Variability of carbamazepine and valproate concentrations in elderly nursing home residents

PURPOSE

Measuring antiepileptic drug (AED) concentrations is common practice in nursing homes. Phenytoin (PHT) concentrations fluctuate substantially in many nursing home residents under constant dose conditions; however, the stability of other AED concentrations has not been studied. We investigated the variability of carbamazepine (CBZ) and valproate (VPA) concentrations under constant dose conditions in US nursing home residents.

METHODS

A database of elderly persons (≥65 years) in 119 nursing homes throughout the US was reviewed for residents with at least one measurement of total PHT, CBZ or VPA. Inclusion criteria for this study were three or more serum concentration measurements while on the same dose of CBZ or VPA, a two-month minimum stay, and no interfering co-medications (inducers or inhibitors). Enrollment occurred over a 2-year period. Data were collected on residents for a minimum of 6 months.

KEY FINDINGS

Of the 593 residents identified, 245 had CBZ or VPA concentrations measured and 44 (18%) met inclusion criteria (22 on CBZ and 22 VPA). Some subjects had little variability in AED concentrations, others had large fluctuations. Total CBZ concentrations within individuals varied as little as 0mg/L to as much as 6.3mg/L and total VPA concentrations as little as 10.0mg/L to as much as 77.6mg/L.

SIGNIFICANCE

The variability of PHT, CBZ, and VPA concentrations in many but not all nursing home residents implies that a re-evaluation of the role of AED concentration measurements in the management of patients is needed. Strategies for use and interpretation of AED concentration measurements need to be reevaluated.

Co-administration of morphine and oxycodone vaccines reduces the distribution of 6-monoacetylmorphine and oxycodone to brain in rats

Opioid conjugate vaccines have shown promise in animal models as a potential treatment for opioid addiction. Individual vaccines are quite specific and each targets only a limited number of structurally similar opioids. Since opioid users can switch or transition between opioids, we studied a bivalent immunization strategy of combining 2 vaccines that could target several of the most commonly abused opioids; heroin, oxycodone and their active metabolites. Morphine (M) and oxycodone (OXY) haptens were conjugated to keyhole limpet hemocyanin (KLH) through tetraglycine (Gly)(4) linkers at the C6 position. Immunization of rats with M-KLH alone produced high titers of antibodies directed against heroin, 6-monoacetylmorphine (6-MAM) and morphine. Immunization with OXY-KLH produced high titers of antibodies against oxycodone and oxymorphone. Immunization with the bivalent vaccine produced consistently high antibody titers against both immunogens. Bivalent vaccine antibody titers against the individual immunogens were higher than with the monovalent vaccines alone owing, at least in part, to cross-reactivity of the antibodies. Administration of a single concurrent intravenous dose of 6-MAM and oxycodone to rats immunized with the bivalent vaccine increased 6-MAM, morphine and oxycodone retention in serum and reduced the distribution of 6-MAM and oxycodone to brain. Vaccine efficacy correlated with serum antibody titers for both monovalent vaccines, alone or in combination. Efficacy of the individual vaccines was not compromised by their combined use. Consistent with the enhanced titers in the bivalent group, a trend toward enhanced pharmacokinetic efficacy with the bivalent vaccine was observed. These data support the possibility of co-administering two or more opioid vaccines concurrently to target multiple abusable opioids without compromising the immunogenicity or efficacy of the individual components.

Using PharmGKB to train text mining approaches for identifying potential gene targets for pharmacogenomic studies

The main objective of this study was to investigate the feasibility of using PharmGKB, a pharmacogenomic database, as a source of training data in combination with text of MEDLINE abstracts for a text mining approach to identification of potential gene targets for pathway-driven pharmacogenomics research. We used the manually curated relations between drugs and genes in PharmGKB database to train a support vector machine predictive model and applied this model prospectively to MEDLINE abstracts. The gene targets suggested by this approach were subsequently manually reviewed. Our quantitative analysis showed that a support vector machine classifiers trained on MEDLINE abstracts with single words (unigrams) used as features and PharmGKB relations used for supervision, achieve an overall sensitivity of 85% and specificity of 69%. The subsequent qualitative analysis showed that gene targets "suggested" by the automatic classifier were not anticipated by expert reviewers but were subsequently found to be relevant to the three drugs that were investigated: carbamazepine, lamivudine and zidovudine. Our results show that this approach is not only feasible but may also find new gene targets not identifiable by other methods thus making it a valuable tool for pathway-driven pharmacogenomics research.

Dosing equation for tacrolimus using genetic variants and clinical factors

AIM

To develop a dosing equation for tacrolimus, using genetic and clinical factors from a large cohort of kidney transplant recipients. Clinical factors and six genetic variants were screened for importance towards tacrolimus clearance (CL/F).

METHODS

Clinical data, tacrolimus troughs and corresponding doses were collected from 681 kidney transplant recipients in a multicentre observational study in the USA and Canada for the first 6 months post transplant. The patients were genotyped for 2,724 single nucleotide polymorphisms using a customized Affymetrix SNP chip. Clinical factors and the most important SNPs (rs776746, rs12114000, rs3734354, rs4926, rs3135506 and rs2608555) were analysed for their influence on tacrolimus CL/F.

RESULTS

The CYP3A5*1 genotype, days post transplant, age, transplant at a steroid sparing centre and calcium channel blocker (CCB) use significantly influenced tacrolimus CL/F. The final model describing CL/F (l h(-1)) was: 38.4 ×[(0.86, if days 6-10) or (0.71, if days 11-180)]×[(1.69, if CYP3A5*1/*3 genotype) or (2.00, if CYP3A5*1/*1 genotype)]× (0.70, if receiving a transplant at a steroid sparing centre) × ([age in years/50](-0.4)) × (0.94, if CCB is present). The dose to achieve the desired trough is then prospectively determined using the individuals CL/F estimate.

