Metabolite Profiling of Clozapine in Patients Switching Versus Maintaining Treatment: A Retrospective Pilot Study.

PURPOSE/BACKGROUND: Pharmacokinetics may be of relevance for the risk of clozapine discontinuation. We compared metabolite profiles, accounting for smoking habits, in patients switching versus maintaining clozapine treatment at therapeutic concentrations. METHODS/PROCEDURES: Adult patients with clozapine serum levels above 1070 nmol/L (350 ng/mL) were retrospectively included from a Norwegian therapeutic drug monitoring service during 2018-2020. Inclusion criteria were (1) known smoking habits, (2) blood sample drawn within 10 to 30 hours after last clozapine intake, and (3) detectable levels of N -desmethylclozapine, clozapine -N -oxide, clozapine-5 N -glucuronide, or clozapine- N + - glucuronide. Patients comedicated with cytochrome P450 enzyme inducers, inhibitors, or valproic acid were excluded. The high-resolution mass spectrometry assay enabled detection of 21 clozapine metabolites. Metabolite profiles were compared between patients switching treatment (switchers), measured as clozapine being replaced by another antipsychotic drug in blood samples, versus maintaining clozapine treatment (nonswitchers) during the study period. FINDINGS/RESULTS: Of the 84 patients fulfilling the study criteria, 7 patients (8.3%) were identified as clozapine switchers. After correcting for smoking habits, the clozapine-5 N -glucuronide/clozapine ratio was 69% lower ( P < 0.001), while the clozapine- N + -glucuronide/clozapine-5 N -glucuronide ratio was 143% higher ( P = 0.026), respectively, in switchers versus nonswitchers. The other metabolite ratios did not significantly differ between switchers and nonswitchers. IMPLICATIONS/CONCLUSIONS: The present study found a significantly reduced 5 N -glucuronidation phenotype in patients switching from clozapine at therapeutic serum concentrations (>1070 nmol/L) to other antipsychotic drugs. This may indicate that glucuronidation, as a potential detoxification mechanism, is related to clozapine tolerability. However, the causality of this observation needs to be investigated in future studies with larger patient populations.

Effect of Valproic Acid on the Metabolic Spectrum of Clozapine in Patients With Schizophrenia.

BACKGROUND: Valproic acid (VPA) is frequently used with clozapine (CLZ) as mood stabilizer and/or seizure prophylaxis. Valproic acid is known to reduce N-desmethylclozapine (N-DMC) but not CLZ levels. This leads to the hypothesis that VPA induces the CLZ metabolism via non-N-desmethylation pathways. Therefore, we aimed to investigate the effect of concurrent VPA use on the serum concentrations of a spectrum of CLZ metabolites in patients, adjusting for smoking. METHODS: In total, 288 patients with an overall number of 737 serum concentration measurements of CLZ and metabolites concurrently using VPA (cases, n = 22) or no interacting drugs (controls, n = 266) were included from a routine therapeutic drug monitoring service. Linear mixed model analyses were performed to compare the dose-adjusted concentrations (C/D) of CLZ, N-DMC, CLZ 5N/N+-glucuronides, and metabolite-to-parent ratios in cases versus controls. RESULTS: After adjusting for covariates, the N-DMC (-40%, P < 0.001) and N+-glucuronide C/Ds (-78%, P < 0.001) were reduced in cases versus controls, while the CLZ C/D was unchanged (P > 0.7). In contrast, the 5N-glucuronide C/D (+250%, P < 0.001) and 5N-glucuronide-to-CLZ ratios (+120%, P = 0.01) were increased in cases versus controls. CONCLUSIONS: Our findings show that complex changes in CLZ metabolism underly the pharmacokinetic interaction with VPA. The lower levels of N-DMC seem to be caused by VPA-mediated induction of CLZ 5N-glucuronide formation, subsequently leading to reduced substrate availability for N-desmethylation. Whether the changes in CLZ metabolism caused by VPA affects the clinical outcome warrants further investigation.

Reduction in N-Desmethylclozapine Level Is Determined by Daily Dose But Not Serum Concentration of Valproic Acid-Indications of a Presystemic Interaction Mechanism.

