The role of active metabolites in drug toxicity

Can TK-TD modelling bridge the gap between in vitro and in vivo mammalian toxicity data?

Repeated dietary dose testing is used to assess longer term toxicity of chemicals, such as pesticides, to mammals. However, the internal pesticide concentration varies significantly as feeding rate relative to body size fluctuates over time. Toxicokinetic-toxicodynamic (TK-TD) models can estimate internal toxicant concentration over time and link this directly to observed effects on endpoints such as the growth rate of laboratory rats. Using TK-TD models it is therefore possible to predict the effects that would result from a constant internal concentration of a pesticide. This presents the possibility of comparison with data from in vitro experiments, potentially facilitating quantitative in vitro to in vivo extrapolation (QIVIVE). We used in vivo TK-TD models to identify relevant internal concentrations and then estimated the experimental conditions required to replicate these in cultured cells, using in vitro TK models. Cell population growth was measured, with a view to extrapolating through time and comparing effect sizes with in vivo predictions. However, observed cell proliferation was not significantly affected by the tested concentrations of any of the five pesticides in this study and so extrapolation was not possible. In light of this negative result, we highlight areas for future work towards QIVIVE of graded sublethal effects in mammals. The most pressing objective is improving the accuracy of in vivo TK predictions, which could be achieved with dietary dosing in TK studies.

Novel insights into molecular and cellular aspects of delayed drug hypersensitivity reactions

INTRODUCTION

Delayed drug hypersensitivity reactions (DDHRs) represent a major health problem. They are unpredictable and can cause life-long disability or even death. The pathophysiology of DDHRs is complicated, multifactorial, and not well understood mainly due to the lack of validated animal models or in vitro systems. The role of the immune system is well demonstrated but its exact pathophysiology still a matter of debate.

AREA COVERED

This review summarizes the current understanding of DDHRs pathophysiology and abridges the available new evidence supporting each hypothesis. A comprehensive literature search for relevant publications was performed using PubMed, Google Scholar, and Medline databases with no date restrictions and focusing on the most recent 10 years.

EXPERT OPINION

Although multiple milestones have been achieved in our understanding of DDHRs pathophysiology as a result of the development of useful experimental models, many questions are yet to be fully answered. A deeper understanding of the mechanistic basis of DDHRs would not only facilitate the development of robust and reliable diagnostic assays for diagnosis, but would also inform therapy by providing specific target(s) for immunomodulation and potentially permit pre-therapeutic risk assessment to pursue the common goal of safe and effective drug therapy.

A Text Mining Protocol for Extracting Drug-Drug Interaction and Adverse Drug Reactions Specific to Patient Population, Pharmacokinetics, Pharmacodynamics, and Disease

Drug-drug interactions (DDIs) and adverse drug reactions (ADR) are experienced by many patients, especially by elderly population due to their multiple comorbidities and polypharmacy. Databases such as PubMed contain hundreds of abstracts with DDI and ADR information. PubMed is being updated every day with thousands of abstracts. Therefore, manually retrieving the data and extracting the relevant information is tedious task. Hence, automated text mining approaches are required to retrieve DDI and ADR information from PubMed. Recently we developed a hybrid approach for predicting DDI and ADR information from PubMed. There are many other existing approaches for retrieving DDI and ADR information from PubMed. However, none of the approaches are meant for retrieving DDI and ADR specific to patient population, gender, pharmacokinetics, and pharmacodynamics. Here, we present a text mining protocol which is based on our recent work for retrieving DDI and ADR information specific to patient population, gender, pharmacokinetics, and pharmacodynamics from PubMed.

The importance of sample size with regard to the robustness of postmortem reference values

Evaluating postmortem toxicological results is a challenging task due to multiple factors affecting blood concentrations after death. In order to improve the diagnostic accuracy in cases of suspected fatal intoxication different compilations of postmortem reference drug concentrations are often used. However, it is not clear what constitutes a reliable postmortem reference value. The current study presents reference concentrations for 13 substances from seven substance groups according to a standardized protocol. The reference concentrations were gathered from 3767 autopsy cases and subdivided into intoxications by one substance only (Group A, n=611), multi-substance intoxications (Group B, n=1355) and postmortem controls, in which incapacitation by drugs were excluded (Group C, n=1801). In particular, this study presents statistical information about the precision and conformity change with various sample sizes. Based on the present data >10 detections are usually needed, for the substances examined, to differentiate between intoxication cases and controls. Repeated samplings show that the median of small samples (N=≤5) has a high variation (normalized interquartile range 138-75%) and that a high number of detections (N=>20) in each group are needed to reduce the variation.

T-Cell Activation by Low Molecular Weight Drugs and Factors That Influence Susceptibility to Drug Hypersensitivity

Drug hypersensitivity reactions adversely affect treatment outcome, increase the length of patients' hospitalization, and limit the prescription options available to physicians. In addition, late stage drug attrition and the withdrawal of licensed drugs cost the pharmaceutical industry billions of dollars. This significantly increases the overall cost of drug development and by extension the price of licensed drugs. Drug hypersensitivity reactions are characterized by a delayed onset, and reactions tend to be more serious upon re-exposure. The role of drug-specific T-cells in the pathogenesis of drug hypersensitivity reactions and definition of the nature of the binding interaction of drugs with HLA and T-cell receptors continues to be the focus of intensive research, primarily because susceptibility is associated with expression of one or a small number of HLA alleles. This review critically examines the mechanisms of T-cell activation by drugs. Specific examples of drugs that activate T-cells via the hapten, the pharmacological interaction with immune receptors and the altered self-peptide repertoire pathways, are discussed. Furthermore, the impacts of drug metabolism, drug-protein adduct formation, and immune regulation on the development of drug antigen-responsive T-cells are highlighted. The knowledge gained from understanding the pathways of T-cell activation and susceptibility factors for drug hypersensitivity will provide the building blocks for the development of predictive in vitro assays that will prevent or help to minimize the incidence of these reactions in clinic.

