Population pharmacokinetics of gabapentin in infants and children

Development and Validation of an LC-MS/MS Method for Quantifying Gabapentin in Plasma: Application to a Pharmacokinetic Study in Cats

Gabapentin (GBP), a γ-aminobutyric acid analogue used for neuropathic pain and epilepsy, can reduce cat stress and improve veterinary exam compliance. A sensitive LC-MS/MS assay was established and rigorously validated for quantifying GBP in feline plasma. The method employed acetonitrile-mediated protein precipitation to efficiently extract GBP and its internal standard, pregabalin (PGB). Chromatographic separation was achieved within 7 min using a C18 column (2.1 × 50 mm, 1.7 μm) with a gradient mobile phase comprising 0.1% (v/v) formic acid in water and acetonitrile. The pharmacokinetics of 25  mg/kg GBP was studied by single-dose oral and intravenous administration. The results demonstrated that the method exhibited satisfactory precision, accuracy and linearity. The pharmacokinetic results showed that the Tmax, Cmax, T1/2 and AUC0-t of GBP in cats after oral administration of 25  mg/kg were (1.83 ± 0.75) h, (13.94 ± 3.75) μg/mL, (5.60 ± 1.79) h and (115.54 ± 27.56) (μg/mL) h, respectively. The results of the study indicated that after intravenous administration of 25 mg/kg GBP, the C0, T1/2 and AUC0-t were (58.82 ± 15.34) μg/mL, (3.87 ± 0.64) h and (160.44 ± 32.65) (μg/mL) h, respectively. The oral bioavailability of GBP in cats was (78.71 ± 18.55)%. In this study, a selective and sensitive LC-MS/MS method for the quantification of GBP in cat plasma was developed and validated. This method was successfully employed to assess the pharmacokinetics following the oral and intravenous administration of GBP in cats.

Development of a Population Pharmacokinetic Gabapentin Model Leveraging Therapeutic Drug Monitoring Concentrations

Background/Objectives: Gabapentin has variable pharmacokinetics (PK), which contributes to difficulty in dosing and increased risk of adverse events. The objective of this study was to leverage gabapentin concentrations from therapeutic drug monitoring (TDM) to develop a population PK (popPK) model and characterize significant covariates that impact gabapentin PK. Methods: Data were retrospectively collected from 82 hospitalized adult patients with TDM gabapentin concentrations. Renal function indicators (i.e., estimated glomerular filtration rate, creatinine clearance, acute kidney injury), body weight parameters (i.e., actual body weight, ideal body weight, adjusted body weight, lean body weight, body mass index, obesity status), fasting plasma glucose levels, and diagnosis of type 2 diabetes were tested as potential covariates. A popPK model was developed in MONOLIX (2020R1, Lixoft, France). Results: A one-compartment model best described gabapentin PK with first-order absorption, dose-dependent bioavailability, first-order elimination, and no lag time. Population parameter estimates for the volume of distribution (Vd), and clearance (Cl) were 44.61 L, and 5.73 L/h, respectively. Serum creatinine was a significant covariate on Cl. Conclusions: The popPK model highlights the importance of renal function in the interindividual variability of gabapentin PK and suggests that diabetes and body weight parameters have no impact on gabapentin PK. Moreover, our study supports the utility of leveraging data obtained from clinical TDM for popPK model development.

Dose rationale for gabapentin and tramadol in pediatric patients with chronic pain

Despite off-label use, the efficacy and safety of gabapentin and tramadol in pediatric patients (3 months to <18 years old) diagnosed with chronic pain has not been characterized. However, generating evidence based on randomized clinical trials in this population has been extremely challenging. The current investigation illustrates the use of clinical trial simulations (CTSs) as a tool for optimizing doses and protocol design for a prospective investigation in pediatric patients with chronic pain. Pharmacokinetic (PK) modeling and CTSs were used to describe the PKs of gabapentin and tramadol in the target population. In the absence of biomarkers of analgesia, systemic exposure (AUC, Css) was used to guide dose selection under the assumption of a comparable exposure-response (PKPD) relationship for either compound between adults and children. Two weight bands were identified for gabapentin, with doses titrated from 5 to 63 mg/kg. This yields gabapentin exposures (AUC0-8 ) of approximately 35 mg/L*h (1200 mg/day adult dose equivalent). For tramadol, median steady state concentrations between 200 and 300 ng/mL were achieved after doses of 2-5 mg/kg, but concentrations showed high interindividual variability. Simulation scenarios showed that titration steps are required to explore therapeutically relevant dose ranges taking into account the safety profile of both drugs. Gabapentin can be used t.i.d. at doses between 7-63 and 5-45 mg/kg for patients receiving gabapentin weighing <15 and ≥15 kg, respectively, whereas a t.i.d. regimen with doses between 1 and 5 mg/kg can be used for tramadol in patients who are not fast metabolisers.

Perioperative Indications for Gabapentinoids in Pediatrics: A Narrative Review

In recent years, there has been increased interest in using gabapentinoids (gabapentin and pregabalin) as part of multimodal medication plans or enhanced recovery after surgery protocols to mitigate several perioperative clinical challenges. Outcomes explored in the context of using gabapentinoids perioperatively in children are variable and include acute complications of pain, anxiety, nausea and vomiting, and emergence agitation, as well as the long-term postoperative outcome of chronic postsurgical pain. This narrative review describes the current literature regarding perioperative use of gabapentinoids in pediatric patients and aims to describe the role of gabapentinoids in the perioperative setting for each specific indication.

Gabapentin in Infants: Critical Evaluation of a Novel Sedative/Analgesic Medication

Chronic pain and agitation can complicate the clinical course of critically ill infants. Randomized controlled trials of analgesia and sedation in neonatal intensive care have focused on relatively short durations of exposure. To date, clinicians have few options to treat chronic visceral pain and hyperalgesia. Gabapentin has emerged as a common therapy for a diverse group of pain syndromes and neurologic conditions in adults. In neonates, case reports and series describe the successful treatment of visceral hyperalgesia arising from gastrointestinal insults with or without concomitant neurologic morbidities. Additionally, a case report and series describe the utility of gabapentin for neonatal abstinence syndrome refractory to standard pharmacotherapy. The adverse effect profile of gabapentin, most notably bradycardia and sedation, compares favorably to alternative analgesics and sedatives. However, the long-term impacts of prolonged gabapentin therapy have not been studied. Therefore, candidates for therapy must be selected carefully, and response must be assessed objectively. Future studies must assess the short-term and long-term benefits and risks of gabapentin compared to standard therapies for chronic pain and agitation in infants and refractory neonatal abstinence syndrome.

