Analysis of genotype and haplotype effects of ABCB1 (MDR1) polymorphisms in the risk of medically refractory epilepsy in an Indian population

Impact of ABCB1 genetic polymorphism on carbamazepine dose requirement among Southern Indian persons with epilepsy

OBJECTIVES

Carbamazepine (CBZ) is one of the oldest, yet first line drugs for treating epilepsy. However, there is a large inter-individual difference in requirement of maintenance dose and one third of persons treated with antiepileptic drugs (AEDs) exhibit drug resistance to therapy. One of the proposed mechanisms for the drug resistance was increased expression of efflux transporter P-glycoprotein. The pharmacogenetic studies of drug transporters (ABCB1) done in combination therapies of AEDs were inconclusive. Hence, we have attempted to study the impact of ABCB1 3435C>T genetic polymorphism and CBZ monotherapy in persons with epilepsy (PWE) from South India, which is a genetically distinct population. With this background, this study was aimed to determine the dose of CBZ in ABCB1 3435C>T genotypes and to determine the distribution of ABCB1 3435C>T genotypes (which codes P-glycoprotein) between responders and non-responders to CBZ therapy.

METHODS

A cross sectional study was conducted in 200 persons with epilepsy, who were categorised as responders and non-responders according to ILAE (international league against epilepsy) criteria. Eligible participants were enrolled from the epilepsy clinic of the neurology department and five ml of blood was collected. DNA extraction and genotyping were done by phenol-chloroform method and real time polymerase chain reaction (RT-PCR), respectively.

RESULTS

The mean maintenance dose of carbamazepine was statistically significant among different genotypes (p<0.05) of ABCB1 3435C>T (526 vs. 637 mg/day in CC vs. TT genotype). There was no significant association between ABCB1 3435C>T polymorphism (p=0.827) and CBZ resistance in PWE. Duration of disease and age of onset were found to be significant in predicting the response to CBZ therapy.

CONCLUSIONS

We report that ABCB1 3435C>T polymorphism is significantly associated with an increase in dose requirement of CBZ in persons with epilepsy from South India.

Demographic and Clinical Predictors of Drug Response in Epileptic Children in Jeddah

Epilepsy is a chronic neurological disease of the brain. Over 20 antiseizure medications are available on the market, but a third of patients still have drug-resistant epilepsy. This study was designed to assess the impact of the demographic and clinical characteristics of epileptic children on their likelihood of developing drug resistance. This study was a multicenter, hospital-based, cross-sectional, case-control study of pediatric patients diagnosed with epilepsy in Jeddah, Saudi Arabia. The study included 101 children with epilepsy. Fifty-six patients showed good response to antiseizure medications (ASMs), and forty-five patients had a poor response. A statistically significant good response to ASMs was reported among younger patients, those who did not report parental consanguinity, those who did not have a family history of epilepsy, and those diagnosed with partial seizures, with no reported adverse effects. The levetiracetam regimen was statistically significant regarding the responsiveness to ASMs. Patients on a monotherapy regimen elicited a significantly better response to levetiracetam than patients on polytherapy (p < 0.001). No significant association was found between the response to ASMs and the sex, nationality, body mass index, complete blood count, or vitamin B12 level. In conclusion, the ASM response in epileptic patients can be predicted by knowing the patient's demographic and epileptic history. However, the complete blood count and vitamin B12 level failed to predict patients' response to ASMs.

Potential clinical and biochemical markers for the prediction of drug-resistant epilepsy: A literature review

Drug resistance is a major challenge in the treatment of epilepsy. Drug-resistant epilepsy (DRE) accounts for 30% of all cases of epilepsy and is a matter of great concern because of its uncontrollability and the high burden, mortality rate, and degree of damage. At present, considerable research has focused on the development of predictors to aid in the early identification of DRE in an effort to promote prompt initiation of individualized treatment. While multiple predictors and risk factors have been identified, there are currently no standard predictors that can be used to guide the clinical management of DRE. In this review, we discuss several potential predictors of DRE and related factors that may become predictors in the future and perform evidence rating analysis to identify reliable potential predictors.

Association between G2677T/A polymorphism in ABCB1 gene and the risk of drug resistance epilepsy: An updated systematic review and meta-analysis

PURPOSE

Epilepsy is a common serious brain condition characterized by the abnormal electrical activity of neurons. In most cases, epileptic patients respond to antiepileptic drugs. Approximately, one-third of patients prove medically intractable. The ABCB1 gene is a superfamily of ATP-binding cassette (ABC) transporters that encode a drug-transport protein, lead to cells and organs protects and eliminates toxic agents. We performed this meta-analysis to assess the association between G2677T/A in the ABCB1 gene and the risk of drug resistance in epileptic patients.

METHODS

Two online libraries (PubMed and Scopus) were used to identify studies that report the relationship between G2677T/A polymorphism in the MDR1 gene and the risk of antiepileptic drug resistance. The meta-analysis was performed using Review Manager 5.3 software. The pooled odds ratios and 95 % confidence intervals (CIs) were calculated using a random or fixed effects model according to the heterogeneity between studies.

RESULTS

A total of 33 eligible studies were included in this meta-analysis which 4192 patients were drug-resistant and 5079 patients were drug-responsive. As a result, a significant association was observed in overall population for the genetic model GG+GA vs AA (OR with 95 % CI = 0,56 [0.34,0.93]; P = 0.02). The subgroup ethnicity analysis showed a significant decrease in the risk of AEDs resistance in the Caucasian population.

CONCLUSION

In conclusion, our analysis demonstrates that G2677T/A polymorphism in the ABCB1 gene decreases the risk of drug resistance. More studies are needed in the different ethnic groups to clarify the role of polymorphism in AEDs resistance.

Impact of ABCB1 Polymorphisms on Lacosamide Serum Concentrations in Uygur Pediatric Patients With Epilepsy in China

BACKGROUND

P-glycoprotein, encoded by ABCB1 (or MDR1), may contribute to drug resistance in epilepsy by limiting gastrointestinal absorption and brain access to antiseizure medications. The study aimed to evaluate the impact of ABCB1 polymorphisms on lacosamide (LCM) serum concentrations in Uygur pediatric patients with epilepsy.

METHODS

The serum concentrations of LCM were determined by ultrahigh performance liquid chromatography, and the ABCB1 polymorphism was analyzed through polymerase chain reaction-fluorescence staining in situ hybridization. The χ2 test and the Fisher exact test were used to analyze the allelic and genotypic distributions of ABCB1 polymorphisms between the drug-resistant and drug-responsive patient groups. Differences in steady-state and dose-corrected LCM serum concentrations between different genotypes were analyzed using the one-way analysis of variance and the Mann-Whitney test.

