Idiosyncratic Drug Reactions Research Articles

Solar Burn Reactivation Induced by Methotrexate

Solar burn reactivation, a rare and idiosyncratic drug reaction, has been reported with the use of a variety of drugs. This reaction is believed to be the result of exposure to ultraviolet light during the subsiding phase of an acute inflammatory reaction. It affects areas of the body that have been previously sunburned. We describe a 16-year-old girl who was receiving treatment for acute lymphoblastic leukemia and experienced a second-degree solar burn reactivation reaction to methotrexate. The patient had a mild sunburn on her face and shoulders the day she went to the oncology clinic for her interim maintenance chemotherapy with vincristine 1.5 mg/m(2)/dose and methotrexate 100 mg/m(2)/dose. Three days later, she returned to the clinic with a 2-day history of fever (<or= 100.2 degrees F), nausea, vomiting, and malaise; the sunburn on her face and shoulders also had become severe, without further sun exposure. Laboratory results revealed elevated blood urea nitrogen and serum creatinine concentrations, and her methotrexate level was elevated at 0.9 mM. The patient was diagnosed with acute renal failure, dehydration, methotrexate toxicity, and second-degree solar burn reactivation reaction. She was admitted to the children's hospital and treated with sodium bicarbonate, acetaminophen with codeine, ondansetron, and silvadene cream. On hospital day 3, the patient's methotrexate level decreased to less than 0.1 mM. The sunburn continued to heal, and after a 14-day hospital stay, complicated by a streptococcal infection, grade 3 mucositis, bacteremia, and mild gastritis and duodenitis, the patient recovered and was discharged. Use of the Naranjo adverse drug reaction probability scale indicated a probable relationship (score of 6) between the patient's solar burn reactivation and methotrexate. Although methotrexate-induced solar burn reactivation is rare, clinicians should be aware of this potential adverse reaction and consider delaying administration of methotrexate by 5-7 days if a patient reports ultraviolet-related erythema in the past 2-4 days or presents with a notable sunburn.

Suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of experimental drug-induced liver injury: a review

Importance of the field: Idiosyncratic drug reactions resulting in drug-induced liver injury (DILI) account for ∼ 13% of acute liver failure cases in the US. Idiosyncratic drug reactions are the third most common cause of liver transplantation, exceeded only by acetaminophen and indeterminate causes. Clinical evidence suggests that idiosyncratic DILI is triggered by drug hapten-altered self proteins resulting in hepatocellular injury. An example of this type of DILI is hepatitis that develops in susceptible individuals following administration of halogenated volatile anesthetics, dihydralazine, carbamazepine or diclofenac.Areas covered in this review: In this review, we describe research in animal models that supports a critical role for suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of immune-mediated DILI.What the reader will gain: The reader will gain insights into the roles of IL-4 in the development of experimental DILI. The reader will gain tools to assist in the translation of these findings to those in patients with immune-mediated DILI, as well as other inflammatory diseases of the liver. The reader will then be made aware of gaps in knowledge in the pathogenesis of DILI where research could result in significant advances in the care of these complicated patients.Take home message: In experimental immune-mediated DILI, IL-4 suppresses regulatory responses to CYP2E1 autoantigens but induces pro-inflammatory responses to drug haptens.

A Mouse Model of Severe Halothane Hepatitis Based on Human Risk Factors

Halothane (2-bromo-2-chloro-1,1,1-trifluoro-ethane) is an inhaled anesthetic that induces severe, idiosyncratic liver injury, i.e., "halothane hepatitis," in approximately 1 in 20,000 human patients. We used known human risk factors (female sex, adult age, and genetics) as well as probable risk factors (fasting and inflammatory stress) to develop a murine model with characteristics of human halothane hepatitis. Female and male BALB/cJ mice treated with halothane developed dose-dependent liver injury within 24 h; however, the liver injury was severe only in females. Livers had extensive centrilobular necrosis, inflammatory cell infiltrate, and steatosis. Fasting rendered mice more sensitive to halothane hepatotoxicity, and 8-week-old female mice were more sensitive than males of the same age or than younger (4-week-old) females. C57BL/6 mice were insensitive to halothane, suggesting a strong genetic predisposition. In halothane-treated females, plasma concentration of tumor necrosis factor-alpha was greater than in males, and neutrophils were recruited to liver more rapidly and to a greater extent. Anti-CD18 serum attenuated halothane-induced liver injury in female mice, suggesting that neutrophil migration, activation, or both are required for injury. Coexposure of halothane-treated male mice to lipopolysaccharide to induce modest inflammatory stress converted their mild hepatotoxic response to a pronounced, female-like response. This is the first animal model of an idiosyncratic adverse drug reaction that is based on human risk factors and produces reproducible, severe hepatitis from halothane exposure with lesions characteristic of human halothane hepatitis. Moreover, these results suggest that a more robust innate immune response underlies the predisposition of female mice to halothane hepatitis.

