Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Mechanism-Based Inactivation of Cytochrome P450 3A by Evodol

1. Evodol is one of the furanoids isolated from the fruits of Evodia rutaecarpa that has been widely prescribed for the treatment of gastrointestinal diseases in China. The aim of this study was to investigate the inhibitory effect of evodol on CYP3A.2. A 30-min preincubation of evodol with human liver microsomes raised an obvious left IC₅₀ shift, 3.9-fold for midazolam 1'-hydroxylation and 3.2-fold for testosterone 6β-hydroxylation. Evodol inactivated CYP3A in a time-, concentration- and NADPH-dependent manner, with K_I and k_inact of 5.1 μM and 0.028 min^-1 for midazolam 1'-hydroxylation and 3.0 μM and 0.022 min^-1 for testosterone 6β-hydroxylation.3. Co-incubation of ketoconazole attenuated the inactivation while inclusion of glutathione (GSH) and catalase/superoxide dismutase displayed no such protection.4. cis-Butene-1, 4-dial (BDA) intermediate derived from evodol were trapped by glutathione and N-acetyl-lysine in microsomes and characterized by HR-MS spectra. The BDA intermediate was believed to play a key role in CYP3A inactivation. CYP3A4 and 2C9 were the primary enzymes contributing to the bioactivation of evodol.5. To sum up, for the first time evodol was characterized as a mechanism-based inactivator of CYP3A.

Removal processes of individual and a mixture of organic micropollutants in the presence of Scenedesmus obliquus

Organic micropollutants (OMPs) need to be removed from wastewater as they can negatively affect aquatic organisms. It has been demonstrated that microalgae-based technologies are efficient in removing OMPs from wastewater. In this study, the removal processes and kinetics of six persistent OMPs (diclofenac, clarithromycin, benzotriazole, metoprolol, carbamazepine and mecoprop) were studied during cultivation of Scenedesmus obliquus in batch mode. These OMPs were added as individual compounds and in a mixture. Short experiments (8 days) were performed to avoid masking of OMP removal processes by light and nutrient limitation. The results show that diclofenac, clarithromycin, and benzotriazole were mainly removed by photodegradation (diclofenac), biodegradation (benzotriazole), or a combination of these two processes (clarithromycin). Peroxidase was involved in intracellular and extracellular biodegradation when benzotriazole was present as individual compound. Carbamazepine, metoprolol and mecoprop showed no biodegradation or photodegradation, and neglectable removal (<5%) by bioadsorption and bioaccumulation. Using an OMP mixture had an adverse effect on the photodegradation of clarithromycin and diclofenac, with reduced first-order kinetic constants compared to the individual compounds. Benzotriazole biodegradation was inhibited by the presence of the OMP mixture. This indicates that the presence of OMPs inhibits the photodegradation and biodegradation of some individual OMPs. These results will improve our understanding of removal processes of individual and mixtures of OMPs by microalgae-based technologies for wastewater treatment.

Psoralen induces hepatotoxicity by covalently binding to glutathione-S-transferases and the hepatic cytochrome P450

BACKGROUND

Psoraleae Fructus has been widely used in China and its surroundings; however, Psoraleae Fructus and its compound preparation have been reported recently to cause liver injury in clinics. Thus, its safe use has attracted increasing attention. The possible mechanism is related to the metabolism of psoralen, but it still needs further clarification.

PURPOSE

The present study was designed to evaluate the toxicity of psoralen and investigate the potentially related molecular mechanisms using chemical biology methods combined with animal experiments to provide evidence for the rational clinical use of psoralen.

METHODS

An in vivo experiment was conducted with a time series of 20-80 mg/kg psoralen to verify its toxic performance. Target capture and click reactions were used to investigate direct targets of psoralen. Selectivity for different glutathione-S-transferase (GST) subtypes in the liver and inhibition of cytochrome P450 (CYP450) were also detected.

RESULTS

Psoralen build-up in the liver is the primary cause of liver damage. Our study revealed the mechanism by which psoralen induces liver injury. Psoralen can bind directly to CYP2D6, CYP3A4, GST-α, and GST-μ and inhibit their activities, causing the depletion of glutathione (GSH) in vivo, which in turn induces hepatic damage. The special structure of α,β-unsaturated lactones in psoralen facilitates its attachment to its target; therefore, complementing psoralen with GSH can efficiently protect the liver from damage.

CONCLUSIONS

Psoralen causes a disorder in drug metabolism by inhibiting the activity of CYPs and GSTs, causing exhaustion of GSH, and subsequently leading to liver damage. The co-administration of GSH and psoralen is an effective way to avoid liver injury in clinical settings.

The Mechanism-Based Inactivation of CYP3A4 by Ritonavir: What Mechanism?

Ritonavir is the most potent cytochrome P450 (CYP) 3A4 inhibitor in clinical use and is often applied as a booster for drugs with low oral bioavailability due to CYP3A4-mediated biotransformation, as in the treatment of HIV (e.g., lopinavir/ritonavir) and more recently COVID-19 (Paxlovid or nirmatrelvir/ritonavir). Despite its clinical importance, the exact mechanism of ritonavir-mediated CYP3A4 inactivation is still not fully understood. Nonetheless, ritonavir is clearly a potent mechanism-based inactivator, which irreversibly blocks CYP3A4. Here, we discuss four fundamentally different mechanisms proposed for this irreversible inactivation/inhibition, namely the (I) formation of a metabolic-intermediate complex (MIC), tightly coordinating to the heme group; (II) strong ligation of unmodified ritonavir to the heme iron; (III) heme destruction; and (IV) covalent attachment of a reactive ritonavir intermediate to the CYP3A4 apoprotein. Ritonavir further appears to inactivate CYP3A4 and CYP3A5 with similar potency, which is important since ritonavir is applied in patients of all ethnicities. Although it is currently not possible to conclude what the primary mechanism of action in vivo is, it is unlikely that any of the proposed mechanisms are fundamentally wrong. We, therefore, propose that ritonavir markedly inactivates CYP3A through a mixed set of mechanisms. This functional redundancy may well contribute to its overall inhibitory efficacy.