RESULTS

A total of 131 Uygur children with epilepsy were analyzed, and of them, 41 demonstrated drug resistance. The frequency of the GT genotype of ABCB1 G2677T/A was significantly higher in the drug-resistant group than that in the drug-responsive group (P < 0.05, OR = 1.966, 95% CI, 1.060-3.647). Patients with the G2677T/A-AT genotype had a statistically significantly lower concentration-to-dose (CD) value than patients with the G2677T/A-GG genotype (mean: 0.6 ± 0.2 versus 0.8 ± 0.5 mcg/mL per mg/kg, P < 0.001). Significantly lower LCM serum concentrations were observed in ABCB1 C3435T CT and TT genotype carriers than those in the CC carriers (P = 0.008 and P = 0.002), and a significantly lower LCM CD value was observed in ABCB1 C3435T CT genotype carriers than that in the CC carriers (P = 0.042).

CONCLUSIONS

ABCB1 G2677T/A and C3435T polymorphisms may affect LCM serum concentrations and treatment efficacy in Uygur pediatric patients with epilepsy, leading to drug resistance in pediatric patients.

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.

Pharmacogenetics of Drug-Resistant Epilepsy (Review of Literature)

Pharmacogenomic studies in epilepsy are justified by the high prevalence rate of this disease and the high cost of its treatment, frequent drug resistance, different response to the drug, the possibility of using reliable methods to assess the control of seizures and side effects of antiepileptic drugs. Candidate genes encode proteins involved in pharmacokinetic processes (drug transporters, metabolizing enzymes), pharmacodynamic processes (receptors, ion channels, enzymes, regulatory proteins, secondary messengers) and drug hypersensitivity (immune factors). This article provides an overview of the literature on the influence of genetic factors on treatment in epilepsy.

Multi-omic strategies applied to the study of pharmacoresistance in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common type of focal epilepsy in adults, and hippocampal sclerosis (HS) is a frequent histopathological feature in patients with MTLE. Pharmacoresistance is present in at least one-third of patients with MTLE with HS (MTLE+HS). Several hypotheses have been proposed to explain the mechanisms of pharmacoresistance in epilepsy, including the effect of genetic and molecular factors. In recent years, the increased knowledge generated by high-throughput omic technologies has significantly improved the power of molecular genetic studies to discover new mechanisms leading to disease and response to treatment. In this review, we present and discuss the contribution of different omic modalities to understand the basic mechanisms determining pharmacoresistance in patients with MTLE+HS. We provide an overview and a critical discussion of the findings, limitations, new approaches, and future directions of these studies to improve the understanding of pharmacoresistance in MTLE+HS. However, it is important to point out that, as with other complex traits, pharmacoresistance to anti-seizure medications is likely a multifactorial condition in which gene-gene and gene-environment interactions play an important role. Thus, studies using multidimensional approaches are more likely to unravel these intricate biological processes.

Predictive association of ABCB1 C3435T genetic polymorphism with the efficacy or safety of lopinavir and ritonavir in COVID-19 patients

Lopinavir and ritonavir are substrates of permeability glycoprotein encoded by ABCB1. The efficacy and safety of these drugs is unknown in COVID-19 patients affected by ABCB1 genetic variability. Patients carrying one or two copies of the ABCB1 C3435T were predictively considered as risk phenotypes. It was predicted that risk phenotypes due to carrying either one or two copies of ABCB1 C3435T were highly prevalent in Europe (76.8%; 95% CI: 75-78), followed by America (67%; 95% CI: 65-69), Asia (63.5%; 95% CI: 62-65) and Africa (41.4%; 95% CI: 37-46), respectively. It is hypothesized that a considerable proportion of COVID-19 patients treated with lopinavir/ritonavir inheriting ABCB1 C3435T genetic polymorphism may be predisposed to either therapeutic failure or toxicity.

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

Possible interplay between the theories of pharmacoresistant epilepsy

Epilepsy is one of the oldest known neurological disorders and is characterized by recurrent seizure activity. It has a high incidence rate, affecting a broad demographic in both developed and developing countries. Comorbid conditions are frequent in patients with epilepsy and have detrimental effects on their quality of life. Current management options for epilepsy include the use of anti-epileptic drugs, surgery, or a ketogenic diet. However, more than 30% of patients diagnosed with epilepsy exhibit drug resistance to anti-epileptic drugs. Further, surgery and ketogenic diets do little to alleviate the symptoms of patients with pharmacoresistant epilepsy. Thus, there is an urgent need to understand the underlying mechanisms of pharmacoresistant epilepsy to design newer and more effective anti-epileptic drugs. Several theories of pharmacoresistant epilepsy have been suggested over the years, the most common being the gene variant hypothesis, network hypothesis, multidrug transporter hypothesis, and target hypothesis. In our review, we discuss the main theories of pharmacoresistant epilepsy and highlight a possible interconnection between their mechanisms that could lead to the development of novel therapies for pharmacoresistant epilepsy.

Association of ABCB1 gene polymorphism (C1236T and C3435T) with refractory epilepsy in Iraqi patients

The mechanisms of refractory epilepsy (RE) are most likely multifactorial, involving environmental, genetic, as well as disease- and drug-related factors. We aimed to study is to investigate the possible association of two ABCB1 gene polymorphism (C3435T and C1236T) with the development of RE in Iraqi patients. One hundred patients with either generalized tonic-clonic seizures, myoclonic epilepsy, or absence epilepsy comprised of 60 patients responsive to AEDs and 40 patients who were refractory to treatment who used multi AEDs for at least one month were studied. Fifty family-unrelated age- and sex-matched healthy subjects represent the control group. ABCB1 gene fragments corresponding to two targeted polymorphisms were amplified with conventional polymerase chain reaction using specific sets of primers. Genotyping was performed by restriction fragment length polymorphism (RFLP) technique. Epileptic patients refractory to AEDs showed a significantly higher frequency of CC genotypes of C3435T polymorphism than controls. Allele C was significantly higher in patients than controls and far more frequent among patients with RE. C1235T polymorphism had no significant role neither in the incidence of epilepsy nor in the AEDs resistance. The CT haplotype was more frequent among patients refractory to AEDs. In contrast, the haplotype block TT was more frequent among responsive (41.3%) than refractory patients (28.7%) (p = 0.068). The CC genotype and C allele of the C3435T polymorphism can increase the risk of RE. The haplotype block CT of C3435T and C1236T can predispose for epilepsy as well as the drug resistance.