CONCLUSIONS

The CYP3A5*1 genotype and four clinical factors were important for tacrolimus CL/F. An individualized dose is easily determined from the predicted CL/F. This study is important towards individualization of dosing in the clinical setting and may increase the number of patients achieving the target concentration. This equation requires validation in an independent cohort of kidney transplant recipients.

Steady-state carbamazepine pharmacokinetics following oral and stable-labeled intravenous administration in epilepsy patients: effects of race and sex

Carbamazepine is a widely prescribed antiepileptic drug. Owing to the lack of an intravenous formulation, its absolute bioavailability, absolute clearance, and half-life in patients at steady state have not been determined. We developed an intravenous, stable-labeled (SL) formulation in order to characterize carbamazepine pharmacokinetics in patients. Ninety-two patients received a 100-mg infusion of SL-carbamazepine as part of their morning dose. Blood samples were collected up to 96 hours after drug administration. Plasma drug concentrations were measured with liquid chromatography-mass spectrometry, and concentration-time data were analyzed using a noncompartmental approach. Absolute clearance (l/hr/kg) was significantly lower in men (0.039 ± 0.017) than in women (0.049 ± 0.018; P = 0.007) and in African Americans (0.039 ± 0.017) when compared with Caucasians (0.048 ± 0.018; P = 0.019). Half-life was significantly longer in men than in women as well as in African Americans as compared with Caucasians. The absolute bioavailability was 0.78. Sex and racial differences in clearance may contribute to variable dosing requirements and clinical response.

A sensitive capillary GC-MS method for analysis of topiramate from plasma obtained from single-dose studies

Topiramate (Topamax®) is an antiepileptic medication used as adjunctive and monotherapy in patients with epilepsy and for migraine prophylaxis. A GC-MS assay was developed that was capable of detecting topiramate plasma concentrations following a single rectal or oral dose administration. Topiramate plasma samples were prepared by solid-phase extraction and were quantified by GC-MS analysis. The topiramate standard curves were split from 0.1 to 4 µg/mL and from 4 to 40 µg/mL in order to give a more accurate determination of the topiramate concentration. The accuracy of the standards ranged from 94.6 to 107.3% and the precision (%CV) ranged from 1.0 to 5.3% for both curves at all concentrations. The %CV for quality controls was <7.6%. The assay is both accurate and precise and will be used to complete future pharmacokinetic studies.

Genetic and environmental correlates of topiramate-induced cognitive impairment

Topiramate is an antiepileptic drug that has marked treatment-limiting side effects on specific aspects of cognitive performance in both patients and healthy volunteers. Because these severe side effects occur only in certain individuals, identifying genetic or environmental variables that influence cognitive response would be of great utility in determining whether to administer this drug to a patient. We gave an acute 100 mg oral dose of topiramate to 158 healthy volunteers and measured how the drug changed their performance on a diverse battery of cognitive tests. We found a wide range of responses to topiramate, and we demonstrated that not all tests in the battery were equally affected. There was no correlation between the effect of topiramate and either education level or baseline cognitive performance. Of interest, there was an up to 55-fold variation in the topiramate plasma levels of the participants. Our genome-wide association study (GWAS) of cognitive response did not reveal any genome-wide significant associations; the study was powered to find variants explaining at least 25% of the variation in cognitive response. Combining the results of this GWAS with a retrospective study of cognitive complaints in 290 epilepsy patients who received topiramate as part of their treatment also did not result in a significant association. Our results support the need for additional genetic studies of topiramate that use larger sample sizes.

Effects of age and dementia on temporal cycles in spontaneous speech fluency

Spontaneous speech of healthy adults consists of alternating periods of fluent and hesitant segments, forming temporal cycles in speech fluency. The regularity of these cycles may be related to the functioning of brain networks during speech planning and execution. This paper investigates the theoretical link between human cognitive functioning and temporal cycles in speech production using a quantitative time series analysis to characterize the regularity and frequency of temporal cycles in adults with differing levels and etiology of cognitive decline. We compare spontaneous speech of adults without a neurological diagnosis, both older and younger, to that of adults with frontotemporal lobar degeneration (FTLD). Two measures of temporal cycle frequency (mean and mode) calculated from the power spectrum of speech fluency represented as a time series were found to be associated with subjects' age, regardless of diagnosis of dementia. Two measures of periodicity (g-statistic and rhythmicity-index), as well as mean frequency, differentiated between adults with and without dementia. Our study confirms the presence of regular temporal cycles in spontaneous speech and suggests that temporal cycle characteristics are affected in different ways by declines in cognitive functioning due to dementia and aging.

Zonisamide discontinuation due to psychiatric and cognitive adverse events: a case-control study

OBJECTIVES

Zonisamide (ZNS) is an antiepileptic drug (AED) that has been associated with psychiatric adverse events (PAE) and cognitive adverse events (CAE); controlled studies evaluating these adverse events are limited. Our objectives were to 1) determine the incidence of PAE and CAE leading to the discontinuation of ZNS and 2) identify risk factors for PAE and CAE associated with the discontinuation of ZNS.

METHODS

All patients exposed to ZNS at MINCEP Epilepsy Care between March 2000 and September 2008 were identified. Reasons for discontinuing ZNS were documented. Separate case-control studies were performed to identify risk factors associated with the discontinuation of ZNS due to PAE or CAE via multivariate binary logistic regression.

