A model of isoniazid-induced hepatotoxicity in rabbits

Hepatoprotective potential of diosmin against hepatotoxic effect of isoniazid and rifampin in wistar rats

OBJECTIVE

The treatment of tuberculosis with isoniazid and rifampin is associated with hepatocellular damage. Therefore, the study was designed to evaluate the hepatoprotective potential of diosmin against hepatotoxic effect of isoniazid and rifampin in Wistar rats.

METHODS

Hepatotoxicity was induced by administering isoniazid and rifampin (100 mg/kg), whereas diosmin was given as treatment control. Markers of liver function (ALT, AST, ALP and bilirubin), inflammatory cytokines (TNFα, IL-6 and IL-1β), apoptosis (caspase-3), oxidative stress parameters (LPO, GSH, CAT and SOD) and histological changes in liver were assessed in normal, hepatotoxic control and treatment groups.

RESULTS

The administration of isoniazid and rifampin significantly increased markers of liver dysfunction (ALT, AST, ALP and bilirubin), cytokines (TNFα, IL-6 and IL-1β) and apoptosis (caspase-3). However, daily dosing of diosmin significantly reduced these markers of liver dysfunction, inflammatory cytokines and apoptosis to near normal levels. Additionally, markers of hepatocellular oxidative stress parameters were significantly altered as evident from increased LPO level and decreased endogenous antioxidants such as GSH, SOD and CAT in isoniazid-and rifampin-treated hepatotoxic group. It was observed that diosmin treatment reduced high levels of LPO and demonstrated significant improvement in antioxidant levels. Histological studies of liver also supported our biochemical findings, which are also manifested as diosmin treatment exhibited protection against hepatocellular degeneration and inflammation.

CONCLUSION

Results of the present study demonstrate hepatoprotective potential of diosmin against isoniazid-and rifampin-treated hepatotoxicity. Thus, we conclude that diosmin may be used along with anti-tubercular drugs (isoniazid and rifampin) in tuberculosis patients to overcome their hepatotoxic adverse effect.

Exposure to Fluoride induces apoptosis in liver of ducks by regulating Cyt-C/Caspase 3/9 signaling pathway

Fluorine being a well-known and essential element for normal physiological functions of tissues of different organisms is frequently used for growth and development of body. The mechanisms of adverse and injurious impacts of fluoride are not clear and still are under debate. Therefore, this study was executed to ascertain the potential mechanisms of sodium fluoride in liver tissues of ducks. For this purpose, a total of 14 ducks were randomly divided and kept in two groups including control group and sodium fluoride treated group. The ducks in control group were fed with normal diet while the ducks in other group were exposed to sodium fluoride (750 mg/kg) for 28 days. The results showed that exposure to sodium fluoride induced deleterious effects in different liver tissues of ducks. The results indicated that mRNA levels of Cas-3, Cas-9, p53, Apaf-1, Bax and Cyt-c were increased in treated ducks with significantly higher mRNA level of Cas-9 and lower levels of the mRNA level of Bcl-2 as compared to untreated control group (P < 0.01). The results showed that protein expression levels of Bax and p53 were increased while protein expression level of Bcl-2 was reduced in treated ducks. No difference was observed in protein expression level of Cas-3 between treated and untreated ducks. The results of this study suggest that sodium fluoride damages the normal structure of liver and induces abnormal process of apoptosis in hepatocyte, which provide a new idea for elucidating the mechanisms of sodium fluoride induced hepatotoxicity in ducks.

Isoniazid causes heart looping disorder in zebrafish embryos by the induction of oxidative stress

Background

The cardiotoxicity of isoniazid on zebrafish embryos and its underlying mechanism is unclear.

Methods

Here, we exposed zebrafish embryos at 4 h post-fertilization to different levels of isoniazid and recorded the morphology and number of malformed and dead embryos under the microscope.

Results

The high concentration of isoniazid group showed more malformed and dead embryos than the low concentration of isoniazid group and control group. The morphology of the heart and its alteration were visualized using transgenic zebrafish (cmlc2: GFP) and confirmed by in situ hybridization. The negative effects of isoniazid on the developing heart were characterized by lower heart rate and more heart looping disorders. Mechanistically, PCR showed decreased expression of heart-specific transcription factors when exposed to isoniazid. Oxidative stress was induced by isoniazid in cardiomyocytes, mediated by decreased activities of catalase and superoxide dismutase, which were rescued by scavengers of reactive oxygen species.

Conclusion

In conclusion, this study demonstrated that isoniazid led to heart looping disturbance by the downregulation of cardiac-specific transcription factors and induction of cardiomyocyte apoptosis.

