Pharmacokinetic profile of long-acting injectable risperidone at steady-state: comparison with oral administration

Consideration of Long-Acting Injectable Antipsychotics for Polypharmacy Regimen in the Treatment of Schizophrenia: Put It on the Table or Not?

Antipsychotic monotherapy (APM) is considered best-acceptable treatment option regardless of antipsychotic class and formulation types for treating schizophrenia. However, antipsychotic polypharmacy (APP) has been also widely utilized in routine clinical practice. Despite APP has some clinical benefits it has also numerous pitfalls in relation with increased total number and doses of APs leading to adverse events as well as decrease of treatment adherence and persistence resulting in poor clinical outcomes. Recent introduction of long-acting injectable antipsychotics (LAIs) to the market has offered a chance for better medication adherence/persistence and also provided a simplification of treatment regime leading to more stabilized treatment for schizophrenia patients. When we cannot stay away from APP in the treatment of schizophrenia, clinicians need to find more proper APP regimens and thereby utilization of APP in efficient way should be a practical strategy to benefit schizophrenia patient in a real world treatment setting. With this regard, LAIs can be one of available APP regimen for treatment of schizophrenia in routine practice since their clinical utility and pharmacokinetic stability over oral APs have been well-elaborated today. However, when we have to commence LAIs as a part of APP with oral APs or other LAIs, every effort should be made before doing so whether or not validated and available treatment options or other clinical factors were not done or evaluated yet. Any treatment guidelines do not support APP regardless of the formulation of APP regimen or address two or more LAIs for treatment of schizophrenia till today.

Modeling Complex Pharmacokinetics of Long-Acting Injectable Products Using Convolution-Based Models With Nonparametric Input Functions

The interest in the development and the therapeutic use of long-acting injectable (LAI) products for chronic or long-term treatments has grown exponentially. The complexity and the multiphase drug release process represent serious issues for an effective modeling of the PK properties of LAI products. The objective of this article is to show how convolution-based models with piecewise-linear approximation of the nonlinear drug release function can provide an enhanced modeling tool for (1) characterizing the complex PK profiles of LAI formulations with completely different drug release properties, and (2) addressing key questions supporting the optimal development of LAI products by simulating the PK time course resulting from different dosing strategies. Convolution-based modeling and simulation were implemented in NONMEM, and 3 case studies were presented to assess the performances of this new modeling approach using PK data of LAI products developed using different technologies and administered using different routes: microsphere technology and aqueous nanosuspension intramuscularly administered and biodegradable polymer subcutaneously administered. The performance of the convolution-based modeling approach was compared with the performance of conventional parametric models using a reference data set on theophylline. The results of the comparison indicated that the nonparametric input function provided a more accurate description of the data either in terms of global measure of goodness of fit (ie, Akaike information criterion and Bayesian information criterion) or in terms of performance of the fitted model (ie, the percent prediction error on C_{ma x} and AUC_0-t ).

Long-acting atypical antipsychotics in schizophrenia: A systematic review and meta-analyses of effects on functional outcome

OBJECTIVE

Impairment in psychosocial function is common in schizophrenia. Long-acting injectable atypical antipsychotics are thought to enhance psychosocial function by boosting adherence. However, no systematic review has examined the effects of long-acting injectable atypical antipsychotics on psychosocial function in clinical trials.

METHODS

We searched major databases including Medline/PubMed, PsychINFO, EMBASE, CINAHL, Scopus, Web of Science, Cochrane Central Register of Controlled Trials and Clinical Trial Registries for randomised controlled trials that compared long-acting injectable atypical antipsychotics to placebo, oral antipsychotic medications or long-acting injectable atypical antipsychotics for all years till 2018, with no language limits. We performed a systematic review of findings on change in psychosocial function and its predictors in the included reports. Data on change in psychosocial functioning were meta-analysed using a random-effects model.

