Idiosyncratic drug reactions: possible role of reactive metabolites generated by leukocytes

Environmental occurrence, risk, and removal strategies of pyrazolones: A critical review

Pyrazolones, widely used as analgesic and anti-inflammatory pharmaceuticals, have become a significant concern because of their persistence and widespread presence in engineered (e.g., wastewater treatment plants) and natural environments. Thus, the urgent task is to ensure the effective and cost-efficient removal of pyrazolones. Advanced oxidation processes are the most commonly used removal method. Furthermore, the biodegradation of pyrazolones has been exploited using microbial communities or pure strains; however, screening for efficient degrading bacteria and clarifying the biodegradation mechanisms required further research. In this critical review, we overview the environmental occurrence of pyrazolones, their potential ecological health risks, and their corresponding removal techniques (e.g., O3 oxidation, photocatalysis, and Fenton-like process). We also emphasize the prospects for the risk and contamination control of pyrazolones in various environments using physicochemical-biochemical coupling technology. Collectively, the environmental occurrence of pyrazolones poses significant public health concerns, necessitating heightened attention and the implementation of effective methods to minimize their environmental risks.

The many roles of myeloperoxidase: From inflammation and immunity to biomarkers, drug metabolism and drug discovery

This review provides a practical guide to myeloperoxidase (MPO) and presents to the reader the diversity of its presence in biology. The review provides a historical background, from peroxidase activity to the discovery of MPO, to its role in disease and drug development. MPO is discussed in terms of its necessity, as specific individuals lack MPO expression. An underlying theme presented throughout brings up the question of the benefit and burden of MPO activity. Enzyme structure is discussed, including accurate masses and glycosylation sites. The catalytic cycle of MPO and its corresponding pathways are presented, with a discussion of the importance of the redox couples of the different states of MPO. Cell lines expressing MPO are discussed and practically summarized for the reader, and locations of MPO (primary and secondary) are provided. Useful methods of MPO detection are discussed, and how these can be used for studying disease processes are implied through the presentation of MPO as a biomarker. The presence of MPO in neutrophil extracellular traps is presented, and the activators of the former are provided. Lastly, the transition from drug metabolism to a target for drug development is where the review concludes.

Integration analysis of GWAS and expression quantitative trait loci to identify candidate genes and pathways for clozapine-related neutropaenia

AIMS

Little is known about the genetic basis of clozapine-related neutropaenia. This study aims to explore candidate genes and pathways involved in clozapine-related neutropaenia.

METHODS

This study conducted a two-stage integrative analysis of the summary statistics from the genome-wide association study (GWAS, n = 552) of the lowest absolute neutrophil count (ANC) during clozapine treatment and the summary data of the expressed quantitative trait locus (eQTL). First, we use the probabilistic Mendelian randomization (PMR-Egger) to identify genes whose expression is causally related to ANC, and then use Bayesian co-localization analysis to investigate whether there are shared causal variants between them [posterior probability for hypotheses 4 (PP.H4) > 0.80]. Finally, gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis were conducted to explore the pathways that may be associated with ANC during clozapine treatment.

RESULTS

PMR-Egger analysis identified 146 genes that may be causally associated with ANC after Bonferroni correction (P-value < 3.25e-6). Bayesian co-localization analysis identified six further genes whose gene expression shared common variants with ANC, including NT5E (PP.H4 = 0.96), GLDC (PP.H4 = 0.82), NUDT17 (PP.H4 = 0.88), MSH4 (PP.H4 = 0.88), PTER (PP.H4 = 0.89) and SERPINB6 (PP.H4 = 0.83). Enrichment analysis identified 52 GO terms and seven pathways associated with ANC, such as NAD metabolic process, drug catabolic process and glyoxylate and dicarboxylate metabolism.

CONCLUSION

This study identified multiple candidate genes and pathways that may be involved in clozapine-related neutropaenia, providing novel clues for the mechanism of clozapine-related neutropaenia.

Mechanisms of Immunotoxicity: Stressors and Evaluators

The immune system defends the body against certain tumor cells and against foreign agents such as fungi, parasites, bacteria, and viruses. One of its main roles is to distinguish endogenous components from non-self-components. An unproperly functioning immune system is prone to primary immune deficiencies caused by either primary immune deficiencies such as genetic defects or secondary immune deficiencies such as physical, chemical, and in some instances, psychological stressors. In the manuscript, we will provide a brief overview of the immune system and immunotoxicology. We will also describe the biochemical mechanisms of immunotoxicants and how to evaluate immunotoxicity.

Genetics of clozapine-associated neutropenia: recent advances, challenges and future perspective

Clozapine is the only effective antipsychotic for treatment-resistant schizophrenia but remains widely under prescribed, at least in part due to its potential to cause agranulocytosis and neutropenia. In this article, we provide an overview of the current understanding of the genetics of clozapine-associated agranulocytosis and neutropenia. We now know that the genetic etiology of clozapine-associated neutropenia is complex and is likely to involve variants from several genes including HLA-DQB1, HLA-B and SLCO1B3/SLCO1B7. We describe recent findings relating to the Duffy-null genotype and its association with benign neutropenia in individuals with African ancestry. Further advances will come from sequencing studies, large, cross-population studies and in understanding the molecular mechanisms underlying these associations.