RESULTS

A total of 544 patients were exposed to ZNS during the study period. PAE and CAE were the most frequently identified reasons for terminating ZNS therapy. The incidence of PAE severe enough to be associated with the discontinuation of ZNS was 6.9%; the incidence of CAE was 5.8%. Factors associated with termination of ZNS therapy due to PAE were past psychiatric history (p = 0.005), symptomatic generalized epilepsy (p = 0.027), and lower maximum ZNS serum concentration (mean = 17.9 mg/L vs 34.7 mg/L, p < 0.001). Independent variables associated with discontinuing ZNS due to CAE were greater number of concomitant AEDs (p = 0.011) and lower maximum ZNS serum concentration (mean = 16.6 mg/L vs 30.6 mg/L, p = 0.002).

CONCLUSIONS

We have identified clinically relevant risk factors associated with the discontinuation of ZNS. Our findings support the concept that selected patients are relatively more vulnerable to CNS adverse events when exposed to ZNS.

Characterization of the time course of carbamazepine deinduction by an enzyme turnover model

BACKGROUND AND OBJECTIVE

Carbamazepine is a potent inducer of drug metabolizing enzymes, which results in a number of clinically significant drug-drug interactions. Deinduction occurs when long-term carbamazepine therapy is discontinued. The goal of this study was to develop a population pharmacokinetic model to describe the time course of carbamazepine deinduction.

PATIENTS AND METHODS

Stable-labelled carbamazepine was administered intravenously on three occasions during the deinduction period to 15 patients with epilepsy for whom carbamazepine therapy was being discontinued. Data were analysed using a nonlinear mixed-effects model (NONMEM). An enzyme turnover model consisting of a one-compartment model linked with a hypothetical enzyme compartment was applied to characterize the time course of carbamazepine deinduction. Model evaluation was performed using the bootstrap approach and a visual predictive check.

RESULTS

In the final model, the deinduction process was accomplished by decreasing the rate of enzyme synthesis, resulting in a decrease in the relative amount of enzymes. The estimated rate constant for enzyme degradation was 0.00805 h-1, corresponding to a half-life of the combined enzymes of 86.1 hours (3.6 days).

CONCLUSION

An enzyme turnover model adequately characterized the experimental data. Based on the predicted enzyme half-life from the final model, the deinduction process should be completed within 2 weeks after carbamazepine therapy is terminated.

Epilepsy in the elderly

The elderly, often defined as those 65 years or older, are the most rapidly growing segment of the population, and onset of epilepsy is higher in this age group than in any other. This paper reviews recent developments, including a new proposed definition of epilepsy, a transgenic mouse model of Alzheimer's disease that exhibits complex partial seizures, evidence that the highest incidence of epilepsy may occur after admission to a nursing home, a challenge to the vitamin D hypothesis of osteoporosis associated with antiepileptic drugs (AEDs), evidence that the genetic complement of hepatic isoenzymes is more predictive of metabolic rate than age, and data showing that there is considerable variability in serum levels of AEDs in many nursing home residents during constant dosing conditions.

CYP2C9*1B promoter polymorphisms, in linkage with CYP2C19*2, affect phenytoin autoinduction of clearance and maintenance dose

The commonly prescribed antiepileptic drug phenytoin has a narrow therapeutic range and wide interindividual variability in clearance explained in part by CYP2C9 and CYP2C19 coding variants. After finding a paradoxically low urinary phenytoin metabolite (S)/(R) ratio in subjects receiving phenytoin maintenance therapy with a CYP2C9*1/*1 and CYP2C19*1/*2 genotype, we hypothesized that CYP2C9 regulatory polymorphisms (rPMs), G-3089A and -2663delTG, in linkage disequilibrium with CYP2C19*2 were responsible. These rPMs explained as much as 10% of the variation in phenytoin maintenance dose in epileptic patients, but were not correlated with other patients' warfarin dose requirements or with phenytoin metabolite ratio in human liver microsomes. We hypothesized the rPMs affected CYP2C9 induction by phenytoin, a pregnane X receptor (PXR), and constitutive androstane receptor (CAR) activator. Transfection studies showed that CYP2C9 reporters with wild-type versus variant alleles had similar basal activity but significantly greater phenytoin induction by cotransfected PXR, CAR, and Nrf2 and less Yin Yang 1 transcription factor repression. Phenytoin induction of CYP2C9 was greater in human hepatocytes with the CYP2C9 wild type versus variant haplotype. Therefore, CYP2C9 rPMs affect phenytoin-dependent induction of CYP2C9 and phenytoin metabolism in humans, with an effect size comparable with that for CYP2C9*2 and 2C9*3. These findings may also be relevant to the clinical use of other PXR, CAR, and Nrf2 activators.

Safety of an IV formulation of carbamazepine

An intravenous formulation of carbamazepine (CBZ) was administered to 113 (60 male; 53 female) persons with epilepsy aged 19-87 years. Subjects received 100mg of study drug as replacement for 100mg of their usual morning dose of CBZ. There were no significant changes in blood pressure or heart rate suggesting that this formulation can be developed as replacement therapy for persons unable to take oral CBZ.

