An evaluation of risperidone drug interactions

Classics in Chemical Neuroscience: Risperidone

After the identification of the influence of serotonergic receptors in ameliorating the negative symptoms associated with schizophrenia, atypical antipsychotics were developed by incorporating dopamine and serotonin antagonism. Risperidone, sold under the trade name Risperdal, was the second atypical antipsychotic developed following clozapine but quickly became a first-line treatment for acute and chronic schizophrenia because of its preferential side effect profile. Despite initial Food and Drug Administration approval 25 years ago, risperidone continues to be a fundamental treatment for schizophrenia, bipolar I disorder, and autism-related irritability. It is on the World Health Organization's List of Essential Medicines for its balance of efficacy, safety, tolerability, and cost-effectiveness. In this review, we highlight the history and importance of risperidone as an atypical antipsychotic, in addition to its chemical synthesis, manufacturing, drug metabolism and pharmacokinetics, pharmacology, structure-activity relationship, indications, and adverse effects.

A drug-drug conditioning paradigm reveals multiple antipsychotic-nicotine interactions

Clinical studies indicate a reciprocal impact between nicotine use and antipsychotic medications in patients with schizophrenia. The present study used a conditioned avoidance response (CAR) test (a behavioral test of antipsychotic effect) and examined the specific drug-drug interactions between nicotine and haloperidol or clozapine. Following acquisition of the avoidance response, rats were first tested under either vehicle, nicotine (0.2, 0.4 mg/kg, sc), haloperidol (0.025, 0.05 mg/kg, sc), clozapine (5.0, 10.0 mg/kg, sc), or a combination of nicotine and haloperidol or nicotine and clozapine for seven consecutive days. Afterward, they were challenged with nicotine (0.2 mg/kg), haloperidol (0.025 mg/kg), or clozapine (5.0 mg/kg) in the CAR to assess if haloperidol or clozapine affected the behavioral effect of nicotine on avoidance response and if nicotine altered the avoidance suppressive effect of haloperidol and clozapine. During the seven avoidance drug test days, nicotine did not alter the avoidance suppressive effect of haloperidol or clozapine. However, in the challenge test, prior nicotine treatment (0.2 mg/kg) attenuated haloperidol's (0.05 mg/kg) sensitized effect on avoidance response. On the other hand, prior haloperidol treatment increased nicotine's (0.2 mg/kg) avoidance disruptive effect, and even engendered nicotine 0.4 mg/kg to exhibit an "acquired" avoidance suppressive effect. The combined nicotine and clozapine treatment did not produce any detectable interactive effects on avoidance response and motor activity. These findings suggest that nicotine is capable of altering the long-term antipsychotic efficacy of haloperidol, while haloperidol can alter the behavioral effects of nicotine. Clozapine and nicotine are less likely to influence each other.

Clinically significant drug interactions with atypical antipsychotics

Atypical antipsychotics [also known as second-generation antipsychotics (SGAs)] have become a mainstay therapeutic treatment intervention for patients with schizophrenia, bipolar disorders and other psychotic conditions. These agents are commonly used with other medications--most notably, antidepressants and antiepileptic drugs. Drug interactions can take place by various pharmacokinetic, pharmacodynamic and pharmaceutical mechanisms. The pharmacokinetic profile of each SGA, especially with phase I and phase II metabolism, can allow for potentially significant drug interactions. Pharmacodynamic interactions arise when agents have comparable receptor site activity, which can lead to additive or competitive effects without alterations in measured plasma drug concentrations. Additionally, the role of drug transporters in drug interactions continues to evolve and may effect both pharmacokinetic and pharmacodynamic interactions. Pharmaceutical interactions occur when physical incompatibilities take place between agents prior to drug absorption. Approximate therapeutic plasma concentration ranges have been suggested for a number of SGAs. Drug interactions that markedly increase or decrease the concentrations of these agents beyond their ranges can lead to adverse events or diminished clinical efficacy. Most clinically significant drug interactions with SGAs occur via the cytochrome P450 (CYP) system. Many but not all drug interactions with SGAs are identified during drug discovery and pre-clinical development by employing a series of standardized in vitro and in vivo studies with known CYP inducers and inhibitors. Later therapeutic drug monitoring programmes, clinical studies and case reports offer methods to identify additional clinically significant drug interactions. Some commonly co-administered drugs with a significant potential for drug-drug interactions with selected SGAs include some SSRIs. Antiepileptic mood stabilizers such as carbamazepine and valproate, as well as other antiepileptic drugs such as phenobarbital and phenytoin, may decrease plasma SGA concentrations. Some anti-infective agents such as protease inhibitors and fluoroquinolones are of concern as well. Two additional important factors that influence drug interactions with SGAs are dose and time dependence. Smoking is very common among psychiatric patients and can induce CYP1A2 enzymes, thereby lowering expected plasma levels of certain SGAs. It is recommended that ziprasidone and lurasidone are taken with food to promote drug absorption, otherwise their bioavailability can be reduced. Clinicians must be aware of the variety of factors that can increase the likelihood of clinically significant drug interactions with SGAs, and must carefully monitor patients to maximize treatment efficacy while minimizing adverse events.

