Current Drug Metabolism - Volume 10, Issue 9, 2009

Drug intolerance constitutes an important cause of patient morbidity and mortality. Intolerance events can be detected at the final stages of drug development, during clinical trials or even after drug approval. Besides the risks derived from their clinical outcome, drug intolerance events may result in stopping drugs for further development, restriction of drug use or drug withdrawal from the market, in spite of the fact that many of the drugs causing intolerance are safe and efficient for most patients. For this reason, the identification of the mechanisms involved in drug intolerance and the early detection of at-risk individuals are key issues. Examples for severe drug intolerance events include among others hepatotoxicity, which ranges from sub-clinical elevations in liver enzyme concentrations to acute liver failure with a mean mortality of 10% for jaundiced patients with acute toxic hepatocellular damage [1]. Acute gastrointestinal bleeding is another common adverse effect, especially for non-steroidal antiinflammatory drugs, with a mortality rate between 7 and 11% [2]; and hypersensitivity reactions, including anaphylaxis, are also commonly caused by non-steroidal antiinflammatory drugs [3]. Besides licensed drugs, herbal and natural supplements are recognized as causing intolerance events with increasing frequency as patients turn more and more to alternative medicine [4]. The mechanisms underlying the interindividual variability in the susceptibility to drug intolerance are poorly understood. In recent years growing evidence points to a genetic basis for such susceptibility, either related to drug bioactivation or biodisposition, to variability in drug targets such as enzymes, transporters or receptors, as well as to genetic variability in general signaling and detoxication mechanisms [5-10]. Although studies are in progress, with few exceptions there is a paucity of published data on the relationship between genetic polymorphisms and drug intolerance. In addition, recent evidence suggests that pharmacogenomic studies are insufficient to predict adverse drug reactions [8] and that the combination of pharmacogenomic and metabolomics studies may be far more informative [11]. This special issue of Current Drug Metabolism on drug intolerance provides a collection of review articles covering basic and clinical topics related to drug intolerance, and identifies further aspects that should be investigated in detail. The paper by Andreu and coworkers discusses the potential role of metabolomics in drug intolerance. Hopefully the combination of metabolomics with pharmacogenomics will give essential information to identify at-risk subjects and to clarify the mechanisms related to bioactivation in drug intolerance. Two papers discuss major drug intolerance mechanisms. These include drug-induced liver injury in a review by Andrade and coworkers, and hypersensitivity reactions in a review by Cornejo and colleagues. These two review papers include clinical and diagnostic criteria as well as an update of causal mechanisms including genetic and non-genetic risk factors. Finally, two review papers exemplify aspects on drug intolerance. The paper by Van Asseldonk et al. discusses therapeutic drug monitoring, pharmacogenomics and drug intolerance of thiopurines, and the paper by Agundez and coworkers discusses basic and clinical aspects on the mechanisms involved in aspirin intolerance, including gastrointestinal complications and hypersensitivity. Further advances in drug intolerance research can be expected with the combined information obtained from proteomics, genomics, metabolomics, bioinformatics, immunology, toxicology and pharmacology. We hope that in the next few years the research effort dedicated to these studies will result in widely used tools capable of increasing the efficiency and safety of drug therapy, and/or to identify individuals with increased susceptibility to develop drug intolerance events. Hopefully, this information will also be useful to recover drugs that have been withdrawn from the market, for selective use in non-susceptible patients. I would like to thank, as Guest Editor of the special issue of Current Drug Metabolism, all the authors who kindly contributed to this issue and to our reviewers.

Adverse drug reactions appear during the clinical use of a drug and constitute a health problem, as they are an important cause of patient morbidity and mortality. In addition, they constitute a major drawback for drug development. Intolerance processes occurring after administration of low drug doses are known as idiosyncratic reactions or as hypersensitivity reactions; the most commonly accepted mechanism for immunological activation is the hapten hypothesis. Most drugs are not reactive per se towards proteins, hence in a number of cases bioactivation seems to be a prerequisite for adduct formation and the subsequent hypersensitivity reaction. Although biotransformation is normally associated with a decreased toxicity, metabolites are sometimes more toxic and reactive than the parent drug. Drug metabolizing enzymes develop their activities especially in liver, where reactive metabolites bind to proteins inducing hepatotoxicity, whereas in skin keratinocytes exhibit the highest biotransformation capability. In the present review, some specific examples of the toxicological consequences of drug biotransformation are given. They include nimesulide, metamizol, celecoxib, paracetamol, dapsone, sulfamethoxazole, amodiaquine, nevirapine, troglitazone, zileuton, felbamate, panadiplon, benzbromarone, fipexide and flutamide. In general, these examples are taken from the recent scientific literature, mostly published during the last decade.

