Do In Vitro Assays Predict Drug Candidate Idiosyncratic Drug-Induced Liver Injury Risk?

In vitro assays are commonly used during drug discovery to try to decrease the risk of idiosyncratic drug-induced liver injury (iDILI). But how effective are they at predicting risk? One of the most widely used methods evaluates cell cytotoxicity. Cytotoxicity assays that used cell lines that are very different from normal hepatocytes, and high concentrations of drug, were not very accurate at predicting idiosyncratic drug reaction risk. Even cytotoxicity assays that use more biologically normal cells resulted in many false-positive and false-negative results. Assays that quantify reactive metabolite formation, mitochondrial injury, and bile salt export pump (BSEP) inhibition have also been described. Although evidence suggests that reactive metabolite formation and BSEP inhibition can play a role in the mechanism of iDILI, these assays are not very accurate at predicting risk. In contrast, inhibition of the mitochondrial electron transport chain appears not to play an important role in the mechanism of iDILI, although other types of mitochondrial injury may do so. It is likely that there are many additional mechanisms by which drugs can cause iDILI. However, simply measuring more parameters is unlikely to provide better predictive assays unless those parameters are actually involved in the mechanism of iDILI. Hence, a better mechanistic understanding of iDILI is required; however, mechanistic studies of iDILI are very difficult. There is substantive evidence that most iDILI is immune mediated; therefore, the most accurate assays may involve those that determine immune responses to drugs. New methods to manipulate immune tolerance may greatly facilitate development of more suitable methods.

Hepatic effects of repeated oral administration of diclofenac to hepatic cytochrome P450 reductase null (HRN™) and wild-type mice.

Hepatic NADPH-cytochrome P450 oxidoreductase null (HRN™) mice exhibit normal hepatic and extrahepatic biotransformation enzyme activities when compared to wild-type (WT) mice, but express no functional hepatic cytochrome P450 activities. When incubated in vitro with [(14)C]-diclofenac, liver microsomes from WT mice exhibited extensive biotransformation to oxidative and glucuronide metabolites and covalent binding to proteins was also observed. In contrast, whereas glucuronide conjugates and a quinone-imine metabolite were formed when [(14)C]-diclofenac was incubated with HRN™ mouse liver, only small quantities of P450-derived oxidative metabolites were produced in these samples and covalent binding to proteins was not observed. Livers from vehicle-treated HRN™ mice exhibited enhanced lipid accumulation, bile duct proliferation, hepatocellular degeneration and necrosis and inflammatory cell infiltration, which were not present in livers from WT mice. Elevated liver-derived alanine aminotransferase, glutamate dehydrogenase and alkaline phosphatase activities were also observed in plasma from HRN™ mice. When treated orally with diclofenac for 7 days, at 30 mg/kg/day, the severities of the abnormal liver histopathology and plasma liver enzyme findings in HRN™ mice were reduced markedly. Oral diclofenac administration did not alter the liver histopathology or elevate plasma enzyme activities of WT mice. These findings indicate that HRN™ mice are valuable for exploration of the role played by hepatic P450s in drug biotransformation, but poorly suited to investigations of drug-induced liver toxicity. Nevertheless, studies in HRN™ mice could provide novel insights into the role played by inflammation in liver injury and may aid the evaluation of new strategies for its treatment.

Multiple compound-related adverse properties contribute to liver injury caused by endothelin receptor antagonists.

