Expert Opinion on Drug Metabolism & Toxicology

Dash, Ajit; Inman, Walker; Hoffmaster, Keith; Sevidal, Samantha; Kelly, Joan; Obach, R Scott; Griffith, Linda G; Tannenbaum, Steven R

doi: 10.1517/17425250903160664pmid: 19637986

Assessment of drug–liver interactions is an integral part of predicting the safety profile of new drugs. Existing model systems range from in vitro cell culture models to FDA-mandated animal tests. Data from these models often fail, however, to predict human liver toxicity, resulting in costly failures of clinical trials. In vitro screens based on cultured hepatocytes are now commonly used in early stages of development, but many toxic responses in vivo seem to be mediated by a complex interplay among several different cell types. We discuss some of the evolving trends in liver cell culture systems applied to drug safety assessment and describe an experimental model that captures complex liver physiology through incorporation of heterotypic cell–cell interactions, 3D architecture and perfused flow. We demonstrate how heterotypic interactions in this system can be manipulated to recreate an inflammatory environment and apply the model to test compounds that potentially exhibit idiosyncratic drug toxicity. Finally, we provide a perspective on how the range of existing and emerging in vitro liver culture approaches, from simple to complex, might serve needs across the range of stages in drug discovery and development, including applications in molecular therapeutics.

Lai, Yurong

doi: 10.1517/17425250903127234pmid: 19611403

Increasing challenges include the application of in vitro/in vivo models for prediction of safety and pharmacokinetic profiles for the compounds entering drug development that are mainly secreted from bile. Species differences in biliary excretion have long been recognized and complicate the extrapolation of human pharmacokinetics from preclinical species or in vitro models. The currently available literature has been reviewed with a focus on drug–drug interactions mediated by hepatic drug transporters and the available tools for transporter assessments. Therefore, our incomplete understanding of interspecies differences and in vitro liver models poses a serious challenge in predicting human pharmacokinetics and/or safety profiles when the compounds are predominantly secreted from bile. This review outlines species differences in hepatobiliary secretion and the recent effort in understanding the absolute difference in hepatobiliary transporter expressions across species. In addition, understanding the expression of hepatobiliary transporters between in vitro hepatocyte models and in vivo to improve the prediction of biliary secretion is also discussed.

Croyle, Maria A

doi: 10.1517/17425250903136748pmid: 19732028

Virus infections are on the rise. Although the first description of CYP expression during virus infection was recorded 50 years ago, mechanistic studies of this phenomenon only began to appear in the last decade due to breakthroughs in molecular biology, genomic and transgenic technology. This review describes the relationship(s) among CYP-mediated drug metabolism, virus infection and the immune response and evaluates in vitro and in vivo models for mechanistic studies. The first studies that assessed CYP expression during infection focused on inflammatory mediators and the innate immune response at early time points. Recent studies assessing virus infection and its effect on hepatic CYP expression noted more long-term effects. An obvious approach toward understanding how viruses affect hepatic CYP3A expression and function would be to assess key regulators of CYP during infection. Improvements in techniques to identify post-translational modifications of CYP and systems that focus on virus–receptor interactions which allow subtraction and addition of immunological and regulatory elements that drive CYP will demonstrate that long-term changes in drug metabolism start from the time the virus enters the circulation, are reinforced by virus binding to cellular targets and further solidified by changes in cellular processes long after the virus is cleared.

Steventon, Glyn B; Mitchell, Stephen C

doi: 10.1517/17425250903179318pmid: 19653802

Phenylalanine 4-monooxygenase is the key enzyme in the sulfoxidation of the thioether drug S-carboxymethyl-l-cysteine and its thioether metabolites, S-methyl-l-cysteine, N-acetyl-S-carboxymethyl-l-cysteine and N-acetyl-S-methyl-l-cysteine in humans, and a number of other mammalian species. The kinetics constants of the sulfoxidation reaction (Km, Vmax and CLE) have been investigated in cytosolic fractions derived from rat and human liver, in cytosolic fractions of HepG2 cells and using both human and mouse cDNA expressed phenylalanine 4-monooxygenase. Differences in Km, Vmax and CLE of S-carboxymethyl-l-cysteine have been seen in HepG2 cells and human and mouse cDNA expressed phenylalanine 4-monooxygenase when compared to both rat and human hepatic cytosolic fractions. The association of the genetic polymorphism in the sulfoxidation of S-carboxymethyl-l-cysteine is highlighted with particular reference to this biotransformation reaction as being a biomarker of disease susceptibility in Parkinson's, Alzheimer's and motor neurone diseases and in rheumatoid arthritis. The possible underlying molecular genetics of the sulfoxidation polymorphism is also discussed in relation to the known allelic frequencies of phenylalanine 4-monooxygenase. Finally, the new found role phenylalanine 4-monooxygenase plays in xenobiotic metabolism is discussed.