Dosing Recommendations for Pediatric Patients With Renal Impairment

A treatment gap exists for pediatric patients with renal impairment. Alterations in renal clearance and metabolism of drugs render standard dosage regimens inappropriate and may lead to drug toxicity, but these studies are not routinely conducted during drug development. The objective of this study was to examine the clinical evidence behind current renal impairment dosage recommendations for pediatric patients in a standard pediatric dosing handbook. The sources of recommendations and comparisons included the pediatric dosing handbook (Lexicomp), the U.S. Food and Drug Administration-approved manufacturer's labels, and published studies in the literature. One hundred twenty-six drugs in Lexicomp had pediatric renal dosing recommendations. Only 14% (18 of 126) of Lexicomp pediatric renal dosing recommendations referenced a pediatric clinical study, and 15% of manufacturer's labels (19 of 126) described specific dosing regimens for renally impaired pediatric patients. Forty-two products had published information on pediatric renal dosing, but 19 (45%) were case studies. When pediatric clinical studies were not referenced in Lexicomp, the renal dosing recommendations followed the adult and pediatric dosing recommendations on the manufacturer's label. Clinical evidence in pediatric patients does not exist for most renal dosing recommendations in a widely used pediatric dosing handbook, and the adult renal dosing recommendations from the manufacturer's label are currently the primary source of pediatric renal dosing information.

Diagnosis and Treatment of Chronic Neuropathic and Mixed Pain in Children and Adolescents: Results of a Survey Study amongst Practitioners

Validated diagnostic tools to diagnose chronic neuropathic and mixed pain in children are missing. Therapeutic options are often derived from therapeutics for adults. To investigate the international practice amongst practitioners for the diagnosis and treatment of chronic, neuropathic pain in children and adolescents, we performed a survey study among members of learned societies or groups whose members are known to treat pediatric pain. The survey included questions concerning practitioners and practice characteristics, assessment and diagnosis, treatment and medication. We analyzed 117 returned questionnaires, of which 41 (35%) were fully completed and 76 (65%) were partially completed. Most respondents based the diagnosis of neuropathic pain on physical examination (68 (58.1%)), patient history (67 (57.3%)), and underlying disease (59 (50.4%)) combined. Gabapentin, amitriptyline, and pregabalin were the first-choice treatments for moderate neuropathic pain. Tramadol, ibuprofen, amitriptyline, and paracetamol were the first-choice treatments for moderate mixed pain. Consensus on the diagnostic process of neuropathic pain in children and adolescents is lacking. Drug treatment varies widely for moderate, severe neuropathic, and mixed pain. Hence, diagnostic tools and therapy need to be harmonized and validated for use in children.

Population pharmacokinetics of gabapentin in patients with neuropathic pain: Lack of effect of diabetes or glycaemic control

AIMS

Gabapentin (GBP) is widely used to treat neuropathic pain, including diabetic neuropathic pain. Our objective was to evaluate the role of diabetes and glycaemic control on GBP population pharmacokinetics.

METHODS

A clinical trial was conducted in patients with neuropathic pain (n = 29) due to type 2 diabetes (n = 19) or lumbar/cervical disc herniation (n = 10). All participants were treated with a single oral dose GBP. Blood was sampled up to 24 hours after GBP administration. Data were analysed with a population approach using the stochastic approximation expectation maximization algorithm. Weight, body mass index, sex, biomarkers of renal function and diabetes, and genotypes for the main genetic polymorphisms of SLC22A2 (rs316019) and SLC22A4 (rs1050152), the genes encoding the transporters for organic cations OCT2 and OCTN1, were tested as potential covariates.

RESULTS

GBP drug disposition was described by a 1-compartment model with lag-time, first-order absorption and linear elimination. The total clearance was dependent on estimated glomerular filtration rate. Population estimates (between-subject variability in percentage) for lag time, first-order absorption rate, apparent volume of distribution and total clearance were 0.316 h (10.6%), 1.12 h^-1 (10.7%), 140 L (7.7%) and 14.7 L/h (6.97%), respectively. No significant association was observed with hyperglycaemia, glycated haemoglobin, diabetes diagnosis, age, sex, weight, body mass index, SLC22A2 or SLC22A4 genotypes.

CONCLUSION

This population pharmacokinetics model accurately estimated GBP concentrations in patients with neuropathic pain, using estimated glomerular filtrationrate as a covariate for total clearance. The distribution and excretion processes of GBP were not affected by hyperglycaemia or diabetes.

Gabapentin Use for Hospitalized Neonates

BACKGROUND

Despite some clinician advocacy for the use of gabapentin to treat neonatal irritability of presumed neurological origin, the extent of gabapentin administration to hospitalized neonates is unknown. We aimed to identify trends in gabapentin utilization among infants hospitalized in neonatal intensive care units (NICUs) across the United States and to evaluate the associations between clinical diagnoses and gabapentin treatment.

METHODS

We analyzed neonates admitted to the NICU using the Pediatric Health Information System (2005 to 2016) to measure treatment timing, duration, and frequency. We used modified Poisson regression with a robust between-cluster variance estimator to calculate a probability (adjusted relative risk) for gabapentin administration.

RESULTS

Of 278,403 neonates, 374 were administered gabapentin (0.13%). The median treatment duration was 16 days (25th to 75th percentile: 8; 40). Gabapentin use increased from 0% in 2005 to 0.39% in 2016. Treatment was prescribed to neonates at 31 of 48 studied hospitals; 73% of total treated infants localized to five neonatal intensive care units. Term (0.16%) and ≤28 weeks' gestation preterm infants (0.22%) were most likely to receive gabapentin. Varying by gestational age, a diagnosis of chromosomal abnormalities, severe bronchopulmonary dysplasia, hemorrhagic stroke, and neonatal abstinence syndrome were associated with higher treatment with gabapentin. The majority (88.8%) of treated infants did not have a seizure diagnosis.

CONCLUSION

Gabapentin use in NICU in the United States increased in recent years and varies markedly between institutions. Term infants, ≤28 weeks' gestation preterm infants, and neonates with chronic genetic, neurological, and gastrointestinal diagnoses were more likely to receive gabapentin.

Non-Invasive Extraction of Gabapentin for Therapeutic Drug Monitoring by Reverse Iontophoresis: Effect of pH, Ionic Strength, and Polyethylene Glycol 400 in the Receiving Medium

Background:

Monitoring of plasma concentrations is a necessity for narrow therapeutic index potent drugs. Development of non-invasive methods can save the patients from the trauma of needles and hence is considered as a research priority.

Introduction:

Gabapentin, an anti-epileptic drug requires therapeutic monitoring because of its narrow therapeutic index. The objective of the study was to develop a suitable method for the non-invasive extraction of gabapentin for the same.

Methods:

Transdermal reverse iontophoresis was performed using pig ear skin as a barrier membrane. Three compartment iontophoretic cells were used for the extraction study. Extractions were carried out under low intensity electric field (current intensity- 0.5 mA/cm2, electrical field approximately 5 V). The donor compartment was charged with aqueous gabapentin (10 µg/ml in phosphate buffer of pH 7.4). For studying the effect of receiving vehicle (pH, ionic strength, and enhancer) on the extraction efficiency of gabapentin, the two receiver chambers were charged with media having varying concentration of these factors. Drug content was determined by HPLC.

Results:

Compared to other pHs, cumulative extraction of gabapentin at pH 5 was significantly higher at both anode and cathode (p<0.001). At low ionic strength, extraction of gabapentin increased linearly with the increase in concentration of ions up to a certain value but at very high ionic strength the pattern reversed. Similar results were obtained with enhancer (polyethylene glycol 400). Extraction increased with increase in polyethylene glycol 400 up to 3% and then decreased.