Changes in activity patterns after the oral administration of brotizolam in institutionalized elderly patients with dementia

BACKGROUND

Little is known about the side effects of sedative-hypnotic agents in elderly dementia patients with sleep disorders. The present study describes activity pattern changes after a single dose of brotizolam in elderly patients with dementia.

METHODS

We conducted retrospective analysis of prospectively collected data from a case series at Asakayama Hospital (Osaka, Japan) between September 2008 and September 2009. Around-the-clock movements of dementia patients who were administered a single dose of brotizolam were recorded by the integrated circuit tag monitoring system during a 4-week baseline and 7-day peri-administration period. Diurnal and nocturnal activity levels and the onset times of the least-active and most-active phases were then measured.

RESULTS

Seven patients (four men, three women; age range 59-85 years) were analyzed. All seven patients had disturbed activity patterns during the peri-administration period. Compared with the pre-administration period, the incidence of reversed rest-activity pattern increased significantly in the post-administration period, as measured by the distance moved per hour (P < 0.000). Patients with advanced stages of dementia had prolonged and delayed activity responses.

CONCLUSIONS

Findings showed changes in activity levels and reversed active/resting phases after a single dose of brotizolam in elderly patients with dementia. Use of brotizolam in elderly patients with dementia, especially in advanced stages, calls for closer attention and longer observation periods.

Relationship between response to risperidone, plasma concentrations of risperidone and CYP3A4 polymorphisms in schizophrenia patients

In this study, we examined the relationships between plasma concentrations of risperidone and 9-hydroxyrisperidone and polymorphisms of CYP3A4. All 130 schizophrenia patients (45 men, 85 women, age 15-60 years) who met DSM-IV criteria were given risperidone for 8 weeks. Clinical efficacy was determined using the Positive and Negative Syndrome Scale (PANSS). CYP3A4*1G was found to be associated with the change in total PANSS scores (Kruskal-Wallis test, P = 0.021), which was not significant on adjusting for multiple testing. Our study has, for the first time, conducted a genetic association study of the CYP3A4 gene with risperidone response. Further studies on larger groups and on the effects of the longer term risperidone treatment are needed to confirm these results.

New atypical antipsychotics for schizophrenia: iloperidone

The optimal treatment of schizophrenia poses a challenge to develop more effective treatments and safer drugs, to overcome poor compliance, discontinuation and frequent switching with available antipsychotics. Iloperidone is a new dopamine type 2/serotonin type 2A (D(2)/5-HT(2A)) antagonist structurally related to risperidone, expected to give better efficacy with less extrapyramidal symptoms than D(2) receptor antagonist antipsychotics. In double-blind phase III trials iloperidone reduced the symptoms of schizophrenia at oral doses from 12 to 24 mg. It was more effective than placebo in reducing positive and negative syndrome total score and Brief Psychiatric Rating scale scores; it was as effective as haloperidol and risperidone in post-hoc analysis. Its long-term efficacy was equivalent to that of haloperidol. The most common adverse events were dizziness, dry mouth, dyspepsia and somnolence, with few extrapyramidal symptoms and metabolic changes in short- and long-term studies in adults. Akathisia was rare, but prolongation of the corrected QT (QTc) interval was comparable to haloperidol and ziprasidone, which is of particular concern. Further comparative studies are needed to clarify the benefit/risk profile of iloperidone and its role in the treatment of schizophrenia.

