UGT2B7_−161C>T Polymorphism Is Associated With Lamotrigine Concentration-to-Dose Ratio in a Multivariate Study

UGT1A4*3 polymorphism influences serum concentration and therapeutic effect of lamotrigine for epilepsy treatment: A meta-analysis

BACKGROUND

Lamotrigine as a broad-spectrum antiepileptic drug, is widely applied and its clinical efficacy is highly recognized. However, significant differences are observed in blood drug concentration of lamotrigine among individuals, which may have an impact on its efficacy. UGT1A4 is the main metabolic enzyme. However, it was inconsistent for the influence of UGT1A4 genetic polymorphism on concentration and efficacy of lamotrigine therapy. This study aimed to evaluate the influences of UGT1A4*3 genetic polymorphisms on lamotrigine concentration and therapeutic effect through meta-analysis.

METHODS

The literature search was conducted in Medline, Embase, PubMed, Web of Science, Wan Fang Database, China National Knowledge Infrastructure, China Science and Technology Journal Database until January 2024. The primary outcome included the mean serum concentration, concentration-to-dose-ratio by body weight (CDR), or efficacy related to different UGT1A4*3 genotype for lamotrigine therapy. Data were collected to access the Mean Difference or odds ratio with 95% confidence interval. Meta-analysis was performed by RevMan 5.2.

RESULTS

A total of eleven studies were enrolled. The meta-analysis for mean serum concentration of lamotrigine showed no significant difference between patients carrying TT genotypes and TG and GG genotypes group (MD: 0.12, 95% [-0.35, 0.58], P = 0.62). There was significant difference in CDR (MD: 0.49, 95% [0.03, 0.94], P = 0.04) and therapeutic efficacy (OR: 7.18, 95% [4.01, 12.83], P<0.00001) of lamotrigine, however no significant difference was found in subgroup analysis of CDR of children (MD: 0.03, 95% [-0.35, 0.42], P = 0.87) between patients carrying TT genotypes and TG and GG genotypes group.

CONCLUSIONS

Polymorphism of UGT1A4*3 influenced the CDR and therapeutic efficacy of lamotrigine for antiepileptic therapy. Genotype analysis provided reference for personalized medication in the future. However, more high-quality evidences are necessary for precise and definitive conclusion.

Influence of UGT2B7, UGT1A4 and ABCG2 Polymorphisms on the Pharmacokinetics and Therapeutic Efficacy of Lamotrigine in Patients with Epilepsy

BACKGROUND AND OBJECTIVES

A substantial inter-individual variability has been observed in the pharmacokinetics of lamotrigine. The aim of the study was to investigate the impact of genetic polymorphism of the metabolizing enzymes (UGT2B7, UGT1A4) and transporter (ABCG2) on the pharmacokinetics and therapeutic efficacy of lamotrigine in patients with epilepsy.

METHODS

The genetic analysis of single-nucleotide polymorphisms was conducted using polymerase chain reaction sequence. High-performance liquid chromatography/tandem mass spectrometry was employed to measure the plasma concentrations of lamotrigine. The efficacy of lamotrigine was assessed by evaluating the reduction rate of epileptic seizure frequency.

RESULTS

This study included a cohort of 331 patients who were treated with lamotrigine as monotherapy. A linear correlation was observed between the lamotrigine concentration and daily dose taken (r = 0.58, p < 2.2e-16). Statistically significant differences were found in both the median plasma concentration and dose-adjusted concentration (C/D ratio) when comparing the ineffective to the effective group (p < 0.05). Multivariate analysis showed that UGT1A4 rs2011425, ABCG2 rs2231142 polymorphisms and age had a significant relationship with the lamotrigine concentrations (p < 0.05). Age was a predictive factor for C/D ratio (p < 0.001). Lamotrigine concentration and weight were good predictive factors for effective seizure outcomes (odds ratio [OR] = 0.715, 95% CI 0.658-0.776, p < 0.001; OR = 0.926, 95% CI 0.901-0.951, p < 0.001, respectively). The cut-off values of lamotrigine trough concentrations for clinical outcomes in the age-related groups were determined as 2.49 μg/ml (area under the receiver-operating characteristic curve [AUC]: 0.828, 95% CI 0.690-0.966), 2.70 μg/ml (AUC: 0.805, 95% CI 0.745-0.866) and 3.25 μg/ml (AUC: 0.807, 95% CI 0.686-0.928) for the adult group, adolescent group, and toddler and school-age group, respectively.

CONCLUSIONS

UGT1A4 rs2011425 and ABCG2 rs2231142 were correlated with lamotrigine concentrations. Lower lamotrigine trough concentration was found in the ineffective group and the troughs were associated with seizure outcomes.

UGT2B7 c.-161C>T polymorphism frequency in Croatian population

Uridine diphosphate glucuronosyltransferase-2B7 (UGT2B7), enzyme responsible for the elimination of a number of xenobiotics through glucuronidation, is expressed in the gut, kidneys, intestines, and brain. However, data on the frequency of UGT2B7 polymorphisms in the Croatian population are limited. The aim of this study was to assess the frequency of the UGT2B7 c.-161C>T (rs7668258) polymorphism in the Croatian population and to compare it with reported frequencies in other populations. This polymorphism is in complete linkage disequilibrium with the UGT2B7 c.802C>T (UGT2B7*2, rs7439366) variant, which is important in clinical medicine. The study reports data of 501 participants from University Hospital Centre Zagreb. All data were collected and analysed retrospectively. Genotyping was performed by real-time polymerase chain reaction (PCR) using the TaqMan® Drug Metabolism Genotyping Assay for UGT2B7 c.-161C>T (rs7668258). We found that 120 (23.95 %) participants were carriers of the UGT2B7 c.-161CC genotype and 255 (50.9 %) were heterozygous carriers (UGT2B7 c.-161CT), while 126 (25.15 %) were homozygous carriers of the variant allele (UGT2B7 c.-161TT). The frequency of the variant UGT2B7 c.-161C>T allele in this study was T=0.506. The frequency of the UGT2B7 c.-161C>T allelic variants and genotypes in the Croatian population is similar to other European populations.

Effects of genetic polymorphism of drug-metabolizing enzymes on the plasma concentrations of antiepileptic drugs in Chinese population

As a chronic brain disease, epilepsy affects ~50 million people worldwide. The traditional antiepileptic drugs (AEDs) are widely applied but showing various problems. Although the new AEDs have partially solved the problems of traditional AEDs, the current clinical application of traditional AEDs are not completely replaced by new drugs, particularly due to the large individual differences in drug plasma concentrations and narrow therapeutic windows among patients. Therefore, it is still clinically important to continue to treat patients using traditional AEDs with individualized therapeutic plans. To date, our understanding of the molecular and genetic mechanisms regulating plasma concentrations of AEDs has advanced rapidly, expanding the knowledge on the effects of genetic polymorphisms of genes encoding drug-metabolizing enzymes on the plasma concentrations of AEDs. It is increasingly imperative to summarize and conceptualize the clinical significance of recent studies on individualized therapeutic regimens. In this review, we extensively summarize the critical effects of genetic polymorphisms of genes encoding drug-metabolizing enzymes on the plasma concentrations of several commonly used AEDs as well as the clinical significance of testing genotypes related to drug metabolism on individualized drug dosage. Our review provides solid experimental evidence and clinical guidance for the therapeutic applications of these AEDs.

