Methotrexate blocks dihydrofolate reductase, thus preventing conversion of the vitamin folate to its active form. Prednisone is an anti-inflammatory agent, which will inhibit proliferation of lymphocytes. Vincristine is a vinca alkaloid, which blocks tubulin assembly, which is necessary for cell division. 6-mercaptopurine is a base analog of guanine. One problem is interfering with normal growth.
Chemotherapy generally targets rapidly dividing cells.
The joint pain may be an indication of gout due to rapid turnover of the leukemia cells, resulting in increased purine metabolism as well as acidosis and dehydration. Since allopurinol will also block the normal catabolic pathway for the cancer drug 6-mercaptopurine, the latter will remain effective longer. Thus, lower doses must be used to prevent toxicity.