Deferiprone-Induced Agranulocytosis : A Critical Review of Five Rechallenged Cases

The most serious adverse effect of deferiprone, the first orally active iron chelator, is agranulocytosis afflicting an estimated 1.6% of patients. Among the 13 reported patients who had experienced deferiprone-induced agranulocytosis or severe neutropenia, 5 were rechallenged. We studied the onset, clinical and rechallenge course of all 5 patients in an attempt to characterise the mechanisms involved in deferiprone-induced agranulocytosis, to verify whether rechallenge in future patients is ethically justified. Deferiprone-induced agranulocytosis showed no trend of dose dependency: of all patients who had experienced agranulocytosis 23% were treated with 50 mg/kg/day, 46% with 75 to 90 mg/kg/day, and 31 % with > 90 mg/kg/day. Available data including bone marrow aspiration in some patients support the hypothesis that an early myeloid precursor is the target cell affected by deferiprone. All 5 rechallenged patients re-experienced agranulocytosis/neutropenia. The lag period to agranulocytosis/neutropenia following reinduction was significantly shorter (13.2 ± 21.7 weeks compared with 46.4 ± 14.2 weeks in the first episode; p < 0.05). All but one of the rechallenged patients re-experienced agranulocytosis or neutropenia 2 to 4 weeks following re-exposure to deferiprone, suggesting a possible immune mechanism. We found that deferiprone was oxidised in vitro by hypochlorous acid, the major neutrophil oxidant to produce a myelotoxic metabolite. This reactive species demonstrated neutrophil toxicity and a dose-dependent lymphotoxic curve. However, we found no differences in the toxicity of this reactive species to neutrophils from 2 patients with a history of deferiprone-induced agranulocytosis when compared with controls. In combination with the clinical characteristics, these results suggest a reactive metabolite-induced immune-mediated reaction. These 5 rechallenged cases ethically preclude the rechallenge of additional cases.

A Case of Sublingual Ranula That Responded Successfully to Localized Injection Treatment with OK-432 after Healing from Drug Induced Hypersensitivity Syndrome

A ranula is a mucus retention cyst or pseudocyst caused by leakage of mucus from the sublingual gland and generally occurs in the oral floor. In addition, drug induced hypersensitivity syndrome (DIHS) is a rare but well-recognized serious adverse effect characterized by fever, skin rashes, generalized lymphadenopathy, hepatitis, and hepatosplenomegaly and oral stomatitis. This paper presents the first case of successfully treated sublingual ranula with localized injection of OK-432 after healing from drug induced hypersensitivity syndrome, which has previously been unreported in the literature. We present the case of a 38-year-old Japanese woman with sublingual ranula that responded successfully to localized injection treatment with OK-432 after healing from drug induced hypersensitivity syndrome. She was affected with cutaneous myositis and interstitial lung disease when she was 26 years old. At the age 34 years, she received additional oral treatment of diaminodiphenyl-sulfone due to deterioration of the cutaneous myositis, which resulted in drug induced hypersensitivity syndrome (DIHS) with severe oral stomatitis. Local injection of OK-432 to the ranula may be a very safe and useful treatment method even if the patient has a history of drug allergy and has connective tissue disease such as cutaneous myositis.

Clozapine treatment of refractory schizophrenia during essential chemotherapy: a case study and mini review of a clinical dilemma

BACKGROUND

Clozapine remains the antipsychotic of choice for refractory schizophrenia. Given the particular side effects of clozapine including neutropenia and myelosuppression, safety and efficacy of add-on chemotherapy for patients who are already under clozapine treatment remain unknown.

OBJECTIVE

We present evidence from a patient with a diagnosis of refractory schizophrenia on clozapine medication, who required essential chemotherapy for chronic lymphocytic leukemia (CLL). We have also reviewed literature regarding this challenging clinical dilemma.

METHOD

We report details about a patient with treatment-resistant schizophrenia who was given chemotherapy (fludarabine, cyclophosphamide and rituximab) for CLL in the course of concomitant treatment with clozapine and granulocyte-colony stimulating factor (G-CSFs). In addition, we have reviewed literature using the PUBMED data base.

RESULTS

Current evidence remains insufficient to provide authoritative guide to clinicians regarding the efficacy and safety of the combined use of clozapine and chemotherapy. However, general conclusion from our case and of the published evidence is that a combination of clozapine use and chemotherapeutic agents do not cause additional hematological worsening with no decreasing efficacy concerns raised.

CONCLUSION

Continuing with clozapine in the course of chemotherapy may be relatively safer for patients who responded well to clozapine concomitant with G-CSF treatment.

Dapsone Hypersensitivity Syndrome: A Clinico-Epidemiological Review

Diaminodiphenyl sulphone (dapsone) is a drug of choice in the treatment of leprosy. It is also useful for the treatment of many neutrophilic and other dermatoses. Dapsone hypersensitivity syndrome is a rare but well recognized serious adverse effect characterized by fever, skin rashes, generalized lymphadenopathy, hepatitis, and hepato-splenomegaly. Twenty-six patients with dapsone hypersensitivity syndrome were studied for clinical profile, outcome, and prognosis. The male:female ratio was 2.2:1, and the mean age was 33.19 years (range 13 to 64 years). The interval between start of dapsone therapy and appearance of symptoms varied from 2-7 weeks (mean 29.82 days). Twenty-four patients received dapsone as a part of multi-drug therapy for leprosy; the other two patients received dapsone for lichen planus and acne vulgaris. Exfoliative dermatitis was the most common cutaneous manifestation followed by erythematous maculo-papular eruption and Stevens-Johnson syndrome-like lesion. The other common systemic manifestations were: fever (26 cases), itching (22 cases), lymphadenopathy (21 cases), jaundice (21 cases), pallor (20 cases), hepatomegaly (19 cases), and pedal edema (14 cases). Investigation profile revealed elevated levels of serum liver enzymes in 100% of patients, elevated erythrocyte sedimentation rate in 92.3%, raised bilirubin in 84.6%, leucocytosis in 69.23%, low hemoglobin (<9 gm/dl) in 46.15% and hypoproteinemia in 42.3%. Eosinophilia, hemolytic anemia, and reticulocytosis count were found in 4 patients each. All the patients had favorable outcomes except three who died due to hepatic failure. Medical personnel must be aware of this potentially fatal syndrome, because it can cause considerable morbidity and mortality.

