Drug-drug interactions involving antidepressants: focus on venlafaxine

Selection of an antidepressant is influenced by many factors, including the patient's current drug regimen and the drug's potential for drug-drug interactions. Many psychotropic agents are known to be involved in drug-drug interactions because they are metabolized by various cytochrome pigment 450 (CYP) isoenzymes. In vitro testing with human hepatic microsomal preparations and monoclonal antibody techniques has allowed for the identification and investigation of many of these isoenzymes. Also, screening of substrates (both drug and probe) at the level of the various enzymes expressed in the human liver has allowed for the development of models that predict the risk for drug-drug interactions in vivo. Antidepressants are metabolized by and are competitive inhibitors of several isoenzymes: CYP1A2, CYP2D6, CYP3A3/4, CYP2C8/9, CYP2C19, and others. Of these, CYP2D6 has been the most thoroughly investigated and is the most extensively characterized, whereas CYP3A3/4 are more abundant and play a major role in the metabolism of many commonly used drugs. CYP2D6, but not CYP3A3/4, is subject to genetic polymorphism, which has been identified through the administration of a probe drug (sparteine, debrisoquin, or dextromethorphan). This analysis allows for the determination of an individual's "metabolizer status." This article discusses the CYP isoenzyme system in general terms and presents selected in vitro information that has been used to determine the likelihood of in vivo drug-drug interactions with various antidepressants. Of the marketed antidepressants, venlafaxine seems to have one of the most favorable drug-interaction profiles, and data specific to it are highlighted. In vitro and in vivo data indicate that venlafaxine either does not significantly inhibit or weakly inhibits the activity of isoenzymes CYP2C9, CYP2D6, CYP1A2, or CYP3A3/4.

Pharmacokinetics and drug interactions: update for new antipsychotics

Advances in our understanding of schizophrenia have led to a new generation of antipsychotic agents. These medications not only demonstrate reduced extrapyramidal symptoms but also possess pharmacologic profiles that can be especially advantageous in treating the negative symptoms of schizophrenia. The pharmacokinetics of many of the newer agents are compared and contrasted with typical neuroleptics. Changes in the pharmacokinetics and dosage of the newer agents are also reviewed. A particular emphasis is placed on the metabolism of the newer agents and their potential for drug-drug pharmacokinetic interactions. Clozapine, the archetypal atypical agent, has a complex pharmacokinetic profile with extremely large interpatient variability and many well-documented drug-drug interactions. Thus, clozapine presents special challenges in dose optimization and requires vigilant clinical monitoring for cardiovascular, neurologic, and hematologic adverse effects. Olanzapine demonstrates a very low potential for drug-drug interactions; it requires extremely high inhibitory concentrations at cytrochrome P450 (CYP) systems, typically 30-fold above the usual concentrations observed at steady-state oral high-dose therapy. The metabolic pathways of olanzapine include N-glucuronidation, reducing its overall sensitivity to drugs that might induce or inhibit its own metabolism via CYP or flavin-containing monooxygenase (FMO) systems. Plasma olanzapine concentrations at steady state typically demonstrate only a fourfold to fivefold variability among patients at a standard dose of medications. Sertindole and risperidone demonstrate polymorphic metabolism characteristics mirroring the CYP 2D6 phenotype. The inhibitory potentials of sertindole at CYP 2D6 and CYP 3A are modest and not likely to be of clinical significance. However, in those patients taking CYP 2D6 inhibitors or in those who are genotypic poor metabolizers, concentrations achieved by sertindole and its metabolites might result in moderate inhibition of CYP 3A.

Serotonin selective reuptake inhibitor drug interactions and the cytochrome P450 system

The article focuses on the effects of the serotonin selective reuptake inhibitors (SSRIs) on specific drug metabolizing isoenzymes: CYP2D6, CYP3A3/4, CYP1A2, CYP2C9, and CYP2C19. Both in vitro and in vivo data regarding the inhibition potential of the SSRIs at each of these isoenzyme systems are reviewed. In general, the magnitude of the in vivo interactions between the SSRIs and substrates for these isoenzyme systems mirrors to a large extent their in vitro inhibitory potencies. However, in vitro work is limited owing to pharmacokinetic considerations, the effect of metabolites on the isoenzymes, and the likelihood that several isoenzymes are co-responsible for the metabolism of a substrate.