Genetic variations associated with pharmacoresistant epilepsy (Review)

Epilepsy is a common, serious neurological disorder worldwide. Although this disease can be successfully treated in most cases, not all patients respond favorably to medical treatments, which can lead to pharmacoresistant epilepsy. Drug-resistant epilepsy can be caused by a number of mechanisms that may involve environmental and genetic factors, as well as disease- and drug-related factors. In recent years, numerous studies have demonstrated that genetic variation is involved in the drug resistance of epilepsy, especially genetic variations found in drug resistance-related genes, including the voltage-dependent sodium and potassium channels genes, and the metabolizer of endogenous and xenobiotic substances genes. The present review aimed to highlight the genetic variants that are involved in the regulation of drug resistance in epilepsy; a comprehensive understanding of the role of genetic variation in drug resistance will help us develop improved strategies to regulate drug resistance efficiently and determine the pathophysiological processes that underlie this common human neurological disease.

ABCB1 Polymorphisms and Drug-Resistant Epilepsy in a Tunisian Population

Background

Epilepsy is one of the most common neurological disorders with about 30% treatment failure rate. An interindividual variations in efficacy of antiepileptic drugs (AEDs) make the treatment of epilepsy challenging, which can be attributed to genetic factors such as ATP-Binding Cassette sub-family B, member1 (ABCB1) gene polymorphisms.

Objective

The main objective of the present study is to evaluate the association of ABCB1 C1236T, G2677T, and C3435T polymorphisms with treatment response among Tunisian epileptic patients.

Materials and Methods

One hundred epileptic patients, originated from north of Tunisia, were recruited and categorized into 50 drug-resistant and 50 drug-responsive patients treated with antiepileptic drugs (AEDs) as per the International League Against Epilepsy. DNA of patients was extracted and ABCB1 gene polymorphisms studied using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method.

Results

The C1236T, G2677T, and C3435T polymorphisms were involved into AED resistance. Significant genotypic (C1236T TT (p ≤ 0.001); G2677T TT (p = 0.001); C3435T TT (p ≤ 0.001)) and allelic associations (C1236T T (3.650, p ≤ 0.001); G2677TT (1.801, p = 0.044); C3435T T (4.730, p ≤ 0.001)) with drug resistance epilepsy (DRE) were observed. A significant level of linkage disequilibrium (LD) was also noted between ABCB1 polymorphisms. Patients with the haplotypes CT and TT (C1236T-G2677T); GT, TC, and TT (G2677T-C3435T); CT and TT (C1236T-C3435T); CTT, TTC, TGT, and TTT (C1236T-G2677T-C3435T) were also significantly associated to AED resistance.

Conclusions

The response to antiepileptics seems to be modulated by TT genotypes, T alleles, and the predicted haplotypes for the tested SNPs in our population. Genetic analysis is a valuable tool for predicting treatment response and thus will contribute to personalized medicine for Tunisian epileptic patients.

A Systems Biology Approach for Personalized Medicine in Refractory Epilepsy

Epilepsy refers to a common chronic neurological disorder that affects all age groups. Unfortunately, antiepileptic drugs are ineffective in about one-third of patients. The complex interindividual variability influences the response to drug treatment rendering the therapeutic failure one of the most relevant problems in clinical practice also for increased hospitalizations and healthcare costs. Recent advances in the genetics and neurobiology of epilepsies are laying the groundwork for a new personalized medicine, focused on the reversal or avoidance of the pathophysiological effects of specific gene mutations. This could lead to a significant improvement in the efficacy and safety of treatments for epilepsy, targeting the biological mechanisms responsible for epilepsy in each individual. In this review article, we focus on the mechanism of the epilepsy pharmacoresistance and highlight the use of a systems biology approach for personalized medicine in refractory epilepsy.

Clinical reappraisal of the influence of drug-transporter polymorphisms in epilepsy

Although novel antiepileptic drugs (AEDs) have been recently released, the issue of drug resistance in epileptic patients remains unsolved and largely unpredictable. Areas covered: We aim to assess the clinical impact of genetic variations that may influence the efficacy of medical treatment in epilepsy patients. Indeed, many genes, including genes encoding drug transporters (ABCB1), drug targets (SCN1A), drug-metabolizing enzymes (CYP2C9, CYP2C19), and human leucocyte antigen (HLA) proteins, may regulate the mechanisms of drug resistance in epilepsy. This review specifically focuses on the ABC genes, which encode multidrug resistance-associated proteins (MRPs) and may reduce the blood-brain barrier penetration of anticonvulsant AEDs. Expert opinion: Drug resistance remains a crucial problem in epilepsy patients. Pharmacogenomic studies may improve our understanding of drug responses and drug resistance by exploring the impact of gene variants and predicting drug responses and tolerability.

Associations between MDR1 C3435T polymorphism and drug-resistant epilepsy in the Polish population

Epilepsy is a common neurological disorder. About one-third of epileptic patients demonstrate multidrug resistance (MDR) phenotype and develop drug-resistant epilepsy (DRE). Single nucleotide polymorphism (SNP) C3435T (rs1045642), identified in the MDR1 gene, is associated with an increased intestinal expression of P-glycoprotein (P-gp) which affects the levels of anti-epileptic drugs in plasma. The reported study was designed to explore associations between the MDR1-C3435T gene SNP and the risk of DRE in the Polish population. The C3435T polymorphism of MDR1 gene was investigated by the PCR-RFLP technique in 74 patients with DRE and 70 age- and sex-matched non-DRE controls. Blood samples were obtained from patients with drug-resistant epilepsy, treated at the Department of Neurological Surgery, Medical University in Warsaw between the years 2011 and 2012. Odds ratios (ORs) and 95 % confidence intervals (CIs) were calculated for each genotype and allele. Genotype distribution of C3435T polymorphism of MDR1 gene was compared between the DRE patients and controls with significant differences (p < 0.05) between the two investigated groups. A possible association was observed between DRE and the presence of 3435C allele. The 3435C allele was found in 69 % of DRE cases and in 48 % of the used controls. The variant 3435T allele of MDR1 decreased the risk of drug-resistant epilepsy [odds ratio (OR) 0.41; 95 % confidence interval (CI) 0.26-0.67]. The results indicate that the C3435T polymorphism of MDR1 gene may be associated with the incidence of DRE observed in the Polish population.

