Suppression of the immune system is a requirement dur-ing organ transplantation because of the propensity of the recipient to reject the foreign tissue by immunologi-cal mechanisms. Since transplantation is usually per-formed in patients with a poor prognosis for survival, the use of immunosuppressive agents has potentially great therapeutic benefit, because it provides the only real hope of continued life for many individuals. Immunosuppression, however, is frequently an adverse reaction when these drugs are used as antineoplastic drugs.
In the past, immunosuppression could be achieved only through the use of nonspecific cytotoxic drugs (e.g., cyclophosphamide or azathioprine), which are particu-larly toxic to rapidly proliferating cells, such as those of the bone marrow, gonadal tissue, and gastrointestinal tract. Consequently, serious side effects, including bone marrow depression, overwhelming infections, and steril-ity, limited their usefulness as immunosuppressants. The concurrent use of corticosteroids with the immunosup-pressants increased the risk of additional toxicity. With the development of the immunosuppressants cy-closporine and tacrolimus it is now possible to avoid much of this toxicity. Because of their relatively low tox-icity, these drugs have revolutionized the field of trans-plantation. It is now possible to successfully transplant tissues to patients not previously considered as candi-dates for transplantation.