Influence of CYP2C9 genotypes on the formation of a hepatotoxic metabolite of valproic acid in human liver microsomes

Formation of potentially toxic metabolites of drugs in reactions catalyzed by human drug-metabolizing enzymes

Data are presented on the formation of potentially toxic metabolites of drugs that are substrates of human drug metabolizing enzymes. The tabular data lists the formation of potentially toxic/reactive products. The data were obtained from in vitro experiments and showed that the oxidative reactions predominate (with 96% of the total potential toxication reactions). Reductive reactions (e.g., reduction of nitro to amino group and reductive dehalogenation) participate to the extent of 4%. Of the enzymes, cytochrome P450 (P450, CYP) enzymes catalyzed 72% of the reactions, myeloperoxidase (MPO) 7%, flavin-containing monooxygenase (FMO) 3%, aldehyde oxidase (AOX) 4%, sulfotransferase (SULT) 5%, and a group of minor participating enzymes to the extent of 9%. Within the P450 Superfamily, P450 Subfamily 3A (P450 3A4 and 3A5) participates to the extent of 27% and the Subfamily 2C (P450 2C9 and P450 2C19) to the extent of 16%, together catalyzing 43% of the reactions, followed by P450 Subfamily 1A (P450 1A1 and P450 1A2) with 15%. The P450 2D6 enzyme participated in an extent of 8%, P450 2E1 in 10%, and P450 2B6 in 6% of the reactions. All other enzymes participate to the extent of 14%. The data show that, of the human enzymes analyzed, P450 enzymes were dominant in catalyzing potential toxication reactions of drugs and their metabolites, with the major role assigned to the P450 Subfamily 3A and significant participation of the P450 Subfamilies 2C and 1A, plus the 2D6, 2E1 and 2B6 enzymes contributing. Selected examples of drugs that are activated or proposed to form toxic species are discussed.

Ethnic Aspects of Valproic Acid P-Oxidation

The safety of the use of psychotropic drugs, widely used in neurological and psychiatric practice, is an urgent problem in personalized medicine. This narrative review demonstrated the variability in allelic frequencies of low-functioning and non-functional single nucleotide variants in genes encoding key isoenzymes of valproic acid P-oxidation in the liver across different ethnic/racial groups. The sensitivity and specificity of pharmacogenetic testing panels for predicting the rate of metabolism of valproic acid by P-oxidation can be increased by prioritizing the inclusion of the most common risk allele characteristic of a particular population (country).

Factors Influencing Plasma Concentrations of Valproic Acid in Pediatric Patients with Epilepsy and the Clinical Significance of CYP2C9 Genotypes in Personalized Valproic Acid Therapy

BACKGROUND

The aim of this study was to investigate the factors affecting plasma valproic acid (VPA) concentration in pediatric patients with epilepsy and the clinical significance of CYP2C9 gene polymorphisms in personalized dosing using therapeutic drug monitoring and pharmacogenetic testing.

METHODS

The medical records of children with epilepsy who underwent therapeutic drug monitoring at our institution between July 2022 and July 2023 and met the inclusion criteria were reviewed. Statistical analysis was performed to determine whether age, sex, blood ammonia, liver function, kidney function, and other characteristics affected the concentration-to-dose ratio of VPA (CDRV) in these patients. To investigate the effect of CYP2C9 polymorphisms on CDRV, DNA samples were collected from patients and the CYP2C9 genotypes were identified using real-time quantitative PCR.

RESULTS

The mean age of 208 pediatric patients with epilepsy was 5.50 ± 3.50 years. Among these patients, 182 had the CYP2C9 *1/*1 genotype, with a mean CDRV (mcg.kg/mL.mg) of 2.64 ± 1.46, 24 had the CYP2C9 *1/*3 genotype, with a mean CDRV of 3.28 ± 1.74, and 2 had the CYP2C9 *3/*3 genotype, with a mean CDRV of 6.46 ± 3.33. There were statistical differences among these 3 genotypes (P < 0.05). The CDRV in these patients were significantly influenced by age, aspartate aminotransferase, total bilirubin, direct bilirubin, globulin, albumin/globulin ratio, prealbumin, creatinine, and CYP2C9 polymorphisms. In addition, multivariate linear regression analysis identified total bilirubin, direct bilirubin, and CYP2C9 polymorphisms as independent risk factors for high CDRV.

CONCLUSIONS

Liver problems and mutations in the CYP2C9 gene increase VPA levels. This underscores the importance of considering these factors when prescribing VPA to children with epilepsy, thereby enhancing the safety and efficacy of the therapy.

Toxicity prediction using target, interactome, and pathway profiles as descriptors

In silico methods are essential to the safety evaluation of chemicals. Computational risk assessment offers several approaches, with data science and knowledge-based methods becoming an increasingly important sub-group. One of the substantial attributes of data science is that it allows using existing data to find correlations, build strong hypotheses, and create new, valuable knowledge that may help to reduce the number of resource intensive experiments. In choosing a suitable method for toxicity prediction, the available data and desired toxicity endpoint are two essential factors to consider. The complexity of the endpoint can impact the success rate of the in silico models. For highly complex endpoints such as hepatotoxicity, it can be beneficial to decipher the toxic event from a more systemic point of view. We propose a data science-based modelling pipeline that uses compounds` connections to tissue-specific biological targets, interactome, and biological pathways as descriptors of compounds. Models trained on different combinations of the collected, compound-target, compound-interactor, and compound-pathway profiles, were used to predict the hepatotoxicity of drug-like compounds. Several tree-based models were trained, utilizing separate and combined target, interactome and pathway level variables. The model using combined descriptors of all levels and the random forest algorithm was further optimized. Descriptor importance for model performance was addressed and examined for a biological explanation to define which targets or pathways can have a crucial role in toxicity. Descriptors connected to cytochromes P450 enzymes, heme degradation and biological oxidation received high weights. Furthermore, the involvement of other, less discussed processes in connection with toxicity, such as the involvement of RHO GTPase effectors in hepatotoxicity, were marked as fundamental. The optimized combined model using only the selected descriptors yielded the best performance with an accuracy of 0.766. The same dataset using classical Morgan fingerprints for compound representation yielded models with similar performance measures, as well as the combination of systems biology-based descriptors and Morgan fingerprints. Consequently, adding the structural information of compounds did not enhance the predictive value of the models. The developed systems biology-based pipeline comprises a valuable tool in predicting toxicity, while providing novel insights about the possible mechanisms of the unwanted events.

