Chronic myeloid leukemia in myasthenia gravis after long-term treatment with 6-mercaptopurine

A woman with myasthenia gravis and a thymoma did not respond sufficiently to thymectomy. She was treated with 6-mercaptopurine. Withdrawal of this treatment was several times followed by an aggravation of myasthenic symptoms. After more than 12 1/2 years treatment she developed Ph1-positive chronic myeloid leukemia (CML). No other case of CML following immunosuppressive treatment has been described. Because the therapeutic agent is potentially leukemogenic, the possibility cannot be definitely excluded that the development of CML is not a mere coincidence.