Atypical kinetics of cytochrome P450 enzymes in pharmacology and toxicology

Atypical kinetics are observed in metabolic reactions catalyzed by cytochrome P450 enzymes (P450). Yet, this phenomenon is regarded as experimental artifacts in some instances despite increasing evidence challenging the assumptions of typical Michaelis-Menten kinetics. As P450 play a major role in the metabolism of a wide range of substrates including drugs and endogenous compounds, it becomes critical to consider the impact of atypical kinetics on the accuracy of estimated kinetic and inhibitory parameters which could affect extrapolation of pharmacological and toxicological implications. The first half of this book chapter will focus on atypical non-Michaelis-Menten kinetics (e.g. substrate inhibition, biphasic and sigmoidal kinetics) as well as proposed underlying mechanisms supported by recent insights in mechanistic enzymology. In particular, substrate inhibition kinetics in P450 as well as concurrent drug inhibition of P450 in the presence of substrate inhibition will be further discussed. Moreover, mounting evidence has revealed that despite the high degree of sequence homology between CYP3A isoforms (i.e. CYP3A4 and CYP3A5), they have the propensities to exhibit vastly different susceptibilities and potencies of mechanism-based inactivation (MBI) with a common drug inhibitor. These experimental observations pertaining to the presence of these atypical isoform- and probe substrate-specific complexities in CYP3A isoforms by several clinically-relevant drugs will therefore be expounded and elaborated upon in the second half of this book chapter.

Methysticin Acts as a Mechanism-Based Inactivator of Cytochrome P450 2C9

Methysticin is one of the naturally occurring bioactive constituents extracted from Piper methysticum Forst. In the present study, we intended to investigate the inhibitory effect of methysticin on cytochrome P450 (P450) enzymes. Methysticin exhibited time-, concentration-, and NADPH-dependent inhibition on CYP2C9 using diclofenac as a probe substrate. Approximately 85% of CYP2C9 activity was inhibited by methysticin at 50 μM after a 30 min preincubation with human liver microsomes in the presence of NADPH. The kinetic parameters K_I, k_inact, and t_1/2,inact were 13.32 ± 1.35 μM, 0.054 ± 0.005 min^-1, and 12.83 ± 3.23 min, respectively. Sulfaphenazole (competitive inhibitor of CYP2C9) displayed a significant protective effect on methysticin-induced CYP2C9 inactivation. However, the inclusion of catalase/superoxide dismutase or glutathione (GSH) showed no such protection. A carbene intermediate was postulated to be involved in methysticin-induced CYP2C9 inactivation as K₃Fe(CN)₆ recovered 14.96% of CYP2C9 activity. A methysticin-derived ortho-quinone intermediate dependent on NADPH was trapped by GSH, and this intermediate was believed to be involved in CYP2C9 inactivation. CYP1A2, 2C9, and 3A4 were the major enzymes responsible for methysticin bioactivation. Taken together, the present work demonstrated that methysticin was a mechanism-based inactivator of CYP2C9. Both ortho-quinone and carbene intermediates appeared to be involved in the inactivation of CYP2C9 induced by methysticin.

Metabolic Activation of the Acrylamide Michael Acceptor Warhead in Futibatinib to an Epoxide Intermediate Engenders Covalent Inactivation of Cytochrome P450 3A

Futibatinib (FUT) is a potent inhibitor of fibroblast growth factor receptor (FGFR) 1-4 that is currently under clinical investigation for intrahepatic cholangiocarcinoma. Unlike its predecessors, FUT possesses an acrylamide warhead which enables it to bind covalently to a free cysteine residue in the FGFR kinase domain. However, it remains uninterrogated if this electrophilic α,β-unsaturated carbonyl scaffold could also directly or indirectly engender off-target covalent binding to nucleophilic centres on other cellular proteins. Here, we discovered that FUT inactivated both cytochrome P450 3A (CYP3A) isoforms with K _I, k _inact, and partition ratio of 12.5 and 51.4 µM, 0.25 and 0.06 min^-1 and ~52 and ~58 for CYP3A4 and CYP3A5, respectively. Along with its time-, concentration- and cofactor-dependent inhibitory profile, FUT also exhibited several cardinal features that were consistent with mechanism-based inactivation. Moreover, the nature of inactivation was unlikely to be pseudo-irreversible and instead arose from the covalent modification of the P450 apoprotein and/or its heme moiety due to the lack of substantial enzyme activity recovery following dialysis and chemical oxidation as well as the absence of the diagnostic Soret peak in spectral analyses. Finally, utilizing GSH trapping and high-resolution mass spectrometry, we illuminated that while the acrylamide moiety in FUT could nonenzymatically conjugate to GSH via Michael addition, it was not implicated in the covalent inactivation of CYP3A. Rather, we surmised that it likely stemmed from the metabolic activation of its acrylamide covalent warhead to a highly electrophilic epoxide intermediate that could covalently modify CYP3A and culminate in its catalytic inactivation. Significance Statement In this study, we reported for the first time the inactivation of CYP3A by FUT. Furthermore, using FUT as an exemplary targeted covalent inhibitor, our study revealed the propensity for its acrylamide Michael acceptor moiety to be metabolically activated to a highly electrophilic epoxide. Due to the growing resurgence of covalent inhibitors and the well-established toxicological ramifications associated with epoxides, we advocate that closer scrutiny be adopted when profiling the reactive metabolites of compounds possessing an α,β-unsaturated carbonyl scaffold.

Evaluation of a Three-Dimensional Primary Human Hepatocyte Spheroid Model: Adoption and Industrialization for the Enhanced Detection of Drug-Induced Liver Injury

Drug-induced liver injury is a leading cause of compound attrition during both preclinical and clinical drug development, and early strategies are in place to tackle this recurring problem. Human-relevant in vitro models that are more predictive of hepatotoxicity hazard identification, and that could be employed earlier in the drug discovery process, would improve the quality of drug candidate selection and help reduce attrition. We present an evaluation of four human hepatocyte in vitro models of increasing culture complexity (i.e., two-dimensional (2D) HepG2 monolayers, hepatocyte sandwich cultures, three-dimensional (3D) hepatocyte spheroids, and precision-cut liver slices), using the same tool compounds, viability end points, and culture time points. Having established the improved prediction potential of the 3D hepatocyte spheroid model, we describe implementing this model into an industrial screening setting, where the challenge was matching the complexity of the culture system with the scale and throughput required. Following further qualification and miniaturization into a 384-well, high-throughput screening format, data was generated on 199 compounds. This clearly demonstrated the ability to capture a greater number of severe hepatotoxins versus the current routine 2D HepG2 monolayer assay while continuing to flag no false-positive compounds. The industrialization and miniaturization of the 3D hepatocyte spheroid complex in vitro model demonstrates a significant step toward reducing drug attrition and improving the quality and safety of drugs, while retaining the flexibility for future improvements, and has replaced the routine use of the 2D HepG2 monolayer assay at GlaxoSmithKline.