Upregulation of breast cancer resistance protein and major vault protein in drug resistant epilepsy

PURPOSE

Identifying factors involved in the development of drug resistant epilepsy (DRE) remains a challenge. Candidate gene studies have shown modulation of resistance to drugs by various multidrug resistance proteins in DRE. However the resistance to drugs in DRE could be more complex and multifactorial involving molecules in different pharmacokinetic processes. In this study for the first time we have analyzed the relative expression of four molecules with different drug resistance mechanisms in two most common DRE pathologies, mesial temporal lobe epilepsy (MTLE) and focal cortical dysplasia (FCD) with respect to each other and also with different non-epileptic controls.

METHODS

Brain tissues resected from MTLE (n=16) and FCD type I and II (n=12) patients who had undergone surgery were analysed for mRNA levels of multidrug resistance-associated protein 1(MRP1), major vault protein (MVP), breast cancer resistance protein (BCRP), and one drug metabolising enzyme (UGT1A4) as compared to non-epileptic controls which were tissues resected from tumor periphery (n=6) and autopsy tissues (n=4) by quantitative PCR.

RESULTS

We found significant upregulation of MVP and BCRP whereas MRP1 and UGT1A4 were unaltered in both pathologies. While upregulation of BCRP was significantly higher in MTLE (9.34±0.45; p<0.05), upregulation of MVP was significantly higher in FCD (2.94±0.65; p<0.01).

CONCLUSION

We propose that upregulation of BCRP and MVP is associated with MTLE and FCD and these molecules not only may have the potential to predict pathology specific phenotypes but may also have therapeutic potential as adjunct treatment in these pathologies.

Relationship between ABCB1 3435TT genotype and antiepileptic drugs resistance in Epilepsy: updated systematic review and meta-analysis

Background

Antiepileptic drugs (AEDs) are effective medications available for epilepsy. However, many patients do not respond to this treatment and become resistant. Genetic polymorphisms may be involved in the variation of AEDs response. Therefore, we conducted an updated systematic review and a meta-analysis to investigate the contribution of the genetic profile on epilepsy drug resistance.

Methods

We proceeded to the selection of eligible studies related to the associations of polymorphisms with resistance to AEDs therapy in epilepsy, published from January 1980 until November 2016, using Pubmed and Cochrane Library databases. The association analysis was based on pooled odds ratios (ORs) and 95% confidence intervals (CIs).

Results

From 640 articles, we retained 13 articles to evaluate the relationship between ATP-binding cassette sub-family C member 1 (ABCB1) C3435T polymorphism and AEDs responsiveness in a total of 454 epileptic AEDs-resistant cases and 282 AEDs-responsive cases. We found a significant association with an OR of 1.877, 95% CI 1.213–2.905. Subanalysis by genotype model showed a more significant association between the recessive model of ABCB1 C3435T polymorphism (TT vs. CC) and the risk of AEDs resistance with an OR of 2.375, 95% CI 1.775–3.178 than in the dominant one (CC vs. TT) with an OR of 1.686, 95% CI 0.877–3.242.

Conclusion

Our results indicate that ABCB1 C3435T polymorphism, especially TT genotype, plays an important role in refractory epilepsy. As genetic screening of this genotype may be useful to predict AEDs response before starting the treatment, further investigations should validate the association.

Association between ABCB1 genetic polymorphism and the effect on epilepsy following phenytoin treatment

The aim of the study was to analyze the effect of ABCB1 genetic polymorphisms on the efficacy of phenytoin (PHT) treatment in epilepsy patients. In total, 200 epilepsy patients who were administered PHT were divided into the responsive and pharmaco-resistance groups depending on the clinical data of PHT treatment in epilepsy patients. The serum concentration of PHT was detected by high-performance liquid chromatography (HPLC). ABCB1 polymorphisms were analyzed by the polymerase chain reaction restriction-fragment length polymorphism method. The C1236T, C3435T and G2677T/A haplotypes were reconstructed for the ABCB1 gene using SHEsis programs. One-way analysis of variance was used for data analysis. In ABCB1 C1236T, the rate of the CC genotype in pharmaco-resistance (17.5%) was higher than that of the responsive group (2.1%), while the rate of the TT genotype in pharmaco-resistance (41.6%) was lower than that of the responsive group (55.4%) (P<0.05). In ABCB1 G2677T/A, the rate of the GG genotype in pharmaco-resistance (29.6%) was higher than that of the responsive group (9.7%), while the rate of the TT genotype in pharmaco-resistance (4.6%) was lower than that of the responsive group (30.4%) (P<0.05). The rate of the TTC haploid in pharmaco-resistance (24.1%) was higher than that of the responsive group (8.8%) (P<0.05). The PHT serum concentration had no statistical significance in the patients with different genotypes. In conclusion, there was no association between ABCB1 genetic polymorphism and PHT serum concentration, although the polymorphisms affected the efficacy of PHT treatment in patients with epilepsy.

Effects of major transporter and metabolizing enzyme gene polymorphisms on carbamazepine metabolism in Chinese patients with epilepsy

AIM

The present study aimed to evaluate the effects of SNPs of major transporter and metabolizing enzyme genes on carbamazepine (CBZ) metabolism in Chinese patients with epilepsy.

MATERIALS & METHODS

For 210 epileptic patients treated with CBZ as monotherapy, nine SNPs in candidate genes ABCB1, CYP3A4, CYP3A5, POR and EPHX1 were analyzed by PCR-RFLP or direct sequencing. Serum concentrations of CBZ, carbamazepine-10,11-epoxide (CBZE) and carbamazepine-10,11-trans dihydrodiol (CBZD) were determined by HPLC. Dose-adjusted concentrations of CBZ (CDRCBZ), CBZE (CDRCBZE), CBZD (CDRCBZ D) and CBZD:CBZE ratio were used as evaluation parameters for CBZ metabolism.

RESULTS

The ABCB1 c.3435C>T was significantly associated with the CDR of CBZ and its major metabolites. CYP3A4*1G and CYP3A5*3 could influence CBZ metabolism, while POR*28 had no effect on it. The EPHX1 c.416A>G and c.128G>C variants were significantly associated with CBZD:CBZE ratio.

CONCLUSION

Our data suggest that certain polymorphisms of major transporter and metabolizing enzyme genes could in part influence interindividual variability of CBZ metabolism in Chinese patients with epilepsy.

The roles of variants in human multidrug resistance (MDR1) gene and their haplotypes on antiepileptic drugs response: a meta-analysis of 57 studies

Objective

Previous studies reported the associations between the ATP-binding cassette sub-family B member 1 (ABCB1, also known as MDR1) polymorphisms and their haplotypes with risk of response to antiepileptic drugs in epilepsy, however, the results were inconclusive.