RESULTS

A total of 26 studies were included in systematic review, and 19 studies with 8616 adults, 68.1% males were meta-analysed. Long-acting injectable atypical antipsychotics were superior to placebo (standardised mean difference = 0.39; 95% confidence interval = [0.32, 0.47]; p < 0.001; I² = 0%; 9 studies) and oral antipsychotic medications (standardised mean difference = 0.16; 95% confidence interval = [0.01, 0.31]; p = 0.04; I² = 77%; 10 studies) for improved psychosocial function and superiority was maintained in short- and long trials. Poor psychosocial function was predicted by longer treatment duration, severe symptoms, poor cognition and poor insight. Functioning was assessed by either a single or a combination of measures, but was not the primary outcome in most studies. Other sources of bias include poor blinding and reporting of randomisation.

CONCLUSION

Long-acting injectable atypical antipsychotics are beneficial for recovery of psychosocial function in comparison with placebo, but the magnitude of superiority over oral antipsychotic treatment was small. Severe psychopathology at baseline predicted poor psychosocial function. Future effectiveness trials in which post-randomisation involvement is kept to a minimum, and psychosocial function is included as primary outcome a priori, are needed to capture the real-world impact of long-acting injectable atypical antipsychotics and to address methodological biases.

A Receiver Operating Characteristic Framework for Non-adherence Detection Using Drug Concentration Thresholds-Application to Simulated Risperidone Data in Schizophrenic Patients

Non-adherence to antipsychotic medication is a primary factor in disease relapse in schizophrenic patients. We sought to evaluate if plasma concentrations of the antipsychotic risperidone can be used as a predictor of treatment adherence and to identify the optimal plasma concentration threshold to reliably distinguish between adherent and non-adherent patients. A population pharmacokinetic model was used to simulate plasma risperidone steady-state trough concentrations in 1000 virtual patients, where 60% of the patients were 100% adherent to their medication, while 40% of the patients were non-adherent to their medication. The probability of adherence was assessed by receiver operating characteristic (ROC) analysis on C_trough. The area under the ROC curve (AUC_ROC) was used to identify the optimal C_trough threshold. Single vs multiple C_trough at steady state was also evaluated. After a single risperidone C_trough measurement, the AUC_ROC (95% CI) was estimated to be 0.71 (0.69-0.72) and the optimal C_trough threshold accounting for the lowest number of adherent and non-adherent misclassifications was estimated to be 11.9 ng/mL. After multiple C_trough measurements, the AUC_ROC (95% CI) increased up to 0.85 (0.84-0.87) for three C_trough measurements. The optimal probability threshold to reliably discriminate between adherent and non-adherent patients was estimated to be 0.51. Using this model which is reflective of typical adherence to antipsychotic medication, we found that three consecutive steady-state C_trough measurements are needed for an accurate and precise diagnostic test to discriminate between patients who are adherent or non-adherent to treatment.

Age Impacts Olanzapine Exposure Differently During Use of Oral Versus Long-Acting Injectable Formulations: An Observational Study Including 8,288 Patients

PURPOSE

Olanzapine is a commonly prescribed antipsychotic available as oral and long-acting injectable (LAI) formulations. Data are lacking on the use and safety of olanzapine-LAI in older patients. The aim of this study was to investigate the effect of increasing age on olanzapine exposure during oral versus LAI administration in a real-life setting.

METHODS

This observational study was based on routine therapeutic drug monitoring data collected during 2005-2017. As a measure of exposure, absolute concentrations and concentration/dose ratios of olanzapine were defined as outcome variables. Linear mixed-model analyzes were used to allow for inclusion of multiple samples per patient and adjustment for covariate effects.

RESULTS

Olanzapine concentrations and doses from 8,288 patients (21,378 measurements) were included. The number of patients on oral treatment was 7,893 (42%, 50 years or older), while 395 were using olanzapine-LAI (27%, 50 years or older). In contrast to oral use, where the dose-adjusted concentration of olanzapine increased significantly for patients 50 years or older (P < 0.001), increasing age had no effect on olanzapine concentration following LAI administration (P = 0.550). The effects of smoking habits and gender were equal in oral and olanzapine-LAI users.