PAMAM-G4 protect the N-(2-hydroxyphenyl)-2-propylpentanamide (HO-AAVPA) and maintain its antiproliferative effects on MCF-7

Our work group designed and synthesized a promissory compound N-(2-hydroxyphenyl)-2-propylpentanamide (HO-AAVPA). The HO-AAVPA is a HDAC1 inhibitor and antiproliferative in cancer cell lines. However, HO-AAVPA is poor water solubility and enzymatically metabolized. In this work, the fourth-generation poly(amidoamine) dendrimer (PAMAM-G4) was used as a drug deliver carrier of HO-AAVPA. Moreover, HO-AAVPA and HO-AAVPA-PAMAM complex were submitted to forced degradation studies (heat, acid, base, oxidation and sunlight). Also, the HO-AAVPA-PAMAM-G4 complex was assayed as antiproliferative in a breast cancer cell line (MCF-7). The HO-AAVPA-PAMAM-G4 complex was obtained by docking and experimentally using three pH conditions: acid (pH = 3.0), neutral (pH = 7.0) and basic (pH = 9.0) showing that PAMAM-G4 captureand protect the HO-AAVPA from forced degradation, it is due to sunlight yielded a by-product from HO-AAVPA. In addition, the PAMAM-G4 favored the HO-AAVPA water solubility under basic and neutral pH conditions with significant difference (F_(2,18) = 259.9, p < 0.001) between the slopes of the three conditions being the basic condition which solubilizes the greatest amount of HO-AAVPA. Finally, the HO-AAVPA-PAMAM-G4 complex showed better antiproliferative effects on MCF-7 (IC₅₀ = 75.3 μM) than HO-AAVPA (IC₅₀ = 192 μM). These results evidence that PAMAM-G4 complex improve the biological effects of HO-AAVPA.

Machine learning advances the integration of covariates in population pharmacokinetic models: Valproic acid as an example

Background and Aim: Many studies associated with the combination of machine learning (ML) and pharmacometrics have appeared in recent years. ML can be used as an initial step for fast screening of covariates in population pharmacokinetic (popPK) models. The present study aimed to integrate covariates derived from different popPK models using ML.

Methods: Two published popPK models of valproic acid (VPA) in Chinese epileptic patients were used, where the population parameters were influenced by some covariates. Based on the covariates and a one-compartment model that describes the pharmacokinetics of VPA, a dataset was constructed using Monte Carlo simulation, to develop an XGBoost model to estimate the steady-state concentrations (Css) of VPA. We utilized SHapley Additive exPlanation (SHAP) values to interpret the prediction model, and calculated estimates of VPA exposure in four assumed scenarios involving different combinations of CYP2C19 genotypes and co-administered antiepileptic drugs. To develop an easy-to-use model in the clinic, we built a simplified model by using CYP2C19 genotypes and some noninvasive clinical parameters, and omitting several features that were infrequently measured or whose clinically available values were inaccurate, and verified it on our independent external dataset.

Results: After data preprocessing, the finally generated combined dataset was divided into a derivation cohort and a validation cohort (8:2). The XGBoost model was developed in the derivation cohort and yielded excellent performance in the validation cohort with a mean absolute error of 2.4 mg/L, root-mean-squared error of 3.3 mg/L, mean relative error of 0%, and percentages within ±20% of actual values of 98.85%. The SHAP analysis revealed that daily dose, time, CYP2C19*2 and/or *3 variants, albumin, body weight, single dose, and CYP2C19*1*1 genotype were the top seven confounding factors influencing the Css of VPA. Under the simulated dosage regimen of 500 mg/bid, the VPA exposure in patients who had CYP2C19*2 and/or *3 variants and no carbamazepine, phenytoin, or phenobarbital treatment, was approximately 1.74-fold compared to those with CYP2C19*1/*1 genotype and co-administered carbamazepine + phenytoin + phenobarbital. The feasibility of the simplified model was fully illustrated by its performance in our external dataset.

Conclusion: This study highlighted the bridging role of ML in big data and pharmacometrics, by integrating covariates derived from different popPK models.

Genetics and antiepileptic mood stabilizer treatment response in bipolar disorder: what do we know?

Antiepileptic mood stabilizers (AED-MS) are often used to treat bipolar disorder (BD). Similar to other mood disorder medications, AED-MS treatment response varies between patients. Identification of biomarkers associated with treatment response may ultimately help with the delivery of individualized treatment and lead to improved treatment efficacy. Here, we conducted a narrative review of the current knowledge of the pharmacogenomics of AED-MS (valproic acid, lamotrigine and carbamazepine) treatment response in BD, including genetic contributions to AED-MS pharmacokinetics. Genes involved in neurotransmitter systems and drug transport have been shown to be associated with AED-MS treatment response. As more studies are conducted, and experimental and analytical methods advance, knowledge of AED-MS pharmacogenomics is expected to grow and contribute to precision medicine in BD.

Impact of genetic and non-genetic factors on hepatic CYP2C9 expression and activity in Hungarian subjects

CYP2C9, one of the most abundant hepatic cytochrome P450 enzymes, is involved in metabolism of 15–20% of clinically important drugs (warfarin, sulfonylureas, phenytoin, non-steroid anti-inflammatory drugs). To avoid adverse events and/or impaired drug-response, CYP2C9 pharmacogenetic testing is recommended. The impact of CYP2C9 polymorphic alleles (CYP2C9*2, CYP2C9*3) and phenoconverting non-genetic factors on CYP2C9 function and expression was investigated in liver tissues from Caucasian subjects (N = 164). The presence of CYP2C9*3 allele was associated with CYP2C9 functional impairment, and CYP2C9*2 influenced tolbutamide 4′-hydroxylase activity only in subjects with two polymorphic alleles, whereas the contribution of CYP2C8*3 was not confirmed. In addition to CYP2C9 genetic polymorphisms, non-genetic factors (co-medication with CYP2C9-specific inhibitors/inducers and non-specific factors including amoxicillin + clavulanic acid therapy or chronic alcohol consumption) contributed to the prediction of hepatic CYP2C9 activity; however, a CYP2C9 genotype–phenotype mismatch still existed in 32.6% of the subjects. Substantial variability in CYP2C9 mRNA levels, irrespective of CYP2C9 genotype, was demonstrated; however, CYP2C9 induction and non-specific non-genetic factors potentially resulting in liver injury appeared to modify CYP2C9 expression. In conclusion, complex implementation of CYP2C9 genotype and non-genetic factors for the most accurate estimation of hepatic CYP2C9 activity may improve efficiency and safety of medication with CYP2C9 substrate drugs in clinical practice.

CYP2C9*3/*3 Gene Expression Affects the Total and Free Concentrations of Valproic Acid in Pediatric Patients with Epilepsy

Purpose

To perform therapeutic drug monitoring (TDM) of total and free plasma valproic acid (VPA) concentrations in pediatric patients with epilepsy and to analyze related factors.