Effects of the CYP2D6*10 alleles and co-medication with CYP2D6-dependent drugs on risperidone metabolism in patients with schizophrenia

OBJECTIVE

Risperidone is converted to 9-hydroxyrisperidone by CYP2D6. Two parameters were used to examine the influences of CYP2D6 polymorphism and of co-medication on risperidone metabolism: the risperidone:9-hydroxyrisperidone concentration ratio (R:9-OHR ratio) and the sum of the risperidone and 9-hydroxyrisperidone concentrations divided by the dose (C:D ratio). We evaluated the effect of the CYP2D6*10 allele, which is a prevalent mutant allele among East Asians.

METHODS

Genotyping using the P450 microarray system was performed for 89 Japanese patients with schizophrenia receiving risperidone. The patients with CYP2D6*1/*1, *1/*2, or *2/*2 were classified as Group 1, those with one CYP2D6*10 allele (CYP2D6*1/*10 or *2/*10) were classified as Group 2, and those with two CYP2D6*10 alleles were classified as Group 3. The R:9-OHR and C:D ratios were analyzed using two-way ANOVAs with the CYP2D6 genotype and co-medication with CYP2D6-dependent drugs as independent variables.

RESULTS

Both the "genotype" and the "co-medication" factors had significant impacts on the R:9-OHR ratio (p = 0.011, p < 0.001). The "genotype" factor also had a significant impact on the C:D ratio (p = 0.032). However, the "co-medication" factor did not have a significant impact on the C:D ratio (p = 0.129).

CONCLUSIONS

The CYP2D6*10 polymorphism and the presence of co-medication exerted significant influences on the pharmacokinetics of risperidone.

Effects of CYP2D6 and CYP3A5 genotypes on the plasma concentrations of risperidone and 9-hydroxyrisperidone in Korean schizophrenic patients

This study was conducted to evaluate the effects of the CYP2D6 and CYP3A5 genotypes on the steady-state plasma levels of risperidone (RIS), 9-hydroxyrisperidone (9-OH-RIS), and the active moiety (RIS plus 9-OH-RIS) in Korean schizophrenic patients. Sixty-four Korean schizophrenic patients were enrolled. CYP2D6 and CYP3A5 genotypes were determined, and the plasma levels of RIS and 9-OH-RIS were measured using high-performance liquid chromatography. The dose-normalized plasma concentrations of RIS, 9-OH-RIS, and the active moiety were compared according to the CYP2D6 and CYP3A5 genotypes. Among the patients, 57 were CYP2D6 extensive metabolizers (EMs; CYP2D6*1/*1, *1/*10, and *10/*10) and 7 were CYP2D6 poor metabolizers (PMs; CYP2D6*1/*5 and *10/*5). For the CYP3A5 genotype, 30 patients were CYP3A5*1 expressors (*1/*1 [n = 1] and *1/*3 [n = 29]) and 34 patients were CYP3A5 nonexpressors (*3/*3). The plasma levels of RIS (2.03 ng/mL per milligram for EMs vs 5.57 ng/mL per milligram for PMs, P < 0.001) and 9-OH-RIS (5.06 ng/mL per milligram for EMs vs 0.22 ng/mL per milligram for PMs, P < 0.001) were significantly different among CYP2D6 genotype groups, but the CYP2D6 EMs (7.09 ng/mL per milligram) and PMs (5.79 ng/mL per milligram) did not show no difference in the levels of the active moiety (P = 0.470). CYP3A5 nonexpressors exhibited higher plasma concentrations of both RIS and 9-OH-RIS than its expressors. In the case of 9-OH-RIS, CYP3A5 nonexpressors exhibited significantly higher concentrations than CYP3A5 expressors (5.42 vs 3.51 ng/mL per milligram, P = 0.022). In addition, concentrations of the active moiety were also significantly different between the CYP3A5 nonexpressors (8.39 ng/mL per milligram) and expressors (5.30 ng/mL per milligram, P = 0.005). In conclusion, both CYP2D6 and CYP3A5 genotypes affected plasma levels of RIS and 9-OH-RIS, whereas the active moiety levels were influenced only by the CYP3A5 genotype but not by the CYP2D6 genotype.