Structural Alerts, Reactive Metabolites, and Protein Covalent Binding: How Reliable Are These Attributes as Predictors of Drug Toxicity?

In an increasing number of cases, a deeper understanding of the biochemical basis for idiosyncratic adverse drug reactions (IADRs) has aided to replace a vague perception of a chemical class effect with a sharper picture of individual molecular peculiarity. Considering that IADRs are too complex to duplicate in a test tube, and their idiosyncratic nature precludes prospective clinical studies, it is currently impossible to predict which new drugs will be associated with a significant incidence of toxicity. Because it is now widely appreciated that reactive metabolites, as opposed to the parent molecules from which they are derived, are responsible for the pathogenesis of some IADRs, the propensity of drug candidates to form reactive metabolites is generally considered a liability. Procedures have been implemented to monitor reactive-metabolite formation in discovery with the ultimate goal of eliminating or minimizing the liability via rational structural modification of the problematic chemical series. While such mechanistic studies have provided retrospective insight into the metabolic pathways which lead to reactive metabolite formation with toxic compounds, their ability to accurately predict the IADR potential of new drug candidates has been challenged. There are several instances of drugs that form reactive metabolites, but only a fraction thereof cause toxicity. This review article will outline current approaches to evaluate bioactivation potential of new compounds with particular emphasis on the advantages and limitation of these assays. Plausible reason(s) for the excellent safety record of certain drugs susceptible to bioactivation will also be explored and should provide valuable guidance in the use of reactive-metabolite assessments when nominating drug candidates for development.

A Strategy to Minimize Reactive Metabolite Formation: Discovery of (S)-4-(1-Cyclopropyl-2-methoxyethyl)-6-[6-(difluoromethoxy)-2,5-dimethylpyridin-3-ylamino]-5-oxo-4,5-dihydropyrazine-2-carbonitrile as a Potent, Orally Bioavailable Corticotropin-Releasing Factor-1 Receptor Antagonist

Detailed metabolic characterization of 8, an earlier lead pyrazinone-based corticotropin-releasing factor-1 (CRF(1)) receptor antagonist, revealed that this compound formed significant levels of reactive metabolites, as measured by in vivo and in vitro biotransformation studies. This was of particular concern due to the body of evidence suggesting that reactive metabolites may be involved in idiosyncratic drug reactions. Further optimization of the structure-activity relationships and in vivo properties of pyrazinone-based CRF(1) receptor antagonists and studies to assess the formation of reactive metabolites led to the discovery of 19e, a high affinity CRF(1) receptor antagonist (IC(50) = 0.86 nM) wherein GSH adducts were estimated to be only 0.1% of the total amount of drug-related material excreted through bile and urine, indicating low levels of reactive metabolite formation in vivo. A novel 6-(difluoromethoxy)-2,5-dimethylpyridin-3-amine group in 19e contributed to the potency and improved in vivo properties of this compound and related analogues. 19e had excellent pharmacokinetic properties in rats and dogs and showed efficacy in the defensive withdrawal model of anxiety in rats. The lowest efficacious dose was 1.8 mg/kg. The results of a two-week rat safety study with 19e indicated that this compound was well-tolerated.

