Diazepam, metformin, omeprazole and simvastatin: a full discussion of individual and mixture acute toxicity.

High consumption of drugs, combined with their presence in the environment, raises concerns about its consequences. Even though researches are often engaged in analyzing substances separately, that is not the environmental reality. Therefore, the aim of this study was to investigate the acute toxicity of the pharmaceuticals simvastatin, metformin, omeprazole and diazepam, and all possible mixtures between them, to the organism Aliivibrio fischeri, verifying possible synergistic or antagonistic effects and assessing byproducts formation. In terms of individual toxicity, omeprazole is the most toxic of the active ingredients, followed by simvastatin, diazepam and, finally, metformin. When the toxicity of mixtures was tested, synergism, antagonism and hormesis were perceived, most probably generated due to byproducts formation. Moreover, it was observed that even when compounds are at concentrations below the non-observed effect concentration (NOEC), there may be toxicity to the mixture. Hence, this work points to the urgent need for more studies involving mixtures, since chemicals are subject to interactions and modifications, can mix, and potentiate or nullify the toxic effect of each other.

Omeprazole does not alter human sperm motility, viability or DNA integrity in vitro.

The effect of omeprazole, a commonly used drug belongs to proton--pump inhibitor class, on human sperm function is still undetermined. Here, we hypothesised that addition of omeprazole to the ejaculated human semen may affect sperm parameters, and hence sperm function. Therefore, we assessed the in vitro effect of omeprazole on human sperm motility, viability and DNA integrity. Sixty-six normozoospermic semen samples were collected randomly from men who attended the andrology laboratory at King Abdullah University Hospital. Sperm motility, viability and DNA breaks were assessed in the presence (1-hr incubation at 37°C) of omeprazole at 5, 10, 20 and 50 µM compared to control (0 µM). None of the examined sperm parameters, at any tested omeprazole concentration, showed significant difference (p > 0.05) compared with the control. In conclusion, omeprazole at 5, 10, 20 and 50 µM does not alter human sperm motility, viability or DNA integrity in vitro.

Suppression of contractile activity in the small intestine by indomethacin and omeprazole.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used to treat a number of conditions, and proton pump inhibitors (PPIs) are often used to prevent NSAID-induced gastric mucosal damage; however, the effects of NSAIDs on intestinal motility are poorly understood. The purpose of the present study is to determine the effects of a prototypical NSAID, indomethacin, either alone or in conjunction with the PPI omeprazole, on intestinal motility. Rats were randomly divided into four groups treated with vehicle, omeprazole, indomethacin, or a combination of indomethacin and omeprazole. Intestinal motility and transit were measured along with inflammatory mediators in the intestinal smooth muscle, markers of mucosal damage, and bacterial counts in the intestinal wall. Indomethacin, but not omeprazole, caused mucosal injury indicated by lower gut bleeding; however, both omeprazole and indomethacin suppressed contractile activity and frequency in the distal part of the small intestine. Cotreatment with omeprazole did not reduce indomethacin-induced intestinal bleeding. Furthermore, although indomethacin caused increased inflammation as indicated by increased edema development and inflammatory mediators, cotreatment with omeprazole did not reduce inflammation in the intestinal smooth muscle or prevent the increased bacterial count in the intestinal wall induced by indomethacin. We conclude that both NSAID and PPI treatment suppressed contractile activity in the distal regions of the small intestine. The suppression of intestinal contractility was associated with increased inflammation in both cases; however, indomethacin and omeprazole appear to affect intestinal motility by different mechanisms.

In vivo analysis of acute eletropharmacological effects of proton pump inhibitors using halothane-anesthetized dogs: a translational study of cardiovascular adverse events.

