Drug-induced liver injury: past, present and future

Pharmacogenetic & pharmacokinetic biomarker for efavirenz based ARV and rifampicin based anti-TB drug induced liver injury in TB-HIV infected patients

BACKGROUND

Implication of pharmacogenetic variations and efavirenz pharmacokinetics in concomitant efavirenz based antiviral therapy and anti-tubercular drug induced liver injury (DILI) has not been yet studied. We performed a prospective case-control association study to identify the incidence, pharmacogenetic, pharmacokinetic and biochemical predictors for anti-tubercular and antiretroviral drugs induced liver injury (DILI) in HIV and tuberculosis (TB) co-infected patients.

METHODS AND FINDINGS

Newly diagnosed treatment naïve TB-HIV co-infected patients (n = 353) were enrolled to receive efavirenz based ART and rifampicin based anti-TB therapy, and assessed clinically and biochemically for DILI up to 56 weeks. Quantification of plasma efavirenz and 8-hydroxyefaviernz levels and genotyping for NAT2, CYP2B6, CYP3A5, ABCB1, UGT2B7 and SLCO1B1 genes were done. The incidence of DILI and identification of predictors was evaluated using survival analysis and the Cox Proportional Hazards Model. The incidence of DILI was 30.0%, or 14.5 per 1000 person-week, and that of severe was 18.4%, or 7.49 per 1000 person-week. A statistically significant association of DILI with being of the female sex (p = 0.001), higher plasma efavirenz level (p = 0.009), efavirenz/8-hydroxyefavirenz ratio (p = 0.036), baseline AST (p = 0.022), ALT (p = 0.014), lower hemoglobin (p = 0.008), and serum albumin (p = 0.007), NAT2 slow-acetylator genotype (p = 0.039) and ABCB1 3435TT genotype (p = 0.001).

CONCLUSION

We report high incidence of anti-tubercular and antiretroviral DILI in Ethiopian patients. Between patient variability in systemic efavirenz exposure and pharmacogenetic variations in NAT2, CYP2B6 and ABCB1 genes determines susceptibility to DILI in TB-HIV co-infected patients. Close monitoring of plasma efavirenz level and liver enzymes during early therapy and/or genotyping practice in HIV clinics is recommended for early identification of patients at risk of DILI.

Structural alert/reactive metabolite concept as applied in medicinal chemistry to mitigate the risk of idiosyncratic drug toxicity: a perspective based on the critical examination of trends in the top 200 drugs marketed in the United States

Because of a preconceived notion that eliminating reactive metabolite (RM) formation with new drug candidates could mitigate the risk of idiosyncratic drug toxicity, the potential for RM formation is routinely examined as part of lead optimization efforts in drug discovery. Likewise, avoidance of "structural alerts" is almost a norm in drug design. However, there is a growing concern that the perceived safety hazards associated with structural alerts and/or RM screening tools as standalone predictors of toxicity risks may be over exaggerated. In addition, the multifactorial nature of idiosyncratic toxicity is now well recognized based upon observations that mechanisms other than RM formation (e.g., mitochondrial toxicity and inhibition of bile salt export pump (BSEP)) also can account for certain target organ toxicities. Hence, fundamental questions arise such as: When is a molecule that contains a structural alert (RM positive or negative) a cause for concern? Could the molecule in its parent form exert toxicity? Can a low dose drug candidate truly mitigate metabolism-dependent and -independent idiosyncratic toxicity risks? In an effort to address these questions, we have retrospectively examined 68 drugs (recalled or associated with a black box warning due to idiosyncratic toxicity) and the top 200 drugs (prescription and sales) in the United States in 2009 for trends in physiochemical characteristics, daily doses, presence of structural alerts, evidence for RM formation as well as toxicity mechanism(s) potentially mediated by parent drugs. Collectively, our analysis revealed that a significant proportion (∼78-86%) of drugs associated with toxicity contained structural alerts and evidence indicating that RM formation as a causative factor for toxicity has been presented in 62-69% of these molecules. In several cases, mitochondrial toxicity and BSEP inhibition mediated by parent drugs were also noted as potential causative factors. Most drugs were administered at daily doses exceeding several hundred milligrams. There was no obvious link between idiosyncratic toxicity and physicochemical properties such as molecular weight, lipophilicity, etc. Approximately half of the top 200 drugs for 2009 (prescription and sales) also contained one or more alerts in their chemical architecture, and many were found to be RM-positive. Several instances of BSEP and mitochondrial liabilities were also noted with agents in the top 200 category. However, with relatively few exceptions, the vast majority of these drugs are rarely associated with idiosyncratic toxicity, despite years of patient use. The major differentiating factor appeared to be the daily dose; most of the drugs in the top 200 list are administered at low daily doses. In addition, competing detoxication pathways and/or alternate nonmetabolic clearance routes provided suitable justifications for the safety records of RM-positive drugs in the top 200 category. Thus, while RM elimination may be a useful and pragmatic starting point in mitigating idiosyncratic toxicity risks, our analysis suggests a need for a more integrated screening paradigm for chemical hazard identification in drug discovery. Thus, in addition to a detailed assessment of RM formation potential (in relationship to the overall elimination mechanisms of the compound(s)) for lead compounds, effects on cellular health (e.g., cytotoxicity assays), BSEP inhibition, and mitochondrial toxicity are the recommended suite of assays to characterize compound liabilities. However, the prospective use of such data in compound selection will require further validation of the cellular assays using marketed agents. Until we gain a better understanding of the pathophysiological mechanisms associated with idiosyncratic toxicities, improving pharmacokinetics and intrinsic potency as means of decreasing the dose size and the associated "body burden" of the parent drug and its metabolites will remain an overarching goal in drug discovery.