Effect of rifampin, an inducer of CYP3A and P-glycoprotein, on the pharmacokinetics of risperidone

The authors studied the effect of rifampin, a dual inducer of CYP3A and P-glycoprotein, on the pharmacokinetics and pharmacodynamics of risperidone in humans. Ten healthy male subjects were treated daily for 7 days with 600 mg rifampin or with placebo. On day 6, a single dose of 1 mg risperidone was administered. Plasma risperidone and 9-hydroxyrisperidone concentrations were measured. Rifampin significantly decreased the mean area under the plasma concentration-time curve by 51% for risperidone, by 43% for 9-hydroxyrisperidone, and by 45% for the active moieties (risperidone + 9-hydroxyrisperidone). Rifampin also decreased the peak plasma concentration of risperidone by 38%, 9-hydroxyrisperidone by 46%, and the active moieties by 41%. The apparent oral clearance of risperidone approximately doubled after rifampin treatment. Thus, rifampin reduced the exposure to risperidone, probably because of a decrease in its bioavailability through the induction of CYP3A and probably P-glycoprotein.

Priapism associated with atypical antipsychotic medications: a review

Priapism defined as persistent, painful and prolonged penile erection, was previously thought to be associated only with the use of the older, conventional first generation or typical antipsychotic medications as well as some other medications, notably, trazodone. The mechanism of priapism associated with antipsychotics is not clear but is thought to be related to alpha-adrenergic blockage that is mediated by the alpha receptors in the corpora cavernosa of the penis. Atypical antipsychotics, also known as second-generation antipsychotics, owing to their favorable side effect profile, are being prescribed with increasing frequency and are not as frequently considered to cause priapism. Some case reports reporting this side effect with their use, however, are found. Pubmed and Ovid databases were searched to obtain all articles and case reports of antipsychotic drug-induced priapism. Key search words included 'priapism', 'antipsychotics' and 'drug-induced priapism'. References of all identified studies were also reviewed. A total of 50 publications were obtained. Most of the atypical antipsychotics have been reported to cause priapism. These cases have occurred in patients shortly after having been started on the antipsychotic medications as well as in those who have been on them for an extended period of time without modification in dosage, and have also occurred sometimes, with the addition of another antipsychotic, lithium or serotonin-specific reuptake inhibitor. Priapism has been documented with nearly all the atypical antipsychotic medications. It is, however, a rarely reported side effect and therefore, underappreciated. Priapism can cause irreversible erectile dysfunction and is a urologic emergency. Clinicians should monitor patients on these medications for this rare, yet significant side effect. Furthermore, caution must be used when adding new drugs to the regimen and patients should be closely monitored for this side effect. Educating patients about the risk of developing priapism would help increase awareness of the side effect and promote early reporting thereby, decreasing long-term morbidity.

Rifampin, a cytochrome P450 3A inducer, decreases plasma concentrations of antipsychotic risperidone in healthy volunteers

BACKGROUND

Although cytochrome P450 (CYP) 2D6 is often thought to be the only CYP responsible for the metabolism of risperidone, many reports suggest that CYP3A may be involved too. Rifampin, a potent CYP3A inducer, has been known to markedly decrease plasma concentrations of various drugs, which are concomitantly administered during treatment.

OBJECTIVE

To examine the effect of rifampin on plasma concentrations of a single oral dose of risperidone in healthy Thai male volunteers.

METHODS

In an open, randomized two-phase crossover study, separated by a 2-week period, 10 healthy Thai male volunteers received a single oral dose of 4-mg risperidone alone or with 600 mg rifampin, orally once daily for 5 days. Serial blood samples were collected at specific time points for a 48-h period. Risperidone was measured in plasma using high performance liquid chromatography with ultraviolet detection. Pharmacokinetic parameters were determined by using non-compartmental analysis.

RESULTS

Co-administration with 600-mg rifampin once daily for 5 days was associated with a significant decrease in risperidone area under the curve (AUC(0-48)) and maximal concentration (C(max)) by 72% (157 x 49 +/- 48 x 80 vs. 42 x 66 +/- 7 x 81 ng/L/h; P<0 x 01) and 50% (32 x 44 +/- 6 x 05 vs. 16 x 16 +/- 2 x 73 ng/mL; P<0 x 05), respectively when compared with risperidone alone.

CONCLUSIONS

Rifampin when used concurrently with risperidone significantly decreases the plasma concentration of risperidone. Our results provide in vivo evidence of the involvement of CYP3A in the metabolism of risperidone, in addition to CYP2D6. Thus, co-administration of risperidone with CYP3A inducer(s), including rifampin should be recognized or avoided in clinical practice.

Evaluation of antipsychotic drugs as inhibitors of multidrug resistance transporter P-glycoprotein

RATIONALE

The multidrug resistance transporter, P-glycoprotein (P-gp), is involved in efflux transport of several antipsychotics in the blood-brain barrier (BBB).

