- Methotrexate (25 mg per week for at least 3 months, or until drug toxicity or intolerance develops) and/or a thiopurine antimetabolite (either 2.5 mg/kg azathioprine or 1.5 mg/kg 6-mercaptopurine in patients homozygous wild-type for the thiopurine-S-methyltransferase [TPMT] gene, or either 1.5 mg/kg azathioprine or 1 mg/kg 6-mercaptopurine in patients heterozygous for TPMT, or doses of these drugs capable of producing a 6-thioguanine nucleotide level of 230-400 without producing a 6-methylmercaptopurine nucleotide level above 5700 for at least 3 months, or until drug allergy, intolerance or toxicity develops); if a patient is homozygous mutant for the TPMT gene, thiopurines would be contraindicated and their use would not be a requirement for enrollment in this protocol