Drug metabolism by leukocytes and its role in drug-induced lupus and other idiosyncratic drug reactions

Characterization of short-lived electrophilic metabolites of the anticancer agent laromustine (VNP40101M)

Laromustine (VNP40101M; 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-(methylamino) carbonylhydrazine) is a novel sulfonylhydrazine alkylating agent. Phase 1 metabolism of laromustine was reported recently and showed that laromustine undergoes rearrangement, dehalogenation, and hydrolysis at physiological pH to form active moieties. (1) A mechanism for the rearrangement was proposed on the basis of fragmentation ions. (1) (,) (2) In this article, we report the phase II conjugates of VNP40101M and VNP4090CE which were formed after incubation of VNP40101M or VNP4090CE with pooled human liver microsomes (HLM) and cofactors nicotinamide adenine dinucleotide phosphate (NADPH), glutathione (GSH), N-acetylecysteine (NAC), and cysteine (CYS). Eight novel phase II conjugates (M-1 to M-8) were identified and characterized by hydrogen-deuterium exchange (H-D), stable isotope ((13)C-labeled VNP40101M), and MS(n) experiments. M-4 and M-5 were further confirmed by nuclear magnetic resonance spectroscopy (NMR). The short-lived CH(3)SO(2)CH(2)CH(2)-, methylformamide and CH(3)SO(2)NHN═CHCH(2)- moieties were generated from VNP40101M. The reactive intermediates CH(3)SO(2)CH(2)CH(2)- and methylformamide formed conjugates with GSH, CYS, and NAC. The CH(3)SO(2)NHN═CHCH(2)- moiety formed conjugates with GSH and NAC. M-2, M-4, and M-6 were only detected from the incubation of VNP40101M because VNP4090CE does not contain a methylformamide group. All other conjugates were formed by both VNP40101M and VNP4090CE. The in vitro studies found that VNP40101M and VNP4090CE undergo activation in human liver microsomes. The results from this study showed that laromustine produces several reactive intermediates that may play a role in the toxicities seen in the clinical trials.

Involvement of the immune system in idiosyncratic drug reactions

There is strong evidence that most idiosyncratic drug reactions (IDRs) are immune-mediated and are caused by reactive metabolites of a drug rather than by the drug itself. Several hypotheses have been proposed by which a drug could induce an immune response. The major hypotheses are the hapten hypothesis and the danger hypothesis; however, the characteristics and spectrum of IDRs are different with different drugs, and this likely reflects mechanistic differences; therefore, no one hypothesis is likely to explain all IDRs. Some IDRs appear to involve epigenetic effects, direct activation of antigen-presenting cells, or disturbing the normal balance of the immune system. It has been suggested that many cases of idiosyncratic liver injury are not immune-mediated, and other mechanisms such as mitochondrial injury may be involved. It is essential that any hypothesis be consistent with the clinical characteristics of the IDR. Although the characteristics of most idiosyncratic liver injury do not suggest that mitochondria are the major target, it is quite possible that milder mitochondrial injury could stimulate an immune-mediated reaction. The observation that IDRs can vary widely among different drugs and different patients is most easily explained by an immune mechanism in which the target of the immune response is different.

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*.

Incidence and risk factors of aplastic anemia in Latin American countries: the LATIN case-control study

Background Associations between aplastic anemia and numerous drugs, pesticides and chemicals have been reported. However, at least 50% of the etiology of aplastic anemia remains unexplained.

DESIGN AND METHODS

This was a case-control, multicenter, multinational study, designed to identify risk factors for agranulocytosis and aplastic anemia. The cases were patients with diagnosis of aplastic anemia confirmed through biopsy or bone marrow aspiration, selected through an active search of clinical laboratories, hematology clinics and medical records. The controls did not have either aplastic anemia or chronic diseases. A total of 224 patients with aplastic anemia were included in the study, each case was paired with four controls, according to sex, age group, and hospital where the case was first seen. Information was collected on demographic data, medical history, laboratory tests, medications, and other potential risk factors prior to diagnosis.

RESULTS

The incidence of aplastic anemia was 1.6 cases per million per year. Higher rates of benzene exposure (>/=30 exposures per year) were associated with a greater risk of aplastic anemia (odds ratio, OR: 4.2; 95% confidence interval, CI: 1.82-9.82). Individuals exposed to chloramphenicol in the previous year had an adjusted OR for aplastic anemia of 8.7 (CI: 0.87-87.93) and those exposed to azithromycin had an adjusted OR of 11.02 (CI 1.14-108.02). Conclusions The incidence of aplastic anemia in Latin America countries is low. Although the research study centers had a high coverage of health services, the underreporting of cases of aplastic anemia in selected regions can be discussed. Frequent exposure to benzene-based products increases the risk for aplastic anemia. Few associations with specific drugs were found, and it is likely that some of these were due to chance alone.