Drug-induced liver injury has been observed in patients treated with the endothelin receptor antagonists sitaxentan and bosentan, but not following treatment with ambrisentan. The aim of our studies was to assess the possible role of multiple contributory mechanisms in this clinically relevant toxicity. Inhibition of the bile salt export pump (BSEP) and multidrug resistance-associated protein 2 was quantified using membrane vesicle assays. Inhibition of mitochondrial respiration in human liver-derived HuH-7 cells was determined using a Seahorse XF(e96) analyzer. Cytochrome P450 (P450)-independent and P450-mediated cell toxicity was assessed using transfected SV40-T-antigen-immortalized human liver epithelial (THLE) cell lines. Exposure-adjusted assay ratios were calculated by dividing the maximum human drug plasma concentrations by the IC50 or EC50 values obtained in vitro. Covalent binding (CVB) of radiolabeled drugs to human hepatocytes was quantified, and CVB body burdens were calculated by adjusting CVB values for fractional drug turnover in vitro and daily therapeutic dose. Sitaxentan exhibited positive exposure-adjusted signals in all five in vitro assays and a high CVB body burden. Bosentan exhibited a positive exposure-adjusted signal in one assay (BSEP inhibition) and a moderate CVB body burden. Ambrisentan exhibited no positive exposure-adjusted assay signals and a low CVB body burden. These data indicate that multiple mechanisms contribute to the rare, but potentially severe liver injury caused by sitaxentan in humans; provide a plausible rationale for the markedly lower propensity of bosentan to cause liver injury; and highlight the relative safety of ambrisentan.

Systems toxicology: modelling biomarkers of glutathione homeostasis and paracetamol metabolism.

One aim of systems toxicology is to deliver mechanistic, mathematically rigorous, models integrating biochemical and pharmacological processes that result in toxicity to enhance the assessment of the risk posed to humans by drugs and other xenobiotics. The benefits of such 'in silico' models would be in enabling the rapid and robust prediction of the effects of compounds over a range of exposures, improving in vitro-in vivo correlations and the translation from preclinical species to humans. Systems toxicology models of organ toxicities that result in high attrition rates during drug discovery and development, or post-marketing withdrawals (e.g., drug-induced liver injury (DILI)) should facilitate the discovery of safe new drugs. Here, systems toxicology as applied to the effects of paracetamol (acetaminophen, N-acetyl-para-aminophenol (APAP)) is used to exemplify the potential of the approach.

Glutathione metabolism modeling: a mechanism for liver drug-robustness and a new biomarker strategy.

BACKGROUND: Glutathione metabolism can determine an individual's ability to detoxify drugs. To increase understanding of the dynamics of cellular glutathione homeostasis, we have developed an experiment-based mathematical model of the kinetics of the glutathione network. This model was used to simulate perturbations observed when human liver derived THLE cells, transfected with human cytochrome P452E1 (THLE-2E1 cells), were exposed to paracetamol (acetaminophen). METHODS: Human liver derived cells containing extra human cytochrome P4502E1 were treated with paracetamol at various levels of methionine and in the presence and absence of an inhibitor of glutamyl-cysteine synthetase (GCS). GCS activity was also measured in extracts. Intracellular and extracellular concentrations of substances involved in glutathione metabolism were measured as was damage to mitochondria and proteins. A bottom up mathematical model was made of the metabolic pathways around and including glutathione. RESULTS: Our initial model described some, but not all the metabolite-concentration and flux data obtained when THLE-2E1 cells were exposed to paracetamol at concentrations high enough to affect glutathione metabolism. We hypothesized that the lack of correspondence could be due to upregulation of expression of glutamyl cysteine synthetase, one of the enzymes controlling glutathione synthesis, and confirmed this experimentally. A modified model which incorporated this adaptive response adequately described the observed changes in the glutathione pathway. Use of the adaptive model to analyze the functioning of the glutathione network revealed that a threshold input concentration of methionine may be required for effective detoxification of reactive metabolites by glutathione conjugation. The analysis also provided evidence that 5-oxoproline and ophthalmic acid are more useful biomarkers of glutathione status when analyzed together than when analyzed in isolation, especially in a new, model-assisted integrated biomarker strategy. CONCLUSION: A robust mathematical model of the dynamics of cellular changes in glutathione homeostasis in cells has been developed and tested in vitro. GENERAL SIGNIFICANCE: Mathematical models of the glutathione pathway that help examine mechanisms of cellular protection against xenobiotic toxicity and the monitoring thereof, can now be made.

Mass spectrometric characterization of circulating covalent protein adducts derived from a drug acyl glucuronide metabolite: multiple albumin adductions in diclofenac patients.