Investigations on the Influence of Zidovudine in the Pharmacokinetics of Isoniazid and Its Hepatotoxic Metabolites in Rats

The HIV-infected patients are co-infected with many bacterial infections in which tuberculosis is most common found worldwide. These patients are often administered with combined therapy of anti-retroviral and anti-tubercular drugs which leads to several complications including hepatotoxicity or adverse drug interactions. The drug-drug interactions between the anti-retroviral and anti-tubercular drugs are not clearly defined and hence, this study was conducted to evaluate the pharmacokinetic drug-drug interactions of Zidovudine (AZT) with Isoniazid (INH) and its hepatotoxic metabolites. Seventy two rats were randomly divided into two major groups with their sub-groups each comprising 6 animals. The Group I received INH alone at a dose of 25 mg/kg; b.w and Group II received AZT (50 mg/kg; b.w) along with INH orally. Pharmacokinetic studies of INH and its metabolites i.e., acetyl hydrazine (ACHY) and hydrazine (HYD) shows that INH and ACHY attains maximum plasma concentration ( C_max) within 30 minutes and HYD attains C_max at 1 hour after INH administration and all these analytes disappear from plasma within 4 hours. Pharmacokinetic studies also revealed that AZT treatment did not showed any drug-drug interactions and have no effect on the T_1/2, plasma clearance, AUC, C_max and T_max of INH and its hepatotoxic metabolites.

Evaluation of ameliorative ability of Silibinin against zidovudine and isoniazid-induced hepatotoxicity and hyperlipidaemia in rats: Role of Silibinin in Phase I and II drug metabolism

HIV/AIDS patients have suppressed immune system, making them vulnerable to many opportunistic infections including tuberculosis (TB). The patients who are co-infected with TB undergo combined regimens with anti-retroviral drugs such as zidovudine (AZT) and anti-tubercular drug such as isoniazid (INH) for therapy leading to hepatotoxicty. Silibinin (SBN), extracted from Silybum marianum commonly called as "Milk thistle" is used against several drugs-induced hepatotoxicity. The present study evaluates the ameliorative effect of SBN against AZT alone, INH alone, and INH + AZT-induced toxic insults to liver of rats. Wistar albino rats (n = 6/groups) were given INH and AZT (25 and 50 mg mg/kg b.w.) respectively either alone or in combination for a sub-chronic period of 45 days orally. Another group of rats received SBN (100 mg/kg b.w.) along with INH and AZT. The group that received propylene glycol served as control. AZT alone, INH alone and INH + AZT treatments showed parenchymal cell injury and cholestasis by highly significant increase in the activities of marker enzymes (aspartate and alanine transaminase, alkaline phosphatase, argino succinic acid lyase), bilirubin and protein. The presence of hyperlipidaemia was observed by analyzing lipid profiles in serum/liver/adipose tissue, gene expression (RT-PCR) of Phase-I and II metabolizing enzymes and western blot. Transmission electron microscopy study also revealed large vacuoles with membraneous debri, pleomorphic mitochondria, disruption of endoplasmic reticulum, presence of lipid droplets, breakage in cellular and nuclear membrane. SBN simultaneous treatment showed ameliorative effect against INH + AZT-induced hepatotoxicity and hyperlipidemia in rats.

Isoniazid metabolism and hepatotoxicity

Isoniazid (INH) is highly effective for the management of tuberculosis. However, it can cause liver injury and even liver failure. INH metabolism has been thought to be associated with INH-induced liver injury. This review summarized the metabolic pathways of INH and discussed their associations with INH-induced liver injury.

Role of Inflammatory and Oxidative Stress, Cytochrome P450 2E1, and Bile Acid Disturbance in Rat Liver Injury Induced by Isoniazid and Lipopolysaccharide Cotreatment

Isoniazid (INH) remains the core drug in tuberculosis management, but serious hepatotoxicity and potentially fatal liver injury continue to accompany INH consumption. Among numerous theories that have been established to explain INH-induced liver injury, an inflammatory stress theory has recently been widely used to explain the idiosyncrasy. Inflammatory stress usually sensitizes tissues to a drug's toxic consequences. Therefore, the present study was conducted to verify whether bacterial lipopolysaccharide (LPS)-induced inflammation may have a role in enhancing INH hepatotoxicity. While single INH or LPS administration showed no major toxicity signs, INH-LPS cotreatment intensified liver toxicity. Both blood biomarkers and histological evaluations clearly showed positive signs of severe liver damage accompanied by massive necrosis, inflammatory infiltration, and hepatic steatosis. Furthermore, elevated serum levels of bile acid associated with the repression of bile acid synthesis and transport regulatory parameters were observed. Moreover, the principal impact of cytochrome P450 2E1 (CYP2E1) on INH toxicity could be anticipated, as its protein expression showed enormous increases in INH-LPS-cotreated animals. Furthermore, the crucial role of CYP2E1 in the production of reactive oxygen species (ROS) was clearly obvious in the repression of hepatic antioxidant parameters. In summary, these results confirmed that this LPS-induced inflammation model might prove valuable in revealing the hepatotoxic mechanisms of INH and the crucial role played by CYP2E1 in the initiation and propagation of INH-induced liver damage, information which could be very useful to clinicians in understanding the pathogenesis of drug-induced liver injury.