Tools for optimising pharmacotherapy in psychiatry (therapeutic drug monitoring, molecular brain imaging and pharmacogenetic tests): focus on antidepressants

Objectives: More than 40 drugs are available to treat affective disorders. Individual selection of the optimal drug and dose is required to attain the highest possible efficacy and acceptable tolerability for every patient.Methods: This review, which includes more than 500 articles selected by 30 experts, combines relevant knowledge on studies investigating the pharmacokinetics, pharmacodynamics and pharmacogenetics of 33 antidepressant drugs and of 4 drugs approved for augmentation in cases of insufficient response to antidepressant monotherapy. Such studies typically measure drug concentrations in blood (i.e. therapeutic drug monitoring) and genotype relevant genetic polymorphisms of enzymes, transporters or receptors involved in drug metabolism or mechanism of action. Imaging studies, primarily positron emission tomography that relates drug concentrations in blood and radioligand binding, are considered to quantify target structure occupancy by the antidepressant drugs in vivo. Results: Evidence is given that in vivo imaging, therapeutic drug monitoring and genotyping and/or phenotyping of drug metabolising enzymes should be an integral part in the development of any new antidepressant drug.Conclusions: To guide antidepressant drug therapy in everyday practice, there are multiple indications such as uncertain adherence, polypharmacy, nonresponse and/or adverse reactions under therapeutically recommended doses, where therapeutic drug monitoring and cytochrome P450 genotyping and/or phenotyping should be applied as valid tools of precision medicine.

Uridine Glucuronosyltransferase 2B7 Polymorphism-Based Pharmacogenetic Dosing of Epirubicin in FEC Chemotherapy for Early-Stage Breast Cancer

BACKGROUND

Epirubicin is metabolized by uridine glucuronosyltransferase 2B7 (UGT2B7). Patients homozygous for the minor allele (CC) in the UGT2B7 -161 promoter polymorphism have lower clearance and significantly higher rates of leukopenia compared to wild-type homozygote (TT) or heterozygote (CT) patients. This study was designed to determine if TT and CT genotype patients could tolerate a higher epirubicin dose compared to CC genotype patients.

PATIENTS AND METHODS

We studied women with histologically confirmed non-metastatic, invasive breast cancer who were scheduled to receive at least three cycles of FE₁₀₀C in the (neo)adjuvant setting. Patients received standard-dose FE₁₀₀C during the first 21-day cycle. Based on genotype, the epirubicin dose was escalated in the second and third cycles to 115 and 130 mg/m² or to 120 and 140 mg/m² for CT and TT genotype patients, respectively. The main outcome measurements were myelosuppression and dose-limiting toxicity. These were analyzed for relationships with the three genotypes.

RESULTS

Forty-five patients were enrolled (10 CC, 21 CT, and 14 TT genotypes) and received 100 mg/m² of epirubicin in the first cycle. Twelve and 10 TT patients were dose escalated at the second and third cycles, respectively; 16 CT patients were dose escalated at the second and third cycles. Leukopenia, but not febrile neutropenia, was genotype and dose dependent and increased in patients with CT and TT genotypes as their dose was increased. However, the third-cycle leukopenia rates were comparable to patients with the CC genotype receiving standard-dose epirubicin.

CONCLUSION

Pharmacogenetically guided epirubicin dosing is well tolerated and allowed dose escalation without increased toxicity.

Coffee and cigarette smoking interactions with lamotrigine

The objective of this analysis was to determine possible interactions between lamotrigine (LTG) and coffee or cigarette use. As part of the statistical analysis of factors influencing LTG pharmacokinetics (PK) in the Equigen chronic dose study, we collected prospective data from enrolled patients on their use of coffee and cigarettes. Subjects were part of a crossover replication study of generic LTG products with rigorous blood sampling and were instructed to not change their typical consumption of these products for the duration of the study. A total of 35 subjects were enrolled, with 33 subjects having sufficient data for analysis. Higher consumption of coffee was associated with a significantly lower area under the curve (AUC) and maximum concentration (Cmax) of lamotrigine (LTG). Higher cigarette use did not result in a significant change in AUC or Cmax. Coffee, but not cigarette use, either induces LTG metabolism or inhibits LTG absorption.

How Can Drug Metabolism and Transporter Genetics Inform Psychotropic Prescribing?

Many genetic variants in drug metabolizing enzymes and transporters have been shown to be relevant for treating psychiatric disorders. Associations are strong enough to feature on drug labels and for prescribing guidelines based on such data. A range of commercial tests are available; however, there is variability in included genetic variants, methodology, and interpretation. We herein provide relevant background for understanding clinical associations with specific variants, other factors that are relevant to consider when interpreting such data (such as age, gender, drug-drug interactions), and summarize the data relevant to clinical utility of pharmacogenetic testing in psychiatry and the available prescribing guidelines. We also highlight areas for future research focus in this field.

Comprehensive Measurements of Intrauterine and Postnatal Exposure to Lamotrigine

OBJECTIVE

The aim of this study was to measure and investigate correlations of lamotrigine concentrations in maternal as well as umbilical cord blood, amniotic fluid, and breast milk to account for the distribution of the drug.

METHODS

Concentrations of lamotrigine were measured in 19 mother-infant pairs at the time of delivery. To account for the penetration ratio into amniotic fluid, cord blood and breast milk, the concentration of lamotrigine in the particular environment was divided by the concentration in maternal serum. A no-intercept model was applied for associations between maternal serum concentrations, amniotic fluid, umbilical cord blood, and breast milk concentrations.

RESULTS

The mean daily dosage of lamotrigine was 351.32 mg (range 50-650 mg). We detected associations between maternal serum and amniotic fluid (β = 0.088, p < 0.001), as well as umbilical cord (β = 0.939, p < 0.001) and breast milk (β = 0.964, p < 0.001). The median penetration ratio into amniotic fluid, cord blood, and breast milk was 0.68, 0.92, and 0.77, respectively.

CONCLUSIONS

Lamotrigine concentrations in amniotic fluid, cord blood, and breast milk give evidence that the fetus/newborn is constantly exposed to lamotrigine. Maternal serum concentrations predicted exposure via amniotic fluid, umbilical cord, and breast milk. Data suggest that therapeutic drug monitoring can be recommended as part of the clinical routine in psychopharmacotherapy for pregnant or breastfeeding women.