Drug-induced hypersensitivity syndrome: clinical and biologic disease patterns in 24 patients

Drug-induced hypersensitivity syndrome (DIHS), also called drug rash with eosinophilia and systemic symptoms (DRESS), is a severe reaction usually characterized by fever, rash, and multiorgan failure, occurring 1-8 weeks after drug introduction. It is an immune-mediated reaction involving macrophage and T-lymphocyte activation and cytokine release, although no consensus has been reached as to its etiology. The skin, hematopoietic system, and liver are frequently involved. DIHS can mimic severe sepsis, viral infection, adult-onset Still disease (AOSD), or lymphoproliferation.We describe 24 consecutive patients with DIHS who were hospitalized between September 2004 and March 2008. Criteria for inclusion in this observational study were suspected drug reaction, eosinophilia >or=500/microL and/or atypical lymphocytes, involvement of at least 2 organs (skin being 1 of them), with suggestive chronology and exclusion of other diagnoses. Our cohort of 12 women and 12 men had a median age of 49 years (range, 22-82 yr), and 11 had skin phototype V or VI. Patients with mild or no rash were immunocompromised (7/24)- defined as treatment with prednisone (>or=10 mg/d) and another immunosuppressant drug, or human immunodeficiency virus infection. All patients were febrile (>38 degrees C), 14 had localized or generalized edema, 7 had pharyngitis, 8 had lymphadenopathy, 22 had hepatitis, 4 had nephritis, 2 had noninfectious and nonlithiasic angiocholitis or cholecystitis. Ten patients were hypotensive, 5 of whom had associated laboratory signs and/or imaging findings suggestive of acute myocardial dysfunction. Half of the patients had hemogram abnormalities, including eosinophilia. Nine DIHS patients fulfilled the Fautrel criteria for AOSD diagnosis, including glycosylated ferritin <20% in 4/11, with or without laboratory characteristics of hemophagocytosis. Twenty DIHS episodes occurred during the less sunny months of October to March.We determined 25-hydroxyvitamin D3 (25[OH]D3) levels in 18 patients and found that 9 patients had vitamin D deficiency (<25 nmol/L or <10 microg/L) and 5 had vitamin D insufficiency (25-50 nmol/L). Moreover, 25(OH)D3 levels were inversely correlated with ferritin values. After culprit-drug withdrawal, outcomes were favorable for all patients, including those with cardiac abnormalities under slow tapering of glucocorticoids.We recommend looking for the frequent but underdiagnosed hypersensitivity myocarditis with noninvasive diagnostic tools, such as N-terminal probrain natriuretic peptide, and promptly withdrawing the culprit drug and starting glucocorticoids. Vitamin D deficiency might be a DIHS risk or severity factor, especially for patients with high skin phototype and during the winter. Because DIHS clinical and laboratory patterns share similarities with AOSD and hemophagocytosis, DIHS should be included in their differential diagnoses.

Reactive metabolites and agranulocytosis

Central to most hypotheses of the mechanism of idiosyncratic drug-induced blood dyscrasias is the involvement of reactive metabolites. In view of the reactive nature of the majority of such metabolites, it is likely that they are formed by, or in close proximity to the blood cells affected. The major oxidative system of neutrophils generates hypochlorous acid. We have demonstrated that the drugs associated with the highest incidence of agranulocytosis are oxidized to reactive metabolites by hypochlorous acid and/or activated neutrophils. There are many mechanisms by which such reactive metabolites could induce agranulocytosis. In the case of aminopyrine-induced agranulocytosis, most cases appear to involve drug-dependent anti-neutrophil antibodies, and these are likely to be induced by cell membrane antigens modified by the reactive metabolite of aminopyrine. The target of agranulocytosis associated with many other drugs is usually neutrophil precursors and may involve cytotoxicity or a cell-mediated immune reaction induced by a reactive metabolite. In the case of aplastic anaemia, there is evidence in some cases for involvement of cytotoxic T cells, which could either be induced by metabolites generated by neutrophils, or more likely, by reactive metabolites generated by stem cells.

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.

Enzyme-Mediated Protein Haptenation of Dapsone and Sulfamethoxazole in Human Keratinocytes: I. Expression and Role of Cytochromes P450

Cutaneous drug reactions (CDRs) are among the most common adverse drug reactions and are responsible for numerous minor to life-threatening complications. Several arylamine drugs, such as sulfamethoxazole (SMX) and dapsone (DDS), undergo bioactivation, resulting in adduction to cellular proteins. These adducted proteins may initiate the immune response that ultimately results in a CDR. Recent studies have demonstrated that normal human epidermal keratinocytes (NHEKs) can bioactivate these drugs, resulting in protein haptenation. We sought to identify the enzyme(s) responsible for this bioactivation in NHEKs. Using immunofluorescence confocal microscopy and an adduct-specific enzyme-linked immunosorbent assay (ELISA), we found that N-acetylation of the primary amine of SMX and DDS markedly reduced the level of protein haptenation in NHEKs. Detection of mRNA and/or protein confirmed the presence of CYP3A4, CYP3A5, and CYP2E1 in NHEKs. In contrast, although a faint band suggestive of CYP2C9 protein was detected in one NHEK sample, a CYP2C9 message was not detectable. We also examined the ability of chemical inhibitors of cytochromes P450 (aminobenzotriazole and 1-dichloroethylene) and cyclooxygenase (indomethacin) to reduce protein haptenation when NHEKs were incubated with SMX or DDS by either confocal microscopy or ELISA. These inhibitors did not significantly attenuate protein adduction with either SMX or DDS, indicating that cytochromes P450 and cyclooxygenase do not play important roles in the bioactivation of these xenobiotics in NHEKs and thus suggesting the importance of other enzymes in these cells.