AH, Taye N, Mogare DG, Chattopadhyay S, Das A. Specific receptor for hydrazine: mapping the in situ release of hydrazine in live cells and in an in vitro enzymatic assay

New chemodosimetric reagent for the specific detection of hydrazine in physiological conditions as well as for the mapping of its in situ generation in live Hct116 and HepG2 cells by enzymatic transformations.

The effect of ageing on isoniazid pharmacokinetics and hepatotoxicity in Fischer 344 rats

Isoniazid is the first-line treatment for tuberculosis; however, its use is limited by hepatotoxicity. Age-related differences in isoniazid pharmacokinetics and hepatotoxicity are uncertain. We aimed to investigate these in young (3 ± 0 months, n = 26) and old (23.0 ± 0.2 months, n = 27) male Fischer 344 rats following a low- or high-dose toxic regimen of isoniazid or vehicle (4 doses/day over 2 days; low: 100, 75, 75, 75 mg/kg; high: 150, 105, 105, 105 mg/kg i.p. every 3 h). Fifteen hours after the last dose, animals were euthanized and sera and livers were prepared for analysis. Isoniazid treatment increased serum hepatotoxicity markers (alanine and aspartate transaminase) in young animals but not in old animals, and only reached significance with the high dose in young animals. Isoniazid treatment caused a trend towards an increase in necrosis in young animals with both doses. In contrast, microvesicular steatosis was increased in old isoniazid-treated animals, reaching significance only with the low dose (steatosis prevalence in old: vehicle 1/9, isoniazid 4/5; P < 0.05). Among isoniazid-treated animals, concentrations of toxic intermediates acetylhydrazine and hydrazine were higher in old than young animals (P < 0.05). With both doses, hepatic cytochrome P450 2E1 activity was higher in young animals compared with old (P < 0.05). There were no other age effects seen on any of the other measured enzymes involved in isoniazid metabolism (N-acetyl transferase, amidase, glutathione-S-transferase). These results show age-related changes in isoniazid pharmacokinetics may contribute towards differential patterns of toxicity and confirm that standard hepatotoxicity markers do not detect isoniazid-induced microvesicular steatosis.

Circadian variation in murine hepatotoxicity to the antituberculosis agent «Isoniazide»

The circadian time is an important process affecting both pharmacokinetics and pharmacodynamics of drugs. Consequently, the desired and/or undesired effects vary according to the time of drug administration in the 24 h scale. This study investigates whether the toxicity in liver as well as oxidative stress varies according to the circadian dosing-time of isoniazid (INH) in mice. A potentially toxic INH dose (120 mg/kg) was injected by i.p. route to different groups of animals at three different circadian times: 1, 9, and 17 Zeitgeber time (ZT). INH administration at 1 ZT resulted in a maximum hepatotoxicity assessed by the significant increase in both serum transaminase (ALAT: alanine aminotransferase) and (ASAT: aspartate aminotransferase) and antioxidant enzyme activities (catalase: CAT and superoxide dismutase: SOD). The highest malondialdehyde (MDA) level indicating an induction of lipid peroxidation resulting in oxidative damage was also observed at 1 ZT. Liver histopathology from INH groups at 9 ZT and at 1 ZT showed moderate to severe cytoplasma vacuolation, hepatocyte hypertrophy, ballooning, and necrosis. The circadian variation in INH toxicity may help realize a chronotherapy protocol in humans based on the selection of the best time associated to optimal tolerance or least side effects.

Antioxidant, Hepatoprotective Potential and Chemical Profiling of Propolis Ethanolic Extract from Kashmir Himalaya Region Using UHPLC-DAD-QToF-MS

The aim of this study was to examine hepatoprotective effect of ethanolic extract of propolis (KPEt) from Kashmir Himalaya against isoniazid and rifampicin (INH-RIF) induced liver damage in rats. Hepatic cellular injury was initiated by administration of INH-RIF combination (100 mg/kg) intraperitoneal (i.p.) injection for 14 days. We report the protective effects of KPEt against INH-RIF induced liver oxidative stress, inflammation, and enzymatic and nonenzymatic antioxidants. Oral administration of KPEt at both doses (200 and 400 mg/kg body weight) distinctly restricted all modulating oxidative liver injury markers and resulted in the attenuation of INH-RIF arbitrated damage. The free radical scavenging activity of KPEt was evaluated by DPPH, nitric oxide, and superoxide radical scavenging assay. The components present in KPEt identified by ultra high performance liquid chromatography diode array detector time of flight-mass spectroscopy (UHPLC-DAD-QToF-MS) were found to be flavonoids and phenolic acids. The protective efficacy of KPEt is possibly because of free radical scavenging and antioxidant property resulting from the presence of flavonoids and phenolic acids.