Hypersensitivity reactions to non beta-lactam antimicrobial agents, a statement of the WAO special committee on drug allergy

Antibiotics are used extensively in the treatment of various infections. Consequently, they can be considered among the most important agents involved in adverse reactions to drugs, including both allergic and non-allergic drug hypersensitivity [J Allergy Clin Immunol 113:832–836, 2004]. Most studies published to date deal mainly with reactions to the beta-lactam group, and information on hypersensitivity to each of the other antimicrobial agents is scarce. The present document has been produced by the Special Committee on Drug Allergy of the World Allergy Organization to present the most relevant information on the incidence, clinical manifestations, diagnosis, possible mechanisms, and management of hypersensitivity reactions to non beta-lactam antimicrobials for use by practitioners worldwide.

Dapsone Hypersensitivity Syndrome That Occurred during Treatment of Pediatric Patient with Erythema Elevatum Diutinum

Herein, we report a case of an 8-year-old girl with dapsone hypersensitivity syndrome (DHS) that occurred during the treatment of erythema elevatum diutinum. She had fever, gross hematuria, and malaise for three weeks after initiation of dapsone therapy. Five days after stopping dapsone treatment, she returned to the emergency clinic because of high fever, emesis, diarrhea, upper respiratory symptoms, and worsening of exanthematous eruptions. A diagnosis of DHS was made, and it improved with oral prednisone. We recommend that pediatric patients who are treated with dapsone need to be observed carefully for the development of DHS.

Investigating the mechanisms of aromatic amine-induced protein free radical formation by quantitative structure-activity relationships: implications for drug-induced agranulocytosis

Aromatic amine drugs have been associated with agranulocytosis (neutrophil depletion) for which the mechanism is unknown. We have previously shown that the metabolism of two aromatic amine drugs by human myeloperoxidase (MPO) results in phenyl radical metabolite formation and also in protein free radical formation on MPO. Because the concentration of drug required to produce a maximum signal for MPO protein free radical (MPO*) detection was different for each drug, this prompted us to consider that other aromatic amines may also show varying degrees of ability to induce MPO* formation. Immunoassay experiments using the immuno-spin-trapping technique were performed, which evaluated the potency of different aromatic amines containing the aniline substructure to generate the MPO*. Each reaction contained equal amounts of H(2)O(2), 5,5-dimethyl-1-pyrroline-N-oxide, MPO, and variable concentrations of aniline derivatives. Several physicochemical parameters for aniline derivatives were used to derive quantitative structure-activity relationship equations, which showed that the Hammett constant (sigma) best correlated with the MPO* formation for all aniline derivatives. More statistically robust equations were derived if the anilines were separated into mono- and disubstituted groups. However, some aniline derivatives did not induce MPO* formation. Using electron spin resonance spectroscopy, we evaluated the ability of all aniline derivatives tested to produce phenyl radical metabolites, as previously shown by spin trapping for the aromatic amine drugs. Interestingly, we found that only those aniline derivatives that produced a phenyl radical also formed MPO*. We propose that the phenyl radical is the reactive free radical metabolite responsible for generating the MPO*.

Acetaminophen (paracetamol) inhibits myeloperoxidase-catalyzed oxidant production and biological damage at therapeutically achievable concentrations

The heme peroxidase enzyme myeloperoxidase (MPO) is released by activated neutrophils and monocytes, where it uses hydrogen peroxide (H(2)O(2)) to catalyze the production of the potent oxidants hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypothiocyanous acid (HOSCN) from halide and pseudohalide (SCN(-)) ions. These oxidants have been implicated as key mediators of tissue damage in many human inflammatory diseases including atherosclerosis, asthma, rheumatoid arthritis, cystic fibrosis and some cancers. It is shown here that acetaminophen (paracetamol), a phenol-based drug with analgesic and antipyretic actions, is an efficient inhibitor of HOCl and HOBr generation by isolated MPO-H(2)O(2)-halide systems. With physiological halide concentrations, acetaminophen concentrations required for 50% inhibition of oxidant formation (IC(50)) were 77+/-6microM (100mMCl(-)) and 92+/-2microM (100mMCl(-) plus 100microMBr(-)), as measured by trapping of oxidants with taurine. The IC(50) for inhibition of HOCl generation by human neutrophils was ca. 100microM. These values are lower than the maximal therapeutic plasma concentrations of acetaminophen (< or =150microM) resulting from typical dosing regimes. Acetaminophen did not diminish superoxide generation by neutrophils, as measured by lucigenin-dependent chemiluminescence. Inhibition of HOCl production was associated with the generation of fluorescent acetaminophen oxidation products, consistent with acetaminophen acting as a competitive substrate of MPO. Inhibition by acetaminophen was maintained in the presence of heparan sulfate and extracellular matrix, materials implicated in the sequestration of MPO at sites of inflammation in vivo. Overall, these data indicate that acetaminophen may be an important modulator of MPO activity in vivo.

Bioactivation, protein haptenation, and toxicity of sulfamethoxazole and dapsone in normal human dermal fibroblasts

Cutaneous drug reactions (CDRs) associated with sulfonamides are believed to be mediated through the formation of reactive metabolites that result in cellular toxicity and protein haptenation. We evaluated the bioactivation and toxicity of sulfamethoxazole (SMX) and dapsone (DDS) in normal human dermal fibroblasts (NHDF). Incubation of cells with DDS or its metabolite (D-NOH) resulted in protein haptenation readily detected by confocal microscopy and ELISA. While the metabolite of SMX (S-NOH) haptenated intracellular proteins, adducts were not evident in incubations with SMX. Cells expressed abundant N-acetyltransferase-1 (NAT1) mRNA and activity, but little NAT2 mRNA or activity. Neither NAT1 nor NAT2 protein was detected. Incubation of NHDF with S-NOH or D-NOH increased reactive oxygen species formation and reduced glutathione content. NHDF were less susceptible to the cytotoxic effect of S-NOH and D-NOH than are keratinocytes. Our studies provide the novel observation that NHDF are able to acetylate both arylamine compounds and bioactivate the sulfone DDS, giving rise to haptenated proteins. The reactive metabolites of SMX and DDS also provoke oxidative stress in these cells in a time- and concentration-dependent fashion. Further work is needed to determine the role of the observed toxicity in mediating CDRs observed with these agents.

