Current Drug Metabolism - Volume 10, Issue 6, 2009

Resveratrol, a naturally occurring polyphenol, shows pleiotropic health beneficial effects, including antioxidant, anti-inflammatory, anti-aging, cardioprotective and neuroprotective activities. Due to the several protective effects and since this compound is widely distributed in the plant kingdom, resveratrol can be envisaged as a chemopreventive/ curative agent introduced almost daily with the diet. Currently, a number of preclinical findings suggest resveratrol as a promising nature's weapon for cancer prevention and treatment. A remarkable progress in elucidating the molecular mechanisms underlying anti-cancer properties of resveratrol has been achieved in the last years. Concerning the resveratrol mechanism of action as a protective (vs. normal cells and tissues) and toxic (vs. cancer cells) compound, many studies focus on its antioxidant capacity as well as on its ability to trigger and favor the apoptotic cascade in malignant cells. However, a generalized mechanism of action able to explain this dual effect of resveratrol has not yet been clearly established. In addition to these important functions, resveratrol is reported to exhibit several other biological/biochemical protective effects on heart, circulation, brain and age-related diseases which are summarized in this Review.

Methotrexate (MTX) is a folate inhibitor that is used successfully in treating a great variety of malignancies and autoimmune diseases. However, there are certain drawbacks to its use, as a significant percentage of patients do not respond to the therapy or develop serious adverse effects. In the last ten years there has been a growing body of evidence pointing to the clinical significance of polymorphisms in genes that are involved in the metabolism, transport, and function of folates and MTX. Indeed, many studies have shown that both the toxicity and the efficacy of this drug can be modified by the presence of these genetic alterations. However, there is also a great deal of contradictory evidence that has as yet prevented the acceptance of any definitive conclusions. This review aims to summarize current knowledge on the pharmacogenetics of MTX, dealing with the reasons for numerous reported discrepancies. The particular focus will be on studies analyzing interactions between single nucleotide polymorphisms (SNPs) along a certain pathway rather than following a single-SNP design. New approaches and future directions in the study of MTX pharmacogenetics will also be discussed.

In humans, four members of the CYP2C subfamily (CYP2C8, CYP2C9, CYP2C18, and CYP2C19) metabolize more than 20% of all therapeutic drugs as well as a number of endogenous compounds. The CYP2C enzymes are found predominantly in the liver, where they comprise ∼20% of the total cytochrome P450. A variety of xenobiotics such as phenobarbital, rifampicin, and hyperforin have been shown to induce the transcriptional expression of CYP2C genes in primary human hepatocytes and to increase the metabolism of CYP2C substrates in vivo in man. This induction can result in drug-drug interactions, drug tolerance, and therapeutic failure. Several drug-activated nuclear receptors including CAR, PXR, VDR, and GR recognize drug responsive elements within the 5' flanking promoter region of CYP2C genes to mediate the transcriptional upregulation of these genes in response to xenobiotics and steroids. Other nuclear receptors and transcriptional factors including HNF4α, HNF3γ, C/EBPα and more recently RORs, have been reported to regulate the constitutive expression of CYP2C genes in liver. The maximum transcriptional induction of CYP2C genes appears to be achieved through a coordinative cross-talk between drug responsive nuclear receptors, hepatic factors, and coactivators. The transcriptional regulatory mechanisms of the expression of CYP2C genes in extrahepatic tissues has received less study, but these may be altered by perturbations from pathological conditions such as ischemia as well as some of the receptors mentioned above.

The heme oxygenase/biliverdin reductase (HO/BVR) axis catalyzes the degradation of heme, but this system and its byproducts, carbon monoxide (CO) and bilirubin, have also been shown to exert cytoprotective effects by activating pro-survival pathways and scavenging free radicals. Naturally occurring substances that upregulate the inducible isoform of HO (HO-1) have therefore been proposed as potential new drugs for the treatment of free radical-induced disease. A number of existing drugs have also been shown to regulate the HO/BVR system, and this capacity is considered an additional mechanism for their therapeutic activity. However, upregulation of the HO/BVR axis is not always beneficial for cells: the heme depletion and accumulation of CO and bilirubin it causes are potentially toxic. Therefore, new pharmacological modulators of HO/BVR activity must act in a dose-dependent manner. This would allow dose titration to achieve a desired pharmacologic effect without producing toxicity. Unfortunately, this goal is more complicated than it seems because toxicity has to be defined in terms of each of the main products of heme metabolism. Furthermore, sensitivity to the therapeutic/ toxic effects of these products is likely to be tissue- or cell-type specific. The solution may lie in the use of novel drug-delivery systems that allow targeted delivery of low doses of the HO/BVR modulator to selected tissues.