Infigratinib is a Reversible Inhibitor and Mechanism-based Inactivator of Cytochrome P450 3A4

Infigratinib (INF) is a promising selective inhibitor of fibroblast growth factor receptors 1-3 that has recently been accorded both orphan drug designation and priority review status by the U.S Food and Drug Administration for the treatment of advanced cholangiocarcinoma. Its propensity to undergo bioactivation to electrophilic species was recently expounded upon. However, other than causing aberrant idiosyncratic toxicities, these reactive intermediates may elicit mechanism-based inactivation (MBI) of cytochrome P450 enzymes (CYP450). In this study, we investigated the interactions between INF and the most abundant hepatic cytochrome P450 3A4 (CYP3A4). Our findings revealed that apart from being a potent noncompetitive reversible inhibitor of CYP3A4, INF inactivated CYP3A4 in a time-, concentration- and NADPH-dependent manner with K _I, k _inact and partition ratio of 2.45 µM, 0.053 min^-1 and 41 respectively when rivaroxaban was employed as the probe substrate. Co-incubation with testosterone (alternative CYP3A substrate) or ketoconazole (direct CYP3A inhibitor) attenuated the rate of inactivation whereas the inclusion of glutathione and catalase did not confer such protection. The lack of enzyme activity recovery following dialysis for 4 hours and oxidation with potassium ferricyanide, coupled with the absence of the characteristic Soret peak signature collectively substantiated that inactivation of CYP3A4 by INF was not mediated by the formation of quasi-irreversible metabolite-intermediate complexes but rather through irreversible covalent adduction to the prosthetic heme and/or apoprotein. Finally, glutathione trapping and high-resolution mass spectrometry experimental results unravelled two plausible bioactivation mechanisms of INF arising from the generation of a p-benzoquinone diimine and epoxide reactive intermediate. Significance Statement The potential of infigratinib (INF) to cause mechanism-based inactivation (MBI) of CYP3A4 was unknown. We report the reversible noncompetitive inhibition and irreversible covalent MBI of CYP3A4 by INF and proposed two potential bioactivation pathways implicating p-benzoquinone diimine and epoxide reactive intermediates. Findings from this study lay the groundwork for future investigation of clinically-relevant drug-drug interactions between INF and concomitant substrates of CYP3A4.

Mechanism-Based Inactivation of Cytochrome P450 3A4 and 3A5 by the Fibroblast Growth Factor Receptor Inhibitor Erdafitinib

Erdafitinib (ERD) is a first-in-class pan inhibitor of fibroblast growth factor receptor 1-4 that has garnered global regulatory approval for the treatment of advanced or metastatic urothelial carcinoma. Although it has been previously reported that ERD elicits time-dependent inhibition (TDI) of cytochrome P450 (P450) 3A4 (CYP3A4), the exact biochemical nature underpinning this observation remains obfuscated. Moreover, it is also uninterrogated if CYP3A5-its highly homologous counterpart-could be susceptible to such interactions. Mechanism-based inactivation (MBI) of P450 is a unique subset of TDI that hinges on prior bioactivation of the drug to a reactive intermediate and possesses profound clinical and toxicological implications due to its irreversible nature. Here, we investigated and confirmed that ERD inactivated both CYP3A isoforms in a time-, concentration-, and NADPH-dependent manner with K_I, k_inact, and partition ratio of 4.01 and 10.04 μM, 0.120 and 0.045 min^-1, and 32 and 55 for both CYP3A4 and CYP3A5, respectively, when rivaroxaban was employed as the probe substrate. Co-incubation with an alternative substrate or direct inhibitor of CYP3A attenuated the rate of inactivation, whereas the addition of glutathione or catalase did not induce such protection. The lack of enzyme activity recovery following dialysis for 4 h and oxidation with potassium ferricyanide combined with the lack of a Soret peak in spectral scans collectively substantiated that ERD is an irreversible covalent MBI of CYP3A. Finally, glutathione trapping and high-resolution mass spectrometry experiments illuminated a plausible bioactivation mechanism of ERD by CYP3A arising from metabolic epoxidation of its quinoxaline ring.

Binimetinib Is a Potent Reversible and Time-Dependent Inhibitor of Cytochrome P450 1A2

Binimetinib is a selective MEK1/2 inhibitor, which is indicative of melanoma. We aimed to investigate the inhibitory effect of binimetinib on cytochrome P450 using human liver microsomes. Binimetinib was demonstrated to display reversible and time-dependent inhibitory effects on human CYP1A2. Binimetinib can inhibit the activity of phenacetin deethylation with IC₅₀ of 5.6 μM. A 30 min preincubation of binimetinib with NADPH-supplemented human liver microsomes raised a significant left IC₅₀ shift (6.5-fold), from 5.69-0.88 μM. The inactivation parameters K_inact and K_I were 0.063 min^-1 and 15.47 μM, and the half-life of inactivation was 11 min. Glutathione (GSH) and catalase/superoxide exhibited minor or no protective effect on binimetinib-induced enzyme inactivation. Trapping experiment by GSH induced a detection of GSH adduct, of which the formation was believed to be through the oxidation of electron-rich 1,4-benzenediamine to reactive 1,4-diiminoquinone species. Cytochrome P450 3A4, 2C9, and 2D6 were involved in the bioactivation of binimetinib. In conclusion, binimetinib was proven to display reversible and time-dependent inhibitory effect on CYP1A2, which may have implications for the toxicity of binimetinib.

Design and Optimization of an Acyclic Amine Series of TRPV4 Antagonists by Electronic Modulation of Hydrogen Bond Interactions

Investigation of TRPV4 as a potential target for the treatment of pulmonary edema associated with heart failure generated a novel series of acyclic amine inhibitors displaying exceptional potency and PK properties. The series arose through a scaffold hopping approach, which relied on use of an internal H-bond to replace a saturated heterocyclic ring. Optimization of the lead through investigation of both aryl regions revealed approaches to increase potency through substituents believed to enhance separate intramolecular and intermolecular H-bond interactions. A proposed internal H-bond between the amine and neighboring benzenesulfonamide was stabilized by electronically modulating the benzenesulfonamide. In the aryl ether moiety, substituents para to the nitrile demonstrated an electronic effect on TRPV4 recognition. Finally, the acyclic amines inactivated CYP3A4 and this liability was addressed by modifications that sterically preclude formation of a putative metabolic intermediate complex to deliver advanced TRPV4 antagonists as leads for discovery of novel medicines.