Long-term use of proton pump inhibitors (PPIs) is known to clinically induce hypomagnesemia, increasing the risk toward QT-interval prolongation and lethal ventricular arrhythmias, whereas PPIs can directly modulate cardiac ionic currents in the in vitro experiments. In order to fill the gap between those information, we assessed acute cardiohemodynamic and electrophysiological effects of sub- to supra-therapeutic doses (0.05, 0.5 and 5 mg/kg/10 min) of typical PPIs omeprazole, lansoprazole and rabeprazole, using halothane-anesthetized dogs (n = 6 for each drug). The low and middle doses of omeprazole and lansoprazole increased or tended to increase the heart rate, cardiac output and ventricular contraction, whereas the high dose plateaued and decreased them. Meanwhile, the low and middle doses of omeprazole and lansoprazole decreased the total peripheral vascular resistance, whereas the high dose plateaued and increased it. Rabeprazole decreased the mean blood pressure in a dose-related manner; moreover, its high dose decreased the heart rate and tended to reduce the ventricular contractility. On the other hand, omeprazole prolonged the QRS width. Omeprazole and lansoprazole tended to prolong the QT interval and QTcV, and rabeprazole mildly but significantly prolonged them in a dose-related manner. High dose of each PPI prolonged the ventricular effective refractory period. Omeprazole shortened the terminal repolarization period, whereas lansoprazole and rabeprazole hardly altered it. In effects, PPIs can exert multifarious cardiohemodynamic and electrophysiological actions in vivo, including mild QT-interval prolongation; thus, PPIs should be given with caution to patients with reduced ventricular repolarization reserve.

Proton pump inhibitors exacerbate NSAID-induced small intestinal injury by inducing dysbiosis.

BACKGROUND & AIMS: Proton pump inhibitors (PPIs) and nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most commonly used classes of drugs, with the former frequently coprescribed to reduce gastroduodenal injury caused by the latter. However, suppression of gastric acid secretion by PPIs is unlikely to provide any protection against the damage caused by NSAIDs in the more distal small intestine. METHODS: Rats were treated with antisecretory doses of omeprazole or lanzoprazole for 9 days, with concomitant treatment with anti-inflammatory doses of naproxen or celecoxib on the final 4 days. Small intestinal damage was blindly scored, and changes in hematocrit were measured. Changes in small intestinal microflora were evaluated by denaturing gradient gel electrophoresis and reverse-transcription polymerase chain reaction. RESULTS: Both PPIs significantly exacerbated naproxen- and celecoxib-induced intestinal ulceration and bleeding in the rat. Omeprazole treatment did not result in mucosal injury or inflammation; however, there were marked shifts in numbers and types of enteric bacteria, including a significant reduction (∼80%) of jejunal Actinobacteria and Bifidobacteria spp. Restoration of small intestinal Actinobacteria numbers through administration of selected (Bifidobacteria enriched) commensal bacteria during treatment with omeprazole and naproxen prevented intestinal ulceration/bleeding. Colonization of germ-free mice with jejunal bacteria from PPI-treated rats increased the severity of NSAID-induced intestinal injury, as compared with mice colonized with bacteria from vehicle-treated rats. CONCLUSIONS: PPIs exacerbate NSAID-induced intestinal damage at least in part because of significant shifts in enteric microbial populations. Prevention or reversal of this dysbiosis may be a viable option for reducing the incidence and severity of NSAID enteropathy.

Ultrastructure of ECL cells in Mastomys after long-term treatment with H2 receptor antagonist loxtidine.

Gastric ECL-cell hyperplasia and carcinoids (ECLoma) develop after 1 year in rats treated with omeprazole or 2 months in Mastomys treated with loxtidine. The aim of this study was to examine the ultrastructure of ECL cells in Mastomys after loxtidine treatment with an attempt to evaluate whether an impairment of autophagy was involved in the tumorigenesis. Mastomys were given loxtidine for 8 or 27 weeks. Morphological analysis of ECL cells showed that (1) cell size was not increased after 8 or 27 weeks; (2) secretory vesicles, a hallmark feature of welldifferentiated ECL cells, were unchanged after 8 weeks but reduced after 27 weeks; (3) granules were reduced after 8 or 27 weeks; (4) microvesicles were unchanged after the treatment; and (5) vacuoles and lipofuscin bodies were found occasionally after 8 weeks but not at 27 weeks. In addition, the appearance of ECL-cell ultrastructure differed between loxtidine-treated Mastomys and rats treated with omeprazole or subjected to antrectomy, but was similar between Mastomys treated with loxtidine for 27 weeks and mice deficient in CCK(2) receptor. We suggest that the ultrastructure of ECL cells in Mastomys after long-term treatment with loxtidine displayed an impaired formation of vacuoles and lipofuscin bodies, markers of the autophagic pathway.

Bone remodeling after administration of proton pump (H+/K+-ATPase) inhibitors and alendronate in ovariectomized rats.