Antidepressant drug interactions and the cytochrome P450 system. The role of cytochrome P450 2D6

The selective serotonin reuptake inhibitors (SSRIs) and venlafaxine display the following rank order of in vitro potency against the cytochrome P450 (CYP) isoenzyme CYP2D6 as measured by their inhibition sparteine and/or dextromethorphan metabolism: paroxetine > fluoxetine identical to norfluoxetine > or = sertraline > or = fluvoxamine > venlafaxine. On this basis, paroxetine would appear to have the greatest and fluvoxamine and venlafaxine the least potential for drug interactions with CYP2D6-dependent drugs. In vivo, inhibitory potency is affected by the plasma concentration of the free (unbound) drug, a potentially important consideration since many CYP2D6-metabolised drugs exhibit nonlinear (saturable) kinetics, and by the presence of metabolites, which might accumulate and interact with the CYP system. Under steady-state conditions, paroxetine and fluoxetine are approximately clinically equipotent inhibitors of CYP2D6 in vivo (as determined through their effects on desipramine metabolism); sertraline, in contrast, shows lower steady-state plasma concentrations than fluoxetine and, hence, a less pronounced inhibition of CYP2D6. Of the drugs that are metabolised by CYP2D6, secondary amine tricyclic antidepressants, antipsychotics (e.g. phenothiazines, and risperidone), codeine, some antiarrhythmics (e.g. flecainide) and beta-blockers form the focus of clinical attention with regard to their potential interactions with the SSRIs. Coadministration of desipramine and fluoxetine (20 mg/day) at steady-state produced an approximately 4-fold elevation in peak plasma desipramine concentrations, while the long half-life of the active metabolite norfluoxetine was responsible for a significant and long lasting (approximately 3 weeks) elevation of plasma desipramine concentrations after discontinuation of fluoxetine. Similarly, coadministration of desipramine with paroxetine produced an approximately 3-fold increase in plasma desipramine concentration. In contrast, coadministration of desipramine and sertraline (50 mg/day) for 4 weeks resulted in a considerably more modest (approximately 30%) elevation in plasma desipramine concentrations. Coadministration of fluoxetine (60 mg/day, as a loading dose) [equivalent to serum concentrations obtained with 20 mg/day at steady-state] with imipramine or desipramine resulted in approximately 3- to 4-fold increases in plasma area under the curve (AUC) values for both imipramine and desipramine (illustrating a significant drug interaction potential at multiple isoenzymes). Consistent with its minimal in vitro effect on CYP2D6, fluvoxamine shows minimal in vivo pharmacokinetic interaction with desipramine, but does interact with imipramine (approximately 3- to 4-fold increase in AUC) through inhibition of CYP3A3/4, CYP1A2, and CYP2C19. Thus, the extent of the in vivo interaction between the SSRIs and tricyclic antidepressants mirrors to a large extent their in vitro inhibitory potencies against CYP2D6 and other isoenzyme systems, especially if one takes into account pharmacokinetic factors.

A loading-dose strategy for converting from oral to depot haloperidol

OBJECTIVE

The author's aim was to evaluate the safety and efficacy of a loading-dose regimen for initiating use of a depot medication, haloperidol decanoate, with patients who had been maintained on oral haloperidol. Patients were given a loading dose of about 20 times their oral maintenance dose in divided injections during the first two weeks of conversion to depot medication. The dose of haloperidol decanoate was gradually reduced, dropping to about ten times the oral dose in the third and fourth months. No supplemental oral medication was used.

METHODS

Haloperidol decanoate was initiated using the loading-dose regimen in 16 chronically ill patients. Lower initial doses of haloperidol decanoate were used in two other groups of patients, one that received supplemental oral haloperidol and one that did not. Plasma levels of haloperidol, severity of illness, and side effects were monitored from baseline to 56 days after the beginning of depot therapy.

RESULTS

Patients who received the loading-dose regimen showed statistically significant clinical improvement and reduced side effects over baseline by the 28th day. The second group of patients also maintained therapeutic response but improved no further. The third group relapsed during the first month and were returned to a regimen of oral haloperidol by the second month.

CONCLUSIONS

A loading-dose regimen for initiating treatment with haloperidol decanoate is safe and effective and can be useful in a clinical setting.

Neuroleptic malignant syndrome associated with clozapine monotherapy

Neuroleptic malignant syndrome is thought to be a result of dopamine receptor blockade in the striatum. Clozapine has only weak affinity for dopamine type 1 and 2 receptors, and therefore it was thought this drug would not precipitate the syndrome. However, six cases of the syndrome have been reported in patients receiving clozapine monotherapy. A review of the pathoetiology of symptoms occurring in the syndrome is included.