Drug-induced liver injury (DILI) is a severe adverse effect. The majority of DILI cases are idiosyncratic and several mechanisms have been postulated to explain why some subjects develop DILI with drugs that are safe for the majority of individuals. Major mechanisms proposed for DILI are based on the production of reactive metabolites, immune-mediated hepatotoxicity, a “danger signal” hypothesis and/or alterations in mitochondrial function. These mechanisms are compatible with the hypothesis for genetic variability in drug metabolism or bioactivation and are a major determinant for DILI. In this review we summarize present knowledge on underlying mechanisms, and clinical expression as well as genetic and non-genetic factors that modulate the risk of developing DILI. With regard to DILI pharmacogenomics, we summarize current evidence on the role of polymorphisms in genes coding for the drug-metabolizing enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, CYP3A5, NAT2, GSTM1, GSTT1, UGT1A1, UGT1A3, UGT1A9 and UGT2B7. Conclusive evidence for association with DILI risk has been obtained for non-mutated CYP2E1, slow NAT2 and slow GSTM1 genotypes. For the rest of the genes additional pharmacogenomics and toxicogenomics studies are required. We identify potential sources of heterogeneity in studies carried out so far as well as new genetic targets which require further investigation.

NSAIDs are the most important group of drugs involved in hypersensitivity drug reactions, and include heterogeneous compounds with very different chemical structures. These reactions can be IgE dependent (immediate reactions), T cell-mediated (nonimmediate), or induced by a non-specific immunological mechanism related with the blocking of the COX-1 enzyme and the shunting to the lipooxygenase pathway (cross-intolerant reactions). Cutaneous symptoms are the most frequent, with ibuprofen, naproxen and diclofenac being common culprit drugs worldwide, although others can be involved because patterns of consumption and exposure rates vary between countries. A very important proportion of immunological reactions are immediate, with urticaria and anaphylaxis being the typical clinical manifestations. Non-immediate reactions comprise a number of heterogeneous entities ranging from mild exanthema to severe TEN or DRESS syndrome, as well as organ-specific reactions such as hepatitis or pneumonitis. Cross-intolerant reactions appear to non-chemically related drugs, and involve respiratory airways, skin or both. In vivo diagnostic tests are based on the capacity of the skin to respond to the culprit drug, but their sensitivity is in many instances rather low. The approach for in vitro testing consists of either detecting specific IgE antibodies or studying the proliferation of T lymphocytes toward the eliciting drug. No appropriate tests are yet available for the in vitro validation of cross-intolerance reactions, although techniques based on the stimulation of basophils have been proposed. Based on these findings, the diagnostic approach is often based on the controlled administration of the drug to assess tolerance. In this work we review current knowledge on hypersensitivity reactions to NSAIDs, including diagnostic approach and genetic studies.

Thiopurines such as azathioprine, 6-mercaptopurine and 6-thioguanine are antimetabolites that have been used for several decades in the treatment of several diseases including inflammatory bowel diseases. Additional anti-inflammatory properties of these thiopurines have been discovered in recent years. Thiopurine metabolism is complex due to the involvement of multiple enzymes, of which the activities are genetically determined and cell type dependent. Single nucleotide polymorphisms in the genes encoding these enzymes have been correlated with altered activities and drug intolerance. Detailed implications of these will be reviewed. Over the years several methods of therapeutic drug monitoring have been developed in an attempt to relate thiopurine drug availability with efficacy and intolerance. In this respect, monitoring pharmacologically active 6-thioguanine nucleotide concentrations is most widely used. So far, however, the clinical usefulness of these methods is hampered by methodological limitations. Some drug interactions may optimize the metabolization of thiopurines and consequently increase its efficacy and decrease drug intolerance. This review focuses on the clinical relevance and usefulness of therapeutic drug monitoring of thiopurines and provides suggestions to optimize thiopurine therapy in the treatment of inflammatory bowel diseases.