Enzyme-mediated protein haptenation of dapsone and sulfamethoxazole in human keratinocytes: II. Expression and role of flavin-containing monooxygenases and peroxidases

Arylamine compounds, such as sulfamethoxazole (SMX) and dapsone (DDS), are metabolized in epidermal keratinocytes to arylhydroxylamine metabolites that auto-oxidize to arylnitroso derivatives, which in turn bind to cellular proteins and can act as antigens/immunogens. Previous studies have demonstrated that neither cytochromes P450 nor cyclooxygenases mediate this bioactivation in normal human epidermal keratinocytes (NHEKs). In this investigation, we demonstrated that methimazole (MMZ), a prototypical substrate of the flavin-containing monooxygenases (FMOs), attenuated the protein haptenation observed in NHEKs exposed to SMX or DDS. In addition, recombinant FMO1 and FMO3 were able to bioactivate both SMX and DDS, resulting in covalent adduct formation. Western blot analysis confirmed the presence of FMO3 in NHEKs, whereas FMO1 was not detectable. In addition to MMZ, 4-aminobenzoic acid hydrazide (ABH) also attenuated SMX- and DDS-dependent protein haptenation in NHEKs. ABH did not alter the bioactivation of these drugs by recombinant FMO3, suggesting its inhibitory effect in NHEKs was due to its known ability to inhibit peroxidases. Studies confirmed the presence of peroxidase activity in NHEKs; however, immunoblot analysis and reverse transcription-polymerase chain reaction indicated that myeloperoxidase, lactoperoxidase, and thyroid peroxidase were absent. Thus, our results suggest an important role for FMO3 and yet-to-be identified peroxidases in the bioactivation of sulfonamides in NHEKs.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Current trends in drug-induced autoimmunity

There are several hypotheses for the mechanism of drug-induced autoimmunity. It could be looked upon as an inappropriate activation of the immune system. One hypothesis is that drugs can inhibit DNA methylation leading to the activation of T cells. Another is that reactive metabolites can interfere with tolerance. Still other hypotheses focus on activation of antigen presenting cells (APCs). Monocytes can form chemically reactive metabolites and such metabolites could lead to their activation. One type of interaction between APCs and T cells involves the reversible formation of an imine bond between an amine on T cells and an aldehyde on the APCs. Several drugs, such as penicillamine and hydralazine, react irreversibly with aldehydes, and they are also associated with a high incidence of drug-induced lupus. This type of interaction may lead to activation of APCs and the induction of autoimmunity. However, there is no clear evidence to suggest that there is one simple mechanism by which drugs induce autoimmunity, and it is likely that several mechanisms may be involved and they may be different for different drugs.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

Toxic rise of clozapine plasma concentrations in relation to inflammation

Recently a small number of patients were observed in two psychiatric hospitals in the Netherlands with clozapine intoxications that complicate or mimic infections. These patients were on chronic medication and normally had stable clozapine blood plasma levels. This article presents four of these cases. Medline was searched for reports of similar cases. A hypothesis was formulated and tested by literature study. Immune modulatory and toxic effects of clozapine protein reactive metabolites or haptens, may play a role in the development of inflammation. Clozapine has a direct influence on different cytokines resembling an inflammatory reaction. Infection or inflammation could induce bioactivation of clozapine into its nitrenium ion that can exert a toxic reaction that induces apoptosis and gives rise to elevated cytokine levels. Clozapine can function as a hapten and induce an IgG, IgM or IgE mediated hypersensitivity reaction. The cytokines released during infection or inflammation downregulate the clozapine metabolism in the P450 system through CYP 1A2. Clozapine plasma levels should be monitored closely if an inflammatory or infectious process is suspected.

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.

Evaluation of hydralazine and procainamide effects on fibroblast membrane fluidity

In this study the membrane fluidity of fibroblasts under different pharmacological treatment was investigated. Two drugs, hydralazine and procainamide, were used to treat the immortalized mouse NIH 3T3 and hamster B14 fibroblasts. Membrane lipid dynamics was measured by fluorescence spectroscopy and electron spin resonance techniques. Two kinds of fluorescent probes (TMA-DPH and 12-(9-anthroyloxy)-stearic acid (12-AS)) and two spin labels (5-doxylstearic acid (5-DS) and 12-doxylstearic acid (12-DS)) were used to monitor fluidity in the upper polar and in the hydrophobic core regions of the lipid bilayer. The drugs influenced the membrane hydrophobic core, of which hydralazine induced fluidization and procainamide increased the rigidity. The membrane fluidity at the surface of the lipid bilayer was not modified by the drugs which indicates that both drugs intercalated mainly into the inner core of the cell membrane.

Structure toxicity relationships--how useful are they in predicting toxicities of new drugs?

This chapter provides just a few newer examples of structural moieties found in drugs that have been associated with reactive metabolite formation and toxicities. For a discussion of several other structures in drugs that undergo metabolic activation to reactive intermediates, the reader is directed to previous volumes in this series and other chapters in this book, as well as a previous condensed review (Nelson, 1982). Since that review, some new knowledge allows us to better predict that some structural moieties are more likely than others to form drug reactive metabolites that may be involved in causing toxic effects in humans. For example, most aniline-, thiophene-, and nitroaromatic-containing drugs have had a relatively high incidence of adverse effects, and it would be prudent in the drug discovery process to avoid these substructures if possible. However, as illustrated by the case of olanzapine, these structures may be important for potent activity, and could therefore be beneficial in some cases. The glitazones represent a new class of drugs with a unique thiazolidinedione structure. This raises two important points. First, it demonstrates how limited our knowledge base is in regard to structure toxicity relationships when new structures are introduced. Our approaches must be very empirical and are far from quantitative for the reasons outlined in the introduction. Secondly, the glitazones point out the importance of benefit/risk considerations. This was a new structural class of drugs with a unique spectrum of action that is very beneficial in the treatment of a major disease. Despite some suspected risk of toxicity, based on early trials, troglitazone was approved for use with careful monitoring. This author believes that was the right decision, as was the decision to withdraw the drug when the risk became unacceptable, especially with the introduction of safer alternatives. If this were just another NSAID (e.g., bromfenac), there would be little reason for approval. In summary, as I pointed out previously (Nelson, 1982), with our limited knowledge of structure toxicity relationships, we can only make reasonable judgments as to risk assessment of a new drug in humans, and hope that we neither release a dangerous chemical entity nor, as importantly, abort an effective one.