Toxic epidermal necrolysis following thalidomide and dexamethasone treatment for multiple myeloma: a case report

Dear Editor, Toxic epidermal necrolysis (TEN) is a life-threatening idiosyncratic adverse drug reaction characterized by fullthickness epidermal necrosis and eventual development of subepidermal bullae [1, 2]. Carbamazapine, phenytoin, and allopurinol are frequent causes of TEN in Asia [1]. Although thalidomide has been suggested as a cause of TEN, to the best of our knowledge, only two cases had been reported worldwide [3, 4]. We present the case of a 65-year-old male patient who was initially diagnosed with Immunoglobulin G-κ (IgG-κ) multiple myeloma stage IIIA, and had been treated with combination chemotherapies including a VAD (vincristine, doxorubicin, and dexamethasone) regimen as well as an MP (melphalan and prednisone) regimen. About 3 years after finishing MP treatment, the patient presented with bone pain and a serum monoclonal protein level of 4,170 mg/dl. We initiated treatment with 200 mg thalidomide per day for 12 days and 40 mg dexamethasone per day for 4 days, and the dose of thalidomide was then increased to 400 mg per day. Eleven days after the boost, the patient’s course was complicated by the development of sloughing from the periorbital and perioral regions. Three days after the onset of the initial event, the patient was admitted to the emergency department with fever, dysphagia, and itching. The same day, oral mucositis, angioedema of lips and eyebrow, and diffuse erythematous to purpuric patches and plaques appeared on the face, neck, trunk, and extremities. On the third day of hospitalization, a skin biopsy showed subepidermal bullous change with an infiltration of lymphocytes and polymorphonuclear cells in the epidermis which suggested TEN (Fig. 1). On the fifth day, the clinical course was complicated by the worsening rash over the entire trunk, and the patient was started on 400 mg/kg intravenous immunoglobulin (IV-Globulin S®, Green Cross Inc., Korea) per day for five consecutive days. In addition, the patient was intensively treated as a burn patient. On the sixth day, focal skin detachment was noticed and, after 2 days, was followed by generalized detachment from the face to the lower extremities. The patient’s severity-ofillness score for TEN (SCORTEN) was 4 with an expected W. K. Eo : S. H. Cheon : S. H. Lee : J. S. Jeong Integrative Cancer Center, Kyung Hee University, East-West Neo Medical Center, Seoul, South Korea

D-Penicillamine-Induced Autoimmunity: Relationship to Macrophage Activation

Idiosyncratic drug reactions represent a serious health problem, and they remain unpredictable largely due to our limited understanding of the mechanisms involved. Penicillamine-induced autoimmunity in Brown Norway (BN) rats represents one model of an idiosyncratic reaction, and this drug can also cause autoimmune reactions in humans. We previously demonstrated that penicillamine binds to aldehydes on the surface of macrophages. There is evidence that an imine bond formed by aldehyde groups on macrophages and amine groups on T cells is one type of interaction between these two cells that is involved in the induction of an immune response. We proposed that the binding of penicillamine with aldehyde groups on macrophages could lead to their activation and in some patients could lead to autoimmunity. In this study, the transcriptome profile of spleen macrophages 6 h after penicillamine treatment was used to detect effects of penicillamine on macrophages with a focus on 20 genes known to be macrophage activation biomarkers. One biological consequence of macrophage activation was investigated by determining mRNA levels for IL-15 and IL-1 beta which are crucial for NK cell activation, as well as levels of mRNA for selected cytokines in spleen NK cells. Up-regulation of the macrophage activating cytokines, IFN-gamma and GM-CSF, and down-regulation of IL-13 indicated activation of NK cells, which suggests a positive feedback loop between macrophages and NK cells. Furthermore, treatment of a murine macrophage cell line, RAW264.7, with penicillamine increased the production of TNF-alpha, IL-6, and IL-23, providing additional evidence that penicillamine activates macrophages. Hydralazine and isoniazid cause a lupus-like syndrome in humans and also bind to aldehyde groups. These drugs were also found to activate RAW264.7 macrophages. Together, these data support the hypothesis that drugs that bind irreversibly with aldehydes lead to macrophage activation, which in some patients can lead to an autoimmune syndrome.

Bioactivation of Minocycline to Reactive Intermediates by Myeloperoxidase, Horseradish Peroxidase, and Hepatic Microsomes: Implications for Minocycline-Induced Lupus and Hepatitis