Patients and Methods

Pediatric epileptic patients treated in 2015-2019 in our hospital were assessed. Total and free plasma VPA concentrations were obtained by UPLC and LC-MS/MS, respectively. Regression analysis was performed to examine the associations of free plasma VPA with total plasma VPA and plasma protein binding rate. The impacts of individual situation, CYP2C9 genotype, and drug combination on VPA concentration were examined.

Results

Of the 251 patients, 81 had lower total concentrations than effective therapeutic levels; 86 and 31 patients had infections and central nervous system dysplasia, respectively. VPA's daily doses and free drug concentrations were significantly lower in the CYP2C9 *3/*3 genotype group versus the CYP2C9 *1/*3 and CYP2C9 *1/*1 groups (P<0.05). Free and total VPA concentrations were linked by Y = 0.0004 X² + 0.042 X + 0.3035 (r=0.6981); VPA plasma protein binding rate and free VPA concentration were related by Y = 0.0003 X² - 0.0127 X + 0.9777 (r=0.8136). Both total and free VPA concentrations were significantly decreased in patients simultaneously administered phenobarbital, meropenem and biapenem (P<0.05), with therapeutic failure after meropenem/biapenem co-administration.

Conclusion

Free VPA amounts have nonlinear relationships with total VPA amounts and plasma protein binding rate in epileptic children. Additionally, CYP2C9 *3/*3 expression affects VPA metabolism. Since phenobarbital affects VPA metabolism, TDM is recommended. Meanwhile, carbapenem-co-administration with VPA should be prohibited.

Pharmacogenetics of Carbamazepine and Valproate: Focus on Polymorphisms of Drug Metabolizing Enzymes and Transporters

Pharmacogenomics can identify polymorphisms in genes involved in drug pharmacokinetics and pharmacodynamics determining differences in efficacy and safety and causing inter-individual variability in drug response. Therefore, pharmacogenomics can help clinicians in optimizing therapy based on patient's genotype, also in psychiatric and neurological settings. However, pharmacogenetic screenings for psychotropic drugs are not routinely employed in diagnosis and monitoring of patients treated with mood stabilizers, such as carbamazepine and valproate, because their benefit in clinical practice is still controversial. In this review, we summarize the current knowledge on pharmacogenetic biomarkers of these anticonvulsant drugs.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Personalized Medicine Using Cutting Edge Technologies for Genetic Epilepsies

Epilepsy is the most common chronic neurologic disorder in the world, affecting 1-2% of the population. Besides, 30% of epilepsy patients are drug-resistant. Genomic mutations seem to play a key role in its etiology and knowledge of strong effect mutations in protein structures might improve prediction and the development of efficacious drugs to treat epilepsy. Several genetic association studies have been undertaken to examine the effect of a range of candidate genes for resistance. Although, few studies have explored the effect of the mutations into protein structure and biophysics in the epilepsy field. Much work remains to be done, but the plans made for exciting developments will hold therapeutic potential for patients with drug-resistance. In summary, we provide a critical review of the perspectives for the development of individualized medicine for epilepsy based on genetic polymorphisms/mutations in light of core elements such as transcriptomics, structural biology, disease model, pharmacogenomics and pharmacokinetics in a manner to improve the success of trial designs of antiepileptic drugs.

Functional Measurement of CYP2C9 and CYP3A4 Allelic Polymorphism on Sildenafil Metabolism

Aim

We aimed to systematically examine the effects of enzymatic activity of 38 human CYP2C9 alleles and 21 human CYP3A4 alleles, including wild-type CYP2C9.1 and CYP3A4.1, which contain the 24 CYP2C9 novel alleles (*36–*60) and 6 CYP3A4 novel alleles (*28–*34) newly found in the Chinese population, on sildenafil metabolism through in vitro experiment.

Methods

The recombinant cytochrome P450 alleles protein of CYP2C9 and CYP3A4 expressed in insect baculovirus expression system were reacted with 10–500 µM sildenafil for 30 minutes at 37°C, and the reaction was terminated by cooling to −80°C immediately. Next, we used ultra-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) detection system to detect sildenafil and its active metabolite N-desmethyl sildenafil.

Results

The intrinsic clearance (Vmax/Km) values of most CYP2C9 variants were significantly altered when compared with the wild-type CYP2C9*1, with most of these variants exhibiting either reduced Vmax and/or increased Km values. Four alleles (CYP2C9*11, *14, *31, *49) exhibited no markedly decreased relative clearance (1-fold). The relative clearance of the remaining thirty-three variants exhibited decrease in different levels, ranging from 1.81% to 88.42%. For the CYP3A4 metabolic pathway, when compared with the wild-type CYP3A4*1, the relative clearance values of four variants (CYP3A4*3, *10, *14 and *I335T) showed significantly higher relative clearance (130.7–134.9%), while five variants (CYP3A4*2, *5, *24, *L22V and *F113I) exhibited sharply reduced relative clearance values (1.80–74.25%), and the remaining nine allelic variants showed no statistical difference. In addition, the kinetic parameters of two CYP3A4 variants (CYP3A4*17 and CYP3A4*30) could not be detected, due to the defect of the CYP3A4 gene.

Conclusion

These findings were the first evaluation of all these infrequent CYP2C9 and CYP3A4 alleles for sildenafil metabolism; when treating people who carry these CYP2C9 and CYP3A4 variants, there should be more focus on the relation of dose intensity, side effects and therapeutic efficacy when administering sildenafil. The study will provide fundamental data on effect of CYP2C9 and CYP3A4 allelic variation on sildenafil metabolism for further clinical research.

Clinical Relevance of Pharmacogenetics in Serotonin Syndrome

Serotonin syndrome is a predictable life-threatening condition that is caused by serotonergic stimulation of the central and peripheral nervous systems. A patient's genetic profile can amplify exposure risk as many serotonergic drugs are metabolized by CYP450 enzymes, and these enzymes may be altered in functionality. We report a case of an elderly man who presented with serotonin syndrome after a dose change in valproic acid 5 weeks prior. His medication list consisted of low-dose serotonergic agents, which is unusual as most cases of serotonin syndrome involve higher doses. A review of his pharmacogenetic profile is presented to retrospectively evaluate the additive risk for serotonin syndrome and implications on resuming serotonergic agents.