Description of an Innovative Pediatric Individualized Therapeutics Clinic: Working toward Precision Drug Therapy

The GOLDILOKs^® (Genomic and Ontogeny-Linked Dose Individualization and cLinical Optimization for KidS) Clinic aims to provide families and physicians with data to make more informed decisions with regard to pharmacological therapy by using innovative therapy and genomic technologies. The objectives are two-fold: (1) To describe the utility of the GOLDILOKs^® Clinic to referring prescribers by evaluating the type of referrals made to the GOLDILOKs^® Clinic and (2) to assess the most often utilized technologies (e.g., genotyping) completed to formulate therapy recommendations. Patient data from July 2010 to June 2016 was retrospectively reviewed following Institutional Review Board (IRB) approval. The GOLDILOKs^® Clinic evaluated 306 patients and had increases in annual referrals from 14 in 2010–2011 to 84 in 2016–2017. The children that were referred were predominately Caucasian (82%) and male (59%) with an average age of 12.4 ± 5.9 years. Subspecialty versus primary care referrals accounted for 82% and 18% of referrals, respectively. Adverse drug reactions (n = 166) and poor medication response (n = 179) were the major reasons for referral. However, it must be noted that patients could have multiple reasons for referral. Pharmacogenetic results were extensively used to provide guidance for future therapy in patients with medication-related problems. Genotyping of drug metabolizing enzymes and drug target receptors was performed in 221 patients (72.2%). Recommendations were fully accepted by 63% and partially accepted by 22% of internal provider referrals.

Acute Liver Failure/Injury Related to Drug Reaction With Eosinophilia and Systemic Symptoms: Outcomes and Prognostic Factors

BACKGROUND

Drug reaction with eosinophilia and systemic symptoms (DRESS) is a rare severe adverse drug-induced reaction with multiorgan involvement. The outcome and prediction of those patients who develop severe acute liver injury (sALI) or acute liver failure (ALF) remain little known.

METHODS

A multicenter retrospective study of patients admitted with a diagnosis of DRESS-related sALI or ALF. Histological review was performed on liver core biopsies from native livers.

RESULTS

Sixteen patients (11 women, 5 men; mean age, 39±17.2 years) were classified as having definite (n=13) or probable (n=3) DRESS. At admission, 3 patients had hepatic encephalopathy; median levels of prothrombin time, INR, and total bilirubin were, respectively, 33% (Q1-Q3, 21-41), 2.74 (1.98-4.50), and 94 μmol/L (Q1-Q3, 39.5-243.5). Nine patients received corticosteroid therapy. Overall, 9 patients improved spontaneously and 7 worsened (liver transplantation [LT] (n=5), deceased (n=2)). Transplantation-free and post-LT survival was 56% and 60%, respectively. After LT, DRESS recurrence was observed in 3 of 5 patients. Systemic corticosteroid therapy was not significantly associated with a clinical improvement. In the multivariate analysis, factor V level less than 40% at day 0 and factor V levels of 40% or greater at admission but decreasing at day 2 were associated with worse outcome. Pathological findings (n=7) revealed atypical lymphoid infiltrates, Kupffer cell hyperplasia with erythrophagocytosis, and an inconstant presence of eosinophils.

CONCLUSIONS

The spontaneous prognosis of patients with sALI/ALF due to DRESS is poor and was not improved by corticosteroid therapy. Histology is helpful to establish diagnosis. Dynamic variables regarding factor V values are predictive of a poor outcome.

Multi-functional scaling methodology for translational pharmacokinetic and pharmacodynamic applications using integrated microphysiological systems (MPS)

Microphysiological systems (MPS) provide relevant physiological environments in vitro for studies of pharmacokinetics, pharmacodynamics and biological mechanisms for translational research. Designing multi-MPS platforms is essential to study multi-organ systems. Typical design approaches, including direct and allometric scaling, scale each MPS individually and are based on relative sizes not function. This study's aim was to develop a new multi-functional scaling approach for integrated multi-MPS platform design for specific applications. We developed an optimization approach using mechanistic modeling and specification of an objective that considered multiple MPS functions, e.g., drug absorption and metabolism, simultaneously to identify system design parameters. This approach informed the design of two hypothetical multi-MPS platforms consisting of gut and liver (multi-MPS platform I) and gut, liver and kidney (multi-MPS platform II) to recapitulate in vivo drug exposures in vitro. This allows establishment of clinically relevant drug exposure-response relationships, a prerequisite for efficacy and toxicology assessment. Design parameters resulting from multi-functional scaling were compared to designs based on direct and allometric scaling. Human plasma time-concentration profiles of eight drugs were used to inform the designs, and profiles of an additional five drugs were calculated to test the designed platforms on an independent set. Multi-functional scaling yielded exposure times in good agreement with in vivo data, while direct and allometric scaling approaches resulted in short exposure durations. Multi-functional scaling allows appropriate scaling from in vivo to in vitro of multi-MPS platforms, and in the cases studied provides designs that better mimic in vivo exposures than standard MPS scaling methods.

In vitro testing for diagnosis of idiosyncratic adverse drug reactions: Implications for pathophysiology

Idiosyncratic drug reactions (IDRs) represent a major health problem, as they are unpredictable, often severe and can be life threatening. The low incidence of IDRs makes their detection during drug development stages very difficult causing many post-marketing drug withdrawals and black box warnings. The fact that IDRs are always not predictable based on the drug's known pharmacology and have no clear dose-effect relationship with the culprit drug renders diagnosis of IDRs very challenging, if not impossible, without the aid of a reliable diagnostic test. The drug provocation test (DPT) is considered the gold standard for diagnosis of IDRs but it is not always safe to perform on patients. In vitro tests have the advantage of bearing no potential harm to patients. However, available in vitro tests are not commonly used clinically because of lack of validation and their complex and expensive procedures. This review discusses the current role of in vitro diagnostic testing for diagnosis of IDRs and gives a brief account of their technical and mechanistic aspects. Advantages, disadvantages and major challenges that prevent these tests from becoming mainstream diagnostic tools are also discussed here.