Covalent protein modifications by electrophilic acyl glucuronide (AG) metabolites are hypothetical causes of hypersensitivity reactions associated with certain carboxylate drugs. The complex rearrangements and reactivities of drug AG have been defined in great detail, and protein adducts of carboxylate drugs, such as diclofenac, have been found in liver and plasma of experimental animals and humans. However, in the absence of definitive molecular characterization, and specifically, identification of signature glycation conjugates retaining the glucuronyl and carboxyl residues, it cannot be assumed any of these adducts is derived uniquely or even fractionally from AG metabolites. We have therefore undertaken targeted mass spectrometric analyses of human serum albumin (HSA) isolated from diclofenac patients to characterize drug-: derived structures and, thereby, for the first time, have deconstructed conclusively the pathways of adduct formation from a drug AG and its isomeric rearrangement products in vivo. These analyses were informed by a thorough understanding of the reactions of HSA with diclofenac AG in vitro. HSA from six patients without drug-: related hypersensitivities had either a single drug-: derived adduct or one of five combinations of 2-8 adducts from among seven diclofenac N-acylations and three AG glycations on seven of the protein's 59 lysines. Only acylations were found in every patient. We present evidence that HSA modifications by diclofenac in vivo are complicated and variable, that at least a fraction of these modifications are derived from the drug's AG metabolite, and that albumin adduction is not inevitably a causation of hypersensitivity to carboxylate drugs or a coincidental association.

Assessment of gadoxetate DCE-MRI as a biomarker of hepatobiliary transporter inhibition.

Drug-induced liver injury (DILI) is a clinically important adverse drug reaction, which prevents the development of many otherwise safe and effective new drugs. Currently, there is a lack of sensitive and specific biomarkers that can be used to predict, assess and manage this toxicity. The aim of this work was to evaluate gadoxetate-enhanced MRI as a potential novel biomarker of hepatobiliary transporter inhibition in the rat. Initially, the volume fraction of extracellular space in the liver was determined using gadopentetate to enable an estimation of the gadoxetate concentration in hepatocytes. Using this information, a compartmental model was developed to characterise the pharmacokinetics of hepatic uptake and biliary excretion of gadoxetate. Subsequently, we explored the impact of an investigational hepatobiliary transporter inhibitor on the parameters of the model in vivo in rats. The investigational hepatobiliary transporter inhibitor reduced both the rate of uptake of gadoxetate into the hepatocyte, k1 , and the Michaelis-Menten constant, Vmax , characterising its excretion into bile, whereas KM values for biliary efflux were increased. These effects were dose dependent and correlated with effects on plasma chemistry markers of liver dysfunction, in particular bilirubin and bile acids. These results indicate that gadoxetate-enhanced MRI provides a novel functional biomarker of inhibition of transporter-mediated hepatic uptake and clearance in the rat. Since gadoxetate is used clinically, the technology has the potential to provide a translatable biomarker of drug-induced perturbation of hepatic transporters that may also be useful in humans to explore deleterious functional alterations caused by transporter inhibition.

Bioactivation of the cannabinoid receptor antagonist rimonabant to a cytotoxic iminium ion metabolite.

The cannabinoid type 1 receptor (CB1r) antagonist rimonabant was approved in 2006 for the treatment of obesity but was withdrawn in 2008 due to serious drug-related psychiatric disorders. In vitro metabolism studies with rimonabant have revealed high levels of reactive metabolite formation, which resulted in irreversible time-dependent P450 3A4 inhibition and in covalent binding to microsomal proteins. In the present study, an in vitro approach has been used to explore whether metabolic bioactivation of rimonabant might result in cell toxicity. A panel of SV40-T-antigen-immortalized human liver derived (THLE) cells that had been transfected with vectors encoding various human cytochrome P450 enzymes (THLE-1A2, 2C9, 2C19, 2D6, and 3A4) or with an empty vector (THLE-Null) were exposed to rimonabant. Cell toxicity and covalent binding to cellular proteins were evaluated, as was metabolite formation. Rimonabant exhibited markedly potentiated dose and time dependent cytotoxicity to THLE-3A4 cells, compared to that of all other THLE cell lines. This was accompanied by high levels of covalent binding of [(14)C]-rimonabant to THLE-3A4 cell proteins (1433 pmol drug equivalents/mg protein) and the formation of several metabolites that were not generated by THLE-Null cells. These included N-aminopiperidine (NAP) and an iminium ion species. However, no toxicity was observed when THLE cells were incubated with NAP. Glutathione depletion did not alter the observed potent cell cytotoxicity of rimonabant to THLE-3A4 cells. Preincubation of THLE-3A4 cells with the cytochrome P450 3A4 inhibitor ritonavir blocked the selective toxicity of rimonabant to these cells. In addition, ritonavir pretreatment blocked the metabolism of the compound in the cells and thereby significantly decreased the covalent binding of [(14)C]-rimonabant to THLE-3A4 cell proteins. We conclude that the potent toxicity of rimonabant in THLE-3A4 cells occurs by a mechanistic sequence, which is initiated by cytochrome P450 3A4 mediated formation of a highly cytotoxic reactive iminium ion metabolite that binds covalently to cellular proteins.