Population Pharmacokinetics of Valproic Acid Concentrations in Elderly Nursing Home Residents

The objective of this study was to identify factors that affect valproic acid (VPA) apparent clearance (CL/F) in elderly nursing home residents. Inclusion criteria included residency in a nursing home for at least 2 months, aged 65 years or older, a stable dosing regimen of VPA for at least 4 weeks, VPA concentration, and complete dosing information. CL/F was analyzed by a nonlinear mixed effects model. A one-compartment model with first-order absorption and elimination was used. Both volume and absorption rate constant were fixed (14 L and 1 hr, respectively). Covariates were tested by forward inclusion and backward elimination. Interindividual variability in clearance was estimated using an exponential error model and expressed as a coefficient of variation. Residual error was estimated using a combined additive and constant coefficient of variation error model. The study consisted of 405 observations from 146 (52 men, 94 women) elderly nursing home residents. CL/F was not affected by age or weight. The population CL/F was 0.843 L/hr. CL/F was 1) 27% lower in female residents; 2) 41% greater when the resident was on concomitant metabolic inducers carbamazepine or phenytoin cotherapy; and 3) 25% greater when the syrup formulation was used. Variability in CL/F was 32.9%. Coefficient of variation and standard deviation of the residual error were 18.2% and 10.6 mg/L, respectively. The increased CL/F in patients taking VPA syrup may be the result of a decreased bioavailability (F) rather than an increased CL that could be associated with pathology requiring use of the syrup rather than an inherent property of the drug formulation. The results from this study may be useful for individualizing dose regimens in the nursing home population based on patient-specific factors.

Carbamazepine dose-concentration relationship in elderly nursing home residents

PURPOSE

To describe the dose-concentration relationships of carbamazepine (CBZ) in elderly nursing home residents and the effect of sex, age, and type of co-medications.

RESULTS

This is a cross-sectional study of elderly (> or = 65 years) nursing home residents across the United States (N=92). Data collection was from 1 June 1998 to 31 December 2000. The mean CBZ dose was 9.2+/-5.4 mg/(kg day(-1)) (+/-Standard Deviation) and serum concentration was 5.9+/-2.2mg/L. The daily dose was significantly lower in the oldest-old age group (> or = 85 years, mean 476.9 mg/day (95% confidence interval CI) 326.5-627.3) as compared to the dose in the young-old (65-74 years, mean 724.4 mg/day (CI) 603.4-845.4) (p=0.016). Adjusted for body weight, doses were similar on a mg/(kg day(-1)) basis. The majority of observed CBZ serum concentrations were at the lower end (67.4%) or below (20.7%) the suggested therapeutic range for younger adult outpatients.

CONCLUSIONS

Total daily CBZ doses and patient weight decreased with age. The average dose for elderly nursing home residents was approximately 9 mg/(kg day(-1)). Carbamazepine serum concentrations were lower than those used for younger adults, suggesting that these patients may be more sensitive to CBZ.

Factors affecting antiepileptic drug pharmacokinetics in community-dwelling elderly

Because aging is associated with changes in physiological processes, it is widely believed that antiepileptic drug pharmacodynamics and pharmacokinetics in elderly patients differ from those in younger adults. In order to better characterize these differences, this chapter reports on preliminary results from an investigation of the effect of age on steady-state phenytoin (PHT) and carbamazepine (CBZ) pharmacokinetics. Parenteral formulations of stable-labeled PHT, fosphenytoin (FOS), and CBZ were administered to elderly (> or =65 years of age) and adult (18-64 years of age) patients on maintenance regimens of PHT or CBZ; a labeled 100-mg dose was infused over 10 min, then the remainder of the patient's AED dose was administered as unlabeled drug. Blood samples were collected just before administration of the labeled drug and for up to 192 h afterward. Samples were then assayed for the concentrations of labeled and unlabeled drug. Preliminary results from 60 patients on PHT therapy (41 elderly, mean age 76 years; 19 younger adults, mean age 41 years) indicate that PHT bioavailability did not differ between the two age groups; however, absorption and elimination half-lives were more variable in the elderly patients. The elimination half-life for the entire patient population was approximately twofold longer than the value reported in the product labeling (40-50 h vs 22 h). Preliminary results from 67 patients on CBZ therapy (14 elderly, mean age 70 years; 53 younger adults, mean age 41 years) showed no apparent difference between elderly and adult patients in any parameter; however, the mean CBZ elimination half-life for the combined groups (21 h) was longer than previous estimates. These results indicate that the effect of age on CBZ and PHT absorption may result in greater variability in plasma concentrations in elderly patients, whereas the effect on half-life is modest.

Pharmacokinetics of Antiepileptic Drugs in Elderly Nursing Home Residents

With approximately 10% of elderly nursing home residents taking antiepileptic drugs (AEDs), it is critical to understand the pharmacokinetics, dosing, and possible adverse reactions of these AEDs. In this chapter, five AEDs commonly prescribed to nursing home residents will be discussed. Phenytoin (PHT), the most commonly used AED in this population, is extensively metabolized by the cytochrome P450 enzyme system, is highly protein bound, and interacts with many concomitant medications. Up to 45% of nursing home residents who receive PHT have concentrations below the range (subtherapeutic) used in adults (<65 years), while approximately 10% of residents have concentrations that are potentially toxic (>20 microg/ml). In addition, serum PHT concentrations can vary greatly within an individual resident and may be subtherapeutic one day and potentially toxic the next. Valproic acid is taken by approximately 9-17% of nursing home residents who are administered AEDs, with over half using it for nonseizure indications. Doses are approximately 16 mg/kg/day in elderly nursing home residents, but doses and serum concentrations are lower in the oldest age group (> or =85 years). A majority of residents are maintained at serum concentrations considered subtherapeutic for epilepsy, whereas relatively few (approximately 3%) are maintained at toxic levels. The average (+/-SD) carbamazepine (CBZ) dose is 8.8 +/- 4.7 mg/kg/day, yielding a mean serum concentration of 6.3 +/- 2.2 mg/liter. Subtherapeutic concentrations are found in up to 20% of serum measurements, while 2.5% of serum measurements are in the toxic range. CBZ is highly bound to serum albumin and alpha1-acid glycoprotein and is metabolized to carbamazepine-10,11-epoxide, an active metabolite thought to be responsible for some side effects. Phenobarbital (PB) is frequently combined with PHT. This combination can cause devastating side effects because both PB and PHT can produce cognitive side effects. Gabapentin is one of the newer AEDs frequently administered to nursing home residents. Its lack of both hepatic metabolism and protein binding potentially makes it a safer drug in this population.

