Interleukin-12 in Treating Patients With Multiple Myeloma

DISEASE CHARACTERISTICS: Histologically proven plateau phase multiple myeloma at original diagnosis: Bone marrow plasmacytosis with greater than 10% plasma cells, sheets of plasma cells, or biopsy proven plasmacytoma Must be in stable plateau phase requiring objective response, no evidence of continuing improvement by any criteria, and less than 20% variation in M protein at least 4 weeks prior to study Must have at least one of the following at original diagnosis: M protein in serum or urine X-ray evidence of osteolytic lesion Measurable or evaluable M protein Serum M protein greater than 1.0 g/dL Urine M protein greater than 200 mg/24 hours M protein less than these values will be considered evaluable (serum less than 1 g/dL or urine less than 200 mg/24 hours) Nonsecretory patients are ineligible PATIENT CHARACTERISTICS: Age: 18 and over Performance Status: ECOG 0-2 Life Expectancy: Not specified Hematopoietic: WBC at least 2,500/mm3 Absolute neutrophil count at least 1,250/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin less than 1.5 times upper limit of normal (ULN) SGOT less than 1.5 times ULN Renal: Creatinine clearance at least 50 mL/min Cardiovascular: No New York Heart Association class III or IV congestive heart failure Other: Not pregnant or nursing Fertile patients must use effective contraception No uncontrolled peptic ulcer disease No inflammatory bowel disease No history of significant autoimmune disease (rheumatoid arthritis or systemic lupus erythematosus) PRIOR CONCURRENT THERAPY: Biologic therapy: At least 60 days since prior biologic response modifiers At least 60 days since prior bone marrow transplantation Chemotherapy: At least 60 days since prior chemotherapy Endocrine therapy: No concurrent corticosteroids Radiotherapy: Not specified Surgery: Not specified