In vitro exploration of potential mechanisms of toxicity of the human hepatotoxic drug fenclozic acid.

The carboxylic acid NSAID fenclozic acid exhibited an excellent preclinical safety profile and promising clinical efficacy, yet was withdrawn from clinical development in 1971 due to hepatotoxicity observed in clinical trials. A variety of modern in vitro approaches have been used to explore potential underlying mechanisms. Covalent binding studies were undertaken with [(14)C]-fenclozic acid to investigate the possible role of reactive metabolites. Time-dependent covalent binding to protein was observed in NADPH-supplemented liver microsomes, although no metabolites were detected in these incubations or in reactive metabolite trapping experiments. In human hepatocytes, covalent binding was observed at lower levels than in microsomes and a minor uncharacterizable metabolite was also observed. In addition, covalent binding was observed in incubations undertaken with dog and rat hepatocytes, where a taurine conjugate of the drug was detected. Although an acyl glucuronide metabolite was detected when liver microsomes from human, rat and dog were supplemented with UDPGA, there was no detectable UDPGA-dependent covalent binding. No effects were observed when fenclozic acid was assessed for P450-dependent and P450-independent cytotoxicity to THLE cell lines, time-dependent inhibition of five major human cytochrome P450 enzymes, inhibition of the biliary efflux transporters BSEP and MRP2 or mitochondrial toxicity to THLE or HepG2 cells. These data suggest that Phase 1 bioactivation plays a role in the hepatotoxicity of fenclozic acid and highlight the unique insight into mechanisms of human drug toxicity that can be provided by investigations of biotransformation and covalent binding to proteins.

Use of In Vivo Imaging and Physiologically-Based Kinetic Modelling to Predict Hepatic Transporter Mediated Drug-Drug Interactions in Rats.

Gadoxetate, a magnetic resonance imaging (MRI) contrast agent, is a substrate of organic-anion-transporting polypeptide 1B1 and multidrug resistance-associated protein 2. Six drugs, with varying degrees of transporter inhibition, were used to assess gadoxetate dynamic contrast enhanced MRI biomarkers for transporter inhibition in rats. Prospective prediction of changes in gadoxetate systemic and liver AUC (AUCR), resulting from transporter modulation, were performed by physiologically-based pharmacokinetic (PBPK) modelling. A tracer-kinetic model was used to estimate rate constants for hepatic uptake (khe), and biliary excretion (kbh). The observed median fold-decreases in gadoxetate liver AUC were 3.8- and 1.5-fold for ciclosporin and rifampicin, respectively. Ketoconazole unexpectedly decreased systemic and liver gadoxetate AUCs; the remaining drugs investigated (asunaprevir, bosentan, and pioglitazone) caused marginal changes. Ciclosporin decreased gadoxetate khe and kbh by 3.78 and 0.09 mL/min/mL, while decreases for rifampicin were 7.20 and 0.07 mL/min/mL, respectively. The relative decrease in khe (e.g., 96% for ciclosporin) was similar to PBPK-predicted inhibition of uptake (97-98%). PBPK modelling correctly predicted changes in gadoxetate systemic AUCR, whereas underprediction of decreases in liver AUCs was evident. The current study illustrates the modelling framework and integration of liver imaging data, PBPK, and tracer-kinetic models for prospective quantification of hepatic transporter-mediated DDI in humans.