Structure of Cytochrome P450 2C9*2 in Complex with Losartan: Insights into the Effect of Genetic Polymorphism

The human CYP2C9 plays a crucial role in the metabolic clearance of a wide range of clinical therapeutics. The *2 allele is a prevalent genetic variation in CYP2C9 that is found in various populations. A marked reduction of catalytic activity toward many important drug substrates has been demonstrated by CYP2C9*2, which represents an amino acid variation at position 144 from arginine to cysteine. The crystal structure of CYP2C9*2 in complex with an antihypertensive drug losartan was solved using X-ray crystallography at 3.1-Å resolution. The Arg144Cys variation in the *2 complex disrupts the hydrogen-bonding interactions that were observed between the side chain of arginine and neighboring residues in the losartan complex of CYP2C9 and the wild-type (WT) ligand-free structure. The conformation of several secondary structural elements is affected, thereby altering the binding and orientation of drug and important amino acid side chains in the distal active site cavity. The new structure revealed distinct interactions of losartan in the compact active site of CYP2C9*2 and differed in occupancy at the other binding sites previously identified in the WT-losartan complex. Furthermore, the binding studies in solution using losartan illustrated lower activity of the CYP2C9*2 compared with the WT. Together, the findings yield valuable insights into the decreased hydroxylation activity of losartan in patients carrying CYP2C9*2 allele and provide a useful framework to investigate the effect of a single-nucleotide polymorphism that leads to altered metabolism of diverse drug substrates. SIGNIFICANCE STATEMENT: The *2 allele of the human drug-metabolizing enzyme CYP2C9 is found in different populations and results in significantly reduced activity toward various drug substrates. How the CYP2C9*2 variant induces altered drug metabolism is poorly understood given that the Arg144Cys variation is located far away from the active site. This work yield insight into the effect of distal variation using multitude of techniques that include X-ray crystallography, isothermal titration calorimetry, enzymatic characterization, and computational studies.

Pharmacogenetics and the treatment of epilepsy: what do we know?

Seizure control with antiepileptic drugs (AEDs) as well as susceptibility to adverse drug reactions varies among individuals with epilepsy. This interindividual variability is partly determined by genetic factors. However, genetic testing to predict the efficacy and toxicity of AEDs is limited and genetic variability is, as yet, largely unexplainable. Accordingly, genetic testing can only be advised in a very limited number of cases in clinical routine. Currently, by applying different methodologies, many trials have been undertaken to evaluate cost benefits of preventive pharmacogenetic analysis for patients. There is significant progress in sequencing technologies, and focus is on next-generation sequencing-based methods, like exome and genome sequencing. In this review, an overview of the current scientific knowledge considering the pharmacogenetics of AEDs is given.

Valproic Acid and the Liver Injury in Patients with Epilepsy: An Update

BACKGROUND

Valproic acid (VPA) as a widely used primary medication in the treatment of epilepsy is associated with reversible or irreversible hepatotoxicity. Long-term VPA therapy is also related to increased risk for the development of non-alcoholic fatty liver disease (NAFLD). In this review, metabolic elimination pathways of VPA in the liver and underlying mechanisms of VPA-induced hepatotoxicity are discussed.

METHODS

We searched in PubMed for manuscripts published in English, combining terms such as "Valproic acid", "hepatotoxicity", "liver injury", and "mechanisms". The data of screened papers were analyzed and summarized.

RESULTS

The formation of VPA reactive metabolites, inhibition of fatty acid β-oxidation, excessive oxidative stress and genetic variants of some enzymes, such as CPS1, POLG, GSTs, SOD2, UGTs and CYPs genes, have been reported to be associated with VPA hepatotoxicity. Furthermore, carnitine supplementation and antioxidants administration proved to be positive treatment strategies for VPA-induced hepatotoxicity.

CONCLUSION

Therapeutic drug monitoring (TDM) and routine liver biochemistry monitoring during VPA-therapy, as well as genotype screening for certain patients before VPA administration, could improve the safety profile of this antiepileptic drug.

Estimation of apparent clearance of valproic acid in adult Saudi patients

Background Valproic acid is one of several antiepileptic medications requiring therapeutic drug monitoring due to its complex and wide pharmacokinetic interindividual variability. Objective The objective of this study was to determine the population pharmacokinetics of valproic acid in adult Saudi patients and to identify factors that explain its pharmacokinetic variability. Setting Tertiary referral teaching hospital, Riyadh, Saudi Arabia. Method A retrospective chart review was performed at King Saud University Medical City of patients who received oral valproic acid. The population pharmacokinetic models were developed using Monolix 4.4. After development of the base model, we investigated several covariates including age, sex, weight, total daily dose, and cotherapy with carbamazepine and phenytoin. Main outcome measures the pharmacokinetic parameters of valproic acid and the variables that contributing towards its inter-individual variability. Results The analysis included a total of 54 valproic acid plasma concentrations from 54 patients (42.5% male). The data were sufficiently described by a one-compartment model with linear absorption and elimination processes. Average parameter estimates for valproic acid apparent clearance (CL/F) and apparent volume of distribution (V/F) were 0.14 L/h and 37.7 L (fixed), respectively. The inter-individual variability (coefficients of variation) in CL/F was 12%. The most significant covariates for valproic acid CL/F were age, body weight, total daily dose, and cotherapy with carbamazepine and phenytoin. Conclusion This model showed significant inter-individual variability between subjects. Our findings showed that patient age, body weight, total daily dose, and cotherapy with carbamazepine and phenytoin are the most significant covariates of valproic acid clearance. Collectively, healthcare providers should take these factors in consideration for optimal valproic acid dosage regimen.

Relevance of CYP2C9 Function in Valproate Therapy

BACKGROUND

Genetic polymorphisms of drug metabolizing enzymes can substantially modify the pharmacokinetics of a drug and eventually its efficacy or toxicity; however, inferring a patient's drug metabolizing capacity merely from his or her genotype can lead to false prediction. Non-genetic host factors (age, sex, disease states) and environmental factors (nutrition, comedication) can transiently alter the enzyme expression and activities resulting in genotypephenotype mismatch. Although valproic acid is a well-tolerated anticonvulsant, pediatric patients are particularly vulnerable to valproate injury that can be partly attributed to the age-related differences in metabolic pathways.

METHODS

CYP2C9 mediated oxidation of valproate, which is the minor metabolic pathway in adults, appears to become the principal route in children. Genetic and non-genetic variations in CYP2C9 activity can result in significant inter- and intra-individual differences in valproate pharmacokinetics and valproate induced adverse reactions.

RESULTS

The loss-of-function alleles, CYP2C9*2 or CYP2C9*3, display significant reduction in valproate metabolism in children; furthermore, low CYP2C9 expression in patients with CYP2C9*1/*1 genotype also leads to a decrease in valproate metabolizing capacity. Due to phenoconversion, the homozygous wild genotype, expected to be translated to CYP2C9 enzyme with normal activity, is transiently switched into poor (or extensive) metabolizer phenotype.