Cutaneous and systemic manifestations of drug-induced vasculitis

OBJECTIVE

To evaluate the literature for published cases of drug-induced vasculitis with cutaneous and/or systemic manifestations.

DATA SOURCES

The MEDLINE database was searched from 1965 to December 1999 for articles focusing on drugs and vasculitis, using various search terminologies (e.g., Churg-Strauss syndrome, Goodpasture's syndrome, Henoch-Schönlein purpura, various drugs suspected to induce vasculitis). Cases were included when they met the established criteria as described in the methodology.

DATA SYNTHESIS

Drugs found to be most frequently associated with vasculitis were propylthiouracil, hydralazine, colony-stimulating factors, allopurinol, cefaclor, minocycline, D-penicillamine, phenytoin, isotretinoin, and methotrexate. The interval between the first exposure and appearance of symptoms was reported to be extremely variable (hours to years). Vasculitis has occurred after drug dosage increases and after rechallenge with the suspected drug. In the majority of cases, vasculitis has resolved after discontinuing the drug. Patients with more severe, often life-threatening, manifestations have required treatment with corticosteroids, plasmapheresis, hemodialysis, or cyclophosphamide. Death was the result in 10% of all published cases, with a predominance in patients in whom multiple organ systems were involved.

CONCLUSIONS

Clinicians need to be suspect of drug-induced vasculitis to enable prompt diagnosis and treatment. This should improve patient outcomes based on the data referenced for this article.

Immunohistochemical detection of protein adducts of 2,4-dinitrochlorobenzene in antigen presenting cells and lymphocytes after oral administration to mice: lack of a role of Kupffer cells in oral tolerance

Although current studies suggest that most drug-induced allergic reactions (DIARS) are caused by immunogenic conjugates formed from the reaction of a reactive metabolite of a drug with cellular proteins, it is not clear why these reactions are relatively rare. One possible pathway that may explain the low incidence of DIARS in many cases is oral tolerance, an antigen-specific immunological hyporesponsiveness induced by oral administration of antigens. The mechanism of oral tolerance, however, is not clearly understood and is difficult to study directly with drugs, because animal models of DIARS have been elusive. We chose 2,4-dinitrochlorobenzene (DNCB) as a model compound to circumvent this problem because animal models of allergic reactions have been established for this compound. DNCB forms immunogenic 2,4-dinitrophenylated (DNP) protein conjugates that can induce immune reactions and it causes oral tolerance when it is fed to animals prior to sensitization. We hypothesized that DNP-protein conjugates may have a role in oral tolerance. To test this idea, we have begun to identify cells bearing these conjugates after the oral administration of DNCB. Female C57BL/6J mice were fed DNCB and tissues were examined after 6 and 24 h. Immunohistochemical analysis indicated the presence of DNP-protein conjugates in enterocytes of the small intestine, in macrophages and lymphocytes of the mesenteric lymph nodes, in dendritic cells and lymphocytes of the spleen, and in Kupffer cells and other sinusoidal cells of the liver. It was found that Kupffer cell depletion did not affect oral tolerance to DNCB. The findings suggest that the cells bearing DNP-protein conjugates, other than Kupffer cells, in the liver and other tissues may be important in the induction of oral tolerance against DNCB. Protein adducts of drugs administered orally may also be present in these cells, and they may have a role in the downregulation of DIARS in many individuals.

Antioxidants and fatty acids in the amelioration of rheumatoid arthritis and related disorders

The generation of reactive oxygen species (free radicals) is an important factor in the development and maintenance of rheumatoid arthritis in humans and animal models. One source of free radicals is nitric oxide produced within the synoviocytes and chondrocytes and giving rise to the highly toxic radical peroxynitrite. Several cytokines, including tumour necrosis factor-alpha (TNFalpha) are involved in the formation of free radicals, partly by increasing the activity of nitric oxide synthase. Indeed, nitric oxide may mediate some of the deleterious effects of cytokines on bone resorption. Aspirin, tetracyclines, steroids and methotrexate can suppress nitric oxide synthase. Dietary antioxidants include ascorbate and the tocopherols and beneficial effects of high doses have been reported especially in osteoarthritis. There is also evidence for beneficial effects of beta-carotene and selenium, the latter being a component of the antioxidant enzyme glutathione peroxidase. The polyunsaturated fatty acids (PUFA) include the n-3 compounds, some of which are precursors of eicosanoid synthesis, and the n-6 group which can increase formation of the pro-inflammatory cytokines TNFalpha and interleukin-6, and of reactive oxygen species. Some prostaglandins, however, suppress cytokine formation, so that n-3 PUFA often oppose the inflammatory effects of some n-6-PUFA. gamma-linolenic acid (GLA) is a precursor of prostaglandin E1, a fact which may account for its reported ability to ameliorate arthritic symptoms. Fish oil supplements, rich in n-3 PUFA such as eicosapentaenoic acid have been claimed as beneficial in rheumatoid arthritis, possibly by suppression of the immune system and its cytokine repertoire. Some other oils of marine origin (e.g. from the green-lipped mussel) and a range of vegetable oils (e.g. olive oil and evening primrose oil) have indirect anti-inflammatory actions, probably mediated via prostaglandin E1. Overall, there is a growing scientific rationale for the use of dietary supplements as adjuncts in the treatment of inflammatory disorders such as rheumatoid arthritis and osteoarthritis.