CONCLUSION

While the dose-adjusted systemic exposure of olanzapine increases by age after oral administration, these novel findings from a large patient population show that systemic exposure of olanzapine-LAI is unaffected by age, probably due to the lacking influence of age-related changes in gastrointestinal absorption and/or presystemic metabolism. From a pharmacokinetic point of view, it is therefore no reason to restrict the use of olanzapine-LAI in older patients requiring long-term treatment.

Pharmacokinetics of risperidone in different application forms - Comparing long-acting injectable and oral formulations

We aimed to explore the differences in the pharmacokinetics of risperidone between oral and long-acting injectable (LAI) formulations using a large database of therapeutic drug monitoring (TDM). Plasma concentrations of risperidone (RIS), its active metabolite (9-OH-RIS) and the active moiety (AM) (RIS+9-OH-RIS), their concentration-to-dose (C/D) ratios and ratio of RIS/9-OH-RIS (an index of CYP2D6 metabolic activity) were used to compare patients receiving risperidone orally (n = 851) and those treated with LAI RIS (n = 63). Patients taking CYP inducers or inhibitors or with liver/renal impairment were eliminated. Our study demonstrated that patients on LAI RIS, despite slightly higher RIS doses in the oral group, showed no significant differences in total AM or 9-OH-RIS. Conversely, RIS concentration, RIS C/D ratio and total C/D ratio were slightly higher in the LAI RIS group, reaching significance due to the large sample size. More importantly, the median ratio of RIS/9-OH-RIS was 0.52 in LAI RIS vs. 0.25 in the oral group, providing a significant difference (p < 0.001). After controlling for confounding factors, we replicated the RIS/9-OH-RIS ratio increases in patients with LAI RIS, probably reflecting a decrease in first-pass metabolism. More studies are required to establish the clinical use of TDM for patients on LAI RIS.

Effects of Continuing Oral Risperidone vs. Switching from Risperidone to Risperidone Long-Acting Injection on Cognitive Function in Stable Schizophrenia Patients: A Pilot Study

OBJECTIVES

Risperidone is the first new generation antipsychotic drug to become available as a long-acting injection (LAI). The purpose of this study was to evaluate the effects of switching from oral risperidone to risperidone LAI (RLAI) on cognitive function in stable schizophrenia patients compared with the effects of continuing oral risperidone.

METHODS

Sixteen stable patients who had received risperidone monotherapy for at least 3 months were enrolled (the RLAI group). Before and 24 weeks after switching to RLAI, the Japanese-language version of the Brief Assessment of Cognition in Schizophrenia (BACS-J) and the Positive and Negative Syndrome Scale (PANSS) were administered. To exclude the possibility of learning effects on the BACS-J results, 14 patients with stable schizophrenia who continued oral risperidone treatment were also assessed (the RIS group).

RESULTS

The two groups did not differ with respect to changes in the PANSS score, and no emergent side effects, including extrapyramidal symptoms, were observed. The BACS-J score for verbal memory exhibited greater improvement in the RLAI group than in the RIS group (p = 0.047).

CONCLUSION

The results of this preliminary study suggest that switching from oral risperidone to RLAI may improve verbal capability more than continuing with oral risperidone. However, these findings must be replicated in a larger, double-blind study.

Dopamine supersensitivity psychosis in schizophrenia: Concepts and implications in clinical practice

Dopamine supersensitivity psychosis (DSP) is observed in patients with schizophrenia under antipsychotic treatment, and it is characterized by rebound psychosis, an uncontrollable psychotic episode following a stable state and tardive dyskinesia. DSP, first described in patients taking typical antipsychotics in the late 1970s, sometimes appears even in patients who are treated with current atypical antipsychotics. It was recently demonstrated that DSP can have a negative impact on the long-term prognosis of schizophrenia patients and that DSP could be involved in the etiology of some cases of treatment-resistant schizophrenia. Accumulating evidence suggests that an up-regulation of dopamine D2 receptors (DRD2) in the brain caused by long-term exposure to antipsychotics is related to the DSP phenomenon. The present review describes the clinical characteristics and the etiology of DSP in the era of second-generation antipsychotics for patients with schizophrenia. Based on the mechanism of DSP, several potential treatments for patients presenting with a DSP episode or the dopamine supersensitivity state are also discussed.

