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

The role of leukocyte-generated reactive metabolites in the pathogenesis of idiosyncratic drug reactions

Evidence strongly suggests that many adverse drug reactions, including idiosyncratic drug reactions, involve reactive metabolites. Furthermore, certain functional groups, which are readily oxidized to reactive metabolites, are associated with a high incidence of adverse reactions. Most drugs can probably form reactive metabolites, but a simple comparison of covalent binding in vitro is unlikely to provide an accurate indication of the relative risk of a drug causing an idiosyncratic reaction because it does not provide an indication of how efficiently the metabolite is detoxified in vivo. In addition, the incidence and nature of adverse reactions associated with a given drug is probably determined in large measure by the location of reactive metabolite formation, as well as the chemical reactivity of the reactive metabolite. Such factors will determine which macromolecules the metabolites will bind to, and it is known that covalent binding to some proteins, such as those in the leukocyte membrane, is much more likely to lead to an immune-mediated reaction or other type of toxicity. Some reactive metabolites, such as acyl glucuronides, circulate freely and could lead to adverse reactions in almost any organ; however, most reactive metabolites have a short biological half-life, and although small amounts may escape the organ where they are formed, these metabolites are unlikely to reach sufficient concentrations to cause toxicity in other organs. Many idiosyncratic drug reactions involve leukocytes, especially agranulocytosis and drug-induced lupus. We and others have demonstrated that drugs can be metabolized by activated neutrophils and monocytes to reactive metabolites. The major reaction appears to be reaction with leukocyte-generated hypochlorous acid. Hypochlorous acid is quite reactive, and therefore it is likely that many other drugs will be found that are metabolized by activated leukocytes. Some neutrophil precursors contain myeloperoxidase and the NADPH oxidase system, and it is likely that these cells can also oxidize drugs. Therefore, although there is no direct evidence, it is reasonable to speculate that reactive metabolites generated by activated leukocytes, or neutrophil precursors in the bone marrow, could be responsible for drug-induced agranulocytosis and aplastic anemia. This could involve direct toxicity or an immune-mediated reaction. These mechanisms are not mutually exclusive, and it may be that both mechanisms contribute to the toxicity, even in the same patient. In the case of drug-induced lupus, a prevalent hypothesis for lupus involves modification of class II MHC antigens.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Phytohemagglutinin-stimulated lymphocytes from patients with systemic lupus erythematosus demonstrate DNA damage

DNA samples from control and lupus lymphocytes were studied for DNA integrity and single-strand breaks by agarose gel electrophoresis following digestion with the enzyme S1 nuclease. S1 nuclease digests single-strand gaps in double-stranded DNA. Gel patterns of phytohemagglutinin (PHA)-stimulated control and lupus lymphocyte DNAs were identical in the absence of S1 nuclease incubation. DNA isolated from PHA-stimulated control lymphocytes was relatively resistant to S1 nuclease digestion in 14 of 16 samples. However, 15 of 16 DNA samples from PHA-stimulated lupus lymphocytes demonstrated dramatically greater S1 nuclease digestion than paired control DNAs from lymphocytes analyzed at the same time under the same conditions. Increased S1 sensitivity suggests that more single-strand DNA breaks were found in PHA-stimulated lupus lymphocytes and/or the lupus DNA was more damaged than control DNA. We suggest that structural changes found in DNA from stimulated T lymphocytes of lupus patients are consistent with an endogenous antigen-mediated disorder.

Systemic lupus erythematosus (SLE)-like syndromes associated with carbamazepine therapy

Records of 80 cases of systemic lupus erythematosus (SLE)-like syndromes associated with use of carbamazepine entered in the single case files of Central Drug Monitoring, Ciba-Geigy, Basel, were reviewed. Included in these were 11 cases reported in the literature. These were assessed in light of the current concepts of both idiopathic and drug-induced forms of SLE. Even allowing for under-reporting of adverse drug reactions, the number of such cases is far below the prevalence rates for SLE. This adverse drug reaction remains allocated to the category 'isolated cases' i.e. frequency is less than 0.001% of the cases treated. Antinuclear antibody (ANA) and anti-double-stranded DNA (anti-dsDNA) tests were not helpful in differentiating between idiopathic and drug-induced SLE. IgG antihistone antibodies in the absence of high titres of dsDNA antibodies is the only reliable test. Symptoms of SLE-like syndromes which manifest after carbamazepine therapy usually subside after discontinuation of the drug. Persistence of symptoms support the diagnosis of idiopathic SLE.

Metabolism of drugs by activated leukocytes: implications for drug-induced lupus and other drug hypersensitivity reactions

Despite their importance, little is known about the mechanism of idiosyncratic reactions, many such reactions have characteristics that suggest an immune-mediated mechanism. This is particularly true of drug-induced lupus which is an autoimmune syndrome. Certain functional groups are associated with a high incidence of idiosyncratic reactions, probably reflecting the ease with which they are metabolized to reactive metabolites. Although the liver is the principal organ of drug metabolism, most reactive metabolites generated in the liver would not reach other organs in significant concentrations. Because of the function of leukocytes, especially monocytes, in the induction of an immune response, the generation of reactive metabolites by monocytes would seem likely to lead to an immune-mediated adverse reaction. We have found that drugs that are associated with drug-induced lupus are oxidized to reactive metabolites by the myeloperoxidase system of monocytes. The initial step in drug-induced lupus could be haptenization of a protein on the surface of monocytes by these reactive metabolites. Other types of idiosyncratic drug reactions may involve a similar mechanism and the same drugs that induce lupus are usually associated with a high incidence of other types of idiosyncratic reactions. for example, procainamide, which causes the highest incidence of drug-induced lupus, also causes a relatively high incidence of agranulocytosis. Even some of the therapeutic effects of drugs may involve the production of reactive metabolites by myeloperoxidase or thyroid peroxidase.

Expression of N-acetyl transferase in a human monocytic cell-line, U937

1. N-acetyl transferase (NAT) catalyses the acetylation of arylamine and hydrazine drugs and other xenobiotics. The activity of one isozyme (polymorphic NAT) varies amongst individuals but the other (monomorphic NAT) does not. 2. The human monocytic cell-line U937 transcribes the gene for monomorphic N-acetyl transferase. 3. Although the gene for polymorphic N-acetyl transferase is present in these cells, its expression is not detected. 4. It is concluded that U937 cells are a useful model for studying the metabolism of arylamines and hydrazines by human monomorphic N-acetyl transferase.