Of the tetracyclines, minocycline is unique in causing a significant incidence of a lupus-like syndrome and autoimmune hepatitis. It is also unique among the tetracyclines in having a para-N,N-dimethylaminophenol ring. Many drugs that cause autoimmune reactions are oxidized to reactive metabolites by the myeloperoxidase (MPO) system of macrophages. In this study, we showed that minocycline is oxidized to reactive intermediates by MPO/H(2)O(2)/Cl(-), HOCl, horseradish peroxidase/H(2)O(2), or hepatic microsomes. When trapped with N-acetylcysteine (NAC), two adducts with protonated molecular ions at m/z 619 were isolated and analyzed by NMR. One represents attack of the aromatic D ring by NAC meta to the N,N-dimethylamino group, which implies that the reactive intermediate was a quinone iminium ion. The NMR of the other adduct, which was not observed when minocycline was oxidized by hepatic microsomes, indicates that the NAC is attached at the junction of the B and C rings. In the oxidation by HOCl, we found an intermediate with a protonated molecular ion of m/z 510 that represents the addition of HOCl to minocycline. The HOCl presumably adds across the double bond of the B ring, and reaction of this intermediate with NAC led to the second NAC adduct. We were surprised to find that the same NAC adduct was not observed after oxidation of tetracycline with HOCl, even though this part of the tetracycline structure is the same as for minocycline. We propose that one or more of these reactive metabolites are responsible for the idiosyncratic drug reactions that are specific to this tetracycline.

Severe idiosyncratic drug reaction (Lyells syndrome) after ingesting dihydroartemisinin

Lyells syndrome also called Toxic epidermal necrolysis is the extreme form of idiosyncratic drug reaction that is called Steven Johnsons Syndrome: The condition results in an extensive loss of the skin with mucous membrane involvement. Lyells syndrome has been induced by many agents. The commonest agent in the literature being sulphonamides. However, in our search of the medical literature there was no report of dihydroarthemisinin as a cause of Lyells syndrome. We report three patients seen at two tertiary health institutions with Lyells syndrome after treatment for malaria with dihydroarthemisinin. This resulted from administration of dihydroarthemisinin with chloroquine in two patients and dihydroarthemisinin with Amodiaquine in one patient. The first patient was a seven year old child who developed 90% cutaneous involvement and died from hemorrhagic shock. The second was a 28 old female that developed a 76% body surface involvement and died from septicemia. The third patient was a pregnant 37 year old woman that developed 52% body involvement and died from septic shock. In these patients the earliest symptoms were not recognized and there was considerable delay before referral. In view of the recent WHO recommendation ofArthemisinin Combination Treatment (ACT) for malaria, we expect more cases of Steven Johnson Syndrome and Lyells syndrome from ACT treatment. The aim of this report is to raise the awareness of clinicians to this potentially fatal complication.

Covalent Binding of Penicillamine to Macrophages: Implications for Penicillamine-Induced Autoimmunity

Idiosyncratic drug reactions (IDRs) represent a major clinical problem, and at present, the mechanisms involved are still poorly understood. One animal model that we have used for mechanistic studies of IDRs is penicillamine-induced autoimmunity in Brown Norway (BN) rats. Previous work in our lab found that macrophage activation preceded the clinical autoimmune syndrome. It is thought that one of the interactions between T cells and macrophages involves reversible Schiff base formation between an amine on T cells and an aldehyde on macrophages, but the identity of the molecules involved is unknown. It is also known that penicillamine reacts with aldehyde groups to form a thiazolidine ring, which unlike a Schiff base, is essentially irreversible. Such binding could lead to macrophage activation. Generalized macrophage activation could lead to the observed autoimmune reaction. Hydralazine and isoniazid also react with aldehydes to form stable hydrazones, and they also cause an autoimmune lupus-like syndrome. In this study, isolated spleen cells from male BN rats were incubated with biotin-aldehyde-reactive probe (ARP, a hydroxylamine), biotin-hydrazide, or D-penicillamine. At all concentrations, ARP, hydrazide, and penicillamine preferentially "stained" macrophages relative to other spleen cells. In addition, preincubation of cells with penicillamine or hydralazine decreased ARP staining of macrophages, which further indicates that most of the ARP binding to macrophages involves binding to aldehyde groups. This provides support for the hypothesis that the interaction between aldehyde-containing signaling molecules on macrophages and penicillamine could be the initial event of penicillamine-induced autoimmunity. Several of the proteins to which ARP binds were identified, and some such as myosin are attractive candidates to mediate macrophage activation.