Incorporating new approach methodologies into regulatory nonclinical pharmaceutical safety assessment.

New approach methodologies (NAMs) based on human biology enable the assessment of adverse biological effects of pharmaceuticals and other chemicals. Currently, however, it is unclear how NAMs should be used during drug development to improve human safety evaluation. A series of 5 workshops with 13 international experts (regulators, preclinical scientists, and NAMs developers) was conducted to identify feasible NAMs and to discuss how to exploit them in specific safety assessment contexts. Participants generated four "maps" of how NAMs can be exploited in the safety assessment of the liver, respiratory, cardiovascular, and central nervous systems. Each map shows relevant endpoints measured and tools used (e.g., cells, assays, platforms), and highlights gaps where further development and validation of NAMs remains necessary. Each map addresses the fundamental scientific requirements for the safety assessment of that organ system, providing users with guidance on the selection of appropriate NAMs. In addition to generating the maps, participants offered suggestions for encouraging greater NAM adoption within drug development and their inclusion in regulatory guidelines. A specific recommendation was that pharmaceutical companies should be more transparent about how they use NAMs in-house. As well as giving guidance for the four organ systems, the maps provide a template that could be used for additional organ safety testing contexts. Moreover, their conversion to an interactive format would enable users to drill down to the detail necessary to answer specific scientific and regulatory questions.

Characterization of THLE-cytochrome P450 (P450) cell lines: gene expression background and relationship to P450-enzyme activity.

The hepatic SV40 large T-antigen immortalized human liver epithelial (THLE) cell line and sublines transfected with cytochromes P450 (P450s) are increasingly being used for evaluation of potential drug-induced liver injury. So far, the available information on transporter and enzyme expression in these transfected cell systems is scattered. The purpose of this study was to characterize THLE cell lines with respect to transporter and enzyme expression. The mRNA expression of 96 typical drug absorption, distribution, metabolism and excretion genes, which encode a selection of transporters, phase I and II drug-metabolizing enzymes, and nuclear hormone receptors, was investigated in five THLE cell lines transfected with individual human P450s and in mock-transfected THLE-null cells using real-time polymerase chain reaction. The majority of the analyzed genes was either absent or expressed at low levels in the THLE-null and THLE-P450 cells, apart from housekeeping genes and the individual transfected P450s. Enzyme activity measurements provided confirmatory functional data for CYP2C9 and CYP3A4. Comparison with gene expression in human liver revealed an overall much lower gene expression in the THLE cell lines. The low levels of expression of a broad range of P450 genes in the THLE cell lines highlight the value of studies undertaken with P450-expressing cell lines for investigation of mechanisms of P450 metabolite-mediated hepatotoxicity. However, when attempting to translate between data obtained in THLE cell lines in vitro and functional consequences in vivo, it is important to take account of their limited expression of genes encoding many other drug-metabolizing enzymes and hepatic transporters.

Mitigating the inhibition of human bile salt export pump by drugs: opportunities provided by physicochemical property modulation, in silico modeling, and structural modification.

The human bile salt export pump (BSEP) is a membrane protein expressed on the canalicular plasma membrane domain of hepatocytes, which mediates active transport of unconjugated and conjugated bile salts from liver cells into bile. BSEP activity therefore plays an important role in bile flow. In humans, genetically inherited defects in BSEP expression or activity cause cholestatic liver injury, and many drugs that cause cholestatic drug-induced liver injury (DILI) in humans have been shown to inhibit BSEP activity in vitro and in vivo. These findings suggest that inhibition of BSEP activity by drugs could be one of the mechanisms that initiate human DILI. To gain insight into the chemical features responsible for BSEP inhibition, we have used a recently described in vitro membrane vesicle BSEP inhibition assay to quantify transporter inhibition for a set of 624 compounds. The relationship between BSEP inhibition and molecular physicochemical properties was investigated, and our results show that lipophilicity and molecular size are significantly correlated with BSEP inhibition. This data set was further used to build predictive BSEP classification models through multiple quantitative structure-activity relationship modeling approaches. The highest level of predictive accuracy was provided by a support vector machine model (accuracy = 0.87, κ = 0.74). These analyses highlight the potential value that can be gained by combining computational methods with experimental efforts in early stages of drug discovery projects to minimize the propensity of drug candidates to inhibit BSEP.