Assessment of nonsteroidal anti-inflammatory drug-induced hepatotoxicity

INTRODUCTION

Liver toxicity related to NSAIDs is of outstanding importance because of the wide use of these drugs. NSAIDs are responsible for roughly 10% of the total of cases of drug-induced hepatotoxicity. The assessment of NSAID-induced hepatotoxicity, presently based on clinical and analytical biomarkers, is critical for early diagnosis and immediate withdrawal of the causing drug.

AREAS COVERED

The review presents an overview of current knowledge of the assessments of NSAID-induced hepatotoxicity with emphasis on the causative drugs, the NSAID-specific mechanisms involved, and a summary of genetic and non-genetic risk factors. Additionally, the authors discuss genetic factors which show NSAID-specific risk, namely CYP2C, UGT2B7, GSTM1 and GSTT1, as well as HLA alleles. The paper includes a list of the NSAID 'usual suspects' that cause hepatotoxicity based on the integrated information of drug-induced hepatotoxicity databases.

EXPERT OPINION

The ultimate goal of this research is pre-prescription testing. Unfortunately, genetic testing, alone, is not sufficient to predict NSAID-induced hepatotoxicity. The development of genetic biomarkers capable of identifying at-risk individuals will not be complete until we develop the ability to fully characterize patients' phenomes and the phenome-genome interaction in patients with NSAID-induced hepatotoxicity. Additionally, a characterization of the metabolic profile of the causative drug in patients with NSAID-induced hepatotoxicity would add crucial information which is presently disregarded in most studies. The full development of robust biomarkers will require the combination of several disciplines including causal statistics, phenomics, genomics, transcriptomics and metabonomics.

Databases in the area of pharmacogenetics

In the area of pharmacogenetics and personalized health care it is obvious that databases, providing important information of the occurrence and consequences of variant genes encoding drug metabolizing enzymes, drug transporters, drug targets, and other proteins of importance for drug response or toxicity, are of critical value for scientists, physicians, and industry. The primary outcome of the pharmacogenomic field is the identification of biomarkers that can predict drug toxicity and drug response, thereby individualizing and improving drug treatment of patients. The drug in question and the polymorphic gene exerting the impact are the main issues to be searched for in the databases. Here, we review the databases that provide useful information in this respect, of benefit for the development of the pharmacogenomic field.

Genetic variations of bile salt transporters as predisposing factors for drug-induced cholestasis, intrahepatic cholestasis of pregnancy and therapeutic response of viral hepatitis

INTRODUCTION

Drug-induced cholestasis, intrahepatic cholestasis of pregnancy and viral hepatitis are acquired forms of liver disease. Cholestasis is a pathophysiologic state with impaired bile formation and subsequent accumulation of bile salts in hepatocytes. The bile salt export pump (BSEP) (ABCB11) is the key export system for bile salts from hepatocytes.

AREAS COVERED

This article provides an introduction into the physiology of bile formation followed by a summary of the current knowledge on the key bile salt transporters, namely, the sodium-taurocholate co-transporting polypeptide NTCP, the organic anion transporting polypeptides (OATPs), BSEP and the multi-drug resistance protein 3. The pathophysiologic consequences of altered functions of these transporters, with an emphasis on molecular and genetic aspects, are then discussed.