Conclusion:

Extraction flux can be optimized by manipulation of the receiver media.

Non-inferiority double-blind randomised controlled trial comparing gabapentin versus tramadol for the treatment of chronic neuropathic or mixed pain in children and adolescents: the GABA-1 trial-a study protocol

INTRODUCTION

Gabapentin is currently used 'off-label' in children and adolescents with chronic neuropathic pain, and reliable evidence of its effects and optimal dosing are lacking.

OBJECTIVES

The GABA-1 trial aims to compare the efficacy and safety of gabapentin liquid formulation relative to tramadol and to explore the pharmacokinetics of both drugs in the treatment of chronic, neuropathic or mixed pain in the paediatric population.

METHODS AND ANALYSIS

The trial is a multicentre, double-blind, double-dummy, randomised, active-controlled, non-inferiority trial. Participants aged from 3 months to <18 years of age with moderate to severe (≥4/10 in age-appropriate pain scales) chronic neuropathic or mixed pain will be recruited in 14 clinical sites in eight European countries. A total of 94 subjects will be randomised to receive gabapentin and tramadol placebo or tramadol and gabapentin placebo throughout 16-19 weeks (including 3 weeks of titration [optimisation period], 12 weeks of treatment at a stable dose [maintenance period] and 1-4 weeks of tapering [discontinuation period]). The primary objective is to assess the efficacy of gabapentin relative to tramadol for the treatment of moderate to severe chronic neuropathic or mixed pain by comparing the difference in average pain scores (assessed by age-appropriate pain scales) between intervention arms after 15 weeks of treatment. Secondary objectives include the assessment of the safety, quality of life and global satisfaction with treatment and the description of the pharmacokinetic-pharmacodynamic relationship of gabapentin liquid formulation and tramadol oral drops to validate the recommended paediatric doses. Only rescue pain medication by paracetamol and/or ibuprofen is allowed during the trial.

ETHICS AND DISSEMINATION

Ethic approval was obtained in the eight participating countries. Results will be submitted for publication in a peer-reviewed journal and presented at one or more scientific conferences.

TRIAL REGISTRATION NUMBERS

2014-004851-30 and NCT02722603.

TRIAL STATUS

Ongoing research study, currently recruiting.

Gabapentin as add-on to morphine for severe neuropathic or mixed pain in children from age 3 months to 18 years - evaluation of the safety, pharmacokinetics, and efficacy of a new gabapentin liquid formulation: study protocol for a randomized controlled trial

Background

Gabapentin has shown efficacy in the treatment of chronic neuropathic or mixed pain in adults. Although pediatric pain specialists have extensive experience with gabapentin for the treatment of neuropathic pain, its use is off-label. Its efficacy and safety in this context have never been shown. The aim of this trial is to compare gabapentin with placebo as add-on to morphine for the treatment of severe chronic mixed or neuropathic pain in children. This trial is part of the European Union Seventh Framework Programme project Gabapentin in Paediatric Pain (GAPP) to develop a pediatric use marketing authorization for a new gabapentin suspension.

Methods/design

The GAPP-2 study is a randomized, double-blind, placebo-controlled, multicenter superiority phase II study in children with severe chronic neuropathic or mixed pain. Its primary objective is to evaluate the efficacy of a gabapentin liquid formulation as adjunctive therapy to morphine. Sixty-six eligible children 3 months to 18 years of age with severe pain (pain scores ≥ 7), stratified in three age groups, will be randomized to receive gabapentin (to an accumulating dose of 45 to 63 mg/kg/day, dependent on age) or placebo, both in addition to morphine, for 12 weeks. Randomization will be preceded by a short washout period, and treatment will be initiated by a titration period of 3 weeks. After the treatment period, medication will be tapered during 4 weeks. The primary endpoint is the average pain scores in the two treatment groups (average of two measures each day for 3 days before the end-of-study visit [V10] assessed by age-appropriate pain scales (Face, Legs, Activity, Cry, Consolability scale; Faces Pain Scale–Revised; Numeric Rating Scale). Secondary outcomes include percentage responders to treatment (subjects with 30% reduction in pain scale), number of episodes of breakthrough pain, number of rescue interventions, number of pain-free days, participant dropouts, quality of life (Pediatric Quality of Life Inventory), and acceptability of treatment. Outcomes will be measured at the end-of-study visit after 12 weeks of treatment at the optimal gabapentin dose. Groups will be compared on an intention-to-treat basis.

Discussion

We hope to provide evidence that the combination of morphine and gabapentin will provide better analgesia than morphine alone and will be safe. We also aim to obtain confirmation of the recommended pediatric dose.

Trial registration

EudractCT, 2014-004897-40. Registered on 7 September 2017.

ClinicalTrials.gov, NCT03275012. Registered on 7 September 2017.

Electronic supplementary material

The online version of this article (10.1186/s13063-018-3169-3) contains supplementary material, which is available to authorized users.

Development of spectrofluorimetric method for determination of certain antiepileptic drugs through condensation with ninhydrin and phenyl acetaldehyde

A new and simple spectrofluorimetric method was developed and validated for determination of gabapentin and pregabalin in their pure forms and pharmaceutical formulations with high sensitivity and selectivity. The proposed method was based on the formation of fluorescent products as a result of condensation reaction of investigated drugs with ninhydrin and phenyl acetaldehyde in Teorell and Stenhagen buffer medium. The fluorescent products had emission maxima at 470 nm after excitation at 375 nm. Parameters affecting the progress of the reaction and stability of the formed products were carefully investigated individually and optimized. The fluorescence-concentration plots were rectilinear over the range of 5.0-25.0 μg ml^-1 for gabapentin and 2.0-30.0 μg ml^-1 for pregabalin with excellent correlation coefficients (0.9999 for gabapentin and pregabalin). The lower detection limits were 0.45 and 0.19 μg ml^-1 for gabapentin and pregabalin, respectively. The developed method has been validated in respect to linearity, accuracy, precision, selectivity and sensitivity. In addition, the proposed method was further applied for quantitative analysis of drugs in their pharmaceutical formulations with high degree of accuracy and precision. The obtained results had an excellent agreement with the reference method, indicating no significant difference in accuracy and precision.

LC-MS/MS Quantification of Tramadol and Gabapentin Utilizing Solid Phase Extraction

An accurate, highly sensitive, and precise method for quantitative analysis of tramadol (TMD) and gabapentin (GBP) by high performance liquid chromatography and tandem mass spectrometry in human plasma was proposed and validated successfully using venlafaxine and pregabalin as internal standards (ISTDs), respectively. An aliquot of 200 μL of plasma was mixed with internal standard dilution and extraction was performed by using solid phase extraction (SPE) technique. Peak resolution was achieved on Phenomenex PFP column (50×4.6 mm, 2.6 μm). The total analytical run time was 3.8 min. Both analytes were monitored using multiple reaction monitoring (MRM) scan and the mass spectrometer was operated in positive polarity mode. The method was validated for specificity, sensitivity, precision, accuracy, and other analytical parameters. The results found were satisfactory over the linear calibration range of 1-500 ng/mL and 10-6000 ng/mL for TMD and GBP, respectively. The developed method can be ready to use by scientific community for quantification of analytes in plasma samples from various clinical studies of different dose strengths.