OBJECTIVES

In the present study, we evaluated the inhibitory effect of the antipsychotics, i.e., risperidone, olanzapine, quetiapine, clozapine, haloperidol, chlorpromazine, a major metabolite of risperidone, 9-OH-risperidone, and a positive control inhibitor, PSC833, on the cellular uptake of a prototypic substrate of P-gp, rhodamine (Rhd) 123, in LLC-PK1 and L-MDR1 cells.

MATERIALS AND METHODS

After incubation of the antipsychotics (1-100 microM) and the positive (10 microM PSC833) or negative (1% dimethyl sulfoxide) controls with 5 microM Rhd 123 for 1 h, the effects of the antipsychotics on the intracellular accumulation of Rhd 123 were examined using a flow cytometric method.

RESULTS

All the antipsychotics showed various degrees of inhibitory effects on P-gp activity. The rank order of the concentration of inhibitor to cause 50% of the maximal increment of intracellular Rhd 123 fluorescence (EC(50)) was: PSC833 (0.5 microM) < olanzapine (3.9 microM) < chlorpromazine (5.8 microM) < risperidone (6.6 microM) < haloperidol (9.1 microM) < quetiapine (9.8 microM) < 9-OH-risperidone (12.5 microM) < clozapine (30 microM). Considering that the antipsychotics' plasma concentrations are generally lower than 1 microM, the present results suggest that olanzapine and risperidone are the only agents that may inhibit P-gp activity in the BBB. However, most of the antipsychotics are extensively accumulated in tissues. In addition, when given orally, the drug concentrations in the gastrointestinal tract are likely to be high.

CONCLUSIONS

Pharmacokinetic interactions due to inhibition of P-gp activity by the antipsychotics appear possible and warrant further investigation.

Successful treatment for obsessive-compulsive disorder with addition of low-dose risperidone to fluvoxamine: implications for plasma levels of catecholamine metabolites and serum brain-derived neurotrophic factor levels

The authors report on the successful treatment of obsessive-compulsive disorder (OCD) in three patients with the addition of risperidone to ongoing fluvoxamine treatment. Plasma homovanillic acid (HVA), but not 3-methoxy-4-hydroxyphenylglycol (MHPG) levels decreased after risperidone administration, and plasma levels of fluvoxamine did not change. In addition, serum brain-derived neurotrophic factor (BDNF) levels were not altered after the recovery from obsessive-compulsive symptoms, indicating that serum BDNF levels might not predict the patient's response to risperidone treatment. Taken together, a combination treatment of risperdone and fluvoxamine might improve obsessive-compulsive symptoms. In short, fluvoxamine enhances the activity of the serotonergic system by inhibiting serotonin transporters, and risperidone decreases that of the dopaminergic system by blocking D2 receptors.

An overview of psychotropic drug-drug interactions

The psychotropic drug-drug interactions most likely to be relevant to psychiatrists' practices are examined. The metabolism and the enzymatic and P-glycoprotein inhibition/induction profiles of all antidepressants, antipsychotics, and mood stabilizers are described; all clinically meaningful drug-drug interactions between agents in these psychotropic classes, as well as with frequently encountered nonpsychotropic agents, are detailed; and information on the pharmacokinetic/pharmacodynamic results, mechanisms, and clinical consequences of these interactions is presented. Although the range of drug-drug interactions involving psychotropic agents is large, it is a finite and manageable subset of the much larger domain of all possible drug-drug interactions. Sophisticated computer programs will ultimately provide the best means of avoiding drug-drug interactions. Until these programs are developed, the best defense against drug-drug interactions is awareness and focused attention to this issue.

Psychotropic drug interactions with valproate

Valproate is a well-established anticonvulsant that is increasingly being employed, often in combination with other psychotropics, for its mood-stabilizing properties. This compound is metabolized by conjugation, beta-oxidation, and cytochrome P450 oxidation (CYP2C9, CYP2C19, and CYP2A6) and also acts as a broad-spectrum inhibitor of a variety of hepatic enzymes including glucoronyltransferase, epoxide hydrolase, and the CYP2C enzymes. In addition, it exhibits saturable protein binding and competes with many drugs for protein binding sites. It is therefore not surprising that valproate has been noted to interact with psychotropic medications of all classes. This article provides an overview of the noted pharmacokinetic psychotropic interactions with valproate, with a particular focus on the mechanisms of these interactions and potential clinical consequences.