EXPERT OPINION

Knowledge of the role of hepatocellullar transporters, especially BSEP, in acquired cholestasis is continuously increasing. A common variant of BSEP (p.V444A) is now a well-established susceptibility factor for acquired cholestasis and recent evidence suggests that the same variant also influences the therapeutic response and disease progression of viral hepatitis C. Studies in large independent cohorts are now needed to confirm the relevance of p.V444A. Genome-wide association studies should lead to the identification of additional genetic factors underlying cholestatic liver disease.

Genetic polymorphism and toxicology--with emphasis on cytochrome p450

Individual susceptibility to environmental, chemical, and drug toxicity is to some extent determined by polymorphism in drug-metabolizing enzymes, in particular the cytochromes P450 (CYPs). This polymorphism is in particular translated into risk differences concerning drugs metabolized by the highly polymorphic enzymes CYP2C9, CYP2C19, and CYP2D6, whereas CYP enzymes active in procarcinogen activation are relatively well conserved without important functional polymorphisms. Examples of drug toxicities that can be predicted by P450 polymorphism include those exerted by codeine, tramadol, warfarin, acenocoumarol, and clopidogrel. The polymorphic CYP2A6 has a role in nicotine metabolism and smoking behavior. Besides this genetic variation, genome-wide association studies now allow for the identification of an increasing number of predictive genetic biomarkers among, e.g., human leukocyte antigens and to some extent drug transporters that provide useful information regarding the choice of the drug and drug dosage in order to avoid toxicity. The translation of this information into the clinical practice has been slow; however, an increasing number of pharmacogenomic drug labels are assigned, where the predictive genotyping before drug treatment can be mandatory, recommended, or only for informational purposes. In this review, we provide an update of the field with emphasis on CYP polymorphism.

Pharmacogenomics and adverse drug reactions

Adverse drug reactions (ADRs) observed during drug development have been the cause for discontinuing development of many drugs. In addition, serious but rare ADRs observed after marketing have led to withdrawal of some drugs. A priori identification of individuals at risk of developing ADRs for a given drug will help develop strategies to reduce the risk for ADRs in these patients. US FDA initiatives and efforts at reducing ADRs to make drugs safer are described, including updating of drug labels to include genomic information intended to reduce ADRs. Pharmacogenomics can also be harnessed to identify individuals at risk of developing serious ADRs and to treat these individuals with alternative therapy, thus converting ADRs that are traditionally considered unavoidable to avoidable ADRs.

Polymorphisms of glutathione S-transferase mu 1, theta 1, and pi 1 genes and prognosis in Hodgkin lymphoma

We examined the influence of the glutathione S-transferase mu 1 (GSTM1), theta 1 (GSTT1), and pi 1 (GSTP1) polymorphisms, which are involved in the metabolism of alkylating agents and anthracyclines, on the outcome of patients with Hodgkin lymphoma (HL) treated with conventional chemotherapy. Genomic DNA from 125 consecutive cases was analyzed by polymerase chain reaction and enzymatic digestion for polymorphism determination. The GSTM1 undeleted genotype was associated with more advanced tumor stage and worse disease-free survival. The GSTT1 undeleted genotype was associated with higher recurrence rate. In contrast, higher toxicity of chemotherapy was attributed to absence of the GSTT1 gene. Concerning overall survival, lower tumor stage (p = 0.006) and International Prognostic Score (p = 0.02), lower peripheral leukocyte count (p = 0.0003), higher serum albumin level (p = 0.08), and GSTT1 undeleted genotype (p = 0.04) were predictive of a better outcome of patients. In multivariate analysis comparing staging and GST polymorphism, only tumor stage and GSTT1 genotype remained in the model. Our results suggest that the GSTT1 polymorphism influences the outcome of Brazilian patients with HL. However, studies of toxicity, pharmacokinetics, and protein function may clarify whether carriers of the distinct genotypes should receive different doses of chemotherapeutic agents.

Safe and effective medicines for all: is personalized medicine the answer?

An improvement in drug treatment and clinical outcome is one of the major challenges in clinical medicine. The development of evidence-based standards of care has led to a significant improvement, but, by definition, strictly standardized cohorts in clinical trials have to ignore individual differences. Personalized medicine is defined as the application of genomic and molecular data to better target the delivery of healthcare, facilitate the discovery and clinical testing of new products, and help determine a person's predisposition to a particular disease or condition. After the deciphering of the human genome, however, the high expectations in individualized medicine were not always fulfilled. However, personalized medicine has become indispensable in the treatment of malignant diseases and there is increasing evidence for its benefit in other areas. This article outlines the impact of pharmacogenetics and pharmacogenomics, especially with regard to personalized medicine, in major medical indications and reflects the obstacles and chances taken in current daily practice.