Pharmacological profile of risperidone

The pharmacodynamics and pharmacokinetics of risperidone, an important atypical antipsychotic drug with potent serotonin-5-HT2 and dopamine-D2 receptor blocking effects, are presented. The pharmacology of atypical versus typical antipsychotic drugs is discussed in the contest of a pathophysiological conceptualization for schizophrenia which incorporates a parkinsonian model with an important role for serotonin and excitatory amino acid neurotransmission. In the normal therapeutic dose range, risperidone displays dose-linear pharmacokinetics in humans and reaches steady-state within 24 hours. Risperidone metabolism yields a active metabolite, 9-OH-risperidone, that has a similar pharmacological profile to the patient compound and therefore contributes to the clinical efficacy of the drug.

Thiothixene pharmacokinetic interactions: a study of hepatic enzyme inducers, clearance inhibitors, and demographic variables

Fifty-nine plasma thiothixene concentrations were measured in 42 patients as part of routine therapeutic drug monitoring. Data collection included concomitant medications, smoking history, and demographic variables. A retrospective analysis was performed to assess the effect of these parameters on oral thiothixene clearance. When groups of patients were categorized by concomitant medications (i.e., no interacting drugs, enzyme/clearance inducers, and enzyme/clearance inhibitors), thiothixene clearance was found to be significantly increased by enzyme inducing drugs (e.g., anticonvulsants) and decreased by clearance inhibiting agents (e.g., cimetidine). Tobacco smoking significantly increased the hepatic clearance of thiothixene within the no interactions and inhibitor groups, but not in the inducer group. Significantly more patients in the inducer group had nondetectable plasma concentrations of thiothixene than the other groups. When the entire patient population was dichotomized by age, patients less than 50 years old had a significantly greater mean clearance (48.2 +/- 37.8 liters/min) versus those greater than or equal to 50 (20.0 +/- 12.6 liters/min). Men in this cohort exhibited a significantly higher clearance (49.2 +/- 38.7 liters/min) than did the women (22.0 +/- 13.5 liters/min). By taking into account these potential sources of pharmacokinetic variability when monitoring plasma thiothixene concentrations, more appropriate dosing of thiothixene may be achieved. Controlled, prospective studies are needed to validate these findings.

Ethnic comparison of haloperidol and reduced haloperidol plasma levels: Taiwan Chinese versus American non-Chinese

Steady-state haloperidol (HAL) and reduced HAL (RHAL) plasma levels were measured in Chinese and non-Chinese schizophrenic patients. The patients (n = 38) were matched according to age (+/- 1 yr) and by HAL dose. In general, Chinese patients had higher mean plasma HAL levels and lower RHAL/HAL ratios compared to non-Chinese patients (23.6 +/- 14.9 ng/ml versus 17.1 +/- 10.1 ng/ml, p less than 0.05; 0.52 +/- 0.44 versus 0.82 +/- 0.62, p less than 0.05). Six groups were formed according to HAL dose (number per group): 10 mg/day (6); 20 (11); 30 (11); 40 (4); 50 (3); and 60 (3). No significant differences were found in age, weight and dose/weight. In each dose group, HAL plasma levels were generally higher in the Chinese patients than in the non-Chinese patients, though significance was only detected in the 30 mg group (26.1 +/- 7.0 ng/ml versus 18.5 +/- 5.1 ng/ml, p = 0.035) and a slight trend in the 40 mg group (36.0 +/- 15.0 ng/ml versus 23.5 +/- 10.4 ng/ml, p = 0.074). RHAL/HAL ratios were generally lower in the Chinese patients than in the non-Chinese patients, with a strong trend toward the significance level in the 20 mg and 30 mg groups (0.22 +/- 0.13 versus 0.58 +/- 0.57, p = 0.066 and 0.43 +/- 0.26 versus 0.71 +/- 0.34, p = 0.062). This study further suggests the possibility of different metabolic rates between Chinese and non-Chinese patients. Possible differences in the enzyme systems which relate to the metabolism of HAL and RHAL between Chinese and non-Chinese populations are discussed.