Influence of genetic variants and antiepileptic drug co-treatment on lamotrigine plasma concentration in Mexican Mestizo patients with epilepsy

Genetic and nongenetic factors may contribute to lamotrigine (LTG) plasma concentration variability among patients. We simultaneously investigated the association of UGT1A1, UGT1A4, UGT2B7, ABCB1, ABCG2, and SLC22A1 variants, as well as antiepileptic drug co-treatment, on LTG plasma concentration in 97 Mexican Mestizo (MM) patients with epilepsy. UGT1A4*1b was associated with lower LTG dose-corrected concentrations. Patients with the UGT2B7-161T allele were treated with 21.22% higher LTG daily dose than those with CC genotype. Two novel UGT1A4 variants (c.526A>T; p.Thr185= and c.496T>C; p.Ser166Leu) were identified in one patient. Patients treated with LTG + valproic acid (VPA) showed higher LTG plasma concentration than patients did on LTG monotherapy or LTG + inducer. Despite the numerous drug-metabolizing enzymes and transporter genetic variants analyzed, our results revealed that co-treatment with VPA was the most significant factor influencing LTG plasma concentration, followed by UGT1A4*1b, and that patients carrying UGT2B7 c.-161T required higher LTG daily doses. These data provide valuable information for the clinical use of LTG in MM patients with epilepsy.

Psychotropic drug use in perinatal women with bipolar disorder

Optimal dose management of psychotropic drugs during the perinatal period reduces the risk for recurrence of mood episodes in women with Bipolar Disorder. Physiological changes during pregnancy are associated with decreases in the plasma concentrations of the majority of mood stabilizing medications. Regular symptom and drug concentration monitoring for lithium and anticonvulsants with reflexive dose adjustment improves the probability of sustained symptom remission across pregnancy. The elimination clearance trajectory across pregnancy for psychotropics dictates the frequency of laboratory monitoring and dose adjustment. The literature on the pharmacokinetics of lithium, lamotrigine, carbamazepine and atypical antipsychotics during pregnancy and postpartum are reviewed, recommendations for symptom and laboratory monitoring are proposed and recommendations for dose adjustments are presented.

Efficacy of antiepileptic drugs in the era of pharmacogenomics: A focus on childhood

BACKGROUND

In recent years advances in the field of pharmacogenomics have expanded the concept for more individualized treatments. Our aim is to provide literature data about the relationship between genetic polymorphisms and efficacy of antiepileptic drugs in children.

METHODS

Pubmed was used as the main medical database source. Only original research papers were considered. No year-of-publication restriction was placed. Quality of evidence was assessed according to American Academy of Neurology guidelines.

RESULTS

A total of 12 cross-sectional and case-control studies fulfilled our selection criteria. ABCB1 gene was associated with drug responsiveness in 2 out of 6 studies and ABCC2 gene in 1 out of 1 studies. SCN1A gene was also associated with seizure control in 4 out of 5 studies. Cytochrome P450 genes were found to significantly affect drug responsiveness in 2 out of 4 studies, while polymorphisms of uridinediphosphateglucuronosyltransferaseUGT2B7 gene predisposed to drug-resistance in 1 out of 2 studies.

CONCLUSION

Variability in genes coding for sodium channels, drug transporters and cytochrome P450 enzymes can have a significant impact on response to antiepileptic drugs. Larger prospective studies with better stratification of samples are needed to shed light on these associations.

Adjustment of the area under the concentration curve by terminal rate constant for bioequivalence assessment in a parallel-group study of lamotrigine

AIM

A new strength of lamotrigine extended-release formulation unexpectedly failed to show bioequivalence with the existing strengths at the same dose in a parallel-group study. We report the post-hoc analyses conducted to identify the cause and propose an approach for future evaluations in similar situations.

METHODS

A seemingly bimodal distribution of the half-life among the study participants prompted the use of terminal-phase-rate-constant-adjusted area under the concentration curve as the endpoint for bioequivalence assessment. Population pharmacokinetic modelling was also performed to assess the bimodal distribution of apparent clearance and the potential treatment effects on bioavailability.

RESULTS

The cause for failing to achieve bioequivalence appeared to be a biased representation of a bimodal clearance distribution between the groups. The pharmacokinetic modelling with a mixture routine identified two subpopulations: 88% had a mean clearance of 1.99 l h-1 ; 12% had a mean clearance of 0.64 l h-1 . The low-clearance population was unequally represented by 13% and 4% of subjects in the reference and test groups, respectively, and treatment appeared to have no significant effect on oral bioavailability. The bioequivalence comparison using the adjusted area concluded with a 90% confidence interval of 0.91-1.06, suggesting that treatment had no significant effect on bioavailability and the formulations would meet regulatory criteria for bioequivalence.

CONCLUSIONS

The adjustment of the area under the concentration curve adjusted by terminal-phase rate constant should be considered for situational application in bioequivalence assessment when there are multiple clearance subpopulations in a parallel-group study.

Relationship Between UGT1A4 and UGT2B7 Polymorphisms and the Steady-State Plasma Concentrations of Lamotrigine in Patients With Treatment-Resistant Depressive Disorder Receiving Lamotrigine as Augmentation Therapy

BACKGROUND

In a previous study, the authors had shown that in treatment-resistant depressive disorder, an early therapeutic response to lamotrigine augmentation therapy is dependent on its plasma concentrations. Lamotrigine is mainly metabolized by UGT1A4 and UGT2B7, and polymorphisms of said UGTs that affect enzyme activities have been reported. This study investigated the effect of these polymorphisms on the steady-state plasma concentrations (Css) of lamotrigine in patients with treatment-resistant depressive disorder receiving lamotrigine as augmentation therapy.

METHODS

The subjects were 103 depressed patients who had already shown insufficient response to at least 3 psychotropics including antidepressants, mood stabilizers, and atypical antipsychotics. The diagnoses were major depressive disorder (n = 46), bipolar II disorder (n = 44), and bipolar I disorder (n = 13). They received augmentation therapy with lamotrigine for 8 weeks. The final doses of lamotrigine were 100 mg/d for 67 subjects who were not taking valproate and 75 mg/d for 36 subjects taking valproate, respectively. Blood sampling was performed at the 8th week. Plasma concentrations of lamotrigine were measured by high-performance liquid chromatography. The genotypes of UGT1A4 142T>G, UGT2B7 -161C>T, and UGT2B7 372A>G were identified by polymerase chain reaction analyses.

RESULTS

There were no significant relationships between these polymorphisms and the Css of lamotrigine in the subjects regardless of valproate comedication.

CONCLUSIONS

This study suggests that these genetic polymorphisms do not affect the Css of lamotrigine in patients with treatment-resistant depressive disorder receiving lamotrigine as augmentation therapy.

Meta-analysis of the Influence of UGT Genetic Polymorphisms on Lamotrigine Concentration

Uridine 5'-diphospho-glucuronosyltransferases (UGTs) are involved in the metabolism of lamotrigine, but whether the UGT1A4 and UGT2B7 genetic polymorphisms affect lamotrigine concentration remains controversial. Thus, the objective of this meta-analysis was to analyse the influence of UGT1A4 and UGT2B7 genetic polymorphisms on lamotrigine concentration. Through searching, screening, selection, data extraction and quantitative analyses, the influence of UGT1A4 and UGT2B7 genetic polymorphisms on lamotrigine concentration-to-dose ratio (CDR) was assessed by meta-analysis of nine studies. Neither UGT1A4 70C>A nor 142T>G significantly affected lamotrigine CDR values (standardized difference in means [SDM] = 0.433, 95% confidence interval [CI] = -0.380-1.302; SDM = -0.458, 95% CI = -1.141-0.224, respectively). Only the UGT2B7 -161C>T homozygous variant had significantly higher CDR values than the wild-type (WT) and heterozygous variant (SDM = 0.634, 95% CI = 0.056-1.222). In conclusion, CDR of lamotrigine was significantly higher for the UGT2B7 -161C>T homozygous variant than for the WT and heterozygous variant. Thus, UGT2B7 -161C>T homozygous variant needs to receive reduced dose.