Hospitalizations for aplastic anemia and agranulocytosis in Saskatchewan: incidence and associations with antecedent prescription drug use

A population-based study to estimate the incidence of hospitalizations for aplastic anemia and agranulocytosis and to evaluate associations between these disorders and antecedent prescription drug use was performed using the Saskatchewan administrative health care databases. Hospital charts and cancer registration data were utilized to apply strict diagnostic criteria to case identification. Estimates of the annual incidence of aplastic anemia and agranulocytosis were 2.7 and 3.0 per million, which are similar to those from other recent population-based studies with comparable case identification criteria. Only a small number of cases were available to evaluate potential drug causes, leading to an analysis of limited power. Nevertheless, significant associations were found between aplastic anemia and tocainide, gold salts, and antidepressants; and between agranulocytosis and anti-thyroids, trimethoprim-sulfamethoxazole, beta-lactams, sulfasalazine, chlorpromazine, carbamazepine, glyburide, and procainamide, although the associations with antibiotics may indicate a response to the disorders rather than a possible cause. These findings are generally consistent with the results of other case-control investigations.

HLA-restricted, processing- and metabolism-independent pathway of drug recognition by human alpha beta T lymphocytes

T cell recognition of drugs is explained by the hapten-carrier model, implying covalent binding of chemically reactive drugs to carrier proteins. However, most drugs are nonreactive and their recognition by T cells is unclear. We generated T cell clones from allergic individuals specific to sulfamethoxazole, lidocaine (nonreactive drugs), and cef-triaxone (per se reactive beta-lactam antibiotic) and compared the increase of intracellular free calcium concentration ([Ca2+]i) and the kinetics of T cell receptor (TCR) downregulation of these clones by drug-specific stimulations. All drugs tested induced an MHC-restricted, dose- and antigen-presenting cell (APC)-dependent TCR downregulation on specific CD4(+) and CD8(+) T cell clones. Chemically nonreactive drugs elicited an immediate and sustained [Ca2+]i increase and a rapid TCR downregulation, but only when these drugs were added in solution to APC and clone. In contrast, the chemically reactive hapten ceftriaxone added in solution needed > 6 h to induce TCR downregulation. When APC were preincubated with ceftriaxone, a rapid downregulation of the TCR and cytokine secretion was observed, suggesting a stable presentation of a covalently modified peptide. Our data demonstrate two distinct pathways of drug presentation to activated specific T cells. The per se reactive ceftriaxone is presented after covalent binding to carrier peptides. Nonreactive drugs can be recognized by specific alphabeta+ T cells via a nonconventional presentation pathway based on a labile binding of the drug to MHC-peptide complexes.

Mechanism of clozapine-induced agranulocytosis : current status of research and implications for drug development

Clozapine is an atypical antipsychotic agent that has several advantages over conventional antipsychotics, not least of which is its superior efficacy. However, the high risk of agranulocytosis (0.8% of patients) associated with clozapine therapy has resulted in restricted indications for its use.The mechanism of clozapine-induced agranulocytosis is not clear. The target cells affected are the myeloid precursors, although the mature neutrophil may also be targeted simultaneously. There is no convincing evidence of direct toxicity of the parent compound or its stable metabolites (demethyl-clozapine and clozapine N-oxide). Clozapine is also metabolised by liver microsomes, peripheral blood neutrophils and their bone marrow precursors to a chemically reactive intermediate that has been postulated to be a nitrenium ion. This toxic metabolite has been shown to covalently bind to neutrophil proteins, suggesting that it may be involved in the pathogenesis of the toxicity. However, it is not clear how toxicity is mediated. The nitrenium ion may bind to essential cellular proteins and disrupt neutrophil function or, alternatively, it may act as a hapten and initiate an immune reaction resulting in immune-mediated destruction of the neutrophil. Indirect evidence exists to support both mechanisms, although clear direct evidence is still lacking. The role of cytokines and apoptosis in the pathogenesis of the agranulocytosis is unclear.The reason why only approximately 1% of individuals who are treated with clozapine are affected by agranulocytosis has not been elucidated. Evidence exists to implicate both the major histocompatibility complex antigens and heat shock protein variants in determining individual susceptibility, although more patients of different ethnic backgrounds need to be studied.The ultimate aim of research into clozapine-induced agranulocytosis should be to either prospectively predict which individuals are going to develop agranulocytosis and/or to develop analogues that retain efficacy but are not toxic. The former is complicated by the fact that predisposition may be multifactorial, and thus prediction may require multiple tests that may be of statistical but not absolute validity. The latter depends on identifying the mechanism of toxicity and the chemical characteristics of clozapine that are responsible for the toxicity. This knowledge may allow rational design of new analogues that do not cause agranulocytosis.

Free radicals produced during the oxidation of hydrazines by hypochlorous acid

Hypochlorous acid (HOCl) derived from activated neutrophils and monocytes has been implicated in the activation of hydrazine-containing drugs to toxic intermediates. However, reactive intermediates formed during the reaction between HOCl and these drugs have not been identified. We investigated the oxidation of the hydrazine derivatives isoniazid, iproniazid, and hydralazine by HOCl. The reaction between HOCl and all three hydrazines resulted in O2 consumption, indicating that free radicals were produced, but the rate and extent of O2 consumption were different for each hydrazine. Moreover, reduction of nitroblue tetrazolium (NBT) was observed only during the reaction between HOCl and isoniazid, suggesting that different radical species may be produced from HOCl reaction with each hydrazine. The oxidation of iproniazid by HOCl in the presence of the radical trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) resulted in the formation of a carbon-centered radical adduct. In contrast, the reaction between HOCl and hydralazine resulted in the formation of a nitrogen-centered DMPO radical adduct. The oxidation of isoniazid by HOCl resulted in the formation of two oxygen-centered radical adducts, DMPO-OOH and DMPO-OH. Myeloperoxidase-catalyzed oxidation of these hydrazines in the presence of Cl- and H2O2 produced radical species that were identical to those observed with HOCl. Thus, some of the toxic side effects of these drugs may be the result of the production of free-radical intermediates from reaction with neutrophil-derived oxidants, such as HOCl. The types of radicals produced and the consequences of generating these reactive species are discussed.