Paradoxical urinary phenytoin metabolite (S)/(R) ratios in CYP2C19*1/*2 patients

Phenytoin (PHT) is primarily metabolized to 5-(4'-hydroxyphenyl)-5-phenylhydantoin (p-HPPH), accounting for 67-88% of an administered dose in humans. p-HPPH is formed by the cytochrome (CYP) 450 enzymes CYP2C9 and CYP2C19, then glucuronidated and excreted into the urine. CYP2C9 catalyses the prochiral formation of (R) and (S)-p-HPPH, and is approximately 40 times more stereoselective towards the formation of the (S) isomer whereas CYP2C19 is not stereoselective. Because of differential stereoselectivity, polymorphisms in the genes can alter the (S)/(R)-p-HPPH ratios. Genotyping for CYP2C9 and CYP2C19 was accomplished by a Taqman based assay. Twelve and twenty-four hour urine samples were collected from 45 epilepsy patients taking PHT under steady-state conditions and (S)/(R) ratios of p-HPPH were determined by chiral HPLC separation. The mean urinary (S)/(R) ratio in the 12-24h urine collection in subjects homozygous for CYP2C9*1/*1, CYP2C19*1/*1 was 24.2+/-3.1(n=21), whereas ratios in CYP2C9*1/*2 and CYP2C9*1/*3 subjects, were 11.1+/-3.3(n=7) and 2.7+/-0.6(n=2), respectively. One CYP2C9*2/*3 patient had a ratio of 2.1. Unexpectedly, CYP2C9*1/*1, CYP2C19*1/*2 subjects had a mean (S)/(R) ratio as low as 12.9+/-1.7(n=12). Our results are generally consistent with single dose PHT studies. However, the (S)/(R)-p-HPPH ratios for the CYP2C9*1/*1, CYP2C19*1/*2 subjects, expected to be in the range of 30-40, were only 12.9, suggesting some undetected linkage disequilibrium between CYP2C9 and CYP2C19 genes that could affect PHT elimination. Furthermore, our study suggests that measurement of urine ratios cannot be used as a marker for genotype determination.

Inherent correlation between dose and clearance in therapeutic drug monitoring settings: possible misinterpretation in population pharmacokinetic analyses

During the course of therapeutic drug monitoring (TDM), doses are adjusted to attain a target concentration range and a correlation between clearance (CL) and dose is introduced. In population pharmacokinetic analyses of such TDM data, CL has frequently been modeled as a function of dose. This paper demonstrates by simulation methodology that the TDM process does indeed introduce a correlation between dose and CL which can be interpreted as a nonlinearity. Using literature values of carbamazepine pharmacokinetics, three steady-state concentrations were simulated following a standard 1000 mg total daily dose (TDD) regimen in 100 in silico subjects. A set of clinical rules was established to adjust the TDD based on these three concentrations, as might be done in the clinical setting. Another set of concentrations using these TDM-derived TDDs for each subject (600-1600 mg) was simulated. A standard population pharmacokinetic analysis of the post-TDM data was conducted using NONMEM. This process was replicated 100 times to estimate the type II error rate. When CL was modeled without TDD, plots of WRES versus PRED demonstrated a clear pattern, as did the delta plots of CL (CL minus TVCL) versus TDD, suggesting the covariate TDD should be incorporated into the model. After TDD was included in the model for CL, the objective function value decreased by an average of 75.7 (p < 0.001). In addition, the inter-individual variability in CL expressed as a coefficient of variation decreased by an average of 9.9% and the diagnostic plots improved. Although CL was simulated to be independent of TDD, it was identified as an important covariate using standard approaches in a simulated TDM setting in 100% of the replicated simulation studies. The TDM process introduces a correlation between CL and TDD that can be misinterpreted as nonlinearity in the system.

Valproic acid doses, concentrations, and clearances in elderly nursing home residents

VPA daily dose and total VPA concentrations for 146 elderly (> or =65 years) nursing home residents collected from June 1998 to December 2000 in homes located throughout the United States are presented. Average age was 78.5+/-8.0 years old. The mean VPA daily dose was 16.2+/-11.2mg/kg and mean total VPA concentration was 48.5+/-24.8 mg/L. The majority (56.2%) of the VPA residents are being maintained at total VPA levels <50mg/L. Mean daily dose (19.4+/-11.4, 16.3+/-12.1, and 11.3+/-7.6 mg/kg/day; p=0.003) and total VPA concentration (56.4+/-25.8, 47.7+/-22.6, and 38.7+/-23.1mg/kg/day; p=0.003) decreased by age groups (65-74, 75-84, and > or =85 years). Daily dose and total VPA concentration were not different in residents receiving inhibitory or inducing co-medications, between men and women, or by albumin level. Total VPA clearance was similar between men and women, among age groups, or according to inducing or inhibiting co-medications.

