Current Drug Metabolism - Volume 14, Issue 9, 2013

Xenobiotics are converted by cytochrome P450 (CYP450) into highly reactive metabolites (RMs) that covalently bind to the catalytic site of the enzyme itself, subsequently causing mechanism-based inhibition. This phenomenon is one of the fates of RMs in the liver. Depending on their affinity to nucleophiles (high-electron density compounds), RMs also may act as hepatotoxic agents by binding to intracellular macromolecules. The present study summarized 29 mechanism-based inhibitors (drugs) with clinical hepatotoxicity. Eighteen of these drugs cause hepatotoxicity (7 through idiosyncratic drug-induced liver injury) via their RMs. The liver injury caused by remaining 11 drugs, namely, fluoxetine, verapamil, furan-containing compounds, and human immunodeficiency virus protease inhibitors, cannot be excluded via RMs because of limited data. A regular pattern for RM-induced hepatotoxicity is summarized: (a) formation of RM-protein adducts that trigger immune responses; (b) covalent binding of RMs to intracellular macromolecules (mitochondria is a commonly victim) may lead to reactive oxygen species (ROS) overproduction, respiratory chain dysfunction, cell stress, and so on; and (c) RM overproduction, which results in glutathione (GSH) depletion. The binding mechanism of RMs to CYP450s and the quantitative parameters (KI, Kinact, and Kinact/KI) of the mechanism-based inhibitors of CYP450s are weakly correlated with the occurrence of hepatotoxicity, while the induction of CYP450 expression (11/29 drugs) may contribute to hepatotoxicity via excessive ROS and RM generation. These results suggest that mechanism-based inhibition is an indicator of RM formation and may thus be used to identify drugs with RM-induced hepatotoxic potential (particularly idiosyncratic drug-induced liver injury).

One key issue in the pharmaceutical development of new compounds is knowledge on metabolism, the enzymes involved and the potential hepatotoxicity of a drug. Primary cultured hepatocytes are a valuable in vitro model for drug metabolism studies. However, human hepatocytes show phenotypic instability and have restricted accessibility and high batch-to-batch functional variability, which seriously complicates their use in routine testing. Therefore, several liver-derived cell models have been developed for drug metabolism and hepatotoxicity screening to circumvent these drawbacks. Hepatoma cell lines offer important advantages, availability, an unlimited life span and a stable phenotype, thus rendering them suitable models for such studies. However, currently available human hepatoma cell lines are not a good alternative to cultured hepatocytes as they show very limited expression for most drug-metabolising enzymes. Other approaches have been developed to generate immortalised hepatic cells with metabolic competence (use of plasmids encoding immortalising genes to transform human hepatocytes, cell lines obtained from transgenic animals, hepatocytomes or hydrid cells). Recombinant models heterologously expressing cytochrome P450 enzymes in hepatoma cells have also been generated, and are widely used in drug metabolism and toxicity evaluations. In recent years, new approaches to up-regulate the expression of drug-biotransformation enzymes in human cell lines (i.e., transfection with the expression vectors encoding key hepatic transcription factors) have also been investigated. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism and hepatotoxicity studies, and the state-of-the-art strategies pursued to generate metabolically competent hepatic cell lines.

Tumour progression is characterized by a rapid cell growth accompanied by changes in the microenvironment, largely due to hypoxia. The angiogenic switch involves changes in the expression of genes that play key roles in tumour progression, invasion, metastasis and therapeutic response, contributing to tumour aggressiveness. The effect of hypoxia on the cellular concentrations of drug metabolizing enzymes and transporters is much less understood. A brief summary of the signaling mechanisms triggered by hypoxia is presented, followed by a review of the known effects of hypoxia on drug metabolism and transport. Most of the studies available have focused on Cytochromes P450 and ATP-binding cassette transporters, while influx transporters of the SLC family have been less investigated. Given its potential to contribute both to the understanding of the pathogenesis of disease and to the optimization of therapeutics, it is rather surprising that this area of research is still underdeveloped. An increasing number of studies focusing on this subject are bound to provide key information for drug development and optimization of therapeutics.