Liver injury in patients with DRESS: A clinical study of 72 cases

BACKGROUND

Drug reaction with eosinophilia and systemic symptoms (DRESS) is a syndrome involving multiple systems. Liver injury is the most common visceral manifestation.

OBJECTIVE

The purpose of this study was to investigate the types of liver injury and factors associated with DRESS.

METHODS

A retrospective cohort study was conducted in Taiwan using a DRESS database compiled from December 2000 to March 2013.

RESULTS

Seventy-two cases were included in this study. Among them, 62 (86.1%) cases involved liver injury, 6 of which (9.7%) were liver injury before skin presentation. The distribution of liver injury patterns at initial presentation was 23 cholestatic type (37.1%), 17 mixed type (27.4%), and 12 hepatocellular type (19.4%). Patients with hepatocellular-type injuries were younger, with a median age of 31.5 (P = .044). Individuals with liver function results more than 10 times the upper limit were more likely to have fever (P = .026), took more time to recover, and had fewer eosinophils in the dermis (P = .002).

LIMITATIONS

The study was a retrospective cohort study with limited cases.

CONCLUSIONS

Liver injury is common in DRESS and frequently associated with atypical lymphocytosis. The cholestatic type is the most common type. Patients with cholestatic-type injuries were older and more frequently had interface changes in skin pathology.

Factors affecting the development of adverse drug reactions (Review article)

OBJECTIVES

To discuss the effect of certain factors on the occurrence of Adverse Drug Reactions (ADRs).

DATA SOURCES

A systematic review of the literature in the period between 1991 and 2012 was made based on PubMed, the Cochrane database of systematic reviews, EMBASE and IDIS. Key words used were: medication error, adverse drug reaction, iatrogenic disease factors, ambulatory care, primary health care, side effects and treatment hazards.

SUMMARY

Many factors play a crucial role in the occurrence of ADRs, some of these are patient related, drug related or socially related factors. Age for instance has a very critical impact on the occurrence of ADRs, both very young and very old patients are more vulnerable to these reactions than other age groups. Alcohol intake also has a crucial impact on ADRs. Other factors are gender, race, pregnancy, breast feeding, kidney problems, liver function, drug dose and frequency and many other factors. The effect of these factors on ADRs is well documented in the medical literature. Taking these factors into consideration during medical evaluation enables medical practitioners to choose the best drug regimen.

CONCLUSION

Many factors affect the occurrence of ADRs. Some of these factors can be changed like smoking or alcohol intake others cannot be changed like age, presence of other diseases or genetic factors. Understanding the different effects of these factors on ADRs enables healthcare professionals to choose the most appropriate medication for that particular patient. It also helps the healthcare professionals to give the best advice to patients. Pharmacogenomics is the most recent science which emphasizes the genetic predisposition of ADRs. This innovative science provides a new perspective in dealing with the decision making process of drug selection.

Glutathione depletion in a liver microsomal assay as an in vitro biomarker for reactive metabolite formation

Glutathione (GSH) plays a major role in cytoprotection, acting as a nucleophile trap for reactive species derived from xenobiotics. This has led to the development of an assay for the detection of reactive species generated by liver microsomal metabolism of xenobiotics. This assay has been used extensively to study reactive metabolites which initiate toxicity through a direct (non-immunological) mechanism, but there are few data on its ability to detect reactive metabolites that initiate toxicity through neo-antigen formation, or to detect xenobiotics that cause GSH loss by oxidation mediated by a redox cycling process. Accordingly, the ability of rat and human liver microsomes to metabolize xenobiotics to GSH-depleting metabolites has been investigated further. Of the five neo-antigen-forming xenobiotics tested, four (amodiaquine, phenobarbitone, procainamide, and sulphanilamide) displayed GSH reactivity that was either dependent or independent (amodiaquine) on metabolism. The other neo-antigen-forming xenobiotic (carbamazepine) was inactive in all microsomal samples tested. Four quinones believed to exert toxcity through arylation (1,4-benzoquinone) and/or redox cycling (duroquinone, menadione, mitomycin c) displayed GSH reactivity, as did nitrofurantoin and diquat, two other redox cycling xenobiotics. Induction of the mixed function oxidase system with Aroclor afforded little advantage when using rat liver microsomes, whilst there was considerable inter-individual variation in the ability of human liver microsomes to mediate metabolism-dependent GSH depletion. It is concluded that the liver microsome GSH depletion assay may be of general utility as a screen for a number of xenobiotic-derived reactive species.

EIDOS: a mechanistic classification of adverse drug effects

The mechanisms of adverse drug effects have not been adequately classified. Here, we propose a comprehensive mechanistic classification of adverse drug effects that considers five elements: the Extrinsic chemical species (E) that initiates the effect; the Intrinsic chemical species (I) that it affects; the Distribution (D) of these species in the body; the (physiological or pathological) Outcome (O); and the Sequela (S), which is the adverse effect. This classification, which we have called EIDOS, describes the mechanism by which an adverse effect occurs; it complements the DoTS classification of adverse effects (based on clinical pharmacology), which takes into account Dose responsiveness, Time course, and Susceptibility factors. Together, these two classification systems, mechanistic and clinical, comprehensively delineate all the important aspects of adverse drug reactions; they should contribute to areas such as drug development and regulation, pharmacovigilance, monitoring therapy, and the prevention, diagnosis, and treatment of adverse drug effects.