Cederbaum, Arthur

doi: 10.1517/17425250903143769pmid: 19671018

The transcription factor Nrf2 regulates the expression of important cytoprotective enzymes. Induction of CYP2E1 is one of the central pathways by which ethanol generates oxidative stress. CYP2E1 can be induced by ethanol and several low molecular mass chemicals such as pyrazole. This review discusses biochemical and toxicological effects of CYP2E1 and the effects of Nrf2 in modulating these actions of CYP2E1. Besides ethanol, CYP2E1 metabolizes and activates many other toxicologic important compounds. One approach to try to understand the basic effects and actions of CYP2E1 was to establish HepG2 cell lines that constitutively express human CYP2E1. Ethanol, polyunsaturated fatty acids and iron were toxic to the HepG2 cells, which express CYP2E1 (E47 cells) but not control C34HepG2 cells, which do not express CYP2E1. Toxicity was associated with enhanced oxidant stress and could be prevented by antioxidants and potentiated if glutathione was removed. The E47 cells had higher glutathione levels and a twofold increase in catalase, cytosolic and microsomal glutathione transferase, and heme oxygenase-1 than control HepG2 cells due to activation of their respective genes. These activations were prevented by antioxidants, suggesting that reactive oxygen species generated by CYP2E1 were responsible for the upregulation of these antioxidant genes. This upregulation may reflect an adaptive mechanism to remove CYP2E1-derived oxidants. Increases in Nrf2 protein and mRNA were observed in livers of chronic alcohol-fed mice or rats and of pyrzole-treated rats or mice, conditions known to elevate CYP2E1. E47 cells showed increased Nrf2 mRNA and protein expression compared with control HepG2 C34 cells. Upregulation of antioxidant genes in E47 cells is dependent on Nrf2 and is prevented by siRNA-Nrf2. Blocking Nrf2 by siRNA-Nrf2 decreases glutathione and increases reactive oxygen species and lipid peroxidation, resulting in decreased mitochondrial membrane potential and loss of cell viability of E47 cells, but not C34 cells. Nrf2 is activated and levels of Nrf2 protein and mRNA are increased when CYP2E1 is elevated. These results suggest that Nrf2 plays a key role in the adaptive response against increased oxidative stress caused by CYP2E1 in the HepG2 cells. However, it is not clear whether Nrf2 is protective against CYP2E1 toxicity in vivo as pyrazole which elevates CYP2E1 in wild-type mice did not elevate CYP2E1 in Nrf2 knockout mice, although pyrazole produced toxicity in the Nrf2 knockout mice.

Roy, Kunal; Roy, Partha Pratim

doi: 10.1517/17425250903158940pmid: 19708826

Cytochrome P450 (CYP450) enzymes are predominantly involved in the Phase I metabolism of xenobiotics. Metabolic inhibition and induction can give rise to clinically important drug–drug interactions. Metabolic stability is a prerequisite for sustaining the therapeutically relevant concentrations, and very often drug candidates are sacrificed due to poor metabolic profiles. Computational tools such as quantitative structure–activity relationships are widely used to study different metabolic end points successfully to accelerate the drug discovery process. There are a lot of computational studies on clinically important CYPs already reported in recent years. But other clinically significant families are to yet be explored computationally. Powerfulness of quantitative structure–activity relationship will drive computational chemists to develop new potent and selective inhibitors of different classes of CYPs for the treatment of different diseases with least drug–drug interactions. Furthermore, there is a need to enhance the accuracy, interpretability and confidence in the computational models in accelerating the drug discovery pathways.

Clinckers, Ralph; Smolders, Ilse; Vermoesen, Katia; Michotte, Yvette; Danhof, Meindert; Voskuyl, Rob; Della Pasqua, Oscar

doi: 10.1517/17425250903146903pmid: 19611404

Biophase concentrations of antiepileptic drugs can differ significantly from pharmacokinetics in plasma. A crucial determinant in the disposition of antiepileptic drugs to the brain is represented by the blood-brain barrier. There is growing evidence that this barrier can alter the availability of antiepileptic drugs at the target site. The permeability of the blood-brain barrier becomes particularly relevant in epileptic conditions and in drug refractory situations. In vivo, intracerebral microdialysis is a valuable technique to determine biophase drug concentrations as it enables investigation of antiepileptic drug transport and distribution in the brain as a function of time. The present review illustrates that intracerebral microdialysis is an indispensable tool for the assessment of the pharmacokinetics of antiepileptic drugs. In addition, we demonstrate how microdialysis data can be used in conjunction with mechanism-based pharmacokinetic/pharmacodynamic modeling for dose selection and optimization of the therapeutic regimen for novel compounds.