BACKGROUND: Valproic acid (VPA) is frequently used together with clozapine (CLZ) as mood-stabilizer or for the prevention of seizures in patients with psychotic disorders. VPA is known to reduce levels of the pharmacologically active CLZ-metabolite N-desmethylclozapine (N-DMC), but factors determining the degree of this interaction are unknown. Here, we investigated the relationship between VPA dose and serum concentration on N-DMC levels in a large patient population adjusting for sex, age, and smoking habits as covariates. METHODS: A total of 763 patients with steady-state serum concentrations of CLZ and N-DMC concurrently using VPA (cases, n = 76) or no interacting drugs (controls, n = 687) were retrospectively included from a therapeutic drug monitoring service at Diakonhjemmet Hospital, Oslo, between March 2005 and December 2016. In addition to information about prescribed doses, age, sex, smoking habits, and use of other interacting drugs were obtained. The effects of VPA dose and serum concentration on dose-adjusted N-DMC levels were evaluated by univariate correlation and multivariate linear mixed-model analyses adjusting for covariates. RESULTS: The dose-adjusted N-DMC levels were approximately 38% lower in VPA users (cases) versus nonusers (controls) (P < 0.001). Within the VPA cases, a negatively correlation between VPA dose and dose-adjusted N-DMC levels was observed with an estimated reduction of 1.42% per 100-mg VPA dose (P = 0.033) after adjusting for sex, age, and smoking. By contrast, there was no correlation between VPA serum concentration and dose-adjusted N-DMC levels (P = 0.873). CONCLUSIONS: The study shows that VPA dose, not concentration, is of relevance for the degree of reduction in N-DMC level in clozapine-treated patients. Presystemic induction of UGT enzymes or efflux transporters might underlie the reduction in N-DMC level during concurrent use of VPA. Our findings indicate that a VPA daily dose of 1500 mg or higher provides a further 21% reduction in N-DMC concentration. This is likely a relevant change in the exposure of this active metabolite where low levels are associated with implications of CLZ therapy.

Absolute and Dose-Adjusted Serum Concentrations of Clozapine in Patients Switching vs. Maintaining Treatment: An Observational Study of 1979 Patients.

BACKGROUND: Clozapine is an effective drug for the management of schizophrenia that has not responded to other agents, but some patients experience insufficient or adverse effects and discontinue treatment. OBJECTIVE: We investigated a potential association between clozapine serum concentrations and switching to other antipsychotics in a large real-world patient population from a therapeutic drug monitoring service. METHODS: Absolute and dose-adjusted serum concentrations (concentration-to-dose ratios [C/D ratios]) of clozapine during dosing between 100 and 1000 mg/day were measured in 1979 Norwegian patients during the period 2005-2019. These variables were compared in patients switching to other antipsychotic drugs versus maintaining clozapine treatment using linear mixed models. Smoking habits were known for 49% of the patients. To prevent potential nonadherence affecting clozapine switching, only patients with serum concentrations above 50% of the lower reference range were included. RESULTS: In total, 190 patients (9.6%) switched from clozapine to another antipsychotic drug during the study period, whereas the remaining patients were not detected as switchers and were interpreted as maintaining treatment. Patients switching treatment had 23.5% lower absolute concentrations (954 vs. 1245 nmol/L; p < 0.001) and 15.7% lower daily doses (305 vs. 362 mg/day; p < 0.001) of clozapine than did nonswitchers, making the clozapine C/D ratio 9.7% lower in switchers than in nonswitchers after correcting for smoking habits (2.80 vs. 3.10 nmol/L/mg/day; p = 0.032). CONCLUSIONS: The present study suggests that decreased absolute and dose-adjusted serum concentrations of clozapine were associated with clozapine discontinuation. The significantly reduced clozapine concentrations regardless of prescribed dose in switchers versus nonswitchers may indicate a pharmacokinetic mechanism underlying the risk of clozapine discontinuation.

Rates of complete nonadherence among atypical antipsychotic drugs: A study using blood samples from 13,217 outpatients with psychotic disorders.