Polymorphisms in drug-related enzymes and receptors are often strongly related to altered drug response and to the risk of developing drug intolerance. Aspirin, usually available as an over-the-counter drug, is one of the most used drugs worldwide and is a common cause of drug intolerance events. Aspirin undergoes polymorphic metabolism. Among the enzymes involved in aspirin biodisposition a major role is played by the enzymes UDP-glucuronosyltransferase UGT1A6, cytochrome P450 CYP2C9 and the xenobiotic/ medium chain fatty acid:CoA ligase ACSM2, although other UGTs and ACSMs enzymes may significantly contribute to aspirin metabolism. UGT1A6, CYP2C9 and ACSM2 are polymorphic, as well as PTGS1 and PTGS2, the genes coding for the enzymes cyclooxygenases COX1 and COX2, respectively. The present review analyzes the importance of genetic variations in enzymes involved in the metabolism and in the effects of aspirin and common polymorphisms related to aspirin intolerance, and it raises hypotheses on genetic factors related to altered response to aspirin that require further investigation. Major polymorphisms related to aspirin biodisposition are rs2070959, rs1105879 and rs6759892 for the UGT1A6 gene, rs1133607 for the ACSM2 gene, and rs1799853, rs1057910, rs28371686, rs9332131 and rs28371685 for the CYP2C9 gene. Regarding aspirin effects, major PGTS1 targets are rs3842787 and rs5789 for European subjects, and rs3842789 and rs3842792 for African subjects. For the PTGS2 gene nonsynonymous SNPs are likely to be of low relevance because of the influence of transcriptional and posttranscriptional factors. Combined studies for the above mentioned polymorphisms and those corresponding to other genes related to aspirin intolerance will provide excellent tools to identify individuals with a high risk to develop intolerance to aspirin.

Human CYP2C8 is a key member of the CYP2C family and metabolizes more than 60 clinical drugs. A number of active site residues in CYP2C8 have been identified based on homology modeling and site-directed mutagenesis studies. In the structure of CYP2C8, the large active site cavity exhibits a trifurcated topology that approximates a T or Y shape, which is consistent with the finding that CYP2C8 can efficiently oxidize relatively large substrates such as paclitaxel and cerivastatin. The active site cavity of CYP2C8 contains at least 48 amino acid residues and many of them are important for substrate binding. The structures of CYP2C8 in complex with distinct ligands have revealed that the enzyme can bind divergent substrates and inhibitors without extensive conformational changes. CYP2C8 is a major catalyst in the metabolism of paclitaxel, amodiaquine, troglitazone, amiodarone, verapamil and ibuprofen, with a secondary role in the biotransformation of cerivastatin and fluvastatin. CYP2C8 also metabolises endogenous compounds such as retinoids and arachidonic acid. Many drugs are inhibitors of CYP2C8 and inhibition of this enzyme may result in clinical drug interactions. The pregnane X receptor, constitutive androstane receptor, and glucocorticoid receptor are likely to involve the regulation of CYP2C8. A number of genetic mutations in the CYP2C8 gene have been identified in humans and some of them have functional impact on the clearance of drugs. Further studies are needed to delineate the role of CYP2C8 in drug development and clinical practice.

Environmental pollution by discharge of dye-containing effluents represents a serious ecological concern in many countries. Public demands for colour-free discharges to receiving waters have made decolouration of a variety of industrial wastewater a top priority. The current existing techniques for dye removal have several drawbacks such as high cost, low efficiency, use of large amounts of chemicals and formation of toxic sub-products. This has impelled the search for alternative methods such as those based on oxidative enzymes. This approach is believed to be a promising technology since it is cost-effective, environmentally friendly and does not produce sludge. Enzymatic transformation of synthetic dyes can be described as the conversion of dye molecules by enzymes into simpler and generally colourless molecules. Detailed characterisation of the metabolites produced during enzymatic transformation of synthetic dyes as well as ecotoxicity studies is of great importance to assess the effectiveness of the biodegradation process. However, most reports on the biotreatment of dyes mainly deal with decolouration and there are few reports on the reduction in toxicity or on the identification of the biodegradation products. This implies a limitation to assess their true technical potential.

Hypericum perforatum L. (St. John's wort) extracts have gained popularity as an alternative to conventional antidepressant drugs for mild to moderate forms of depressive disorders. New potential psychiatric uses for extracts in obsessive-compulsive disorder, generalised anxiety disorder and alcohol dependence have also been suggested on the basis of animal studies. The neurochemical mechanisms of these central actions are still debated but several components have antidepressant-like and anxiolytic-like effects in animals, or interact with neurotransmitter systems believed to be causally involved in depression, anxiety and in psychiatric illness generally. However, these data should interpreted taking account of the pharmacokinetic data on the main components, particularly those of their brain distribution and concentrations and the relationships with blood concentrations; the (scant) data so far suggest that the acylphloroglucinol hyperforin, the flavonol quercetin and its glycosylated forms and their metabolites, the biflavones amentoflavone and its I3,II8-analog biapigenin and the naphthodianthrones hypericin and pseudohypericin pass the blood-brain barrier poorly in animals. The brain concentrations of all these high-molecular weight, poorly water-soluble compounds after pharmacologically effective doses of the extracts are therefore far below those effective on neurotransmitter receptors and the mechanisms which are obviously important in the central effects of conventional, pharmacologically related drugs. Additional pharmacokinetic data on the brain concentrations of these and other constituents and their metabolites are therefore required for a more meaningful interpretation of the central effects of St. John's Wort extracts.