Advances in the Study of the Potential Hepatotoxic Components and Mechanism of Polygonum multiflorum

The roots of Polygonum multiflorum (PM) (He Shou Wu in Chinese) are one of the most commonly used tonic traditional Chinese medicines (TCMs) in China. PM is traditionally valued for its antiaging, liver- and kidney-tonifying, and hair-blackening effects. However, an increasing number of hepatotoxicity cases induced by PM attract the attention of scholars worldwide. Thus far, the potential liver injury compounds and the mechanism are still uncertain. The aim of this review is to provide comprehensive information on the potential hepatotoxic components and mechanism of PM based on the scientific literature. Moreover, perspectives for future investigations of hepatotoxic components are discussed. This study will build a new foundation for further study on the hepatotoxic components and mechanism of PM.

Uptake and metabolism of clarithromycin and sulfadiazine in lettuce

Antibiotics are introduced into agricultural fields by the application of manure or biosolids, or via irrigation using reclaimed wastewater. Antibiotics can enter the terrestrial food chains through plant uptake, which forms an alternative pathway for human exposure to antibiotics. However, previous studies mainly focused on detecting residues of the parent antibiotics, while ignoring the identification of antibiotics transformation products in plants. Here, we evaluated the uptake and metabolism of clarithromycin (CLA) and sulfadiazine (SDZ) in lettuce under controlled hydroponic conditions. The antibiotics and their metabolites were identified by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC-QToF-MS/MS) and ultra-performance liquid chromatograph Micromass triple quadrupole mass spectrometry (UPLC-QqQ-MS/MS). The structure of CLA, SDZ and N-acetylated SDZ were confirmed with synthesized standards, verifying the reliability of the identification method. Eight metabolites of CLA and two metabolites of SDZ were detected in both the leaves and roots of lettuce. The metabolites of CLA included phases I and II transformation products, while only phase II metabolites of SDZ were observed in lettuce. The proportion of CLA metabolites was estimated to be greater than 70%, indicating that most of the CLA was metabolized in plant tissues. The proportion of SDZ metabolites was lower than 12% in the leaves and 10% in the roots. Some metabolites might have the ability to increase or acquire antibacterial activity. Therefore, in addition to the parent compounds, metabolites of antibiotics in edible vegetables are also worthy of study for risk assessment and to determine the consequences of long-term exposure.

Glucosylation and Glutathione Conjugation of Chlorpyrifos and Fluopyram Metabolites Using Electrochemistry/Mass Spectrometry

Xenobiotics and their reactive metabolites are conjugated with native biomolecules such as glutathione and glucoside during phase II metabolism. Toxic metabolites are usually detoxified during this step. On the other hand, these reactive species have a potential health impact by disrupting many enzymatic functions. Thus, it is crucial to understand phase II conjugation reactions of xenobiotics in order to address their fate and possible toxicity mechanisms. Additionally, conventional methods (in vivo and in vitro) have limitation due to matrix complexity and time-consuming. Hence, developing fast and matrix-free alternative method is highly demandable. In this work, oxidative phase I metabolites and reactive species of chlorpyrifos (insecticide) and fluopyram (fungicide) were electrochemically produced by using a boron-doped diamond electrode coupled online to electrospray mass spectrometry (ESI-MS). Reactive species of the substrates were trapped by biomolecules (glutathione and glucoside) and phase II conjugative metabolites were identified using liquid chromatography (LC)-MS/MS, and/or Triple time of flight (TripleTOF)-MS. Glutathione conjugates and glucosylation of chlorpyrifos, trichloropyridinol, oxon, and monohydroxyl fluopyram were identified successfully. Glutathione and glucoside were conjugated with chlorpyrifos, trichloropyridinol, and oxon by losing a neutral HCl. In the case of fluopyram, its monohydroxyl metabolite was actively conjugated with both glutathione and glucoside. In summary, seven bioconjugates of CPF and its metabolites and two bioconjugates of fluopyram metabolites were identified using electrochemistry (EC)/MS for the first time in this work. The work could be used as an alternative approach to identify glutathione and glucosylation conjugation reactions of other organic compounds too. It is important, especially to predict phase II conjugation within a short time and matrix-free environment.

4'-Hydroxy-6,7-methylenedioxy-3-methoxyflavone: A novel flavonoid from Dulacia egleri with potential inhibitory activity against cathepsins B and L

A new flavone, 4'-hydroxy-6,7-methylenedioxy-3-methoxyflavone 1, and two other nucleosides, ribavirin 2 and adenosine 3, were isolated from the leaves of Dulacia egleri. The nucleosides were identified by spectroscopic techniques (1D, 2D-NMR) while the structure of the flavonoid was established by 1D, 2D-NMR analysis, including HRESIMS data. The results obtained in the biological assays showed that the compound 1 was able to inhibit cathepsins B and L with IC₅₀ of 14.88 ± 0.18 μM and 3.19 ± 0.07 μM, respectively. The mechanism of inhibition for both enzymes were determined showing to be competitive at cathepsin B with Ki = 12.8 ± 0.6 μM and non-linear non-competitive with positive cooperativity inhibition at cathepsin L with K_i = 322 ± 33 μM, αK_i = 133 ± 15 μM, βK_i = 5.14 ± 0.41 μM and γK_i = 13.2 ± 13 μM.

Cobicistat: Review of a Pharmacokinetic Enhancer for HIV Infection

PURPOSE

This article reviews the clinical pharmacology, pharmacodynamic and pharmacokinetic (PK) properties, clinical efficacy and tolerability, drug interactions, and dosing and administration of cobicistat.

METHODS

Searches of MEDLINE and International Pharmaceutical Abstracts from 1964 to February 2015 were conducted using the search terms cobicistat and GS-9350. Relevant information was extracted from the identified clinical trials and review articles. Abstracts from the Conference on Retroviruses and Opportunistic Infections (2014-2015) and the Interscience Conference on Antimicrobial Agents and Chemotherapy (2013-2014) were also searched.