Monitoring gene changes during antiepileptic drug therapy to widen the safety window and reduce pharmacoresistance

A novel and potentially effective way to decrease pharmacoresistance is to widen an antiepileptic drug's (AED) safety window by altering its efficacy-toxicity relationship to reduce the frequency or eliminate the occurrence of drug-induced toxicities. The first step in this process is to identify specific AED toxicities that are common, objective, reliable to assess, and exhibit both interpatient and interdrug variability. Next, one or more of the fundamental sources of biological variability (e.g., DNA, RNA, proteins, or metabolites) is selected for study. By identifying a relationship between variations in these fundamental sources and specific AED toxicities, one aims to identify pathways underlying the genesis of the toxicity (mechanisms) or biomarkers related to elevated risk for AED toxicity (predictors). These types of studies have many methodological challenges. However, if properly conducted, the resulting data could be used to design interventions that directly block the toxicity mechanisms or preselect individuals who are at high risk of developing these toxicities. Both approaches would conceivably widen the safety window of efficacious AEDs and reduce the incidence of pharmacoresistance. This article describes a general framework for this approach and provides an example of the methodology using differential gene expression measured using RNA-based microarray technology in children experiencing VPA-associated weight gain.

Identification of metabolites of the tryptase inhibitor CRA-9249: Observation of a metabolite derived from an unexpected hydroxylation pathway

The metabolites of the tryptase inhibitor CRA-9249 were identified after exposure to liver microsomes. CRA-9249 was found to be degraded rapidly in liver microsomes from rabbit, dog, cynomolgus monkey, and human, and less rapidly in microsomes from rat. The key metabolites included cleavage of an aryl ether, in addition to an unexpected hydroxylation of the amide side chain adjacent to the amide nitrogen. The chemical structures of both metabolites were confirmed by synthesis and comparison to material isolated from the liver microsomes. Several suspected hydroxylated metabolites were also synthesized and analyzed as part of the structure identification process.

The development of a higher throughput reactive intermediate screening assay incorporating micro-bore liquid chromatography–micro-electrospray ionization–tandem mass spectrometry and glutathione ethyl ester as an in vitro conjugating agent

An in vitro reactive intermediate screening assay, incorporating the use of the close analog of glutathione, glutathione ethyl ester (GSH-EE) as a conjugating agent, was developed to identify compounds that form reactive intermediates in an in vitro metabolite generating system. The biological assay consisted of substrate [s] = 10 microM, human liver microsomes, an NADPH generating system and glutathione ethyl ester. Conjugates were extracted from the biological matrix using a combination of protein precipitation and a semi-automated 96-well plate solid phase extraction (SPE) procedure. A micro-bore liquid chromatography-micro-electrospray ionization-tandem mass spectrometry (microLC-microESI-MS/MS) method detected glutathione ethyl ester conjugates using selected reaction monitoring (SRM) to simultaneously monitor for multiple MH+ to [MH - 129]+ transitions, where the 129 mass unit (Da) represents the neutral loss of the pyroglutamate moiety from GSH-EE. The multiple MH+ to [MH - 129]+ transitions (SRM mass table) were generated for potential reactive intermediates of each compound. Glutathione (GSH) and GSH-EE conjugate standards were used to evaluate MS detection sensitivity. Based on direct comparison of standard curve data, an approximate 10-fold increase in sensitivity was observed for conjugates containing GSH-EE moiety versus GSH. In vitro experiments were conducted using literature substrates acetaminophen, rosiglitazone, clozapine, diclofenac and either GSH-EE or GSH as a reactive intermediate conjugating agent. An increase in detection sensitivity was observed for each GSH-EE conjugate and in the case of acetaminophen-GSH-EE the peak area increase was approximately 80-fold. Twelve drug compounds, each having known biotransformation mechanisms, were used to further test the detection capabilities of the assay and establish a concordance to literature data. When GSH was used in the assay, conjugates were detected for 4 out of the 12 test compounds (33%). When GSH-EE was used in the assay, conjugates were detected for 10 out of the 12 test compounds (83%).

Immune mechanism of drug hypersensitivity

Drug hypersensitivity reactions can lead to a great variety of different diseases. The main cause is a specific interaction of antibodies or T cells with a drug. In addition to the hapten concept, some drugs can bind directly to T-cell receptors and stimulate them. Based on recent investigation on different exanthemas, an extended classification of the Gell and Coombs type IV reaction is proposed.

DRESS syndrome associated with HHV-6 reactivation

Drug reaction with eosinophilia and systemic symptoms (DRESS) syndrome is a serious condition associated with drugs. We report the case of a patient with a febrile skin eruption associated with eosinophilia and hepatitis following drug intake. Serological testing for human herpes virus 6 (HHV-6) was positive. Skin biopsy was compatible with DRESS syndrome. Despite discontinuation of all medications and initiating of corticosteroids, the patient developed acute renal and cardiac failure leading to death. Diagnosis, pathophysiology, and treatment are discussed.

Cross-reactivity with drugs at the T cell level

PURPOSE OF REVIEW

Cross-reactivity with drugs is an important clinical problem in drug hypersensitivity. Once a patient is labeled 'drug-allergic' all drugs of the same class are withheld and future therapeutic interventions are limited. Here we review cross-reactivity with drugs at the T cell level.

RECENT FINDINGS

Analysis of T cell recognition of various classes of drugs (beta-lactam antibiotics, sulfonamides, local anesthetics) using T cell clones suggests that at the T cell level the whole structure, in particular the core and to a lesser degree side chains, are recognized.

SUMMARY

It is necessary to differentiate cross-reactivity mediated by T cells and antibodies as only the latter seem to recognize side chains exclusively.