Improving the decision-making process in structural modification of drug candidates: reducing toxicity

The rule of three, relating to activity-exposure-toxicity, presents the single most difficult challenge in the design and advancement of drug candidates to the development stage. Absorption, distribution, metabolism and excretion (ADME) studies are widely used in drug discovery to optimize this balance of properties necessary to convert lead compounds into drugs that are both safe and effective for human patients. Idiosyncratic drug reactions (IDRs; referred to as type B reactions, which are mainly caused by reactive metabolites) are one type of adverse drug reaction that is important to human health and safety. This review highlights the strategies for the decision-making process involving substructures that, when found in drugs, can form reactive metabolites and are involved in toxicities in humans; the tools used to reduce IDRs are also discussed. Several examples are included to show how toxicity studies have influenced and guided drug design. Investigations of reactive intermediate formation in subcellular fractions with the use of radiolabeled reagents are also discussed.

Ramiprii-induced Agranulocytosis Confirmed by a Lymphocyte Cytotoxicity Test

A 50-year-old man who had chronic renal failure presented with neutropenic fever four days after ramipril was initiated. Agranulocytosis due to other causes was ruled out after a bone marrow aspiration and biopsy examination were performed. A relationship between the drug and the adverse effect was suggested. It was established by a novel lymphocyte cytotoxicity test.

Tolerability of Paracetamol

The excellent tolerability of therapeutic doses of paracetamol (acetaminophen) is a major factor in the very wide use of the drug. The major problem in the use of paracetamol is its hepatotoxicity after an overdose. Hepatotoxicity has also been reported after therapeutic doses, but critical analysis indicates that most patients with alleged toxicity from therapeutic doses have taken overdoses. Importantly, prospective studies indicate that therapeutic doses of paracetamol are an unlikely cause of hepatotoxicity in patients who ingest moderate to large amounts of alcohol. Controlled clinical trials have found that paracetamol is very well tolerated by the gastrointestinal tract. While variable results have been found in case control studies, most studies have shown no change or a small increase in the relative risk of perforations, ulcer or bleeding in the upper gastrointestinal tract. However, associations between the use of paracetamol and gastrointestinal toxicity, as well as with chronic renal disease and asthma, are very likely to reflect biases in some case control studies. In particular, such biases may be caused by the perceived high tolerability of paracetamol in these diseases. The consequent use of paracetamol in these diseases states then leads to an apparent association between paracetamol and the disease. Despite metabolism of paracetamol to reactive compounds, hypersensitivity reactions are rare, although urticaria occurs in occasional patients. Paracetamol appears to be well tolerated during pregnancy although prospective studies are required.

Benefits and risks of deferiprone in iron overload in Thalassaemia and other conditions: comparison of epidemiological and therapeutic aspects with deferoxamine

Deferiprone is the only orally active iron-chelating drug to be used therapeutically in conditions of transfusional iron overload. It is an orphan drug designed and developed primarily by academic initiatives for the treatment of iron overload in thalassaemia, which is endemic in the Mediterranean, Middle East and South East Asia and is considered an orphan disease in the European Union and North America. Deferiprone has been used in several other iron or other metal imbalance conditions and has prospects of wider clinical applications. Deferiprone has high affinity for iron and interacts with almost all the iron pools at the molecular, cellular, tissue and organ levels. Doses of 50-120 mg/kg/day appear to be effective in bringing patients to negative iron balance. It increases urinary iron excretion, which mainly depends on the iron load of patients and the dose of the drug. It decreases serum ferritin levels and reduces the liver and heart iron content in the majority of chronically transfused iron loaded patients at doses >80 mg/kg/day. It is metabolised to a glucuronide conjugate and cleared through the urine in the metabolised and a non-metabolised form, usually of a 3 deferiprone: 1 iron complex, which gives the characteristic red colour urine. Peak serum levels of deferiprone are observed within 1 hour of its oral administration and clearance from blood is within 6 hours. There is variation among patients in iron excretion, the metabolism and pharmacokinetics of deferiprone. Deferiprone has been used in more than 7500 patients aged from 2-85 years in >50 countries, in some cases daily for >14 years. All the adverse effects of deferiprone are considered reversible, controllable and manageable. These include agranulocytosis with frequency of about 0.6%, neutropenia 6%, musculoskeletal and joint pains 15%, gastrointestinal complains 6% and zinc deficiency 1%. Discontinuation of the drug is recommended for patients developing agranulocytosis. Deferiprone is of similar therapeutic index to subcutaneous deferoxamine but is more effective in iron removal from the heart, which is the target organ of iron toxicity and mortality in iron-loaded thalassaemia patients. Deferiprone is much less expensive to produce than deferoxamine. Combination therapy of deferoxamine and deferiprone has been used in patients not complying with subcutaneous deferoxamine or experiencing toxicity or not excreting sufficient amounts of iron with use of either drug alone. New oral iron-chelating drugs are being developed, but even if successful these are likely to be more expensive than deferiprone and are not likely to become available in the next 5-8 years. About 25% of treated thalassaemia patients in Europe and more than 50% in India are using deferiprone. For most thalassaemia patients worldwide who are not at present receiving any form of chelation therapy the choice is between deferiprone and fatal iron toxicity.

