Clinical use of the immunosuppressive drug azathioprine is limited by potentially serious toxic effects related to depression of bone marrow function. The immunosuppressive and toxic properties of azathioprine are regarded as being properties of the cytotoxicity of its metabolite, 6-mercaptopurine (6-MP). However, azathioprine has an immunosuppressive effect additional to that attributable to 6-MP alone, and we propose that this is associated with an action of the methylnitroimidazolyl substituent. This suggests a route to the rational design of nontoxic immunosuppressants by replacing the 6-MP component of azathioprine with nontoxic thiols. We have synthesized and tested in vitro 24 such analogues, with two being further tested in vivo. In the human mixed lymphocyte reaction, virtually all compounds showed some degree of activity, 10 compounds being more active than azathioprine. In vivo, two compounds were more effective than azathioprine at prolonging graft survival in mice. In an oral toxicity study in male CD1 mice at doses equivalent to those at which azathioprine caused severe bone marrow depression both analogues had no toxic effects. Our results show that the immunosuppressive effects and bone marrow toxicity of azathioprine are not a consequence of release of 6-MP alone, and with appropriate modification can be separated, an approach which may lead to less toxic immunosuppressive drugs.
The mixed lymphocyte reaction of Lesch-Nyhan patients (HGPRT deficient) was used to study the immunosuppressive effects of azathioprine and 6-mercaptopurine (6-MP). Mitogen stimulated lymphocytes of these patients are highly resistant to azathioprine and 6-MP. When both stimulator and responder lymphocytes in the MLR were HGPRT deficient, azathioprine (36 ,uM) was much more inhibitory than 6-MP (100 ,uM). Azathioprine produced inhibition of 98.2% and 78.5% compared with the values of 63.9% and 30.6% for 6-MP. The difference in inhibitory activity between azathioprine and 6-MP was reduced when normal stimulator lymphocytes were cultured with HGPRT deficient responder lymphocytes in the MLR. These results provide very strong evidence that the nucleotide metabolites of azathioprine and 6-MP are unnecessary for immunosuppression. They also suggest that azathioprine and 6-MP interfere with antigenic triggering of the MLR.
1 The effect of interleukin-2 on the response of the human MLR to azathioprine was studied. Azathioprine (36 ,LM) inhibited the MLR by 86% and interleukin-2 (25%) stimulated the MLR by 99%. 2 Only partial relief (16%) of azathioprine (36 giM) inhibition was found in the interleukin-2 treated MLR compared with azathioprine treatment alone. 3 However, [3H]-thymidine incorporation by azathioprine (36 ,IM) + interleukin-2 treated MLR was 4.3 times greater than that exposed to azathioprine alone. As the residual number of immunocompetent cells after drug exposure is pathologically more important than the fraction suppressed, this result may be of clinical relevance. 4 These results show that the major mode of action of azathioprine is not the inhibition of interleukin-2 production.
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