Antibiotic safety assessment

Antibiotics usually have positive risk-benefit ratios, their adverse effects being generally mild and reversible on treatment cessation. However, severe adverse drug reactions (ADR), associated with significant mortality and morbidity have resulted in the withdrawal of several active antibiotics, including new fluoroquinolones. Adverse reactions to antibiotics are often poorly documented. The purpose of this article is to examine current tools for investigating and preventing antibiotic toxicity and to suggest future lines of investigation. Structure/ADR relationships have been investigated with various antibiotics (beta-lactams, macrolides, quinolones, etc.) in an attempt to reduce the risk of adverse reactions. Some reactions can be linked to the drug's stereochemical composition. In the case of quinolones for instance, particularly ofloxacin and its derivatives, experimental data show that individual enantiomers have different toxicities. Another major factor that influences the risk of ADRs in a given population is metabolic variability, due to genetic differences in the relevant drug-metabolizing enzymes. Idiosyncratic antibiotic toxicity can be caused by a chemically reactive metabolite. Recent advances in molecular biology, and especially in individual genomic characterization (DNA chip technology, etc.), could in future be useful for identifying patients who are at a special risk of ADR. Finally, certain pharmacokinetic parameters (AUC, Cmax, etc.) can be used to predict adverse effects.

Pharmacological interaction of drugs with antigen-specific immune receptors: the p-i concept

PURPOSE OF REVIEW

Drug allergies are examples of immune reactions to small molecular compounds. In many drug allergies drug specific CD4+ and CD8+ T-cells can be detected, which recognize small chemicals via their alphabeta-T-cell receptor in a major histocompatibility complex dependent way. In this review a new concept of drug presentation to T-cells is presented.

RECENT FINDINGS

Drugs were stimulatory for T-cells if they bound covalently to peptides or proteins, but also if the drug had structural features allowing it to bind in a labile way (noncovalently) to the major histocompatibility peptide complex. This latter binding method has some similarities to superantigen stimulations and can explain allergies to drugs that are not metabolized. It has been described in patients with maculopapular, bullous and neutrophilic drug eruption, as well as in contact dermatitis.

SUMMARY

Noncovalent drug presentation leads to the stimulation of immune cells, namely T-cells. The drug needs two surface molecules (one inert serving as a scaffold, major histocompatibility complex, and one reactive, T-cell receptor) to exert its function. Although two receptor structures are involved, the process is reminiscent of a pharmacological interaction between a drug and its receptors and, from the phrase pharmacological interaction with immune receptors, was thus termed the p-i concept.

Advancing the medical management of epilepsy: disease modification and pharmacogenetics

Despite the recent development of new antiepilepsy drugs, a significant number of children are still unable to achieve seizure freedom without side effects. Understanding the factors behind individual variability in antiepilepsy drug tolerability and dose response and incorporating these factors into a treatment plan would represent an important advance in epilepsy pharmacotherapy. A more thorough understanding of the epileptogenic process may allow clinicians to select antiepilepsy drugs that interrupt or modify various steps in the epileptogenic progression (ie, disease modification). Additionally, advances in the understanding of human genetics may allow for selection of antiepilepsy drugs and dosage regimens based on a patient's clinical characteristics and genotype (ie, pharmacogenetics). This article focuses on these two areas of potential improvement in the medical treatment of patients with epilepsy. Such methods of tailoring antiepilepsy drug therapy would be preferable to the trial-and-error system that is currently used.

Cytochrome P-450 mRNA expression in human liver and its relationship with enzyme activity

CYP activity and protein contents have been measured in human liver using different techniques. In contrast, CYP mRNA data are scarce and the relationships between CYP mRNA contents and activities have not been established. These studies deserve further attention because mRNA determinations by RT-PCR require a very small amount of material (e.g., liver needle biopsy) and could provide important data regarding CYP expression regulation. In this study we measured in 12 human liver samples the mRNA contents of 10 CYPs by quantitative RT-PCR and the metabolic activities using specific substrates. mRNA contents and activities showed high correlation coefficients for CYP1A1, CYP1A2, CYP3A4, CYP2D6, and CYP2B6 (0.96, 0.94, 0.69, 0.61, and 0.52, respectively), but no significant correlations were found for CYP2C9, CYP2A6, and CYP2E1. The results suggest that the regulation of CYP1A1, CYP1A2, CYP3A4, CYP2D6, and CYP2B6 expression is essentially pretranslational and that their mRNA levels could allow a good estimate of their activity.

Idiosyncratic reactions: new methods of identifying high-risk patients

This article describes the mechanisms of idiosyncratic drug reactions (IDRs) and provides an analysis of potential methods for identifying patients at high risk for antiepileptic idiosyncratic drug reactions. IDRs may be caused by toxic metabolites, either directly or indirectly (by way of an immunologic response or a free radical-mediated process). Four methods to potentially identify patients at high risk for AED IDRs are discussed: development of an "at-risk" clinical profile for a particular AED: identification of biomarkers that measure the formation of a toxic metabolite by a previously unrecognized bioactivation pathway for a particular AED; identification of biomarkers indicating deficient detoxification abilities [e.g., deficient free radical scavenging enzyme activities or low calculated oxidative protection (COP) ratios 1 and 2]; and identification of at-risk genetic markers. Clinical profiles for patients receiving valproic acid (VPA), felbamate (FBM), and lamotrigine (LTG) and who are at risk for development of AED IDRs are presented. Patients with VPA IDRs have deficient erythrocyte glutathione peroxidase activity, low plasma selenium concentrations, low COP1 ratios, and low COP2 ratios compared with age-matched controls. Patients with FBM-associated aplastic anemia have deficient erythrocyte glutathione peroxidase, superoxide dismutase (SOD), and glutathione reductase activities compared with age-matched controls. Use of at-risk clinical profiles (for VPA, FBM, and LTG) and measurement of erythrocyte glutathione peroxidase activity, erythrocyte SOD activity, and calculation of COP1 and COP2 ratios (for VPA and FBM) are inexpensive, simple methods of identifying high-risk patients for IDRs. Research is needed to further characterize the mechanism of IDRs, to investigate the clinical utility of free radical-scavenging enzyme activity measurement and calculation of COP ratios for other AED IDRs, and to develop additional methods of identifying patients at high risk for AED IDRs.