CONCLUSION

Novel strategy for valproate therapy adjusted to CYP2C9-status (CYP2C9 genotype and CYP2C9 expression) is strongly recommended in childhood. The early knowledge of pediatric patients' CYP2C9-status facilitates the optimization of valproate dosing which contributes to the avoidance of misdosing induced adverse reactions, such as abnormal blood levels of ammonia and alkaline phosphatase, and improves the safety of children's anticonvulsant therapy.

Effects of valproate on reproductive endocrine function in male patients with epilepsy: A systematic review and meta-analysis

BACKGROUND

Valproate (VPA) is a broad spectrum antiepileptic drug (AED) that is generally used as a first line agent for most idiopathic and symptomatic generalized epilepsies. Many studies have indicated that AEDs cause reproductive endocrine disorders in males, but the exact etiology underpinning these dysfunctions is not clear. This meta-analysis and systematic review was intended to evaluate the effect of VPA on reproductive endocrine function in male patients with epilepsy.

METHODS

A literature search was performed using electronic databases up to December 2017 for eligible studies. The differences in the levels of the reproductive factors, luteinizing hormone (LH), follicle-stimulating hormone (FSH), sex hormone binding globulin (SHBG), testosterone, dehydroepiandrosterone sulfate (DHEAS), and androstenedione (ADION) in the male patients with epilepsy treated with VPA (treatment group) were compared with the those of the healthy controls (control group) by the use of the Standardized mean difference (SMD) with 95% confidence intervals (CIs).

RESULTS

Six publications with a total of 316 subjects were identified. The result revealed that the levels of FSH (SMD = -1.33, 95% CI: -2.60 to -0.07, P = 0.039) and testosterone (SMD = -0.45, 95% CI: -0.87 to -0.03, P = 0.038) of the treatment group were decreased significantly compared with the healthy controls. There was an increase in the levels of SHBG (SMD = 0.41, 95% CI: -0.21 to 1.03, P = 0.197), DHEAS (SMD = 0.20, 95% CI: -0.06 to 0.45, P = 0.126) and ADION (SMD = 0.73, 95% CI: -0.10 to 1.57, P = 0.086), and a decrease in that of LH(SMD = -0.71, 95% CI: -1.49 to 0.07, P = 0.075) in the male patients with epilepsy treated with VPA, but the differences did not reach statistical significance (P > 0.05).

CONCLUSIONS

This meta-analysis indicates that VPA may lead to a significant decrease in the levels of FSH and testosterone and alter the concentrations of LH, DHEAS, SHBG, and ADION to some extent, which might contribute to the reproductive endocrine dysfunction in male patients with epilepsy. It is important for clinical neurologists to be cautious when prescribing VPA to reproductive-aged male patients with epilepsy.

Risk Factors for Valproic Acid-induced Hyperammonaemia in Chinese Paediatric Patients with Epilepsy

This study was aimed at identifying genetic and non-genetic risk factors for valproic acid (VPA)-induced hyperammonaemia in Chinese paediatric patients with epilepsy. A total of 210 epileptic patients, treated with VPA as monotherapy, were enrolled and classified into hyperammonaemia and control groups according to their blood ammonia level (cut-off value 50 μmol/L). Serum concentrations of VPA and its major metabolites were simultaneously determined by ultrahigh-performance liquid chromatography-tandem mass spectrometry. Six single nucleotide polymorphisms in the candidate genes, CYP2C9, CYP2A6, CYP2B6 and CPS1, were analysed by a matrix-assisted laser desorption ionization-time of flight mass spectrometry method or nested PCR. Significant differences in age, aspartate transaminase level and the incidence of liver injury were observed between patients of hyperammonaemia and control groups. Genotype distributions of CYP2C9*3, CYP2A6*4 and CPS1 4217C>A allelic variants were also significantly different between the two groups. According to multiple regression analysis, a significant negative correlation was detected between age and the blood ammonia level, while liver injury, the concentration-dose ratio (CDR) of VPA and 2-propyl-4-pentenoic acid (4-ene VPA), and the presence of CYP2A6*4 or CPS1 4217C>A showed positive correlations with the blood ammonia level. In addition, the risk factors for hyperammonaemia identified by logistic regression analysis were as follows: a younger age (odds ratio [OR] = 0.85; 95% confidence interval [CI] = 0.76-0.96; p = 0.007), occurrence of liver injury (OR = 4.60; 95% CI = 1.27-16.74; p = 0.021), higher CDR of 4-ene VPA (OR = 1.08; 95% CI = 1.03-1.14; p = 0.001), and carrying mutant alleles of CYP2C9*3 (OR = 3.42; 95% CI = 1.15-10.19; p = 0.028), CYP2A6*4 (OR = 3.23; 95% CI = 1.40-7.48; p = 0.006) and CPS1 4217C>A (OR = 3.25; 95% CI = 1.52-6.94; p = 0.002). Our findings indicated that multiple genetic and non-genetic risk factors that were identified can be used to predict the development of VPA-induced hyperammonaemia in Chinese paediatric patients with epilepsy.

The pharmacogenomics of valproic acid

Valproic acid is an anticonvulsant and mood-stabilizing drug used primarily in the treatment of epilepsy and bipolar disorder. Adverse effects of valproic acid are rare, but hepatotoxicity is severe in particular in those younger than 2 years old and polytherapy. During valproic acid treatment, it is difficult for prescribers to predict its individual response. Recent advances in the field of pharmacogenomics have indicated variants of candidate genes that affect valproic acid efficacy and safety. In this review, a large number of candidate genes that influence valproic acid pharmacokinetics and pharmacodynamics are discussed, including metabolic enzymes, drug transporters, neurotransmitters and drug targets. Furthermore, pharmacogenomics is an important tool not only in further understanding of interindividual variability but also to assess the therapeutic potential of such variability in drug individualization and therapeutic optimization.

Genetic polymorphisms and valproic acid plasma concentration in children with epilepsy on valproic acid monotherapy

PURPOSE

The aim of the study is to evaluate the association between genetic polymorphisms and valproic acid (VPA) concentration to dose ratio in children with epilepsy on VPA monotherapy.

METHODS

A total of 137 children, aged 3.5-18 years, (89 males and 48 females) with epilepsy on sustained-release VPA monotherapy were enrolled. Trough plasma concentrations of VPA at steady-state were measured using an AXSYM automatic immunity analyzer. The values were divided by body weight and total daily dose to calculate concentration to dose ratio of VPA (CDRV). Forty-eight single nucleotide polymorphisms involved in the pharmacokinetics of VPA were identified by MassARRAY system. The logarithmic transformed CDRV (lnCDRV) was normally distributed, and PLINK software was used to evaluate the association between genetic polymorphisms and lnCDRV using linear regression adjusted for gender and seizure type.