During osteoporosis therapy with alendronate, esophagitis and stomach ulceration may occur, requiring the concurrent use of drugs which decrease gastric juice production. The aim of the present study was to investigate the effect of concurrent administration of proton pump (H+/K+ATP-ase) inhibitors: omeprazole or pantoprazole and alendronate on bone remodeling in ovariectomized rats. The experiments were carried out on 3-month-old Wistar rats, divided into following groups (n = 8-10): NOVX - non-ovariectomized control rats; OVX - ovariectomized control rats; OVX + alendronate; OVX + omeprazole; OVX + omeprazole + alendronate; OVX + pantoprazole; OVX + pantoprazole + alendronate. The drugs were administered for 28 days: alendronate (3 mg/kg p.o.), omeprazole or pantoprazole (3 mg/kg i.p.). Bone remodeling was estimated based on histomorphometric evaluation of the tibia (endosteal and periosteal transverse growth and the osteoid width, transverse cross-section surface of the diaphysis and of the marrow cavity) and the femur (width of trabeculae in the distal epiphysis and metaphysis). Bone mass/100 g body mass and mass of bone mineral/100 mg bone mass in the tibia and femur were also determined. Pantoprazole stronger than omeprazole intensified bone remodeling disorders caused by estrogen deficiency in ovariectomized rats. Bone remodeling disorders were the result of intensification of bone resorption with concurrent inhibition of bone formation and mineralization. Pantoprazole, and to lesser extent omeprazole, weakened the preventive effect of alendronate on bone remodeling in ovariectomized rats.

Antioxidative defense against omeprazole-induced toxicogenetical effects in Swiss mice.

BACKGROUND: Omeprazole (OME), a most frequently used proton pump inhibitor in gastric acidosis, is evident to show many adverse effects, including genetic instability. This study evaluated toxicogenic effects of OME in Mus musculus. METHODS: For this study, 40 male Swiss mice were divided into 8 groups (n = 5) and treated with OME at doses of 10, 20, and 40 mg/kg and/or treated with the antioxidants retinol palmitate (100 IU/kg) and ascorbic acid (2.0 µM/kg). Cyclophosphamide 50 mg/kg, (cytotoxic agent) and the vehicle were served as positive and negative control group, respectively. After 14 days of treatment, the stomach cells along with the bone marrow and peripheral blood lymphocytes were collected and submitted to the comet assay (alkaline version) and micronucleus test. Additionally, hematological and biochemical parameters of the animals were also determined inspect of vehicle group. RESULTS: The results suggest that OME at all doses induced genotoxicity and mutagenicity in the treated cells. However, in association with the antioxidants, these effects were modulated and/or inhibited along with a DNA repair capacity. CONCLUSIONS: Taken together, antioxidants (such as retinol palmitate and ascorbic acid) may be one of the best options to counteract OME-induced cytogenetic instability.

Early use of omeprazole benefits patients with acute myocardial infarction.

BACKGROUND: Bleeding complications are not uncommon in patients with acute myocardial infarction (AMI) during treatments. How to prevent the occurrence of upper gastrointestinal bleeding in AMI patients has become one of the most intractable problems. And there are conflicting data on the efficacy and complication rate of omeprazole treatment. We conducted an intervention study to determine whether using omeprazole could benefit AMI patients. METHODS: A total of 237 patients with AMI were divided into two groups at random: omeprazole group including 114 patients and control group including 123 patients. Omeprazole 40 mg by intravenous drip was given to the patients in omeprazole group when they were admitted to the hospitals. From the second day they were given omeprazole 20 mg per day by oral administration for 7 days. In contrast, no gastric acid inhibitor was given to the patients in control group. The incidence of upper gastrointestinal bleeding, the recanalization rate and overall mortality in both groups were observed. RESULTS: The incidence of upper gastrointestinal bleeding in omeprazole group was 5.3% (6/114) which was much lower than 14.6% (18/123) in control group (P = 0.017), but the recanalization rate had no significant difference between the two groups (P = 0.681). The overall mortality in omeprazole group was lower than that of control group (3.5% vs. 10.6%, P = 0.035). CONCLUSIONS: Our findings suggest that early use of omeprazole in AMI patients could decrease the incidence of upper gastrointestinal bleeding and the overall mortality, without influencing the recanalization rate. Early use of omeprazole might benefit AMI patients.

A Novel Open Access Web Portal for Integrating Mechanistic and Toxicogenomic Study Results.