FINDINGS

Cobicistat is a PK enhancer lacking antiviral activity that, via selective cytochrome P-450 (CYP) 3A inhibition, inhibits the metabolism of certain antiretroviral medications and is used for prolonging their effect. Cobicistat has been studied as a booster of elvitegravir, a second-generation integrase inhibitor, and of the protease inhibitors atazanavir and darunavir. Data on its clinical efficacy and tolerability have been presented in 2 Phase II trials and in 9 Phase III trials, which reported durable efficacy in terms of achievement of sustained suppression of HIV-1 RNA levels to <50 copies/mL for at least 48 weeks. Cobicistat was generally well-tolerated in these studies. Cobicistat may increase serum creatinine levels via the inhibition of proximal renal tubular cell transporters and thus reduce estimated glomerular filtration rate, although it does not appear to affect actual glomerular filtration rate. Given the potent CYP3A inhibition by cobicistat, its coadministration with drugs metabolized by CYP3A may result in increased plasma concentrations of such agents. Moreover, as cobicistat is metabolized predominantly by CYP3A, plasma concentrations may increase or decrease on coadministration with CYP3A inhibitors or inducers, respectively.

IMPLICATIONS

With potent durability through 48 weeks, a tolerability profile comparable to other first- and second-line antiretroviral therapies, and a convenient dosing schedule with low daily pill burden in fixed-dose combination tablets, cobicistat is a potential addition to the management of HIV infection as a PK enhancer. However, the effects of cobicistat on serum creatinine and its considerable drug-interaction potential may warrant additional monitoring.

Mass-spectrometric profiling of porphyrins in complex biological samples with fundamental, toxicological, and pharmacological applications

Rapid, high-throughput, and quantitative evaluations of biological metabolites in complex milieu are increasingly required for biochemical, toxicological, pharmacological, and environmental analyses. They are also essential for the development, testing, and improvement of new commercial chemical products. We demonstrate the application of ultra-high performance liquid chromatography-mass spectrometry (uHPLC-MS), employing an electrospray ionization source and a high accuracy quadrupole time-of-flight mass analyzer, for the identification and quantification of a series of porphyrin derivatives in liver: a matrix of particular relevance in toxicological or pharmacological testing. Exact mass is used to identify and quantify the metabolites. Chromatography enhances sensitivity and alleviates potential saturation issues by fanning out the contents of a complex sample before their injection into the spectrometer, but is not strictly necessary for the analysis. Extraction and sample treatment procedures are evaluated and matrix effects discussed. Using this method, the known mechanism of action of a well-characterized porphyrinogenic agent was verified in liver extracts from treated rats. The method was also validated for use with bacterial cells. This exact-mass method uses workhorse instruments available in many laboratories, providing a highly flexible alternative to existing HPLC- and MS/MS-based approaches for the simultaneous analysis of multiple compounds in biological media.

Mechanism-based inactivation of CYP450 enzymes: a case study of lapatinib

Mechanism-based inactivation (MBI) of CYP450 enzymes is a unique form of inhibition in which the enzymatic machinery of the victim is responsible for generation of the reactive metabolite. This precondition sets up a time-dependency for the inactivation process, a hallmark feature that characterizes all MBI. Yet, MBI itself is a complex biochemical phenomenon that operates in different modes, namely, covalent binding to apoprotein, covalent binding of the porphyrin group and also complexation of the catalytic iron. Using lapatinib as a recent example of toxicological interest, we present an example of a mixed-function MBI that can confound clinical drug-drug interactions manifestation. Lapatinib exhibits both covalent binding to the apoprotein and formation of a metabolite-intermediate complex in an enzyme-selective manner (CYP3A4 versus CYP3A5), each with different reactive metabolites. The clinical implication of this effect is also contingent upon genetic polymorphisms of the enzyme involved as well as the co-administration of other substrates, inhibitors or inducers, culminating in drug-drug interactions. This understanding recapitulates the importance of applying isoform-specific mechanistic investigations to develop customized strategies to manage such outcomes.

Resistance to antifungals that target CYP51

Fungal diseases are an increasing global burden. Fungi are now recognised to kill more people annually than malaria, whilst in agriculture, fungi threaten crop yields and food security. Azole resistance, mediated by several mechanisms including point mutations in the target enzyme (CYP51), is increasing through selection pressure as a result of widespread use of triazole fungicides in agriculture and triazole antifungal drugs in the clinic. Mutations similar to those seen in clinical isolates as long ago as the 1990s in Candida albicans and later in Aspergillus fumigatus have been identified in agriculturally important fungal species and also wider combinations of point mutations. Recently, evidence that mutations originate in the field and now appear in clinical infections has been suggested. This situation is likely to increase in prevalence as triazole fungicide use continues to rise. Here, we review the progress made in understanding azole resistance found amongst clinically and agriculturally important fungal species focussing on resistance mechanisms associated with CYP51. Biochemical characterisation of wild-type and mutant CYP51 enzymes through ligand binding studies and azole IC50 determinations is an important tool for understanding azole susceptibility and can be used in conjunction with microbiological methods (MIC50 values), molecular biological studies (site-directed mutagenesis) and protein modelling studies to inform future antifungal development with increased specificity for the target enzyme over the host homologue.

The ruthenium complexes cis-(dichloro)tetramineruthenium(III) chloride and cis-tetraammine(oxalato)ruthenium(III) dithionate overcome resistance inducing apoptosis on human lung carcinoma cells (A549)

Lung cancer is one of the leading causes of death in the world, and non-small cell lung carcinoma accounts for approximately 75-85 % of all lung cancers. In the present work, we studied the antitumor activity of the compound cis-(dichloro)tetramineruthenium(III) chloride {cis-[RuCl2(NH3)4]Cl} against human lung carcinoma tumor cell line A549. The present study aimed to investigate the relationship between the expression of MDR1 and CYP450 genes in human lung carcinoma cell lines A549 treated with cisCarboPt, cisCRu(III) and cisDRu(III). The ruthenium-based coordinated complexes presented low cytotoxic and antiproliferative activities, with high IC50 values, 196 (±15.49), 472 (±20.29) and 175 (±1.41) for cisCarboPt, cisCRu(III) and cisDRu(III), respectively. The tested compounds induced apoptosis in A549 tumor cells as evidenced by caspase 3 activation, but only at high concentrations. Results also revealed that the amplification of P-gp gene is greater in A549 cells exposed to cisCarboPt and cisCRu(III) than cisDRu(III). Taken together all these results strongly demonstrate that MDR-1 over-expression in A549 cells could be associated to a MDR phenotype of these cells and moreover, it is also contributing to the platinum, and structurally-related compound, resistance in these cells. The identification and characterization of novel mechanisms of drug resistance will enable the development of a new generation of anti-cancer drugs that increase cancer sensitivity and/or represent more effective chemotherapeutic agents.