Ambroxol inhibits peroxynitrite-induced damage of alpha1-antiproteinase and free radical production in activated phagocytic cells

The present study examined the effect of ambroxol on toxic action of peroxynitrite and the respiratory burst in activated phagocytic cells. Ambroxol decreased the inactivation or destruction of alpha1-antiproteinase induced by peroxynitrite (ONOO-) or hypochlorous acid (HOCl), which was similar to penicillamine and glutathione and was greater than diclofenac sodium and naproxen sodium. Ambroxol significantly decreased ONOO--mediated tyrosine nitration and iron plus EDTA-mediated degradation of 2-deoxy-D-ribose. Ambroxol significantly attenuated the production of superoxide, hydrogen peroxide, HOCl, and nitric oxide in fMLP- or IL-1-activated phagocytic cells, while the inhibitory effects of antiinflammatory and thiol compounds were only observed in HOCl production. Ambroxol and antiinflammatory drugs did not show a cytotoxic effect on macrophages. The results suggest that ambroxol protects tissue components against oxidative damage by an action different from antiinflammatory drugs. Ambroxol may interfere with oxidative damage of alpha1-antiproteinase through a scavenging action on ONOO- and HOCl and inhibition of the respiratory burst of phagocytic cells.

Y-931, a novel atypical antipsychotic drug, is less sensitive to oxidative phenomena

The oxidation behavior of Y-931, a potent atypical antipsychotic drug, was compared with that of clozapine and olanzapine. In two enzymatic systems (horseradish peroxidase (HRP)/glutathione (GSH) and HRP/H(2)O(2)/GSH) which generate thiyl radicals, clozapine markedly strengthened the electron paramagnetic resonance (EPR) signal for the radical. Olanzapine, Y-931 and the major metabolites (compounds 1-3) had no or minimal effect on the intensity of this signal. In addition, the redox potential values for the three derivatives were in accord with the EPR spin trapping results. In toxicological experiments in human leukocytes, a concentration-dependent toxicity was observed when neutrophils were incubated with clozapine (1-10 micromol/l) and H(2)O(2) (1 mmol/l). However, Y-931 and olanzapine did not show remarkable toxicity under the conditions.

Electrooxidation potential as a tool in the early screening for new safer clozapine-like analogues

The chemical modification of clozapine (1) has permitted the finding of new analogues, e.g., olanzapine (2), quetiapine (3), 5-(4-methylpiperazin-1-yl)-8-chloropyrido[2,3-b][1,5]benzoxazepine fumarate (9), with a clinical or psychopharmacological profile similar to that of clozapine. However, when developing new derivatives, the designers are discouraged by the development of clozapine-induced agranulocytosis. Different researchers have raised the role played by the oxidizability of the molecule in such a deleterious effect. In the present paper, we examined the oxidation profile (direct scavenging abilities, efficacy in inhibiting lipid peroxidation, and electrooxidation potential) of newly developed methoxy and trifluoromethylsulfonyloxy analogues related to clozapine, some of them being described as putative antipsychotic. The oxazepine derivative 7, unlike the other diazepine derivatives (6, 10--12), was not readily oxidized. Using a statistical predictive model for hematotoxicity previously described, 7 was found in the cluster of potentially nontoxic compounds while diazepine derivatives 6 and 10-12 were classified as potentially toxic compounds. Among these original compounds, 7, which presents a preclinical clozapine-like profile and a low sensitivity to oxidation, could be a promising antipsychotic candidate with low side effects. Considering the tricyclic derivatives examined so far, some elements of structure-oxidation relationship (SOR) might be pointed out. Regarding the nature of the tricyclic ring substituent, from the most to the least sensitive to oxidation, the sequence was as follows: HO > Cl > CH(3)O > CF(3)SO(2)O. The nature of the tricyclic ring influenced also the sensitivity to oxidation; the diazepine moiety appeared to be the most reactive ring compared to oxa- and thiazepine congeners. These parameters could be advantageously integrated in the early design of new safer clozapine-like analogues.

Idiosyncratic reactions: new methods of identifying high-risk patients

This article describes the mechanisms of idiosyncratic drug reactions (IDRs) and provides an analysis of potential methods for identifying patients at high risk for antiepileptic idiosyncratic drug reactions. IDRs may be caused by toxic metabolites, either directly or indirectly (by way of an immunologic response or a free radical-mediated process). Four methods to potentially identify patients at high risk for AED IDRs are discussed: development of an "at-risk" clinical profile for a particular AED: identification of biomarkers that measure the formation of a toxic metabolite by a previously unrecognized bioactivation pathway for a particular AED; identification of biomarkers indicating deficient detoxification abilities [e.g., deficient free radical scavenging enzyme activities or low calculated oxidative protection (COP) ratios 1 and 2]; and identification of at-risk genetic markers. Clinical profiles for patients receiving valproic acid (VPA), felbamate (FBM), and lamotrigine (LTG) and who are at risk for development of AED IDRs are presented. Patients with VPA IDRs have deficient erythrocyte glutathione peroxidase activity, low plasma selenium concentrations, low COP1 ratios, and low COP2 ratios compared with age-matched controls. Patients with FBM-associated aplastic anemia have deficient erythrocyte glutathione peroxidase, superoxide dismutase (SOD), and glutathione reductase activities compared with age-matched controls. Use of at-risk clinical profiles (for VPA, FBM, and LTG) and measurement of erythrocyte glutathione peroxidase activity, erythrocyte SOD activity, and calculation of COP1 and COP2 ratios (for VPA and FBM) are inexpensive, simple methods of identifying high-risk patients for IDRs. Research is needed to further characterize the mechanism of IDRs, to investigate the clinical utility of free radical-scavenging enzyme activity measurement and calculation of COP ratios for other AED IDRs, and to develop additional methods of identifying patients at high risk for AED IDRs.

Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Adverse drug reactions account for between 2 to 5% of all hospital admissions and can prevent the administration of an otherwise effective therapeutic agent. Hypersensitivity or immune-mediated reactions, although less common, tend to be proportionately more serious. There is convincing evidence to implicate the immune system in the pathogenesis of hypersensitivity reactions. Our understanding of the way in which the immune system recognises drugs is based on the hapten hypothesis; the onset of hypersensitivity involves drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and T cell proliferation. Central to this hypothesis is the critical role of drug metabolism, with the balance between metabolic bioactivation and detoxification being one important component of individual susceptibility. The purpose of this review is to classify drug hypersensitivity reactions in terms of their clinical presentation, and also to consider recent advances in our understanding of the chemical, biochemical and, in particular, cellular immunological mechanisms of hypersensitivity. The following topics are reviewed: (i) drug disposition and cellular metabolism; (ii) mechanisms of antigen processing and presentation; (iii) the role of cytokines and co-stimulatory molecules in the induction and maintenance of a polarised immune response; and (iv) the application of the hapten hypothesis, danger hypothesis and serial triggering model to drug hypersensitivity. A greater understanding of the mechanism(s) of hypersensitivity may identify novel therapeutic strategies and help to combat one of the more severe forms of adverse reactions to drugs.

Transfusional iron overload and chelation therapy with deferoxamine and deferiprone (L1)

Iron is essential for all living organisms. Under normal conditions there is no regulatory and rapid iron excretion in humans and body iron levels are mainly regulated from the absorption of iron from the gut. Regular blood transfusions in thalassaemia and other chronic refractory anaemias can result in excessive iron deposition in tissues and organs. This excess iron is toxic, resulting in tissue and organ damage and unless it is removed it can be fatal to those chronically transfused. Iron removal in transfusional iron overload is achieved using chelation therapy with the chelating drugs deferoxamine (DF) and deferiprone (L1). Effective chelation therapy in chronically transfused patients can only be achieved if iron chelators can remove sufficient amounts of iron, equivalent to those accumulated in the body from transfusions, maintaining body iron load at a non-toxic level. In order to maintain a negative iron balance, both chelating drugs have to be administered almost daily and at high doses. This form of administration also requires that a chelator has low toxicity, good compliance and low cost. DF has been a life-saving drug for thousands of patients in the last 40 years. It is mostly administered by subcutaneous infusion (40-60 mg/kg, 8-12 h, 5 days per week), is effective in iron removal and has low toxicity. However, less than 10% of the patients requiring iron chelation therapy worldwide are able to receive DF because of its high cost, low compliance and in some cases toxicity. In the last 10 years we have witnessed the emergence of oral chelation therapy, which could potentially change the prognosis of all transfusional iron-loaded patients. The only clinically available oral iron chelator is L1, which has so far been taken by over 6000 patients worldwide, in some cases daily for over 10 years, with very promising results. L1 was able to bring patients to a negative iron balance at doses of 50-120 mg/kg/day. It increases urinary iron excretion, decreases serum ferritin levels and reduces liver iron in the majority of chronically transfused iron-loaded patients. Despite earlier concerns of possible increased risk of toxicity, all the toxic side effects of L1 are currently considered reversible, controllable and manageable. These include agranulocytosis (0.6%), musculoskeletal and joint pains (15%), gastrointestinal complaints (6%) and zinc deficiency (1%). The incidence of these toxic side effects could in general be reduced by using lower doses of L1 or combination therapy with DF. Combination therapy could also benefit patients experiencing toxicity with DF and those not responding to either chelator alone. The overall efficacy and toxicity of L1 is comparable to that of DF in both animals and humans. Despite the steady progress in iron chelation therapy with DF and L1, further investigations are required for optimising their use in patients by selecting improved dose protocols, by minimising their toxicity and by identifying new applications in other diseases of iron imbalance.

Oxidation sensitivity may be a useful tool for the detection of the hematotoxic potential of newly developed molecules: application to antipsychotic drugs

Some antipsychotic agents have been found to produce agranulocytosis and aplastic anemia. The oxidation phenomena and/or the formation of free radicals has been suggested to be causally related to various hematological disorders, e.g., agranulocytosis. Using five experimental conditions, we tested the oxidative potential of compounds with and without a history of hematological side effects, e.g., agranulocytosis and aplastic anemia. A statistical analysis was undertaken for each experimental condition and a multivariate analysis combining all results was performed. Two peroxidase-induced free radical models did not successfully discriminate between drugs with and without a history of causing hematologic problems (<70%). The lipid peroxidation system provided even less satisfactory discrimination, with only 56.25% correct classification. However, an 87.5% correct classification was obtained when using the oxidation potentials of these drugs determined at pH 4.7 and at pH 7.4. A multivariate analysis taking into account the five variables provided 87.5% success in classification. The two clusters were better discriminated in terms of a "distance coefficient." In a second analysis, the putative antipsychotic pyridobenzodiazepine analogues (JL5, JL8, JL18, and JL25) were classified in the cluster of toxic compounds, while the oxa- and thiazepine analogues (JL2, JL3, and JL13) were classified as nontoxic compounds. On the other hand, a few metabolites of clozapine and fluperlapine were classified in the toxic compound group. The procedure described herein is, to our knowledge, the first which classifies molecules of different structures as well as different pharmacological profiles according to their hematotoxic potential. Such a procedure could be used to predict drug-induced hematological side effects.

Oxidation of clozapine and ascorbate by myeloperoxidase

The kinetics and spectra of the reactions of clozapine with compounds I and II of myeloperoxidase were investigated using both single- and sequential-mixing stopped-flow techniques, steady-state kinetics, and spectrophotometric measurements. The results show conclusively that both compounds I and II are reduced in one-electron reactions with clozapine. At pH 7.0 the rate constant for compound I reacting with clozapine is (1.5 +/- 0.1) x 10(6) M(-1) s(-1) and for compound II (4.8 +/- 0.1) x 10(4) M(-1) s(-1). The physiological pH of 7.4 was found to be optimal for the oxidation of clozapine by compound I. The rate constant for compound I reacting with ascorbate is (1.1 +/- 0.1) x 10(6) M(-1) s(-1) and for compound II (1.1 +/- 0.2) x 10(4) M(-1) s(-1), both obtained at pH 7.0. Experiments with both clozapine and ascorbate present showed that ascorbate acts both as a competitive inhibitor and free radical scavenger.