Silibinin mitigates zidovudine-induced hepatocellular degenerative changes, oxidative stress and hyperlipidaemia in rats

Prolonged zidovudine (AZT) treatment in HIV-infected and AIDS patients is shown to induce liver toxicity leading to complications. Therapeutic regimen that could encounter this adverse effect is unavailable and management of toxicity is often symptomatic or is limited to withdrawal of therapy. In the present investigation, we evaluated the alleviating properties of silibinin (SBN), a flavanolignan obtained from Silybum marianum against subacute AZT-induced hepatotoxicity and oxidative stress in rats. AZT treatment (50 mg/kg body weight (b.w.) periorally (p.o.), daily for 45 days) caused highly significant increases in alanine transaminase, alkaline phosphatase, argininosuccinic acid lyase and bilirubin in serum. Oxidative stress is shown by a highly significant increase in lipid peroxidase and total carbonyl content and decrease in catalase and protein thiols in the liver tissue. Hyperlipidaemia is indicated by highly significant increase in total lipids and free fatty acid in serum. Evaluation of liver by haematoxylin and eosin staining shows parenchymal cell enlargement, inflammatory changes and increase in sinusoidal spaces. Simultaneous treatment of SBN (100 mg/kg b.w. p.o., daily for 45 days) significantly protected the liver against hepatotoxicity, oxidative stress and hyperlipidaemia induced by AZT, and this alleviating property is attributed to hepatoprotective, membrane-stabilizing, antioxidant and free radical scavenging properties of SBN.

Development and validation of liquid chromatography tandem mass spectrometry method for simultaneous quantification of first line tuberculosis drugs and metabolites in human plasma and its application in clinical study

Rifampicin (RIF) and isoniazid (INH), first line drugs for the treatment of tuberculosis, are known to cause hepatotoxicity as a serious adverse side effect. To further understand the pharmacokinetic parameters of these two drugs, we have developed and validated a rapid, sensitive and selective LC-MS/MS method for simultaneous quantification of RIF, INH and their metabolites 25-desacetylrifampicin (DRIF), acetylisoniazid (AcINH) and isonicotinic acid (INA). Analytes were extracted from 20 μl of plasma using solid-phase extraction (SPE) followed by chromatographic separation on Zorbax SB-Aq column (50 mm × 4.6mm, particle size 5 μm) using stepwise gradient elution of 5mM ammonium acetate and 90% acetonitrile with 0.1% formic acid. Separation of all analytes was achieved in the total run time of 6 min. The analytes were detected under positive ionization mode by multiple reaction monitoring (MRM) and quantification of analytes was performed by using deuterium-labelled internal standard. Excellent linearity (r(2) ≥ 0.995) was achieved for the analytes at different concentration ranges. The method was accurate (90-115%), precise (CV %<14) and specific. Matrix effect was in the range of 93-111% except for INA (40-42%) while recovery from SPE was reproducible (CV %<7.4) in the range of 60-86%. Post-preparative stability (48 h, 6°C autosampler) and freeze-thaw stability (3 cycles) were assessed with mean recovered concentration of >85%. The method was successfully applied to a clinical study of 33 healthy subjects to evaluate the effect of concomitant of INH on the pharmacokinetic parameters of RIF as well as the segregation of the subjects into slow or fast acetylators of INH.

Mechanisms of isoniazid-induced idiosyncratic liver injury: emerging role of mitochondrial stress

Idiosyncratic drug-induced liver injury (DILI) is a significant adverse effect of antitubercular therapy with isoniazid (INH). Although the drug has been used for many decades, the underlying mode of action (both patient-specific and drug-specific mechanisms) leading to DILI are poorly understood. Among the patient-specific determinants of susceptibility to INH-associated DILI, the importance of HLA genetic variants has been increasingly recognized, whereas the role of polymorphisms of drug-metabolizing enzymes (NAT2 and CYP2E1) has become less important and remains controversial. However, these polymorphisms are merely correlative, and other molecular determinants of susceptibility have remained largely unknown. Regarding the drug-specific mechanisms underlying INH-induced liver injury, novel concepts have been emerging. Among these are covalent protein adduct formation via novel reactive intermediates, leading to hapten formation and a potential immune response, and interference with endogenous metabolism. Furthermore, INH and/or INH metabolites (e.g. hydrazine) can cause mitochondrial injury, which can lead to mitochondrial oxidant stress and impairment of energy homeostasis. Recent studies have revealed that underlying impairment of complex I function can trigger massive hepatocellular injury induced by otherwise nontoxic concentrations of INH superimposed on these mitochondrial deficiencies. This review discusses these emerging new paradigms of INH-induced DILI and highlights recent insights into the mechanisms, as well as points to the existing large gaps in our understanding of the pathogenesis.