Time-dependent inhibition of CYP3A4 by sertraline, a selective serotonin reuptake inhibitor

Drug-drug interactions associated with selective serotonin reuptake inhibitors (SSRIs) are widely known. A major interaction by SSRIs is the inhibition of cytochrome P450 (P450)-mediated hepatic drug metabolism. The SSRI, sertraline, is also reported to increase the blood concentration of co-administered drugs. The potency of sertraline directly to inhibit hepatic drug metabolism is relatively weak compared with the other SSRIs, implying that additional mechanisms are involved in the interactions. The study examined whether sertraline produces time-dependent inhibition of CYP3A4 and/or other P450 enzymes. Incubation of human liver microsomes with sertraline in the presence of NADPH resulted in marked decreases in testosterone 6β-hydroxylation activities, indicating that sertraline metabolism leads to CYP3A4 inactivation. This inactivation required NADPH and was not protected by glutathione. No significant inactivation was observed for other P450 enzymes. Spectroscopic evaluation revealed that microsomes with and without sertraline in the presence of NADPH gave a Soret peak at 455 nm, suggesting the formation of metabolic intermediate (MI) complexes of sertraline metabolite(s) with the reduced form of P450. This is the first report indicating that sertraline produced time-dependent inhibition of CYP3A4, which may be associated with MI complex formation.

Preclinical strategy to reduce clinical hepatotoxicity using in vitro bioactivation data for >200 compounds

Drug-induced liver injury is the most common cause of market withdrawal of pharmaceuticals, and thus, there is considerable need for better prediction models for DILI early in drug discovery. We present a study involving 223 marketed drugs (51% associated with clinical hepatotoxicity; 49% non-hepatotoxic) to assess the concordance of in vitro bioactivation data with clinical hepatotoxicity and have used these data to develop a decision tree to help reduce late-stage candidate attrition. Data to assess P450 metabolism-dependent inhibition (MDI) for all common drug-metabolizing P450 enzymes were generated for 179 of these compounds, GSH adduct data generated for 190 compounds, covalent binding data obtained for 53 compounds, and clinical dose data obtained for all compounds. Individual data for all 223 compounds are presented here and interrogated to determine what level of an alert to consider termination of a compound. The analysis showed that 76% of drugs with a daily dose of <100 mg were non-hepatotoxic (p < 0.0001). Drugs with a daily dose of ≥100 mg or with GSH adduct formation, marked P450 MDI, or covalent binding ≥200 pmol eq/mg protein tended to be hepatotoxic (∼ 65% in each case). Combining dose with each bioactivation assay increased this association significantly (80-100%, p < 0.0001). These analyses were then used to develop the decision tree and the tree tested using 196 of the compounds with sufficient data (49% hepatotoxic; 51% non-hepatotoxic). The results of these outcome analyses demonstrated the utility of the tree in selectively terminating hepatotoxic compounds early; 45% of the hepatotoxic compounds evaluated using the tree were recommended for termination before candidate selection, whereas only 10% of the non-hepatotoxic compounds were recommended for termination. An independent set of 10 GSK compounds with known clinical hepatotoxicity status were also assessed using the tree, with similar results.

Identification of the unknown transformation products derived from clarithromycin and carbamazepine using liquid chromatography/high-resolution mass spectrometry

RATIONALE

A comprehensive study of the environmental fate of pollutants is more and more required, above all on new contaminants, i.e. pharmaceuticals. As high-resolution mass spectrometry (HRMS(n)) may be a suitable analytical approach for characterization of unknown compounds, its performance was evaluated in this study.

METHODS

The analyses were carried out using liquid chromatography (LC) (electrospray ionization (ESI) in positive mode) coupled with a LTQ-Orbitrap analyzer. High-resolution mass spectrometry was employed to assess the evolution of the drug transformation processes over time; accurate masses of protonated molecular ions and sequential product ions were reported with an error below 5 millimass units, which guarantee the correct assignment of their molecular formula in all cases, while their MS(2) and MS(3) spectra showed several structurally diagnostic ions that allowed characterization of the different transformation products (TPs) and to distinguish the isobaric species.

RESULTS

The simulation of phototransformation occurring in the aquatic environment and identification of biotic and abiotic transformation products of the two pharmaceuticals were carried out in heterogeneous photocatalysis using titanium dioxide, aimed to recreate conditions similar to those found in the environmental samples. Twenty-eight main species were identified after carbamazepine transformation and twenty-nine for clarithromycin.

CONCLUSIONS

This study demonstrates that HRMS, combined with LC, is a technique able to play a key role in the evaluation of the environmental fate of pollutants and allows elucidation of the transformation pathways followed by the two drugs.

A dual function of the furanocoumarin chalepensin in inhibiting Cyp2a and inducing Cyp2b in mice: the protein stabilization and receptor-mediated activation

Chalepensin, a furanocoumarin, is present in several medicinal Rutaceae plants and causes a mechanism-based inhibition of human and mouse cytochrome P450 (P450, CYP) 2A in vitro. To address the in vivo effect, we investigated the effects of chalepensin on multiple hepatic P450 enzymes in C57BL/6JNarl mice. Oral administration of 10 mg/kg chalepensin to mice for 7 days significantly decreased hepatic coumarin 7-hydroxylation (Cyp2a) and increased 7-pentoxyresorufin O-dealkylation (Cyp2b) activities, whereas activities of Cyp1a, Cyp2c, Cyp2e1, and Cyp3a were not affected. Without affecting its mRNA level, the decreased Cyp2a activity was accompanied by an increase in the immunodetected Cyp2a5 protein level. In chalepensin-treated mice, microsomal Cyp2a5 was less susceptible to ATP-fortified cytosolic degradation than that in control mice, resulting in the elevated protein level. The in vitro inactivation through NADPH-fortified pre-incubation with chalepensin also protected microsomal Cyp2a5 against protein degradation. Using cell-based reporter systems, chalepensin at a concentration near unbound plasma concentration activated mouse constitutive androstane receptor (CAR), in agreement with the observed induction of Cyp2b. These findings revealed that suicidal inhibition of Cyp2a5 and the CAR-mediated Cyp2b9/10 induction concurrently occurred in chalepensin-treated mice.