Methods

The Pubmed, Embase, Web of Science, CNKI and Chinese Biomedicine databases were searched up to July 15, 2014. Pooled odds ratios (ORs) and 95% confidence intervals (CIs) were calculated using a fixed-effects or random-effects model based on heterogeneity tests. Meta-regression and Galbraith plot analysis were carried out to explore the possible heterogeneity.

Results

A total of 57 studies involving 12407 patients (6083 drug-resistant and 6324 drug-responsive patients with epilepsy) were included in the pooled-analysis. For all three polymorphisms (C3435T, G2677T/A, and C1236T), we observed a wide spectrum of minor allele frequencies across different ethnicities. A significantly decreased risk of AEDs resistance was observed in Caucasian patients with T allele of C3435T variant, which was still significant after adjusted by multiple testing corrections (T vs C: OR=0.83, 95%CI=0.71-0.96, p=0.01). However, no significant association was observed between the other two variants and AEDs resistance. Of their haplotypes in ABCB1 gene (all studies were in Indians and Asians), no significant association was observed with AEDs resistance. Moreover, sensitivity and Cumulative analysis showed that the results of this meta-analysis were stable.

Conclusion

In summary, this meta-analysis demonstrated that effect of C3435T variant on risk of AEDs resistance was ethnicity-dependent, which was significant in Caucasians. Additionally, further studies in different ethnic groups are warranted to clarify possible roles of haplotypes in ABCB1 gene in AEDs resistance, especially in Caucasians.

ABCB1 gene C3435T polymorphism and drug resistance in epilepsy: evidence based on 8,604 subjects

BACKGROUND

The present study aimed to assess the role of C3435T polymorphism in drug-resistance in epilepsy by a meta-analysis.

MATERIAL AND METHODS

Databases were obtained from the Cochrane Library, MEDLINE, EMBASE, PubMed, Science Direct database, CNKI, and Wanfang up to October 2014. All the case-control association studies evaluating the role of ABCB1 C3435T in pharmacoresistance to anti-epileptic drug (AED) were identified. RevMan 5.0 software was utilized to perform quantitative analyses in an allele model (C vs. T) and a genotype model (CC vs. CT+TT).

RESULTS

From the 189 potential studies, we included 28 articles for the meta-analysis, including 30 independent case-control studies involving 4124 drug-resistant epileptic patients and 4480 epileptic patients for whom drug treatment was effective. We excluded 164 studies because of duplication, lack of genotype data, and non-clinical research. We found that C3435T polymorphism was not significantly associated with drug resistance in epilepsy, either in allele model (C vs. T: OR=1.07; 95%CI: 0.95-1.19) or in genotype model (CC vs. CT+TT: OR=1.05; 95%CI: 0.89-1.24, P=0.55). Subgroup analyses suggested that in Caucasian populations there are significant differences between resistance group (NR) and control group (R) in both allele model (C vs. T: OR=1.09; 95%CI: 1.00-1.18, P=0.05) and genotype model (CC vs. CT+TT: OR=1.20; 95%CI: 1.04-1.40, P=0.01). However, we did not find this association in Asian populations.

CONCLUSIONS

We conclude that the ABCB1 C3435T polymorphism may be a genetic marker for drug resistance in epilepsy in Caucasian populations.

MDR-1 and MRP2 Gene Polymorphisms in Mexican Epileptic Pediatric Patients with Complex Partial Seizures

Although the Pgp efflux transport protein is overexpressed in resected tissue of patients with epilepsy, the presence of polymorphisms in MDR1/ABCB1 and MRP2/ABCC2 in patients with antiepileptic-drugs resistant epilepsy (ADR) is controversial. The aim of this study was to perform an exploratory study to identify nucleotide changes and search new and reported mutations in patients with ADR and patients with good response (CTR) to antiepileptic drugs (AEDs) in a rigorously selected population. We analyzed 22 samples In Material and Methods, from drug-resistant patients with epilepsy and 7 samples from patients with good response to AEDs. Genomic DNA was obtained from leukocytes. Eleven exons in both genes were genotyped. The concentration of drugs in saliva and plasma was determined. The concentration of valproic acid in saliva was lower in ADR than in CRT. In ABCB1, five reported SNPs and five unreported nucleotide changes were identified; rs2229109 (GA) and rs2032582 (AT and AG) were found only in the ADR. Of six SNPs associated with the ABCC2 that were found in the study population, rs3740066 (TT) and 66744T > A (TG) were found only in the ADR. The strongest risk factor in the ABCB1 gene was identified as the TA genotype of rs2032582, whereas for the ABCC2 gene the strongest risk factor was the T allele of rs3740066. The screening of SNPs in ACBC1 and ABCC2 indicates that the Mexican patients with epilepsy in this study display frequently reported ABCC1 polymorphisms; however, in the study subjects with a higher risk factor for drug resistance, new nucleotide changes were found in the ABCC2 gene. Thus, the population of Mexican patients with AED-resistant epilepsy (ADR) used in this study exhibits genetic variability with respect to those reported in other study populations; however, it is necessary to explore this polymorphism in a larger population of patients with ADR.

Genetic association analysis of ATP binding cassette protein family reveals a novel association of ABCB1 genetic variants with epilepsy risk, but not with drug-resistance

Epilepsy constitutes a heterogeneous group of disorders that is characterized by recurrent unprovoked seizures due to widely different etiologies. Multidrug resistance remains a major issue in clinical epileptology, where one third of patients with epilepsy continue to have seizures. Role of efflux transporters in multidrug resistant epilepsy has been attributed to drug-resistant epilepsy although, with discrepant observation in genetic studies. These discrepancies could be attributed to variety of factors such as variable definition of the anti-epileptic drug (AED)-resistance, variable epilepsy phenotypes and ethnicities among the studies. In the present study we inquired the role of multidrug transporters ABCB1 and ABCG2 variants in determining AED-resistance and susceptibility to epilepsy in three well-characterized cohorts comprising of mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) (prototype for AED-resistant epilepsy); juvenile myoclonic epilepsy (JME) (prototype for AED-responsive epilepsy); and healthy non-epileptic controls, in 738 subjects of Malayalam speaking south Indian ancestry. ABCB1 and ABCG2 variants were not found to be associated with drug resistance when AED-resistant and AED-responsive cohorts were compared. However, a significant association was observed between ABCB1 (C3435T) rs1045642 and risk of having epilepsy (MTLE-HS and JME pooled cohort; genotypic p-value = 0.0002; allelic p-value = 0.004). This association was seen persistent with MTLE-HS (genotypic p-value = 0.0008; allelic p-value = 0.004) and also with JME (genotypic p-value = 0.01; allelic p-value = 0.05) cohort individually. In-silico functional prediction indicated that ABCB1 rs1045642 has a deleterious impact on protein coding function and in splicing regulation. We conclude that the ABCB1 and ABCG2 variants do not confer to AED-resistance in the study population. However, ABCB1 rs1045642 increases vulnerability to epilepsy with greater tendency for MTLE-HS in south Indian ancestry from Kerala.