Hepatitis following famotidine: a case report

H2 receptor antagonists can rarely cause idiosyncratic drug reactions leading to acute hepatitis. Famotidine, however, is considered a relatively safe drug with regards to hepatotoxicity. We report a case of a 47 year old male with a history of hepatitis C who developed acute hepatitis on the third day of hospitalization with a dramatic rise in his liver enzymes from normal values at the time of admission. The acute rise in liver enzymes made us consider an adverse drug reaction and famotidine was discontinued. Subsequently his liver enzymes came back to normal in seven days. Thus, physicians should consider famotidine induced hepatitis as a possible etiology of acute liver dysfunction.

The Role of Haptic Macrophages in Regulation of Idiosyncratic Drug Reactions

Idiosyncratic drug reactions (IDR) account for approximately 6%-10% of all adverse drug reactions. The unpredictable and serious nature of these reactions makes them a significant economic burden and safety concern to the health care community and the pharmaceutical industry. Clinical and laboratory evidence suggests that adverse immune responses against drug-protein adducts play a role in the pathogenesis of IDR. However, it remains unclear why only a small percentage of patients are susceptible to developing these reactions. We hypothesized that most patients develop immunological tolerance against drug-protein adducts as a default mechanism, and that IDRs can only occur when this tolerance is deficient or abrogated in susceptible individuals. Using a murine model of 2,4-dinitrochlorobenzene (DNCB)-induced delayed type hypersensitivity (DTH) reaction, our previously published data demonstrated that intravenous pretreatment of mice with dinitrophenyl-bovine serum albumin (DNP-BSA) induced immunological tolerance to subsequent DNCB sensitization, and that hepatic macrophages (Kupffer cells, KC) played an important role in mediating such tolerance. Further mechanistic investigation revealed that KC, acting as incompetent antigen-presenting cells, cannot elicit strong T cell reactions, and that they actively suppress T cell activation through production of prostaglandins. These findings suggest that KCs may play a critical role in regulating immune reactions within the liver and contributing to liver-mediated systemic immune tolerance.

Immune-Mediated Adverse Drug Reactions

Many adverse drug reactions are mediated by the immune system. This can be because the therapeutic effect of the drug targets the immune system. For example, immunosuppressive drugs increase the risk of infections. It is paradoxical that some immunosuppressive drugs can lead to autoimmune reactions. Another mechanism by which drugs can cause an adverse reaction involves an idiosyncratic response to the drug such as an immune-mediated skin rash. These idiosyncratic drug reactions (IDRs) are difficult to study because of the paucity of valid animal models and their unpredictable nature. Therefore, much of our mechanistic knowledge of IDRs is based on inferences from the clinical characteristics of IDRs rather than on controlled mechanistic studies. In general, IDRs are associated with a delay between starting the drug and the onset of the adverse reaction, and the typical delay is different for different types of IDRs. In contrast, on rechallenge, there is usually a rapid onset of the adverse reaction, which is characteristic of an amnestic immune response. The absence of such a rapid response is usually considered evidence that an IDR is not immune-mediated; yet, there are immune-mediated IDRs that do not have an amnestic response. One possible reason for the lack of an amnestic response is if the IDR has a strong autoimmune component leading to deletion of autoimmune memory cells when the drug is withdrawn. Another interesting characteristic of IDRs is that there are many drugs that can cause different types of IDRs in different patients. A possible explanation is that although the immune response is induced by a drug, it is directed against an autoantigen, and interindividual differences in the immune repertoire determine which autoantigen and target organ are affected. Although testing these hypotheses represents a difficult challenge, the importance of these adverse reactions makes it a high priority.

Hepatotoxic Interaction of Sulindac with Lipopolysaccharide: Role of the Hemostatic System

Sulindac (SLD) is a nonsteroidal anti-inflammatory drug (NSAID) that has been associated with a greater incidence of idiosyncratic hepatotoxicity in human patients than other NSAIDs. One hypothesis regarding idiosyncratic adverse drug reactions is that interaction of a drug with a modest inflammatory episode precipitates liver injury. In this study, we tested the hypothesis that lipopolysaccharide (LPS) interacts with SLD to cause liver injury in rats. SLD (50 mg/kg) or its vehicle was administered to rats by gavage 15.5 h before LPS (8.3 x 10(5) endotoxin unit/kg) or its saline vehicle (i.v.). Thirty minutes after LPS treatment, SLD or vehicle administration was repeated. Rats were killed at various times after treatment, and serum, plasma, and liver samples were taken. Neither SLD nor LPS alone caused liver injury. Cotreatment with SLD/LPS led to increases in serum biomarkers of both hepatocellular injury and cholestasis. Histological evidence of liver damage was found only after SLD/LPS cotreatment. As a result of activation of hemostasis induced by SLD/LPS cotreatment, fibrin and hypoxia were present in liver tissue before the onset of hepatotoxicity. Heparin treatment reduced hepatic fibrin deposition and hypoxia and protected against liver injury induced by SLD/LPS cotreatment. These results indicate that cotreatment with nontoxic doses of LPS and SLD causes liver injury in rats, and this could serve as a model of human idiosyncratic liver injury. The hemostatic system is activated by SLD/LPS cotreatment and plays an important role in the development of SLD/LPS-induced liver injury.