Pathophysiologic basis for schizophrenia and the efficacy of antipsychotics

Current concepts of schizophrenia and its treatment are discussed. Schizophrenia entails negative symptoms, such as behavioral and cognitive deficits, attributed to loss of normal functions, and positive, or florid, symptoms that originate from the disturbed function of the remainder of the brain. Schizophrenia type I has positive symptoms predominating, and schizophrenia type II has negative symptoms predominating. Many atypical antipsychotics are now in Phase II and Phase III development; the prototype, clozapine, is now available. These agents challenge traditional views about drug treatment in schizophrenia. Schizophrenia may be explained by a persistent impairment in one or more neurotransmitter or neuromodulatory regulatory mechanisms, resulting in unstable or erratic neurotransmission. A more complex conceptualization of the role of the dopaminergic system makes it possible to understand the lack of biochemical tolerance to the therapeutic effects of antipsychotics, the varied time to onset of effects and prevalence of extrapyramidal symptoms, and the differences in efficacy within and among patients. Drug selection on the basis of patient-specific biological markers and neuropsychological function might expedite treatment responses. Drug therapy should augment homeostatic mechanisms and restore appropriate dopaminergic responses to physiological stimuli. Atypical antipsychotics may act by stabilizing presynaptic activity at a new set point, activating prefrontal dopaminergic systems and inhibiting mesolimbic systems, or exerting pharmacologic or functional effects on other neurotransmitter and neuropeptidergic systems. The traditional view that schizophrenia is simply a manifestation of dopaminergic overactivity is inadequate. New investigative techniques and the study of atypical antipsychotics suggest that a dysregulation hypothesis may be more consistent with the complexities of schizophrenia.

Single- vs multiple-dose pharmacokinetics of clozapine in psychiatric patients

Clozapine plasma levels were monitored in 16 patients during a series of three consecutive treatments (single dose-multiple dose-single dose). Each patient received a single 75-mg dose (3 x 25 mg) with clozapine tablets, and serial plasma samples were collected over 48 hr after the dose. At 48 hr, a multiple-dose regimen was started, consisting of an initial dose escalation period followed by dosing at a constant regimen for at least 6 days. After the last dose, serial plasma samples were again obtained over 72 hr. Drug was then withheld for at least 7 days, a final single 75-mg dose was given, and plasma sampling was repeated. A subset of the patient population (N = 7) was used to test for a food effect during the single-dose treatments. The pharmacokinetic parameters between the initial and the final single dose periods were not significantly different. Similarly, there were no differences within patients when given the dose after fasting (fed 1 hr after dose) or with a meal. In contrast, the terminal elimination rate differed between the single-dose and the multiple-dose treatments (t1/2 m3 = 7.9 hr single dose and 14.2 hr multiple dose) (P less than 0.05) and the dose-normalized area under the plasma concentration/time curves increased 27% with multiple dosing. Since a previous study in patients (Choc et al., Pharm. Res. 4:402-405, 1987) showed dose proportionality of clozapine plasma concentrations during multiple-dose regimens, the present results cannot be described by Michaelis-Menten kinetics.

Haloperidol and reduced haloperidol plasma levels in Chinese vs. non-Chinese psychiatric patients

Haloperidol and reduced haloperidol plasma concentrations were measured in age-matched Chinese and non-Chinese patients (n = 32). Steady-state plasma concentrations were obtained 10-12 hours after the bedtime dose. Haloperidol and reduced haloperidol concentrations were measured by liquid chromatography and radioimmunoassay. Haloperidol plasma concentrations did not significantly differ between the populations, but reduced haloperidol levels were 3 times greater in non-Chinese patients than in Chinese patients. The incidence of extrapyramidal side effects was higher in Chinese patients (18 vs. 10), while non-Chinese patients with extrapyramidal symptoms had higher reduced haloperidol plasma levels. Logistic regression analysis revealed that ethnicity and reduced haloperidol/haloperidol ratios were important variables in predicting extrapyramidal symptoms. These results suggest that the metabolism and disposition of haloperidol and reduced haloperidol could differ among ethnic populations.

Clinical outcome and adverse effect profile associated with concurrent administration of alprazolam and imipramine

Clinical outcome and adverse effects associated with concurrent alprazolam and imipramine administration were studied in 29 patients with major depressive disorder who completed a 6-week trial in which they served as their own controls. Alprazolam was added on Day 8 in gradually escalating, then gradually tapering dosages while imipramine dosages remained unchanged. Significant decreases were observed in scores on the Hamilton Rating Scales for Depression and Anxiety at all later evaluation days with Day 8 as baseline. The mean total Symptom and Side Effects score decreased significantly from Day 8 to Day 22 when alprazolam doses were 1 mg q.i.d. For most side effects, total number of reports remained constant or decreased from Day 1 to later evaluation days. Standing diastolic blood pressures were significantly lower on Day 22 than on Day 1. No significant relationship was found between any rating scale score and plasma concentration data.