Interaction between ABCG2 421C>A polymorphism and valproate in their effects on steady-state disposition of lamotrigine in adults with epilepsy

AIMS

To investigate the impact of glucuronidation enzyme (UGT1A4*3 142T>G, UGT1A4*2 70C>A, UGT2B7 -161C>T) and transporter (MDR1/ABCB1 1236C>T, ABCG2 421C>A) polymorphisms on steady-state disposition of lamotrigine and on the lamotrigine-valproate interaction.

METHODS

Adults with epilepsy on lamotrigine monotherapy (n = 131) or lamotrigine + valproate treatment (n = 74) were genotyped and steady-state lamotrigine and valproate morning troughs were determined as a part of routine therapeutic drug monitoring.

RESULTS

No effect of UGT and MDR1/ABCB1 polymorphisms was observed. In the entire cohort, ABCG2 421A allele had no effect however an interaction between the variant allele and valproate was observed: (i) in lamotrigine-only patients, variant allele (vs. wild type homozygosity) was independently (adjustments: age, sex, body mass index, lamotrigine dose, other polymorphisms) associated with mildly lower lamotrigine troughs [geometric means ratio (GMR) = 0.76, 95% confidence interval (CI) 0.59-0.98], whereas in lamotrigine + valproate patients it was associated with higher troughs (GMR = 1.72, 95%CI 1.14-2.62); (ii) valproate cotreatment was overall associated with markedly higher troughs vs. lamotrigine monotherapy (GMR = 3.49, 95%CI 2.73-4.44), but more so in variant allele carriers (GMR = 5.24, 95%CI 3.38-8.15) than in wild type homozygotes (GMR = 2.32, 95%CI 1.89-2.83); (iii) variant allele effects in two treatment subsets and valproate effects in two genotype subsets differed by 2.36-fold (95%CI 1.39-3.67); (iv) increase in lamotrigine troughs associated with increasing valproate troughs was greater in variant allele carriers than in wild type homozygotes, i.e. variant allele effect increased with increasing valproate troughs.

CONCLUSION

This study is first to indicate a potentially relevant interaction between ABCG2 421C>A polymorphism and valproate in their effects on lamotrigine disposition.

Time course of reversal of valproate-mediated inhibition of lamotrigine

PURPOSE

Conversion to lamotrigine (LTG) monotherapy from sodium valproate (VPA) is complicated by the robust pharmacokinetic interaction between the two AEDs. This study examined changes in LTG serum concentrations immediately following VPA discontinuation.

METHODS

Ten healthy female and male adult subjects were initiated on LTG (Lamictal) 10 mg orally every morning for 30 days and VPA (Depakote ER) 500 mg orally every morning for 14 days. Morning trough (pre-dose) venous blood samples were obtained for determination of LTG and VPA concentrations on study days 14, 15, 16, 18, 20, 22, 24, 26, 28, and 30. Following the collection of the blood sample on day 15, VPA was discontinued.

RESULTS

Despite stable LTG dosage serum concentrations on study day 20, 22, 24, 26, and 28, all were significantly lower compared to baseline (p < 0.05).

CONCLUSIONS

These observations demonstrate that the pharmacokinetic interaction between LTG and VPA is reversible, and that de-inhibition appears to follow a predictable time course. Complete offset, or reversal of this interaction takes place 10-14 days after VPA discontinuation. Our data also confirms the observation that LTG oral clearance may be inhibited by very low concentrations of VPA. These data support the conversion algorithm suggested by the manufacturer, and provide guidance to the clinician. These data provide clinically useful information in developing a dosing algorithm for converting patients to LTG monotherapy.

Effect of antiepileptic drug comedication on lamotrigine concentrations

Aim

To estimate the effect size of concomitant antiepileptic therapy on the concentrations of lamotrigine, a drug often prescribed in combination with other antiepileptic drugs (AED), which can act as enzyme inducers or inhibitors.

Methods

A total of 304 patients with epilepsy, aged 18-70 years, were divided into a lamotrigine monotherapy group and groups receiving lamotrigine with AEDs that act as enzyme inducers, enzyme inhibitors, or both. We compared lamotrigine monotherapy serum concentrations with those where lamotrigine was administered with a metabolic inhibitor valproate, metabolic inducers carbamazepine, oxcarbazepine, phenobarbital, phenytoin, or topiramate, and both an inducer and an inhibitor.

Results

Comparison of trough lamotrigine monotherapy concentrations and lamotrigine polytherapy concentrations showed an almost similar median concentration in case of drug-inducers, and higher lamotrigine concentration in case of comedication with valproate as an inhibitor. A significant difference was confirmed after dose correction (P < 0.001). Significant positive correlations of lamotrigine trough serum concentrations with valproate were observed before and after the dose correction (r = 0.480, P < 0.001 and r = 0.561, P < 0.001, respectively). Positive correlations between the dose-corrected lamotrigine trough concentration and carbamazepine (r = 0.439; P < 0.001) or monohydroxy metabolite of oxcarbazepine (MHD) (r = 0.675; P < 0.001) were also significant.

Conclusion

Higher valproate levels resulted in higher inhibition potency and higher lamotrigine levels. Increased dose-corrected concentrations of inducers carbamazepine and MHD, after the process of induction was finished, did not lower lamotrigine concentrations. These findings can be of clinical significance for optimal AED dosing.

The Impact of Pharmacokinetic Interactions With Eslicarbazepine Acetate Versus Oxcarbazepine and Carbamazepine in Clinical Practice

BACKGROUND

Eslicarbazepine acetate (ESL) is a new anti-epileptic drug (AED) chemically related to oxcarbazepine (OXC) and carbamazepine (CBZ) and is increasingly used in clinical practice. The purpose of the study was to investigate 2-way pharmacokinetic interactions between ESL and other AEDs as compared to OXC and CBZ.

METHODS

Anonymous data regarding age, gender, use of AEDs, daily doses and serum concentration measurements of ESL, OXC, CBZ and lamotrigine (LTG) and other AEDs were retrieved from 2 therapeutic drug monitoring (TDM) databases in Norway. Drugs were categorized according to their known potential for interactions. Concentration/dose (C/D) ratios were calculated.

RESULTS

Data from 1100 patients were available. The C/D ratios of ESL and OXC were unchanged in combination with enzyme-inducing AEDs or valproate (VPA). The C/D ratio of CBZ decreased by 40% and 22% in combination with other enzyme-inducing AEDs or VPA, respectively, pointing to an increased clearance. ESL demonstrated no significant enzyme-inducing effect on LTG metabolism although there was a 20% and 34% decrease in the C/D ratio of LTG in combination with OXC and CBZ, respectively.