Carbamazepine‐induced drug‐induced hypersensitivity syndrome in a 14‐year‐old Japanese boy

Drug-induced hypersensitivity syndrome (DIHS) is a life-threatening idiosyncratic drug reaction, and an early accurate diagnosis is essential for its treatment. We describe a 14-year-old boy with localization-related epilepsy, who developed severe adverse cutaneous and systemic reactions after 3 weeks of carbamazepine administration. During the course of the clinical symptoms, reactivation of human herpesvirus 6 (HHV-6) was proven by detection of the HHV-6 genome in serum and elevation of HHV-6 immunoglobulin G (IgG). He fulfilled the newly established criteria for DIHS. Among eight identified medications that can precipitate DIHS, four are antiepileptic drugs. Establishing a treatment strategy for DIHS is warranted to improve its outcome. Therefore, it is important to raise awareness of DIHS among epileptologists.

Analysis of clinical features and prognostic factors of allopurinol hypersensitivity syndrome

Objective To study the clinical features and prognostic factors of allopurinol hypersensitivity syndrome (AHS). Methods Clinical data on 52 patients with AHS were collected and analyzed retrospectively. Results AHS was an acute severe drug reaction with characteristic symptoms, including generalized skin eruption, fever, lymphadenectasis, eosinophilia, and multiple visceral involvement. Renal inadequacy was observed in 88% of the patients prior to the occurrence of AHS. After the onset of AHS, serum creatinine increased from 271 to 668 μmol/L (t=3.46, P<0.05), while glomerular filtration rate dropped from 37 to 14 ml/min (t=2.87, P<0.05 ) in patients. Multivariate logistic regression analysis revealed that the prognosis of AHS was associated with the severity of renal inadequaey prior to the onset ofAHS (r=2.56, P<0.05 ), severity of skin eruption (r=2.34, P<0.05 ), systemic and regular administration of adequate glucocorticoids (r=2.05 ,P<0.05 ), Conclusions AHS is a severe idiosyncratic drug reaction and commonly seen in patients with renal inadequacy. Severe renal inadequacy and skin eruption often predict a poor prognosis. Systemic glucocorticoid is effective for AHS. Key words: Allopurinol; Drug hypersensitivity; Prognosis

Gene Expression Profiles in Livers from Diclofenac-Treated Rats Reveal Intestinal Bacteria-Dependent and -Independent Pathways Associated with Liver Injury

Diclofenac (DCLF) is a nonsteroidal anti-inflammatory drug that is associated with idiosyncratic adverse drug reactions in humans. Previous studies revealed a crucial role for intestine-derived bacteria and/or lipopolysaccharide (LPS) in DCLF-induced hepatotoxicity. We further explored this mechanism by conducting gene expression analysis of livers from rats treated with a hepatotoxic dose of DCLF (100 mg/kg) with or without oral antibiotic pretreatment. Genes for which expression was altered by DCLF were divided into two groups: genes with expression altered by antibiotic treatment and those unaffected by antibiotics. The former group of genes represented the ones for which DCLF-induced alterations in expression depended on intestinal bacteria. The expression of the latter group of genes was probably changed by direct effect of DCLF rather than by intestinal bacteria. Functional analysis of genes in the former group revealed LPS-related signaling, further suggesting a role for bacterial endotoxin in the liver injury. Functional analysis of genes in the latter group revealed changes in signaling pathways related to inflammation, hypoxia, oxidative stress, the aryl hydrocarbon receptor, and peroxisome proliferator-activated receptor alpha. Neutrophil depletion failed to protect from DCLF-induced hepatotoxicity, suggesting that intestinal bacteria contribute to liver injury in a neutrophil-independent manner. Hypoxia occurred in the livers of rats treated with DCLF, and hypoxia in vitro rendered hepatocytes sensitive to DCLF-induced cytotoxicity. These results support the hypothesis that intestinal bacteria are required for DCLF-induced hepatotoxicity and suggest that hypoxia plays an important role in the pathogenesis.