Significance of MDR1 gene polymorphism C3435T in predicting drug response in epilepsy

Antiepileptic drug (AED) treatment in epilepsy is often compromised by the unpredictability of efficacy and inter-individual variability among patients, which at least in part is the result of genetic variation. The idea to determine an individual's response to a prescribed medicine came into inception around 29 years ago. Pharmacogenetics is used to predict the drug response and efficacy, as well as potential adverse effects. We investigated the functional significance of the C3435T polymorphism of the MDR1 gene in a South Indian population. The patients were divided into responders and non-responders based on their clinical outcome and AED response. The risk of drug resistance was significantly higher in patients bearing TT genotype in comparison to carriers of the homozygous CC genotype [TT vs. CC, χ(2)=12.52; p=0.001, Odds ratio=2.34 (95% CI: 1.942-11.32)]. We suggest that the influence of the C3435T polymorphism in predicting the drug-resistance in epilepsy, might be significant and further investigations focusing on carbamazepine and phenytoin, in various ethnic populations are necessary to clarify the effect of C3435T polymorphism on the multidrug resistance in epilepsy patients.

CYP3A5*3 and C3435T MDR1 polymorphisms in prognostication of drug-resistant epilepsy in children and adolescents

Drug-resistant epilepsies still remain one of the most profound problems of contemporary epileptology. Several mechanisms of drug resistance are possible; among them, genetic factors have a prominent place. Much importance is attached to genes, which encode enzymes that metabolize antiepileptic drugs CYP 3A, which belong to the family of cytochromes P450 and the genome of multidrug resistance, such as multidrug resistance 1 (MDR1) that expresses P-glycoprotein (P-gp), a drug transporter protein. The aim of the study was to assess the relation between polymorphism of gene CYP3A5 and polymorphism C3435T of MDR1 gene with the occurrence of focal, drug-resistant epilepsy in children and youths up to 18 years of age. The study comprised 85 patients, and their age range was from 33 months to 18 years of age, suffering from epilepsy, partly responding well to treatment, partly drug resistant. The polymorphism of both genes has been analysed using the PCR-RFLP method. The study failed to corroborate association between polymorphism CYP3A5∗3 and C3435T polymorphism in MDR1 gene and pharmacoresistant epilepsy. The results of our research do not confirm the prognostic value of the polymorphisms examined in the prognostication of drug resistance in epilepsies.

Linkage disequilibrium between polymorphisms of ABCB1 and ABCC2 to predict the treatment outcome of Malaysians with complex partial seizures on treatment with carbamazepine mono-therapy at the Kuala Lumpur Hospital

PURPOSE

Carbamazepine (CBZ) is used as the first line of treatment of Complex Partial Seizures (CPS) in the Epilepsy Clinic, Neurology Department of Kuala Lumpur Hospital (KLH). More than 30% of the patients remain drug resistant to CBZ mono-therapy. CBZ is transported by the P-glycoprotein (P-gp). The P-gp encoded by the ABCB1 and ABCC2 genes are expressed in drug resistant patients with epilepsy. A few studies have shown significant association between CBZ resistant epilepsy and Linkage Disequilibrium (LD) with adjacent polymorphisms of these genes. Our study is aimed at determining the correlation between patients' response to CBZ mono-therapy to Single Nucleotide Polymorphisms G2677T and C3435T of the ABCB1 gene as well as G1249A and -24C>T of the ABCC2 gene.

METHOD

314 patients with CPS were recruited from the Neurology Department of the KLH based on stringent inclusion and exclusion criteria, of whom 152 were responders and the other 162 were non-responders. DNA was extracted from their blood samples and Taqman technology for allelic discrimination was performed. Results were described as genotype frequencies. The SHEsis analysis platform was used to calculate linkage disequilibrium index and infer haplotype frequencies. Haploview was used to do permutation test to obtain a corrected p-value.

RESULTS

Resistance to treatment with CBZ mono-therapy was significantly associated with the 2677TT and the 3435TT genotypes while it was not significantly associated with the G1249A and -24C>T polymorphisms. The GCGC haplotype combination of the 2677G>T, 3435C>T, 1249G>A and -24C>T respectively was found to be extremely significant (p = 1.10e-20) with good drug response to CBZ mono-therapy.

CONCLUSION

Linkage disequilibrium between the 2677G>T, 3435C>T, 1249G>A and -24C>T SNPs may be used as a reliable screening marker to determine the treatment outcome of CBZ mono-therapy with CPS irrespective of race or gender.

Genetic variability and haplotype profile of MDR1 in Saudi Arabian males

Polymorphisms in multidrug resistance (MDR1) gene play an important role in influencing the pharmacological action and toxicity profile of a large number of therapeutic agents, and in human susceptibility to various diseases. Because of genotypic variability, several studies were directed toward determination of the frequencies of MDR1 polymorphisms and/or haplotypes in different ethnic populations. In this study, we determined the frequencies of the most common three polymorphisms in the MDR1 gene (i.e., C1236T, G2677T, and C3435T) in Saudi Arabians and their haplotypes. Our results showed that the frequencies of 1236T, 2677T, and 3435T were 43.7 %, 40.2 %, and 42.2 %, respectively. In addition, the frequencies of the most common MDR1 haplotypes, C-G-C and T-T-T, were correspondent to 48.8 and 35.5 %. Furthermore, we identified moderate to strong linkage disequilibrium between the loci of these single nucleotide polymorphisms in the studied subjects. These identified frequencies in Saudi Arabians are different from that reported in the other ethnic groups.

Antipsychotic drug dosage and therapeutic response in schizophrenia is influenced by ABCB1 genotypes: a study from a south Indian perspective

Aim: The conventional practice of using trial and error mode to select antipsychotic drugs in treatment of schizophrenia can result in symptom exacerbations, relapse and severe side effects, resulting in higher costs of treatment. P-glycoprotein (ABCB1) is known to regulate the concentration of antipsychotic drugs in the brain. Variable expressivity based on polymorphism in the gene ABCB1 may reflect on the drug response and its relationship to dosage. Materials & methods: All antipsychotic dosages administered to patients were converted to common chlorpromazine equivalents. Response to antipsychotics was based on 50% cutoff in Brief Psychiatric Rating Scale ratings after 1-year of follow-up. Using a case–control study design, ABCB1 polymorphisms were screened in 192 individuals grouped into responders and nonresponders. Results: A strong allelic, genotypic and haplotypic association, was observed, which was predictive of good response to antipsychotics. Individuals carrying the favorable homozygous genotypes of rs1045642 and rs2032582 displayed better response with increased dosage while those carrying risk genotype manifested refractoriness on increased dosage. Conclusion: The study suggests that a priori knowledge of ABCB1 genotypes can provide a significant input into evaluating the patient’s response to medication, and minimizing redundant dosing and refractoriness.