A New Level A Type IVIVC for the Rational Design of Clinical Trials Toward Regulatory Approval of Generic Polymeric Long-Acting Injectables

Chronic neuropsychiatric disorders and diabetes mellitus affect millions of patients and require long-term supervision and expensive medical care. Although repeated drug administration can help manage these diseases, relapses and re-hospitalization owing to patient non-adherence and reduced therapeutic efficacy remain challenging. In response, long-acting injectables, which provide sustained drug release over longer periods at concentrations close to therapeutic ranges, have been proposed. Recent advancements include polymeric long-acting injectables (pLAIs), in which the active pharmaceutical ingredient (API) is encapsulated within U.S. Food and Drug Administration (FDA)-approved biocompatible polymers, such as poly(lactic-co-glycolic acid), or PLGA. Despite significant progress and development in the global pLAI market, FDA guidance for the approval of complex drug products, such as generic pLAIs, is not clearly defined. Although in vitro to in vivo correlation (IVIVC) can facilitate the identification of critical quality attributes (CQAs), drug formulations, and in vitro test platforms for evaluating drug performance in vivo, the application of IVIVC in order to shortlist time- and resource-intensive clinical trials for generic pLAIs has not been reported. Here, we propose a new Level A Type IVIVC that directly correlates the in vitro outcomes, such as drug dissolution, of candidate generic formulations with the clinical characteristics, such as drug absorption, of a reference listed drug (RLD), to help identify the specific generic pLAI formulations with clinical absorptions that are likely to be similar to that of the RLD, thereby reducing the number of clinical trials required for evaluation of clinical bioequivalence (BE). Therefore, the scope of the proposed method is intended only for the rational design of clinical trials, i.e., to shortlist the specific pLAI generic formulations for clinical BE evaluation, and not necessarily to analyze drug performances (i.e., drug safety and effectiveness) in the shortlisted clinical trials or post-approval. Once validated, this method will be of great value to developers of generic pLAIs and regulatory bodies to accelerate their approval of these generic pLAIs.

Does formulation matter? A systematic review and meta-analysis of oral versus long-acting antipsychotic studies

Recently, many authors highlighted the potential advantages of a broader prescription of long-acting injectable antipsychotics (LAIs) based on various assumptions, including favorable pharmacokinetic features. In this systematic review, data from randomized controlled trials comparing LAIs versus the oral formulation of the same antipsychotic were meta-analyzed in order to ascertain whether the route of administration may be associated with a different efficacy and tolerability profile. Of 21 included studies, 18 contributed to the meta-analysis, providing data for risperidone, olanzapine, aripiprazole, zuclopenthixol, fluphenazine and haloperidol. For all drugs, the number of dropouts for any reason (primary outcome) did not differ between the two formulations, except for a small effect in favor of LAI aripiprazole (2 comparisons; 986 patients; relative risk (RR) 0.78; 95% confidence interval (CI) 0.64 to 0.95). Similarly, no differences emerged in terms of dropouts for adverse events, extrapyramidal symptoms, prolactin increase (except for a small advantage for LAI risperidone), weight gain, non-response rate, relapse rate, and dropouts for inefficacy (except for a small advantage for oral olanzapine). Data on aripiprazole proved to be of high quality according to the GRADE approach (Grading of Recommendations, Assessment, Development and Evaluation), therefore we are confident that the effect estimate is close to the true effect. Data on risperidone were of moderate quality, while data on olanzapine, fluphenazine, zuclopenthixol and haloperidol were of low quality. In conclusion, there is no robust evidence to support doctors in choosing LAI instead of oral formulations in order to obtain better tolerability and efficacy.