Demonstration of the Metabolic Pathway Responsible for Nevirapine-Induced Skin Rash

The reverse transcriptase inhibitor, nevirapine (NVP), causes skin rashes and hepatotoxicity. We used a rat model to determine if the rash is caused by the parent drug or a reactive metabolite. By manipulation of metabolic pathways and testing analogues, we eliminated all but one pathway, 12-hydroxylation, which involves the oxidation of an exocyclic methyl group, as being responsible for the rash. Treatment with 12-OH-NVP caused a rash, and an analogue in which the methyl hydrogens were replaced by deuterium to inhibit the 12-OH pathway did not cause a rash; however, quite unexpectedly, blood levels of the deuterated analogue were very low. This is due to partitioning of the benzylic free radial intermediate between oxygen rebound to form 12-OH-NVP and loss of another hydrogen atom to form a reactive quinone methide, which inactivates P450. Cotreatment with the P450 inhibitor, 1-aminobenzotriazole, led to comparable levels of NVP and the deuterated analogue, and the deuterated analogue still caused a lower rash incidence. These data clearly point to the 12-hydroxy pathway being responsible for NVP skin rash. We propose that the hepatotoxicity of NVP in humans is due to the quinone methide formed by P450 in the liver, while the skin rash may be due to the quinone methide formed in the skin by sulfation of 12-OH metabolite followed by loss of sulfate. This is the first example in which a valid animal model of an idiosyncratic drug reaction was used to determine the metabolic pathway responsible for the reaction.

Clozapine-Induced Agranulocytosis and its Genetic Determinants

Clozapine-induced agranulocytosis (CiA) is a potentially life-threatening pharmacological adverse drug reaction, which limits a broader application of this highly effective atypical antipsychotic in schizophrenic patients. Although this adverse reaction has been well known for almost 30 years, only few genetically based determinants can be identified to date. Furthermore, owing to rare occurrence, specific clinical course and complexity of pathomechanisms of antipsychotic-induced agranulocytosis, only a few of the findings met the criteria of replication. The most promising susceptibility genes for CiA include genes involved in the human leukocyte antigen system and in specific metabolizing enzyme systems. However, complex idiosyncratic drug reactions such as CiA are considered to be determined by multiple, possibly interacting genetic variations, rather than by a single causative variant.

Toxicophores, reactive metabolites and drug safety: when is it a cause for concern?

It is generally accepted that bioactivation of relatively inert functional groups (toxicophores) to reactive metabolites is an obligatory step in the pathogenesis of certain idiosyncratic adverse drug reactions (IADRs). IADRs cannot be detected in regulatory animal toxicity studies and, given their low frequency of occurrence in humans (1 in 10,000 to 1 in 100,000), they are often not detected until the drug has gained broad exposure in a large patient population. The detection of IADRs during late clinical trials or after a drug has been released can lead to an unanticipated restriction in its use, and even in its withdrawal. To date, there is neither a consistent nor a well-defined link between bioactivation and IADRs; however, the potential does exist for these processes to be causally related. Thus, the formation of reactive metabolites with a drug candidate is generally considered a liability in most pharmaceutical companies. Procedures have been implemented to evaluate bioactivation potential of new drug candidates with the goal of eliminating or minimizing reactive metabolite formation by rational structural modification of the lead chemical class. While such studies have proven extremely useful in the retrospective analysis of bioactivation pathways of toxic drugs and defining toxicophores, their ability to accurately predict the IADR potential of new drug candidates has been challenged, given that several commercially successful drugs form reactive metabolites, yet, they are not associated with a significant incidence of IADRs. In this article, we review the basic methodology that is currently utilized to evaluate the bioactivation potential of new compounds, with particular emphasis on the advantages and limitation of these assays. Plausible reasons for the excellent safety record of certain drugs susceptible to bioactivation are also explored. Overall, these observations provide valuable guidance in the proper use of bioactivation assessments when selecting drug candidates for development.