Low-dose risperidone augmentation of fluvoxamine treatment in obsessive-compulsive disorder: a double-blind, placebo-controlled study

According to previous data, the addition of risperidone in obsessive-compulsive patients refractory to serotonin reuptake inhibitors (SRIs) is shown to be a safe and effective treatment strategy. The aims of our study were to evaluate the efficacy of risperidone addition, in comparison to placebo, in fluvoxamine-refractory obsessive-compulsive patients and to investigate whether risperidone could boost the efficacy of fluvoxamine in fluvoxamine-responder patients. Subjects were 45 obsessive-compulsive inpatients, consecutively recruited at the Department of Neurosciences at the San Raffaele Hospital, Milan. Thirty-nine patients completed the study. All patients received 12 weeks of a standardized open-label fluvoxamine monotherapy and then continued for 6 weeks with placebo or risperidone in a double-blind design. Results showed a significant effect of risperidone addition, at the end of the double-blind phase (18th week), only for fluvoxamine-refractory patients. Five patients on risperidone (50%) and two (20%) on placebo became responders, with a Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) decrease > or =35%. Risperidone was generally well tolerated, except for a mild transient sedation and a mild increase in appetite. This preliminary study suggests that even very low (0.5 mg) risperidone doses are effective in OC patients who were nonresponders to a standardized treatment with fluvoxamine.

A case report of a poor metabolizer of CYP2D6 presented with unusual responses to nortriptyline medication

We present a case with decreased metabolic activity of CYP2D6, a cytochrome P450 enzyme catalyzing the metabolism of nortriptyline (NT). Conventional dosage regimen led to toxic plasma concentration of NT and adverse effects such as dry mouth, constipation, and dizziness in this case with genotype CYP2D6*5/*10B. This case suggests the clinical usefulness of pharmacogenetic testing in individualized dosage adjustments of NT.

Plasma Risperidone Concentrations During Combined Treatment with Sertraline

The effect of sertraline on the steady-state plasma concentrations of risperidone and its active metabolite 9-hydroxyrisperidone (9-OH-risperidone) was studied in 11 patients with schizophrenia or schizoaffective disorder. To treat concomitant depressive symptoms, additional sertraline, at the dose of 50 mg/d, was administered for 4 weeks to patients stabilized on risperidone (4-6 mg/d). Mean plasma concentrations of risperidone, 9-OH-risperidone, and the active moiety (sum of the concentrations of risperidone and 9-OH-risperidone) did not change significantly during combined treatment with sertraline. At the end of week 4, sertraline dosage was adjusted in some patients on the basis of the individual response and then maintained until the end of week 8. At final evaluation, mean plasma levels of risperidone active moiety were not modified in the 4 patients who were still receiving the initial sertraline dose, but concentrations were slightly but not significantly increased (by a mean 15% over pretreatment) in the subgroup of 5 subjects treated with a final dose of 100 mg/d. In the 2 patients receiving the highest dose of sertraline, 150 mg/d, at week 8 total plasma risperidone concentrations were increased by 36% and 52%, respectively, as compared with baseline values. Sertraline coadministration with risperidone was well tolerated, and no patient developed extrapyramidal symptoms. These findings indicate that sertraline at dosages up to 100 mg/d is not associated with clinically significant changes in plasma risperidone concentrations. However, higher doses of sertraline may elevate plasma risperidone levels, presumably as a result of a dose-dependent inhibitory effect of sertraline on CYP2D6-mediated 9-hydroxylation of risperidone.

Olanzapine penetration into brain is greater in transgenic Abcb1a P-glycoprotein-deficient mice than FVB1 (wild-type) animals

The transmembrane energy-dependent efflux transporter P-glycoprotein (P-gp) limits a range of drugs from penetrating cells and deposits them into the extracellular space. P-gp is highly expressed in several normal tissues, including the luminal surface of capillary endothelial cells in the brain of humans. In this study, we tested whether olanzapine distribution to tissues highly expressing P-gp or devoid of this transporter was similar in Abcb1a (-/-) mice lacking P-gp and control animals. At 1 h following the intraperitoneal injection of 2.5 microg olanzapine/g mouse, olanzapine concentrations were statistically and significantly higher in brain (three-fold), liver (2.6-fold), and kidney (1.8-fold) of Abcb1a (-/-) mice than those of the control FVB Abcb1a (+/+) mice, and not statistically different in plasma, spleen, or penile tissue. Similar differences were also found for the ratios of organ:plasma and organ:spleen between the two groups. This is the first report that the presence of the Abcb1a gene is an important factor controlling brain access to olanzapine. The finding that the brain penetration of olanzapine is limited by P-gp implies that the highly prevalent functional polymorphisms of ABCB1 in humans may be a factor contributing to variability in dose requirements for this antipsychotic drug.