Chemical insults, such as environmental or occupational carcinogenic agents, play a major role in the pathogenesis of many cancers. Many carcinogens exert genotoxic and cytotoxic effects via bioactivation into electrophilic species, a process catalyzed primarily by phase I drug metabolizing enzymes, typically cytochrome P450s. These reactive intermediates can induce DNA and RNA damage, and formation of protein adducts. The reactive species are often detoxified by phase II drug metabolizing enzymes, such as glutathione Stransferases (GSTs), UDP-glucuronosyl transferases (UGTs), sulfotransferase (ST) and N-acetyltransferase (NAT). Phase II enzymes classically conjugate these hydrophobic intermediates to a water-soluble group, thus masking their reactive nature, and allowing subsequent excretion. Therefore, strategies that modulate the levels of phase II enzymes by either pharmacological or nutritional means can lead to enhanced elimination of reactive species. Agents that preferentially activate phase II over phase I enzymes can be beneficial as chemopreventives. Compounds, such as isothiocyanates and dithiolthiones have been shown to act as transcriptional activators of phase II enzymes. A consensus enhancer element, known as antioxidant response element (ARE), in the regulatory domains of many phase II genes and an ARE-binding transcription factor nuclear factor E2-related factor 2 (Nrf2) have been implicated in the action of many chemopreventive agents. In this review, we will discuss the mechanisms of regulation of phase II enzymes, including the signal transduction events elicited by chemopreventive agents. We will also summarize the data available for these agents in preclinical models of tumorigenesis. Some chemopreventive agents have progressed to various stages of clinical trials, e.g. biomarker studies in healthy volunteers or in susceptible populations. These clinical data will be reviewed. Finally, we will provide a commentary on implementation of discovery and development programs for novel chemopreventive agents that are based on rational drug design, with lead optimization towards a safe and efficacious regimen in man.

The polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2) and OCT3 (SLC22A3) mediate facilitated and bidirectional diffusion of small (≤ 500Da) organic cations with broad specificities for endogenous substrates such as choline, acetylcholine and monoamine neurotransmitters, as well as a variety of xenobiotics. Importantly, besides a wide range of clinically used drugs, these also include several toxins like the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) and herbicide paraquat. OCT2-OCT3 display differential tissue distribution: OCT1 is predominantly found in liver of humans, and liver and the kidney in rodents; OCT2 is most strongly expressed in both human and rodent kidney, whereas OCT3 primarily expressed in placenta, but also more widely detected in various tissues, including brain and lung. The physiological roles OCTs as transporters for biogenic amines or acetylcholine in these tissues are still debated, in contrast to their involvement in providing access pathways for harmful/toxic cationic substrates into the body and particular tissues. This review highlights a novel role of human and rodent OCTs as carriers of the toxic fluorescent dye ethidium, as opposed to the less harmful related phenanthridine compound propidium, which is not transported. Additional uptake and efflux pathways for ethidium in pro- and eukaryotes are discussed. OCT-mediated pathways may determine major entry routes for ethidium into the body where toxicity via specific mechanisms may develop in tissues expressing OCTs. Considering the high affinity of OCTs for ethidium (Km = 1-2 μM) and their strong expression in various organs, strict safety guidelines for the handling of ethidium should be reinforced.

Oral anticoagulants, the main drugs used for the prevention and treatment of thromboembolic diseases, are a leading cause of fatal haemorrhagic complications due to the variable reactions that different patients can have when taking the drugs. The reaction to coumarins is affected by both diet such as the patient daily intake of vitamin K, and also by genetically determined levels of critical proteins. Clinically available, warfarin consists of a racemic mixture of two active optical isomers, (R)- and (S)- isoforms, and their pharmacokinetic and pharmaco-dynamic properties differ considerably, because the (S)-enantiomer is three times more potent than the (R)- enantiomer. Metabolism of warfarin occurs through the action of three different cytochrome P-450 enzymes. S-warfarin, predominantly responsible for the anticoagulation effect, is metabolized mostly by the CYP2C9 enzyme. A number of polymorphisms in CYP2C9 gene have been identified but the most important are CYP2C9*2 and CYP2C9*3. The second enzyme that is involved in coumarins metabolism is the vitamin K epoxide reductase (VKORC). This enzyme is the target of coumarins and converts oxidized vitamin K to the reduced active form that is required for the post-translational (gamma) carboxylation of the vitamin K-dependent coagulation factors. VKORC activity levels are affected by several polymorphisms that can be divided into high or low level haplotypes. There is a considerable controversy about the clinical use of genotyping before starting the anticoagulant therapy so, further randomized, prospective and large trials are required to recommend the use of pharmacogenetic testing.

Sulfonylurea drugs including chlorpropamide, gliclazide, tolbutamide, glipizide, glibenclamide (glyburide) and glimepiride are the most widely used oral hypoglycaemic agents in people with type 2 diabetes. This review investigates the impact of genetic polymorphisms on the pharmacokinetics and pharmacodynamics of sulfonylurea drugs. CYP2C9 is the major enzyme involved in sulfonylurea drug metabolism. CYP2C9 variant allele carriers have significant lower apparent clearance of these medicines. CYP2C19 genotype is more influential for gliclazide pharmacokinetics when compared to CYP2C9. Sulfonylurea pharmacodynamics is affected by several genes. Sulfonylurea receptor 1 (SUR1, ABCC8 gene) and K+ inward rectifier Kir6.2 (KCNJ11) have been correlated to significant variation in sulfonylurea response. Diabetics with the SUR1 exon 33 G allele are more sensitive to gliclazide and the rs5210 variant of the KCNJ11 gene was associated with improved clinical efficacy of gliclazide. Carriers of Transcription factor 7-like 2 (TCF7L2) variants are more likely to fail sulfonylurea therapy. On the other hand, patients with HNF-1α mutations had a significant greater response to gliclazide when compared to those with type 2 diabetes. The Arg972 polymorphism of insulin receptor substrate 1 (IRS1) may lead to secondary failure of sulfonylurea therapy. Calpain 10 gene (CAPN10) polymorphism has also been linked to sulfonylurea drug response. Despite the available evidence, larger population studies that investigate the pharmacokinetics and pharmacodynamics of sulfonylurea drugs are needed to investigate the influence of key SNPs amidst all potential contributing factors to variability in response to these which inturn will provide information to optimise sulfonylurea use in people with diabetes.