Lack of liver injury in Wistar rats treated with the combination of isoniazid and rifampicin

Isoniazid (INH) can cause serious idiosyncratic liver injury. An animal model would greatly facilitate mechanistic studies, but it is essential that the mechanism in the model be similar to the liver injury that can occur in humans. We attempted to replicate a previous study in which Wistar rats treated with INH and rifampicin (RMP) developed liver injury, which was promising because of its delayed onset similar to the liver injury that can occur in humans. Wistar rats were treated with either a high dose of INH (150 mg/kg/day) or a combination of INH and RMP (75 mg/kg/day and 50 mg/kg/day, respectively) for up to 4 weeks. However, we did not observe any liver injury or evidence of an inflammatory infiltrate as had been reported; rather, we observed an increase in CTLA4-positive cells in the cervical lymph nodes as well as a decrease in serum CXCL1 and MCP-1. In short, we were unable to reproduce a previously reported model of delayed onset INH-induced liver injury in Wistar rats.

A study of the effect of Nigella sativa (Black seeds) in isoniazid (INH)-induced hepatotoxicity in rabbits

Objective:

To investigate the possibility of hepatoprotective effect of Nigella sativa (NS) in INH-induced hepatotoxicity.

Materials and Methods:

The experiments were carried out on 24 male rabbits. They were divided into 4 groups (6 each); rabbits in group 1 were treated with INH following a standard protocol to induce hepatotoxicity. Rabbits in group 2 received starch. Group 3 received NS 1 g/kg/day before INH treatment. Group 4 rabbits were treated with NS only. Phenobarbital sodium (IP) was given to induce metabolism of INH. INH and NS were given orally. The experiment continued for 12 days; at day 13, animals were sacrificed. Liver function tests, malondialdehyde (MDA) were estimated in serum and in liver homogenates. Liver histopathological examinations were performed.

Results:

Histopathological changes of hepatotoxicity were found in all INH-treated rabbits. The histopathological findings were normal in three rabbits treated with NS before INH, very mild in two, and with moderate changes in one rabbit. Serum alanine aminotransferase (S.ALT) was elevated after INH treatment and returned back to the control value when NS was given before INH. Similar pattern of effect was noticed with serum aspartate aminotransferase (S.AST), S. total bilirubin, S. MDA, and Serum alkaline phosphatase.In liver homogenate, AST, ALT, and MDA were increased with INH treatment compared to the control, then decreased with NS treatment given before INH

Conclusions:

NS has hepatoprotective effects against INH-induced hepatotoxicity in rabbits. NS 1 g/kg proved safe, no adverse effects; no histopathological or biological abnormalities were seen.

Pharmacokinetics, tissue distribution and relative bioavailability of isoniazid-solid lipid nanoparticles

Low levels of isoniazid gain access into plasma following oral administration due to its high aqueous solubility, poor permeability and rapid and extensive hepatic metabolism. Further, a small t(1/2) of 1-4 h indicates its short stay in plasma and the need for repetitive or high doses which may subsequently result in hepatotoxicity and neurotoxicity associated with its use. Isoniazid-solid lipid nanoparticles (SLNs) were prepared to achieve improved bioavailability and prolonged effect, thus minimizing pulsatile plasma concentrations (and associated side effects at peak plasma concentrations). Developed SLNs showed high entrapment efficiency (69%) and small size (d(90) 48.4 nm) such that they are expected to bypass reticulo-endothelial system (RES) pickup resulting in prolonged circulation times and since liver is the major site of metabolism of isoniazid, RES avoidance will reduce its elimination from the body. Single dose (25 mg/kg BW) oral pharmacokinetic studies were performed in plasma and various tissues of rats. A significant improvement (p<0.001) in relative bioavailability in plasma (6 times) and brain (4 times) was observed after administration of isoniazid-SLNs with respect to the free drug solution at the same dose. Insignificant changes in liver concentration coupled with bypass of first pass metabolism and slow release of isoniazid (60%, in 24 h) indicate low incidence of hepatotoxicity. Isoniazid-SLNs showed a 3 times higher LD50.