Pharmacokinetic factors affecting antidepressant drug clearance and clinical effect: evaluation of doxepin and imipramine--new data and review

The selection of a starting dose for an antidepressant, and subsequent clinical titration to an appropriate therapeutic dosage, should be based on pharmacokinetic and pharmacodynamic principles. In the past decade, therapeutic monitoring of antidepressant drugs and use of pharmacokinetic principles have been shown to be an improvement over the dose-response approach. Endogenous (e.g., genetic metabolic phenotype, hepatic blood flow, and protein binding) and exogenous factors (e.g., smoking, dietary habits, concurrent medications) are capable of influencing physiological and pharmacokinetic variables in patients, accounting for the marked interindividual differences in the clearance rates of cyclic antidepressants. Interpatient variability for steady-state concentrations in plasma (Cpss) greater than 20-fold are observed at a fixed dose of imipramine (r2 = 0.525, df = 346, t = 19.541, P less than 0.0001) or doxepin (r2 = 0.506, df = 128, t = 11.403, P less than 0.0001). Analysis of doxepin in plasma vs estimated in oral clearance for 61 patients demonstrates a significant decline in oral clearance as a function of Cpss. At doses approaching the upper range recommended for the treatment of depression, Cpss appear to approach, in at least a few individuals, the maximum metabolic capacity of the patient (Vmax), leading to greater-than-expected increases in concentrations for a given dosage increment. Significant alterations in oral clearance are observed when medications are administered concomitantly. A greater-than-threefold difference in mean oral doxepin clearance rates is observed between two groups of patients receiving additional medications that are either inducers or inhibitors (P less than 0.0001, df = 32, t = 6.687). Pharmacokinetic principles defining and explaining the determinants of oral clearance can provide the clinician with a greater insight into the reasons for therapeutic failure and toxicity.

Pharmacokinetic factors affecting antidepressant drug clearance and clinical effect: evaluation of doxepin and imipramine--new data and review

The selection of a starting dose for an antidepressant, and subsequent clinical titration to an appropriate therapeutic dosage, should be based on pharmacokinetic and pharmacodynamic principles. In the past decade, therapeutic monitoring of antidepressant drugs and use of pharmacokinetic principles have been shown to be an improvement over the dose-response approach. Endogenous (e.g., genetic metabolic phenotype, hepatic blood flow, and protein binding) and exogenous factors (e.g., smoking, dietary habits, concurrent medications) are capable of influencing physiological and pharmacokinetic variables in patients, accounting for the marked interindividual differences in the clearance rates of cyclic antidepressants. Interpatient variability for steady-state concentrations in plasma (Cpss) greater than 20-fold are observed at a fixed dose of imipramine (r2 = 0.525, df = 346, t = 19.541, P less than 0.0001) or doxepin (r2 = 0.506, df = 128, t = 11.403, P less than 0.0001). Analysis of doxepin in plasma vs estimated in oral clearance for 61 patients demonstrates a significant decline in oral clearance as a function of Cpss. At doses approaching the upper range recommended for the treatment of depression, Cpss appear to approach, in at least a few individuals, the maximum metabolic capacity of the patient (Vmax), leading to greater-than-expected increases in concentrations for a given dosage increment. Significant alterations in oral clearance are observed when medications are administered concomitantly. A greater-than-threefold difference in mean oral doxepin clearance rates is observed between two groups of patients receiving additional medications that are either inducers or inhibitors (P less than 0.0001, df = 32, t = 6.687). Pharmacokinetic principles defining and explaining the determinants of oral clearance can provide the clinician with a greater insight into the reasons for therapeutic failure and toxicity.

An evaluation of population pharmacokinetics in therapeutic trials. Part II. Detection of a drug-drug interaction

The use of observational data, collected during the routine clinical care of patients, has been advocated as a means to obtain clinically relevant information regarding the pharmacokinetic parameters of drugs. However, the validity of this approach and its proper role in new drug development is unclear. This study was performed to evaluate the ability of three approaches to estimate population pharmacokinetic parameters: the traditional approach, mixed-effect modeling, and a simple pharmacokinetic screen. The evaluation was performed with data collected during a multicenter, open-label study evaluating the efficacy, safety, and pharmacokinetics of imipramine and alprazolam in combination. The traditional pharmacokinetic study demonstrated a 20% decrease in the clearance of imipramine in the presence of 4 mg/day alprazolam. Mixed-effect modeling extends these findings by suggesting that the interaction is dependent on the simultaneous concentration of alprazolam, a finding that was not possible under the study design typically used for traditional pharmacokinetic studies. Although the simple screen suggests the presence of the drug-drug interaction, limited information regarding pharmacokinetic parameters is available and those parameters that can be estimated are biased.