CONCLUSIONS

Possible pharmacokinetic interactions have been studied for ESL as compared to OXC and CBZ. The pharmacokinetics of ESL is not affected by enzyme-inducing AEDs or VPA and does not affect the metabolism of LTG in contrast to OXC and CBZ. The study demonstrates the value of using TDM databases to explore the potential for pharmacokinetic interactions of new AEDs.

Unexplained spikes in lamotrigine serum concentration: nonlinear elimination?

OBJECTIVES

The objective of this study was to evaluate possible nonlinear lamotrigine (LTG) pharmacokinetics at elevated concentration. LTG is reported to have linear kinetics, so that elimination rate is linearly proportional to blood concentration and a change in dose is accompanied by a proportionate change in serum concentration. We encountered patients in whom LTG serum concentration increased dramatically in response to minor or no change in LTG dose. We studied this phenomenon in patients with LTG toxicity in one clinic.

MATERIALS AND METHODS

Using electronic medical records from 1997 to 2014, we identified patients who developed clinical LTG toxicity with LTG serum concentrations >20 mg/l, after tolerating lamotrigine at lower serum concentrations. We reviewed LTG dose change and other changes that preceded the episode of toxicity.

RESULTS

Twenty-two patients had at least one episode of LTG toxicity with levels higher than 20 mg/l (of 922 patients with available levels). The peak serum concentration varied from 21.1 to 40.3 mg/l (mean 28.7). The increase in level was explained in three patients (post-delivery in one, addition of valproate in two). In the 18 others, the increase was not explained or it was disproportionate to an increase in LTG dose.

CONCLUSIONS

Spikes in LTG levels and associated clinical toxicity may occur unexpectedly, suggesting that elimination kinetics may be nonlinear in some individuals at serum concentrations in the upper range. Measurement and close monitoring of LTG levels is warranted for new symptoms that could be consistent with lamotrigine toxicity, particularly when the baseline serum concentration has been >10 mg/l.

Allele and genotype frequencies of genes relevant to anti-epileptic drug therapy in Mexican-Mestizo healthy volunteers

AIM

To determine allele and genotype frequencies of genes influencing anti-epileptic drug therapy in Mexican-Mestizo (MM) healthy volunteers, and to evaluate whether these are different from those reported for other populations.

SUBJECTS & METHODS

Thirty-nine variants of CYP3A5, EPHX1, NR1I2, HNF4A, UGT1A1, UGT2B7, ABCC2, RALBP1, SCN1A, SCN2A and GABRA1 were genotyped in 300 MM healthy volunteers.

RESULTS

All studied alleles were presented in MM, except for seven UGT1A1 variants (*6-8, 14, 15, 27 and 29). Allele and genotype frequencies showed interethnic variations when compared with European, Asian and African populations. Allele frequencies of greater than 30% were observed in ten genes.

CONCLUSION

The results presented regarding the frequencies and interethnic differences of these polymorphisms should be taken into account for future pharmacogenetic studies of anti-epileptic drugs in MM patients with epilepsy.

Pharmacokinetics of lamotrigine and its metabolite N-2-glucuronide: Influence of polymorphism of UDP-glucuronosyltransferases and drug transporters

AIMS

This study aimed to develop a population pharmacokinetic model for quantitative evaluation of the influence of genetic variants in metabolic enzymes and transporters on lamotrigine pharmacokinetics while taking into account the influence of various clinical, biochemical and demographic factors.

METHODS

We included 100 patients with epilepsy on stable dosing with lamotrigine as mono or adjunctive therapy. Lamotrigine and lamotrigine N-2-glucuronide concentrations were determined in up to two plasma samples per patient. Patients were genotyped for UGT1A4, UGT2B7, ABCB1 and SLC22A1. Population pharmacokinetic analysis was performed by non-linear mixed effects modelling. Prior knowledge from previous pharmacokinetic studies was incorporated to stabilize the modelling process. A parent-metabolite model was developed to get a more detailed view on the covariate effects on lamotrigine metabolism.

RESULTS

With a base model absorption rate (interindividual variability) was estimated at 1.96 h(-1) (72.8%), oral clearance at 2.32 l h(-1) (41.4%) and distribution volume at 77.6 l (30.2%). Lamotrigine clearance was associated with genetic factors, patient's weight, renal function, smoking and co-treatment with enzyme inducing or inhibiting drugs. In patients with UGT2B7-161TT genotype clearance was lower compared with GT and GG genotypes. Clearance was particularly high in patients with UGT2B7 372 GG genotype (compared with AA genotype it was 117%; 95% CI 44.8, 247% higher).

CONCLUSIONS

Variability in lamotrigine pharmacokinetics is large and quantification of its sources may lead to more precise individual treatment. Genotyping for UGT2B7 may be useful in various clinical settings.

UGT1A6- and UGT2B7-related valproic acid pharmacogenomics according to age groups and total drug concentration levels

AIM

The role of UGT1A6 and UGT2B7 polymorphisms and the impact of total drug plasma concentration in valproic acid (VPA) pharmacogenomics.

PATIENTS & METHODS

A total of 134 Greek patients were recruited (76 adults). Patients were genotyped for UGT1A6 19T>G, 541A>G and 552A>C and UGT2B7 802T>C polymorphisms. Patients' demographic and clinical data were registered. Natural logarithm of concentration-to-dose ratio (CDR) was also calculated as the final outcome.

RESULTS

No significant genotype-related differences in VPA metabolism were noted among various subgroups. An increased lnCDR ratio was noted in children patients compared with adults suggesting increased metabolic capability in younger ages.

CONCLUSION

UGT1A6 and UGT2B7 genotypes were not related to significant changes in VPA metabolism, even after controlling for total drug concentration levels. Younger ages were associated with increased VPA clearance rate.

An investigation of the influence of patient-related factors and comedications on lamotrigine clearance in patients with epilepsy

Lamotrigine (LTG) is one of the most widely used antiepileptic drugs. Confusion still exists in the literature as to the relative influence of age, body weight, and concomitant drug therapy on LTG pharmacokinetics. So, the objective of this study is to evaluate the influence of patient-related factors and comedication on LTG apparent oral clearance (CL/F). A therapeutic drug-monitoring database was used to identify steady-state plasma LTG concentrations in 210 patients. LTG CL/F values were calculated for each patient according to the equation CL/F (L/h per kg) = LTG daily dose (mg/kg)/Css (steady state concentration) (mg/L) × 24 h. A linear-regression model was used to assess the influence of gender, dose, age, and body weight in LTG CL/F. The influence of comedication on LTG CL/F was investigated by applying the Bonferroni post-test. The lowest LTG CL/F was found in patients comedicated with valproate (VPA) (mean, 0.0183 L/h per kg), followed by patients receiving VPA + enzyme inducers (0.0271 L/h per kg), patients on LTG monotherapy (0.0298 L/h per kg) and patients comedicated with enzyme inducers (0.056 L/h per kg) LTG CL/F correlated significantly with LTG dose (P < 0.01), but showed no significant relationship with gender, weight, and age. LTG CL/F is influenced by the type of antiepileptic comedication. The correlation with dose may be a spurious finding related to the fact that physicians, in adjusting dosage according to clinical response, are more likely to use larger doses in patients with high clearance values.