Applying toxicogenomics to improving the safety profile of drug candidates and crop protection molecules is most useful when it identifies relevant biological and mechanistic information that highlights risks and informs risk mitigation strategies. Pathway-based approaches, such as gene set enrichment analysis, integrate toxicogenomic data with known biological process and pathways. Network methods help define unknown biological processes and offer data reduction advantages. Integrating the 2 approaches would improve interpretation of toxicogenomic information. Barriers to the routine application of these methods in genome-wide transcriptomic studies include a need for "hands-on" computer programming experience, the selection of 1 or more analysis methods (eg pathway analysis methods), the sensitivity of results to algorithm parameters, and challenges in linking differential gene expression to variation in safety outcomes. To facilitate adoption and reproducibility of gene expression analysis in safety studies, we have developed Collaborative Toxicogeomics, an open-access integrated web portal using the Django web framework. The software, developed with the Python programming language, is modular, extensible and implements "best-practice" methods in computational biology. New study results are compared with over 4000 rodent liver experiments from Drug Matrix and open TG-GATEs. A unique feature of the software is the ability to integrate clinical chemistry and histopathology-derived outcomes with results from gene expression studies, leading to relevant mechanistic conclusions. We describe its application by analyzing the effects of several toxicants on liver gene expression and exemplify application to predicting toxicity study outcomes upon chronic treatment from expression changes in acute-duration studies.

Proton pump inhibitors promote the growth of androgen-sensitive prostate cancer cells through ErbB2, ERK1/2, PI3K/Akt, GSK-3ß signaling and inhibition of cellular prostatic acid phosphatase.

Prostate cancer (PCa) is one of the most common cancer in men. Although hormone-sensitive PCa responds to androgen-deprivation, there are no effective therapies for castration-resistant PCa. It has been recently suggested that proton pump inhibitors (PPIs) may increase the risk of certain cancers; however, association with PCa remains elusive. Here, we evaluated the tumorigenic activities of PPIs in vitro, in PCa cell lines and epithelial cells from benign prostatic hyperplasia (BPH) and in vivo, in PCa mice xenografts. PPIs increased survival and proliferation, and inhibited apoptosis in LNCaP cells. These effects were attenuated or absent in androgen-insensitive DU-145 and PC3 cells, respectively. Specifically, omeprazole (OME) promoted cell cycle progression, increased c-Myc expression, ErbB2 activity and PSA secretion. Furthermore, OME induced the phosphorylation of MAPK-ERK1/2, PI3K/Akt and GSK-3ß, and blunted the expression and activity of cellular prostatic acid phosphatase. OME also increased survival, proliferation and PSA levels in BPH cells. In vivo, OME promoted tumor growth in mice bearing LNCaP xenografts. Our results indicate that PPIs display tumorigenic activities in PCa cells, suggesting that their long-term administration in patients should be carefully monitored.

Toxicogenetic study of omeprazole and the modulatory effects of retinol palmitate and ascorbic acid on Allium cepa.

Omeprazole (OME) is a proton pump inhibitor used for the treatment of various gastric and intestinal disease; however, studies on its effects on the genetic materials are still restricted. The present study aimed to evaluate possible toxicogenic effects of OME in Allium cepa meristems with the application of cytogenetic biomarkers for DNA damage, mutagenic, toxic and cytotoxic effects. Additionally, retinol palmitate (RP) and ascorbic acid (AA) were also co-treated with OME to evaluate possible modulatory effects of OME-induced cytogenetic damages. OME was tested at 10, 20 and 40 µg/mL, while RP and AA at 55 µg/mL and 352.2 µg/mL, respectively. Copper sulphate (0.6 µg/mL) and dechlorinated water were used as positive control and negative control, respectively. The results suggest that OME induced genotoxicity and mutagenicity in A. cepa at all tested concentrations. It was noted that cotreatment of OME with the antioxidant vitamins RP and/or AA significantly (p < 0.05) inhibited and/or modulated all toxicogenic damages induced by OME. These observations demonstrate their antigenotoxic, antimutagenic, antitoxic and anticitotoxic effects in A. cepa. This study indicates that application of antioxidants may be useful tools to overcome OME-induced toxic effects.

Cytotoxic effects of the proton pump inhibitor omeprazole on the non-target marine microalga Tetraselmis suecica.