Original submitted 14 February 2012; Revision submitted 14 May 2012

Drug resistance in epilepsy and the ABCB1 gene: The clinical perspective

Multidrug resistance is one of the most serious problems in the treatment of epilepsy that is likely to have a complex genetic and acquired basis. Various experimental data support the hypothesis that over-expression of antiepileptic drug (AED) transporters may play a pivotal role in drug resistance. Hyyt 6however, key questions concerning their functionality remain unanswered. The idea that P-glycoprotein, encoded by the ABCB1 gene, might mediate at least part of the drug resistance was met with both enthusiasm and skepticism. As in oncology, initial optimism has been clouded subsequently by conflicting results. The first study reporting a positive association between genetic variation in the P-glycoprotein and multidrug-resistant epilepsy was published in 2003. Since then, several other genetic association studies have attempted to verify this result. However, taken overall, the role of P-glycoprotein in drug resistance in epilepsy still remains uncertain. We intend to critically review the inherent problems associated with epilepsy pharmacogenetic studies in general and with ABCB1 polymorphisms studies in particular. The lessons learnt from the ABCB1 studies can help us to guide future association genetics studies to investigate AED resistance, and thereby taking us closer to the cherished dream of personalized AED therapy.

Effects of ABCB1 polymorphisms on plasma carbamazepine concentrations and pharmacoresistance in Chinese patients with epilepsy

P-glycoprotein may play a role in drug resistance in epilepsy by limiting gastrointestinal absorption and brain access of antiepileptic drugs (AEDs). We sought to investigate the effects of ABCB1 polymorphisms on plasma carbamazepine (CBZ) concentrations and pharmacoresistance in Chinese patients with epilepsy. C1236T, G2677T/A, and C3435T polymorphisms of ABCB1 were genotyped by polymerase chain reaction amplification followed by restriction fragment length polymorphism analysis or direct automated DNA sequencing in 84 patients treated with CBZ monotherapy. Patients with 3435-TT (n=15) had lower adjusted CBZ concentrations than those with 3435-CC (n=30) (P=0.026). However there were no associations between all the studied genotypes, haplotypes, or diplotypes involving ABCB1 C1236T, G2677T/A, and C3435T polymorphisms and pharmacoresistance in the patient cohort. Our results suggest that ABCB1 3435-TT is associated with decreased plasma CBZ levels in Chinese patients with epilepsy. However, whether this contributes to CBZ resistance needs to be further investigated in a larger cohort of patients.

Lack of association between ABCB1 gene polymorphisms and pharmacoresistant epilepsy: an analysis in a western Chinese pediatric population

OBJECTIVE

The genetic polymorphisms of the ABCB1 (ATP-binding cassette B1) gene encoding P-glycoprotein have been proposed to be associated with pharmacoresistance phenotype in epilepsy patients. P-glycoprotein, a transmembrane transporter, works as an efflux pump by limiting antiepileptic drugs across the blood brain barrier, with correspondingly lowering drug concentrations in epileptogenic loci. In this study, we analyzed whether the three single nucleotide polymorphisms (C1236T, G2677T/A, and C3435T) in the ABCB1 gene were associated with pharmacoresistant epilepsy in a western Chinese pediatric population.

METHODS

A total of 350 children with epilepsy who had been prescribed antiepileptic drugs for at least 1year were included. Of this patient group 193 were drug responsive and 157 were drug resistant according to the presence of seizures. Genotypes of the three loci of ABCB1 gene were detected in 368 age- and sex-matched normal children and 350 epileptic children using the polymerase chain reaction (PCR)-restriction fragment length polymorphism technique. Normal population sample populace from the same ethnicity and territory was genotyped to check for population stratification. The allele, genotype, haplotype, and diplotype frequencies of ABCB1 polymorphisms were compared between drug-resistant and drug-responsive subjects.

RESULTS

No significant differences were observed in the frequencies of genotype, allele, haplotype, or diplotype of ABCB1 polymorphisms between patients with drug-resistant and drug-responsive epilepsy (p>0.05).

CONCLUSION

The above three polymorphisms in the ABCB1 gene were not found to be significantly associated with drug resistant epilepsy in a western Chinese pediatric population.

Association of ABCB1 gene polymorphisms and their haplotypes with response to antiepileptic drugs: a systematic review and meta-analysis

AIMS

Several studies demonstrated a link between ABCB1 gene variants and the response to treatment in epilepsy, but the results have been inconclusive. Here, we performed the first haplotype meta-analysis to examine the association of haplotypes of ABCB1 common variants with the response to treatment in epilepsy.

MATERIALS & METHODS

We meta-analyzed the studies that evaluated the role of ABCB1 C1236T, G2677T/A and C3435T polymorphisms and their haplotypes in the response to treatment.

RESULTS

Meta-analysis of 23 studies (7067 patients) showed no significant association of ABCB1 alleles, genotypes and haplotypes with the response to treatment in the overall population or in each ethnicity subgroup.

CONCLUSION

Our data suggest that the haplotypes of these loci may not be involved in the response to treatment.

Lack of association of ABCB1 and PXR polymorphisms with response to treatment in epilepsy

It is proposed that overexpression of P-glycoprotein (P-gp), encoded by the ABC subfamily B member 1 (ABCB1) gene, is involved in resistance to antiepileptic drugs (AEDs) in about 30% of patients with epilepsy. Genetic variation and haplotype patterns are population specific which may cause different phenotypes such as response to AEDs. Although several studies examined the link between the common polymorphisms in the ABCB1 gene with resistance to AEDs, the results have been conflicting. This controversy may be caused by the effect of some confounders such as ethnicity and polytherapy. Moreover, expression of the ABCB1 gene is under the control of pregnane X receptor (PXR). Evidence showed that PXR gene contribute to the response to treatment. The aim of this study was to assess the association of ABCB1 and PXR genetic polymorphisms with response to the carbamazepine (CBZ) or sodium valproate (VPA) monotherapy in epilepsy. Genotypes were assessed in 685 Chinese, Indian, and Malay epilepsy patients for ABCB1 (C1236T, G2677T, C3435T) and PXR (G7635A) polymorphisms. No association between these polymorphisms and their haplotypes, and interaction between them, with response to treatment was observed in the overall group or in the Chinese, Indian, and Malay subgroups. Our data showed that these polymorphisms may not contribute to the response to CBZ or VPA monotherapy treatment in epilepsy.

