DISEASE CHARACTERISTICS:

• Histologically confirmed cancer that is potentially incurable by standard chemotherapy, radiotherapy, or surgical procedures

  • Failed prior first-line therapy (patients are also eligible if no effective first-line therapy exists)
• Previously treated or clinically stable brain metastases are allowed
• No leukemia or lymphoma
• No clinically significant pleural effusions or ascites

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Performance status:

• Karnofsky 70-100%

Life expectancy:

• Not specified

Hematopoietic:

• WBC at least 4,000/mm^3
• Platelet count at least 160,000/mm^3
• Hemoglobin greater than 10 g/dL
• Serum and RBC folate normal

Hepatic:

• Bilirubin no greater than 1.5 mg/dL
• SGOT or SGPT less than 2 times upper limit of normal

Renal:

• Creatinine no greater than 1.2 mg/dL OR
• Creatinine clearance at least 60 mL/min

Cardiovascular:

• No unstable angina
• No congestive heart failure
• No cardiac arrhythmia

Other:

• Homocysteine normal
• No grade III or IV edema
• Not pregnant or nursing
• Fertile patients must use effective contraception during and for 6 months after study

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• See Disease Characteristics
• At least 3 weeks since prior chemotherapy (4 weeks for nitrosoureas or mitomycin) and recovered

Endocrine therapy:

• Not specified

Radiotherapy:

• See Disease Characteristics
• At least 3 weeks since prior radiotherapy to bone marrow-containing areas and recovered

Surgery:

• See Disease Characteristics
• No prior pneumonectomy

Other:

• No concurrent folic acid or potentially nephrotoxic agents