This international guideline proposes improving clozapine package inserts worldwide by using ancestry-based dosing and titration. Adverse drug reaction (ADR) databases suggest that clozapine is the third most toxic drug in the United States (US), and it produces four times higher worldwide pneumonia mortality than that by agranulocytosis or myocarditis. For trough steady-state clozapine serum concentrations, the therapeutic reference range is narrow, from 350 to 600 ng/mL with the potential for toxicity and ADRs as concentrations increase. Clozapine is mainly metabolized by CYP1A2 (female non-smokers, the lowest dose; male smokers, the highest dose). Poor metabolizer status through phenotypic conversion is associated with co-prescription of inhibitors (including oral contraceptives and valproate), obesity, or inflammation with C-reactive protein (CRP) elevations. The Asian population (Pakistan to Japan) or the Americas’ original inhabitants have lower CYP1A2 activity and require lower clozapine doses to reach concentrations of 350 ng/mL. In the US, daily doses of 300–600 mg/day are recommended. Slow personalized titration may prevent early ADRs (including syncope, myocarditis, and pneumonia). This guideline defines six personalized titration schedules for inpatients: 1) ancestry from Asia or the original people from the Americas with lower metabolism (obesity or valproate) needing minimum therapeutic dosages of 75–150 mg/day, 2) ancestry from Asia or the original people from the Americas with average metabolism needing 175–300 mg/day, 3) European/Western Asian ancestry with lower metabolism (obesity or valproate) needing 100–200 mg/day, 4) European/Western Asian ancestry with average metabolism needing 250–400 mg/day, 5) in the US with ancestries other than from Asia or the original people from the Americas with lower clozapine metabolism (obesity or valproate) needing 150–300 mg/day, and 6) in the US with ancestries other than from Asia or the original people from the Americas with average clozapine metabolism needing 300–600 mg/day. Baseline and weekly CRP monitoring for at least four weeks is required to identify any inflammation, including inflammation secondary to clozapine rapid titration.

In a previous study we showed that the disposition of clozapine after a single oral dose is unrelated to either debrisoquine or S-mephenytoin hydroxylation polymorphism. The same 14 healthy subjects studied in that investigation were given 150 mg of caffeine. The reciprocal of plasma clozapine AUC (0,24), was correlated with an index of the N3-demethylation of caffeine (r, = 0.84; P = 0.0024), used as a measure of cytochrome P4501A2 (CYP1A2) activity. Ni-and N7-demethylation indices of caffeine also reflect CYP1A2 activity and were also correlated with clozapine clearance (rs = 0.89 and 0.85; P = 0.0013 and 0.0023; respectively). No significant relationships with xanthine oxidase and N-acetyl transferase activity, also assessed by a caffeine test, were found. This study suggests that clozapine is metabolised by CYP1A2 to a major extent.

The polymorphic CYP2C19 enzyme metabolizes psychoactive compounds and is expressed in the adult liver and fetal brain. Previously, we demonstrated that the absence of CYP2C19 is associated with lower levels of depressive symptoms in 1472 Swedes. Conversely, transgenic mice carrying the human CYP2C19 gene (2C19TG) have shown an anxious phenotype and decrease in hippocampal volume and adult neurogenesis. The aims of this study were to: (1) examine whether the 2C19TG findings could be translated to humans, (2) evaluate the usefulness of the 2C19TG strain as a tool for preclinical screening of new antidepressants and (3) provide an insight into the molecular underpinnings of the 2C19TG phenotype. In humans, we found that the absence of CYP2C19 was associated with a bilateral hippocampal volume increase in two independent healthy cohorts (N=386 and 1032) and a lower prevalence of major depressive disorder and depression severity in African-Americans (N=3848). Moreover, genetically determined high CYP2C19 enzymatic capacity was associated with higher suicidality in depressed suicide attempters (N=209). 2C19TG mice showed high stress sensitivity, impaired hippocampal Bdnf homeostasis in stress, and more despair-like behavior in the forced swim test (FST). After the treatment with citalopram and 5-HT receptor agonist 8OH-DPAT, the reduction in immobility time in the FST was more pronounced in 2C19TG mice compared with WTs. Conversely, in the 2C19TG hippocampus, metabolic turnover of serotonin was reduced, whereas ERK1/2 and GSK3β phosphorylation was increased. Altogether, this study indicates that elevated CYP2C19 expression is associated with depressive symptoms, reduced hippocampal volume and impairment of hippocampal serotonin and BDNF homeostasis.