Influence of valproic acid concentration and polymorphism of UGT1A4*3, UGT2B7 -161C > T and UGT2B7*2 on serum concentration of lamotrigine in Chinese epileptic children

PURPOSE

To investigate the impact of valproic acid (VPA) and genetic polymorphism of the major metabolizing enzyme (UGT1A4, UGT2B7) of lamotrigine (LTG) and VPA on LTG concentration in Chinese epileptic children.

METHODS

Three single nucleotide polymorphisms (UGT1A4*3, UGT2B7 -161C > T and UGT2B7*2) were analyzed by polymerase chain reaction-restriction fragment length polymorphism or direct DNA sequencing. The concentrations of LTG and VPA were measured by high-performance liquid chromatography (HPLC) and fluorescence polarization immunoassay, respectively. The adjusted concentration of LTG was defined as the concentration-to-dose-ratio (CDRLTG). Data analysis was performed using IBM SPSS Statistics 21.0.

RESULTS

A total of 56 patients treated with LTG as monotherapy and 158 patients treated with LTG plus VPA were included in this study. In the polytherapy group, LTG concentration showed a good linear relationship with gender, age, daily LTG dose, VPA concentration, and UGT1A4*3 polymorphism, but had no relationship with the polymorphism of UGT2B7 -161C > T or UGT2B7*2. Moreover, LTG concentration and CDRLTG for the UGT1A4*3 were higher compared to UGT1A4*1 (LTG: 7.24 ± 3.51 vs 5.26 ± 3.27 μg/mL, p = 0.001; CDRLTG: 2.75 ± 1.02 vs 2.14 ± 0.96 μg/mL per mg/kg, p < 0.001, respectively). In the monotherapy group, there was no statistical difference between UGT1A4*3 and UGT1A4*1 in LTG concentration or CDRLTG. The patients in the polytherapy group were divided into two subgroups according to VPA concentration (lower/higher: 10-50/50-125 μg/mL). CDRLTG values of the patients carrying the UGT1A4*3 genotype were higher compared to UGT1A4*1*1 (2.86 ± 1.03 vs 2.22 ± 0.94 μg/mL per mg/kg, p = 0.001) only when the VPA concentration was higher.

CONCLUSIONS

UGT1A4*3 polymorphism had an effect on LTG concentration only with VPA co-administration, and the effect was remarkable when VPA concentration was higher.

Polymorphisms of ABCG2, ABCB1 and HNF4α are associated with Lamotrigine trough concentrations in epilepsy patients

Lamotrigine (LTG) is commonly used to control seizure in epilepsy patients and with referenced therapeutic windows in clinical practice. This study is to identify and characterize the function of genetic variants that influence the trough concentrations of LTG in epilepsy patients following monotherapy regimen (37.5-250 mg/d). Twelve single nucleotide polymorphisms (SNPs) involved in LTG metabolism and transport pathways, including UGT2B7, ABCB1, ABCG2, NR1I2 and HNF4α were genotyped in 140 Chinese epilepsy patients. Steady-state trough concentration of LTG was measured by a high-performance liquid chromatography method. Polymorphisms in ABCG2 rs2231142, rs3114020, HNF4α rs2071197 and ABCB1 rs1128503 were found to be associated with LTG CDR (concentration/dose normalized by body weight). In addition, multiple linear regression analysis revealed that ABCG2 rs2231142 had a remarkable effect on LTG concentrations which is stated to be 4.8% of the variability of LTG and may also help to interpret ethnic difference in LTG pharmacokinetics. Our findings provided new insights that SNPs of genes involved in the transport of LTG contribute to interpatient variation in LTG pharmacokinetics. Future studies are necessary to determine whether these SNPs can be used to provide LTG dosing guidance and influence seizure control and adverse reaction of LTG.

Influence of uridine diphosphate glucuronosyltransferase inducers and inhibitors on the plasma lamotrigine concentration in pediatric patients with refractory epilepsy

This study evaluated the influence of concomitant antiepileptic drugs (AEDs) on the plasma concentration of lamotrigine (LTG) in pediatric patients with epilepsy. We retrospectively reviewed 1653 plasma samples from 709 patients (aged 6 months to 16 years) and compared the concentration-to-dose ratio (CD ratio) of LTG among different AED regimens. The median CD ratio of patients receiving LTG monotherapy was 1.25 μg/mL/mg/kg. In patients receiving LTG plus VPA, the CD ratio was increased by about 140%. The CD ratio was elevated from a low VPA concentration (<40 μg/mL) and the increase was VPA concentration-dependent. In contrast, the median CD ratio of patients treated with LTG plus the inducers phenytoin, phenobarbital, and carbamazepine was 0.42, 0.63, and 0.66, respectively, and phenytoin significantly reduced the CD ratio in comparison with phenobarbital or carbamazepine (p < 0.001). Pediatric patients of all ages beyond infancy showed similar susceptibility to VPA or inducers, but infants had higher CD ratios compared with the other age groups. Among other AEDs, topiramate, ethosuximide, and rufinamide reduced the CD ratio. These findings should be useful for estimating interactions between LTG and concomitant AEDs.

Frequencies of UGT1A4*2 (P24T) and *3 (L48V) and their effects on serum concentrations of lamotrigine

The gene encoding uridine diphosphate glucuronosyltransferase (UGT) 1A4 shows considerable polymorphism. Several common drugs are metabolised by UGT1A4, among them lamotrigine (LTG). Experimental and clinical studies suggest that certain variants of UGT1A4 are associated with altered enzyme activity. However, results are conflicting. This clinical study aimed to investigate the frequencies of two common UGT1A4 variants, *2 (P24T) and *3 (L48V), and their potential effects on serum concentrations of LTG. The *2 variant was associated with a trend towards higher serum concentrations, while the *3 variant was associated with significantly lower serum concentrations of LTG. The calculated allele frequencies were in the same range as in earlier studies on Caucasian populations. To our knowledge, this is the first study suggesting a clinical effect of UGT1A4*2. Further study is needed to confirm this finding.

Pregnancy and pharmacogenomics in the context of drug metabolism and response

It is well-known that profound physiological and biochemical changes occur throughout the course of pregnancy. At the same time, the role of pharmacogenomics in modulating the metabolism and response profile to numerous medications has been elucidated. Yet, the clinical impact of pharmacogenomics during pregnancy is less well understood. We present an overview of factors modulating the pharmacokinetics and pharmacodynamics of medications throughout the time span of pregnancy while providing insights on how pharmacogenomics may contribute to interindividual variability in drug metabolism and response amongst pregnant women.

Implications of pharmacogenetics for the therapeutic use of antiepileptic drugs

INTRODUCTION

Epilepsy is a chronic neurological disease manifesting as recurrent seizures. Despite the availability of numerous antiepileptic drugs (AEDs), one-third of the patients are not responsive to treatment. Such inter-individual variability in the response to AEDs may be partly explained by genetic differences. This review summarizes the pharmacogenetics (PGx) of AEDs. In addition, a model-based approach is presented that enables the integration of PGx data with other relevant sources of variability, such as demographic characteristics and co-medications.

