Noninterference of cytochrome P4501A2 in the cytotoxicity of tacrine using genetically engineered V79 Chinese hamster cells for stable expression of the human or rat isoform and two human hepatocyte cell lines

In-vitro stability and metabolism of a tacrine–silibinin codrug

Objectives

A tacrine-silibinin codrug showed promising results in pharmacological and toxicity testing, superior to an equimolar mixture of tacrine and silibinin. The aim of this study was to get more information about its stability, possible degradation products, metabolites, and especially its active principle in vitro and in vivo.

Methods

The stability of the codrug was analysed under in-vitro assay conditions. Additionally, its metabolism was investigated using pooled human liver microsomes. Metabolites were identified via liquid chromatography-high resolution electrospray ionization mass spectrometry. Furthermore, the influence of one of the main cleavage products, tacrine hemi succinamide, on viability and mitochondria of hepatic stellate cells was analysed.

Key findings

The codrug remained stable in culture medium (Dulbecco's modified Eagle's medium) over an incubation period of 24 h, whereas exposition to microsomal enzymes led to rapid cleavage of the ester bond to form silibinin and a tacrine hemi succinamide. In addition, glucuronidated metabolites of both silibinin and the codrug were detected. For the tacrine hemi succinamide, no effects were observed with regard to cell viability and mitochondrial impairment.

Conclusions

This study helps understand and interpret previous results concerning the effects and the absence of toxicity of the tacrine–silibinin codrug and supplies important information for further identification of the active principles of the codrug in vivo.

Study of PMS777, a new type of acetylcholinesterase inhibitor, in human HepG2 cells. Comparison with tacrine and galanthamine on oxidative stress and mitochondrial impairment

Acetylcholinesterase inhibitors are commonly used as cognitive enhancers for dementia in aged people. Among them, tacrine (THA) but not galanthamine, was shown to exhibit hepatotoxicity which reduces its clinical use. PMS777, both a PAF antagonist and a new potent acetylcholinesterase inhibitor was recently demonstrated to reverse scopolamine-induced amnesia in mice without toxicity. In the present study, the effects of THA, galanthamine and PMS777 were compared in HepG2 cells on the oxidative parameters involved in the reported hepatotoxicity of THA. THA (> or = 10 microM) induced an oxidative stress as shown by elevated ROS and MDA production and by a decrease in GSH level. Moreover, mitochondrial membrane potential and redox status were decreased. At low concentrations (< or =10 microM), there was no significant disturbance. None of the oxidative stress markers was affected by PMS777 up to the maximum concentration tested and it is suggested that PMS777 is not cytotoxic for HepG2 cells. Galanthamine was also without cytotoxicity. Our results suggest that the toxic effect of THA above 10 microM may be caused by drug-induced mitochondrial energization impairment and destabilisation of membrane phospholipids associated with an oxidative stress. In contrast by preventing these dysfunctions, PMS777 could be safer than THA.

Alaternin and emodin with hydroxyl radical inhibitory and/or scavenging activities and hepatoprotective activity on tacrine-induced cytotoxicity in HepG2 cells

The antioxidative and hepatoprotective potentials of two anthraquinones, alaternin (2-hydroxyemodin) and emodin, to scavenge and/or inhibit hydroxyl radicals generated by the Fenton reaction and to protect tacrine-induced cytotoxicity in human liver derived HepG2 cells were evaluated, respectively. The inhibitory activity on hydroxyl radical generated in a cell-free chemical system (FeSO4/H2O2) was investigated by a fluorescence spectrophotometer using a highly fluorescent probe, 2',7'-dichlorofluorescein. The hydroxyl radical scavenging activity was determined by electron spin resonance spectroscopy using 5,5-dimethy-1-pyrroline-N-oxide as hydroxyl radicals trapping agents. Tacrine-induced HepG2 cell toxicity was determined by a 3-[4,5-dimethylthiazole-2yl]-2,5-diphenyltertrazolium bromide assay. Although the scavenging activity of alaternin on hydroxyl radical was similar to that of emodin in dose-dependent patterns, the inhibitory activity exhibited by the former on hydroxyl radical generation was stronger than that of the latter, with IC50 values of 3.05 +/- 0.26 microM and 13.29 +/- 3.20 microM, respectively. In addition, the two anthraquinones, alaternin and emodin showed their hepatoprotective activities on tacrine-induced cytotoxicity, and the EC50 values were 4.02 microM and 2.37 microM, respectively. Silymarin, an antihepatotoxic agent used as a positive control exhibited the EC50 value of 2.00 microM. These results demonstrated that both alaternin and emodin had the simultaneous antioxidant and hepatoprotective activities.