Metabolism of clozapine by human neutrophils: evidence for a specific oxidation of clozapine by the myeloperoxidase system with inhibition of enzymatic chlorination cycle

The use of clozapine, an unique antipsychotic drug, raises the real problem of drug-induced polymorphonuclear neutrophil cytotoxicity. Clozapine prescription has been restricted due to a 1-2% incidence of drug-induced agranulocytosis. The exact mechanism of this adverse effect is not yet known. The myeloperoxidase-hydrogen peroxide system could play a key role in the initiation of agranulocytosis. Therefore, we have investigated the clozapine effects on hydrogen peroxide and hypochlorous acid, evaluated the peroxidase-mediated metabolism of clozapine by mass spectrometry analysis because myeloperoxidase uses hydrogen peroxide and chloride producing hypochlorous acid in its chlorination cycle, and thus could oxidise clozapine in its peroxidation cycle. First, evidence for inhibition of hypochlorous acid production and scavenging of hydrogen peroxide by clozapine were demonstrated in vitro, in different cell-free and cellular systems. Results are consistent with an inhibition of the myeloperoxidase chlorination cycle when clozapine is oxidised in the peroxidation cycle. Secondly, ion-spray mass spectrometry analysis allowed us to confirm clozapine oxidation by the myeloperoxidase system. Actually, clozapine N-oxide with a m/z at 343 was formed. It could be the final step of the metabolisation of clozapine via two successive univalent oxidations mediated by peroxidase. We suggest that generation of a free cation radical, CLZ(o+), was the initial step. CLZ(o+) is a very reactive species and may play an important role in the onset of agranulocytosis either by direct toxicity or via an immunological mechanism. However, this assumption does not exclude the possible role of other metabolic ways involving, in particular, N-desmethylclozapine.

Peroxidase-catalysed oxidation of different dibenzazepine derivatives

According to a recent hypothesis suggesting the potential role of free radical formation in the clozapine-induced agranulocytosis, we have evaluated the susceptibility to the peroxidase-mediated oxidation of different dibenzazepine analogues. On the one hand, compounds with an arylamine group such as clozapine or isoclozapine present a high reactivity in the horseradish peroxidase or myeloperoxidase systems and, on the other hand, fluperlapine, though known to induce agranulocytosis, and other dibenzothiazepine and dibenzoxazepine derivatives appear insensitive to oxidation. Consequently, among tricyclic derivatives, the way of diaryloxa- and diarylthiazepine compounds could be an alternative for the development of safer drugs such as antipsychotics.

CD8+ dermal T cells from a sulphamethoxazole-induced bullous exanthem proliferate in response to drug-modified liver microsomes

There is evidence that T lymphocytes play a critical role in the pathogenesis of drug-induced bullous exanthems. Sulphonamides are known to be among the most frequent aetiological agents in these severe drug-induced cutaneous hypersensitivity reactions. Several studies indicate that cytochrome P450-dependent metabolites of sulphonamides act as the nominal allergens. A 70-year-old woman with a severe blistering exanthem caused by cotrimoxazole (sulphamethoxazole and trimethoprim) was studied. We employed an in vitro approach to determine whether cytochrome P450-dependent enzymes activated drug-specific T lymphocytes from this patient. Immunohistochemical analysis of involved skin revealed a majority of epidermal CD8+ T lymphocytes, whereas the dermal infiltrate was composed of both CD4+ and CD8+ T cells. Dermal T lymphocytes isolated from lesional skin proliferated in response to sulphamethoxazole, but not to trimethoprim, in the presence of autologous mononuclear cells used as antigen-presenting cells. The antigen-specific response of sulphamethoxazole-specific T cells was significantly augmented in the presence of murine liver microsomes with P450-dependent catalytic activities. Our observations suggest that some cutaneous hypersensitivity reactions to sulphamethoxazole are due to drug-specific T lymphocytes. Cytochrome P450-dependent enzymes may play a critical role in the formation of the nominal antigen, which is recognized by antigen-specific T cells.

Myeloperoxidase-mediated activation of xenobiotics by human leukocytes

Peripheral blood leukocytes contain a variety of enzymes that are capable of metabolising xenobiotics. The enzyme myeloperoxidase (MPO) appears to be the most important for drug metabolism. MPO is a peroxidase/oxidase and generates the powerful oxidant hypochlorous acid. MPO- or MPO-generated oxidants are capable of oxidizing a wide variety of compounds and a broad range of functional groups, especially those that contain nitrogen and sulfur. Leukocytes have a role in immune response; therefore, reactive intermediates generated by leukocyte metabolism of xenobiotics may have a role in idiosyncratic drug reactions, particularly those that are immune-mediated such as drug-induced lupus or agranulocytosis.

Present status and future prospects of oral iron chelation therapy in thalassaemia and other diseases

In the last few years we have witnessed the emergence of oral chelation which is a new form of therapy for transfusional iron-loaded patients in thalassaemia and other refractory anaemias. The need for a cheap, non-toxic, orally effective iron chelator is paramount because it could potentially save the lives of many thousands of patients. At present, less than 10% of the patients requiring iron chelation therapy worldwide receive the widely used chelating drug desferrioxamine (DF) because of its high cost, oral inactivity and toxicity. The most promising oral iron chelator is 1, 2-dimethyl-3-hydroxypyrid-4-one (L1 or INN: Deferiprone), which has so far been taken by over 450 patients in 15 countries, and in some cases daily for over 4 years with very promising results. L1 was shown at 50-100 mg/kg/day to be effective in bringing patients to negative iron balance. It increases urinary iron excretion, decreases serum ferritin levels and reduces liver iron in multi-transfused iron-loaded patients. Toxic side effects were mainly encountered at high doses (80-100 mg/kg/day) and include transient agranulocytosis (5 cases), transient musculoskeletal and joint pains (10-20%), gastric intolerance (2-6%) and zinc deficiency (1%). The incidence of these toxic side effects was reduced by using lower doses of 50-75 mg/kg/day. The overall efficacy and toxicity of L1 is comparable to that of DF in animals and humans. Further work is required for identifying susceptible individuals to L1 toxicity, and also optimum dose protocols of L1 which can maximise iron excretion and minimise the incidence of toxic side effects.