In vitro approach to assess the potential for risk of idiosyncratic adverse reactions caused by candidate drugs.

Idiosyncratic adverse drug reactions (IADRs) in humans can result in a broad range of clinically significant toxicities leading to attrition during drug development as well as postlicensing withdrawal or labeling. IADRs arise from both drug and patient related mechanisms and risk factors. Drug related risk factors, resulting from parent compound or metabolites, may involve multiple contributory mechanisms including organelle toxicity, effects related to compound disposition, and/or immune activation. In the current study, we evaluate an in vitro approach, which explored both cellular effects and covalent binding (CVB) to assess IADR risks for drug candidates using 36 drugs which caused different patterns and severities of IADRs in humans. The cellular effects were tested in an in vitro Panel of five assays which quantified (1) toxicity to THLE cells (SV40 T-antigen-immortalized human liver epithelial cells), which do not express P450s, (2) toxicity to a THLE cell line which selectively expresses P450 3A4, (3) cytotoxicity in HepG2 cells in glucose and galactose media, which is indicative of mitochondrial injury, (4) inhibition of the human bile salt export pump, BSEP, and (5) inhibition of the rat multidrug resistance associated protein 2, Mrp2. In addition, the CVB Burden was estimated by determining the CVB of radiolabeled compound to human hepatocytes and factoring in both the maximum prescribed daily dose and the fraction of metabolism leading to CVB. Combining the aggregated results from the in vitro Panel assays with the CVB Burden data discriminated, with high specificity (78%) and sensitivity (100%), between 27 drugs, which had severe or marked IADR concern, and 9 drugs, which had low IADR concern, we propose that this integrated approach has the potential to enable selection of drug candidates with reduced propensity to cause IADRs in humans.

Systems biology tools for toxicology.

An important goal of toxicology is to understand and predict the adverse effects of drugs and other xenobiotics. For pharmaceuticals, such effects often emerge unexpectedly in man even when absent from trials in vitro and in animals. Although drugs and xenobiotics act on molecules, it is their perturbation of intracellular networks that matters. The tremendous complexity of these networks makes it difficult to understand the effects of xenobiotics on their ability to function. Because systems biology integrates data concerning molecules and their interactions into an understanding of network behaviour, it should be able to assist toxicology in this respect. This review identifies how in silico systems biology tools, such as kinetic modelling, and metabolic control, robustness and flux analyse, may indeed help understanding network-mediated toxicity. It also shows how these approaches function by implementing them vis-à-vis the glutathione network, which is important for the detoxification of reactive drug metabolites. The tools enable the appreciation of the steady state concept for the detoxification network and make it possible to simulate and then understand effects of perturbations of the macromolecules in the pathway that are counterintuitive. We review how a glutathione model has been used to explain the impact of perturbation of the pathway at various molecular sites, as would be the effect of single-nucleotide polymorphisms. We focus on how the mutations impact the levels of glutathione and of two candidate biomarkers of hepatic glutathione status. We conclude this review by sketching how the various systems biology tools may help in the various phases of drug development in the pharmaceutical industry.

Evaluation of the pharmacokinetics, biotransformation and hepatic transporter effects of troglitazone in mice with humanized livers.