Effects of clozapine on plasma cytokine and soluble cytokine receptor levels

The antipsychotic drug clozapine frequently induces fever during the first weeks of administration. In addition, it has been shown that clozapine increases plasma soluble interleukin-2 receptor (sIL-2r) levels as early as 1 week after treatment is started. These findings suggest that clozapine has immunomodulatory effects. To investigate this issue in more detail, we assessed the time course of rectal temperature, blood cell counts, and cytokine and soluble cytokine receptor plasma levels during 6 weeks of clozapine treatment in 27 schizophrenic patients. Clozapine increased the plasma levels of tumor necrosis factor-alpha (TNF-alpha), soluble TNF receptors p55 and p75, and sIL-2r. These increases were independent of prior or concurrent medication and did also occur in patients who did not experience clozapine-induced fever. However, increases in TNF-alpha and sIL-2r levels were more pronounced in patients with clozapine-induced fever who showed in addition increased plasma IL-6 levels and granulocyte counts. Plasma IL-1 receptor antagonist levels and monocyte and lymphocyte counts were not affected by clozapine treatment. It is concluded that clozapine has consistent in vivo immunomodulatory effects. The results presented suggest that clozapine-induced fever is mediated by pyrogenic cytokines.

Lesions associated with drug reactions

Several of the more common oral lesions secondary to medication use have been presented in this article. Although some of these conditions are specific to the oral tissues, some are not. Numerous cutaneous reactions to medications have mucosal counterparts. It is important for the clinician to recognize that many oral lesions are simply a manifestation of a systemic condition and should be managed with that in mind.

Redox mediation in the peroxidase-catalyzed oxidation of aminopyrine: possible implications for drug-drug interactions

Many drugs, industrial pollutants, and other xenobiotics are known to be oxidized by peroxidases to potentially harmful free-radical intermediates. We have examined the possibility that certain compounds, acting as efficient peroxidase substrates, may stimulate the formation of reactive free radicals by acting as mediators of electron transfer reactions (redox mediators). To explore this hypothesis, we have investigated the interaction of two well-known peroxidase substrates, chlorpromazine and aminopyrine. As shown by ESR and UV-visible spectroscopy, chlorpromazine radical was able to oxidize aminopyrine to aminopyrine cation radical. The rate constant for this rapid, pH-dependent, reaction was estimated to be 1 x 10(7) M-1 s-1 at pH 4.5. Transient-state and steady-state kinetic studies both showed that rate constants for chlorpromazine oxidation to its cation radical by horseradish peroxidase (HRP) were about 100-fold greater than for the corresponding HRP-catalyzed oxidation of aminopyrine to its cation radical. When both aminopyrine and chlorpromazine were present with HRP and H2O2, aminopyrine cation radical formation was stimulated 100-fold. Concomitantly, the accumulation of chlorpromazine cation radical was completely inhibited in the presence of aminopyrine. Similar results were obtained when lactoperoxidase, myeloperoxidase, or the myeloperoxidase mimic HOCl were substituted for HRP. These data suggest that chlorpromazine can act as a redox mediator for peroxidase-catalyzed oxidation of aminopyrine and other chemicals. We suggest that some peroxidase substrates, acting as redox mediators, may stimulate the production of toxic free-radical intermediates from various drugs and other xenobiotics. As such, this may have implications for a number of adverse effects caused by these xenobiotic chemicals.

Effect of serotonin on the chemiluminescence response of rat peripheral blood leucocytes

Serotonin (5-HT) has been shown to exert various immunomodulatory effects. In this study, the effects of 5-HT, 5-hydroxy-DL-tryptophan (5-HTP) and DL-p-chlorophenylalanine (PCPA) on the chemiluminescence (CL) responses of rat peripheral blood leucocytes (PBL) activated by phorbol myristate acetate (PMA), opsonized zymosan or latex beads were assessed. The CL responses were measured following in vitro treatment with 0.01-100 microM 5-HT, and either 1 h after the last i.p. administration of 5-HT (0.05, 0.5, 1, 2.5, 5 or 10 mg/kg for 4 days), 5-HTP (25 or 100 mg/kg for 4 days) or PCPA (200 mg/kg for 4 days, n = 5), or 48 h after a single 200 mg/kg PCPA injection. A concentration-dependent decrease in CL responses was noted following in vitro 5-HT treatment. In vivo treatment of rats with 5-HT produced a reverse bell-curve inhibiting effects on the CL response with a maximal inhibition in rats receiving 1 mg/kg/day 5-HT and a weaker response of PMA-activated PBL. In vivo treatment with high-dose 5-HTP increased CL response of opsonized zymosan-activated PBL, while low-dose 5-HTP decreased CL response of opsonized zymosan and latex beads-activated PBL. No effect was observed in PMA-activated PBL from rats treated with 5-HTP. By contrast, in vivo treatment with PCPA increased CL responses induced by PMA or latex beads, whereas CL responses using opsonized zymosan were not significantly affected. These results suggest that 5-HT modulates the CL response of rat leucocytes to particulate stimuli.