Hepatotoxic effects of therapies for tuberculosis

Hepatotoxic effects attributable to antituberculosis therapy are considered unique among drug-related liver problems because almost all first-line antituberculosis medications have such adverse effects, which vary in severity according to the drug and the regimen. In addition, all regimens for the treatment of active tuberculosis include a combination of medications that must typically be administered for at least 6 months to ensure complete cure of the disease and to minimize the development of drug-resistant bacterial strains. Hepatotoxic effects are a serious problem in patients who are undergoing treatment for tuberculosis, not only because of the morbidity and mortality they directly cause, but also because the liver symptoms can necessitate interruption of therapy or affect a patient's adherence to it, which can limit the efficacy of the antitubercular regimen.

Isoniazid-induced apoptosis in HepG2 cells: generation of oxidative stress and Bcl-2 down-regulation

Isoniazid (INH) is a first-line antibiotic used in the treatment of infections caused by Mycobacterium tuberculosis. However it has a serious limitation of being hepatotoxic. Delineating the mechanism underlying INH-induced hepatotoxicity may be beneficial in devising ways to counteract its toxic manifestations. Studies in human hepatoma HepG2 cells have indicated that INH exposure causes induction of apoptosis. This study was aimed at identifying the key components/pathways of the INH-induced apoptotic pathway using HepG2 cells. HepG2 cells were exposed to increasing concentrations of INH (6.5, 13, 26, and 52 mM). Hydrogen peroxide (0.3 mM) served as positive control. After incubating for specific time intervals cells were harvested and evidences of cytotoxicity, oxidative stress, and apoptosis were sought. The findings indicated that INH exposure causes increased ROS generation along with alteration in levels of enzymatic antioxidants such as Superoxide dismutase, Catalase, and Glucose-6-Phosphate dehydrogenase. Altered Bcl-2/Bax content, cytochrome-c translocation, caspase activation, and DNA fragmentation emphasized involvement of apoptosis.

Effect of vitamin C on oxidative liver injury due to isoniazid in rats

BACKGROUND

The aim of the present study was to investigate the effect of different doses of vitamin C on oxidative liver injury due to isoniazid (INH) in rats.

METHODS

Rats were divided into four subgroups, each containing 10 rats. Group 1 was the control group; group 2, INH 50 mg/kg per day; group 3, INH 50 mg/kg per day + low-dose vitamin C (100 mg/kg per day); group 4, INH 50 mg/kg per day + high-dose vitamin C (1000 mg/kg per day). INH and vitamin C were administered into their stomachs through an oral tube. After 21 days, measurements were made in both serum and homogenized liver tissues. The levels of glutathione (GSH), superoxide dismutase (SOD) and other biochemical variables were measured. Malondialdehyde (MDA), glutathione peroxidase (GSH-px) and vitamin C were measured using commercial kits.

RESULTS

Aspartate amino transferase and alanine aminotransferase in group 2 were higher than those in groups 1, 3 and 4 (P < 0.008 for both). Serum and tissue levels of MDA in group 2 were higher than that in groups 1 and 3 (P < 0.008 for both). There was no difference in the SOD levels between the four groups (P= 0.095). Erythrocyte and tissue GSH in group 2 were higher than that in groups 1 and 3 (P < 0.008 for both). Interestingly, erythrocyte and tissue GSH in group 4 were lower than those in group 1 (P < 0.008 for both). Erythrocyte level of GSH-px in group 2 was higher than that in groups 1 and 3 (P < 0.008 for both).

CONCLUSIONS

INH-induced liver injury is associated with oxidative stress, and co-administration of low-dose vitamin C may reduce this damage effectively in a rat model. The antioxidant effect of high-dose vitamin C does not seem more potent compared to the low dose.

Role of reactive metabolites in drug-induced hepatotoxicity

Drugs are generally converted to biologically inactive forms and eliminated from the body, principally by hepatic metabolism. However, certain drugs undergo biotransformation to metabolites that can interfere with cellular functions through their intrinsic chemical reactivity towards glutathione, leading to thiol depletion, and functionally critical macromolecules, resulting in reversible modification, irreversible adduct formation, and irreversible loss of activity. There is now a great deal of evidence which shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, such as acetaminophen, tamoxifen, isoniazid, and amodiaquine. The main theme of this article is to review the evidence for chemically reactive metabolites being initiating factors for the multiple downstream biological events culminating in toxicity. The major objectives are to understand those idiosyncratic hepatotoxicities thought to be caused by chemically reactive metabolites and to define the role of toxic metabolites.