Omeprazole (OMP) is one of the most commonly used drugs for the treatment of gastro-intestinal disorders. Although it is daily consumed in high quantities and commonly detected in waters worldwide, relatively little is known about its ecotoxicity. The aim of this study was to evaluate the potential acute toxicity of increasing concentrations of OMP on the marine microalga Tetraselmis suecica analysing several cytotoxicity biomarkers by flow cytometry after 24h of exposure. Results showed that OMP caused a decrease in growth and autofluorescence, an increase in cellular volume and intracellular complexity, hyperpolarization of cytoplasmic and mitochondrial membranes and intracellular acidification. In addition, large amounts of reactive oxygen species (ROS) were generated which resulted in a decrease in the percentage of the viable population. However, the viable population showed an increase in their metabolic activity as an early response to overcome the stress. In conclusion, OMP may affect proton pumps in non-target organisms such as microalgae; it disturbed pH homeostasis and provoked an early accumulation of ROS that resulted in a rapid cell death in cells exposed to the highest concentration assayed.

Enantioselective endocrine disrupting effects of omeprazole studied in the H295R cell assay and by molecular modeling.

Enantiomers possess different pharmacokinetic and pharmacodynamic properties and this may not only influence the therapeutic effect of a drug but also its toxicological effects. In the present work we investigated the potential enantioselective endocrine disrupting effects of omeprazole (OME) and its two enantiomers on the human steroidogenesis using the H295R cell line. Differences in production of 16 steroid hormones were analyzed using LC-MS/MS. Additionally, to evaluate the differences in binding modes of these enantiomers, docking and molecular dynamics (MD) simulations of S-omeprazole (S-OME) and R-omeprazole (R-OME) in CYP17A1, CYP19A1 and CYP21A2 were carried out. Exposing H295R cells to OME and its enantiomers resulted in an increase of progesterone (PRO) and 17α-hydroxy-progesterone (OH-PRO) levels. At the same time, a decrease in the corticosteroid and androgen synthesis was observed, indicating inhibition of CYP21A2 and CYP17A1. In both cases, the effect of R-OME was smaller compared to that of the S-OME and a certain degree of enantioselectivity of CYP17A1 and CYP21A2 was suggested. Docking indicated that the N-containing rings of OME possibly could interact with the iron atom of the heme for S-OME in CYP17A1 and S- and R-OME in CYP21A2. However, density functional theory calculations suggest that the direct N-Fe interaction is weak. The study demonstrates enantioselective differences in the endocrine disrupting potential of chiral drugs such as omeprazole. These findings may have potential implications for drug safety and drug design.

Sex differences in liver toxicity-do female and male human primary hepatocytes react differently to toxicants in vitro?

There is increasing amount of evidence for sex variation in drug efficiency and toxicity profiles. Women are more susceptible than men to acute liver injury from xenobiotics. In general, this is attributed to sex differences at a physiological level as well as differences in pharmacokinetics and pharmacodynamics, but neither of these can give a sufficient explanation for the diverse responses to xenobiotics. Existing data are mainly based on animal models and limited data exist on in vitro sex differences relevant to humans. To date, male and female human hepatocytes have not yet been compared in terms of their responses to hepatotoxic drugs. We investigated whether sex-specific differences in acute hepatotoxicity can be observed in vitro by comparing hepatotoxic drug effects in male and female primary human hepatocytes. Significant sex-related differences were found for certain parameters and individual drugs, showing an overall higher sensitivity of female primary hepatocytes to hepatotoxicants. Moreover, our work demonstrated that high content screening is feasible with pooled primary human hepatocytes in suspension.

TCDD and omeprazole prime platelets through the aryl hydrocarbon receptor (AhR) non-genomic pathway.

The role of the aryl hydrocarbon receptor (AhR) in hemostasis has recently gained increased attention. Here, we demonstrate, by qRT-PCR and western blot, that human platelets express both AhR mRNA and AhR protein. AhR protein levels increase in a dose dependent manner when incubated with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or omeprazole. Treatment of platelets with puromycin blocks increased AhR protein synthesis in the presence of AhR activators. Additionally, treatment of platelets with either activator results in phosphorylation of p38MAPK and cPLA2, two key signaling molecules in platelet activation pathways. Using the AhR competitive inhibitors alpha naphthoflavone and CH-223191, we show that phosphorylation of p38MAPK is AhR dependent. Further, inhibition of p38MAPK blocks downstream cPLA2 phosphorylation induced by TCDD or omeprazole. Treatment with AhR activators results in platelet priming, as demonstrated by increased platelet aggregation, which is inhibited by AhR antagonists. Our data support a model of the platelet AhR non-genomic pathway in which treatment with AhR activators results in increased expression of the AhR, phosphorylation of p38MAPK and cPLA2, leading to platelet priming in response to agonist.

