0.1680,p<0.001), while age, gender, and TD showed no significant effects on serum levels. For norclozapine levels, significant positive association was found between TD and norclozapine serum levels(β=4.0495,SE=1.6819,p=0.0182) as well as DD. Clozapine/norclozapine ratio (C/N) showed significant differences between genders, with females having a higher ratio (β=3.554,SE=0.620,p<5.40e-08) compared to males(β=3.207,SE=0.583,p<1.51e-07), while DD, age, and TD of use had no significant effects on the ratio.

Conclusion:TD significantly affects only norclozapine level, while C/N is influenced by gender independently of age, dosage, and TD. Therefore, it may be speculated that C/N has limited benefit in guiding clinical decision-making. Monitoring norclozapine levels may be more beneficial for TDM, as it reflects the effects of clinical variables like TD and DD effectively.