Methapyrilene hepatotoxicity is associated with increased hepatic glutathione, the formation of glucuronide conjugates, and enterohepatic recirculation

The mechanisms by which acute administration of methapyrilene, an H(1)-receptor antihistamine causes periportal necrosis to rats are unknown. This study investigated the role of the hepato-biliary system in methapyrilene hepatotoxicity following daily administration of 150 mg/kg per day over 3 consecutive days. Biliary metabolites of methapyrilene were tentatively identified. In male Han Wistar rats administration of methapyrilene significantly increased hepatic reduced glutathione (GSH) to 140% of control levels 24 h following the last dose. There were no significant changes in the activities of glutathione-related enzymes, glutathione peroxidase (GPx) and reductase (GSH), glutathione S-transferase (GST), and gamma-glutamyl cysteine synthetase (gamma-GCS) over 3 days of methapyrilene administration. Methapyrilene treatment resulted in no significant increase in excretion of biliary oxidized glutathione (GSSG), a sensitive marker of oxidative stress in vivo, following the third dose. [3H]Methapyrilene-derived radioactivity was detected in bile, to a greater extent than in feces, indicating that methapyrilene and/or metabolites underwent enterohepatic recirculation. Cannulation and exteriorization of the bile duct (to interrupt enterohepatic recirculation) afforded some protection against the hepatotoxicity, assessed by clinical chemistry and histopathology. Liquid chromatography-mass spectrometry (LC-MS) analysis of bile indicated the presence of unmetabolized methapyrilene, methapyrilene O-glucuronide and desmethyl methapyrilene O-glucuronide. These data demonstrate that acute methapyrilene hepatotoxicity in vivo is not a consequence of GSH depletion, or oxidative stress, but that enterohepatic recirculation of biliary metabolites may be important. Progressive exposure to non-oxidizing, reactive metabolic intermediates may be responsible for hepatotoxicity.

Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Adverse drug reactions account for between 2 to 5% of all hospital admissions and can prevent the administration of an otherwise effective therapeutic agent. Hypersensitivity or immune-mediated reactions, although less common, tend to be proportionately more serious. There is convincing evidence to implicate the immune system in the pathogenesis of hypersensitivity reactions. Our understanding of the way in which the immune system recognises drugs is based on the hapten hypothesis; the onset of hypersensitivity involves drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and T cell proliferation. Central to this hypothesis is the critical role of drug metabolism, with the balance between metabolic bioactivation and detoxification being one important component of individual susceptibility. The purpose of this review is to classify drug hypersensitivity reactions in terms of their clinical presentation, and also to consider recent advances in our understanding of the chemical, biochemical and, in particular, cellular immunological mechanisms of hypersensitivity. The following topics are reviewed: (i) drug disposition and cellular metabolism; (ii) mechanisms of antigen processing and presentation; (iii) the role of cytokines and co-stimulatory molecules in the induction and maintenance of a polarised immune response; and (iv) the application of the hapten hypothesis, danger hypothesis and serial triggering model to drug hypersensitivity. A greater understanding of the mechanism(s) of hypersensitivity may identify novel therapeutic strategies and help to combat one of the more severe forms of adverse reactions to drugs.

Association analysis of drug metabolizing enzyme gene polymorphisms in HIV-positive patients with co-trimoxazole hypersensitivity

The use of co-trimoxazole in HIV-positive patients has been associated with a high frequency (40-80%) of hypersensitivity reactions. This has been attributed to the bioactivation of the sulphonamide component, sulphamethoxazole (SMX), to its toxic hydroxylamine and nitroso metabolites. The aim of this study was to determine whether functionally significant polymorphisms in the genes coding for enzymes involved in SMX metabolism influence susceptibility to SMX hypersensitivity. HIV-positive patients with (n = 56) and without (n = 89) SMX hypersensitivity were genotyped for allelic variants in CYP2C9, GSTM1, GSTT1, GSTP1 and NAT2 using polymerase chain reaction (PCR) and/or PCR-restriction fragment length polymorphism analysis. The CYP2C9*2/*3 genotype and CYP2C9*3 allele frequencies were nine- and 2.5-fold higher in the hypersensitive group compared to non-sensitive patients, respectively, although they were not statistically significant when corrected for multiple testing. There were no differences in the frequencies of the GSTM1 and GSTT1 null genotypes, and the slow acetylator genotype, between hypersensitive and non-sensitive patients, while GSTP1 frequency was lower (although non-significant) in the hypersensitive group [21% versus 32%, odds ratio (OR) = 0.5, Pc = 0.24]. Comparison of the genotype frequencies in HIV-positive and -negative patients showed that the NAT2 slow acetylator genotype frequency in the HIV-positive patients (74%) was significantly (Pc = 0.0003, OR = 2.3) higher than in control subjects (56%). Our results show that genetic polymorphisms in drug metabolizing enzymes are unlikely to be major predisposing factors in determining individual susceptibility to co-trimoxazole hypersensitivity in HIV-positive patients.

Quantitative RT-PCR measurement of human cytochrome P-450s: application to drug induction studies

A quantitative RT-PCR assay has been developed that is able to measure the mRNA content of the major human CYPs (1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, and 3A5). The technique is highly specific, reproducible, rapid, and sensitive enough to quantitate low and high abundant mRNAs. The PCR primers were selected to specifically match each CYP mRNA, to have a very close annealing temperature, and to render PCR products of similar sizes. The PCR conditions were designed to allow the simultaneous measurement of the various human liver CYPs in a single run. To achieve precise and reproducible quantitation of each cytochrome mRNA, a external standard (luciferase mRNA) is added to the probes to monitor the efficiency of the RT step. The degree of amplification is estimated using appropriate cDNA standards and quantitation of the amplified products by fluorescent measurement. This assay can be used to quantify the most relevant CYPs in human liver and cultured human hepatocytes. CYPs 3A4 and 2E1 were the most abundant mRNAs in human liver (2.5 and 1.7 x 10(8) molecules/microgram of total RNA respectively), whereas 1A1 and 2D6 were the least abundant isoforms (1.2 and 2.1 x 10(6) molecules/microgram of total RNA). A similar pattern was also found in short-term cultured human hepatocytes. This technique is also suitable for assessing CYP mRNA induction by xenobiotics. Cells exposed to 3-methylcholanthrene showed a characteristic increased expression of CYP1A2 and 1A1 mRNAs. Upon incubation with phenobarbital and rifampin (rifampicin), human hepatocytes increased CYP 2B6, 3A4, and 3A5 among others.