Immune response to sulfamethoxazole in patients with AIDS

Antibody- and cell-mediated responses to sulfamethoxazole (SMX) were analyzed in AIDS patients with or without a history of hypersensitivity and in negative controls. In 20 of 20 (P < 0.01) human immunodeficiency virus (HIV)-seropositive patients with skin reactions to cotrimoxazole, we found SMX-specific antibodies, while only 9 of 20 and 17 of 20 HIV-seropositive patients without a history of hypersensitivity to cotrimoxazole had SMX-specific immunoglobulin M (IgM) and IgG, respectively. The levels of specific IgM and IgG were higher in patients with skin reactions than in patients without reactions (IgM, 1.0 +/- 0.19 versus 0.47 +/- 0.23 [P < 0.001]; IgG, 0.68 +/- 0.15 versus 0.47 +/- 0.14 [P < 0.001] [mean optical density values +/- standard deviations]). Seronegative controls with no history of exposure to sulfa compounds did not have SMX-specific IgG or IgM antibodies, and controls with a history of intake of SMX with or without reactions had low levels of IgG and IgM. The SMX-specific IgG subclasses were exclusively IgG1 and IgG3. None of the patients had detectable SMX-specific IgE or IgA antibodies nor did they exhibit a cell-mediated response as measured by a lymphocyte proliferation assay. Antibodies to SMX recognized N-acetyl-sulfonamide, N-(2-thiazolyl)-sulfanilamide, sulfadiazine, and sulfisoxazole but did not recognize sulfanilamide or 3-amino-5-methyl isoxazole in an inhibition assay. It is not known whether the SMX-specific antibodies associated with hypersensitivity reactions to SMX in HIV-seropositive patients have a pathogenic role in these reactions. Sulfanilamide or 3-amino-5-methyl isoxazole, on the other hand, could be potential alternative therapies in HIV-seropositive patients with a history of skin reactions to SMX.

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.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.

Promotion of oxidative damage to arachidonic acid and alpha 1-antiproteinase by anti-inflammatory drugs in the presence of the haem proteins myoglobin and cytochrome C

A mixture of myoglobin and hydrogen peroxide (H2O2) causes peroxidation of arachidonic acid. This peroxidation is greatly accelerated by adding phenylbutazone, which is effective even in the absence of H2O2. A wide range of other drugs was examined for their ability to exert similar pro-oxidant effects. We found that meclofenamic acid and flufenamic acid stimulated myoglobin-dependent lipid peroxidation, but only in the presence of H2O2. Ascorbic acid inhibited peroxidation both in the presence and in the absence of these drugs. Phenylbutazone, meclofenamic acid and flufenamic acid could also cause damage to proteins (as measured by inactivation of alpha 1-antiproteinase) in the presence of myoglobin and H2O2. The mitochondrial protein cytochrome c can also stimulate lipid peroxidation in the presence of H2O2. Phenylbutazone and meclofenamic acid, but not flufenamic acid, enhanced the peroxidation, which was again inhibited by ascorbic acid. However, only phenylbutazone caused inactivation of alpha 1-antiproteinase in the presence of cytochrome c and H2O2. Since respiring mitochondria generate superoxide radicals and H2O2, catalysis of lipid peroxidation and of the formation of drug-derived radicals by cytochrome c could be a mechanism contributing to mitochondrial damage by drugs.

Mechanisms of unpredictable adverse drug reactions

Adverse drug reactions are common problems associated with therapy, and are major sources of morbidity and mortality. There are numerous types of drug reactions, including predictable drug reactions such as side effects, toxicity, drug interactions and secondary effects that can be anticipated when planning therapy. There are also a number of unpredicted adverse effects, which are unexpected consequences of therapy. The least severe unpredicted adverse drug reaction is intolerance, which appears to be an exaggeration of pharmacological or toxic effects of the drug among vulnerable subsets of patients. Some of the most severe and life-threatening adverse drug reactions are allergic. These adverse effects can be mediated by a number of mechanisms, including the development of drug-specific IgE, serum-sickness-like reactions in response to drug-antibody complexes, direct release of inflammatory mediators, or involvement of the immune system by mechanisms that are poorly understood. Idiosyncratic adverse drug reactions are a heterogeneous group of adverse effects that are not predictable from the pharmacological actions of the drug. Many of these reactions occur as a consequence of pharmacogenetic variations in drug bioactivation and drug or metabolite detoxification or clearance. The physician must be vigilant for the possibility of unpredictable adverse drug reactions during or after therapy. Research currently underway may afford the opportunity to predict, and hopefully prevent, some of these adverse reactions in the future.

Lipoic and dihydrolipoic acids as antioxidants. A critical evaluation

A detailed evaluation of the antioxidant and pro-oxidant properties of lipoic acid (LA) and dihydrolipoic acid (DHLA) was performed. Both compounds are powerful scavengers of hypochlorous acid, able to protect alpha 1-antiproteinase against inactivation by HOCl. LA was a powerful scavenger of hydroxyl radicals (OH.) and could inhibit both iron-dependent OH. generation and peroxidation of ox-brain phospholipid liposomes in the presence of FeCl3-ascorbate, presumably by binding iron ions and rendering them redox-inactive. By contrast, DHLA accelerated iron-dependent OH. generation and lipid peroxidation, probably by reducing Fe3+ to Fe2+. LA inhibited this pro-oxidant action of DHLA. However, DHLA did not accelerate DNA degradation by a ferric bleomycin complex and slightly inhibited peroxidation of arachidonic acid by the myoglobin-H2O2 system. Under certain circumstances, DHLA accelerated the loss of activity of alpha-antiproteinase exposed to ionizing radiation under a N2O/O2 atmosphere and also the loss of creatine kinase activity in human plasma exposed to gas-phase cigarette smoke. Neither LA nor DHLA reacted with superoxide radical (O.2-) or H2O2 at significant rates, but both were good scavengers of trichloromethylperoxyl radical (CCl3O2.). We conclude that LA and DHLA have powerful antioxidant properties. However, DHLA can also exert pro-oxidant properties, both by its iron ion-reducing ability and probably by its ability to generate reactive sulphur-containing radicals that can damage certain proteins, such as alpha 1-antiproteinase and creatine kinase.