Interchangeability of Generic Drugs: A Nonparametric Pharmacokinetic Model of Gabapentin Generic Drugs

Substitution by generic drugs is allowed when bioequivalence to the originator drug has been established. However, it is known that similarity in exposure may not be achieved at every occasion for all individual patients when switching between formulations. The ultimate aim of our research is to investigate if pharmacokinetic subpopulations exist when subjects are exposed to bioequivalent formulations. For that purpose, we developed a pharmacokinetic model for gabapentin, based on data from a previously conducted bioavailability study comparing gabapentin exposure following administration of the gabapentin originator and three generic gabapentin formulations in healthy subjects. Both internal and external validation confirmed that the optimal model for description of the gabapentin pharmacokinetics in this comparative bioavailability study was a two‐compartment model with absorption constant, an absorption lag time, and clearance adjusted for renal function, in which each model parameter was separately estimated per administered formulation.

Population pharmacokinetics of gabapentin in healthy Korean subjects with influence of genetic polymorphisms of ABCB1

The objective of this study was to perform population pharmacokinetic (PK) analysis of gabapentin in healthy Korean subjects and to investigate the possible effect of genetic polymorphisms (1236C > T, 2677G > T/A, and 3435C > T) of ABCB1 gene on PK parameters of gabapentin. Data were collected from bioequivalence studies, in which 173 subjects orally received three different doses of gabapentin (300, 400, and 800 mg). Only data from reference formulation were used. Population pharmacokinetics (PKs) of gabapentin was estimated using a nonlinear mixed-effects model (NONMEM). Gabapentin showed considerable inter-individual variability (from 5.2- to 8.7-fold) in PK parameters. Serum concentration of gabapentin was well fitted by a one-compartment model with first-order absorption and lag time. An inhibitory Emax model was applied to describe the effect of dose on bioavailability. The oral clearance was estimated to be 11.1 L/h. The volume of distribution was characterized as 81.0 L. The absorption rate constant was estimated at 0.860 h^-1, and the lag time was predicted at 0.311 h. Oral bioavailability was estimated to be 68.8% at dose of 300 mg, 62.7% at dose of 400 mg, and 47.1% at dose of 800 mg. The creatinine clearance significantly influenced on the oral clearance (P < 0.005) and ABCB1 2677G > T/A genotypes significantly influenced on the absorption rate constant (P < 0.05) of gabapentin. However, ABCB1 1236C > T and 3435C > T genotypes showed no significant effect on gabapentin PK parameters. The results of the present study indicate that the oral bioavailability of gabapentin is decreased when its dosage is increased. In addition, ABCB1 2677G > T/A polymorphism can explain the substantial inter-individual variability in the absorption of gabapentin.

Visceral Hyperalgesia: When to Consider Gabapentin Use in Neonates-Case Study and Review

Visceral hyperalgesia refers to increased pain sensation in response to gastrointestinal sensory stimulus. In neonates with neurological impairments, gabapentin has been successfully used as a treatment for visceral hyperalgesia in neonates. The authors describe a preterm infant with myelomeningocele and persistent neuropathic pain that manifested as irritability, hypertonicity, poor weight gain, and feeding intolerance. After exclusion of other etiologies, the diagnosis of visceral hyperalgesia was suspected and the infant was treated with gabapentin. Following appropriate titration to effect and close monitoring of side effects of gabapentin, he subsequently demonstrated improved tone, decreased irritability with feedings, and appropriate weight gain. In addition, the authors provide a review of the available literature of gabapentin use in neonates and offer suggestions on when to consider starting gabapentin in a neonate with neurological impairment and chronic unexplained gastrointestinal manifestations.

Pharmacokinetic Properties of New Antiepileptic Drugs

Eight new antiepileptic drugs (AEDs) have been approved for use within the United States within the past decade. They are felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, and zonisamide. These afford clinicians with more options to increase efficacy and tolerability in the treatment of patients with epilepsy. Pharmacokinetic properties and drug interactions with other AEDs and other medications taken for comorbidities are individually discussed for each of these new agents. Drug concentrations are not routinely monitored for these newer agents, and there have been few studies designed to investigate their concentration-effect relationships. For most of these medications, the concentrations observed in responders and nonresponders overlap considerably and levels associated with efficacy are often associated with adverse events, complicating the definition of target ranges. Also, epilepsy manifests itself sporadically causing difficulty in clinically monitoring efficacy of medications. Therapeutic drug monitoring provides for the individualization of treatment for these agents, which is important because they demonstrate significant variability in inter- and intraindividual pharmaco-kinetic properties. Therapeutic drug monitoring also allows for identification of noncompliance, drug interactions, and toxicity. Current knowledge of the relationships between efficacy, toxicity, and drug concentrations is discussed.

Developmental pharmacokinetics in pediatric populations

Information on drug absorption and disposition in infants and children has increased considerably over the past 2 decades. However, the impact of specific age-related effects on pharmacokinetics, pharmacodynamics, and dose requirements remains poorly understood. Absorption can be affected by the differences in gastric pH and stomach emptying time that have been observed in the pediatric population. Low plasma protein concentrations and a higher body water composition can change drug distribution. Metabolic processes are often immature at birth, which can lead to a reduced clearance and a prolonged half-life for those drugs for which metabolism is a significant mechanism for elimination. Renal excretion is also reduced in neonates due to immature glomerular filtration, tubular secretion, and reabsorption. Limited data are available on the pharmacodynamic behavior of drugs in the pediatric population. Understanding these age effects provide a mechanistic way to identify initial doses for the pediatric population. The various factors that impact pharmacokinetics and pharmacodynamics mature towards adult values at different rates, thus requiring continual modification of drug dose regimens in neonates, infants, and children. In this paper, the age-related changes in drug absorption, distribution, metabolism, and elimination in infants and children are reviewed, and the age-related dosing regimens for this population are discussed.

A novel computational biostatistics approach implies impaired dephosphorylation of growth factor receptors as associated with severity of autism

The prevalence of autism spectrum disorders (ASDs) has increased 20-fold over the past 50 years to >1% of US children. Although twin studies attest to a high degree of heritability, the genetic risk factors are still poorly understood. We analyzed data from two independent populations using u-statistics for genetically structured wide-locus data and added data from unrelated controls to explore epistasis. To account for systematic, but disease-unrelated differences in (non-randomized) genome-wide association studies (GWAS), a correlation between P-values and minor allele frequency with low granularity data and for conducting multiple tests in overlapping genetic regions, we present a novel study-specific criterion for 'genome-wide significance'. From recent results in a comorbid disease, childhood absence epilepsy, we had hypothesized that axonal guidance and calcium signaling are involved in autism as well. Enrichment of the results in both studies with related genes confirms this hypothesis. Additional ASD-specific variations identified in this study suggest protracted growth factor signaling as causing more severe forms of ASD. Another cluster of related genes suggests chloride and potassium ion channels as additional ASD-specific drug targets. The involvement of growth factors suggests the time of accelerated neuronal growth and pruning at 9-24 months of age as the period during which treatment with ion channel modulators would be most effective in preventing progression to more severe forms of autism. By extension, the same computational biostatistics approach could yield profound insights into the etiology of many common diseases from the genetic data collected over the last decade.