Selection of Atypical Antipsychotics for the Management of Schizophrenia

OBJECTIVE

To review the evidence for selecting one atypical antipsychotic agent over another for management of schizophrenia.

DATA SOURCES

A literature search of MEDLINE (1966–June 2003), EMBASE (1998–June 2003), and the Cochrane Library was conducted using the following terms: schizophrenia, quetiapine, ziprasidone, olanzapine, aripiprazole, and risperidone. Bibliographies of relevant articles were hand-searched for additional references.

STUDY SELECTION AND DATA EXTRACTION

Prospective, randomized, blinded trials and meta-analyses that directly or indirectly compared ≥2 atypical antipsychotic agents in the management of schizophrenia are included in this review. Studies comparing an atypical agent with clozapine are not included.

DATA SYNTHESIS

A small number of prospective, randomized, blinded trials that compare efficacy and tolerability of olanzapine and risperidone have been published. These trials did not reveal clinically meaningful differences in efficacy but did confirm that their adverse effect profiles are slightly different (more weight gain with olanzapine and more extrapyramidal reactions with risperidone). Direct comparisons between other atypical antipsychotics are not available. Systematic reviews (indirect comparisons) of placebo-controlled or traditional antipsychotic-controlled trials suggest similar efficacy for quetiapine, olanzapine, and risperidone when placebo is the comparator and inferior efficacy of quetiapine compared to olanzapine and risperidone when haloperidol is the comparator. The few available economic analyses are difficult to interpret in light of current practice.

CONCLUSIONS

Additional randomized, blinded clinical trials directly comparing efficacy, tolerability, and cost-effectiveness are needed to confirm the proposed differences among atypical antipsychotic agents before recommendations can be made with confidence.

Metabolic drug interactions with new psychotropic agents

New psychotropic drugs introduced in clinical practice in recent years include new antidepressants, such as selective serotonin reuptake inhibitors (SSRI) and 'third generation' antidepressants, and atypical antipsychotics, i.e. clozapine, risperidone, olanzapine, quetiapine, ziprasidone and amisulpride. These agents are extensively metabolized in the liver by cytochrome P450 (CYP) enzymes and are therefore susceptible to metabolically based drug interactions with other psychotropic medications or with compounds used for the treatment of concomitant somatic illnesses. New antidepressants differ in their potential for metabolic drug interactions. Fluoxetine and paroxetine are potent inhibitors of CYP2D6, fluvoxamine markedly inhibits CYP1A2 and CYP2C19, while nefazodone is a potent inhibitor of CYP3A4. These antidepressants may be involved in clinically significant interactions when coadministered with substrates of these isoforms, especially those with a narrow therapeutic index. Other new antidepressants including sertraline, citalopram, venlafaxine, mirtazapine and reboxetine are weak in vitro inhibitors of the different CYP isoforms and appear to have less propensity for important metabolic interactions. The new atypical antipsychotics do not affect significantly the activity of CYP isoenzymes and are not expected to impair the elimination of other medications. Conversely, coadministration of inhibitors or inducers of the CYP isoenzymes involved in metabolism of the various antipsychotic compounds may alter their plasma concentrations, possibly leading to clinically significant effects. The potential for metabolically based drug interactions of any new psychotropic agent may be anticipated on the basis of knowledge about the CYP enzymes responsible for its metabolism and about its effect on the activity of these enzymes. This information is essential for rational prescribing and may guide selection of an appropriate compound which is less likely to interact with already taken medication(s).

Antiepileptic-antipsychotic drug interactions: a critical review of the evidence

The potential for drug-drug interactions in psychiatry and patients with epilepsy is very high. Moreover, antiepileptic drugs are widely used outside epilepsy as psychotropic agents and their spectrum of activity on behavior is of considerable interest to psychopharmacology. In both neurologic and psychiatric practice, pharmacotherapy combinations are commonly used to treat comorbid psychiatric and neurologic disorders, to reduce or control the adverse effects of a medication or to increase its efficacy. This paper focuses on the metabolic pharmacokinetic interactions between two classes of psychotropic drugs: antiepileptic and antipsychotic drugs. The degree of documentation varies for many interactions from clinical case-report experiences to well established research outcomes. The evidence and the clinical significance of these interactions are reviewed. In general, it is better to use as few drugs as possible, as multicolored politherapies increase the possible adverse effects of drug interactions and reduce patient compliance.