Pharmacokinetics of haloperidol: an update

Haloperidol is commonly used in the therapy of patients with acute and chronic schizophrenia. The enzymes involved in the biotransformation of haloperidol include cytochrome P450 (CYP), carbonyl reductase and uridine diphosphoglucose glucuronosyltransferase. The greatest proportion of the intrinsic hepatic clearance of haloperidol is by glucuronidation, followed by the reduction of haloperidol to reduced haloperidol and by CYP-mediated oxidation. In studies of CYP-mediated disposition in vitro, CYP3A4 appears to be the major isoform responsible for the metabolism of haloperidol in humans. The intrinsic clearances of the back-oxidation of reduced haloperidol to the parent compound, oxidative N-dealkylation and pyridinium formation are of the same order of magnitude, suggesting that the same enzyme system is responsible for the 3 reactions. Large variation in the catalytic activity was observed in the CYP-mediated reactions, whereas there appeared to be only small variations in the glucuronidation and carbonyl reduction pathways. Haloperidol is a substrate of CYP3A4 and an inhibitor, as well as a stimulator, of CYP2D6. Reduced haloperidol is also a substrate of CYP3A4 and inhibitor of CYP2D6. Pharmacokinetic interactions occur between haloperidol and various drugs given concomitantly, for example, carbamazepine, phenytoin, phenobarbital, fluoxetine, fluvoxamine, nefazodone, venlafaxine, buspirone, alprazolam, rifampicin (rifampin), quinidine and carteolol. Overall, drug interaction studies have suggested that CYP3A4 is involved in the biotransformation of haloperidol in humans. Interactions of haloperidol with most drugs lead to only small changes in plasma haloperidol concentrations, suggesting that the interactions have little clinical significance. On the other hand, the coadministration of carbamazepine, phenytoin, phenobarbital, rifampicin or quinidine affects the pharmacokinetics of haloperidol to an extent that alterations in clinical consequences would be expected. In vivo pharmacogenetic studies have indicated that the metabolism and disposition of haloperidol may be regulated by genetically determined polymorphic CYP2D6 activity. However, these findings appear to contradict those from studies in vitro with human liver microsomes and from studies of drug interactions in vivo. Interethnic and pharmacogenetic differences in haloperidol metabolism may explain these observations.

The role of reactive drug metabolites in immune-mediated adverse drug reactions

OBJECTIVE

To highlight recent advances in the understanding of adverse drug reactions (ADRs), with a focus on models outlining interactions between drug metabolism, disease processes, and immunity. Specific mechanisms that identify the metabolic pathways responsible for drug bioactivation to reactive drug metabolites (RDMs) involved in the initiation and propagation of specific immune-mediated hypersensitivity reactions are discussed. Drug classes well known to be associated with immune-mediated ADRs are reviewed and the clinical implications of current research are discussed.

DATA SOURCES

Original experimental research and immunologic review articles relevant to ADR diagnosis and etiology.

DATA EXTRACTION

Results of relevant in vitro experiments and clinical reactions to drug therapy were compiled and reviewed. Critical discoveries concerning the identification of RDMs involved in ADRs were highlighted, with respect to RDM involvement in the production of an immune response to drug haptens.

DATA SYNTHESIS

Drug adverse effects are classified according to clinical characteristics, immune interactions, and mechanistic similarities. Cytochrome P450 bioactivation of drug molecules to RDMs is a prerequisite to many ADRs. An electrophilic metabolite may react with cellular macromolecules (i.e., lipids, proteins, nucleic acids), resulting in direct cellular damage and organ toxicity. Covalent binding of an RDM to cellular macromolecules may also result in the formation of a hapten that is capable of eliciting a cellular or humoral immune response against drug or protein epitopes, culminating in the characteristic symptoms of hypersensitivity reactions. Mechanistic details concerning the identification of stable protein-metabolite conjugates and their interaction with the immune system remain unclear. Genetic imbalance between bioactivation and detoxification pathways, as well as reduced cellular defense against RDMs due to disease or concomitant drug therapy, act as risk factors to the onset and severity of ADRs.

CONCLUSIONS

Adverse reactions to drug therapy cause significant morbidity and mortality. Identification of the pathways involved in drug bioactivation and detoxification may elucidate the potential of chemical agents to induce immune-mediated ADRs. Understanding the mechanisms of ADRs to current xenobiotics is helpful in the prevention and management of ADRs, and may prove useful in the design of novel therapeutic agents with reduced incidence of severe adverse events.