A phase 2a, randomized, crossover trial of gabapentin enacarbil for the treatment of postherpetic neuralgia in gabapentin inadequate responders

OBJECTIVE

To compare the efficacy of high-dose (3,600 mg/day) vs low-dose (1,200 mg/day) oral gabapentin enacarbil (GEn) on pain intensity in adults with postherpetic neuralgia (PHN) and a history of inadequate response to ≥1,800 mg/day gabapentin.

DESIGN

Multicenter, randomized, double-blind, crossover study (NCT00617461).

SETTING

Thirty-five outpatient centers in Germany and the United States.

SUBJECTS

Subjects aged ≥18 years with a diagnosis of PHN.

METHODS

During a 2-week baseline period, subjects received open-label treatment with 1,800 mg/day gabapentin. Subjects who had a mean 24-hour average pain intensity score ≥4 during the last 7 days of the baseline period were randomized to receive GEn (1,200 or 3,600 mg/day) for treatment period 1 (28 days), followed by GEn 2,400 mg/day (4 days), and the alternate GEn dose for treatment period 2 (28 days).

RESULTS

There was a modest but significant improvement in pain intensity scores with GEn 3,600 mg vs 1,200 mg (adjusted mean [90% confidence interval] treatment difference, -0.29 [-0.48 to -0.10]; P = 0.013). The difference in efficacy between doses was observed primarily in subjects who received the higher dose during treatment period 2; certain aspects of the study design may have contributed to this outcome. Plasma steady-state gabapentin exposure during GEn treatment was as expected and consistent between treatment periods. No new safety signals or adverse event trends relating to GEn exposure were identified.

CONCLUSIONS

While the overall results demonstrated efficacy in a PHN population, the differences between treatment periods confound the interpretation. These findings could provide insight into future trial designs.

Oxcarbazepine Pharmacokinetics and Tolerability in Children With Inadequately Controlled Epilepsy

This two-part, open-label study evaluated the pharmacokinetics, safety, and tolerability of oxcarbazepine as combination therapy in 112 children 2 to 12 years old with inadequately controlled epilepsy. Part I was a pharmacokinetic study in children stratified by age (2-5 years and 6-12 years) and randomized to receive a single oxcarbazepine dose of 5 mg/kg or 15 mg/kg. Mean specific AUC and t(1/2) values of the active metabolite (MHD) were approximately 30% lower in younger children compared with older children, regardless of dose. Part II was a 4-month safety, tolerability, and pharmacokinetic study in which children received oxcarbazepine doses of 11 to 68 mg/kg/day. The mean specific oxcarbazepine daily dose was 38% higher in younger children compared with older children. Similarly, mean trough plasma MHD concentrations were 34% lower in younger children. Six (5%) children discontinued due to adverse events. Oxcarbazepine was safe and well tolerated. Younger children require higher oxcarbazepine doses because of rapid clearance.

Understanding the pharmacokinetics of anxiolytic drugs

INTRODUCTION

Anxiety disorders are considered the most common mental disorders and they can increase the risk for comorbid mood and substance use disorders, significantly contributing to the global burden of disease. For this reason, anxiolytics are the most prescribed psychoactive drugs, particularly in the Western world.

AREAS COVERED

This review aims to analyze pharmacokinetic profile, plasma level variations so as the metabolism, interactions and possible relation to clinical effect of several drugs which are used primarily as anxiolytics. The drugs analyzed include benzodiazepines, anticonvulsants (pregabalin, gabapentin), buspirone, β-blockers and antihistamines (hydroxyzine). Regarding the most frequently used anxiolytic benzodiazepines, data on alprazolam, bromazepam, chlordesmethyldiazepam, chlordiazepoxide, clotiazepam, diazepam, etizolam, lorazepam, oxazepam, prazepam and clonazepam have been detailed.

EXPERT OPINION

There is a need for a more balanced assessment of the benefits and risks associated with benzodiazepine use, particularly considering pharmacokinetic profile of the drugs to ensure that patients, who would truly benefit from these agents, are not denied appropriate treatment. An optimal pharmacological approach involving an integrative pharmacokinetic and pharmacodynamic optimization strategy would ensure better treatment and personalization of anxiety disorders. So it would be desirable for the development of new anxiolytic drug(s) that are more selective, fast acting and free from the unwanted effects associated with the traditional benzodiazepines as tolerance or dependence.

Clinical pharmacokinetics of gabapentin after administration of gabapentin enacarbil extended-release tablets in patients with varying degrees of renal function using data from an open-label, single-dose pharmacokinetic study

BACKGROUND

Gabapentin enacarbil, a transported acyloxyalkylcarbamate prodrug of gabapentin, provides predictable and dose-proportional gabapentin exposure (AUC). Gabapentin is cleared via renal excretion, and its elimination is proportional to creatinine clearance (CrCL); CrCL can, therefore, be used as a predictor of gabapentin renal clearance. Gabapentin produced from hydrolysis of gabapentin enacarbil is also eliminated via the renal clearance pathway. It was, therefore, anticipated that the pharmacokinetics of gabapentin derived from gabapentin enacarbil would also be affected by renal function.

OBJECTIVE

The objective of this study was to describe a population pharmacokinetic analysis of gabapentin enacarbil in patients with varying degrees of renal function, using data from an open-label study of gabapentin enacarbil in patients with renal impairment (XenoPort, Inc. protocol XP066), to determine whether dosage adjustments are necessary in patients with renal impairment.

METHODS

Men and women >18 years of age with a body mass index ≤34 kg/m(2) and who were, in general, healthy with the exception of renal impairment were enrolled All patients received a single 600-mg gabapentin enacarbil extended-release tablet under fed conditions. After dosing, plasma, urine, and dialysate samples were analyzed. Safety profile evaluations included adverse events, vital signs, ECGs, and laboratory values. Pharmacokinetic data were compared with those from Phase I-III studies in subjects with normal renal function to evaluate the relationship between gabapentin oral clearance (CL/F) and CrCL.

RESULTS

Fifteen patients (11 men and 4 women) were enrolled. One patient had moderate renal impairment (CrCL 30-59 mL/min), 7 patients had severe renal impairment (CrCL <30 mL/min), and 7 patients had end-stage renal disease (CrCL <15 mL/min). Ten patients were white, 4 were African American, and 1 was American Indian or Alaskan Native. Their mean (range) age was 55 (28-76) years, weight was 85.6 (62-134) kg, and body mass index was 28.3 (22-34) kg/m(2). Mean maximum plasma gabapentin concentration was 5.77 μg/mL in patients with moderate and severe renal impairment, and 5.59 μg/mL in patients with end-stage renal disease who were undergoing hemodialysis. Based on the population pharmacokinetic analysis, gabapentin CL/F after administration of gabapentin enacarbil was proportionally related to CrCL, with an approximately 1.6-fold decrease in CL/F for every 2-fold decrease in CrCL. The most frequent adverse event was dizziness (4 of 15 patients). Other adverse events that were assessed as possibly or probably related to treatment were defecation urgency, extremity pain, feeling of relaxation, and muscle weakness; each occurred in 1 patient only. All events were mild or moderate and resolved without sequelae.