Evidence for free radical formation during horseradish peroxidase-catalyzed N-demethylation of crystal violet

Crystal violet (gentian violet) can undergo an oxidative metabolism, catalyzed by horseradish peroxidase, resulting in formaldehyde formation. The N-demethylation reaction was strongly inhibited by reduced glutathione. Evidence for the formation of a crystal violet radical during the horseradish peroxidase catalyzed reaction was the detection of thiyl and ascorbate radicals from glutathione and ascorbate, respectively. The concentration of radicals from both compounds was significantly increased in the presence of crystal violet. Oxygen uptake was stimulated when glutathione was present in the system and this oxygen uptake was dependent on the dye and enzyme concentration. Oxygen uptake did not occur when ascorbate, instead of glutathione, was present in the system. However, when glutathione was present, ascorbate totally inhibited the glutathione-stimulated oxygen uptake in the crystal violet/horseradish peroxidase/hydrogen peroxide system. Although a weak ESR spectrum from a crystal violet-derived free radical was detected when the dye reacted with H2O2 and horseradish peroxidase, using the fast flow technique, this spectrum could not be interpreted.

Drug-specific immune responses induced by procainamide, hydralazine and isoniazid in guinea-pigs

The drug-induced graft vs host reaction (GVHR) hypothesis requires, as its first step, specific T-cell immune responses to the drug-modified self. Procainamide, isoniazid and hydralazine are known to provoke various allergic reactions including GVHR-like adverse effects in man. We now report that drug-specific immune responses can easily be induced by these drugs in guinea-pigs. Twenty-five milligrams of each of these drugs and penicillin G, which is known to make covalent bonds with proteins and to also induce drug-specific immune responses, were mixed with complete Freund's adjuvant (CFA) and subcutaneously (s.c.) injected twice at an interval of 2 weeks into female Hartley guinea-pigs. The antibodies to these drugs were assessed by means of an enzyme-linked immunosorbent assay (ELISA). Two weeks after the last injection, all animals treated with isoniazid, hydralazine and penicillin G produced high titers of antibodies to these drugs. Antibodies to procainamide were also detected, although their antibody titers were low. The specificity of the antibodies produced were tested by the inhibition of ELISA and concentration-dependent inhibition was observed. Delayed type hypersensitivity (DTH) reactions were also observed in the animals treated with procainamide, isoniazid and hydralazine 2 weeks after the last injection. These results suggest that the allergic reactions observed in clinical use are related to the inducing potential of drug-specific immune responses in an animal system. Therefore, immunization of guinea-pigs with test drugs and CFA may give useful information for predicting the occurrence of allergic reactions in man.

Idiosyncratic reactions to antidepressants: a review of the possible mechanisms and predisposing factors

Antidepressants, a widely used group of drugs, are associated with a range of idiosyncratic reactions affecting in particular the liver, skin and both the hematological and central nervous systems. These reactions seem to be mediated by chemically reactive metabolites formed by the cytochrome P450 enzyme system, the toxicity occurring either directly or indirectly via an immune mechanism. Individual susceptibility is determined by factors, both genetic and environmental, which result in inadequate detoxication of the chemically reactive metabolite. Prevention of such reactions will depend on either the development of new compounds which are not converted to toxic metabolites or by prediction of individual susceptibility prior to drug administration.

Direct and metabolism-dependent toxicity of sulphasalazine and its principal metabolites towards human erythrocytes and leucocytes

1. The role of metabolites in sulphasalazine-mediated toxicity has been investigated in vitro by the use of human red blood cells and mononuclear leucocytes as target cells, with methaemoglobin formation and cytotoxicity respectively, being the defined toxic end-points. 2. Of the metabolites of sulphasalazine investigated, only sulphapyridine was bioactivated by human liver microsomes in the presence of NADPH to a metabolite which caused marked methaemoglobinaemia and a small, but statistically significant degree of mononuclear leucocyte cell death. 3. Methaemoglobinaemia was inhibited by ketoconazole but not by ascorbic acid (100 microM), glutathione (500 microM) and N-acetylcysteine (50 microM). In contrast, ascorbic acid and the thiols afforded complete protection for mononuclear leucocytes. 4. Sulphapyridine (100 microM) was converted in vitro to a metabolite (metabolite conversion 6.8 +/- 0.3%), the retention time of which on h.p.l.c. corresponded to synthetic sulphapyridine hydroxylamine. The half-life of sulphapyridine hydroxylamine in phosphate buffer (pH 7.4) was found to be 8.1 min. 5. In the absence of microsomes and NADPH, sulphapyridine hydroxylamine caused a concentration-dependent (10-500 microM) increase in methaemoglobinaemia (2.9%-24.4%) and cytotoxicity (5.4%-51.4%), whereas sulphasalazine, sulphapyridine, 5-hydroxy sulphapyridine and 5-aminosalicylic acid had no effect.

Mechanism of hypersensitivity reactions: proposed involvement of reactive metabolites generated by activated leukocytes

Hypersensitivity drug reactions are a major source of serious adverse drug reactions, yet very little is known about their mechanism. Several drugs are oxidized by activated neutrophils and mononuclear cells to reactive metabolites. Jack Uetrecht explains that the pattern of hypersensitivity reactions associated with these drugs - drug-induced lupus, agranulocytosis, and generalized hypersensitivity reactions - fits a mechanism in which leukocyte-generated reactive metabolites initiate the hypersensitivity reaction. Because activation of the leukocyte is necessary for reactive metabolite formation, one risk factor for a drug hypersensitivity reaction may be an infection or other inflammatory condition.