Antituberculosis drug-induced hepatotoxicity: concise up-to-date review

The cornerstone of tuberculosis management is a 6-month course of isoniazid, rifampicin, pyrazinamide and ethambutol. Compliance is crucial for curing tuberculosis. Adverse effects often negatively affect the compliance, because they frequently require a change of treatment, which may have negative consequences for treatment outcome. In this paper we review the incidence, pathology and clinical features of antituberculosis drug-induced hepatotoxicity, discuss the metabolism and mechanisms of toxicity of isoniazid, rifampicin and pyrazinamide, and describe risk factors and management of antituberculosis drug-induced hepatotoxicity. The reported incidence of antituberculosis drug-induced hepatotoxicity, the most serious and potentially fatal adverse reaction, varies between 2% and 28%. Risk factors are advanced age, female sex, slow acetylator status, malnutrition, HIV and pre-existent liver disease. Still, it is difficult to predict what patient will develop hepatotoxicity during tuberculosis treatment. The exact mechanism of antituberculosis drug-induced hepatotoxicity is unknown, but toxic metabolites are suggested to play a crucial role in the development, at least in the case of isoniazid. Priorities for future studies include basic studies to elucidate the mechanism of antituberculosis drug-induced hepatotoxicity, genetic risk factor studies and the development of shorter and safer tuberculosis drug regimens.

Rifampicin exacerbates isoniazid-induced toxicity in human but not in rat hepatocytes in tissue-like cultures

BACKGROUND AND PURPOSE

Rifampicin has been extensively reported to exacerbate the hepatotoxicity of isoniazid in patients with tuberculosis. However, this was controversially claimed by previous reports using rat models. This study evaluated the effect of rifampicin on isoniazid-induced hepatocyte toxicity by using human and rat hepatocytes in tissue-like culture.

EXPERIMENTAL APPROACH

Hepatocytes in tissue-like gel entrapment were used to examine isoniazid toxicity, as shown by cell viability, intracellular glutathione content and albumin secretion. For demonstration of the differential effects of rifampicin on human and rat hepatocytes, induction by rifampicin of cytochrome P450 (CYP) 2E1, a major enzyme associated with isoniazid hepatotoxicity, was detected by 4-nitrocatechol formation and RT-PCR analysis.

KEY RESULTS

Rifampicin (12 microM) enhanced isoniazid-induced toxicity in human hepatocytes but not in rat hepatocytes. Enhanced CYP 2E1 enzymic activity and mRNA expression were similarly detected in human hepatocytes but not in rat hepatocytes. Both rat and human hepatocytes in gel entrapment were more sensitive to isoniazid treatment compared with the corresponding hepatocytes in a monolayer culture.

CONCLUSIONS AND IMPLICATIONS

The difference in induction of CYP 2E1 by rifampicin between rat and human hepatocytes accounted for the difference in exacerbation of isoniazid hepatocyte toxicity by rifampicin, with more significant toxicity in gel entrapment than in monolayer cultures. Thus, human hepatocytes in tissue-like cultures (gel entrapment) could be an effective model for hepatotoxicity research in vitro, closer to the in vivo situation.

Role of animal models in the study of drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions (DHRs) are a major problem, in large part because of their unpredictable nature. If we understood the mechanisms of these reactions better, they might be predictable. Their unpredictable nature also makes mechanistic studies very difficult, especially prospective clinical studies. Animal models are vital to most biomedical research, and they are almost the only way to test basic hypotheses of DHRs, such as the involvement of reactive metabolites. However, useful animal models of DHRs are rare because DHRs are also unpredictable in animals. For example, sulfonamide-induced DHRs in large-breed dogs appear to be valid because they are very similar to the DHRs that occur in humans; however, the incidence is only approximately 0.25%, and large-breed dogs are difficult to use as an animal model. Two more practical models are penicillamine-induced autoimmunity in the Brown Norway rat and nevirapine-induced skin rash in rats. The toxicity in these models is clearly immune mediated. In other models, such as amodiaquine-induced agranulocytosis/hepatotoxicity and halothane-induced hepatotoxicity, the drug induces an immune response but there is no clinical toxicity. This finding suggests that regulatory mechanisms usually limit toxicity. Many of the basic characteristics of the penicillamine and nevirapine models, such as memory and tolerance, are quite different suggesting that the mechanisms are also significantly different. More animal models are needed to study the range of mechanisms involved in DHRs; without them, progress in understanding such reactions is likely to be slow.