CONCLUSIONS

The data suggest that dosage adjustment for gabapentin enacarbil is necessary in patients with impaired renal function. Gabapentin enacarbil, 600 mg, seemed to be well tolerated in this small selected population.

Effects of genetic variation in the novel organic cation transporter, OCTN1, on the renal clearance of gabapentin

Gabapentin is an anticonvulsant that is widely prescribed for epilepsy and other neuropathic disorders. The pharmacokinetics, particularly the absorption and renal elimination, of gabapentin appear to involve membrane transporters. In this study, we tested the hypothesis that organic cation transporter 1 (OCTN1), a multispecific transporter expressed at the apical membrane in intestine and kidney, plays a role in gabapentin pharmacokinetics and that the common variant of OCTN1, OCTN1-L503F, contributes to variation in the pharmacokinetics of the drug. We observed that OCTN1 facilitates the Na+-independent transport of gabapentin, and that the OCTN1-L503F variant is deficient in gabapentin transport activity in stably transfected HEK-293 cells (fourfold enhanced uptake of gabapentin by OCTN1-503L vs twofold enhanced uptake by OCTN1-L503F, compared to mock-transfected cells). In clinical studies, we found that in subjects homozygous for the L503F variant, gabapentin renal clearance (CL(R)) approximates the glomerular filtration rate (mean+/-SE: 110+/-12 ml/min, n=9), whereas in subjects homozygous for the reference allele, gabapentin undergoes net secretion in the kidney (141+/-7.8 ml/min, n=11, P<0.05). Creatinine clearance and OCTN1 genotype accounted for 56% of the variation in CL(R) and were the only significant predictors of CL(R) (P<0.05). Importantly, OCTN1 genotype was the only significant predictor of net secretion of gabapentin (P<0.008). Oral bioavailability of gabapentin was not affected by OCTN1 genotype. We conclude that OCTN1 contributes to active tubular secretion of gabapentin, and that this effect may be diminished or absent in individuals carrying the OCTN1-L503F polymorphism. These results provide clinical evidence of the role of genetic variation in renal drug transporters in active drug secretion in vivo.

Validated HPLC method for the determination of gabapentin in human plasma using pre-column derivatization with 1-fluoro-2,4-dinitrobenzene and its application to a pharmacokinetic study

A rapid, sensitive and accurate high-performance liquid chromatographic method with UV detection was developed and validated for the quantification of gabapentin in human plasma. Gabapentin was quantified using pre-column derivatization with 1-fluoro-2,4-dinitrobenzene following protein precipitation of plasma with acetonitrile. Amlodipine was used as internal standard. The chromatographic separation was carried out on a Nova-Pak C(18) column using a mixture of 50 mM NaH(2)PO(4) (pH=2.5)-acetonitrile (30:70, v/v) as mobile phase with UV detection at 360 nm. The flow rate was set at 1.5 ml/min. The method was linear over the range of 0.05-5 microg/ml of gabapentin in plasma (r(2)>0.999). The within-day and between-day precision values were in the range of 2-5%. The limit of quantification of the method was 0.05 microg/ml. The method was successfully used to study the pharmacokinetics of gabapentin in healthy volunteers.

Clinical development of antiepileptic drugs for children

A clinical development plan specific to children is a necessary component of every development plan for a new antiepileptic drug (AED). In the last decade, considerable discussion has occurred in the medical and regulatory communities, resulting in specific pediatric drug development legislation. Ethical issues are a foremost consideration in the design and conduct of studies. The timing of clinical studies differs between adults and children. In general, studies in children will not be performed until efficacy and safety has been demonstrated in adults. Exceptions include development of AEDs for seizure types seen only in children. Formulation preparation and dosing selection are often more challenging in children. Clinical trials including pharmacokinetic studies will be conducted in patients. A relatively small number of children, given subdivision into age groups and seizure types, are available for study. Clinical trials must be designed with children in mind, adjusting the length of the trials and the choice of controls. Efficacy extrapolation from adults may be considered for partial seizures in children, but not in infants. Seizure counts remain an appropriate efficacy endpoint; however, ascertainment in infants and younger children may require EEG monitoring. Safety specific to growing and developing children must be evaluated and long-term effects monitored.

Validated LC-MS/MS method for quantification of gabapentin in human plasma: application to pharmacokinetic and bioequivalence studies in Korean volunteers

A sensitive validated liquid chromatography-tandem mass spectrometric method (LC-MS/MS) for gabapentin (GB) in human plasma has been developed and applied to pharmacokinetic (PK) and bioequivalence (BE) studies in human. In a randomized crossover design with a 1-week period, each subject received a 300 mg GB capsule. The procedure involves a simple protein precipitation with acetonitrile and separated by LC with a Gemini C(18) column using acetonitrile-10 mm ammonium acetate (20:80, v/v, pH 3.2) as mobile phase. The GB and internal standard [(S)-(+)-alpha-aminocyclohexanepropionic acid hydrate] were analyzed using an LC-API 2000 MS/MS in multiple reaction monitoring mode. The ionization was optimized using ESI(+) and selectivity was achieved using MS/MS analysis, m/z 172.0 --> 154.0 and m/z 172.0 --> 126.0 for GB and IS, respectively. The assay exhibited good linearity over a working range of 20-5000 ng/mL for GB in human plasma with a lower limit of quantitation of 20 ng/mL. No endogenous compounds were found to interfere with the analysis. The accuracy and precision were shown for concentrations over the standard ranges. This method was successfully applied for the PK and BE studies by analysis of blood samples taken up to 36 h after an oral dose of 300 mg of GB in 24 healthy volunteers.

Gabapentin and breastfeeding: a case report

The aim of this study was to describe the milk-plasma ratio and relative infant dose of gabapentin in a breastfeeding mother and to determine the well-being of her exposed infant. The mother-infant pair was studied over a 24-hour dose interval at steady state. Gabapentin concentrations were quantified by high-performance liquid chromatography. Infant exposure was calculated as concentration in milk multiplied by an estimated milk production of 0.15 L/kg/d and normalized to the weight-adjusted maternal dose. The milk-plasma ratio was 0.86; the relative infant dose was 2.34%. The absolute infant dose was approximately 3% of the children's dose for gabapentin, and the infant plasma level of 0.4 mg/L was approximately 6% of the maternal plasma drug concentration. No adverse effects attributable to gabapentin were noted in the infant. In combination with a previously published report, these limited data support the prescription of gabapentin to a breastfeeding mother after a careful individual risk-benefit analysis.

Racial/ethnic disparities in the treatment of epilepsy: what do we know? What do we need to know?

We examine current understanding of the minority disadvantage in the clinical management of epilepsy. We performed an online literature search using several keywords (race, ethnicity, epilepsy, treatment, and quality of life) and identified additional literature through cross-referencing/manual search. The search produced 58 items published between 1977 and 2005. Of 49 original research studies, 38 were quantitative, 7 were qualitative, and 4 used mixed methods. Three or more articles were published in Epilepsia, Epilepsy &Behavior, Epilepsy Research, Neurology, and Seizure. Research concerning racial/ethnic differences in epilepsy treatment is scarce and limited by methodology, but suggests underutilization of state-of-the-art therapies by minorities. Racial/ethnic minorities also appear to have limited knowledge about epilepsy and its treatment, experience barriers to care, lack social support, and seek alternative therapies for epilepsy. We propose a framework to identify the array of disparities, points of intervention, and interventions.