Advances in antiepileptic drug treatments. A rational basis for selecting drugs for older patients with epilepsy

Incidence of epilepsy increases rapidly after age 65; recent studies indicate that approximately 10% of nursing home residents are being treated with antiepileptic drugs (AEDs). Almost all are being treated with first generation AEDs. The average nursing home patient receives six medications, has age-related changes in protein binding, decreases in hepatic and renal clearance, and may have alterations in gastrointestinal absorption. AEDs that do not have drug-drug interactions, are not metabolized by the liver, and are readily absorbed may offer benefits in this population. New studies are demonstrating that the first generation AEDs have a number of shortcomings for treating older patients, whereas some of the newer AEDs may overcome these limitations. This paper reviews the present knowledge base and compares properties of the first generation AEDs with newer agents to develop a more rational approach for drug selection in older adults.

Phenytoin use in elderly nursing home residents

BACKGROUND

Phenytoin (PHT) dosing regimens are often determined based on experience in those aged <65 years rather than in those aged >or=65 years.

OBJECTIVE

The goal of this study was to determine the impact of sex, age, receipt of concomitant inhibitors or inducers of PHT metabolism, and albumin levels on doses and total serum concentrations of PHT in elderly nursing home residents.

METHODS

Consulting pharmacists to nursing homes located throughout the United States collected data from June 1998 to December 2000. The mean daily dose per person and mean total serum PHT concentration were tested for statistical differences by sex, age group (6-74, 75-84, and >or=85 years), coadministration of PHT inhibitors or inducers, and albumin levels.

RESULTS

Data were collected from 387 residents (259 women, 128 men) of 112 nursing homes in 19 states who received PHT and for whom PHT concentrations were available. The mean (SD) age of the study population was 79.4 (7.8) years; women constituted 67.0% of the study population. The mean (SD) total daily dose and total PHT concentration were 4.9 (1.8) mg/kg and 11.7 (6.4) mg/L, respectively. In general, women received higher mean (SD) daily doses of PHT compared with men (5.1 [1.8] vs 4.6 [1.6] mg/kg, respectively; P=0.017) to achieve similar total serum concentrations (11.6 [6.4] and 12.0 [6.6] mg/L). PHT doses and serum concentrations were similar between age groups. There were no differences in daily doses (mg/kg or mg/d) or total serum concentrations of PHT based on concomitant use of inhibitors or inducers of PHT metabolism or on albumin levels,

CONCLUSIONS

In this study in elderly nursing home residents, women received higher doses of PHT than men to achieve similar total serum PHT concentrations. There were no differences in doses or total serum PHT concentrations by age group, use of concomitant inducers or inhibitors of PHT metabolism, or albumin levels.

Relative bioavailability of topiramate administered rectally

OBJECTIVE

To determine the relative bioavailability and tolerability of a topiramate (TPM) suspension after rectal administration.

DESIGN/METHOD

Seven healthy men and five healthy non-pregnant women were enrolled. A 100 or 200 mg tablet of TPM was given orally and a 200 mg dose was given rectally in a randomized, open-label, crossover study with at least a 2-week washout period between doses. Plasma samples were collected prior to dosing and the following times after each dose: 0.5, 1, 1.5, 2, 3, 4, 6, 8, 12, 16, 24, 48, 72, and 96 h. Relative bioavailability was determined by calculating the ratio of the dose-normalized area under the curve (AUC/D) for the rectal and oral doses.

RESULTS

Ten subjects completed the study. Two of the first seven subjects who received a 200 mg initial oral dose, withdrew because of side effects. The remaining subjects received a 100 mg oral dose. Three subjects received a 200 mg dose orally and rectally, and seven subjects received 100 mg orally and 200mg rectally. The average AUC/D was 0.72+/-0.18 h/l for the rectal dose and 0.76+/-0.20 h/l for the oral dose. The relative bioavailability (n=10) for TPM administered rectally was 0.95+/-0.17 with a range of 0.68-1.2. There were no statistically significant differences between the oral or rectal pharmacokinetic parameters.

CONCLUSIONS

In healthy adults, rectally administered TPM is absorbed to a similar extent as the oral dosage form. Rectal administration is an acceptable route of administration for TPM, when the oral route is temporarily unavailable.

Rapid infusion of sodium valproate in acutely ill children

The purpose of this study was to investigate the clinical safety of sodium valproate and total and unbound valproic acid plasma concentrations after rapid infusion in hospitalized, acutely ill children. Four children (5-15 years) completed the study. Sodium valproate doses (8.3-15.4 mg/kg) were administered in <or=15 minutes. No clinically significant changes in vital signs were observed nor were there any significant adverse events. Both total and unbound valproic acid concentrations were higher at 0.5 hour than at pre-infusion or 6 hours after infusion. Rapid administration of valproate to acutely ill patients can be done safely. Unbound valproic acid concentrations in acutely ill as compared with relatively healthy epilepsy patients were higher and could not be predicted based on their total pre-infusion valproic acid concentrations.