Resveratrol potentiates cytochrome P450 2 d22-mediated neuroprotection in maneb- and paraquat-induced parkinsonism in the mouse

A strong association between polymorphisms of the cytochrome P450 (CYP/Cyp) 2D6 gene and risk to Parkinson's disease (PD) is well established. The present study investigated the neuroprotective potential of Cyp2d22, a mouse ortholog of human CYP2D6, in maneb- and paraquat-induced parkinsonism and the mechanisms involved therein along with the effects of resveratrol on various parameters associated with Cyp2d22-mediated neuroprotection. The animals were treated intraperitoneally with resveratrol (10mg/kg, daily) and paraquat (10mg/kg) alone or in combination with maneb (30 mg/kg), twice a week, for 9 weeks, along with their respective controls. The subsets of animals were also treated intraperitoneally with a Cyp2d22 inhibitor, ketoconazole (100mg/kg, daily). Maneb and paraquat reduced Cyp2d22 and vesicular monoamine transporter type 2 (VMAT-2) expressions, the number of tyrosine hydroxylase-positive cells, and dopamine content and increased paraquat accumulation in the nigrostriatal tissues, oxidative stress, microglial activation, neuroinflammation, and apoptosis. Cyp2d22 inhibitor significantly exacerbated all these neurodegenerative indexes. Resveratrol cotreatment, partially but significantly, ameliorated the neurodegenerative changes by altering Cyp2d22 expression and paraquat accumulation. The results obtained in the study demonstrate that Cyp2d22 offers neuroprotection in maneb- and paraquat-induced dopaminergic neurodegeneration and resveratrol enhances its neuroprotective credentials by influencing Cyp2d22 expression and paraquat accumulation.

Penta-O-galloyl-β-D-glucose attenuates cisplatin-induced nephrotoxicity via reactive oxygen species reduction in renal epithelial cells and enhances antitumor activity in Caki-2 renal cancer cells

Cisplatin shows limited therapeutic efficacy due to serious side effects such as nephrotoxicity and hepatotoxicity. In the present study, we demonstrate that 1,2,3,4,6-penta-O-galloyl-β-d-glucose (PGG) has protective effects against cisplatin-induced cytotoxicity and apoptosis in normal human primary renal epithelial cells (HRCs) while showing synergistic effect against cisplatin-induced cell death in human Caki-2 renal cancer cells. PGG significantly blocked cisplatin-mediated cytotoxicity and reduced cisplatin-induced sub-G1 accumulation in HRCs. Consistently, PGG reduced the number of apoptotic cell populations by TdT-mediated dUTP nick end labeling (TUNEL) and Live/Dead assays in cisplatin-treated HRCs. Furthermore, PGG suppressed PARP cleavage and caspase-3 activation, cytochrome c release, up-regulation of bax and p53 in cisplatin-treated HRCs. Moreover, PGG attenuated reactive oxygen species (ROS) production mediated by cisplatin treatment, suggesting that PGG prevented cisplatin-induced apoptosis by inhibiting ROS generation in HRCs. Notably, PGG significantly enhanced cytotoxicity and PARP cleavage in cisplatin-treated Caki-2 renal cancer cells. Combination Index (CI) revealed synergism between PGG and cisplatin in Caki-2 cells. Taken together, our findings suggest the dual effects of PGG as a protective supplement against cisplatin-induced toxicity in normal renal cells and a combination chemotherapeutic drug with cisplatin in renal cancer cells.

Discovery of common urinary biomarkers for hepatotoxicity induced by carbon tetrachloride, acetaminophen and methotrexate by mass spectrometry-based metabolomics

Liver toxicity represents an important healthcare issue because it causes significant morbidity and mortality and can be difficult to predict before symptoms appear owing to drug therapy or exposure to toxicants. Using metabolomic techniques, we discovered common biomarkers for the prediction of hepatotoxicity in rat urine using mass spectrometry. For this purpose, liver toxicity was induced by 5 days of oral administration of carbon tetrachloride (1 ml kg(-1) per day), acetaminophen (1000 mg kg(-1) per day) and methotrexate (50 mg kg(-1) per day). Serum levels of alkaline phosphatase aspartate aminotransferase, alanine aminotransferase and histopathology in liver tissue were then checked to demonstrate liver toxicity. Global metabolic profiling with UPLC-TOF-MS (ultraperformance liquid chromatography-mass spectrometry), multivariate analysis (partial least square-discriminant analysis, hierarchical analysis) and database searching were performed to discover common biomarkers for liver toxicity induced by these three compounds. Urinary concentrations of the newly discovered biomarkers were then quantified to confirm them as biomarkers of hepatotoxicity with targeted metabolic profiling using GC (gas chromatography)-MS and CE (capillary electrophoresis)-MS. In the results, steroids, amino acids and bile acids were metabolically changed between the control and drug-treated groups in global metabolic profiling; 11β-hydroxyandrosterone, epiandrosterone, estrone, 11-dehydrocorticosterone, glycine, alanine, valine, leucine, dl-ornithine, 3-methylhistidine, cholic acid and lithocholic acid were selected as liver toxicity biomarkers after performing targeted metabolic profiling. In conclusion, we discovered metabolite biomarkers belonging to three different metabolic pathways to check for liver toxicity with mass spectrometry from a metabolomics study that could be used to evaluate hepatotoxicity induced by drugs or other toxic compounds.

Metabolic intermediate complex formation of human cytochrome P450 3A4 by lapatinib

Lapatinib, an oral breast cancer drug, has recently been reported to be a mechanism-based inactivator of cytochrome P450 (P450) 3A4 and also an idiosyncratic hepatotoxicant. It was suggested that formation of a reactive quinoneimine metabolite was involved in mechanism-based inactivation (MBI) and/or hepatotoxicity. We investigated the mechanism of MBI of P450 3A4 by lapatinib. Liquid chromatography-mass spectrometry analysis of P450 3A4 after incubation with lapatinib did not show any peak corresponding to irreversible modifications. The enzymatic activity inactivated by lapatinib was completely restored by the addition of potassium ferricyanide. These results indicate that the mechanism of MBI by lapatinib is quasi-irreversible and mediated via metabolic intermediate complex (MI complex) formation. This finding was verified by the increase in a signature Soret absorbance at approximately 455 nm. Two amine oxidation products of the metabolism of lapatinib by P450 3A4 were characterized: N-hydroxy lapatinib (M3) and the oxime form of N-dealkylated lapatinib (M2), suggesting that a nitroso or another related intermediate generated from M3 is involved in MI complex formation. In contrast, P450 3A5 was much less susceptible to MBI by lapatinib via MI complex formation than P450 3A4. In addition, P450 3A5 had a significantly lower ability than 3A4 to generate M3, consistent with N-hydroxylation as the initial step in the pathway to MI complex formation. In conclusion, our results demonstrate that the primary mechanism for MBI of P450 3A4 by lapatinib is not irreversible modification by the quinoneimine metabolite, but quasi-irreversible MI complex formation mediated via oxidation of the secondary amine group of lapatinib.