Modulation of procainamide toxicity by selenium-enriched yeast in rats

Free radical processes are proposed to play a crucial role in the development of procainamide adverse effects. Therefore, selenium, as a potent antioxidant, may modified procainamide toxicity. To test this hypothesis plasma and liver thiobarbituric acid-reacting substances (TBARS), plasma antioxidant activity (AOA), erythrocyte and liver superoxide dismutase (SOD), catalase, as well as selenium-dependent glutathione peroxidase (Se-GPX) were determined in the following four groups of rats: selenium-treated (Se), procainamide-treated (P), procainamide and selenium-treated (P + Se), and control (C). Morphological studies of leukocytes [tested for lupus erythematosus (LE) cells] and liver were also made. Atypical, i.e. enlarged and swollen, leukocytes resulting from procainamide and selenium treatment were observed. These changes were found in four out of five rats in the Se group, eight out of ten in the P group, and in seven out of ten in the P + Se group. LE-like cells were observed in two rats in the P + Se group. A statistically significant decrease in plasma and liver TBARS by 20% and 36%, respectively, increased activity of SOD by 20%, catalase by 48% and Se-GPX by 15% in erythrocytes, and decreased activity of liver SOD by 17% and catalase by 22% were found in the P + Se group as compared to the P group. These results indicated that selenium exerted antioxidant effects on the procainamide-treated rats. However, selenium did not prevent the development of disturbances in leukocyte morphology, on the contrary, it possibly promoted the conversion of leukocytes to LE 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.

Carbon-centered free radical formation during the metabolism of hydrazine derivatives by neutrophils

The neutrophil-catalyzed metabolism of hydrazine derivatives to carbon-centered radicals was investigated by the spin-trapping technique using alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). Oxidation of methylhydrazine (MeH), dimethylhydrazine (DMH), phenylethylhydrazine or procarbazine by neutrophils from rat peritoneal exudates led to the formation of alkyl radicals. The monosubstituted hydrazine oxidation by phorbol ester (PMA)- or Zymocel-activated neutrophils generated, on average, 2- to 4-fold more POBN-alkyl adducts than di-substituted hydrazines. Supernatant from sonicated neutrophils generated similar yields of radicals. Azide, an inhibitor of myeloperoxidase, effectively reduced the neutrophil-catalyzed radical yield from the oxidation of MeH but not DMH. On the other hand, superoxide dismutase and catalase effectively inhibited radical formation in DMH metabolism by PMA-activated neutrophils, in contrast to MeH metabolism. Our results show that neutrophils are able to metabolize hydrazine derivatives, the pathway depending on the hydrazine substitution. Alkyl radical production during the oxidation of mono-substituted derivatives, such as MeH, was mediated mainly by myeloperoxidase, and that of di-substituted derivatives, such as DMH, was mediated mainly by active oxygen species.

Immunopharmacology and adverse drug reactions

Adverse drug reactions are common and troublesome complications of contemporary pharmacotherapy. Adverse drug reactions are frequently, and often incorrectly, referred to as "allergy". Although there are multiple mechanisms for adverse drug reactions, adverse drug reactions mediated by the immune system account for a disproportionate number of fatal and serious adverse reactions, and constitute a major clinical problem for patients and physicians. The immune system has evolved in multicellular organisms as a defence against infection. Interactions between drugs and the immune system occur as inadvertent consequences of the protective function of the immune system, with drug molecules or drug-carrier haptens being recognized as "non-self" by the immune system. The classical mechanisms for drug hypersensitivity described by Gell and Coombs (Types 1 to 4) include IgE-mediated, cytotoxic, immune complex-mediated and delayed mechanism. These mechanisms provide elegant models for drug-immune interactions that can provide mechanistic explanations for events such as urticaria associated with penicillins. However, these mechanisms do not account for many of the immunologically mediated adverse reactions commonly encountered in clinical practice. Over the last two decades, there has been an increasing awareness of the importance of reactive drug metabolites and drug-protein interactions in the initiation of immunologic events mediating adverse drug reactions. Reactive drug metabolites may produce direct and profound effects on various functions of the immune system. Although some adverse reactions mediated by the immune system occur with equal frequency among adults and children, some of these reactions appear to be markedly more common among children than adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Idiosyncratic drug reactions: a mechanistic evaluation of risk factors

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Oxidative damage to lipids and alpha 1-antiproteinase by phenylbutazone in the presence of haem proteins: protection by ascorbic acid

Phenylbutazone is an anti-inflammatory drug with numerous side-effects that restrict its clinical use. In the presence of myoglobin, or of haemoglobin plus H2O2, phenylbutazone accelerates the peroxidation of lipids (arachidonic acid and rat liver microsomes) and causes the inactivation of alpha-antiproteinase, so that this protein can no longer inhibit elastase. We propose that haem proteins oxidize phenylbutazone into a damaging free radical. Ascorbic acid inhibits these pro-oxidant actions of phenylbutazone.