Comparison of antiepileptic drugs tiagabine, lamotrigine, and gabapentin in mouse models of acute, prolonged, and chronic nociception

Some antiepileptic drugs have been shown to be clinically effective in the treatment of neuropathic pain. This study determined whether the new antiepileptic drug tiagabine, a GABA uptake inhibitor, is efficacious in mice in a broad range of nociceptive tests (hot-plate, formalin, and dynorphin-induced chronic allodynia) and compared tiagabine's potency with two other antiepileptic drugs, gabapentin and lamotrigine. Intraperitoneally administered tiagabine, but not lamotrigine, gabapentin, or i.t. tiagabine, produced dose-dependent antinoception in the hot-plate test. A 5-min pretreatment with tiagabine (2-29 nmol i.t.) dose-dependently inhibited both the acute and late phase formalin behaviors; pretreatment with lamotrigine (4-265 nmol i.t.) inhibited only the late phase. In the formalin assay the GABA(A) antagonist bicuculline reversed the acute phase antinociception, whereas the GABA(B) antagonist saclofen reversed both the acute and late phase tiagabine-induced antinociception. Tiagabine administered i.p. but not i.t. dose-dependently reduced dynorphin-induced chronic allodynia for 120 min. Gabapentin and lamotrigine produced antinociception administered either i.t. or i.p. in a dose-dependent manner. Thus, we have shown that gabapentin and lamotrigine produced antinociception in two mouse models of pain, whereas tiagabine produced antinociception in all three mouse models of pain.

Inhibition of phenytoin hydroxylation in human liver microsomes by several selective serotonin re-uptake inhibitors

Several case reports have indicated that the selective serotonin re-uptake inhibitor (SSRI) fluoxetine increases phenytoin blood levels when given concurrently. The mechanism of this drug-drug interaction has been attributed to inhibition of CYP2C9-catalyzed hydroxylation of phenytoin to its major oxidative metabolite in humans, para-hydroxyphenyl phenyl hydantoin (HPPH). With a bank of human liver microsomes (HLM), four SSRIs (fluoxetine, norfluoxetine, sertraline, and paroxetine) were tested for inhibition of HPPH formation. Initially, the K(m) and V(max) values of phenytoin hydroxylation to HPPH were determined in the individual HLM samples. The average K(m) (n=8) was 9.7+/-2.9 microM. The V(max) varied fivefold, with an average value of 113+/-53 pmol HPPH/min/nmol CYP450. All of the SSRIs inhibited HPPH formation; resulting Ki values were 31.1+/-10.1 microM (fluoxetine) (n=5), 51.1+/-9.4 microM (norfluoxetine) (n=3), 52.2+/-21.5 microM (sertraline) (n=3), and 80.0+/-7.2 microM (paroxetine) (n=3). Sulfaphenazole (10 microM), utilized as a positive control for inhibition of HPPH formation, inhibited phenytoin hydroxylation (>95%) in all HLM samples. Diclofenac hydroxylation to 4'-OH diclofenac, a specific marker for CYP2C9 activity, was determined in HLM1-HLM6 and was highly correlated with HPPH formation in HLM1-HLM6, indicating that phenytoin hydroxylation in human liver microsomes is largely due to CYP2C9. This work presents direct evidence that the effect of fluoxetine on phenytoin blood levels may be explained by inhibition of CYP2C9-catalyzed phenytoin hydroxylation. In light of typical SSRI blood levels observed in patients, this study also suggests that the risk of a SSRI-phenytoin interaction is highest with fluoxetine and norfluoxetine, and less likely with sertraline and paroxetine.

Relative bioavailability of lamotrigine chewable dispersible tablets administered rectally

STUDY OBJECTIVE

To determine the relative bioavailability of lamotrigine (LTG) chewable dispersible tablets after rectal administration.

DESIGN

Two-period, crossover study with a 2-week washout between dosing periods.

SETTING

Clinical research center.

PATIENTS

Twelve healthy adult volunteers.

INTERVENTION

One hundred milligrams of a LTG chewable dispersible tablet was administered by oral and rectal routes.

MEASUREMENTS AND MAIN RESULTS

Plasma samples were collected before and up to 120 hours after drug administration. The samples were analyzed for LTG by high-performance liquid chromatography, and the relative bioavailability was determined. Drug concentrations were lower after rectal than after oral administration. The relative bioavailability (F = AUC(rectal)/AUC(oral)) was 0.52 +/- 0.23 (SD).

CONCLUSION

Drug prepared from LTG chewable dispersible tablets is absorbed rectally, although not to the same extent as when given orally. Rectal administration of suspension of these tablets can be an acceptable route of administration.

Rectal absorption of lamotrigine compressed tablets

PURPOSE

Interruption of oral drug administration poses a significant clinical problem for antiepileptic drugs that have no parenteral formulation. If a drug is absorbed rectally, rectal administration can be a useful alternative when the oral route of administration is not possible. The purpose of this study was to compare the single-dose pharmacokinetics of lamotrigine (LTG) compressed tablets after rectal and oral administration in healthy volunteers.

METHODS

A single LTG compressed tablet (100 mg) was administered orally and rectally to 12 volunteers in this single-dose, two-period, crossover study with a 2-week washout between doses. For rectal administration, tablets were crushed and suspended in 10 mL of water. Plasma samples were collected from 0 to 120 hr after each dose and analyzed for LTG by an HPLC method developed for this investigation.

RESULTS

LTG plasma concentrations were lower after rectal administration versus oral administration. The average area under the curve was 28.90 +/- 9.5 microg/mL/hr after rectal administration and 51.71 +/- 19.2 microg/mL/hr after oral administration. The average maximum LTG concentration was 0.53 +/- 0.14 microg/mL after rectal administration and 1.45 +/- 0.35 microg/mL after oral administration. The relative bioavailability for LTG compressed tablets was 0.63 +/- 0.33 for rectal administration. There were no drug-related rashes or serious side effects.

CONCLUSIONS

LTG suspension prepared from LTG compressed tablets is absorbed rectally, although not to the same extent or rate as when given orally.