Decursin prevents cisplatin-induced apoptosis via the enhancement of antioxidant enzymes in human renal epithelial cells

Adverse effects, nephrotoxicity and hepatotoxicity, of anticancer drugs such as cisplatin have limited the usage for cancer therapy. Therefore, development or identification of supplement agents in anticancer drugs is attractive to reduce side effects and enhance antitumor activity. Here, we found that decursin isolated from Angelica gigas showed protective effects of cisplatin-induced damage in normal human primary renal epithelial cells (HRCs). We found that decursin significantly blocked cisplatin-induced cytotoxicity by 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assay in HRCs. Further, we found that decursin inhibited sub-G1 and cell death by suppression of cleavage of caspase-3, -9 and poly(ADP-ribose) polymerase (PARP) induced by cisplatin treatment in HRCs. Importantly, decursin effectively restored the activities of Cu/Zn superoxide dismutase (SOD), catalase and glutathione peroxidase in cisplatin-treated HRCs. Taken together, our findings demonstrate that decurcin prevents cisplatin-induced cytotoxicity and apoptosis through the activation of antioxidant enzymes in HRCs and suggest further that combination of decursin might suppressed adverse effects of anticancer drugs in cancer patients.

Metabolism considerations for kinase inhibitors in cancer treatment

IMPORTANCE OF THE FIELD

A concerted effort by the pharmaceutical industry over the last decade has led to the successful clinical development of protein kinase inhibitors as effective targeted therapies for certain cancers.

AREAS COVERED IN THIS REVIEW

This review details eight small molecule kinase inhibitors that have been approved for the treatment of cancer in either the US or Europe as of March 2010: imatinib, sorafenib, gefitinib, erlotinib, dasatinib, lapatinib, sunitinib and nilotinib. These eight compounds vary from the relatively specific inhibitor lapatinib to the more promiscuous kinase inhibitors dasatinib and sunitinib.

WHAT THE READER WILL GAIN

A brief discussion on the biology of each inhibitor, selectivity over other kinases and toxicity is provided. A more detailed discussion on the metabolism, drug transporters, drug-drug interactions and possible roles of metabolism in compound toxicity is provided for each compound.

TAKE HOME MESSAGE

The majority of the currently approved kinase inhibitors is heavily influenced by drug transporters and significantly affected by CYP3A4 inhibitors/inducers. At least three, gefitinib, erlotinib and dasatinib, are metabolized to form reactive metabolites capable of covalently-binding biomolecules.

Building a tiered approach to in vitro predictive toxicity screening: a focus on assays with in vivo relevance

One of the greatest challenges facing the pharmaceutical industry today is the failure of promising new drug candidates due to unanticipated adverse effects discovered during preclinical animal safety studies and clinical trials. Late stage attrition increases the time required to bring a new drug to market, inflates development costs, and represents a major source of inefficiency in the drug discovery/development process. It is generally recognized that early evaluation of new drug candidates is necessary to improve the process. Building in vitro data sets that can accurately predict adverse effects in vivo would allow compounds with high risk profiles to be deprioritized, while those that possess the requisite drug attributes and a lower risk profile are brought forward. In vitro cytotoxicity assays have been used for decades as a tool to understand hypotheses driven questions regarding mechanisms of toxicity. However, when used in a prospective manner, they have not been highly predictive of in vivo toxicity. Therefore, the issue may not be how to collect in vitro toxicity data, but rather how to translate in vitro toxicity data into meaningful in vivo effects. This review will focus on the development of an in vitro toxicity screening strategy that is based on a tiered approach to data collection combined with data interpretation.

Approaches for minimizing metabolic activation of new drug candidates in drug discovery

A large body of circumstantial evidence suggests that metabolic activation of drug candidates to chemically reactive electrophilic metabolites that are capable of covalently modifying cellular macromolecules may result in acute and/or immune system-mediated idiosyncratic toxicities in humans. Thus, minimizing the potential for metabolic activation of new drug candidates during the drug discovery and lead optimization stage represents a prudent strategy to help discover and develop the next generation of safe and effective therapeutic agents. In the present chapter, we discuss the scientific methodologies that currently are available to industrial pharmaceutical scientists for assessing and minimizing metabolic activation during drug discovery, their attributes and limitations, and future scientific directions that have the potential to help advance progress in this field. We also propose a roadmap that should help utilize the armamentarium of available scientific tools in a logical way and contribute to addressing metabolic activation issues in the drug discovery-setting in a rapid, scientifically appropriate, and resource-conscious manner.

Biochemical mechanisms in drug-induced liver injury: Certainties and doubts

Drug-induced liver injury is a significant and still unresolved clinical problem. Limitations to knowledge about the mechanisms of toxicity render incomplete the detection of hepatotoxic potential during preclinical development. Several xenobiotics are lipophilic substances and their transformation into hydrophilic compounds by the cytochrome P-450 system results in production of toxic metabolites. Aging, preexisting liver disease, enzyme induction or inhibition, genetic variances, local O₂ supply and, above all, the intrinsic molecular properties of the drug may affect this process. Necrotic death follows antioxidant consumption and oxidation of intracellular proteins, which determine increased permeability of mitochondrial membranes, loss of potential, decreased ATP synthesis, inhibition of Ca²⁺-dependent ATPase, reduced capability to sequester Ca²⁺ within mitochondria, and membrane bleb formation. Conversely, activation of nucleases and energetic participation of mitochondria are the main intracellular mechanisms that lead to apoptosis. Non-parenchymal hepatic cells are inducers of hepatocellular injury and targets for damage. Activation of the immune system promotes idiosyncratic reactions that result in hepatic necrosis or cholestasis, in which different HLA genotypes might play a major role. This review focuses on current knowledge of the mechanisms of drug-induced liver injury and recent advances on newly discovered mechanisms of liver damage. Future perspectives including new frontiers for research are discussed.