Cognitive function and risperidone long-acting injection vs. paliperidone palmitate in schizophrenia: a 6-month, open-label, randomized, pilot trial

BACKGROUND

Recently, long-acting injection (LAI) of second-generation antipsychotics has become a valuable strategy for the treatment of schizophrenia. However, few studies have compared the effects of different LAI antipsychotics on cognitive functions so far. The present study aimed to compare the influence of risperidone LAIs (RLAI) and paliperidone palmitate LAIs (PP) on cognitive function in outpatients with schizophrenia.

METHODS

In this 6-month, open-label, randomized, and controlled study, 30 patients with schizophrenia who were treated with RLAIs were randomly allocated to the RLAI-continued group or the PP group. At baseline and 6 months, the patients were evaluated using the Brief Assessment of Cognition in Schizophrenia (BACS) that was the primary outcome of the study. The Subjective Well-being under Neuroleptic drug treatment-Short form (SWNS), the Positive and Negative Syndrome Scale (PANSS), and the Drug-Induced Extrapyramidal Symptoms Scale (DIEPSS) scores were secondary outcome variables and they were tested at the same time points.

RESULTS

The two groups did not differ in terms of PANSS, DIEPSS, or SWNS total score changes. However, the BACS score for the attention and processing speed item showed higher improvement in the PP group than the RLAI group (p = 0.039).

CONCLUSIONS

The results of this preliminary study suggest that PPs may improve attention and processing speed more than RLAIs. Anyway, a replication in a larger and double-blind study is needed.

TRIAL REGISTRATION

UMIN000014470 . Registered 10 July 2014.

Risperidone long-acting injection: in bipolar I disorder

A long-acting intramuscular formulation of the atypical antipsychotic agent risperidone is now indicated for the maintenance treatment of patients with bipolar I disorder. The formulation utilizes a novel drug delivery system of biodegradable microspheres and is bioequivalent to the oral formulation of the drug. Moreover, fluctuations in plasma drug concentrations at steady state were 1.7-fold lower with long-acting than with oral risperidone. Maintenance treatment with risperidone long-acting injection, as monotherapy in adults with stabilized bipolar I disorder or as an adjunct to standard therapy in adults with stabilized, frequently relapsing bipolar I disorder, was effective in delaying relapse to symptoms in two well designed trials with maintenance phases of 1 or 2 years' duration. The time to relapse to any mood episode (primary endpoint) was significantly longer with risperidone long-acting injection than with placebo in both studies. Risperidone long-acting injection also significantly reduced the risk of relapse relative to placebo in these trials. Maintenance treatment with risperidone long-acting injection was generally well tolerated in patients with bipolar disorder, both as monotherapy and adjunctive therapy, with most adverse reactions being of mild to moderate severity.

Long-acting injectable risperidone and metabolic ratio: a possible index of clinical outcome in treatment-resistant schizophrenic patients

RATIONALE

It is still common to encounter a partial or no response to antipsychotic treatment in clinical practice, but only individual case reports are currently available concerning the efficacy of long-acting risperidone (RLAI) in treatment-resistant schizophrenia. The relationship between RSP and 9-OH-RSP plasma levels, and clinical response or tolerability has not yet been thoroughly assessed.

METHODS

This open-label, non-randomised study involved 30 outpatients with treatment-resistant schizophrenia, who were prescribed RLAI for 6 months, and clinically evaluated using the Brief Psychiatric Rating Scale (BPRS), the Positive and Negative Symptoms Scale (PANSS), the Clinical Global Impression-Improvement Scale (CGI-I), and the Simpson and Angus Scale for Extrapyramidal Side Effects (EPSE). Plasma RSP and 9-OH-RSP levels were determined at steady-state, and the metabolic ratio (MR) was calculated as plasma 9-OH-RSP/RSP levels.

