A re-examination of the genotoxicity and carcinogenicity of azathioprine

Mapping and analysis of chromatin state dynamics in nine human cell types

Chromatin profiling has emerged as a powerful means for genome annotation and detection of regulatory activity. Here we map nine chromatin marks across nine cell types to systematically characterize regulatory elements, their cell type-specificities, and their functional interactions. Focusing on cell type-specific patterns of promoters and enhancers, we define multi-cell activity profiles for chromatin state, gene expression, regulatory motif enrichment, and regulator expression. We use correlations between these profiles to link enhancers to putative target genes, and predict the cell type-specific activators and repressors that modulate them. The resulting annotations and regulatory predictions have implications for interpreting genome-wide association studies. Top-scoring disease SNPs are frequently positioned within enhancer elements specifically active in relevant cell types, and in some cases affect a motif instance for a predicted regulator, thus proposing a mechanism for the association. Our study presents a general framework for deciphering cis-regulatory connections and their roles in disease.

Emerging treatments in multiple sclerosis: azathioprine and mofetil

Global immunosuppression instead of focused selective or specific immunomodulating strategies may still be relevant in diseases with chronic and broad immune dysregulation such as multiple sclerosis (MS). Among classical or new immunosuppressive drugs, two of them, both inhibiting purine synthesis, show an attractive profile for MS treatment. Azathioprine (AZA) is the most anciently and widely used global immunosuppressive drug in MS. Despite founded initial fears, it can be stated today that AZA is usually well tolerated and compatible with normal daily activities, that it requires minimal monitoring and does not significantly increase the risk of cancer induction after 5 years of continuous usage at the conventional 2.5 mg/kg daily dose. The only two presently available well conducted trials of AZA in ambulatory patients with relapsing-remitting MS show marginally significant beneficial results of AZA treatment on relapse frequency and disability. Some preliminary data on brain MRI are also promising. Mycophenolate mofetil (MMF) affects mainly the desired cell types, with a good safety profile, a rapidly reversible activity, and an absence of mutagenic effect and chromosome breakage. However, it remains to be shown that promising experimental results can be converted into significant clinical results in MS. It is presently demonstrated for AZA and it is presumable for MMF that neither drug is able to cure MS. However, it can be anticipated that either drug in combination with other strategies such as recombinant beta interferon could represent a significant adjunct for the therapeutic control of MS, at least in early ambulatory relapsing-remitting MS. Presently, the choice between the old, no longer 'sexy', but well-known drug as AZA and a young, appealing, but still to be better evaluated drug (notably for the long run) as MMF is a matter of personal, community, industrial and scientific inclination.

Acute leukaemia following renal transplantation

Four renal transplant patients on immunosuppressive therapy who presented with acute myeloid leukaemia are described. In two cases, azathioprine may have played an important role as a cofactor in leukaemogenesis. In a third case, the alkylating agent cyclophosphamide may have contributed. All patients were treated for leukaemia with full doses of cytotoxic chemotherapy and, in each case, a functioning renal allograft was preserved throughout the treatment despite attenuation of immunosuppressive therapy. Three patients achieved complete remission. Of the three, one is surviving at 2 years and two expired during the pancytopenic phase of their treatment with no active leukaemia present, and with intact renal function. As increasing expertise in the field of organ transplantation allows patients to survive longer, such patients' exposure to immunosuppressive and potentially leukaemogenic drugs is prolonged. The risk of secondary neoplasia has been previously documented in this population. Two of the four cases reported here suffered from polycystic kidney disease as their underlying condition. While this report suggests that the leukaemias are related to renal transplantation, we cannot rule out an association with the underlying disease which led to the transplant. This report further suggests that the leukaemia that develops in such patients may respond to standard therapy, and that such treatment does not compromise the transplanted kidney.

Carcinogenicity of azathioprine: an S-AR investigation

Besides its use in the treatment of a variety of presumed autoimmune diseases, azathioprine is given as an immunosuppressant to patients who have had renal transplants. Though epidemiological studies have provided "sufficient" evidence of its carcinogenicity in humans, the carcinogenicity tests in rats and mice are considered to be inconclusive because of limitations in the design and results of these tests (IARC, 1981, 1987). Rosenkranz and Klopman (1991) used the CASE program to identify the structural features responsible for its carcinogenicity. They concluded that this genotoxic chemical was a carcinogen due to the presence of the molecular fragment C"-S-C=. The finding was based on the presence of this biophore fragment in five other compounds, namely: 2-amino-5-nitrothiazole, 2-mercaptobenzothiazole, fenthione, 4,4'-thiodianiline and nithiazide. Recently, Ashby (1992) has expressed concern over the validity of their findings. With the aim of contributing to this debate on the mechanism of carcinogenicity of azathioprine, we have analyzed the structural basis of carcinogenicity of azathioprine and the five support compounds using the carcinogenicity predictor of our toxicity prediction program, TOPKAT. The results, more in line with Ashby's concerns, indicate that no molecular fragment involving the S atom is associated with the carcinogenic properties of these molecules. According to the TOPKAT program the carcinogenicity, if any, of azathioprine is due to the NO2 electrophile because its other major structural features are found to be either associated with non-carcinogenicity or do not discriminate carcinogens from non-carcinogens.

Induction of micronuclei in rat bone marrow and peripheral blood following acute and subchronic administration of azathioprine

The frequency of micronuclei was assessed in polychromatic erythrocytes of bone marrow and in polychromatic and normochromatic erythrocytes in peripheral blood of rats following exposure to azathioprine for 28 days. This was compared with the incidence of micronuclei in bone-marrow following exposure to a single dose of azathioprine. The incidence of micronuclei in bone-marrow polychromatic erythrocytes at the maximum tolerated dose (10 mg/kg) following exposure for 28 days was 29.5/1000. The incidence of micronucleated polychromatic erythrocytes in the peripheral blood at this dose was 4.4/1000. At the maximum tolerated dose in the single-dose study (40 mg/kg) the incidence obtained at 48 h post-treatment was 15.7/1000. This supports the view that the use of animals in a subchronic toxicity study is at least as sensitive for assessing in vivo clastogenic activity as an acute study and could reduce animal usage in toxicology assessments.