AREAS COVERED

A comprehensive overview is provided of the data available in the literature on the evidence for correlations between genetic mutations and pharmacokinetic (PK) and/or pharmacodynamics (PD) of AEDs. This information is then used in an integrated manner in the second part, where PGx differences are parameterized as covariates in PK and PKPD models.

EXPERT OPINION

Polymorphisms are profuse in the PK and PD of AEDs. However, understanding of their clinical implication remains limited due to the lack of methodologies that discriminate the contribution of other sources of variability in CNS exposure to drugs. A model-based approach, in which other intrinsic (e.g., demographic covariates) and extrinsic (e.g., drug-drug interactions) factors are evaluated concurrently is needed to ensure optimization and individualization of treatment in epileptic patients.

Pharmacokinetics of lamotrigine in paediatric and young adult epileptic patients--nonlinear mixed effects modelling approach

PURPOSE

The purpose of the study was to examine and describe adjunctive lamotrigine (LTG) pharmacokinetics in paediatric and young adult patients using a nonlinear mixed effects modelling (NONMEM) approach.

METHODS

The study included 53 patients (age range 3-35 years) who were concomitantly treated with carbamazepine (CBZ) and/or valproic acid (VPA). A total of 70 blood samples corresponding to trough levels were available for analysis. Data were modelled, and the final model was evaluated using NONMEM and auxiliary software tools.

RESULTS

The final LTG population model included the effects of concomitant drugs and patient's weight (WT) which stratified the population into three groups: ≤25 kg, >25 to <60 kg and ≥60 kg. Based on the final model, the estimated LTG oral clearance (CL/F) for a typical patient weighing ≤25 kg, >25 to <60 kg or ≥60 kg who was concomitantly treated with CBZ was estimated to be 3.28, 4.23, or 7.15 l/h, respectively. If a patient was concomitantly treated with CBZ + VPA, the CL/F decreased on average by 69.5 % relative to LTG + CBZ co-therapy. VPA was found to decrease the LTG CL/F by 87.6 % compared to co-therapy with only CBZ.

CONCLUSION

The LTG population pharmacokinetic model developed in this study may be a reliable method for individualising the LTG dosing regimen in paediatric and young adult patients on combination therapy during therapeutic drug monitoring.

Relevance of UDP-glucuronosyltransferase polymorphisms for drug dosing: A quantitative systematic review

UDP-glucuronosyltransferases (UGT) catalyze the biotransformation of many endobiotics and xenobiotics, and are coded by polymorphic genes. However, knowledge about the effects of these polymorphisms is rarely used for the individualization of drug therapy. Here, we present a quantitative systematic review of clinical studies on the impact of UGT variants on drug metabolism to clarify the potential for genotype-adjusted therapy recommendations. Data on UGT polymorphisms and dose-related pharmacokinetic parameters in man were retrieved by a systematic search in public databases. Mean estimates of pharmacokinetic parameters were extracted for each group of carriers of UGT variants to assess their effect size. Pooled estimates and relative confidence bounds were computed with a random-effects meta-analytic approach whenever multiple studies on the same variant, ethnic group, and substrate were available. Information was retrieved on 30 polymorphic metabolic pathways involving 10 UGT enzymes. For irinotecan and mycophenolic acid a wealth of data was available for assessing the impact of genetic polymorphisms on pharmacokinetics under different dosages, between ethnicities, under comedication, and under toxicity. Evidence for effects of potential clinical relevance exists for 19 drugs, but the data are not sufficient to assess effect size with the precision required to issue dose recommendations. In conclusion, compared to other drug metabolizing enzymes much less systematic research has been conducted on the polymorphisms of UGT enzymes. However, there is evidence of the existence of large monogenetic functional polymorphisms affecting pharmacokinetics and suggesting a potential use of UGT polymorphisms for the individualization of drug therapy.

Association between lamotrigine concentrations and ABCB1 polymorphisms in patients with epilepsy

BACKGROUND

Epilepsy is treated with a variety of anticonvulsants that are often used concomitantly. Therefore, therapeutic drug monitoring is often necessary. Along with clinical and environmental factors, genetic predisposition has been recognized to be relevant for interindividual variability in drug response. Polymorphic transporter proteins such as P-glycoprotein significantly influence pharmacokinetics and bioavailability of many structurally unrelated drugs. The aim of the study was to evaluate the impact of polymorphisms in the P-glycoprotein-encoding gene ABCB1 (C1236T, G2677T/A, C3435T) on antiepileptic drug disposition.

METHODS

We recruited 222 patients with epilepsy who were prescribed lamotrigine in monotherapy or polytherapy. Lamotrigine plasma concentrations were analyzed and compared with ABCB1 gene variants. The ABCB1 genotyping was performed by real-time polymerase chain reaction methods. The therapeutic drug monitoring was performed by high-performance liquid chromatography-diode array detector (DAD) and immunoassay.

RESULTS

A significant correlation was confirmed between lamotrigine concentration and additional drugs (P < 0.001). In the whole group, statistical analysis showed correlations between lamotrigine concentrations and ABCB1 C1236T variants: 10.1 and 6.5 μmol/L for CC versus CT + TT, respectively (P = 0.021), and for dose corrected lamotrigine 0.068 and 0.053 μmol·L·mg, for CC versus CT + TT, respectively (P = 0.017). Analysis of a specific haplotype showed that 1236C-2677G-3435C carriers had higher lamotrigine concentrations than 1236T-2677G-3435T carriers (P < 0.001), followed by 1236T-2677T-3435C carriers (P < 0.001).

CONCLUSIONS

ABCB1 C1236T, G2677T/A, C3435T polymorphisms have an influence on lamotrigine serum concentrations.

Genetic variability of drug-metabolizing enzymes: the dual impact on psychiatric therapy and regulation of brain function

Polymorphic drug-metabolizing enzymes (DMEs) are responsible for the metabolism of the majority of psychotropic drugs. By explaining a large portion of variability in individual drug metabolism, pharmacogenetics offers a diagnostic tool in the burgeoning era of personalized medicine. This review updates existing evidence on the influence of pharmacogenetic variants on drug exposure and discusses the rationale for genetic testing in the clinical context. Dose adjustments based on pharmacogenetic knowledge are the first step to translate pharmacogenetics into clinical practice. However, also clinical factors, such as the consequences on toxicity and therapeutic failure, must be considered to provide clinical recommendations and assess the cost-effectiveness of pharmacogenetic treatment strategies. DME polymorphisms are relevant not only for clinical pharmacology and practice but also for research in psychiatry and neuroscience. Several DMEs, above all the cytochrome P (CYP) enzymes, are expressed in the brain, where they may contribute to the local biochemical homeostasis. Of particular interest is the possibility of DMEs playing a physiological role through their action on endogenous substrates, which may underlie the reported associations between genetic polymorphisms and cognitive function, personality and vulnerability to mental disorders. Neuroimaging studies have recently presented evidence of an effect of the CYP2D6 polymorphism on basic brain function. This review summarizes evidence on the effect of DME polymorphisms on brain function that adds to the well-known effects of DME polymorphisms on pharmacokinetics in explaining the range of phenotypes that are relevant to psychiatric practice.