Bioavailability of psychotropic drugs: historical perspective and pharmacokinetic overview

The evolution of the federal government's role in the regulation and evaluation of generic psychotropic medications is described. To place many of the methodologic bioequivalence issues for antipsychotic agents into perspective, the pharmacokinetics of these drugs are reviewed. Appropriate methodologies for studying the pharmacokinetics and pharmacodynamics of psychotropic drugs are in early developmental stages. Many of the issues relating to bioequivalence of generic products will not be resolved until a better understanding of these factors is developed.

Effects of smoking on haloperidol and reduced haloperidol plasma concentrations and haloperidol clearance

Plasma concentrations of haloperidol and its reduced metabolite (reduced haloperidol) were investigated in cigarette smokers (N = 23) and nonsmokers (N = 27). Steady-state plasma concentrations were obtained 12 h post bedtime dose. Haloperidol and reduced haloperidol concentrations were determined by RIA. Reduced haloperidol was separated by selective succinylation and liquid chromatography. Patients were clinically assessed with the Clinical Global Impression Scale (CGIS). Smokers had significantly lower haloperidol and reduced haloperidol plasma concentrations than nonsmokers (P less than 0.01, P less than 0.05). Clearance of haloperidol was significantly greater in smokers compared to nonsmokers (P = 0.0052). CGIS assessments did not show significant differences between smokers and nonsmokers. Plasma concentrations should be carefully monitored when patients either start or stop smoking.

Clinical pharmacokinetics of the depot antipsychotics

The clinical pharmacokinetics of the 4 depot antipsychotics for which plasma level studies are available (i.e. fluphenazine enanthate and decanoate, haloperidol decanoate, clopenthixol decanoate and flupenthixol decanoate) are reviewed. The proper study of these agents has been handicapped until recently by the necessity of accurately measuring subnanomolar concentrations in plasma. Their kinetic properties, the relationship of plasma concentrations to clinical effects, and conversion from oral to injectable therapy are discussed. The depot antipsychotics are synthesised by esterification of the active drug to a long chain fatty acid and the resultant compound is then dissolved in a vegetable oil. The absorption rate constant is slower than the elimination rate constant and therefore, the depot antipsychotics exhibit 'flip-flop' kinetics where the time to steady-state is a function of the absorption rate, and the concentration at steady-state is a function of the elimination rate. Fluphenazine is available as both an enanthate and decanoate ester (both dissolved in sesame oil), although the decanoate is more commonly used clinically. The enanthate produces peak plasma concentrations on days 2 to 3 and declines with an apparent elimination half-life (i.e. the half-time of the apparent first-order decline of plasma concentrations) of 3.5 to 4 days after a single injection. The decanoate produces an early high peak which occurs during the first day and then declines with an apparent half-life ranging from 6.8 to 9.6 days following a single injection. After multiple injections of fluphenazine decanoate, however, the mean apparent half-life increases to 14.3 days, and the time to reach steady-state is 4 to 6 weeks. Withdrawal studies with fluphenazine decanoate suggest that relapsing patients have a more rapid plasma concentration decline than non-relapsing patients, and that the plasma concentrations do not decline smoothly but may exhibit 'lumps' due to residual release from previous injection sites or multicompartment redistribution. Cigarette smoking has been found to be associated with a 2.33-fold increase in the clearance of fluphenazine decanoate. In 3 different studies, fluphenazine has been proposed to have a therapeutic range from less than 0.15 to 0.5 ng/ml with an upper therapeutic range of 4.0 ng/ml. Plasma concentrations following the decanoate injection are generally lower than, but clinically equivalent to, those attained with the oral form of the drug. Haloperidol decanoate plasma concentrations peak on the seventh day following injection although, in some patients, this peak may occur on the first day.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of carbamazepine on plasma haloperidol levels

Plasma haloperidol levels were monitored in three schizophrenic patients when carbamazepine was either added or discontinued. The percent decrease in plasma haloperidol levels due to concomitant carbamazepine therapy was between 59% and 61%. The effects of carbamazepine on plasma haloperidol levels were noted to occur in 2 to 3 weeks. Although no adverse effects occurred in the patients during therapy, careful monitoring of clinical symptoms and plasma haloperidol levels is recommended.