RESULTS

rs28898617 (UGT1A3/4/5/6/7/8/9/10, BETA=0.32, P=0.0089) was significantly associated with higher lnCDRV. No other associations were found.

CONCLUSIONS

In pediatric patients taking VPA monotherapy, rs28898617 was associated with a higher normalized VPA plasma concentration. Further studies are warranted to confirm the results.

Association of CYP2C9, CYP2A6, ACSM2A, and CPT1A gene polymorphisms with adverse effects of valproic acid in Chinese patients with epilepsy

OBJECTIVE

To explore the influence of CYP2C9, CYP2A6, ACSM2A, CPT1A gene polymorphisms on valproic acid (VPA) and its role in metabolism-related liver dysfunction in order to guide the clinical safety and rational use of VPA.

METHODS

One hundred two patients taking sodium valproate oral solution were genotyped. To assess the genotypes of relevant genes, the CYP2C9 gene was directly sequenced; for polymorphism classification, multiple Long-PCR electrophoresis was conducted for CYP2A6; and imLDR method was used for ACSM2A and CPT1A. GC-MS-SIM was used to determine the levels of VPA and 2-propyl-4-pentenoic acid (4-ene-VPA) in human plasma simultaneously.

RESULTS

CYP2C9 mutations had a significant impact on 4-ene-VPA concentration, in patients with wild-type CYP2C9 (CYP2C9*1), which has a greater capacity for VPA metabolism than the mutant type (CYP2C9*3), liver dysfunction was substantially higher. Patients with an ACSM2A polymorphism had higher levels of ALT and AST compared with wild-type (p<0.05), but the mutations had no effect on the VPA-related liver dysfunction (p>0.05). Among different CYP2A6 and CPT1A genotype groups, there was no significant correlation in the levels of VPA, 4-ene-VPA, ALT, AST or TB (p>0.05). The content of 4-ene-VPA had no direct correlation with the incidence of liver dysfunction.

CONCLUSIONS

Early detection of CYP2C9 gene polymorphisms may help to predict or prevent liver dysfunction caused by VPA. While the concentration of 4-ene-VPA was not suitable as an early warning index, the results provide clear theoretical guidance for the rational and safe clinical use of VPA.

Increase in the systemic exposure of primary metabolites of Midazolam in rat arising from CYP inhibition or hepatic dysfunction

The main purpose of this study is to demonstrate the possibility of increase in the systemic exposure of drug metabolites by CYP-inhibition or acute hepatitis. Midazolam (MDZ) was used as a model substrate of CYP3A and 1-aminobenzotriazole (ABT) was used as a CYP-inhibitor. After oral pretreatment with ABT, MDZ was intravenously injected to rats and the plasma profiles of MDZ and its primary metabolites, 1'-hydroxy MDZ and 4-hydroxy MDZ, were observed. In the ABT-pretreatment rats, plasma AUCs of both metabolites were much larger than those in control rats, demonstrating a higher systemic exposure of metabolites under CYP-inhibited condition. Furthermore, kinetic analysis revealed that the amount of both metabolites entered into the systemic circulation increased significantly (about 5-times). Increases in the systemic exposure of the primary metabolites of MDZ were also observed in the acute hepatitis rats induced by CCl₄-pretreatment. As underlying mechanisms, it was speculated that ABT inhibited the subsequent metabolism of primary metabolites of MDZ in the hepatocytes and enhanced their release to the systemic circulation. In vitro study with rat liver microsomes supported this speculation. In conclusion, this study showed the complexity of PK profiles of drug metabolites, which might lead to new aspects on their safety issue.

The Impact of Anti-Epileptic Drugs on Growth and Bone Metabolism

Epilepsy is a common neurological disorder worldwide and anti-epileptic drugs (AEDs) are always the first choice for treatment. However, more than 50% of patients with epilepsy who take AEDs have reported bone abnormalities. Cytochrome P450 (CYP450) isoenzymes are induced by AEDs, especially the classical AEDs, such as benzodiazepines (BZDs), carbamazepine (CBZ), phenytoin (PT), phenobarbital (PB), and valproic acid (VPA). The induction of CYP450 isoenzymes may cause vitamin D deficiency, hypocalcemia, increased fracture risks, and altered bone turnover, leading to impaired bone mineral density (BMD). Newer AEDs, such as levetiracetam (LEV), oxcarbazepine (OXC), lamotrigine (LTG), topiramate (TPM), gabapentin (GP), and vigabatrin (VB) have broader spectra, and are safer and better tolerated than the classical AEDs. The effects of AEDs on bone health are controversial. This review focuses on the impact of AEDs on growth and bone metabolism and emphasizes the need for caution and timely withdrawal of these medications to avoid serious disabilities.

A Pharmacokinetic Modeling Approach to Predict the Contribution of Active Metabolites to Human Efficacious Dose

A preclinical drug candidate, MRK-1 (Merck candidate drug parent compound), was found to elicit tumor regression in a mouse xenograft model. Analysis of samples from these studies revealed significant levels of two circulating metabolites, whose identities were confirmed by comparison with authentic standards using liquid chromatography-tandem mass spectrometry. These metabolites were found to have an in vitro potency similar to that of MRK-1 against the pharmacological target and were therefore thought to contribute to the observed efficacy. To predict this contribution in humans, a pharmacokinetic (PK) modeling approach was developed. At the mouse efficacious dose, the areas under the plasma concentration time curves (AUCs) of the active metabolites were normalized by their in vitro potency compared with MRK-1. These normalized metabolite AUCs were added to that of MRK-1 to yield a composite efficacious unbound AUC, expressed as "parent drug equivalents," which was used as the target AUC for predictions of the human efficacious dose. In vitro and preclinical PK studies afforded predictions of the PK of MRK-1 and the two active metabolites in human as well as the relative pathway flux to each metabolite. These were used to construct a PK model (Berkeley Madonna, version 8.3.18; Berkeley Madonna Inc., University of California, Berkeley, CA) and to predict the human dose required to achieve the target parent equivalent exposure. These predictions were used to inform on the feasibility of the human dose in terms of size, frequency, formulation, and likely safety margins, as well as to aid in the design of preclinical safety studies.