Mechanism of clozapine-induced agranulocytosis : current status of research and implications for drug development

Clozapine is an atypical antipsychotic agent that has several advantages over conventional antipsychotics, not least of which is its superior efficacy. However, the high risk of agranulocytosis (0.8% of patients) associated with clozapine therapy has resulted in restricted indications for its use.The mechanism of clozapine-induced agranulocytosis is not clear. The target cells affected are the myeloid precursors, although the mature neutrophil may also be targeted simultaneously. There is no convincing evidence of direct toxicity of the parent compound or its stable metabolites (demethyl-clozapine and clozapine N-oxide). Clozapine is also metabolised by liver microsomes, peripheral blood neutrophils and their bone marrow precursors to a chemically reactive intermediate that has been postulated to be a nitrenium ion. This toxic metabolite has been shown to covalently bind to neutrophil proteins, suggesting that it may be involved in the pathogenesis of the toxicity. However, it is not clear how toxicity is mediated. The nitrenium ion may bind to essential cellular proteins and disrupt neutrophil function or, alternatively, it may act as a hapten and initiate an immune reaction resulting in immune-mediated destruction of the neutrophil. Indirect evidence exists to support both mechanisms, although clear direct evidence is still lacking. The role of cytokines and apoptosis in the pathogenesis of the agranulocytosis is unclear.The reason why only approximately 1% of individuals who are treated with clozapine are affected by agranulocytosis has not been elucidated. Evidence exists to implicate both the major histocompatibility complex antigens and heat shock protein variants in determining individual susceptibility, although more patients of different ethnic backgrounds need to be studied.The ultimate aim of research into clozapine-induced agranulocytosis should be to either prospectively predict which individuals are going to develop agranulocytosis and/or to develop analogues that retain efficacy but are not toxic. The former is complicated by the fact that predisposition may be multifactorial, and thus prediction may require multiple tests that may be of statistical but not absolute validity. The latter depends on identifying the mechanism of toxicity and the chemical characteristics of clozapine that are responsible for the toxicity. This knowledge may allow rational design of new analogues that do not cause agranulocytosis.

Mechanisms of drug reactions: the metabolic track

Hypersensitivity syndrome (HSR) describes a drug-induced symptom complex consisting of fever, rash, and internal organ involvement. Although these reactions are rare, they are very important because of their severity and unpredictability. The metabolic conversion of drugs to chemically-reactive products is now established as a prerequisite for many idiosyncratic drug reactions. In the setting of HSR, an imbalance in the rates of formation of reactive metabolites and of enzymatic detoxification can lead to accumulation of these byproducts. Reactive metabolites could act as haptens eliciting an immune response, covalently bind target proteins causing cell death, or interact with nucleic acids leading to mutations. The lymphocyte toxicity assay (LTA) provides an in vitro assessment of host susceptibility to reactive metabolites of a given drug. It has validated the clinical finding of increased risk of HSR in first degree relatives of patients. It is hoped that the LTA will be used to predict host susceptibility before drug exposure. Ultimately it is hoped that the genetic defects that lead to drug reactions will be identified. This would improve drug development safety and allow primary prevention of serious reactions.

Overview of enzymes of drug metabolism

Most pharmacologically active molecules are lipophilic and remain un-ionized or only partially ionized at physiological pH. Biotransformation means that a lipid-soluble xenobiotic or endobiotic compound is enzymatically transformed into polar, water-soluble, and excretable metabolites. The major organ for drug biotransformation is the liver. The metabolic products often are less active than the parent drug or inactive. However, some biotransformation products (metabolites) may have enhanced activity or toxic effects. Thus biotransformation may include both "detoxication" and "toxication" processes. One of the major enzyme systems that determines the organism's capability of dealing with drugs and chemicals is represented by the cytochrome P450 monooxygenases. Studies in the last 15 years have provided evidence that cytochrome P450 occurs in many different forms or "isozymes" which differ in spectral, chemical, and immunological properties and have different substrate affinities. These isozymes also differ in their regulation and tissue distribution. Recombinant DNA studies indicate that between 40 and 60 structural genes code for different cytochrome P450 isozymes in a single organism. Other enzyme systems include dehydrogenases, oxidases, esterases, reductases, and a number of conjugating enzyme systems including glucuronosyltransferases, sulfotransferases, glutathione S-transferases, etc. Environmental and genetic factors cause interindividual and intraindividual differences in drug metabolism and may alter the balance between toxification and detoxification reactions. Genetic polymorphisms lead to subpopulations of patients with decreased, absent, or even increased activities of certain reactions (e.g., CYP2D6, CYP2C19, N-acetyltransferase polymorphism). Environmental factors such as other drugs, steroids, dietary factors, alcohol, and cigarette smoke can induce or inhibit drug-metabolizing enzymes and cause intraindividual variation.

Kinetic parameters of lymphocyte microsomal epoxide hydrolase in carbamazepine hypersensitive patients. Assessment by radiometric HPLC

Idiosyncratic hypersensitivity reactions with carbamazepine have been postulated to be due to a deficiency of microsomal epoxide hydrolase (HYL1), although this is based on indirect evidence. Using 3H-cis stilbene oxide (0.5 Ci/mmol) as a substrate, we have developed a radiometric HPLC assay sensitive enough to measure the kinetic parameters of HYL1 in lymphocytes. The intra-assay coefficient of variation was 8%. Enzyme activity has been measured in lymphocytes from six carbamazepine hypersensitive patients, six patients on carbamazepine without any adverse effects, and twelve drug-naive healthy volunteers. No significant difference was observed in three kinetic parameters of the enzyme among these three groups. The values for Km, Vmax, and intrinsic clearance ranged from 6.1-89.9 microM, 3.0-23.2 pmoles diol formed/min/mg protein, and 0.147-0.493 microliter/min/mg protein. There was no difference in enzyme activity between patients currently on carbamazepine and healthy volunteers, indicating a lack of induction of lymphocyte HYL1 by carbamazepine. Co-incubation of lymphocytes with 1,1,1-trichloropropene oxide, an inhibitor of hepatic HYL1, resulted in an 82% inhibition of activity, similar to that observed with the hepatic enzyme. The healthy volunteers were genotyped as being either GSTM1 positive (n = 6) or GSTM1 negative (n = 6). This did not affect the kinetic parameters of lymphocyte microsomal epoxide hydrolase. Our results suggest that there is normal HYL1 activity in lymphocytes of hypersensitive patients using cis-stilbene oxide as a substrate.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.