The use of nanoparticles (NPs) has improved the quality of many industrial, pharmaceutical, and medical products. Increased surface reactivity, a major reason for the positive effects of NPs, may, on the other hand, also cause adverse biological effects. Almost all non-biodegradable NPs cause cytotoxic effects but employ quite different modes of action. The relation of biodegradable or loaded NPs to cytotoxic mechanism is more difficult to identify because effects may by caused by the particles or degradation products thereof. This review introduces problems of NPs in conventional cytotoxicity testing (changes of particle parameters in biological fluids, cellular dose, cell line and assay selection). Generation of reactive oxygen and nitrogen species by NPs and of metal ions due to dissolution of the NPs is discussed as a cause for cytotoxicity. The effects of NPs on plasma membrane, mitochondria, lysosomes, nucleus, and intracellular proteins as cellular targets for cytotoxicity are summarized. The comparison of the numerous studies on the mechanism of cellular effects shows that, although some common targets have been identified, other effects are unique for particular NPs or groups of NPs. While titanium dioxide NPs appear to act mainly by generation of reactive oxygen and nitrogen species, biological effects of silver and iron oxide are caused by both reactive species and free metal ions. NPs lacking heavy metals, such as carbon nanotubes and polystyrene particles, interfere with cell metabolism mainly by binding to macromolecules.

Vitamin D is one of the important nuclear steroid transcription regulators that controls transcriptions of a large number of genes. Vitamin D supplement is commonly recommended for the elderly to prevent bone diseases. Amounting new evidence has indicated that vitamin D plays a crucial role in brain development, brain function regulation and neuroprotection. Parkinson's disease (PD) is a degenerative disorder commonly seen in the elderly, characterized by movement disorders including tremor, akinesia, and loss of postural reflexes. The motor symptoms largely result from the continued death of dopaminergic neurons in the substantia nigra, despite use of current therapeutic interventions. The cause and mechanism of neuron death is currently unknown. Vitamin D deficiency is common in patients with PD suggesting its preventive and therapeutic potential. Vitamin D may exert protective and neurotropic effects directly at cellular level, e.g. protection of dopamine system, and/or by regulating gene expression. This review summarizes the epidemiological, genetic and translational evidence implicating vitamin D as a candidate for prevention and treatment for PD.

Colorectal cancer (CRC) is one of the most common solid tumors worldwide. A diet rich in dietary fiber is associated with a reduction in its risk. Butyrate (BT) is one of the main end products of anaerobic bacterial fermentation of dietary fiber in the human colon. This short-chain fatty acid is an important metabolic substrate in normal colonic epithelial cells and has important homeostatic functions at this level, including the ability to prevent/inhibit carcinogenesis. BT is transported into colonic epithelial cells by two specific carrier-mediated transport systems, the monocarboxylate transporter 1 (MCT1) and the sodium-coupled monocarboxylate transporter 1 (SMCT1). In normal colonic epithelial cells, BT is the main energy source for normal colonocytes and it is effluxed by BCRP. Colonic epithelial tumoral cells show a reduction in BT uptake (through a reduction in MCT1 and SMCT1 protein expression), an increase in the rate of glucose uptake and glycolysis becomes their primary energy source. BT presents an anticarcinogenic effect (induction of cell differentiation and apoptosis and inhibition of cell proliferation) but has an apparent opposing effect upon growth of normal colonocytes (the "BT paradox"). Because the cellular effects of BT (e.g. inhibition of histone deacetylases) are dependent on its intracellular concentration, knowledge on the mechanisms involved in BT membrane transport and its regulation seem particularly relevant in the context of the physiological and pharmacological benefits of this compound. This review discusses the mechanisms of BT transport and integrates this knowledge with the effects of BT in tumoral and normal colonocytes.