Hypergastrinaemia evoked by omeprazole stimulates growth of gastric mucosa but not of pancreas or intestines in hamster, guinea pig and chicken.

Treatment of chickens, hamsters and guinea-pigs with the long-acting anti-secretagogue omeprazole resulted in elevated levels of serum gastrin. The chickens received 400 mumol/kg by i.m. injection once daily, the hamsters and guinea-pigs received the same dose by the oral route once daily. In all 3 species omeprazole raised the intragastric pH to 4, measured 12-14 h after the administration of the drug. After 10 weeks of treatment, trophic changes were observed in the stomach of hamster and guinea pig and in the proventriculus of chicken. The trophic changes were manifested in a greatly increased stomach weight and gastric mucosal mass. There were no trophic effects outside the stomach (or proventriculus). The results are in agreement with previous observations in the rat and support the view that long-lasting sustained hypergastrinaemia causes trophic changes in the stomach but not in the pancreas or in the intestines.

Evaluation of omeprazole genotoxicity in a battery of in vitro and in vivo assays.

Omeprazole, a proton pump inhibitor of wide use in the treatment of gastric acid-related disorders, was evaluated for its genotoxic effects in both rat and human cultured cells and in the intact rat. DNA repair synthesis, as revealed by autoradiography, was detected in primary cultures of metabolically competent rat hepatocytes exposed to concentrations ranging from 10 to 100 mg/l, but the responses cannot be considered as clearly positive. Under the same experimental conditions any significant evidence of DNA repair was absent in primary hepatocytes from two human donors. At the same concentrations a modest but dose-related increase of micronucleated cells, that reached the level of statistical significance at 33 mg/l, was present in primary rat hepatocytes and in one of two human donors. In human lymphocytes exposed to subtoxic concentrations ranging from 0.78 to 12.5 mg/l a reproducible concentration dependent clastogenic effect was absent. In partially hepatectomized female rats treated with a single p.o. dose of 1000 mg/kg, the frequency of micronucleated cells was 5.2-fold higher than in controls in the liver, but only 2.0-fold higher in polychromatic erythrocytes of the bone marrow. In rats of the same sex given azoxymethane as initiator of colon carcinogenesis the oral administration for 8 successive weeks of 10 mg/kg omeprazole on alternate days increased the response to azoxymethane, as indicated by the occurrence in colon mucosa of a modest but statistically significant increase in both the average number and size of aberrant crypt foci. Taken as a whole, our results suggest that omeprazole behaves as a weak genotoxic agent for the rat liver. Reliable information about the potential genotoxic risk to humans requires further studies on primary cells from a wide number of donors.

The role of protein tyrosine kinases in CYP1A1 induction by omeprazole and thiabendazole in rat hepatocytes.

Benzimidazoles compounds like omeprazole (OME) and thiabendazole (TBZ) mediate CYP1A1 induction differently from classical aryl hydrocarbon receptor (AhR) ligands, 3-methylcholanthrene (3-MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To clarify the involvement of an intracellular signal pathway in CYP1A1 induction by OME and TBZ, the TBZ, OME and 3-MC signal-transducing pathways were compared by using specific protein tyrosine kinase inhibitors in primary culture of rat hepatocytes. The effect of OME and TBZ (75-250 microM) on cytochrome P450 1A1 (CYP1A1) expression was therefore studied in primary cultures of rat hepatocytes after 24 h, 48 h and 72 h of exposure. Both compounds provoked a dose- and time-dependent increase in CYP1A1 (EROD activity, protein and mRNA levels), but OME was less effective at all the concentrations and times tested. The mechanism of benzimidazole-mediated induction of CYP1A1 was investigated by comparison with 3-MC, a prototypical AhR ligand. As expected, OME and TBZ were unable to displace [(3)H]-TCDD from its binding sites to the AhR in competitive binding studies. Moreover, classic tyrosine kinase inhibitor herbimycin A (HA) inhibited the two benzimidazoles-mediated CYP1A1 inductions, but only partially inhibited the 3-MC-mediated one. Another two tyrosine kinase inhibitors, Lavendustin A (LA) and genistein (GEN), had no effect on CYP1A1 induction by benzimidazoles and 3-MC. These results are consistent with the implication of a tyrosine kinase, most probably the Src tyrosine kinase, in the mechanism of CYP1A1 induction in rat hepatocytes.