Pharmacokinetic variability of four newer antiepileptic drugs, lamotrigine, levetiracetam, oxcarbazepine, and topiramate: a comparison of the impact of age and comedication

BACKGROUND

Newer antiepileptic drugs (AEDs) are widely used in patients with epilepsy. There is still insufficient documentation regarding pharmacokinetic variability of these AEDs in different patient groups.

PURPOSE

The purpose of this study was to compare age and comedication as factors contributing to pharmacokinetic variability between 4 newer AEDs (lamotrigine, levetiracetam, oxcarbazepine, and topiramate) among patients with refractory epilepsy.

METHODS

Data regarding age, gender, use of AEDs, daily doses, and serum concentration measurements were retrieved from a therapeutic drug monitoring database, from patients admitted to the National Center for Epilepsy, Norway, 2007-2008.

RESULTS

In total, 1050 patients were included, 111 younger children (2-9 years), 137 older children (10-17 years), 720 adults (18-64 years), 82 elderly (65-93 years). Fifty percent of the patients were prescribed polytherapy, in 88 different combinations. The interindividual pharmacokinetic variability was extensive, as illustrated by a 10-fold variability in serum concentration compared with dose. Age affected the apparent clearance of levetiracetam to the largest extent, as shown by a 60% increase in younger children and a 40% reduction in the elderly, respectively, compared with adults. Comedication altered the clearance of lamotrigine to the greatest extent ±70% because it is affected by both enzyme inducers and inhibitors. Hepatic enzyme inducers increased the clearance of levetiracetam and topiramate by 25% and oxcarbazepine by 75%. Valproic acid reduced the clearance of topiramate by 25%.

CONCLUSION

Age and comedication are important contributors to pharmacokinetic variability. Age had the greatest impact on levetiracetam, and comedication affected the clearance of each of the 4 AEDs investigated in this study. Pharmacokinetic drug interactions must be carefully considered when multidrug therapies are prescribed. Therapeutic drug monitoring is a valuable tool for individualizing AED therapy.

Influence of the UGT2B7 -161C>T polymorphism on the population pharmacokinetics of lamotrigine in Thai patients

BACKGROUND AND OBJECTIVES

A high inter-individual variability in the pharmacokinetics of lamotrigine has been observed. Lamotrigine is primarily metabolized by UGT1A4 and UGT2B7, both of which show genetic polymorphisms. Both genetic and non-genetic factors may influence the pharmacokinetics of lamotrigine. The aim of this study was to determine the pharmacokinetic parameters of lamotrigine and to investigate the effect of genetic variants of UGT1A4 and UGT2B7 and various non-genetic factors on its pharmacokinetics.

METHODS

Four single nucleotide polymorphisms (SNPs; UGT1A4 142 T>G, UGT1A4 70C>T, UGT2B7 372A>G, UGT2B7 -161C>T) were identified using the TaqMan Allelic Discrimination Assay. Data were analyzed using NONMEM. Model evaluation was performed using the bootstrap approach and predictive check.

RESULTS

A total of 116 samples were obtained from 75 patients and included in the population analysis. The use of enzyme inducers, valproic acid, and the UGT2B7-161 C>T SNP were found to significantly influence lamotrigine apparent clearance (CL/F). Lamotrigine CL/F in patients carrying the UGT2B7 -161 CT or TT SNP was 18% lower than that in patients carrying the UGT2B7 -161 CC SNP.

CONCLUSION

Both genetic and non-genetic factors were found to influence lamotrigine pharmacokinetics. These factors should be considered when determining lamotrigine dosing. The model presented here could be useful for lamotrigine dose adjustment in clinical practice.

Pharmacogenetics of the antiepileptic drugs phenytoin and lamotrigine

Patients treated with antiepileptic drugs can exhibit large interindividual variability in clinical efficacy or adverse effects. This could be partially due to genetic variants in genes coding for proteins that function as drug metabolizing enzymes, drug transporters or drug targets. The purpose of this article is to provide an overview of the current knowledge on the pharmacogenetics of two commonly prescribed antiepileptic drugs with similar mechanisms of action; phenytoin (PHT) and lamotrigine (LTG). These two drugs have been selected in order to model the pharmacogenetics of Phase I and Phase II metabolism for PHT and LTG, respectively. In light of the present evidence, patients treated with PHT could benefit from CYP2C9 and CYP2C19 genotyping/phenotyping. For those under treatment with LTG, UGT1A4 and UGT2B7 genotyping might be of clinical use and could contribute to the interindividual variability in LTG concentration to dose ratio in epileptic patients.

Influence of concomitant antiepileptic drugs on plasma lamotrigine concentration in adult Japanese epilepsy patients

Lamotrigine (LTG) is an antiepileptic drug (AED) that was approved in Japan in 2008. We evaluated the influence of AEDs that induce hepatic enzymes (including phenytoin (PHT), phenobarbital (PB), carbamazepine (CBZ)), valproic acid (VPA), and various combinations of these drugs, on plasma LTG concentration in adult Japanese epilepsy patients. A total of 621 patients (mean age 34.4±11.8 years) were evaluated retrospectively. We calculated the concentration to dose ratio (CD ratio) for LTG with different AED regimens, and employed multiple regression analysis to determine factors influencing the LTG concentration. There was a linear correlation between the dose and concentration of LTG in patients treated with LTG (group I), LTG+VPA (group II), LTG+inducers (group III), or LTG+VPA+inducers (group IV). The mean CD ratio of patients on LTG monotherapy was 1.43±0.4 (μg/mL)/(mg/kg). When LTG was combined with VPA, the CD ratio increased about 2.2-fold, but there was no significant correlation between the CD ratio and VPA concentration. The mean CD ratios calculated in patients receiving LTG+PHT, LTG+PB, and LTG+CBZ were 0.56, 0.84, and 0.91, respectively. Addition of PHT significantly reduced the CD ratio in a concentration-dependent manner, in comparison with PB and CBZ (p<0.005 and p<0.001, respectively). Stepwise multiple regression analysis showed that the coefficient of determination of groups I, II, III, and IV were 0.94, 0.94, 0.90, and 0.91, respectively. In the clinical setting, these findings can help to estimate LTG concentrations and predict the inducing or inhibiting effects of concomitant AEDs.

Host factors affecting antiepileptic drug delivery-pharmacokinetic variability

Antiepileptic drugs (AEDs) are the mainstay in the treatment of epilepsy, one of the most common serious chronic neurological disorders. AEDs display extensive pharmacological variability between and within patients, and a major determinant of differences in response to treatment is pharmacokinetic variability. Host factors affecting AED delivery may be defined as the pharmacokinetic characteristics that determine the AED delivery to the site of action, the epileptic focus. Individual differences may occur in absorption, distribution, metabolism and excretion. These differences can be determined by genetic factors including gender and ethnicity, but the pharmacokinetics of AEDs can also be affected by age, specific physiological states in life, such as pregnancy, or pathological conditions including hepatic and renal insufficiency. Pharmacokinetic interactions with other drugs are another important source of variability in response to AEDs. Pharmacokinetic characteristics of the presently available AEDs are discussed in this review as well as their clinical implications.