Pharmacokinetic variability of new antiepileptic drugs at different ages

Newer generation antiepileptic drugs (AEDs) are increasingly used to treat epilepsies in infants, children, and the elderly. For rational prescribing in these populations, it is essential to understand the pharmacokinetic changes that occur during development and aging. Data obtained in recent years indicate that the apparent oral clearance (CL/F) of lamotrigine, topiramate, levetiracetam, oxcarbazepine, gabapentin, tiagabine, zonisamide, vigabatrin, and felbamate is considerably higher in children than in adults,the magnitude of the difference being on average in the order of 20%to 120%, depending on the drug and the age distribution of the assessed population. Information on the pharmacokinetics of these drugs in newborns is completely lacking or very sparse. Studies in the elderly have demonstrated that significant pharmacokinetic changes also occur at the other extreme of age. On average, CL/F values of newer generation AEDs have been found to be reduced by 10% to 50% compared with those found in young or middle-aged adults. These pharmacokinetic changes are clinically important and con-tribute to age-related differences in dosage requirements.

Rapid quantification of gabapentin in human plasma by liquid chromatography/tandem mass spectrometry

A simple, sensitive and rapid liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed and validated for the quantification of gabapentin, a new antiepileptic drug, in human plasma using its structural analogue, 1,1-cyclohexane diacetic acid monoamide (CAM) as internal standard. The method involved a simple protein precipitation by means of acetonitrile followed by a rapid isocratic elution with 10mM ammonium formate buffer/acetonitrile (20/80, v/v, pH 3.0) on Waters Symmetry C(18 reversed phase chromatographic column and analyzed by mass spectrometry in the multiple reaction monitoring mode. The precursor to product ion transitions of m/z 172-->154 and m/z 200-->182 were used to measure the analyte and the IS, respectively. The assay exhibited a linear dynamic range of 40-10000 ng/mL for gabapentin in human plasma. The limit of detection and lower limit of quantification in human plasma were 10 and 40 ng/mL, respectively. Acceptable precision and accuracy were obtained for concentrations over the standard curve ranges. A run time of 2 min for each sample made it possible to analyze a throughput of more than 400 human plasma samples per day. The validated method has been successfully used to analyze human plasma samples for application in pharmacokinetic, bioavailability or bioequivalence studies.

Clinical Pharmacokinetics of New-Generation Antiepileptic Drugs at the Extremes of Age

In recent years, several new-generation antiepileptic drugs (AEDs) have been introduced in clinical practice. These agents, which include felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide, are being increasingly used in the treatment of epilepsy at the extremes of age. For a rational prescribing of these drugs in specific age groups, major pharmacokinetic changes that occur during development and aging need to be taken into consideration. A review of available evidence indicates that the apparent oral clearance (CL/F) of new-generation AEDs in children is increased by 20-170% (depending on the type of drug and characteristics of the patients studied) compared with adults, with the highest CL/F values usually being observed in the youngest age groups. These findings do not necessarily apply to the first weeks of life, when drug eliminating capacity is still undergoing maturation, as in the case of lamotrigine for which preliminary data suggest that CL/F in neonates aged <2 months can be much lower than in infants aged 2-12 months. At the other extreme of age, in the elderly, CL/F is almost invariably reduced (on average by 10-50%) compared with values found in non-elderly adults. Age-related CL/F changes, together with the large interindividual pharmacokinetic variability, contribute to the need for individualised dosage requirements in these patients. Measurement of serum drug concentrations can be useful as an aid to dosage individualization in these age groups but interpretation of therapeutic drug monitoring data should also take into account the possibility of age-related changes in pharmacodynamic sensitivity and, for neonates and the elderly, alterations in drug binding to serum proteins.

Pharmacokinetic Variability of Newer Antiepileptic Drugs

A new generation of antiepileptic drugs (AEDs) has reached the market in recent years with ten new compounds: felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide. The newer AEDs in general have more predictable pharmacokinetics than older AEDs such as phenytoin, carbamazepine and valproic acid (valproate sodium), which have a pronounced inter-individual variability in their pharmacokinetics and a narrow therapeutic range. For these older drugs it has been common practice to adjust the dosage to achieve a serum drug concentration within a predefined 'therapeutic range', representing an interval where most patients are expected to show an optimal response. However, such ranges must be interpreted with caution, since many patients are optimally treated when they have serum concentrations below or above the suggested range. It is often said that there is less need for therapeutic drug monitoring (TDM) with the newer AEDs, although this is partially based on the lack of documented correlation between serum concentration and drug effects. Nevertheless, TDM may be useful despite the shortcomings of existing therapeutic ranges, by utilisation of the concept of 'individual reference concentrations' based on intra-individual comparisons of drug serum concentrations. With this concept, TDM may be indicated regardless of the existence or lack of a well-defined therapeutic range. The ten newer AEDs all have different pharmacological properties, and therefore, the usefulness of TDM for these drugs has to be assessed individually. For vigabatrin, a clear relationship between drug concentration and clinical effect cannot be expected because of its unique mode of action. Therefore, TDM of vigabatrin is mainly to check compliance. The mode of action of the other new AEDs would not preclude the applicability of TDM. For the prodrug oxcarbazepine, TDM is also useful, since the active metabolite licarbazepine is measured. For drugs that are eliminated renally completely unchanged (gabapentin, pregabalin and vigabatrin) or mainly unchanged (levetiracetam and topiramate), the pharmacokinetic variability is less pronounced and more predictable. However, the dose-dependent absorption of gabapentin increases its pharmacokinetic variability. Drug interactions can affect topiramate concentrations markedly, and individual factors such as age, pregnancy and renal function will contribute to the pharmacokinetic variability of all renally eliminated AEDs. For those of the newer AEDs that are metabolised (felbamate, lamotrigine, oxcarbazepine, tiagabine and zonisamide), pharmacokinetic variability is just as relevant as for many of the older AEDs. Therefore, TDM is likely to be useful in many clinical settings for the newer AEDs. The purpose of the present review is to discuss individually the potential value of TDM of these newer AEDs, with emphasis on pharmacokinetic variability.

Antiepileptic drug monotherapy: pediatric concerns

Selecting the optimal antiepileptic drug (AED) begins with accurate epilepsy classification, including seizure type and epilepsy syndrome if possible. Based on the available data, children with focal epilepsy, with or without secondary generalization, can be treated with a traditional or newer narrow-spectrum or broad-spectrum AED. Children with generalized convulsive epilepsy, mixed epilepsy, or seizures of an unknown type are best treated with a broad-spectrum AED. Children with childhood absence epilepsy can be treated with ethosuximide, valproate, or lamotrigine. In all cases, the best choice among the various AED options requires consideration of factors such as seizure frequency, seizure severity, AED adverse event profile, AED titration schedule, patient comorbidities, prescription plan coverage, and cost. Most children with epilepsy achieve the goal of "no seizures and no side effects" and most children eventually become seizure free without AEDs. If accurate epilepsy classification is made, clear differences in efficacy are not evident among the multiple available AEDs. Better comparative data emphasizing adverse event profiles, comorbidities and longer-term outcome are needed between the traditional and newer AEDs.

