Body mass index as a determinant of clozapine plasma concentrations: A pharmacokinetic-based hypothesis

Knowledge regarding the impact of body composition measures on pharmacokinetics of antipsychotics is limited.

Our aim was to investigate the impact of body weight and body mass index on clozapine pharmacokinetics using a therapeutic drug monitoring database.

A large therapeutic drug monitoring dataset of clozapine plasma concentrations considering three patient subgroups was analysed: a control group (CLZ₀, 20-30 kg/m², n=266), a group with high body mass index (CLZ_high, body mass index ⩾30 kg/m², n=162) and with low body mass index values (CLZ_low, body mass index <20 kg/m², n=27). Comparisons of plasma and dose-adjusted plasma concentrations (C/D) of clozapine were based on the Spearman's correlation (rs), Kruskal Wallis and Mann-Whitney-U tests. For percentages we used the Pearson chi-square test (χ²). To assess effects of confounders we used bootstrapping analysis of covariates.

Regarding demographic characteristics, groups differed only for sex percentage with more females than males in CLZ_low and CLZ_high compared to CLZ₀ (p=0.001 for χ² test). Plasma and C/D values were positively associated with body mass index (rs=0.108, p=0.022 and rs=0.156, p=0.001 respectively). Intergroup differences were observed for plasma and dose-adjusted concentrations of clozapine (p=0.031 and p=0.029 for Kruskal Wallis respectively): post-hoc pairwise comparisons showed higher plasma concentrations and C/D of clozapine in CLZ_high compared to CLZ₀ (p=0.014 and p=0.007 respectively for Mann-Whitney U-test), by mean 21 and 18%, respectively. Differences for C/D values remained after accounting for sex (p=0.02).

In obese patients, bioavailability, distribution or elimination of clozapine may be altered due to increased clozapine deposits in fat tissue and hepatic enzyme activity changes.