Hepatoprotective and free radical scavenging activities of phenolic petrosins and flavonoids isolated from Equisetum arvense

Hepatoprotective activity-guided fractionation of the MeOH extract of Equisetum arvense L. (Equisetaceae) resulted in the isolation of two phenolic petrosins, onitin (1) and onitin-9-O-glucoside (2), along with four flavonoids, apigenin (3), luteolin (4), kaempferol-3-O-glucoside (5), and quercetin-3-O-glucoside (6). Among these, compounds 1 and 4 exhibited hepatoprotective activities on tacrine-induced cytotoxicity in human liver-derived Hep G2 cells, displaying EC(50) values of 85.8 +/ -9.3 microM and 20.2 +/- 1.4 microM, respectively. Silybin, used as a positive control, showed the EC(50) value of 69.0 +/- 3.3 microM. Compounds 1 and 4 also showed superoxide scavenging effects (IC(50) = 35.3 +/- 0.2 microM and 5.9 +/- 0.3 microM, respectively) and DPPH free radical scavenging effect (IC(50) of 35.8 +/- 0.4 microM and 22.7 +/- 2.8 microM, respectively). These results support the use of this plant for the treatment of hepatitis in oriental traditional medicine.

Hepatoprotective phenolic constituents of Rhodiola sachalinensis on tacrine-induced cytotoxicity in Hep G2 cells

Two hepatoprotective phenolic compounds, kaempferol (2) and salidroside (4), were isolated from the roots of Rhodiola sachalinensis together with two inactive compounds cinnamyl alcohol (1) and daucosterol (3) based on the hepatoprotective activity against tacrine-induced cytotoxicity in human liver-derived Hep G2 cells. The EC(50) values of compounds 2 and 4 were 33.5 and 51.3 micro m, respectively. Silybin as a positive control showed an EC(50) value of 68.4 micro m.

Steroid hormones mediate sex difference in brain levels of tacrine and its hypothermic effect in the rat

Tacrine, a reversible cholinesterase (ChE) inhibitor, lowers body temperature by increasing cholinergic activity in the hypothalamus. Its hypothermic effect was significantly greater in female than in male rats at doses of 2.5-12.5 mg/kg. Gonadectomy increased the maximum fall in temperature after tacrine (5 mg/kg) from 1.92+/-0.16 to 2.59+/-0.13 degrees C in males and from 2.96+/-0.25 to 3.63+/-0.27 degrees C in females. Testosterone (10 mg/rat) rats significantly reduced the hypothermia in gonadectomised males and females and abolished the gender difference. Adrenalectomy increased the fall in temperature after tacrine (5 mg/kg) to 2.92+/-0.15 degrees C in males and 4.18+/-0.24 degrees C in females. The sex difference that remained was abolished by four daily injections of corticosterone (5 mg/kg). Plasma ChE can bind tacrine thereby lowering the amount available to the brain. Ovariectomy decreased plasma ChE activity from 2.27+/-0.24 to 1.66+/-0.14, while adrenalectomy reduced it to 1.30+/-0.10 (micromoles acetylthiocholine hydrolysed/ml/h). This enzyme activity was unaffected by gonadectomy and adrenalectomy in males. Brain levels of tacrine, (5 mg/kg), 1 h after injection were 2.41+/-0.35 microg/gm in males and 4.97+/-0.57 microg/gm in females. Gonadectomy increased brain levels in males to 4.05+/-0.51 microg/gm and testosterone restored them to 2.64+/-0.3 microg/gm. The hypothermic effect of tacrine was highly correlated to its brain concentration after the hormonal manipulations. It is concluded that steroids can reduce the pharmacological effects of tacrine by interfering with its entry into the brain.