The role of metabolic activation in drug-induced hepatotoxicity

The importance of reactive metabolites in the pathogenesis of drug-induced toxicity has been a focus of research interest since pioneering investigations in the 1950s revealed the link between toxic metabolites and chemical carcinogenesis. There is now a great deal of evidence that shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, but how these toxic species initiate and propagate tissue damage is still poorly understood. This review summarizes the evidence for reactive metabolite formation from hepatotoxic drugs, such as acetaminophen, tamoxifen, diclofenac, and troglitazone, and the current hypotheses of how this leads to liver injury. Several hepatic proteins can be modified by reactive metabolites, but this in general equates poorly with the extent of toxicity. Much more important may be the identification of the critical proteins modified by these toxic species and how this alters their function. It is also important to note that the toxicity of reactive metabolites may be mediated by noncovalent binding mechanisms, which may also have profound effects on normal liver physiology. Technological developments in the wake of the genomic revolution now provide unprecedented power to characterize and quantify covalent modification of individual target proteins and their functional consequences; such information should dramatically improve our understanding of drug-induced hepatotoxic reactions.

Influence of Silymarin Administration on Hepatic Glutathione-Conjugating Enzyme System in Rats Treated with Antitubercular Drugs

OBJECTIVE

This study evaluated the influence of simultaneous administration of silymarin (SIL), a hepatoprotective and antioxidant agent, on the status of glutathione (GSH) and its metabolising enzymes in the liver tissue of rats treated with antitubercular drugs, i.e. isoniazid (INH), rifampicin (RIF) and pyrazinamide (PYR).

METHODS

Male Wistar albino rats (n = 24) were randomly divided into four groups. Group I received saline as they served as controls. Group II rats were administered antitubercular drugs (INH 25 mg/kg + RIF 50 mg/kg + PYR 140 mg/kg orally) daily for 45 days. Group III animals were treated with SIL (50 mg/kg orally) simultaneously with the antitubercular drugs for the same period. Group IV animals were treated with SIL alone. The status of GSH, glutathione peroxidase (GPx), glutathione reductase (GR) and glutathione-s-transferase (GST) in liver tissue was evaluated at the end of the study.

RESULTS

Administration of antitubercular drugs caused a significant decrease (p < 0.001) in the status of GPx, GST and GR and of non-enzymic (GSH) antioxidants in liver tissue when compared with saline-treated control rats. Simultaneous treatment of SIL with antitubercular drugs completely prevented decreases in the levels of all the above parameters. Treatment with SIL alone enhanced the activities of GST (p < 0.001) and GPx (p < 0.05) and did not alter glutathione levels compared with control.

CONCLUSION

A fall in the status of glutathione and its conjugating enzymes upon administration of antitubercular drugs denotes an impairment of the antioxidant defence mechanism. Simultaneous administration of SIL afforded complete protection of the liver against this abnormality, an effect that could have been due to the strong antioxidant properties of SIL.

Isoniazid-induced seizures with secondary rhabdomyolysis and associated acute renal failure in a dog

Isoniazid-induced seizures resulted in rhabdomyolysis and associated acute renal tubular necrosis in a dog. Rhabdomyolysis and myoglobinuric renal failure, although recognised in the dog, are reported infrequently as a consequence of seizures. The clinical presentation of isoniazid toxicity in a dog is described.

Rifampicin and isoniazid increase acetaminophen and isoniazid cytotoxicity in human HepG2 hepatoma cells

Acetaminophen (APAP) induced a concentration-dependent (0-30 mM) cytotoxic effect in human HepG2 hepatoma cells which was significantly increased when intracellular reduced glutathione (GSH) content was decreased. The cytotoxic effect of APAP (0-30 mM) was significantly lower in a day 3-treated compared to day 1-treated HepG2 cells. A 3-day preincubation of HepG2 cells with 5 microM 3-methylcholanthrene (3MC), 50 microM rifampicin (RFP) or 1 mM isoniazid (INH) significantly increased 15-30 mM APAP cytotoxicity, of about 15-20% for INH and RFP and 35-50% for 3MC. The cytotoxicity of 10 mM APAP was also increased (about 20%) by a 3-day preincubation with INH but was not affected by 3MC and RFP. INH induced a concentration-dependent (0-40 mM) cytotoxic effect in day-1 treated HepG2 cells and not significantly affected by decreases in intracellular GSH concentrations. INH was not cytotoxic in day 3-treated HepG2 cells. A 3-day preincubation of HepG2 cells with 50 mM RFP or 1 mM INH significantly increased 10-40 mM INH cytotoxicity, respectively of about 10% and 10-25%. A 3-day preincubation with 3MC did not modify the cytotoxic effect of INH at these concentrations. This is to our knowledge the first report of increases by INH and RFP of APAP of INH cytotoxicity in vitro in hepatocellular cells of human origin. It is in accordance with clinical observations of severe hepatotoxicity associated with APAP or INH usage in patients receiving multiple drug therapy (INH, RFP) for tuberculosis or in alcoholics.