The pharmacokinetics, biotransformation and hepatic transporter effects of troglitazone were investigated following daily oral dosing, at 300 and 600 mg/kg, for 7 days to control (SCID) and chimeric (PXB) mice with humanized livers. Clinical chemistry revealed no consistent pattern of changes associated with troglitazone treatment in the PXB mouse. Human MRP2 but not mouse mrp2 was down-regulated following troglitazone treatment. Pharmacokinetic analysis revealed similar T(max) values for troglitazone in both mouse groups, a mono- and bi-phasic elimination phase in PXB and SCID mice, respectively, but a 3- to 5- and 2- to 5-fold higher C(max) and AUC, respectively, in PXB mice. Oxidative and conjugative metabolic pathways were identified, with the sulfate being the predominant metabolite in PXB compared to SCID mice (4- to 13-fold increase in liver and blood, respectively). The glucuronide conjugate was predominant in SCID mice. There was no evidence of glutathione conjugation. The primary oxidative pathways were mono- and di-oxidations which may also be attributed to quinone or hydroquinone derivatives. Several metabolites were observed in PXB mice only. As the troglitazone metabolic profiles in the PXB mouse were similar to reported human data the PXB mouse model can provide a useful first insight into circulating human metabolites of xenobiotics metabolized in the liver.

Functional and toxicological consequences of metabolic bioactivation of methapyrilene via thiophene S-oxidation: Induction of cell defence, apoptosis and hepatic necrosis.

UNLABELLED: Methapyrilene, [N,N-dimethyl-N'-pyridyl-N'(2-thienylmethyl)-1,2-ethanediamine] (MP) was withdrawn from, clinical use due to reported periportal hepatic necrosis and hepatocarcinogenicity in the rat, via S-oxidation of the thiophene group. In this study MP is used as a model hepatotoxin to further characterise the functional consequences of S-oxidation of the thiophene group in vivo, in rat models and in vitro, in freshly isolated rat hepatocyte suspensions. In vivo histological studies revealed the early depletion of glutathione (GSH), which was confined to the damaged periportal area, in contrast to an increase in GSH levels in the centrilobular region. Additionally, the induction of cell defence was demonstrated by an increase in the protein levels of heme-oxygenase 1 (HO-1) and glutamate cysteine ligase, catalytic subunit (GCLC) in vivo. Histological examination demonstrated that cytotoxicity progresses initially via apoptosis before an increase in necrosis over the 3-day administration. An apoptotic-like mechanism was observed in vitro via the measurement of cytochrome c release and caspase activation. CONCLUSION: This study provides evidence for a complex pathway of MP-induced hepatotoxicity which progresses through early adaptation, apoptosis, necrosis and inflammation, all underpinned by the zonal induction and depletion of GSH within the liver.

Advancing nonclinical innovation and safety in pharmaceutical testing.

Nonclinical tests are considered crucial for understanding the safety of investigational medicines. However, the effective translation from nonclinical to human application is limited and must be improved. Drug development stakeholders are working to advance human-based in vitro and in silico methods that may be more predictive of human efficacy and safety in vivo because they enable scientists to model the direct interaction of drugs with human cells, tissues, and biological processes. Here, we recommend test-neutral regulations; increased funding for development and integration of human-based approaches; support for existing initiatives that advance human-based approaches; evaluation of new approaches using human data; establishment of guidelines for procuring human cells and tissues for research; and additional training and educational opportunities in human-based approaches.

Can Bile Salt Export Pump Inhibition Testing in Drug Discovery and Development Reduce Liver Injury Risk? An International Transporter Consortium Perspective.

Bile salt export pump (BSEP) inhibition has emerged as an important mechanism that may contribute to the initiation of human drug-induced liver injury (DILI). Proactive evaluation and understanding of BSEP inhibition is recommended in drug discovery and development to aid internal decision making on DILI risk. BSEP inhibition can be quantified using in vitro assays. When interpreting assay data, it is important to consider in vivo drug exposure. Currently, this can be undertaken most effectively by consideration of total plasma steady state drug concentrations (Css,plasma ). However, because total drug concentrations are not predictive of pharmacological effect, the relationship between total exposure and BSEP inhibition is not causal. Various follow-up studies can aid interpretation of in vitro BSEP inhibition data and may be undertaken on a case-by-case basis. BSEP inhibition is one of several mechanisms by which drugs may cause DILI, therefore, it should be considered alongside other mechanisms when evaluating possible DILI risk.