Wiggins, Jon Brendan; Rajapakse, Ramona

doi: 10.1517/17425250903206996pmid: 19743890

Background: 5-Aminosalicylate (5-ASA) agents are the mainstay of oral therapy for ulcerative colitis (UC). Balsalazide, a prodrug of 5-ASA, has recently been approved for the treatment of UC. Objective: To summarize current data on balsalazide and to discuss its impact on management of UC. Methods: A systematic review of published literature was performed on PubMed using the search terms ‘Balsalazide’ and ‘Colazal™’. The Cochrane database was also reviewed. Results: Balsalzide, a 5-ASA prodrug, ulilizes azoreduction by colonic bacteria to achieve a sustained release of active 5-ASA throughout the colon. A recent clinical trial has demonstrated balsalazide 6.7 g/day to be superior to placebo in inducing remission in symptomatic UC. The drug is well tolerated with a safety profile comparable to other oral 5-ASA agents. The current data suggests that symptomatic remission occurs with both greater swiftness and greater frequency when compared with mesalamine. Conclusion: Balsalazide is approved for the treatment of mild-to-moderate active UC. It is efficacious for the induction of remission in mild to moderate UC and has a favorable safety profile, with the added advantages of greater efficacy of remission induction and rapidity of onset.

Abe, Masanori; Okada, Kazuyoshi; Matsumoto, Koichi

doi: 10.1517/17425250903282799pmid: 19761411

The goal of antihypertensive treatment is to reduce cardiovascular and cerebrovascular events associated with high blood pressure. A combination therapy with different antihypertensive agents is more successful than monotherapy in most hypertensive patients, with the added advantage of a better safety profile. Therefore, treatment of hypertensive patients with fixed-dose combination therapy consisting of the angiotensin II receptor blocker losartan along with hydrochlorothiazide (HCTZ) has several potential benefits over monotherapy with each individual component. It provides more effective blood pressure control, a reduction in the likelihood of adverse effects and facilitation of patient compliance due to a simple once-daily regimen. One of the advantages of the combination of losartan with HCTZ is the potential reduction in HCTZ-induced metabolic disorders; in particular, this combination can have attractive benefits for patients of hyperuricemia. Losartan plus HCTZ fixed-dose combination therapy is frequently recommended for the treatment of hypertension and lowers blood pressure in mild-to-moderate and even severe hypertensive patients to a level comparable with other classes of antihypertensive agents in combination with HCTZ. Fixed-dose combination therapy with losartan plus HCTZ is a logical choice as antihypertensive therapy for patients in whom combination therapy is necessary to achieve additional blood pressure reduction.

Gatti, Davide; Viapiana, Ombretta; Idolazzi, Luca; Fracassi, Elena; Adami, Silvano

doi: 10.1517/17425250903029190pmid: 19761412

Neridronic acid (6-amino-1-idroxyesilidene-1,1-bisphosphonate) is a nitrogen-containing bisphosphonate licensed in Italy for the treatment of osteogenesis imperfecta and Paget's disease of bone. The pharmacodynamic profile is similar to that of other nitrogen-containing bisphosphonates and is characterized by its high affinity for bone tissue particularly at sites undergoing a process of remodeling. In growing children affected by osteogenesis imperfect, neridronic acid rapidly increases bone mineral density as measured by dual X-ray absortiometry and this is associated with a significant decrease in fracture cumulative number. Similar results have been obtained also in newborns (< 12 month old) and in adult patients. In Paget's disease of bone, 200 mg intravenous neridronic acid is associated with a 65% rate of full remission and a biochemical response (decrease of > 75% of bone turnover markers) in 95% of the patients. Neridronic acid treatment has been reported to be effective also in other skeletal diseases such as osteoporosis, algodystrophy, hypercalcemia of malignancy and bone metastasis. Neridronic acid has been developed only for parenteral use, and it is the only one used as intramuscular injection. This avoids all the limitations of oral bisphosphonates and may be offered for a home treatment with simple nursing assistance.