Gas chromatography-tandem mass spectrometry analysis of gabapentin in serum

A new highly sensitive analytical method for determining gabapentin [1-(aminomethyl) cyclohexaneacetic acid; Neurontin] in serum using gas chromatography/tandem mass spectrometry (GC-MS/MS) was developed. GC-MS/MS was applied to determine the levels of gabapentin in serum samples of mice at 1 and 6 h after oral or intraperitoneal treatment (300 mg/kg). At 1 h, the concentrations of the drug were 4.02 +/- 0.42 and 4.32 +/- 0.28 microg/mL in mice treated orally and intraperitoneally, respectively. At 6 h, drug levels decreased by about 66% in both groups. The method, coupling two stages of mass analysis, could be very useful in identifying the drug in complex mixtures such as blood and urine. Moreover, it is easy and rapid to perform, and sensitive enough to allow the presence of the drug to be determined at very low detection limits. It is a very reliable method for both clinical and experimental monitoring of gabapentin.

Association Between Patient Age and Gabapentin Serum Concentration-to-Dose Ratio

OBJECTIVES

To evaluate the association between patient age and gabapentin (GBP) concentration-to-dose ratio by a multivariate analysis.

METHODS

The association between patient age and the trough steady-state serum concentration of gabapentin (GBP) normalized to 1 mg/kg body weight or concentration-to-dose ratio (CDR) was retrospectively assessed by analysis of covariance. Potential confounding factors considered were GBP dosage, the number of GBP doses per day, and the presence of concomitant antiepileptic drugs (AEDs). Concentrations that had been measured in predose "trough" samples collected from 66 patients, aged 5-84 years, with partial seizures or neuropathic pain chronically receiving GBP BID (n = 21) or TID (n = 45), alone (n = 15) or in combination with other AEDs (n = 51) were used in this retrospective analysis.

RESULTS

Average GBP CDR was 0.23 +/- 0.18 (mean +/- SD). The GBP CDR increased with age (r2 = 0.46, P < 0.001), and the correlation was improved when only samples from patients taking GBP BID were separately considered (r2 = 0.68, P < 0.001). The ratio was lower in the 10 children younger than 11 years of age (0.07 +/- 0.05) than in 8 adolescents aged 12 to 18 years (0.14 +/- 0.04), lower than in 35 adults aged 19 to 65 years (0.22 +/- 0.13), and lower than in 13 patients older than 65 years of age (0.45 +/- 0.20) by 1-way analysis of variance (F = 19.4, P < 0.001). Analysis of covariance showed a significant influence on GBP CDR of patient age (P < 0.001) and the number of GBP daily doses (P < 0.01), but GBP daily dosage or concomitant AEDs had no significant influence on the ratio.

CONCLUSIONS

In this retrospective study of a small, select group of patients, (1) the GBP CDR increased significantly with age when potential confounding factors such as GBP dosage, number of GBP doses per day, and concomitant AEDs were considered by analysis of covariance, and (2) patients older than 65 years, even without any known renal disease, may have double GBP CDR than younger adults and, therefore, may need half of the GBP dose per body weight to achieve a similar concentration.

Pharmacokinetics of gabapentin in paediatric patients with uncontrolled seizures

PURPOSE

The pharmacokinetics of gabapentin in paediatric patients with uncontrolled seizures was studied.

METHODS

Thirteen paediatric patients (mean age: 9.4 years) with uncontrolled partial seizures were included. Patients received gabapentin orally until doses were individualized to 9.6-39.8 mg/kg/day. Blood samples were obtained just prior to the dose and over 8 h after gabapentin was administered in the fasting state. The plasma concentration of gabapentin was measured by a high-performance liquid chromatography assay. Pharmacokinetic parameters for gabapentin were determined by non-compartment methods using multivariate regression analysis.

RESULTS

Data from nine patients were suitable for pharmacokinetic analysis. The C(max) from 0.9 to 5.8 microg/mL (mean: 2.6 +/- 1.7 microg/mL) and T(max) from 0.5 to 2.0 h (mean: 1.6 +/- 1.0 h). The apparent clearance (Cl/F) ranged from 0.12 to 1.12 L/h/kg (mean: 0.50 +/- 0.29 L/h/kg), and the elimination half-life from 3.2 to 12.2 h (mean: 5.5 +/- 0.8 h). Five patients experienced moderate (n = 4) to severe (n = 1) aggressive behaviour and another gained weight on gabapentin.

CONCLUSIONS

Our data suggests that gabapentin pharmacokinetics can vary substantially among paediatric patients. Gabapentin was well tolerated in patients with uncontrolled partial seizures up to 6 months of therapy.

Influence of Dosage, Age, and Co-medication on Plasma Topiramate Concentrations in Children and Adults with Severe Epilepsy and Preliminary Observations on Correlations with Clinical Response

The influence of dosage, age, and co-medication on plasma topiramate (TPM) concentrations at steady state was investigated in 51 patients aged 3 to 30 years. All patients had chronic active epilepsy, and most were receiving concomitant medication with enzyme-inducing anticonvulsants (carbamazepine and phenobarbital). Plasma TPM concentrations were determined by a specific immunoassay in samples obtained before the morning dose. Thirty-five patients could be evaluated prospectively at different dose levels, and the relationship between plasma TPM concentration and dosage was linear over the assessed dose range (1.8 to 10.0 mg/kg) both in adults and in children. The influence of age on pharmacokinetic parameters could be assessed only for the 42 patients co-medicated with enzyme inducers. In these patients dose-normalized plasma TPM concentrations correlated positively with age (r = 0.59, P < 0.0001), where apparent oral clearance values (CL/F) were inversely related to age (r = 0.73, P < 0.0001). In particular, CL/F values in children aged less than 10 years (112 +/- 82 mL/kg/h, mean +/- SD, n = 14) were almost three times as high as those observed in patients aged >15 to 30 years (42 +/- 16 mL/kg/h, n = 17), whereas the CL/F value in children aged 10 to 15 years (66 +/- 22 mL/kg/h, n = 11) was intermediate between those found in the two other age groups. Patients not receiving enzyme-inducing AEDs showed lower CL/F values than did age- and gender-matched patients on enzyme-inducing co-medication. A preliminary evaluation of the relationship between plasma TPM concentration and therapeutic response could be made in 41 patients. No significant difference in drug concentration was detected between patients showing a greater than 50% reduction in seizure frequency compared with baseline (5.9 +/- 2.2 micrograms/mL, n = 30) and those having no clinical improvement (5.2 +/- 2.2 micrograms/mL, n = 11). Likewise, there was no consistent relationship between plasma TPM concentration and appearance of adverse effects. These results indicate that plasma TPM concentrations are linearly related to dosage both in adults and in children and that children aged <10 years require much greater body weight-adjusted dosage to achieve drug levels comparable to those observed in young adults. The marked increase in TPM clearance caused by enzyme-inducing co-medication was confirmed.