Long-term pharmacoclinical follow-up in schizophrenic patients treated with risperidone. Plasma and red blood cell concentrations of risperidone and its 9-hydroxymetabolite and their relationship to whole blood serotonin and tryptophan, plasma homovanillic acid, 5-hydroxyindoleacetic acid, dihydroxyphenylethyleneglycol and clinical evaluations

The aim of the study was to establish a relationship between the clinical efficacy of risperidone (Risp), the biological levels of Risp and its metabolite, 9-hydroxyrisperidone (9-OH-Risp), and the turnover of blood biogenic amines during a long-term treatment (1 year). Risp is one of the newer atypical antipsychotic drugs with potent serotonin (5HT2), moderate D2 and real alpha 1-alpha 2 adrenoreceptor antagonistic effects. The study has been performed in an open setting and included 17 patients, but only 15 were followed-up from 3 to 12 months. Pharmacokinetic analyses were conducted at the same time as clinical evaluations, grading using the Positive and Negative Syndrome Scale (PANSS), the Clinical Global Impressions (CGI), the Global Assessment of Functioning Scale (GAF), the Quality of Life Scale (QLS) and the Extrapyramidal Symptoms Rating Scale (ESRS) and the determinations of plasma and red blood cell (RBC) Risp and 9-OH-Risp, whole blood 5HT and tryptophan (Trp), plasma homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5HIAA) and dihydroxyphenylethyleneglycol (DHPG). The therapeutic drug monitoring needed oral Risp daily dose of 4.5 +/- 2.3 mg (range 2-8) and the stabilized concentrations (ng/ml) at endpoint in plasma and RBC were 10 +/- 8 (range 1-23) and 3.5 +/- 2 (range 1-8) for Risp and 29 +/- 19 (range 8-70) and 11.5 +/- 6.6 (range 2.6-22.5) for 9-OH-Risp, respectively. 9-OH-Risp appears to be the major active metabolite compound at higher concentrations than Risp. Positive linear correlations were found only between plasma and RBC 9-OH-Risp and the daily dose and the score of the GAF. Statistically significant clinical results showed that Risp is a potent antipsychotic agent efficacious both on positive and negative symptoms and on quality of life. Positive symptoms decreased after about the second month and the negative symptoms improved secondly. Patients (n = 8) who responded to Risp were characterized, on the long-term, by a statistically significant decrease of whole blood 5HT and increase of plasma DHPG.

Extrapyramidal symptoms with ritonavir/indinavir plus risperidone

OBJECTIVE

To report a case of suspected extrapyramidal symptoms (EPS) in a patient initiated on ritonavir and indinavir while taking risperidone for a tic disorder.

CASE SUMMARY

A 35-year-old white man with AIDS received risperidone 2 mg twice daily for treatment of a Tourette's-like tic disorder. Ritonavir and indinavir were initiated, and 1 week later, he experienced significantly impaired swallowing, speaking, and breathing, and worsening of his existing tremors. Ritonavir and indinavir were discontinued. On the same day, the patient increased the risperidone dosage to 3 mg twice daily. Symptoms continued to worsen over the next 3 days. All investigations and laboratory parameters were unremarkable, and vital signs were stable. Risperidone was discontinued and clonazepam initiated. Three days later, the patient's symptoms were significantly improved.

DISCUSSION

The symptoms described herein are consistent with neuroleptic-induced acute dystonia and potentially neuroleptic-induced parkinsonism. We believe this adverse effect occurred as a result of a drug interaction between ritonavir/indinavir and risperidone. Based on the pharmacokinetics of these medications, we hypothesize that inhibition of CYP2D6 and CYP3A4 by ritonavir and indinavir may have resulted in an accumulation of the active moiety of risperidone, which may explain the occurrence of EPS in this patient.

CONCLUSIONS

This is the second published case report describing a suspected drug interaction with ritonavir, indinavir, and risperidone. Caution is warranted when risperidone is prescribed with ritonavir/indinavir, and possibly with other antiretrovirals that inhibit the same pathways.