Influence of glutamine synthetase gene polymorphisms on the development of hyperammonemia during valproic acid-based therapy

PURPOSE

Valproic acid (VPA), which is widely used to treat epilepsy, migraine, and bipolar disorder, can causes severe hyperammonemia. However, the mechanism responsible for this adverse effect is not readily apparent. We previously reported that phenytoin coadministration is a strong risk factor for the development of hyperammonemia during VPA-based therapy. In this study, we focused on glutamine synthetase, which catalyzes the synthesis of glutamine from glutamate and ammonia and examined the association with the development of hyperammonemia during VPA-based therapy.

METHODS

For this study, we recruited 202 Japanese pediatric patients having epilepsy. We selected three polymorphisms (rs10911070, rs10797771, and rs10911021) in the glutamine synthetase (GLUL) gene. Hyperammonemia was defined as a plasma ammonia level exceeding 200 or 170 μg/dL. We evaluated the association between the development of hyperammonemia during VPA-based therapy and the patient characteristics, including three GLUL polymorphisms.

RESULTS

The number of patients who developed hyperammonemia during VPA-based therapy was 20 (9.9%) using the 200 μg/dL cutoff value and 30 (14.9%) using the 170 μg/dL cutoff value. Using a multivariate logistic regression analysis, the GLUL rs10797771 polymorphism and phenytoin coadministration in the 200 μg/dL cutoff value, and female in addition to two factors in the 170 μg/dL cutoff value, had significant associations with a plasma ammonia level elevation during VPA-based therapy.

CONCLUSION

Phenytoin coadministration, GLUL rs10797771 polymorphism in the 200μg/dL cutoff value, and female in addition to two factors in the 170μg/dL cutoff value, are independent risk factors for elevated plasma ammonia levels during VPA-based therapy.

The pharmacogenomics of epilepsy

Genetic factors contribute to the high interindividual variability in response to antiepileptic drugs. However, most genetic markers identified to date have limited sensitivity and specificity, and the value of genetic testing in guiding antiepileptic drug (AED) therapy is limited. The best defined indication for testing relates to HLA-B*15:02 genotyping to identify those individuals of South Asian ethnicity who are at high risk for developing serious adverse cutaneous reactions to carbamazepine. The indication for HLA-A*31:01 testing to identify individuals at risk for skin reactions from carbamazepine, or for CYP2C9 genotyping to identify individuals at risk for serious skin reactions from phenytoin is less compelling. The use of genetic testing to guide epilepsy treatment is likely to increase in the future, as better understanding of the function of epilepsy genes will permit the application of precision medicine targeting the biological mechanisms responsible for epilepsy in the specific individual.

Phenoconversion of CYP2C9 in epilepsy limits the predictive value of CYP2C9 genotype in optimizing valproate therapy

Aim: Since prominent role in valproate metabolism is assigned to CYP2C9 in pediatric patients, the association between children's CYP2C9-status and serum valproate concentrations or dose-requirements was evaluated. Materials & Methods: The contribution of CYP2C9 genotype and CYP2C9 expression in children (n = 50, Caucasian) with epilepsy to valproate pharmacokinetics was analyzed. Results: Valproate concentrations were significantly lower in normal expressers with CYP2C9*1/*1 than in low expressers or in patients carrying polymorphic CYP2C9 alleles. Consistently, the dose-requirement was substantially higher in normal expressers carrying CYP2C9*1/*1 (33.3 mg/kg vs 13.8–17.8 mg/kg, p < 0.0001). Low CYP2C9 expression significantly increased the ratio of poor metabolizers predictable from CYP2C9 genotype (by 46%). Conclusion: Due to the substantial downregulation of CYP2C9 expression in epilepsy, inferring patients’ valproate metabolizing phenotype merely from CYP2C9 genotype results in false prediction.

Influence of uridine diphosphate glucuronosyltransferase 2B7 -161C>T polymorphism on the concentration of valproic acid in pediatric epilepsy patients

BACKGROUND

Valproic acid (VPA) is widely used to treat various types of epilepsy. Interindividual variability in VPA pharmacokinetics may arise from genetic polymorphisms of VPA-metabolizing enzymes. This study aimed to examine the relationships between plasma VPA concentrations and the -161C>T single nucleotide polymorphism in uridine diphosphate glucuronosyltransferase (UGT) 2B7 genes in pediatric epilepsy patients.

METHODS

This study included 78 pediatric epilepsy patients carrying the cytochrome P450 (CYP) 2C9*1/*1 genotype and who were not treated with the enzyme inducers (phenytoin, phenobarbital, and carbamazepine), lamotrigine, and/or topiramate. CYP2C9*3 and UGT2B7 -161C>T polymorphisms were identified using methods based on polymerase chain reaction-restriction fragment length polymorphism. Blood samples were drawn from each patient under steady-state conditions, and plasma VPA concentrations were measured.

RESULTS

Significant differences in adjusted plasma VPA concentrations were observed between carriers of CC, CT, and TT genotypes in the UGT2B7 -161C>T polymorphism (P = 0.039). Patients with the CC genotype had lower adjusted plasma VPA concentrations than those with CT or TT genotype (P = 0.028).

CONCLUSIONS

These data suggest that the UGT2B7 -161C>T polymorphism in pediatric epilepsy patients carrying the CYP2C9*1/*1 genotype affects VPA concentration.

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.

CYP2C9 polymorphism in patients with epilepsy: genotypic frequency analyzes and phenytoin adverse reactions correlation

OBJECTIVE

CYP2C9 is a major enzyme in human drug metabolism and the polymorphism observed in the corresponding gene may affect therapeutic outcome during treatment. The distribution of variant CYP2C9 alleles and prevalence of phenytoin adverse reactions were hereby investigated in a population of patients diagnosed with epilepsy.

METHOD

Allele-specific PCR analysis was carried out in order to determine frequencies of the two most common variant alleles, CYP2C9*2 and CYP2C9*3 in genomic DNA isolated from 100 epileptic patients. We also analyzed the frequency of phenytoin adverse reactions among those different genotypes groups. The data was presented as mean±standard deviation.

RESULTS

The mean age at enrollment was 39.6±10.3 years (range, 17-72 years) and duration of epilepsy was 26.5±11.9 years (range 3-48 years). The mean age at epilepsy onset was 13.1±12.4 years (range, 1 month-62 years). Frequencies of CYP2C9*1 (84%), CYP2C9*2 (9%) and CYP2C9*3 (7%) were similar to other published reports. Phenytoin adverse reactions were usually mild and occurred in 15% patients, without correlation with the CYP2C9 polymorphism (p=0.34).

CONCLUSION

Our findings indicate an overall similar distribution of the CYP2C9 alleles in a population of patients diagnosed with epilepsy in the South of Brazil, compared to other samples. This sample of phenytoin users showed no drug related adverse reactions and CYP2C9 allele type correlation. The role of CYP2C9 polymorphism influence on phenytoin adverse reaction remains to be determined since some literature evidence and our data found negative results.