A capillary GC-MS method for analysis of phenytoin and [13C3]-phenytoin from plasma obtained from pulse dose pharmacokinetic studies

Stable isotope analogues of phenytoin are useful for pulse dose pharmacokinetic studies in epilepsy patients. A simultaneous assay was developed to quantitate phenytoin (5,5-diphenylhydantoin) and its stable isotope analogue [13C3]-phenytoin (5,5-diphenyl-2,4,5-13C3-hydantoin) from plasma. Quantitation was achieved by GC-MS analysis of liquid/liquid extracted plasma samples, with [2H10]-phenytoin (5,5-di(pentadeuterophenyl)-hydantoin) as an internal standard. The total coefficients of variance (C.V.t) were < 7% for phenytoin (2.5-40 micrograms ml-1) and < 10.3% for [13C3]-phenytoin (0.1-6.0 micrograms ml-1). The accuracy of the assay varied from 87.8-100.1% (phenytoin, 2.5-40 micrograms ml-1) and 89.6-116.3% ([13C3]-phenytoin, 0.02-6.0 micrograms ml-1). The assay was tested under in vivo conditions by administration of a pulse dose of the stable isotope analogue to a single rat dosed to steady-state with fosphenytoin, a phenytoin prodrug. The results of the in vivo experiment demonstrate the usefulness of this assay for future pharmacokinetic studies in special population epilepsy patients.

Failure of absorption of gabapentin after rectal administration

PURPOSE

We wished to determine the extent of absorption of gabapentin (GBP) after rectal administration to children on maintenance therapy.

METHODS

Two children scheduled for extensive surgery received GBP rectally and orally. A pharmacokinetic profile was derived after each route of administration.

RESULTS

Serum GBP levels after rectal administration decreased at a rate similar to their rate of decrease after oral administration. However, GBP concentrations were much lower after rectal administration; therefore, we concluded that the aqueous solution was poorly absorbed rectally. The GBP half-life (t1/2) for the 2 children after oral doses were 4.2 and 4.8 h.

CONCLUSIONS

Rectal administration of GBP is not satisfactory when oral administration is interrupted. When oral GBP therapy is temporarily discontinued, clinicians should consider administration of alternative antiepileptic drugs (AEDs) that can be administered parenterally or rectally.

mu-opioid receptor regulates CFTR coexpressed in Xenopus oocytes in a cAMP independent manner

The objective of this study was to characterize the signaling mechanisms of the mu-opioid receptor in its coupling to the cystic fibrosis transmembrane conductance regulator (CFTR) when coexpressed in Xenopus oocytes. Because oocytes do not contain endogenous cAMP-regulated ion channels, the cAMP-modulated CFTR was coexpressed with receptors as a 'reporter' channel. Agonist treatment of oocytes coexpressing mu-opioid receptors, beta2-adrenergic receptors and CFTR produced Cl- currents in a dose-related manner and immunocytochemical analysis confirmed receptor expression. These data suggest that opioid agonists could activate adenylyl cyclase in this system to elevate cAMP levels. Heterotrimeric G protein betagamma-subunits acting on adenylyl cyclase type II would increase cAMP levels. The probable presence of adenylyl cyclase type II and other components of opioid signal transduction such as G(i alpha2), were demonstrated by RT-PCR. However, measurement of cAMP levels in individual oocytes by radioimmunoassay showed that opioid agonist application to oocytes expressing mu-opioid receptors, beta2-adrenergic receptors and CFTR did not increase cAMP levels, whereas application of the beta2-adrenergic agonist, isoproterenol, or IBMX alone did increase cAMP levels. Opioid-induced CFTR activation was not affected by either application of the broad spectrum kinase inhibitor, H7, nor by application of the specific PKA inhibitor, KT5720. Injection of free betagamma-subunits, which could activate the endogenous type II cyclase, was unable to produce measurable currents in oocytes expressing the CFTR. These studies indicate that opioid activation of the CFTR is not mediated through a cAMP/PKA pathway, by either betagamma-subunit activation of an adenylyl cyclase type II or promiscuous coupling to G(s alpha).

Functional expression of adrenergic and opioid receptors in Xenopus oocytes: interaction between alpha 2- and beta 2-adrenergic receptors

We functionally expressed alpha 2-adrenergic, beta 2-adrenergic, and delta-opioid receptors in Xenopus laevis oocytes. We detected receptor function as changes in currents carried by adenosine 3',5'-cyclic monophosphate (cAMP)-regulated chloride channels provided by the cystic fibrosis transmembrane conductance regulator (CFTR) and recorded by two-electrode voltage clamp. Co-application of forskolin and isobutylmethylxanthine (IBMX) or IBMX alone produced currents with a reversal potential indicative of chloride ions only in oocytes previously injected with mRNA encoding CFTR. Isoproterenol produced concentration-dependent responses in oocytes injected with mRNA encoding beta 2-adrenergic receptors and CFTR, and co-administration of propranolol antagonized these responses. Similarly, the alpha 2-adrenergic agonist UK14304 increased IBMX-induced currents only in oocytes injected with mRNA encoding alpha 2-adrenergic receptors and CFTR, and idazoxan antagonized these enhancements. The delta-opioid agonist DADLE produced concentration-related, naloxone-reversible increases in IBMX- and forskolin-induced currents only in oocytes injected with mRNA encoding delta-opioid receptors and CFTR. In oocytes co-injected with alpha 2, beta 2, and CFTR mRNAs, isobolographic analysis revealed an additive interaction between alpha 2- and beta 2-adrenergic receptors. These studies establish the oocyte as a cell system for studying the interactions among cAMP-modulating G protein-coupled receptors and provide another example of alternative coupling of alpha 2-adrenergic and delta-opioid receptors to G proteins, possibly Gs proteins, other than Gi proteins.