RESULTS

At the end of the study, 60% of the patients responded to RLAI (a >or=20% reduction in the PANSS score). Linear regression analysis showed a significant positive relationship between the RSP dose and active moiety (RSP + 9-OH-RSP) (r = 0.4; p = 0.02). There was a significant positive relationship between active moiety and EPSE scores (r = 0.6; p = 0.00). The BPRS responders had a significantly higher mean MR than the non-responders (3.41 +/- 1.87 SD vs 1.60 +/- 0.98 SD) (p = 0.00).

CONCLUSIONS

Therapeutic drug monitoring seems to be useful in optimising the dose of RLAI, especially in the case of tolerability problems. MR might be a better index of clinical response to RLAI than the value of the active moiety, although this needs to be confirmed by further data.

Risperidone long-acting injection: a review of its long term safety and efficacy

A long-acting form of the second-generation antipsychotic drug risperidone is now broadly available for the treatment of schizophrenia and closely related psychiatric conditions. It combines the advantage of previously available depot formulations for first-generation drugs with the favorable characteristics of the modern "atypical" antipsychotics, namely higher efficacy in the treatment of the negative symptoms of schizophrenia and reduced motor disturbances. Published clinical studies show an objective clinical efficacy (as per psychiatric symptom scores and relapse data) that exceeds that of oral atypical antipsychotics when patients are switched to the long-acting injectable form, a low incidence of treatment-emergent extrapyramidal side effects, and very good acceptance by patients. Available data for maintenance treatment of bipolar disorder show equivalence with the oral form instead of superiority, but are still limited. As it seems likely that efficacy benefits are mostly due to the fact that the injectable form reduces the demand for patient compliance to one physician visit every 2 weeks instead of self-administration on a daily or twice-daily basis, additional potential could exist in other psychiatric disorders where atypical antipsychotic drugs are of benefit but where patient adherence to treatment schedules is typically low.

Estimation of the impact of noncompliance on pharmacokinetics: an analysis of the influence of dosing regimens

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Patient compliance is better with formulations that require less frequent dosing than with formulations that require more frequent dosing. Intramuscular risperidone and long-acting oxybutynin are two examples of medicines reformulated for less frequent dosing. However, it is not clear whether better compliance with less frequent dosing regimens translates to improved therapeutic outcome.

WHAT THIS STUDY ADDS

At equivalent daily doses and typical patterns of compliance, fortnightly intramuscular depot administrations of risperidone provide better pharmacokinetic coverage than once-daily oral dosing. Once-daily dosing of oxybutynin is no better at maintaining pharmacokinetic exposure than twice-daily dosing at half strength. The use of simulated compliance data as input to pharmacokinetic models is useful to assess the impact of noncompliance on internal drug exposure.

AIMS

To determine whether, for oxybutynin and risperidone, drug exposure is better with less frequent dosing regimens than with regimens that require more frequent dosing.

METHODS

Pharmacokinetic models of oxybutynin (5 mg twice-daily and 10 mg once-daily) and risperidone (2 mg once-daily orally and 25 mg fortnightly intramuscular injection) were developed. Simulations of multiple doses were performed by use of stochastic models of dose-taking compliance and clinic visit attendance.

RESULTS

At therapeutic concentrations and with typical patterns of noncompliance, intramuscular injections of risperidone resulted in a 41% (SD 12%) greater pharmacokinetic coverage than the oral dose, 76% (SD 10%) vs. 35% (SD 7%). No discernable differences were evident between once- and twice-daily formulations of oxybutynin, 29.2% (SD 10%) vs. 29.0% (SD 13%).

CONCLUSIONS

For equivalent doses for each drug, the longer acting preparation of risperidone, but not oxybutynin, is pharmacokinetically more forgiving of noncompliance than the shorter acting counterparts. Further analysis is required to confirm whether these observations are valid clinically.