Prediction of relative in vivo metabolite exposure from in vitro data using two model drugs: dextromethorphan and omeprazole

Metabolites can have pharmacological or toxicological effects, inhibit metabolic enzymes, and be used as probes of drug-drug interactions or specific cytochrome P450 (P450) phenotypes. Thus, better understanding and prediction methods are needed to characterize metabolite exposures in vivo. This study aimed to test whether in vitro data could be used to predict and rationalize in vivo metabolite exposures using two model drugs and P450 probes: dextromethorphan and omeprazole with their primary metabolites dextrorphan, 5-hydroxyomeprazole (5OH-omeprazole), and omeprazole sulfone. Relative metabolite exposures were predicted using metabolite formation and elimination clearances. For dextrorphan, the formation clearances of dextrorphan glucuronide and 3-hydroxymorphinan from dextrorphan in human liver microsomes were used to predict metabolite (dextrorphan) clearance. For 5OH-omeprazole and omeprazole sulfone, the depletion rates of the metabolites in human hepatocytes were used to predict metabolite clearance. Dextrorphan/dextromethorphan in vivo metabolite/parent area under the plasma concentration versus time curve ratio (AUC(m)/AUC(p)) was overpredicted by 2.1-fold, whereas 5OH-omeprazole/omeprazole and omeprazole sulfone/omeprazole were predicted within 0.75- and 1.1-fold, respectively. The effect of inhibition or induction of the metabolite's formation and elimination on the AUC(m)/AUC(p) ratio was simulated. The simulations showed that unless metabolite clearance pathways are characterized, interpretation of the metabolic ratios is exceedingly difficult. This study shows that relative in vivo metabolite exposure can be predicted from in vitro data and characterization of secondary metabolism of probe metabolites is critical for interpretation of phenotypic data.

Drug-induced toxicity on mitochondria and lipid metabolism: mechanistic diversity and deleterious consequences for the liver

Numerous investigations have shown that mitochondrial dysfunction is a major mechanism of drug-induced liver injury, which involves the parent drug or a reactive metabolite generated through cytochromes P450. Depending of their nature and their severity, the mitochondrial alterations are able to induce mild to fulminant hepatic cytolysis and steatosis (lipid accumulation), which can have different clinical and pathological features. Microvesicular steatosis, a potentially severe liver lesion usually associated with liver failure and profound hypoglycemia, is due to a major inhibition of mitochondrial fatty acid oxidation (FAO). Macrovacuolar steatosis, a relatively benign liver lesion in the short term, can be induced not only by a moderate reduction of mitochondrial FAO but also by an increased hepatic de novo lipid synthesis and a decreased secretion of VLDL-associated triglycerides. Moreover, recent investigations suggest that some drugs could favor lipid deposition in the liver through primary alterations of white adipose tissue (WAT) homeostasis. If the treatment is not interrupted, steatosis can evolve toward steatohepatitis, which is characterized not only by lipid accumulation but also by necroinflammation and fibrosis. Although the mechanisms involved in this aggravation are not fully characterized, it appears that overproduction of reactive oxygen species by the damaged mitochondria could play a salient role. Numerous factors could favor drug-induced mitochondrial and metabolic toxicity, such as the structure of the parent molecule, genetic predispositions (in particular those involving mitochondrial enzymes), alcohol intoxication, hepatitis virus C infection, and obesity. In obese and diabetic patients, some drugs may induce acute liver injury more frequently while others may worsen the pre-existent steatosis (or steatohepatitis).

Rationalization and prediction of in vivo metabolite exposures: the role of metabolite kinetics, clearance predictions and in vitro parameters

IMPORTANCE OF THE FIELD

Due to growing concerns over toxic or active metabolites, significant efforts have been focused on qualitative identification of potential in vivo metabolites from in vitro data. However, limited tools are available to quantitatively predict their human exposures.

AREAS COVERED IN THIS REVIEW

Theory of clearance predictions and metabolite kinetics is reviewed together with supporting experimental data. In vitro and in vivo data of known circulating metabolites and their parent drugs were collected and the predictions of in vivo exposures of the metabolites were evaluated.

WHAT THE READER WILL GAIN

The theory and data reviewed will be useful in early identification of human metabolites that will circulate at significant levels in vivo and help in designing in vivo studies that focus on characterization of metabolites. It will also assist in rationalization of metabolite-to-parent ratios used as markers of specific enzyme activity.

TAKE HOME MESSAGE

The relative importance of a metabolite in comparison to the parent compound as well as other metabolites in vivo can only be predicted using the metabolite's in vitro formation and elimination clearances, and the in vivo disposition of a metabolite can only be rationalized when the elimination pathways of that metabolite are known.

Influence of CYP2C9 polymorphism on metabolism of valproate and its hepatotoxin metabolite in Iranian patients

Sodium valproate (VPA) has 16 known metabolites in humans. The 2-ene-VPA has anti-convulsant efficacy and 4-ene-VPA is reported to contribute in VPA hepatotoxicity. The formation of 4-ene-VPA is catalyzed by cytochrome P450 2C9 (CYP2C9). CYP2C9 allele mutation is closely related to the attenuation of the enzymatic activity and 4-ene-VPA production. In the present work, VPA, 2-ene-VPA, and 4-ene-VPA in serum of patients receiving VPA were determined and the correlation between CYP2C9 polymorphism and 4-ene-VPA concentration was examined. Blood samplings in 68 patients were performed at two time-points (peak and trough) and one sample blood obtained from 50 healthy volunteers for genotype evaluation. Patients were divided into three groups (22 cases of monotherapy, 19 cases of enzyme inducer therapy, and 27 cases of polytherapy). There was a significant reduction in concentration of VPA and 4-ene-VPA between peak and trough time. In peak concentration, there was a significant correlation between 2-ene-VPA and VPA in all groups. The concentration of 4-ene-VPA in the enzyme inducer and polytherapy group was significantly higher than that of the monotherapy group. The allele frequencies of CYP2C9*1, CYP2C9*2, and CYP2C9*3 were 88.97%, 8.09%, and 2.94% in the patient group and 91%, 6%, and 3% in the normal group, respectively. There was no significant difference in allele frequency in two groups. Mutated alleles didn't have any significant effect on 4-ene-VPA production. No patient showed toxic level of 4-ene-VPA or saturation of ß-oxidation pathway. In conclusion, the role of CYP2C9*2 and CYP2C9*3 in attenuation of 4-ene-VPA formation cannot be confirmed.