P-glycoprotein: tissue distribution, substrates, and functional consequences of genetic variations

P-glycoprotein (ABCB1, MDR1) belongs to the ABC transporter family transporting a wide range of drugs and xenobiotics from intra- to extracellular at many biological interfaces such as the intestine, liver, blood-brain barrier, and kidney. The ABCB1 gene is highly polymorphic. Starting with the observation of lower duodenal protein expression and elevated digoxin bioavailability in relation to the 3435C>T single nucleotide polymorphism, hundreds of pharmacokinetic and outcome studies have been performed, mostly genotyping 1236C>T, 2677G>T/A, and 3435C>T. Though some studies pointed out that intracellular concentrations of anticancer drugs, for example, within lymphocytes, might be affected by ABCB1 variants resulting in differential outcome, current knowledge of the functional significance genetic variants of ABC membrane transporters does not allow selection of a particular SNP to predict an individual's pharmacokinetics.

Multidrug resistance 1 (MDR1) gene polymorphisms in childhood drug-resistant epilepsy

Despite considerable progress in the pharmacotherapy of epilepsy, more than 30% of patients are reported to be resistant to antiepileptic drugs. Multidrug resistance 1 (MDR1) gene could play a role in drug resistance in epilepsy. In this study, the authors investigated the association between the MDR1 gene polymorphisms, C3435T and G2677AT, and drug resistance epilepsy by using polymerase chain reaction/restriction fragment length polymorphism and pyrosequencing methods in a group of 39 patients with drug-resistant epilepsy and 92 controls. No associations were found between the polymorphisms of the MDR1 gene and drug-resistant epilepsy. Haplotype analysis showed no significant association. Compound genotype analysis showed that CC3435/GG2677 was significantly higher in the control group compared to the patient group. In conclusion, MDR1 polymorphisms investigated in this study are not associated with antiepileptic drug resistance, but the CC3435/GG2677 compound genotype might have an effect on antiepileptic drug response.

ABCB1 C3435T polymorphism and the risk of resistance to antiepileptic drugs in epilepsy: a systematic review and meta-analysis

OBJECTIVE

The C3435T, a major allelic variant of the ABCB1 gene, is proposed to play a crucial role in drug-resistance in epilepsy. The C/C genotype carriers reportedly are at higher risk of pharmacoresistance to AEDs, but only in some studies. The hypothesis of the C-variant associated risk and resistance to antiepileptic drugs (AEDs) has been hampered by conflicting results from inadequate power in case-control studies. To assess the role of C3435T polymorphism in drug-resistance in epilepsy, a systematic review and meta-analysis was conducted.

METHODS

Databases were obtained from the Cochrane Library, MEDLINE, EMBASE, major American and European conference abstracts, and www.google.my for genetic association studies up to February 2010. All the case-control association studies evaluating the role of ABCB1 C3435T in pharmacoresistance to AEDs were identified. The new definition of treatment outcome from International League Against Epilepsy (ILAE) was used for including studies for sub-analysis. To measure the strength of genetic association for the gene variant, the odds ratios (ORs) with 95% confidence intervals (CIs) were calculated using models of both fixed- and random-effects for comparisons of the alleles and genotypes with co-dominant (C/C vs. T/T, C/T vs. T/T), dominant (C/C+C/T vs. T/T), and recessive (C/C vs. C/T+T/T) models in overall and in ethnicity subgroups. The 19 studies were selected for the next sub-analysis based on the new definition of drug-responsiveness and drug-resistance from ILAE. The same analysis was also performed for treatment outcome and ethnicity subgroups.

RESULTS

A total of 22 association studies including 3231 (47.8%) drug-resistant patients and 3524 (52.2%) drug-responsive patients or healthy controls (genotyped for C3435T) were pooled in this meta-analysis. The allelic association of ABCB1 C3435T with risk of drug-resistance was not significant under fixed-effects model, 1.06 (95% CI 0.98-1.14, p=0.12) and random-effects model, 1.10 (0.93-1.30, p=0.28) in overall and in the subgroup analysis by ethnicity. Similar results were also obtained for all genetic models in the stratified analyses by new definition of drug-resistance by ILAE and ethnicity subgroups. There was no publication bias.

CONCLUSION

We failed to show an association between the ABCB1 C3435T polymorphism and the risk of drug-resistance suggesting a revision in contribution of this polymorphism in the multi-drug transporters hypothesis of pharmacoresistance to AEDs in epilepsy.

Genetic factors associated with drug-resistance of epilepsy: relevance of stratification by patient age and aetiology of epilepsy

Epilepsy drug-resistance may depend on the metabolism of antiepileptic drugs (AEDs), transport to the epileptic focus and/or target sensitivity. Furthermore, drug response depends on multiple characteristics of the patient, the epilepsy, and the antiepileptic drugs used. We have investigated the association between polymorphisms related to antiepileptic drug metabolism (CYP2C9, CYP2C19, and UGT), transport (ABCB1), and targets (SCN1A) both in a crude analysis and after adjusting by clinical factors associated with drug-resistance, and stratifying by patient age or aetiology of epilepsy. Caucasian outpatients (N=289), children (N=80) and adolescent-adults (N=209), with idiopathic (N=69), cryptogenic (N=97) or symptomatic epilepsies (N=123) were selected when they had either drug-resistance (with at least four seizures over the previous year after treatment with more than three appropriate AEDs at appropriate doses) or drug responsiveness (without seizures for at least a year). Samples were genotyped by allelic discrimination using TaqMan probes. No significant association between polymorphisms and drug-resistance was found either in the crude analysis or in the adjusted analysis. However, adults with the ABCB1_3435TT or 2677TT genotypes had a lower risk of drug-resistance than those with the CC or the GG genotypes. Furthermore, patients with symptomatic epilepsies with the ABCB1_3435CT or TT genotypes had a lower risk of drug-resistance than those with the CC genotype. An opposite but insignificant tendency was found in children and in idiopathic epilepsies. Although replication studies will be needed to confirm our results, they suggest that stratification by patient age and by the aetiology of epilepsy could contribute to unmask the association between ABCB1 polymorphisms and drug-resistance of epilepsy.