Long-acting formulations of atypical antipsychotics: time to reconsider when to introduce depot antipsychotics

The current availability of a long-acting injectable formulation of risperidone and the potential future availability of long-acting formulations of other atypical antipsychotics (such as paliperidone) should prompt a reconsideration of at what stage in the treatment of schizophrenia such long-acting agents should be introduced. At present, long-acting formulations, particularly of conventional antipsychotics (depots), are usually reserved for patients with chronic schizophrenia who are at high-risk of noncompliance. Recent and increasing data from other patient groups, such as those with first-episode psychosis, suggest that long-acting risperidone is associated with good efficacy and tolerability leading to high patient acceptance, and treatment continuation rates that are greater than with oral antipsychotics. The benefits of an atypical antipsychotic coupled with the assurance of medication delivery in the form of a long-acting injection imply that these novel formulations should be considered in first-episode patients, for whom optimal outcome is frequently compromised by early treatment discontinuation and poor adherence.

Clinical Pharmacokinetics of Atypical Antipsychotics

In the past, the information about the dose-clinical effectiveness of typical antipsychotics was not complete and this led to the risk of extrapyramidal adverse effects. This, together with the intention of improving patients' quality of life and therapeutic compliance, resulted in the development of atypical or second-generation antipsychotics (SGAs). This review will concentrate on the pharmacokinetics and metabolism of clozapine, risperidone, olanzapine, quetiapine, amisulpride, ziprasidone, aripiprazole and sertindole, and will discuss the main aspects of their pharmacodynamics. In psychopharmacology, therapeutic drug monitoring studies have generally concentrated on controlling compliance and avoiding adverse effects by keeping long-term exposure to the minimal effective blood concentration. The rationale for using therapeutic drug monitoring in relation to SGAs is still a matter of debate, but there is growing evidence that it can improve efficacy, especially when patients do not respond to therapeutic doses or when they develop adverse effects. Here, we review the literature concerning the relationships between plasma concentrations of SGAs and clinical responses by dividing the studies on the basis of the length of their observation periods. Studies with clozapine evidenced a positive relationship between plasma concentrations and clinical response, with a threshold of 350-420 ng/mL associated with good clinical response. The usefulness of therapeutic drug monitoring is well established because high plasma concentrations of clozapine can increase the risk of epileptic seizures. Plasma clozapine concentrations seem to be influenced by many factors such as altered cytochrome P450 1A4 activity, age, sex and smoking. The pharmacological effects of risperidone depend on the sum of the plasma concentrations of risperidone and its 9-hydroxyrisperidone metabolite, so monitoring the plasma concentrations of the parent compound alone can lead to erroneous interpretations. Despite a large variability in plasma drug concentrations, the lack of studies using fixed dosages, and discrepancies in the results, it seems that monitoring the plasma concentrations of the active moiety may be useful. However, no therapeutic plasma concentration range for risperidone has yet been clearly established. A plasma threshold concentration for parkinsonian side effects has been found to be 74 ng/mL. Moreover, therapeutic drug monitoring may be particularly useful in the switch between the oral and the long-acting injectable form. The reviewed studies on olanzapine strongly indicate a relationship between clinical outcomes and plasma concentrations. Olanzapine therapeutic drug monitoring can be considered very useful in assessing therapeutic efficacy and controlling adverse events. A therapeutic range of 20-50 ng/mL has been found. There is little evidence in favour of the existence of a relationship between plasma quetiapine concentrations and clinical responses, and an optimal therapeutic range has not been identified. Positron emission tomography studies of receptor blockade indicated a discrepancy between the time course of receptor occupancy and plasma quetiapine concentrations. The value of quetiapine plasma concentration monitoring in clinical practice is still controversial. Preliminary data suggested that a therapeutic plasma amisulpride concentration of 367 ng/mL was associated with clinical improvement. A therapeutic range of 100-400 ng/mL is proposed from non-systematic clinical experience. There is no direct evidence concerning optimal plasma concentration ranges of ziprasidone, aripiprazole or sertindole.