BACKGROUND: Nonadherence to antipsychotics may cause relapse and hospitalizations in patients with psychotic disorders. The purpose was to quantify and compare the outpatient's nonadherence rates of atypical antipsychotics by objective detection in blood samples. METHODS: Totally, 13,217 outpatients with therapeutic drug monitoring (TDM) data of atypical antipsychotics were included. An event of complete nonadherence was defined as an occurrence of undetectable level of a prescribed antipsychotic in the blood sample submitted for TDM. Patients with such an event(s) were defined as nonadherent of the respective drug treatment (outcome). The rates of nonadherence patients were compared between the drugs by logistic regression. RESULTS: In the study population, 70.2% of the patients were prescribed doses compliant with a schizophrenia diagnosis. The mean olanzapine equivalent dose in the population was 13.4 mg (95% confidence interval (CI): 13.3, 13.6). The frequency of nonadherence patients, regardless of drug, was 3.7% (CI: 3.4-4.0). The nonadherence patient rate was lowest in clozapine-treated patients (2.2%; CI: 1.5-2.8), followed by aripiprazole (2.3%; 1.7-2.8), risperidone (2.4%; 1.6-3.0), quetiapine (2.8%; 2.3-3.2) and olanzapine (4.9%; 4.1-5.3). Users of olanzapine had significantly higher risk of complete nonadherence (Odds ratio: 1.9; CI: 1.6-2.3, p < 0.001) compared to patients treated with other antipsychotics as a group. CONCLUSIONS: In this study, complete nonadherence of atypical antipsychotics, measured as undetectable blood level, was disclosed for ~5% of outpatients with psychotic disorders. The rate of complete nonadherence was significantly higher during olanzapine treatment compared to other atypical antipsychotics. Further studies should investigate if this reflects drug differences in tolerability or other causal relationships.

Undetectable or subtherapeutic serum levels of antipsychotic drugs preceding switch to clozapine.

Adequate antipsychotic treatment intensity is required for defining treatment-resistant schizophrenia (TRS) and justifying clozapine treatment. We investigated the occurrence of undetectable or subtherapeutic serum levels of oral antipsychotics preceding switch to clozapine as an endpoint of TRS. For patients starting clozapine, 12-month retrospective reviews of antipsychotic serum concentration measurements were performed in a Norwegian therapeutic drug monitoring (TDM) database from 2005 to 2017. Undetectable levels in high-sensitive analytical assays defined 'no drug use', while levels <50% of the lower reference range defined 'subtherapeutic use'. Similar data were collected for patients switching to long-acting injectable (LAI) antipsychotics, as a reference of 'no or subtherapeutic drug use'. Nineteen of 353 patients initiating clozapine (5.4%) had a recent history of undetectable antipsychotic drug levels compared to 91 of 1048 (8.7%) initiating LAI. Another 19 patients starting clozapine (5.4%) had recent events of subtherapeutic levels. In conclusion, the present retrospective study may indicate that every 10th patient starting clozapine has a recent history of undetectable or subtherapeutic serum levels of oral antipsychotics. The clinical implications of the present study for the assessment of TRS should be investigated prospectively in further studies.

Impact of antipsychotic polypharmacy on nonadherence of oral antipsychotic drugs - A study based on blood sample analyses from 24,239 patients.

Nonadherence to oral antipsychotic drugs is a major issue in clinical psychiatry giving rise to treatment failure. Further, polypharmacy is common in the treatment of psychotic disorders due to insufficient treatment effect during monotherapy. As a potential circuit problem, we hypothesized that antipsychotic polypharmacy is associated with increased risk of nonadherence. To investigate this, in terms of 'complete' nonadherence, the rates of undetectable serum drug concentrations during prescribing of doses used in psychotic disorders were compared during antipsychotic 'monotherapy' vs 'polypharmacy' treatment using therapeutic drug monitoring (TDM) data of 24,239 patients. A complete nonadherence patient was objectively defined as the detection of at least one event of undetectable serum concentration of a prescribed antipsychotic drug. The rate of complete nonadherence patients was compared between antipsychotic monotherapy and polypharmacy by multivariate logistic regression analyses. The overall rate of complete nonadherence in the population was 6.8% (n = 1,644; 95%CI: 6.5-7.1). Compared to monotherapy patients, the rate of nonadherence increased significantly with the number of co-prescribed antipsychotic drugs. After adjusting for sex (p = 0.091) and age (p < 0.001) as covariates, the rates of nonadherence vs monotherapy were 1.69-fold (95% CI: 1.48-1.92; p < 0.001) for two, 2.60-fold (95% CI: 1.88-3.59; p < 0.001) for three, and 3.54-fold (95% CI: 1.46-8.58; p = 0.005) for four or more co-prescribed antipsychotics, respectively. The present naturalistic study shows that antipsychotic polypharmacy significantly increases the rate of complete nonadherence, which is positively correlated with increasing number of concurrently used antipsychotic drugs. Thus, the intended clinical benefit of combining oral antipsychotic drugs may probably be reduced by increased nonadherence.